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Publications

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2013 Publications

Two-photon interference between disparate sources for quantum networking
A.R. McMillan, L. Labonte, A.S. Clark, B. Bell, O. Alibart, A. Martin, W.J. Wadsworth, S. Tanzilli & J.G. Rarity
Sci. Rep. 3 (2013) 2032
Quantum networks involve entanglement sharing between multiple users. Ideally, any two users would be able to connect regardless of the type of photon source they employ, provided they fulfill the requirements for two-photon interference. From a theoretical perspective, photons coming from different origins can interfere with a perfect visibility, provided they are made indistinguishable in all degrees of freedom. Previous experimental demonstrations of such a scenario have been limited to photon wavelengths below 900 nm, unsuitable for long distance communication, and suffered from low interference visibility. We report two-photon interference using two disparate heralded single photon sources, which involve different nonlinear effects, operating in the telecom wavelength range. The measured visibility of the two-photon interference is 80 ± 4%, which paves the way to hybrid universal quantum networks.

Newton's cradles in optics: From N-soliton fission to soliton chains
R. Driben, B.A. Malomed, A.V. Yulin & D.V. Skryabin
Phys. Rev. A 87 (2013) 063808
A mechanism for creating a Newton's cradle (NC) in nonlinear light wave trains under the action of the third-order dispersion (TOD) is demonstrated. The formation of the NC structure plays an important role in the process of fission of higher-order (N) solitons in optical fibers. After the splitting of the initial N soliton into a nonuniform chain of fundamental quasisolitons, the tallest one travels along the entire chain, through consecutive collisions with other solitons, and then escapes, while the remaining chain of pulses stays as a bound state, due to the radiation-mediated interaction between them. Increasing the initial soliton's order, N, leads to the transmission through, and release of additional solitons with enhanced power, along with the emission of radiation, which may demonstrate a broadband supercontinuum spectrum. The NC dynamical regime remains robust in the presence of extra perturbations, such as the Raman and self-steepening effects, and dispersion !

Experimental characterization of universal one-way quantum computing
B.A. Bell, M.S. Tame, A.S. Clark, R.W. Nock, W.J. Wadsworth & J.G. Rarity
New J. Physics 15 (2013) 053030
We report the characterization of a universal set of logic gates for one-way quantum computing using a four-photon 'star' cluster state generated by fusing photons from two independent photonic crystal fibre sources. We obtain a fidelity for the cluster state of 0.66 ± 0.01 with respect to the ideal case. We perform quantum process tomography to completely characterize a controlled-NOT, Hadamard and T gate all on the same compact entangled resource. Together, these operations make up a universal set of gates such that arbitrary quantum logic can be efficiently constructed from combinations of them. We find process fidelities with respect to the ideal cases of 0.64 ± 0.01 for the CNOT, 0.67 ± 0.03 for the Hadamard and 0.76 ± 0.04 for the T gate. The characterization of these gates enables the simulation of larger protocols and algorithms. As a basic example, we simulate a Swap gate consisting of three concatenated CNOT gates. Our work provides some pragmatic insigh! ts into the prospects for building up to a fully scalable and fault-tolerant one-way quantum computer with photons in realistic conditions.

Emission and Nonradiative Decay of Nanodiamond NV Centers in a Low Refractive Index Environment
F.A. Inam, M.D.W. Grogan, M. Rollings, T. Gaebel, J.M. Say, C. Bradac, T.A. Birks, W.J. Wadsworth, S. Castelletto, J.R. Rabeau & M.J. Steel
ACS NANO 7 (2013) 3833-3843
The nitrogen vacancy (NV) center is the most widely studied single optical defect in diamond with great potential for applications in quantum technologies. Development of practical single-photon devices requires an understanding of the emission under a range of conditions and environments. In this work, we study the properties of a single NV center in nanodiamonds embedded In an air-like silica aerogel environment which provides a new domain for probing the emission behavior of NV centers in nanoscale environments. In this arrangement, the emission rate is governed primarily by the diamond crystal lattice with negligible contribution from the surrounding environment. This is in contrast to the conventional approach of studying nanodiamonds on a glass coverslip. We observe an increase in the mean lifetime due to the absence of a dielectric interface near the emitting dipoles and a distribution arising from the irregularities in the nanodiamond geometry. Our approach result!

Dispersion of nonlinearity in subwavelength waveguides: derivation of pulse propagation equation and frequency conversion effects
X.S. Zhao, A.V. Gorbach & D.V. Skryabin
J. Opt. Soc. Am. B 30 (2013) 812-820
Description of pulse propagation in waveguides with subwavelength features and high refractive index contrasts requires an accurate account of the dispersion of nonlinearity due to the considerable mode profile variation with the wavelength. The corresponding model derived from asymptotic expansion of Maxwell equations contains a complicated network of interactions between different harmonics of the pulse, and therefore is not convenient for analysis. We demonstrate that this model can be reduced to the generalized nonlinear Schrodinger-type pulse propagation equation under the assumption of factorization of the four-frequency dependence of nonlinear coefficients. We analyze two different semiconductor waveguide geometries and find that the factorization works reasonably well within large wavelength windows. This allows us to utilize the pulse propagation equation for the description of a broadband signal evolution. We study the mechanism of modulational instability induced by the dispersion of nonlinearity and find that the power threshold predicted by the simple model with three interacting harmonics is effectively removed when using pulses, while the efficiency of this process grows for shorter pulse durations. Also, we identify the effects of geometrical and material dispersion of nonlinearity on spectral broadening of short pulses in semiconductor waveguides. (C) 2013 Optical Society of America

Femtosecond pulses at 20 GHz repetition rate through spectral masking of a phase modulated signal and nonlinear pulse compression
B.H. Chapman, A.V. Doronkin, J.M. Stone, J.C. Knight, S.V. Popov & J.R. Taylor
Opt. Express 21 (2013) 5671-5676
We present a laser system capable of producing 190 femtosecond pulses at a repetition rate of 20 GHz. The spectral masking of a phase modulated diode laser is used to produce a train of picosecond pulses which are compressed using a fibre-grating compressor followed by subsequent adiabatic soliton compression to the femtosecond regime using a tapered photonic crystal fiber. (c) 2013 Optical Society of America

Predicting hole sizes after fibre drawing without knowing the viscosity
Y. Chen & T.A. Birks
Opt. Mater. Express 3 (2013) 346-356
We report the first analytical description of the drawing of fibres with holes that does not require knowledge of the viscosity (or temperature) of the glass in the furnace. The model yields expressions for the size of a hole that is isolated from other holes and small compared to the outer diameter of the fibre, but includes the effects of surface tension, pressurisation and arbitrary viscosity profiles. The effect of viscosity is represented by the fibre draw tension which, unlike viscosity, can readily be measured in practice by the fibre fabricator. The model matches experiments without recourse to any adjustable fitting parameters. (C) 2013 Optical Society of America

Temperature response of an all-solid photonic bandgap fiber for sensing applications
R.E.P. de Oliveira, J.C. Knight, T. Taru & C.J.S. de Matos
Appl. Optics 52 (2013) 1461-1467
The spectral shift due to temperature in the photonic bandgap (PBG) of an all-solid PBG fiber is investigated, aiming at discrete and distributed temperature sensing. A temperature rise induces a red shift in the bandgap spectra, which can be easily and precisely monitored by measuring the fiber transmission near one of the band edges. Two different situations that are potentially compatible with distributed and quasi-distributed sensing were investigated: heating a 2 m section of a longer (similar to 10 m) fiber, and heating the whole extension of a fiber that is tens of centimeters in length and was spliced to conventional fibers on both sides. The latter setup yielded bandgap spectral shifts up to similar to 35 pm/degrees C. Aiming at discrete sensing, a short (similar to 50 mm) fiber section was subjected to a tight bend so as to exhibit increased temperature sensitivity. Choosing the position of the bend allows for reconfiguration, on demand, of the sensor. A semi-analytical method to identify the spectral position of bandgaps was used to model the fiber transmission, as well as the bandgap shift with temperature, with consistent results. (C) 2013 Optical Society of America

Beam delivery and pulse compression to sub-50 fs of a modelocked thin-disk laser in a gas-filled Kagome-type HC-PCF fiber
F. Emaury, C.F. Dutin, C.J. Saraceno, M. Trant, O.H. Heckl, Y.Y. Wang, C. Schriber, F. Gerome, T. Sudmeyer, F. Benabid & U. Keller
Opt. Express 21 (2013) 4986-4994
We present two experiments confirming that hypocycloid Kagome-type hollow-core photonic crystal fibers (HC-PCFs) are excellent candidates for beam delivery of MW peak powers and pulse compression down to the sub-50 fs regime. We demonstrate temporal pulse compression of a 1030-nm Yb:YAG thin disk laser providing 860 fs, 1.9 mu J pulses at 3.9 MHz. Using a single-pass grating pulse compressor, we obtained a pulse duration of 48 fs (FWHM), a spectral bandwidth of 58 nm, and an average output power of 4.2 W with an overall power efficiency into the final polarized compressed pulse of 56%. The pulse energy was 1.1 mu J. This corresponds to a peak power of more than 10 MW and a compression factor of 18 taking into account the exact temporal pulse profile measured with a SHG FROG. The compressed pulses were close to the transform limit of 44 fs. Moreover, we present transmission of up to 97 mu J pulses at 10.5 ps through 10-cm long fiber, corresponding to more than twice the critical peak power for self-focusing in silica. (C) 2013 Optical Society of America

Flexible delivery of Er:YAG radiation at 2.94 mu m with negative curvature silica glass fibers: a new solution for minimally invasive surgical procedures
A. Urich, R.R.J. Maier, F. Yu, J.C. Knight, D.P. Hand & J.D. Shephard
Biomed. Opt. Express 4 (2013) 193-205
We present the delivery of high energy microsecond pulses through a hollow-core negative- curvature fiber at 2.94 mu m. The energy densities delivered far exceed those required for biological tissue manipulation and are of the order of 2300 J/cm(2). Tissue ablation was demonstrated on hard and soft tissue in dry and aqueous conditions with no detrimental effects to the fiber or catastrophic damage to the end facets. The energy is guided in a well confined single mode allowing for a small and controllable focused spot delivered flexibly to the point of operation. Hence, a mechanically and chemically robust alternative to the existing Er:YAG delivery systems is proposed which paves the way for new routes for minimally invasive surgical laser procedures. (c) 2012 Optical Society of America

2012 Publications

Time-Dependent Degradation of Photonic Crystal Fiber Attenuation Around OH Absorption Wavelengths
I. Gris-Sanchez & J.C. Knight
30 (2012) 3597-3602
We present a study of the time-dependent degradation of attenuation in Photonic Crystal Fibers in the wavelength region from 1350 nm to 1450 nm. Changes in spectral attenuation were monitored over 16 weeks of exposure to a laboratory environment in different solid core PCF's as well as in a hollow-core bandgap fiber. Increasing spectral attenuation was observed at 1364 nm and at 1384 nm, wavelengths corresponding to known OH absorption features in silica. We also observe the appearance of a broad attenuation peak around 1398 nm. The observed degradation is shown to decrease exponentially from the ends of the fiber, and is attributed to ingress of contaminants from the fiber ends. This attribution is supported by measurements on a fiber stored with sealed ends.

Soliton-plasmon resonances as Maxwell nonlinear bound states
C. Milian, D.E. Ceballos-Herrera, D.V. Skryabin & A. Ferrando
Opt. Lett. 37 (2012) 4221-4223
We demonstrate that soliplasmons (soliton-plasmon bound states) appear naturally as eigenmodes of nonlinear Maxwell's equations for a metal/Kerr interface. Conservative stability analysis is performed by means of finite element numerical modeling of the time-independent nonlinear Maxwell equations. Dynamical features are in agreement with the presented nonlinear oscillator model. (C) 2012 Optical Society of America

From zero dispersion to group index matching: How tapering fibers offers the best of both worlds for visible supercontinuum generation
J.M. Stone & J.C. Knight
Opt. Fib. Tech. 18 (2012) 315-321
We provide an experimental study of the nonlinear optical effects which can be enhanced in order to produce visible supercontinua in photonic crystal fibers. We examine individual uniform fibers and discuss the physical origins of the resulting supercontinua. We then examine tapered supercontinuum fibers which exploit the advantages of our individual uniform fibers at different points along the length of the taper. We demonstrate reproducible control of taper shape. (C) 2012 Elsevier Inc. All rights reserved.

Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying
M. Dowson, M. Grogan, T. Birks, D. Harrison & S. Craig
J.ApEnergy 97 (2012) 396-404
When developing sustainable building fabric technologies, it is essential that the energy use and CO2 burden arising from manufacture does not outweigh the respective in-use savings. This study investigates this paradigm by carrying out a streamlined life cycle assessment (LCA) of silica aerogel. This unique, nanoporous translucent insulation material has the lowest thermal conductivity of any solid, retaining up to four times as much heat as conventional insulation, whilst being highly transparent to light and solar radiation. Monolithic silica aerogel has been cited as the 'holy grail' of future glazing technology. Alternatively, translucent granular aerogel is now being produced on a commercial scale. In each case, many solvents are used in production, often accompanied by intensive drying processes, which may consume large amounts of energy and CO2. To date, there has been no peer-reviewed LCA of this material conducted to the ISO 14000 standard. Primary data for this 'cradle-to-factory gate' LCA is collected for silica aerogel made by low and high temperature supercritical drying. In both cases, the mass of raw materials and electricity usage for each process is monitored to determine the total energy use and CO2 burden. Findings are compared against the predicted operational savings arising from retrofitting translucent silica aerogel to a single glazed window to upgrade its thermal performance. Results should be treated as a conservative estimate as the aerogel is produced in a laboratory, which has not been developed for mass manufacture or refined to reduce its environmental impact. Furthermore, the samples are small and assumptions to upscale the manufacturing volume occur without major changes to production steps or equipment used. Despite this, parity between the CO2 burden and CO2 savings is achieved in less than 2 years, indicating that silica aerogel can provide a measurable environmental benefit. (C) 2011 Elsevier Ltd. All rights reserved.

Design and fabrication of hollow-core photonic crystal fibers for high-power ultrashort pulse transportation and pulse compression
Y.Y. Wang, X. Peng, M. Alharbi, C.F. Dutin, T.D. Bradley, F. Gerome, M. Mielke, T. Booth & F. Benabid
Opt. Lett. 37 (2012) 3111-3113
We report on the recent design and fabrication of kagome-type hollow-core photonic crystal fibers for the purpose of high-power ultrashort pulse transportation. The fabricated seven-cell three-ring hypocycloid-shaped large core fiber exhibits an up-to-date lowest attenuation (among all kagome fibers) of 40 dB/km over a broadband transmission centered at 1500 nm. We show that the large core size, low attenuation, broadband transmission, single-mode guidance, and low dispersion make it an ideal host for high-power laser beam transportation. By filling the fiber with helium gas, a 74 mu J, 850 fs, and 40 kHz repetition rate ultrashort pulse at 1550 nm has been faithfully delivered at the fiber output with little propagation pulse distortion. Compression of a 105 mu J laser pulse from 850 fs down to 300 fs has been achieved by operating the fiber in ambient air. (C) 2012 Optical Society of America

Porous silicon nanocrystals in a silica aerogel matrix
J. Amonkosolpan, D. Wolverson, B. Goller, S. Polisski, D. Kovalev, M. Rollings, M. Grogan & T. Birks
Nano Res. Lett. 7 (2012) 397
Silicon nanoparticles of three types (oxide-terminated silicon nanospheres, micron-sized hydrogen-terminated porous silicon grains and micron-size oxide-terminated porous silicon grains) were incorporated into silica aerogels at the gel preparation stage. Samples with a wide range of concentrations were prepared, resulting in aerogels that were translucent (but weakly coloured) through to completely opaque for visible light over sample thicknesses of several millimetres. The photoluminescence of these composite materials and of silica aerogel without silicon inclusions was studied in vacuum and in the presence of molecular oxygen in order to determine whether there is any evidence for non-radiative energy transfer from the silicon triplet exciton state to molecular oxygen adsorbed at the silicon surface. No sensitivity to oxygen was observed from the nanoparticles which had partially H-terminated surfaces before incorporation, and so we conclude that the silicon surface has become substantially oxidised. Finally, the FTIR and Raman scattering spectra of the composites were studied in order to establish the presence of crystalline silicon; by taking the ratio of intensities of the silicon and aerogel Raman bands, we were able to obtain a quantitative measure of the silicon nanoparticle concentration independent of the degree of optical attenuation.

Hollow-core Optical Fiber Gas Lasers (HOFGLAS): a review [Invited]
A.V.V. Nampoothiri, A.M. Jones, C. Fourcade-Dutin, C.C. Mao, N. Dadashzadeh, B. Baumgart, Y.Y. Wang, M. Alharbi, T. Bradley, N. Campbell, F. Benabid, B.R. Washburn, K.L. Corwin & W. Rudolph
Opt. Mater. Express 2 (2012) 948-961
The development of hollow core photonic crystal fibers with low losses over a broad spectral region in the near IR enabled the demonstration of a novel laser type - Hollow-core Optical Fiber Gas Laser (HOFGLAS). The laser combines attractive features of fiber lasers such as compactness and long interaction length of pump and laser radiation with those of gas lasers such as the potential for high output power and narrow line width. This paper summarizes recent developments and describes the demonstration of C2H2 and HCN prototype lasers. Avenues to extend laser emission further into the IR are discussed. (C) 2012 Optical Society of America

"Photonic lantern" spectral filters in multi-core Fiber
T.A. Birks, B.J. Mangan, A. Diez, J.L. Cruz & D.F. Murphy
Opt. Express 20 (2012) 13996-14008
Fiber Bragg gratings are written across all 120 single-mode cores of a multi-core optical Fiber. The Fiber is interfaced to multimode ports by tapering it within a depressed-index glass jacket. The result is a compact multimode "photonic lantern" filter with astrophotonic applications. The tapered structure is also an effective mode scrambler. (C) 2012 Optical Society of America

Ultrafast laser inscription of a 121-waveguide fan-out for astrophotonics
R.R. Thomson, R.J. Harris, T.A. Birks, G. Brown, J. Allington-Smith & J. Bland-Hawthorn
Opt. Lett. 37 (2012) 2331-2333
Using ultrafast laser inscription, we report the fabrication of a prototype three-dimensional 121-waveguide fan-out device capable of reformatting the output of a 120-core multicore fiber (MCF) into a one-dimensional linear array. When used in conjunction with an actual MCF, we demonstrate that the reformatting function using this prototype would result in an overall through put loss of approximate to 7.0 dB. However, if perfect coupling from the MCF into the fan-out could be achieved, the reformatting function would result in an overall loss of only approximate to 1.7 dB. With adequate development, similar devices could efficiently reformat the output of so-called "photonic lanterns" fabricated using highly multicore fibers. (C) 2012 Optical Society of America

Saturation spectroscopy of iodine in hollow-core optical fiber
A. Lurie, P.S. Light, J. Anstie, T.M. Stace, P.C. Abbott, F. Benabid & A.N. Luiten
Opt. Express 20 (2012) 11906-11917
We present high-resolution spectroscopy of I-2 vapor that is loaded and trapped within the core of a hollow-core photonic crystal fiber (HC-PCF). We compare the observed spectroscopic features to those observed in a conventional iodine cell and show that the saturation characteristics differ significantly. Despite the confined geometry it was still possible to obtain sub-Doppler features with a spectral width of similar to 6 MHz with very high contrast. We provide a simple theory which closely reproduces all the key observations of the experiment. (c) 2012 Optical Society of America

Fiber four-wave mixing source for coherent anti-Stokes Raman scattering microscopy
S. Lefrancois, D. Fu, G.R. Holtom, L.J. Kong, W.J. Wadsworth, P. Schneider, R. Herda, A. Zach, X.S. Xie & F.W. Wise
Opt. Lett. 37 (2012) 1652-1654
We present a fiber-format picosecond light source for coherent anti-Stokes Raman scattering microscopy. Pulses from a Yb-doped fiber amplifier are frequency converted by four-wave mixing (FWM) in normal-dispersion photonic crystal fiber to produce a synchronized two-color picosecond pulse train. We show that seeding the FWM process overcomes the deleterious effects of group-velocity mismatch and allows efficient conversion into narrow frequency bands. The source generates more than 160 mW of nearly transform-limited pulses tunable from 775 to 815 nm. High-quality coherent Raman images of animal tissues and cells acquired with this source are presented. (C) 2012 Optical Society of America

Low loss silica hollow core fibers for 3-4 mu m spectral region
F. Yu, W.J. Wadsworth & J.C. Knight
Opt. Express 20 (2012) 11153-11158
We describe a silica hollow-core fiber for mid-infrared transmission with a minimum attenuation of 34 dB/km at 3050 nm wavelength. The design is based on the use of a negative curvature core wall. Similar fiber designed for longer wavelengths has a transmission band extending beyond 4 mu m. (C) 2012 Optical Society of America

Bessel-Like Beams Generated by Photonic Crystal Fibre
Y. Chen, J.M. Stone, W.J. Wadsworth, J.C. Knight & T.A. Birks
IEEE (2012) 6326429
We report a photonic crystal fibre device whose output far field resembles that of an ideal Bessel beam over a wide wavelength range. The Bessel-like beam self-heals when the central peak is obstructed.

Low loss Kagome hollow-core photonic crystal fiber for high power fast laser beam transportation and pulse compression
Y.Y. Wang, X. Peng, M. Alharbi, C.F. Dutin, T.D. Bradley, M. Mielke, T. Booth & F. Benabid
IEEE (2012) 6326399
A Kagome hollow-core PCF with a record loss of 40dB/km is used to deliver 74 mu J and 850fs laser pulse with little propagation pulse-distortion, and to compress a 105 mu J laser pulse from 900fs to 300fs.

Optical properties of low loss (70dB/km) Kagome hollow core photonic crystal fiber for Rb and Cs based optical applications
T.D. Bradley, M. Alharbi, Y.Y. Wang, C. Fourcade-Dutin & F. Benabid
IEEE (2012) 6325759
We report on hollow-core Kagome fiber with a record loss of 70dB/km and operating at similar to 800nm. We show experimentally that the bending-loss is limited by coupling between the guiding core mode and the modes in the cladding holes.

Millijoule laser pulse delivery for spark ignition through kagome hollow-core fiber
B. Beaudou, F. Gerome, Y.Y. Wang, M. Alharbi, T.D. Bradley, G. Humbert, J.L. Auguste, J.M. Blondy & F. Benabid
Opt. Lett. 37 (2012) 1430-1432
We report on power handling oriented design of kagome lattice hollow-core fiber and demonstrate through it for the first time nanosecond laser pulses induced spark ignition in a friendly manner. Two different core designs and transmission bands are investigated and evaluated. The energy threshold damage was measured to be in excess of the 10 mJ level and the output power density is approaching the TW/cm(2) after focusing; demonstrating the outstanding ability of such fiber for high power delivery. (C) 2012 Optical Society of America

Solitons in semiconductor microcavities reply
D.V. Skryabin, D.N. Krizhanovskii, M.S. Skolnick, E.A. Cerda-Mendez & R. Hartley
Nat. Photonics 6 (2012) 204-204

Polychromatic Cherenkov radiation and supercontinuum in tapered optical fibers
C. Milian, A. Ferrando & D.V. Skryabin
J. Opt. Soc. Am. B 29 (2012) 589-593
We numerically demonstrate that bright solitons in tapered optical fibers can emit polychromatic Cherenkov radiation providing they remain spectrally close to the zero dispersion wavelength during propagation along the fiber. The prime role in this phenomenon is played by the soliton self-frequency shift driving efficiency of the radiation and tuning of its frequency. Depending on tapering and input pulse power, the radiation is emitted either as a train of pulses at different frequencies or as a single temporally broad and strongly chirped pulse. (C) 2012 Optical Society of America

Delivery of high energy Er:YAG pulsed laser light at 2.94 mu m through a silica hollow core photonic crystal fibre
A. Urich, R.R.J. Maier, B.J. Mangan, S. Renshaw, J.C. Knight, D.P. Hand & J.D. Shephard
Opt. Express 20 (2012) 6677-6684
In this paper the delivery of high power Er: YAG laser pulses through a silica hollow core photonic crystal fibre is demonstrated. The Er: YAG wavelength of 2.94 mu m is well beyond the normal transmittance of bulk silica but the unique hollow core guidance allows silica to guide in this regime. We have demonstrated for the first time the ability to deliver high energy pulses through an all-silica fibre at 2.94 mu m. These silica fibres are mechanically and chemically robust, biocompatible and have low sensitivity to bending. A maximum pulse energy of 14 mJ at 2.94 mu m was delivered through the fibre. This, to our knowledge, is the first time a silica hollow core photonic crystal fibre has been shown to transmit 2.94 mu m laser light at a fluence exceeding the thresholds required for modification (e.g. cutting and drilling) of hard biological tissue. Consequently, laser delivery systems based on these fibres have the potential for the realization of novel, minimally-invasive surgical procedures. (C) 2012 Optical Society of America

Modulational instability in a silicon-on-insulator directional coupler: role of the coupling-induced group velocity dispersion
W. Ding, O.K. Staines, G.D. Hobbs, A.V. Gorbach, C. de Nobriga, W.J. Wadsworth, J.C. Knight, D.V. Skryabin, M.J. Strain, M. Sorel & R.M. De La Rue
Opt. Lett. 37 (2012) 668-670
We report frequency conversion experiments in silicon-on-insulator (SOI) directional couplers. We demonstrate that the evanescent coupling between two subwavelength SOI waveguides is strongly dispersive and significantly modifies modulational instability (MI) spectra through the coupling induced group velocity dispersion (GVD). As the separation between two 380-nm-wide silicon photonic wires decreases, the increasing dispersion of the coupling makes the GVD in the symmetric supermode more normal and suppresses the bandwidth of the MI gain observed for larger separations. (C) 2012 Optical Society of America

Experimental characterization of photonic fusion using fiber sources
B. Bell, A.S. Clark, M.S. Tame, M. Halder, J. Fulconis, W.J. Wadsworth & J.G. Rarity
New J. Physics 14 (2012) -
We report on the fusion of photons from two independent photonic crystal fiber sources into polarization entangled states using a fiber-based polarizing beamsplitter. We achieve fidelities of up to F = 0.74 ± 0.01 with respect to the maximally entangled Bell state |phi(+)> using a low pump power of 5.3 mW with a success rate of 3.2 fourfold detections per second. By increasing the pump power we find that success rates of up to 111.6 fourfold detections per second can be achieved, with entanglement still present in the fused state. We characterize the fusion operation by providing a full quantum process reconstruction. Here a model is developed to describe the generation of entanglement, including the main causes of imperfection, and we show that this model fits well with the experimental results. Our work shows how non-ideal settings limit the success of the fusion, providing useful information about the practical requirements for an operation that may be used to build large entangled states in bulk and on-chip quantum photonic waveguides.

High-resolution optical spectroscopy in a hollow-core photonic crystal fiber
C. Perrella, P.S. Light, T.M. Stace, F. Benabid & A.N. Luiten
Phys. Rev. A 85 (2012) 012518
In this paper, we present detailed high-resolution spectroscopy of rubidium (Rb) vapor confined within a hollow-core photonic crystal fiber (HC-PCF). We find a very low level of additional frequency broadening associated with this confinement, with spectral features being only 1 MHz broader than the natural linewidth of the excited state. We show that this additional broadening is consistent solely with the atoms' transit across the fiber's optical mode. This low level of decoherence opens the door to a wide variety of applications including compact frequency standards and new types of quantum optical devices based on alkali-metal-loaded HC-PCFs. We highlight the low level of decoherence through observation of electromagnetically induced transparency in the confined vapor.

Control of nanoparticle aggregation in aerogel hosts
M.D.W. Grogan, S.C. Heck, L.M. Xiao, R. England, S.A. Maier & T.A. Birks
J. Non-Cryst. Solids 358 (2012) 241-245
Plasmonic aerogel containing 50 nm gold nanoparticles is made using a modified 2-step method that maintains control over the gel time while preventing nanoparticle aggregation. Strong narrow surface plasmon resonances verify that the nanoparticles are well dispersed within the silica matrix, and enable applications in sensing. SERS, nonlinear optics or plasmonic gain. Discrepancies between measured and simulated resonance wavelengths are attributed to the breakdown of the effective index approximation, due to the short-scale penetration of the resonance electric field into the host medium. (C) 2011 Elsevier B.V. All rights reserved.

Design and fabrication of hollow-core photonic crystal fibers for high power fast laser beam transportation and pulse compression
Y.Y. Wang, X. Peng, M. Alharbi, C.F. Dutin, T.D. Bradley, M. Mielke, T. Booth & F. Benabid
SPIE 8269 (2012) -
We report on recent design and fabrication of Kagome type hollow-core photonic crystal fiber (HC-PCF) for the purpose of high power fast laser beam transportation. The fabricated seven-cell three-ring hypocycloid-shaped large core fiber exhibits an up-to-date lowest attenuation (among all Kagome fibers) of 40dB/km over a broadband transmission centered at 1500nm. We show that the large core size, low attenuation, broadband transmission, single modedness, low dispersion and relatively low banding loss makes it an ideal host for high power laser beam transportation. By filling the fiber with helium gas, a 74 mu J, 850fs and 40kHz repetition rate ultra-short pulse at 1550nm has been faithfully delivered with little propagation pulse distortion. Compression of a 105 mu J laser pulse from 850fs to 300fs has been achieved by operating the fiber in ambient air.

Evanescent coupling assisted four-wave mixing in a silicon-oninsulator directional coupler
W. Ding, O. Staines, G.D. Hobbs, A.V. Gorbach, C.E. de Nobriga, W.J. Wadsworth, J.C. Knight, D.V. Skryabin, M. Strain, M. Sorel & R.M. De La Rue
SPIE 8554 (2012) -
Four-wave mixing (FWM) has been extensively explored in optical fibers and more recently in on-chip silicon-on-insulator (SOI) waveguides. A phase-matched FWM with a pair of degenerate pump photons generating and amplifying signal and idler photons is referred as modulational instability (MI). Following theory of FWM in waveguide arrays, we utilize evanescent couplings between neighboring waveguides to control the phase-matching condition in FWM. In experiments, a set of single-channel SOI nanowaveguides with the waveguide width decreasing from 380nm to 340nm demonstrate that changing the waveguide group velocity dispersion (GVD) at the pump wavelength from being anomalous to being normal makes MI gain gradually disappear. We also perform the same experiment with an array of two 380nm-wide SOI waveguide, and demonstrate that for the large separation of 900nm and 800nm, MI gain is present as for the single waveguide; while for the small separation of 400nm, the MI gain dis!

Observation of bright polariton solitons in a semiconductor microcavity
M. Sich, D.N. Krizhanovskii, M.S. Skolnick, A.V. Gorbach, R. Hartley, D.V. Skryabin, E.A. Cerda-Mendez, K. Biermann, R. Hey & P.V. Santos
Nat. Photonics 6 (2012) 50-55
Microcavity polaritons are composite half-light half-matter quasiparticles, which have recently been demonstrated to exhibit rich physical properties, such as non-equilibrium condensation, parametric scattering and superfluidity. At the same time, polaritons have important advantages over photons for information processing, because their excitonic component leads to weaker diffraction and stronger interparticle interactions, implying, respectively, tighter localization and lower powers for nonlinear functionality. Here, we present the first experimental observations of bright polariton solitons in a strongly coupled semiconductor microcavity. The polariton solitons are shown to be micrometre-scale localized non-diffracting wave packets with a corresponding broad spectrum in momentum space. Unlike the solitons known in Bose condensed atomic gases, they are non-equilibrium and rely on a balance between losses and external pumping. Microcavity polariton solitons are excited on picosecond timescales, and thus have further benefits for information processing over light-only solitons in semiconductor cavity lasers, which have nanosecond response times.

Progress in hollow core photonic crystal fiber for atomic vapour based coherent optics
T.D. Bradley, Y.Y. Wang, M. Alharbi, C.F. Dutin, B.J. Mangan, N.V. Wheeler & F. Benabid
SPIE 8273 (2012) 82730O
We report on progress in different hollow core photonic crystal fiber (HC-PCF) design and fabrication for atomic vapor based applications. We have fabricated a Photonic bandgap (PBG) guiding HC-PCF with a record loss of 107dB/km at 785nm in this class of fiber. A double photonic bandgap (DPBG) guiding HC-PCF with guidance bands centred at 780nm and 1064nm is reported. A 7-cell 3-ring Kagome HC-PCF with hypocycloid core is reported, the optical loss at 780nm has been reduced to 70dB/km which to the best of our knowledge is the lowest optical loss reported at this wavelength using HC-PCF. Details on experimental loading of alkali metal vapours using a far off red detuned laser are reported. This optical loading has been shown to decrease the necessary loading time for Rb into the hollow core of a fiber. The quantity of Rb within the fiber core has been enhanced by a maximum of 14% through this loading procedure.

2011 Publications

Surface-induced nonlinearity enhancement in subwavelength rod waveguides
A. Marini, R. Hartley, A.V. Gorbach & D.V. Skryabin
Phys. Rev. A 84 (2011) 063839
We develop a perturbative theory to describe optical propagation in subwavelength rod waveguides. In this approach, we account for loss and nonlinearity in the boundary conditions. A comparison to the traditional perturbative approach used in optical fibers reveals that the surface contribution provides a significant nonlinearity enhancement in the subwavelength regime. We further compare the nonlinearity enhancement of metallic, dielectric, and semiconductor waveguides, in addition to determining the attenuation coefficient of metallic nanowires.

High-performance iodine fiber frequency standard
A. Lurie, F.N. Baynes, J.D. Anstie, P.S. Light, F. Benabid, T.M. Stace & A.N. Luiten
Opt. Lett. 36 (2011) 4776-4778
We have constructed a compact and robust optical frequency standard based around iodine vapor loaded into the core of a hollow-core photonic crystal fiber (HC-PCF). A 532nm laser was frequency locked to one hyperfine component of the R(56) 32-0 (127)I(2) transition using modulation transfer spectroscopy. The stabilized laser demonstrated a frequency stability of 2.3 x 10(-12) at 1 s, almost an order of magnitude better than previously reported for a laser stabilized to a gas-filled HC-PCF. This limit is set by the shot noise in the detection system. We present a discussion of the current limitations to the performance and a route to improve the performance by more than an order of magnitude. (C) 2011 Optical Society of America

Low-noise, high-brightness, tunable source of picosecond pulsed light in the near-infrared and visible
P.J. Mosley, S.A. Bateman, L. Lavoute & W.J. Wadsworth
Opt. Express 19 (2011) 25337-25345
We have built a flexible source of picosecond pulsed light in both the near-infrared and visible spectral regions. A photonic crystal fiber (PCF) was pumped with a pulsed 1064 nm fiber laser to generate four-wave mixing (FWM) sidebands at 947 nm and 1213 nm. This process was seeded at the idler wavelength with a tunable diode laser to limit the spectral width of the sidebands to less than 0.5 nm. Subsequently the idler was mixed efficiently with the residual pump in a nonlinear crystal to yield their sum frequency at 567 nm. All three outputs were tunable by adjusting the seed wavelength and all had very low pulse-to-pulse amplitude noise. This technique could be extended to different wavelength ranges by selecting different seed lasers and PCF. (C) 2011 Optical Society of America

A complex multi-notch astronomical filter to suppress the bright infrared sky
J. Bland-Hawthorn, S.C. Ellis, S.G. Leon-Saval, R. Haynes, M.M. Roth, H.G. Lohmannsroben, A.J. Horton, J.G. Cuby, T.A. Birks, J.S. Lawrence, P. Gillingham, S.D. Ryder & C. Trinh
Nat. Coms. 2 (2011) -
A long-standing and profound problem in astronomy is the difficulty in obtaining deep near-infrared observations due to the extreme brightness and variability of the night sky at these wavelengths. A solution to this problem is crucial if we are to obtain the deepest possible observations of the early Universe, as redshifted starlight from distant galaxies appears at these wavelengths. The atmospheric emission between 1,000 and 1,800 nm arises almost entirely from a forest of extremely bright, very narrow hydroxyl emission lines that varies on timescales of minutes. The astronomical community has long envisaged the prospect of selectively removing these lines, while retaining high throughput between them. Here we demonstrate such a filter for the first time, presenting results from the first on-sky tests. Its use on current 8 m telescopes and future 30 m telescopes will open up many new research avenues in the years to come.

Hydrophobic photonic crystal fibers
L.M. Xiao, T.A. Birks & W.H. Loh
Opt. Lett. 36 (2011) 4662-4664
We propose and demonstrate hydrophobic photonic crystal fibers (PCFs). A chemical surface treatment for making PCFs hydrophobic is introduced. This repels water from the holes of PCFs, so that their optical properties remain unchanged even when they are immersed in water. The combination of a hollow core and a water-repellent inner surface of the hydrophobic PCF provides an ultracompact dissolved-gas sensor element, which is demonstrated for the sensing of dissolved ammonia gas. (C) 2011 Optical Society of America

Focus issue introduction: nonlinear optics
B. Boulanger, S.T. Cundiff, D.J. Gauthier, M. Karlsson, Y.Q. Lu, R.A. Norwood, D. Skryabin & T. Taira
Opt. Express 19 (2011) 23561-23566
It is now fifty years since the original observation of second harmonic generation ushered in the field of nonlinear optics, close on the heels of the invention of the laser. This feature issue celebrates this anniversary with papers that span the range from new nonlinear optical materials, through the increasingly novel methods that have been developed for phase matching, to emerging areas such as nonlinear metamaterials and plasmonic enhancement of optical properties. It is clear that the next fifty years of nonlinear optics will witness a proliferation of new applications with increasing technological impact. (C) 2011 Optical Society of America

In-Line Gas Sensor Based on a Photonic Bandgap Fiber With Laser-Drilled Lateral Microchannels
H. Lehmann, H. Bartelt, R. Willsch, R. Amezcua-Correa & J.C. Knight
IEEE Sensors J. 11 (2011) 2926-2931
The fabrication, characterization, and gas sensor application of a hollow core photonic bandgap fiber (HC-PBGF) with laser drilled lateral microchannels will be described. The HC-PBGF was tailor-made for gas sensing applications in the near infrared region from 1.5 to 1.7 mu m wavelengths, covering the first harmonic absorptions of quite a number of natural gas components. Laser-drilled, lateral microchannels makes access to the light guiding core of the HC-PBGF and offers the ability to realize fast-responding, distributed gas sensor cells with large optical path lengths. The application of those cells in a distributed sensor arrangement using white light spectroscopy combined with chemometrical methods as interrogation method will be demonstrated.

Parametric polariton solitons in coherently pumped semiconductor microcavities
O.A. Egorov, D.V. Skryabin & F. Lederer
Phys. Rev. B 84 (2011) 165305
We demonstrate the existence of parametric solitons due to four-wave mixing in a coherently pumped semiconductor microcavity operating in the strong-coupling regime. These spatial solitons are localized in the direction perpendicular to the pump momentum and form periodic trains of pulses in the direction parallel to it. The parametric solitons constitute a family continuously parameterized by the energies and momenta of the signal and idler components. They also play a profound role in the formation of two-dimensional polariton solitons.

Parametric polariton solitons in coherently pumped semiconductor microcavities
O.A. Egorov, D.V. Skryabin & F. Lederer
Phys. Rev. B 84 (2011) 165305
We demonstrate the existence of parametric solitons due to four-wave mixing in a coherently pumped semiconductor microcavity operating in the strong-coupling regime. These spatial solitons are localized in the direction perpendicular to the pump momentum and form periodic trains of pulses in the direction parallel to it. The parametric solitons constitute a family continuously parameterized by the energies and momenta of the signal and idler components. They also play a profound role in the formation of two-dimensional polariton solitons.

Temporal pulse compression in a xenon-filled Kagome-type hollow-core photonic crystal fiber at high average power
O.H. Heckl, C.J. Saraceno, C.R.E. Baer, T. Sudmeyer, Y.Y. Wang, Y. Cheng, F. Benabid & U. Keller
Opt. Express 19 (2011) 19142-19149
In this study we demonstrate the suitability of Hollow-Core Photonic Crystal Fibers (HC-PCF) for multiwatt average power pulse compression. We spectrally broadened picosecond pulses from a SESAM mode-locked thin disk laser in a xenon gas filled Kagome-type HC-PCF and compressed these pulses to below 250 fs with a hypocycloid-core fiber and 470 fs with a single cell core defect fiber. The compressed average output power of 7.2 W and 10.2 W at a pulse repetition rate of approximately 10 MHz corresponds to pulse energies of 0.7 mu J and 1 mu J and to peak powers of 1.6 MW and 1.7 MW, respectively. Further optimization of the fiber parameters should enable pulse compression to below 50 fs duration at substantially higher pulse energies. (C) 2011 Optical Society of America

Highly-efficient, octave spanning soliton self-frequency shift using a specialized photonic crystal fiber with low OH loss
S.A. Dekker, A.C. Judge, R. Pant, I. Gris-Sanchez, J.C. Knight, C.M. de Sterke & B.J. Eggleton
Opt. Express 19 (2011) 17766-17773
We report the first demonstration of efficient, octave spanning soliton self-frequency shift. In order to achieve this we used a photonic crystal fiber with reduced OH absorption and widely spaced zero-dispersion wavelengths. To our knowledge, this is the largest reported frequency span for a tunable, fiber-based source. In addition, we observe the generation of light above 2 m m directly from a Ti:Sapphire laser in the form of Cerenkov emission by the soliton when the red-shift saturates at the edge of the anomalous dispersion region. (C) 2011 Optical Society of America

Optofluidic microchannels in aerogel
L.M. Xiao & T.A. Birks
Opt. Lett. 36 (2011) 3275-3277
We report optofluidic waveguides made by filling microchannels in aerogel with water. The aerogel cladding is a nanoporous material with an extremely low refractive index of similar to 1.05, giving a large index step from the water core. Channels were formed by removing embedded optical fibers, which could be nonuniform or multiple. The porosity of the aerogel allowed air to be displaced from the channel, preventing the trapping of bubbles. The attenuation of red light in the highly multimode water core waveguide was no greater than 1.5 dB/cm. (C) 2011 Optical Society of America

Intrinsically narrowband pair photon generation in microstructured fibres
A. Clark, B. Bell, J. Fulconis, M.M. Halder, B. Cemlyn, O. Alibart, C.L. Xiong, W.J. Wadsworth & J.G. Rarity
13 (2011) 065009
In this paper, we study the tailoring of photon spectral properties generated by four-wave mixing in a birefringent photonic crystal fibre (PCF). The aim is to produce intrinsically narrow-band photons and hence to achieve high non-classical interference visibility and generate high-fidelity entanglement without any requirement for spectral filtering, leading to high effective detection efficiencies. We show unfiltered Hong-Ou-Mandel interference visibilities of 77% between photons from the same PCF and 80% between separate sources. We compare results from modelling the PCF to these experiments and analyse photon purities.

Double-clad hollow core photonic crystal fiber for coherent Raman endoscope
S. Brustlein, P. Berto, R. Hostein, P. Ferrand, C. Billaudeau, D. Marguet, A. Muir, J. Knight & H. Rigneault
Opt. Express 19 (2011) 12562-12568
Performing label free coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) in endoscope imaging is a challenge, with huge potential clinical benefit. To date, this goal has remained inaccessible because of the inherent coherent Raman noise that is generated in the fiber itself. By developing double-clad hollow core photonic crystal fiber, we demonstrate coherent anti-Stokes Raman scattering and stimulated Raman scattering in an, endoscope-like. scheme. Both the excitation beams and the collected CARS and SRS signals travel through the same fiber. No CARS and SRS signals are generated within the hollow core fiber even for temporally overlapping pump and Stokes beams, leading to excellent image quality. The CARS and SRS signals generated in the sample are coupled back into a high numerical aperture multimode cladding surrounding the central photonic crystal cladding. We demonstrate this scheme by imaging molecular vibrational bonds of organic crystal deposited on a glass surface. (C)2011 Optical Society of America

Reducing spectral attenuation in small-core photonic crystal fibers
I. Gris-Sanchez, B.J. Mangan & J.C. Knight
Opt. Mater. Express 1 (2011) 179-184
We describe a modified fabrication process to reduce spectral attenuation in highly nonlinear photonic crystal fibers (PCF) by reducing the effect of OH(-) content in the silica glass. In particular we show outstanding results for small core sizes of 2 mu m diameter including an attenuation of 10dB/km at the OH(-) peak wavelength of 1384nm, by annealing the preform prior to the fiber draw. (C)2010 Optical Society of America

Dispersion of nonlinearity and modulation instability in subwavelength semiconductor waveguides
A.V. Gorbach, X. Zhao & D.V. Skryabin
Opt. Express 19 (2011) 9345-9351
Tight confinement of light in subwavelength waveguides induces substantial dispersion of their nonlinear response. We demonstrate that this dispersion of nonlinearity can lead to the modulational instability in the regime of normal group velocity dispersion through the mechanism independent from higher order dispersions of linear waves. A simple phenomenological model describing this effect is the nonlinear Schrodinger equation with the intensity dependent group velocity dispersion. (C) 2011 Optical Society of America

High finesse microfiber knot resonators made from double-ended tapered fibers
L.M. Xiao & T.A. Birks
Opt. Lett. 36 (2011) 1098-1100
We fabricated optical microfiber knot resonators from thin tapered fibers (diameter down to 1 mu m) linked to untapered fiber at both ends. We demonstrated a finesse of about 100, over twice as high as previously reported for microfiber resonators. Low-loss encapsulation of microfiber knot resonators in hydrophobic silica aerogel was also investigated. (C) 2011 Optical Society of America

Guidance in Kagome-like photonic crystal fibres I: analysis of an ideal fibre structure
L. Chen, G.J. Pearce, T.A. Birks & D.M. Bird
Opt. Express 19 (2011) 6945-6956
Propagation of light in a square-lattice hollow-core photonic crystal fibre is analysed as a model of guidance in a class of photonic crystal fibres that exhibit broad-band guidance without photonic bandgaps. A scalar governing equation is used and analytic solutions based on transfer matrices are developed for the full set of modes. It is found that an exponentially localised fundamental mode exists for a wide range of frequencies. These analytic solutions of an idealised structure will form the basis for analysis of guidance in a realistic structure in a following paper. (C) 2011 Optical Society of America

Stable spatial plasmon solitons in a dielectric-metal-dielectric geometry with gain and loss
A. Marini, D.V. Skryabin & B. Malomed
Opt. Express 19 (2011) 6616-6622
Using a combination of numerical and analytical techniques we demonstrate that a metal stripe surrounded by the active and passive dielectrics supports propagation of stable spatial surface-plasmon solitons. Our analytical methods include the multiple scale reduction of the Maxwell's equations to the coupled Ginzburg-Landau system, and the soliton perturbation theory developed in the framework of the latter. (C) 2011 Optical Society of America

Carrier-envelope offset frequency measurement for tunable femtosecond lasers using resonant dispersive waves
J.H. Peng, F. Zhu, F. Benabid & A.V. Sokolov
Opt. Lett. 36 (2011) 891-893
We report that carrier-envelope offset frequency measurement for tunable femtosecond lasers is fulfilled by directly using the resonant dispersive waves generated in a photonic crystal fiber. (C) 2011 Optical Society of America

Ultrafast laser inscription of an integrated photonic lantern
R.R. Thomson, T.A. Birks, S.G. Leon-Saval, A.K. Kar & J. Bland-Hawthorn
Opt. Express 19 (2011) 5698-5705
We used ultrafast laser inscription to fabricate three-dimensional integrated optical transitions that efficiently couple light from a multimode waveguide to a two-dimensional array of single mode waveguides and back. Although the entire device has an average insertion loss of 5.7 dB at 1539 nm, only approximate to 0.7 dB is due to mode coupling losses. Based on an analysis which is presented in the paper, we expect that our device should convert a multimode input into an array of single modes with a loss of. approximate to 2.0 dB, assuming the input coupling losses are zero. Such devices have applications in astrophotonics and remote sensing. (C) 2011 Optical Society of America

Nonlinear switching in arrays of semiconductor on metal photonic wires
C. Milian & D.V. Skryabin
Appl. Phys. Lett. 98 (2011) 111104
Using three-dimensional finite element modeling of the time independent Maxwell equations we demonstrate nonlinearity induced suppression of the power transfer in the arrays of nanoscale semiconductor on metal photonic wires. Nonlinear switching of the plasmonic modes can be achieved over few micron propagation distances using few hundred watts of the peak power. (C) 2011 American Institute of Physics.

Coherent supercontinuum generation in photonic crystal fiber with all-normal group velocity dispersion
L.E. Hooper, P.J. Mosley, A.C. Muir, W.J. Wadsworth & J.C. Knight
Opt. Express 19 (2011) 4902-4907
We demonstrate supercontinuum generation in a photonic crystal fiber with all-normal group velocity dispersion. Pumping a short section of this fiber with compressed pulses from a compact amplified fiber laser generates a 200 nm bandwidth continuum with typical self-phase-modulation characteristics. We demonstrate that the supercontinuum is compressible to a duration of 26 fs. It therefore has a high degree of coherence between all the frequency components, and is a single pulse in the time domain. A smooth, flat spectrum spanning 800 nm is achieved using a longer piece of fiber. (C) 2011 Optical Society of America

Structure of plasmonic aerogel and the breakdown of the effective medium approximation
M.D.W. Grogan, S.C. Heck, K.M. Hood, S.A. Maier & T.A. Birks
Opt. Lett. 36 (2011) 358-360
A method for making aerogel doped with gold nanoparticles (GNPs) produces a composite material with a well-defined localized surface plasmon resonance peak at 520 nm. The width of the extinction feature indicates the GNPs are well dispersed in the aerogel, making it suited to optical study. A simple effective medium approximation cannot explain the peak extinction wavelengths. The plasmonic field extends on a scale where aerogel cannot be considered isotropic, so a new model is required: a 5 nm glass coating on the GNPs models the extinction spectrum of the composite material, with air (aerogel), methanol (alcogel), or toluene filling the pores. (C) 2011 Optical Society of America

Mid-infrared gas filled photonic crystal fiber laser based on population inversion
A.M. Jones, A.V.V. Nampoothiri, A. Ratanavis, T. Fiedler, N.V. Wheeler, F. Couny, R. Kadel, F. Benabid, B.R. Washburn, K.L. Corwin & W. Rudolph
Opt. Express 19 (2011) 2309-2316
We demonstrate for the first time an optically pumped gas laser based on population inversion using a hollow core photonic crystal fiber (HC-PCF). The HC-PCF filled with (C2H2)-C-12 gas is pumped with similar to 5 ns pulses at 1.52 mu m and lases at 3.12 mu m and 3.16 mu m in the mid-infrared spectral region. The maximum measured laser pulse energy of similar to 6 nJ was obtained at a gas pressure of 7 torr with a fiber with 20 dB/m loss near the lasing wavelengths. While the measured slope efficiencies of this prototype did not exceed a few percent due mainly to linear losses of the fiber at the laser wavelengths, 25% slope efficiency and pulse energies of a few mJ are the predicted limits of this laser. Simulations of the laser's behavior agree qualitatively with experimental observations. (C)2011 Optical Society of America

Large Core Photonic Microcells for Coherent Optics and Laser Metrology
N.V. Wheeler, M.D.W. Grogan, Y.Y. Wang, D.F. Murphy, T.A. Birks & F. Benabid
SPIE (2011) -
A photonic microcell (PMC) is a length of gas-filled hollow core-photonic crystal fiber (HC-PCF) which is hermetically sealed at both ends by splicing to standard single mode fiber. We describe advances in the fabrication technique of PMCs which enable large core Kagome-lattice HC-PCFs to be integrated into PMC form. The modified fabrication technique uses fiber-tapering to accommodate the large dimensions of the fiber and enables low loss splices with single mode fiber by reducing mode field mismatch. Splice losses as low as 0.6 dB are achieved between 1-cell defect Kagome HC-PCF and single mode fiber. Relative to the previously reported PMCs, which were based on photonic bandgap HC-PCF, the present Kagome HC-PCF based PMC provides broad optical transmission, surface mode-free guidance and larger core at the cost of slightly increased fiber attenuation (similar to 0.2 dB/m). Therefore, the integration of this fiber into PMC form opens up new applications for PMC-based devices. The advantage of the large core dimensions and surface mode free guidance for quantum optics in gas-filled HC-PCF are demonstrated by generation of narrow sub-Doppler features in an acetylene-filled large core PMC.

Low loss broadband transmission in hypocycloid-core Kagome hollow-core photonic crystal fiber
Y.Y. Wang, N.V. Wheeler, F. Couny, P.J. Roberts & F. Benabid
Opt. Lett. 36 (2011) 669-671
We report on the fabrication of a seven-cell-core and three-ring-cladding large-pitch Kagome-lattice hollow-core photonic crystal fiber (HC-PCF) with a hypocycloid-shaped core structure. We demonstrate experimentally and theoretically that the design of this core shape enhances the coupling inhibition between the core and cladding modes and offers optical attenuation with a baseline of ?180?dB/km over a transmission bandwidth larger than 200?THz. This loss figure rivals the state-of-the-art photonic bandgap HC-PCF while offering an approximately three times larger bandwidth and larger mode areas. Also, it beats the conventional circular-core-shaped Kagome HC-PCF in terms of the loss. The development of this novel (to our knowledge) HC-PCF has potential for a number of applications in which the combination of a large optical bandwidth and a low loss is a prerequisite© 2011 Optical Society of America

Stable low-loss optical nanofibres embedded in hydrophobic aerogel
L.M. Xiao, M.D.W. Grogan, W.J. Wadsworth, R. England & T.A. Birks
Opt. Express 19 (2011) 764-769
Nanofibres, optical fibres narrower than the wavelength of light, degrade in hours on exposure to air. We show that encapsulation in hydrophobic silica aerogel (refractive index 1.05) provides protection and stability (over 2 months) without sacrificing low attenuation, strong confinement and accessible evanescent field. The measured attenuation was <0.03 dB/mm, over 10 x lower than reported with other encapsulants. This enables many nanofibre applications based on their extreme small size and strong external evanescent field, such as optical sensors, nonlinear optics, nanofibre circuits and high-Q resonators. The aerogel is more than a waterproof box, it is a completely-compatible gas-permeable material in intimate contact with the nanofibre and hydrophobic on both the macroscopic and molecular scales. Its benefits are illustrated by experiments on gas sensing (exploiting the aerogel's porosity) and supercontinuum generation (exploiting its ultra-low index). (C)2011 Optical Society of America

Fabrication and Characterization of Ultra-large Core Size (> 100 mu m) Kagome Fiber for Laser Power Handling
Y. Cheng, Y.Y. Wang, J.L. Auguste, F. Gerome, G. Humbert, J.M. Blondy & F. Benabid
(2011) -
Kagome fibers were fabricated with core size larger than 100 mu m. The fibers combined low loss over broad transmission and with small bend loss, making it a potential solution for high power and fast laser delivery.

Spatio-Temporal Nonlinear Optics in Arrays of Subwavelength Waveguides
W. Ding, A.V. Gorbach, O.K. Staines, C.E. de Nobriga, G.D. Hobbs, W.J. Wadsworth, J.C. Knight, D.V. Skryabin, A. Samarelli, M. Sorel & R.M. De La Rue
(2011) -
Spectral broadening in an array of subwavelength silicon waveguides pumped with fs pulses is studied. Adjusting input pulse position, different spectral patterns are observed and explained using the resonant emission from temporal supermode solitons.

Low loss and broadband hollow-core photonic crystal fibers
Y.Y. Wang, F. Gerome, G. Humbert, J.M. Blondy & F. Benabid
7946 (2011) -
We report on recent developments on fabrication and optical guidance of Kagome-lattice hollow-core photonic crystal fiber (HC-PCF). These include the design and fabrication of a hypocycloid-shaped core Kagome HC-PCF that combines a record optical attenuation with a baseline exhibiting similar to 180 dB/km over a transmission bandwidth larger than 200 THz. These results are corroborated with theoretical simulations which show that both the core-shape and the cladding ring number play role in inhibited coupling, inducing core-mode confinement for the fundamental transmission band. We also show that the inhibited coupling is weaker for the first higher-order transmission band by theoretically and experimentally comparing Kagome HC-PCF with a single anti-resonant ring hollow-core fiber.

Sub-natural Raman linewidth and high power CW Raman-Stokes laser in hydrogen filled HC-PCF
M. Alharbi, A. Husakou & F. Benabid
(2011) -
We report on several tens of Watts CW Raman-Stokes in hydrogen gas with linewidth as low as 80 KHz. This represents a narrowing of the Raman-linewidth by a factor of more than 4 orders of magnitude. (C)2011 Optical Society of America

Spatial and Temporal Dynamics of Raman Coherence in Hydrogen-filled HC-PCF
Y.Y. Wang, A. Husakou, B.J. Mangan & F. Benabid
(2011) -
The spatial and temporal dynamics of molecular Raman coherence is experimentally and theoretically investigated using delayed probe-pump technique in hydrogen-filled hollow-core photonic crystal fiber.

Linear and nonlinear optical properties of hollow core photonic crystal fiber
F. Benabid & P.J. Roberts
J. Mod. Opt. 58 (2011) 87-124
We review the optical guidance properties of hollow-core photonic crystal fibers. We follow a historical perspective to introduce the two major optical guidance mechanisms that were identified as operating in these fibers: photonic bandgap guidance and inhibited coupling guidance. We then review the modal properties of these fibers and assess the transmission loss mechanisms in photonic bandgap guiding hollow-core photonic crystal fiber. We dedicate a section to a review of the technical basics of hollow-core photonic crystal fiber fabrication and photonic microcell assembly. We review some of the early results on the use of hollow-core photonic crystal fiber for laser guiding micro-sized particles, as well as the generation of stimulated Raman scattering, electromagnetically induced transparency and laser frequency stabilization when the fiber core is filled with a gas-phase material. We conclude this review with a non-exhaustive list of prospects where hollow-core photonic crystal fiber could play a central role.

Time- and Frequency-Domain Measurements of Solitons in Subwavelength Silicon Waveguides Using Cross-correlation
W. Ding, A.V. Gorbach, W.J. Wadsworth, J.C. Knight, D.V. Skryabin, M.J. Strain, M. Sorel & R.M. De La Rue
(2011) -
Time-domain measurements of dispersion-and nonlinearity-induced chirpings of femtosecond pulses in silicon-on-insulator nanowires reveal nonlinear dispersion compensation. Spectral measurements show pronounced dispersive wave emission by solitons.

Positron and positronium studies of silica aerogel
C.J. Edwardson, M.D.W. Grogan, T.A. Birks & P.G. Coleman
J. Phys. Conf. 262 (2011) -
Variable-energy positron annihilation spectroscopy (VEPAS) has been used to probe the structure of gold nanoparticles suspended in a silica aerogel matrix by measuring the annihilation parameters S and W, and the ortho-positronium (o-Ps) fraction parameter F, as a function of incident positron energy. Correction methods have been developed to improve the sensitivity of F by removing background contributions to the number of recorded o-Ps annihilation events. Charging effects have also been observed and investigated.

Surface-induced nonlinearity enhancement of TM modes in planar subwavelength waveguides
D.V. Skryabin, A.V. Gorbach & A. Marini
J. Opt. Soc. Am. B 28 (2011) 109-114
Using an asymptotic expansion of Maxwell equations and boundary conditions, we derive an amplitude equation for nonlinear TM modes in planar metal and dielectric waveguides. Our approach reveals that the physics of the significant enhancement of the nonlinear response in subwavelength waveguides with respect to their weakly guiding counterparts is hidden in surface effects. The corresponding enhancement factor is determined by the products of the surface discontinuities of the transverse field components and of the surface values of the longitudinal components of the electric field summed over all the interfaces. We present an insightful expression for the enhancement factor induced by the surface plasmon polaritons and discuss numerical and analytical results for the subwavelength dielectric and metal slot waveguides. Our theory also includes diffraction effects along the unbound direction in these waveguides. (C) 2010 Optical Society of America

2010 Publications

Time and frequency domain measurements of solitons in subwavelength silicon waveguides using a cross-correlation technique
W. Ding, A.V. Gorbach, W.J. Wadsworth, J.C. Knight, D.V. Skryabin, M.J. Strain, M. Sorel & R.M. De La Rue
Opt. Express 18 (2010) 26625-26630
We report time domain measurements of the group-velocity-dispersion-induced and nonlinearity-induced chirping of femtosecond pulses in subwavelength silicon-on-insulator waveguides. We observe that at a critical input power level, these two effects compensate each other leading to soliton formation. Formation of the fundamental optical soliton is observed at a peak power of a few Watts inside the waveguide. Interferometric cross-correlation traces reveal compression of the soliton pulses, while spectral measurements show pronounced dispersive waves emitted by solitons into the wavelength range of normal group velocity dispersion.

PHOTONIC CRYSTAL WAVEGUIDES Compressing slow solitons
D.V. Skryabin & J.C. Knight
Nat. Photonics 4 (2010) 806-807
Time-domain measurements have confirmed the existence and compression of optical solitons in nanoscale planar photonic crystal waveguides, giving hope for the future prospects of on-chip nonlinear optical circuits.

Supermode dispersion and waveguide-to-slot mode transition in arrays of silicon-on-insulator waveguides
C.E. de Nobriga, G.D. Hobbs, W.J. Wadsworth, J.C. Knight, D.V. Skryabin, A. Samarelli, M. Sorel & R.M. De La Rue
Opt. Lett. 35 (2010) 3925-3927
In this Letter, we report group index measurements of the supermodes of an array of two strongly coupled silicon-on-insulator waveguides. We observe coupling-induced dispersion that is greater than the material and waveguide dispersion of the individual waveguides. We demonstrate that the system transforms from supporting the two supermodes associated with two coupled waveguides to the single mode of a slot waveguide within the investigated spectral range. During the cutoff of the antisymmetric supermode, an anti-crossing between the symmetric TM and antisymmetric TE supermodes has been observed. (C) 2010 Optical Society of America

Quantum theory of phase correlations in optical frequency combs generated by stimulated Raman scattering
W. Chunbai, M.G. Raymer, Y.Y. Wang & F. Benabid
Phys. Rev. A 82 (2010) 053834
We explore theoretically the phase correlation between multiple generated sidebands in a Raman optical frequency comb under conditions of spontaneous initiation from quantum zero-point noise. We show that there is a near- deterministic correlation between sideband phases in each laser shot which may lead to synthesis of attosecond pulse trains. (C) 2010 American Physical Society.

Ultrashort pulse compression and delivery in a hollow-core photonic crystal fiber at 540 nm wavelength
P.J. Mosley, W.C. Huang, M.G. Welch, B.J. Mangan, W.J. Wadsworth & J.C. Knight
Opt. Lett. 35 (2010) 3589-3591
We have fabricated a bandgap-guiding hollow-core photonic crystal fiber (PCF) capable of transmitting and compressing ultrashort pulses in the green spectral region around 532 nm. When propagating subpicosecond pulses through 1 m of this fiber, we have observed soliton-effect temporal compression by up to a factor of 3 to around 100 fs. This reduces the wavelength at which soliton effects have been observed in hollow-core PCF by over 200 nm. We have used the pulses delivered at the output of the fiber to machine micrometer-scale features in copper. (C) 2010 Optical Society of America

Spatiotemporal nonlinear optics in arrays of subwavelength waveguides
A.V. Gorbach, W. Ding, O.K. Staines, C.E. de Nobriga, G.D. Hobbs, W.J. Wadsworth, J.C. Knight, D.V. Skryabin, A. Samarelli, M. Sorel & R.M. De La Rue
Phys. Rev. A 82 (2010) 041802
We report numerical and experimental investigations of spatiotemporal nonlinear optical effects leading to spectral broadening in an array of subwavelength silicon waveguides pumped with infrared femtosecond pulses. Adjusting an input pulse position across the array, we observe different patterns in the output spectra. We explain these observations using a theory of the resonant (Cherenkov) radiation emitted by temporal solitons belonging to different spatial supermodes of the array. We also demonstrate strong nonperturbative coupling of temporal dispersion and discrete diffraction in the subwavelength arrays.

Polariton solitons due to saturation of the exciton-photon coupling
O.A. Egorov, D.V. Skryabin & F. Lederer
Phys. Rev. B 82 (2010) 165326
We study the properties of bright polariton solitons existing above the excitonic resonance in semiconductor microcavities operating in the strong-coupling regime. Saturation of the exciton-photon coupling due to electron-hole pair screening plays the crucial role in their formation and essentially determines the properties of these localized structures. Taking into account the finite mass of coherent excitons leads to modulational instabilities due to the generation of free excitons with large momenta. This instability can develop across the soliton profile resulting in either an imprinted stationary nanoscale periodic pattern or explosive soliton oscillations.

Coupled-mode approach to surface plasmon polaritons in nonlinear periodic structures
A. Marini, A.V. Gorbach & D.V. Skryabin
Opt. Lett. 35 (2010) 3532-3534
We present a coupled-mode theory describing light propagation in an array of nonlinear plasmonic waveguides. Our model predicts a two-band dependence of the propagation constant versus transverse quasi-momentum and existence of discrete and gap plasmon solitons. (C) 2010 Optical Society of America

Silica aerogel core waveguide
M.D.W. Grogan, S.G. Leon-Saval, R. England & T.A. Birks
Opt. Express 18 (2010) 22497-22502
We have selectively filled the core of hollow photonic crystal fibre with silica aerogel. Light is guided in the aerogel core, with a measured attenuation of 0.2 dB/cm at 1540 nm comparable to that of bulk aerogel. The structure guides light by different mechanisms depending on the wavelength. At long wavelengths the effective index of the microstructured cladding is below the aerogel index of 1.045 and guidance is by total internal reflection. At short wavelengths, where the effective cladding index exceeds 1.045, a photonic bandgap can guide the light instead. There is a small region of crossover, where both index- and bandgap-guided modes were simultaneously observed. (C) 2010 Optical Society of America

Quantum-Fluctuation-Initiated Coherence in Multioctave Raman Optical Frequency Combs
Y.Y. Wang, W. Chunbai, F. Couny, M.G. Raymer & F. Benabid
Phys. Rev. Lett. 105 (2010) 123603
We show experimentally and theoretically that the spectral components of a multioctave frequency comb spontaneously created by stimulated Raman scattering in a hydrogen-filled hollow-core photonic crystal fiber exhibit strong self- coherence and mutual coherence within each 12 ns driving laser pulse. This coherence arises in spite of the field’s initiation being from quantum zero-point fluctuations, which causes each spectral component to show large phase and energy fluctuations. This points to the possibility of an optical frequency comb with nonclassical correlations between all comb lines.(C) 2010 American Physical Society.

Parametric instabilities of microcavity polaritons in a periodic potential
A.V. Gorbach & D.V. Skryabin
Phys. Rev. B 82 (2010) 125313
We analyze parametric instabilities of microcavity polartions in the presence of one-dimensional periodic potentials. Instabilities appear in a certain window of pump momentum orientations with respect to the potential so that the momentum projection onto the direction of periodicity is large enough. Ring-shaped domains of parametrically amplified perturbations form a periodic chain in momentum space. Independently from the orientation of the pump momentum, the chain maintains its orientation along the direction of periodicity of the system. In the physical space the amplified structures emerge as stripes modulated along the direction of periodicity and of finite width in the orthogonal direction.

Two-Dimensional Localization of Exciton Polaritons in Microcavities
O.A. Egorov, A.V. Gorbach, F. Lederer & D.V. Skryabin
Phys. Rev. Lett. 105 (2010) 073903
We report two-dimensional localization of exciton polaritons in a coherently pumped planar semiconductor microcavity operating in the strong-coupling regime. Two-dimensional polariton solitons exist despite the opposite dispersion signs along the orthogonal in plane directions. Nonlinearities compensating the opposing dispersions have different physical origins and are due to the repulsion of polaritons on one side and due to parametric four-wave mixing on the other. Both of these nonlinearities can support their respective families of one-dimensional solitons, which coexist with each other and with the two-dimensional solitons.

High power red and near-IR generation using four wave mixing in all integrated fibre laser systems
L. Lavoute, J.C. Knight, P. Dupriez & W.J. Wadsworth
Opt. Express 18 (2010) 16193-16205
We demonstrate high power generation of visible red and near IR light by four wave mixing in photonic crystal fibres (PCFs) pumped at 1064 nm with picosecond pulses (30 - 80 ps). 30% conversion efficiency is demonstrated in a single pass using fibre lengths less than 1 m, with signal wavelengths from 650 nm to 820 nm selectable by choice of PCF. An all fibre integrated system delivers 2.16 W at 740 nm with a pulse repetition frequency of 20 MHz. We discuss the overall parameter space for this type of wavelength conversion in PCF with different fibre designs suitable for delivering a particular wavelength at low or high power. (C) 2010 Optical Society of America

Matched cascade of bandgap-shift and frequency-conversion using stimulated Raman scattering in a tapered hollow-core photonic crystal fibre
B. Beaudou, F. Couny, Y.Y. Wang, P.S. Light, N.V. Wheeler, F. Gerome & F. Benabid
Opt. Express 18 (2010) 12381-12390
We report on a novel means which lifts the restriction of the limited optical bandwidth of photonic bandgap hollow-core photonic crystal fiber on generating high order stimulated Raman scattering in gaseous media. This is based on H-2-filled tapered HC-PCF in which the taper slope is matched with the effective length of Raman process. Raman orders outside the input-bandwidth of the HC-PCF are observed with more than 80% quantum-conversion using a compact, low-power 1064 nm microchip laser. The technique opens prospects for efficient sources in spectral regions that are poorly covered by currently existing lasers such as mid-IR. (C) 2010 Optical Society of America

Large-core acetylene-filled photonic microcells made by tapering a hollow-core photonic crystal fiber
N.V. Wheeler, M.D.W. Grogan, P.S. Light, F. Couny, T.A. Birks & F. Benabid
Opt. Lett. 35 (2010) 1875-1877
We report on kagome-lattice photonic microcells with low losses, large outer diameters, and large cores. The large (40-70 mu m) cores are accommodated by tapering the fibers and splicing the reduced ends to a single-mode fiber. We demonstrate the repeatability of this process and obtain splice losses of 0.6 dB by optimizing the taper transition length. Narrow electromagnetically induced transparencies and saturable absorption are demonstrated in an acetylene-filled photonic microcell. (C) 2010 Optical Society of America

Vortex Lattices in Coherently Pumped Polariton Microcavities
A.V. Gorbach, R. Hartley & D.V. Skryabin
Phys. Rev. Lett. 104 (2010) 213903
We propose a new class of vortex lattices supported by the parametric conversion of polaritons in wide aperture semiconductor microcavities operating in the strong coupling regime and pumped by a coherent beam. We present numerical and analytical results confirming the existence and robustness of the polaritonic vortex lattices in practically relevant settings and discuss their melting scenarios.

Bend Loss in All-Solid Bandgap Fibers Revisited
Y.F. Li, D.M. Bird & T.A. Birks
J. Lightwave Technol. 28 (2010) 1368-1372
The rate of decay of the fundamental guided mode into the surrounding cladding in all-solid photonic bandgap fibers is investigated. Both a simple 1-D model and numerical calculations of realistic fibers show that the decay rate varies asymmetrically across the bandgaps, with larger values near the upper edge of the bandgap. The implications for modeling bend loss in these fibers are discussed.

Photonic microcell unleashes the full potential of gas lasers
F. Benabid, F. Couny & Y.Y. Wang
Laser Focus World 46 (2010) 55-60
A photonic-crystal fiber microcell with a laser-gas-filled core provides long interaction lengths and small mode areas, providing an integrated laser with tailorable wavelength.

Photonic BandGap Fiber With Multiple Hollow Cores
B.J. Mangan, A.C. Muir & J.C. Knight
J. Lightwave Technol. 28 (2010) 1287-1290
We report the fabrication and characterization of the first photonic bandgap fiber with multiple hollow waveguiding cores. Perspectives for scaling to highly multicore designs are discussed.

Colloquium: Looking at a soliton through the prism of optical supercontinuum
D.V. Skryabin & A.V. Gorbach
Rev. Mod. Phys. 82 (2010) 1287
A traditional view on solitons in optical fibers as robust particlelike structures suited for information transmission has been significantly altered and broadened over the past decade when solitons have been found to play the major role in generation of octave broad supercontinuum spectra in photonic crystal and other types of optical fibers. This remarkable spectral broadening is achieved through complex processes of dispersive radiation being scattered from, emitted, and transformed by solitons. Thus solitons have emerged as the major players in nonlinear frequency conversion in optical fibers. Unexpected analogies of these processes have been found with dynamics of ultracold atoms and ocean waves. This Colloquium focuses on recent understanding and new insights into physics of soliton-radiation interaction and supercontinuum generation.

Higher order guided mode propagation in solid-core photonic bandgap fibers
V. Pureur, J.C. Knight & B.T. Kuhlmey
Opt. Express 18 (2010) 8906-8915
We investigate higher order core-modes of solid-core photonic bandgap fibers experimentally and theoretically. We observe that for some wavelengths ranges the second mode is guided while the fundamental mode is not. We interpret this behavior in terms of the band diagrams and full numerical simulations, in good agreements with experiments. The sole guidance of the second, ring shaped modes observed at the edges of bandgaps could be of use for generation of vortex beams. (C) 2010 Optical Society of America

Compact and portable multiline UV and visible Raman lasers in hydrogen-filled HC-PCF
Y.Y. Wang, F. Couny, P.S. Light, B.J. Mangan & F. Benabid
Opt. Lett. 35 (2010) 1127-1129
We report on the realization of compact UV visible multiline Raman lasers based on two types of hydrogen-filled hollow-core photonic crystal fiber. The first, with a large pitch Kagome lattice structure, offers a broad spectral coverage from near IR through to the much sought after yellow, deep-blue and UV, whereas the other, based on photonic bandgap guidance, presents a pump conversion concentrated in the visible region. The high Raman efficiency achieved through these fibers allows for compact, portable diode-pumped solid-state lasers to be used as pumps. Each discrete component of this laser system exhibits a spectral density several orders of magnitude larger than what is achieved with supercontinuum sources and a narrow line-width, making it an ideal candidate for forensics and biomedical applications. (C) 2010 Optical Society of America

Amplified spontaneous emission of surface plasmon polaritons and limitations on the increase of their propagation length
P.M. Bolger, W. Dickson, A.V. Krasavin, L. Liebscher, S.G. Hickey, D.V. Skryabin & A.V. Zayats
Opt. Lett. 35 (2010) 1197-1199
Amplified spontaneous emission of surface plasmon polaritons (SPPs) at the interface of a resonant gain medium has been observed. The amplification is accompanied by significant spectral narrowing and limits the gain available for compensation of SPP propagation losses. The effect is similar to the deteriorating influence of amplified spontaneous emission in laser resonators. (C) 2010 Optical Society of America

Slow and Superluminal Light Pulses Via EIT in a 20-m Acetylene-Filled Photonic Microcell
N.V. Wheeler, P.S. Light, F. Couny & F. Benabid
J. Lightwave Technol. 28 (2010) 870-875
We have developed an all-fiber system where we generate electromagnetically induced transparencies in a 20-m acetylene-filled photonic microcell. Using this system, pulses of probe light were delayed and advanced by up to 5 and 1 ns, respectively. The delay/advance is tunable through the probe detuning and the coupling Rabi frequency. Through optimization of experimental parameters such as acetylene pressure, coupling laser power and decoherence rates it is shown that a pulse delay of 7 ns/m is possible. Limitations imposed on the fiber length by resonance group velocity dispersion and spectral reshaping are also discussed. In addition to optical buffering, we suggest a slow-light-based fiber optical gyroscope with an enhanced SNR of similar to 92.

Ultrahigh-birefringent squeezed lattice photonic crystal fiber with rotated elliptical air holes
F. Beltran-Mejia, G. Chesini, E. Silvestre, A.K. George, J.C. Knight & C.M.B. Cordeiro
Opt. Lett. 35 (2010) 544-546
We report an experimental realization of a highly birefringent photonic crystal fiber as a result of compressing a regular hexagonal structure. The experimental measurements estimate a group birefringence of approximately 5.5 X 10(-3) at 1550 nm in good agreement with the numerical results. We study the influence of compressing the regular structure at different directions and magnifications, obtaining a method to realistically enhance the phase birefringence while moderating the group birefringence. (C) 2010 Optical Society of America

Compact and Portable Multiline UV & Visible Raman Lasers in Hydrogen-filled HC-PCF
Y.Y. Wang, F. Couny, P.S. Light, B.J. Mangan & F. Benabid
QELS 2010 (2010) -
We present two compact multi-line Raman-lasers based on two types of HC-PCF photonic microcells. Each discrete component of the laser exhibits high spectral power density and narrow linewidth for forensics and biomedical applications. (C) 2010 Optical Society of America

Ultra-High Reflectivity Hollow Core PCF Microcell using a Tapered Micro-Mirror
N.V. Wheeler, M.D.W. Grogan, T.D. Bradley, F. Couny, T.A. Birks & F. Benabid
QELS 2010 (2010) -
Ultra-high reflectivity is achieved at the end-face of a hollow-core PCF via the insertion and fusion of a metal-coated tapered single-mode-fiber to the core of the hollow fiber. Saturable absorption is demonstrated using this device. (C) 2010 Optical Society of America

Low loss broadband transmission in optimized core-shape Kagome Hollow-Core PCF
Y.Y. Wang, F. Couny, P.J. Roberts & F. Benabid
QELS 2010 (2010) -
A Kagome-lattice hollow-core photonic crystal fiber with a hypocycloid-shaped core enhances the coupling-inhibition between the core-and cladding-modes, thus providing an optical attenuation <250dB/km. This rivals the state-of-the-art photonic bandgap hollow-core fibers is demonstrated. (C)2010 Optical Society of America

One-dimensional polariton solitons and soliton waveguiding in microcavities
D.V. Skryabin, O.A. Egorov, A.V. Gorbach & F. Lederer
J. SpMi 47 (2010) 5-9
We extend our recent results [O.A. Egorov et al. Phys. Rev. Lett. 102, 153904 (2009)] on half-light-half-matter polariton solitons in planar semiconductor microcavities operating in the strong coupling regime. We initiate discussion on the structure of the solitons in the momentum space and its link to the instability of the upper branch of the polariton bistability loop. Numerical results showing the soliton excitation by a seed pulse are presented. (C) 2009 Elsevier Ltd. All rights reserved.

Aerogel Package for Fused Fibre Couplers
L.M. Xiao, M.D.W. Grogan, R. England, W.J. Wadsworth & T.A. Birks
QELS 2010 (2010) -
Fused fibre couplers were encapsulated in hydrophobic aerogel. This provides an epoxy-free all-silica package that is low-loss, waterproof, supports the whole coupler waist and is stable up to 250 degrees C. (C)2010 Optical Society of America

Supermode Dispersion of Strongly Coupled Silicon-on-Insulator Waveguides
C.E. de Nobriga, W.J. Wadsworth, A.V. Gorbach, D.V. Skryabin, J.C. Knight, A. Samarelli, M. Sorel & R.M. De La Rue
QELS 2010 (2010) -
We report measurement of the group index dispersion of the supermodes of a three channel array of strongly coupled silicon-on-insulator waveguides, and compare results with numerical simulations. We observe strong coupling-induced dispersion. (C) 2010 Optical Society of America

High Power 55 Watts CW Raman Fiber-Gas-Laser
F. Couny, B.J. Mangan, A.V. Sokolov & F. Benabid
QELS 2010 (2010) -
We demonstrate, for the first time, a Raman fiber-gas-laser with 55W output power by coupling 85W of a high-power fiber-laser in a H2-filled hollow-core-photonic-crystal-fiber. The laser characteristics include pure Stokes output and high mode quality. (C) 2010 Optical Society of America

Supercontinuum Generation in Tapered Fibres Embedded in Silica Aerogel
M.D. Rollings, M.D.W. Grogan, L.M. Xiao, R. England, T.A. Birks & W.J. Wadsworth
QELS 2010 (2010) -
We demonstrate spectral broadening in tapered optical fibres embedded in silica aerogel. The aerogel provides protection to the taper without noticeably affecting the refractive and dispersive properties or power handling. (C) 2010 Optical Society of America

Ultra-long-lived Molecular Coherence in Hydrogen Filled Hollow-core Photonic Crystal Fiber
Y.Y. Wang, F. Couny, B.J. Mangan & F. Benabid
QELS 2010 (2010) -
A delayed probe-pump technique shows the persistence of molecular coherence of rotational Raman resonance in hydrogen-filled hollow-core photonic crystal fiber over a time-scale similar to 42 times longer than the dephasing time. (C) 2010 Optical Society of America

Coupling efficiency and transmission through hollow-core photonic bandgap fibers
J.M. Estudillo-Ayala, B.J. Mangan, W.J. Wadsworth & J.C. Knight
7839 (2010) -
We study the transmission of light through different lengths of Hollow-core bandgap fiber. We demonstrate 95% transmission of 5 picosecond pulses at 1064nm through fiber lengths of 1m, but only 77% transmission through longer lengths of 10m. This variation is not consistent with the measured attenuation of the "fundamental" low-loss mode of the fiber as being below 20dB/km in this spectral range, because the light transmitted through the short fiber not exclusively in the fundamental fiber mode. We conclude that great care is required to understand coupling efficiencies using short fiber lengths.

Gas Sensing with a Sub-Micron Tapered Fibre Embedded in Hydrophobic Aerogel
L.M. Xiao, M.D.W. Grogan, R. England, W.J. Wadsworth & T.A. Birks
QELS 2010 (2010) -
Tapered fibres with waist diameters down to 0.7 mu m embedded in hydrophobic aerogel are demonstrated as evanescent-field gas sensors. The porous aerogel is permeable to gases but protects the sub-micron fibre from contamination and damage. (C) 2010 Optical Society of America

Plasmonic Aerogel Doped with Gold Nanoparticles
M.D.W. Grogan, M.D. Rollings, L.M. Xiao, W.J. Wadsworth, R. England, S.A. Maier & T.A. Birks
QELS 2010 (2010) -
We have developed a new technique to incorporate gold nanoparticles into silica aerogel without aggregation. Such a doped "plasmonic" aerogel can be used for nonlinear optical experiments and plasmonic sensing. (C) 2010 Optical Society of America

Third Harmonic Generation in Uniform Fibre Nanotapers via Intermodal Coupling
M. Delgado-Pinar, Y.F. Li, D.M. Bird, T.A. Birks & W.J. Wadsworth
QELS 2010 (2010) -
Third harmonic generation in fibre nanotapers by intermodal coupling is experimentally characterized. UV radiation is generated from a pulsed 1064 nm laser in a 3 cm taper of 492 nm diameter. (C) 2010 Optical Society of America

Large Core Acetylene-Filled Photonic Microcells Made by Tapering the Hollow-Core Fiber
N.V. Wheeler, M.D.W. Grogan, P.S. Light, F. Couny, T.A. Birks & F. Benabid
QELS 2010 (2010) -
We fabricated low insertion loss and large core (40-70 mu m) kagome lattice acetylene-filled photonic microcells by tapering the large outer diameter kagome fibers and splicing their reduced ends to a single mode fiber. (C) 2010 Optical Society of America

All-Normal Dispersion Photonic Crystal Fiber for Coherent Supercontinuum Generation
L.E. Hooper, P.J. Mosley, A.C. Muir, W.J. Wadsworth & J.C. Knight
QELS 2010 (2010) -
We describe supercontinuum generation in a short photonic crystal fiber with all-normal group velocity dispersion. We observe a 200 nm broad self phase modulation spectrum, which is expected to have high temporal coherence. (C)2010 Optical Society of America

Ultrashort Pulse Delivery in Hollow-Core Photonic Bandgap Fiber at 540 nm
W.C. Huang, M.G. Welch, P.J. Mosley, B.J. Mangan, W.J. Wadsworth & J.C. Knight
QELS 2010 (2010) -
We report the transmission and compression of ultrashort pulses with a wavelength of 540 nm in hollow-core photonic bandgap fiber. We have observed pulses as short as 115 fs after 1 m of fiber. (C) 2007 Optical Society of America

2009 Publications

Modal interferometer based on hollow-core photonic crystal fiber for strain and temperature measurement
S.H. Aref, R. Amezcua-Correa, J.P. Carvalho, O. Frazao, P. Caldas, J.L. Santos, F.M. Araujo, H. Latifi, F. Farahi, L.A. Ferreira & J.C. Knight
Opt. Express 17 (2009) 18669-18675
In this work, sensitivity to strain and temperature of a sensor relying on modal interferometry in hollow-core photonic crystal fibers is studied. The sensing structure is simply a piece of hollow-core fiber connected in both ends to standard single mode fiber. An interference pattern that is associated to the interference of light that propagates in the hollow core fundamental mode with light that propagates in other modes is observed. The phase of this interference pattern changes with the measurand interaction, which is the basis for considering this structure for sensing. The phase recovery is performed using a white light interferometric technique. Resolutions of ± 1.4 mu epsilon and ± 0.2 degrees C were achieved for strain and temperature, respectively. It was also found that the fiber structure is not sensitive to curvature. (C) 2009 Optical Society of America

Mode field converter based on embedded photonic crystal fiber
W.M. Sun, X.Q. Liu, Q. Chai, J.Z. Zhang, F.H. Fu, Y. Jiang & T.A. Birks
Chin. Opt. Lett. 7 (2009) 921-923
Six high-index cores are embedded around the central solid core of the photonic crystal fiber to form a fiber embedded photonic crystal fiber (FEPCF), which is investigated based on the beam propagation method. In this structure, the Gaussian mode could be transferred to the ring mode. So FEPCF could used as a mode convertor.

Optical frequency comb generation in gas-filled hollow core photonic crystal fibres
F. Couny & F. Benabid
J. Opt. A: Pure Appl. Opt. 11 (2009) 103002
The efficiency of gas-based nonlinear processes is often limited by the diffraction of the pump laser as it propagates through the nonlinear medium. As a consequence, phenomena with strong nonlinear response requirements, such as high harmonic generation or Raman sideband generation, lack the required laser-matter interaction to fulfil their potential. Indeed, the conversion efficiency of these techniques is usually low and the experimental set-up cumbersome. The advent of hollow core photonic crystal fibre technology drafts new territories for nonlinear optics, and in particular offers new alternatives for sub-femtosecond pulse generation. The air-guiding fibre combines unprecedented laser confinement over long interaction lengths and, when filled with an adequate nonlinear gas, offers improved conversion efficiency and up to a million-fold reduction of the pump power threshold. This paper presents a review of the types of hollow core PCF available for nonlinear applications and the results obtained for efficient Raman conversion in H-2-filled hollow core PCF that led to the observation of a multi-octave frequency comb spanning from similar to 325 to similar to 2300 nm using a single pump laser with relatively low power. The generated ultra-broad spectrum creates a simple route towards a compact source of attosecond pulses.

High harmonic generation in a gas-filled hollow-core photonic crystal fiber
O.H. Heckl, C.R.E. Baer, C. Krankel, S.V. Marchese, F. Schapper, M. Holler, T. Sudmeyer, J.S. Robinson, J.W.G. Tisch, F. Couny, P. Light, F. Benabid & U. Keller
Appl. Phys. B 97 (2009) 369-373
High harmonic generation (HHG) of intense infrared laser radiation (Ferray et al., J. Phys. B: At. Mol. Opt. Phys. 21:L31, 1988; McPherson et al., J. Opt. Soc. Am. B 4:595, 1987) enables coherent vacuum-UV (VUV) to soft-X-ray sources. In the usual setup, energetic femtosecond laser pulses are strongly focused into a gas jet, restricting the interaction length to the Rayleigh range of the focus. The average photon flux is limited by the low conversion efficiency and the low average power of the complex laser amplifier systems (Keller, Nature 424:831, 2003; Sudmeyer et al., Nat. Photonics 2:599, 2008; Roser et al., Opt. Lett. 30:2754, 2005; Eidam et al., IEEE J. Sel. Top. Quantum Electron. 15:187, 2009) which typically operate at kilohertz repetition rates. This represents a severe limitation for many experiments using the harmonic radiation in fields such as metrology or high-resolution imaging. Driving HHG with novel high-power diode-pumped multi-megahertz laser systems has the potential to significantly increase the average photon flux. However, the higher average power comes at the expense of lower pulse energies because the repetition rate is increased by more than a thousand times, and efficient HHG is not possible in the usual geometry. So far, two promising techniques for HHG at lower pulse energies were developed: external build-up cavities (Gohle et al., Nature 436:234, 2005; Jones et al., Phys. Rev. Lett. 94:193, 2005) and resonant field enhancement in nanostructured targets (Kim et al., Nature 453:757, 2008). Here we present a third technique, which has advantages in terms of ease of HHG light extraction, transverse beam quality, and the possibility to substantially increase conversion efficiency by phase-matching (Paul et al., Nature 421:51, 2003; Ren et al., Opt. Express 16:17052, 2008; Serebryannikov et al., Phys. Rev. E (Stat. Nonlinear Soft Matter Phys.) 70:66611, 2004; Serebryannikov et al., Opt. Lett. 33:977, 2008; Zhang et al., Nat. Phys. 3:270, 2007). The interaction between the laser pulses and the gas occurs in a Kagome-type Hollow-Core Photonic Crystal Fiber (HC-PCF) (Benabid et al., Science 298:399, 2002), which reduces the detection threshold for HHG to only 200 nJ. This novel type of fiber guides nearly all of the light in the hollow core (Couny et al., Science 318:1118, 2007), preventing damage even at intensities required for HHG. Our fiber guided 30-fs pulses with a pulse energy of more than 10 mu J, which is more than five times higher than for any other photonic crystal fiber (Hensley et al., Conference on Lasers and Electro-Optics (CLEO), IEEE Press, New York, 2008).

Amplification of surface plasmon polaritons in the presence of nonlinearity and spectral signatures of threshold crossover
A. Marini, A.V. Gorbach, D.V. Skryabin & A.V. Zayats
Opt. Lett. 34 (2009) 2864-2866
We describe the effects of nonlinearity on propagation of surface plasmon polaritons (SPPs) at an interface between a metal and an amplifying medium of the externally pumped two-level atoms. Using Maxwell equations we derive the nonlinear dispersion law and demonstrate that the nonlinear saturation of the linear gain leads to formation of stationary SPP modes with the intensities independent from the propagation distance. Transition to the regime of stationary propagation is similar to the threshold crossover in lasers and leads to narrowing of the SPP spectrum. (C) 2009 Optical Society of America

Tapered fibers embedded in silica aerogel
L.M. Xiao, M.D.W. Grogan, S.G. Leon-Saval, R. Williams, R. England, W.J. Wadsworth & T.A. Birks
Opt. Lett. 34 (2009) 2724-2726
We have embedded thin tapered fibers (with diameters down to 1 mu m) in silica aerogel with low loss. The aerogel is rigid but behaves refractively like air, protecting the taper without disturbing light propagation along it. This enables a new class of fiber devices exploiting volume evanescent interactions with the aerogel itself or with dopants or gases in the pores. (C) 2009 Optical Society of America

Double photonic bandgap hollow-core photonic crystal fiber
P.S. Light, F. Couny, Y.Y. Wang, N.V. Wheeler, P.J. Roberts & F. Benabid
Opt. Express 17 (2009) 16238-16243
We report on the design, fabrication and characterization of hollow-core photonic crystal fiber with two robust bandgaps that bridge the benchmark laser wavelengths of 1064nm and 1550nm. The higher-order bandgap arises due to the extremely thin struts of the silica cladding and their fine-tuning relative to the apex size. The optimum strut thickness was found to be approximately one hundredth of the cladding pitch. (C) 2009 Optical Society of America

10 kHz accuracy of an optical frequency reference based on (C2H2)-C-12-filled large-core kagome photonic crystal fibers
K. Knabe, S. Wu, J.K. Lim, K.A. Tillman, P.S. Light, F. Couny, N. Wheeler, R. Thapa, A.M. Jones, J.W. Nicholson, B.R. Washburn, F. Benabid & K.L. Corwin
Opt. Express 17 (2009) 16017-16026
Saturated absorption spectroscopy reveals the narrowest features so far in molecular gas-filled hollow-core photonic crystal fiber. The 48-68 mu m core diameter of the kagome-structured fiber used here allows for 8 MHz full-width half-maximum sub-Doppler features, and its wavelength-insensitive transmission is suitable for high-accuracy frequency measurements. A fiber laser is locked to the (C2H2)-C-12 nu(1) + nu(3) P(13) transition inside kagome fiber, and compared with frequency combs based on both a carbon nanotube fiber laser and a Cr:forsterite laser, each of which are referenced to a GPS-disciplined Rb oscillator. The absolute frequency of the measured line center agrees with those measured in power build-up cavities to within 9.3 kHz (1 sigma error), and the fractional frequency instability is less than 1.2 x 10(-11) at 1 s averaging time. (C) 2009 Optical Society of America

10 kHz accuracy of an optical frequency reference based on 12C2H2-filled large-core kagome photonic crystal fibers.
K. Knabe, W. Shun, J.K. Lim, K.A. Tillman, P.S. Light, F. Couny, N. Wheeler, R. Thapa, A.M. Jones, J.W. Nicholson, B.R. Washburn, F. Benabid & K.L. Corwin
Opt. Express 17 (2009) 16017-16026
Saturated absorption spectroscopy reveals the narrowest features so far in molecular gas-filled hollow-core photonic crystal fiber. The 48-68 ?m core diameter of the kagome-structured fiber used here allows for 8 MHz full-width half-maximum sub-Doppler features, and its wavelengthinsensitive transmission is suitable for high-accuracy frequency measurements. A fiber laser is locked to the 12C2H2 v1 + v3 P(13) transition inside kagome fiber, and compared with frequency combs based on both a carbon nanotube fiber laser and a Cr:forsterite laser, each of which are referenced to a GPS-disciplined Rb oscillator. The absolute frequency of the measured line center agrees with those measured in power build-up cavities to within 9.3 kHz (1 sigma error), and the fractional frequency instability is less than 1.2 × 10(-11) at 1 s averaging time. ©2009 Optical Society of America

Gap polariton solitons
A.V. Gorbach, B.A. Malomed & D.V. Skryabin
Phys. Lett. A 373 (2009) 3024-3027
We report the existence, and study mobility and interactions of gap polariton solitons in a microcavity with a periodic potential, where the light field is strongly coupled to excitons. Gap solitons are formed due to the interplay between the repulsive exciton-exciton interaction and cavity dispersion. The analysis is carried out in an analytical form. using the coupled-mode (CM) approximation, and also by means of numerical methods. (C) 2009 Elsevier B.V. All rights reserved.

A phase-stabilized carbon nanotube fiber laser frequency comb
J.K. Lim, K. Knabe, K.A. Tillman, W. Neely, Y.S. Wang, R. Amezcua-Correa, F. Couny, P.S. Light, F. Benabid, J.C. Knight, K.L. Corwin, J.W. Nicholson & B.R. Washburn
Opt. Express 17 (2009) 14115-14120
A frequency comb generated by a 167 MHz repetition frequency erbium-doped fiber ring laser using a carbon nanotube saturable absorber is phase-stabilized for the first time. Measurements of the in-loop phase noise show an integrated phase error on the carrier envelope offset frequency of 0.35 radians. The carbon nanotube fiber laser comb is compared with a CW laser near 1533 nm stabilized to the v(1) + v(3) overtone transition in an acetylene-filled kagome photonic crystal fiber reference, while the CW laser is simultaneously compared to another frequency comb based on a Cr:Forsterite laser. These measurements demonstrate that the stability of a GPS-disciplined Rb clock is transferred to the comb, resulting in an upper limit on the locked comb's frequency instability of 1.2 x 10(-11) in 1 s, and a relative instability of < 3 x 10(-12) in 1 s. The carbon nanotube laser frequency comb offers much promise as a robust and inexpensive all-fiber frequency comb with potential for scaling to higher repetition frequencies. (C) 2009 Optical Society of America.

More than threefold expansion of highly nonlinear photonic crystal fiber cores for low-loss fusion splicing
Z. Chen, C. Xiong, L.M. Xiao, W.J. Wadsworth & T.A. Birks
Opt. Lett. 34 (2009) 2240-2242
We have formed low-loss fusion splices from highly nonlinear (HNL) photonic crystal fibers (PCFs) with small cores and high air-filling fractions to fibers with much larger mode field diameters (MFDs). The PCF core was locally enlarged by the controlled collapse of holes around the core while keeping other holes open. The fiber was then cleaved at the enlarged core and spliced to the large MFD fiber with a conventional electric arc fusion splicer. Splice losses as low as 0.36 dB were achieved between a PCF and a standard single-mode fiber (SMF) with MFDs of 1.8 mu m and 5.9 mu m, respectively. (c) 2009 Optical Society of America.

Continuum generation by dark solitons
C. Milian, D.V. Skryabin & A. Ferrando
Opt. Lett. 34 (2009) 2096-2098
We demonstrate that the dark soliton trains in optical fibers with a zero of the group-velocity dispersion can generate broad spectral distribution (continuum) associated with the resonant dispersive radiation emitted by solitons. This radiation is either enhanced or suppressed by the Raman scattering depending on the sign of the third-order dispersion. (C) 2009 Optical Society of America.

Ultrasensitive UV-tunable grating in all-solid photonic bandgap fibers
J.M. Lazaro, B.T. Kuhlmey, J.C. Knight, J.M. Lopez-Higuera & B.J. Eggleton
Opt. Commun. 282 (2009) 2358-2361
We study the shift of a long period grating's resonance wavelength with UV induced refractive index changes in an all-solid photonic bandgap fiber. A long period grating is mechanically imprinted in an all-solid photonic bandgap fiber with Germanium doped silica high-index rods in a lower-index silica background. The index of the high-index rods is modified through UV exposure, and we observe that the long period grating's resonance shifts with the bandgaps. With a sensitivity of 21,000 nanometers per refractive index unit and a 8.8 nm resonance width changes of refractive index of 3 x 10(-6) are in principle detectable Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

Control of transient regime of stimulated Raman scattering using hollow-core PCF
F. Couny, O. Carraz & F. Benabid
J. Opt. Soc. Am. B 26 (2009) 1209-1215
Accurate control over the transient and steady-state regimes of stimulated Raman amplification is achieved in hydrogen-filled hollow-core photonic crystal fiber via the control of the fiber length and the internal gas pressure. The experimental evolution of the characteristic time that determines the limit between the two scattering regimes is shown to closely follow the theoretical prediction. Transient amplification is observed for pump-laser pulse lengths longer than 10 times the Raman dephasing time, opening new prospects for the generation of a coherent optical frequency comb using ultralong pump-laser pulses (>100 ns). (C) 2009 Optical Society of America

Solitons in Hollow Core Photonic Crystal Fiber: Engineering Nonlinearity and Compressing Pulses
M.G. Welch, K. Cook, R.A. Correa, F. Gerome, W.J. Wadsworth, A.V. Gorbach, D.V. Skryabin & J.C. Knight
J. Lightwave Technol. 27 (2009) 1644-1652
We have demonstrated nonlinear propagation in a 3-cell hollow core photonic crystal fiber. The reduced core size increases the nonlinear coefficient of the guided mode. However, the reduction in the expected soliton energy is small (a factor of approximately 2) as the dispersion of this fiber is also increased by the smaller core. We also demonstrate soliton compression using a 35 m 7-cell tapered fiber, compressing picosecond input pulses by over an order of magnitude.

Dual-Wavelength-Pumped Supercontinuum Generation in an All-Fiber Device
C.L. Xiong, Z.L. Chen & W.J. Wadsworth
J. Lightwave Technol. 27 (2009) 1638-1643
We experimentally demonstrate supercontinuum generation with a novel dual-wavelength pumping scheme. The dual-wavelength pump source and supercontinuum generation were realized one after another in a fusion spliced all-fiber device. The spectrum, extending from as short as 360 nm to beyond 1750 nm, is very flat. The all-fiber configuration makes the source very compact.

Accurate measurement of the dispersion of hollow-core fibers using a scalable technique
M.G. Welch, C.E. de Nobriga, R.A. Correa, W.J. Wadsworth & J.C. Knight
Opt. Express 17 (2009) 9006-9011
A scalable and accurate technique for measuring the group index and dispersion of optical fibers is used to provide the first accurate measurements of dispersion slope in hollow-core photonic band-gap fibers. We present data showing group index, group-velocity dispersion and dispersion slope in hollow-core fibers guiding at both 800 nm and 1064 nm wavelength. (C) 2009 Optical Society of America

Spatial solitons in periodic nanostructures
A.V. Gorbach & D.V. Skryabin
Phys. Rev. A 79 (2009) 053812
We present the first-principles theory of the existence and stability of TE and TM spatial solitons in a subwavelength periodic semiconductor-dielectric structure. We have found that for the wavelength of 1550 nm and the interface separation close to and less than 100 nm the band structure of the linear TE and TM waves becomes similar to the band structure of a homogeneous medium. The properties of TE solitons change accordingly. However, the transverse profiles of the TM solitons continue to reflect the subwavelength geometry of the structure and develop dominant intensity peaks inside the low-index dielectric slots. Our stability analysis based on the linearized Maxwell equations indicates that the nonlinear TM waves can be approximated as the evanescently coupled modes of the slot waveguides with the low-index dielectric core and the high-index semiconductor cladding. Transition to the regime where the slot waveguides start to determine properties of TM waves is associated with the so-called Brewster condition.

Broadband tunable optical parametric amplification from a single 50 MHz ultrafast fiber laser
Y.W. Tzeng, Y.Y. Lin, C.H. Huang, J.M. Liu, H.C. Chui, H.L. Liu, J.M. Stone, J.C. Knight & S.W. Chu
Opt. Express 17 (2009) 7304-7309
We have demonstrated a 0.7 mu m - 1.9 mu m wavelength-tunable light source based on a single-pass optical parametric amplification (OPA) in a multiperiod magnesium oxide-doped periodically poled lithium niobate crystal. The OPA pump was a frequency-doubled ultrafast ytterbium-doped fiber oscillator, and the residual 1040 nm laser power after frequency doubling was recycled to generate a supercontinuum seeding source. Compared with conventional OPAs, this system is free from timing jitter between the pump laser and the seeding source. Over 50% conversion efficiency was obtained with 10 nJ pump energy. Combined with a 50 MHz repetition rate, this versatile source is ideal for biomedical and spectroscopic applications. (C) 2009 Optical Society of America

Bright Cavity Polariton Solitons
O.A. Egorov, D.V. Skryabin, A.V. Yulin & F. Lederer
Phys. Rev. Lett. 102 (2009) 153904
The lower branch of the dispersion relation of exciton polaritons in semiconductor microcavities, operating in the strong-coupling regime, contains sections of both positive and negative curvature along one spatial direction. We show that this leads to the existence of stable one-dimensional bright microcavity solitons supported by the repulsive polariton nonlinearity. To achieve localization along the second transverse direction we propose to create a special soliton waveguide by changing the cavity detuning and hence the boundary of the soliton existence in such a way that the solitons are allowed only within the stripe of the desired width.

Narrowband high-fidelity all-fibre source of heralded single photons at 1570 nm
A.R. McMillan, J. Fulconis, M. Halder, C. Xiong, J.G. Rarity & W.J. Wadsworth
Opt. Express 17 (2009) 6156-6165
An all-fibre heralded single photon source operating at 1570 nm has been demonstrated. The device generates correlated photon pairs, widely spaced in frequency, through four-wave mixing in a photonic crystal fibre. Separation of the pair photons and narrowband filtering is all achieved in fibre. The output heralded single photon rate was 9.2 x 10(4) per second, with a counts-to-accidentals ratio of 10.4 and a heralding fidelity of 52%. Furthermore, narrowband filtering ensured that the output single photon state was near time-bandwidth limited with a coherence length of 4 ps. Such a source is well suited to quantum information processing applications. (C) 2009 Optical Society of America

Electromagnetically induced transparency in acetylene molecules with counterpropagating beams in V and lambda schemes
P.S. Light, F. Benabid, F. Couny, G.J. Pearce & D.M. Bird
Appl. Phys. Lett. 94 (2009) -
We report on the experimental observation of electromagnetically induced transparency in V and lambda energy level schemes using counterpropagating coupling and probe beam geometry. The observation was achieved using an acetylene photonic microcell. The conditions required for this observation are explored theoretically, and we show that the use of counterpropagating beams in electromagnetically induced transparency may have applications as a spectroscopic technique where velocity discrimination is desirable. ©2009 American Institute of Physics

Characterization of Photonic Crystal Fibre mode converter using low coherence interferometry
P. Nandi, Z. Chen, A. Witkowska, W.J. Wadsworth, J.G. Rarity, W.J. Wadsworth, T.A. Birks & J.C. Knight
Opt. Lett. 34 (2009) 1123-1125
The relative group delay of the different modes present in an all-fiber LP11 mode converter at a central wavelength of 750 nm is observed using low coherence interferometric imaging. We have simultaneously measured the relative group delay and computed the intensity and the phase distribution of the modes emitted from the mode converter end face using a Fourier technique, providing unequivocal identification of the modes involved. © 2009 Optical Society of America

Coupling induced anomalous group velocity dispersion in nonlinear arrays of silicon photonic wires
C.J. Benton & D.V. Skryabin
Opt. Express 17 (2009) 5879-5884
We demonstrate that the group velocity dispersion (GVD) of the supermodes in a small array of silicon photonic wires can differ dramatically from the single wire GVD. This enables soliton propagation and modulational instability to be seen at wavelengths where single wires have strongly normal GVD. (C) 2009 Optical Society of America

Frequency stabilisation of a fibre-laser comb using a novel microstructured fibre
C.R. Locke, E.N. Ivanov, P.S. Light, F. Benabid & A.N. Luiten
Opt. Express 17 (2009) 5897-5904
There is great interest in developing high performance optical frequency metrology based around mode-locked fibre lasers because of their low cost, small size and long-term turnkey operation when compared to the solid-state alternative. We present a method for stabilising the offset frequency of a fibre-based laser comb using a 2f - 3f technique based around a unique fibre that exhibits strong resonant dispersive wave emission. This fibre requires lower power than conventional highly non-linear fibre to generate a suitable signal for offset frequency stabilisation and this in turn avoids the complexity of additional nonlinear steps. We generate an offset frequency signal from the mixing of a wavelength-shifted second harmonic comb with a third harmonic of the comb. Additionally, we have stabilised the repetition rate of the laser to a level better than 10(-14)/root tau, limited by the measurement system noise floor. We present the means for complete and precise measurement of the transfer function of the laser frequency controls. (C) 2009 Optical Society of America

Nonclassical 2-photon interference with separate intrinsically narrowband fibre sources
M. Halder, J. Fulconis, B. Cemlyn, A. Clark, C. Xiong, W.J. Wadsworth & J.G. Rarity
Opt. Express 17 (2009) 4670-4676
In this paper, we demonstrate a source of photon pairs based on four-wave-mixing in photonic crystal fibres. Careful engineering of the phase matching conditions in the fibres enables us to create photon pairs at 597 nm and 860 nm in an intrinsically factorable state showing no spectral correlations. This allows for heralding one photon in a pure state and hence renders narrow band filtering obsolete. The source is narrow band, bright and achieves an overall detection efficiency of up to 21% per photon. For the first time, a Hong-Ou-Mandel interference with unfiltered photons from separate fibre sources is presented. (C) 2009 Optical Society of America

All-optical-fiber polarization-based quantum logic gate
A.S. Clark, J. Fulconis, J.G. Rarity, W.J. Wadsworth & J.L. O'Brien
Phys. Rev. A 79 (2009) 030303
We report an experimental demonstration of a quantum controlled-NOT gate constructed entirely in optical fiber and operating on polarization-encoded single-photon qubits. We operated this gate using two heralded optical fiber single-photon sources and found an average logical fidelity of 90% and an average process fidelity of 0.83 <= (F) over bar <= 0.91. On the basis of a simple model we are able to conclude that imperfections are primarily due to the photon sources, meaning that the gate itself works with very high fidelity. Such all fiber quantum information processing will likely have important applications in future quantum networks.

Low bend loss in tightly-bent fibers through adiabatic bend transitions
L. Yao, A. Birks & J.C. Knight
Opt. Express 17 (2009) 2962-2967
We demonstrate low bend loss for tightly bent optical fibers by winding the fiber around a mandrel designed to follow an adiabatic transition path into the bend. Light in the fundamental core-guided mode is smoothly transferred to a single cladding mode of the bent fiber, and back to the core mode as it leaves the bent region again. Design of the transition is based on modeling of the propagation and coupling characteristics of the core and cladding modes, which clearly illustrate the physical processes involved. (c) 2009 Optical Society of America

All-fiber devices based on photonic crystal fibers with integrated electrodes
G. Chesini, C.M.B. Cordeiro, C.J.S. de Matos, M. Fokine, I.C.S. Carvalho & J.C. Knight
Opt. Express 17 (2009) 1660-1665
A special kind of microstructured optical fiber is proposed and fabricated in which, in addition to the holey region (solid core and silica-air cladding), two large holes exist for electrode insertion. Either Bi-Sn or Au-Sn alloys were selectively inserted into the large holes forming two parallel, continuous and homogeneous internal electrodes. We demonstrate the production of a monolithic device and its use to externally control some of the guidance properties (e. g. polarization) of the fiber. (C) 2009 Optical Society of America

Light and gas confinement in hollow-core photonic crystal fibre based photonic microcells
F. Benabid, P.J. Roberts, F. Couny & P.S. Light
J. Eur. Opt. Soc. Rapid Pub. 4 (2009) 09004
We review the recent progress on the understanding of optical guidance mechanisms in hollow-core photonic crystal fibres, and on the quantum and nonlinear optical applications of photonic microcells based on this fibre form. Two classes of hollow-core photonic crystal fibre are identified: one guides via a photonic bandgap and the other guides by virtue of an inhibited coupling between core and cladding mode constituents. For the former fibre type, we explore how the bandgap is formed using a photonic analogue of the tight-binding model and how it is related to the anti-resonant reflection optical waveguide guidance. For the second type of fibre, which can guide over a broad wavelength range, we examine the nature of the inhibited coupling. We describe a technique for the fabrication of photonic microcells that can accommodate vacuum pressures, and we finish by showing the latest results on electromagnetically induced transparency in a rubidium filled hollow-core photonic crystal fibre, the CW-pumped hydrogen Raman laser and the generation of multi-octave spanning stimulated Raman scattering spectral combs. [DOI: 10.2971/jeos.2009.09004]

Modifying photonic crystal fibres
T.A. Birks, M.D.W. Grogan, Z. Chen, L.M. Xiao, S.G. Leon-Saval, C. Xiong & R. England
(2009) 96-97
We use controlled hole collapse in photonic crystal fibres to reduce the splice loss between dissimilar fibres. We also describe a hollow-core PCF as a support for a waveguide core made from silica aerogel.

2008 Publications

Dark polariton solitons in semiconductor microcavities
A.V. Yulin, O.A. Egorov, F. Lederer & D.V. Skryabin
Phys. Rev. A 78 (2008) 061801
We report the existence of dark polariton solitons and study their stability and symmetry breaking in semiconductor microcavities operating in the strong-coupling regime. These half-light half-matter solitons are potential candidates for applications in all-optical signal processing. Their excitation time and required pump powers are a few orders of magnitude less than those of their weakly coupled light-only counterparts.

Square-lattice large-pitch hollow-core photonic crystal fiber
F. Couny, P.J. Roberts, T.A. Birks & F. Benabid
Opt. Express 16 (2008) 20626-20636
We report on the design, fabrication and characterization of silica square-lattice hollow core photonic crystal fibers optimized for low loss guidance over an extended frequency range in the mid-IR region of the optical spectrum. The fiber’s linear optical properties include an ultra-low group velocity dispersion and a polarization cross-coupling as low as -13.4dB over 10m of fiber.

Hollow Core PCFs enable high nonlinearity at low light levels
F. Benabid, F. Couny, P.S. Light & P.J. Roberts
Laser Focus World 44 (2008) 61-64
The advent of photonic microcells that consist of a length of gas-filled hollow-core photonic-crystal fiber hermetically sealed to a solid conventional optical fiber enables the development of novel fiber lasers and frequency combs.

Fourth-order dispersion mediated solitonic radiations in HC-PCF cladding
F. Benabid, F. Biancalana, P.S. Light, F. Couny, A. Luiten, P.J. Roberts, J. Peng & A.V. Sokolov
Opt. Lett. 33 (2008) 2680-2682
We observe experimentally, for the first time to our knowledge, the simultaneous emission of two strong conjugate resonant dispersive waves by optical solitons. The effect is observed in a small waveguiding glass feature within the cladding of a Kagome hollow-core photonic crystal fiber. We demonstrate theoretically that the phenomenon is attributed to the unusually high fourth-order dispersion coefficient of the waveguiding feature.

New white-light supercontinuum sources have no blue deficiency
J. Clowes & J. Knight
Laser Focus World 44 (2008) 59-61

Tailoring the Nonlinear Response of Hollow-core Photonic Bandgap Fibres
M. Welch, R.A. Correa, F. Gerome, Renshaw S, & J. Knight
AIP Conf. Proc. 1055 (2008) 54-57
We have fabricated 7-cell and 3-cell core hollow-core photonic bandgap fibres with core sizes ranging from ~16.7 µm to 6.5 µm. A numerical study of the nonlinear coefficient of fibres with different core sizes is carried out. We show that the nonlinearity is more effectively increased by a 3-cell core design than by reducing the size of a seven-cell core.

Experimental reconstruction of bands in solid core photonic bandgap fibres using acoustic gratings
B.T. Kuhlmey, F. Luan, L.B. Fu, D.I. Yeom, B.J. Eggleton, A.M. Wang & J.C. Knight
Opt. Express 16 (2008) 13845-13856
We present the first characterisation of cladding modes of a low-index contrast all-solid photonic bandgap fiber using an acousto-optic long-period grating. We experimentally measure the relative band diagrams of the cladding, visualise the fields of the cladding modes in the near field, and find both to be in good agreement with simulations. Our measurements and simulations show that the bands of the cladding are very sensitive to actual details of the structure.

Spatiotemporal quasisolitons and resonant radiation in arrays of silicon-on-insulator photonic wires
C.J. Benton, A.V. Gorbach & D.V. Skryabin
Phys. Rev. A 78 (2008) 033818
We have analyzed the conditions for low-power spatiotemporal soliton formation in arrays of evanescently coupled silicon-on-insulator photonic wires. We have verified that pronounced soliton effects can be observed in the presence of realistic loss, two- photon absorption, and higher-order dispersions. A soliton in an N-wire array can excite N resonant frequencies, but some of these may be suppressed due to the soliton having zero projection onto the corresponding radiation supermodes. This results in pronounced differences between the radiation spectra observed from solitons excited at the edge and in the center of arrays.

Trends in stimulated Brillouin scattering and optical phase conjugation
M. Ostermeyer, H.J. Kong, V.I. Kovalev, R.G. Harrison, A.A. Fotiadi, P. Megret, M. Kalal, O. Slezak, J.W. Yoon, J.S. Shin, D.H. Beak, S.K. Lee, Z. Lu, S. Wang, D. Lin, J.C. Knight, N.E. Kotova, A. Straber, A. Scheikh-Obeid, T. Riesbeck, S. Meister, H.J. Eichler, Y. Wang, W. He, H. Yoshida, H. Fujita, M. Nakatsuka, T. Hatae, H. Park, C. Lim, T. Omatsu, K. Nawata, N. Shiba, O.L. Antipov, M.S. Kuznetsov & N.G. Zakharov
Laser and Particle Beams 26 (2008) 297-362
An overview on current trends in stimulated Brillouin scattering and optical phase conugation is given. This report is based on the results of the "Second International Workshop on stimulated Brillouin scattering and phase conjugation" held in Potsdam/Germany in September 2007. The properties of stimulated Brillouin scattering are presented for the compensation of phase distortions in combination with novel laser technology like ceramics materials but also for e.g., phase stabilization, beam combination, and slow light. Photorefractive nonlinear mirrors and resonant refractive index gratings are addressed as phase conjugating mirrors in addition.

Discrete cavity solitons due to saturable nonlinearity
A.V. Yulin, A.R. Champneys & D.V. Skryabin
Phys. Rev. A 78 (2008) 011804
Discrete localized structures are found in optical cavities with focusing saturable nonlinearity. Families of both bright and gray solitons exhibit multistability as they develop internal shelves in the pinning region around a Maxwell point. Both saturability and discreteness are required for these solitons to be observable, but they can occur for either zero or finite losses.

Core-Surround Shaping of Hollow-Core Photonic Crystal Fiber Via HF Etching
Y.Y. Wang, P.S. Light & F. Benabid
IEEE Photonics Technol. Lett. 20 (2008) 1018-1020
We report on a technique to pattern the shape of the core surround of a hollow-core photonic crystal fiber. Different shapes are fabricated and a relationship between surface mode location and core surround thickness is experimentally observed.

Vortex solitons in an off-resonant Raman medium
A.V. Gorbach, D.V. Skryabin & C.N. Harvey
Phys. Rev. A 77 (2008) 063810
We investigate the existence and linear stability of coupled vortex solitons supported by cascaded four-wave mixing in a Raman active medium excited away from the resonance. We present a detailed analysis for the two-and three-component vortex solitons and demonstrate the formation of stable and unstable vortex solitons, and associated spatiotemporal helical beams, under the conditions of the simultaneous frequency and vortex comb generation.

Over 4000 nm bandwidth of mid-IR supercontinuum generation in sub-centimeter segments of highly nonlinear tellurite PCFs
P. Domachuk, N.A. Wolchover, M. Cronin-Golomb, A. Wang, A.K. George, C.M.B. Cordeiro, J.C. Knight & F.G. Omenetto
Opt. Express 16 (2008) 7161-7168
We report broad bandwidth, mid-IR supercontinuum generation using a sub-cm (8 mm) length of highly nonlinear tellurite microstructured photonic crystal fiber (PCF). We pump the fiber at telecommunication wavelengths by using 1550 nm, 100 fs pulses of energy E=1.9 nJ. When coupled in the PCF, these pulses result in a supercontinuum (SC) bandwidth of 4080 nm extending from 789 to 4870 nm measured at 20 dBm below the peak spectral power. This bandwidth is comparable or in excess of previously reported spectra for other nonlinear glass fiber formulations despite the significantly shorter fiber length. In addition, besides offering a convenient pump wavelength, short fiber lengths enable smoother SC spectra, lower dispersion, and reduced material absorption at longer wavelengths making the use of this PCF particularly interesting. (C) 2008 Optical Society of America.

Novel process eases production of hollow-core fiber
R.A. Correa & J. Knight
Laser Focus World 44 (2008) 67-69
A new way of drawing fiber that omits the use of a core tube and interstitial fibers results in a faster procedure that reduces the time it takes to fabricate a preform and draw the first working fibers to one day, and improves performance.

Modal coupling in surface-corrugated long-period-grating fiber tapers
W. Ding & S.R. Andrews
Opt. Lett. 33 (2008) 717-719
We describe the properties of a surface-corrugated long-period-grating fiber taper fabricated using contact optical lithography and wet etching techniques. The preservation of cylindrical symmetry in this device facilitates investigation of the modal behavior. Comparison of the measured and calculated transmission spectra reveals that the widely used coupled-mode theory is not applicable. Instead, a mode-projection model, in which modal propagation and coupling are treated separately within the grating, explains the experiments very well.(c) 2008Optical Society of America.

Metallic mode confinement in microstructured fibres
J. Hou, D. Bird, A. George, S. Maier, B.T. Kuhlmey & J.C. Knight
Opt. Express 16 (2008) 5983-5990
We report the first long, uniform, optical fibers in which visible light is guided in a single mode by metallic reflection. We describe the fabrication, experiment and characterization of these metallic optical fibers and compare them with theoretical calculations. (c) 2008 Optical Society of America.

Soliton self-frequency shift, non-solitonic radiation and self-induced transparency in air-core fibers
A.V. Gorbach & D.V. Skryabin
Opt. Express 16 (2008) 4858-4865
We report numerical investigation of several effects accompanying propagation of femtosecond pulses in air-core photonic crystal fibers. We have found that the strong Raman response of air does not always result in the large soliton self-frequency shift, because it can simultaneously stimulate energy losses into non-solitonic radiation. We demonstrate that the pronounced spectral tails seen in many recent experiments on the short wavelength side of the soliton spectra can be associated with emission of Airy waves by the decelerating solitons. For pulse durations close to 10fs all radiation effects due to Raman response of air become negligible for a special choice of the peak power leading to propagation in the self-induced transparency regime. (c) 2008 Optical Society of America.

Mid-infrared gas sensing using a photonic bandgap fiber
N. Gayraud, L.W. Kornaszewski, J.M. Stone, J.C. Knight, D.T. Reid, D.P. Hand & W.N. MacPherson
Appl. Optics 47 (2008) 1269-1277
We demonstrate methane sensing based on Fourier transform infrared spectroscopy using a hollow-core photonic bandgap fiber guiding in the mid-infrared and idler pulses from a femtosecond optical parametric oscillator. Transmission measurements are presented for several fibers, and sensing is demonstrated using a fiber whose bandgap overlaps the methane fundamental absorption lines. The gas filling process of the air core is described, and qualitative methane concentrations measurements to 1000 ppm (parts in 10(6)) are reported. Operation down to 50 ppm based on our current experiment is predicted. (c) 2008 Optical Society of America.

Solitonic dynamics of ultrashort pulses in a highly nonlinear photonic-crystal fiber visualized by spectral interferometry
D.A. Sidorov-Biryukov, A. Fernandez, L. Zhu, A. Verhoef, P. Dombi, A. Pugzlys, E.E. Serebryannikov, A.M. Zheltikov, J.C. Knight & A. Baltuska
Opt. Lett. 33 (2008) 446-448
A linear technique of phase measurement based on spectral interferometry is employed to visualize the fine details in the spectral phase of a soliton produced in a highly nonlinear photonic crystal fiber (PCF) with a resolution better than 0.1 nm. Gigahertz features have been resolved in the spectral phase of the soliton PCF output, allowing the accuracy of time-domain soliton envelope reconstruction to be improved on the timescale of a few femtoseconds. (C) 2008 Optical Society of America.

Polarized supercontinuum in birefringent photonic crystal fibre pumped at 1064 nm and application to tuneable visible/UV generation
C. Xiong & W.J. Wadsworth
Opt. Express 16 (2008) 2438-2445
We generate a flat, polarized and single mode supercontinuum (SC) spanning 450-1750 nm in a highly birefringent photonic crystal fibre (PCF) pumped by a 1064 nm microchip laser. More than 99% of the total power is kept in a single linear polarization. The measured power coupling penalty due to the elliptical core is less than 6% (0.25 dB). As one of its applications, we demonstrate tuneable visible/UV generation in the nonlinear crystal BIBO pumped by this polarized SC source. A tuneable range of 400-525 nm is obtained by critical phase matching in BIBO. We also show the results of visible/UV generation in BIBO pumped by the signal wavelength of polarized four-wave mixing (FWM) in PCF. (C) 2008 Optical Society of America.

Visibly "white" light generation in uniform photonic crystal fiber using a microchip laser
J.M. Stone & J.C. Knight
Opt. Express 16 (2008) 2670-2675
We report on visibly white supercontinuum generation in photonic crystal fibers using a sub ns pump source at 1064nm. The spectra extend from below 400nm to 2450nm, some 50nm further into the blue than previously reported spectra. The extra bandwidth which is achieved by a simple modification to the fiber structure gives a higher apparent color temperature and a truly "white" visual appearance. The mechanism for the generation of the deeper blue to ultraviolet frequencies is outlined and our modified fiber is compared with fibers which have been conventionally used for supercontinuum generation. (C) 2008 Optical Society of America.

High power tunable femtosecond soliton source using hollow-core photonic bandgap fiber, and its use for frequency doubling
F. Gerome, P. Dupriez, J. Clowes, J.C. Knight & W.J. Wadsworth
Opt. Express 16 (2008) 2381-2386
We report a high power tunable femtosecond soliton-based source using a simple combination of fiber-amplified pulses at 1064nm and hollow-core photonic bandgap fiber. Compression of 5.5ps input pulses, strongly chirped by self phase modulation in the amplifier, results in stable 520fs-soliton formation with 77% conversion efficiency after only 8m propagation in the hollow-core fiber. The Raman self-frequency shift of the solitons was used to provide 33nm wavelength tuneability. The transformlimited output pulses were frequency doubled using a nonlinear crystal with high conversion efficiency of 60% to demonstrate a femtosecond green laser tunable from 534nm to 548nm with 180nJ pulse energy.(C) 2008 Optical Society of America.

Highly efficient generation of broadband cascaded four-wave mixing products
A. Cerqueira, J.M.C. Boggio, A.A. Rieznik, H.E. Hernandez-Figueroa, H.L. Fragnito & J.C. Knight
Opt. Express 16 (2008) 2816-2828
We report and investigate on a highly efficient technique to generate broadband cascaded four-wave mixing (FWM) products. It consists of launching two strong pump waves near the zero-dispersion wavelength of very short (of order of few meters) optical fibers. Simulations based on split step fourier method (SSFM) and experimental data demonstrate the efficiency of this approach. Multiple FWM products have been investigated by using conventional fibers and ultra-flattened dispersion photonic crystal fibers. Measured results present bandwidths of 300 nm with up to 118 FWM products. We have also demonstrated a flat bandwidth of 110 nm covering the C and L bands, with a small variation of only 1.2 dB between the powers of FWM products, achieved by using highly nonlinear fibers (HNLFs). The use of dispersion tailored photonic crystal fibers has been shown interesting for improving the multiple FWM efficiency and reducing the separation between the pump wavelengths. (c) 2008 Optical Society of America.

Control of surface modes in low loss hollow-core photonic bandgap fibers
R. Amezcua-Correa, F. Gerome, S.G. Leon-Saval, N.G.R. Broderick, T.A. Birks & J.C. Knight
Opt. Express 16 (2008) 1142-1149
We report on the fabrication and characterization of hollow-core photonic bandgap fibers that do not suffer from surface mode coupling within the photonic bandgap of the cladding. This enables low attenuation over the full spectral width of the bandgap - we measured a minimum loss of 15 dB/km and less than 50 dB/km over 300 nm for a fiber operating at 1550 nm. As a result of the increased bandwidth, the fiber has reduced dispersion and dispersion slope - by a factor of almost 2 compared to previous fibers. These features are important for several applications in high-power ultrashort pulse compression and delivery. Realizing these advances has been possible due to development of a modified fabrication process which makes the production of low-loss hollow-core fibers both simpler and quicker than previously. (c) 2008 Optical Society of America.

2007 Publications

Characterization of frequency noise on a broadband infrared frequency comb using optical heterodyne techniques
K. Kim, J.W. Nicholson, M. Yan, J.C. Knight, N.R. Newbury & S.A. Diddams
Opt. Express 15 (2007) 17715-17723
We measure the frequency noise across a Cr: forsterite infrared frequency comb through the optical heterodyne beat of different comb teeth against stable continuous wave (CW) lasers. This sensitive measurement shows strong correlations of the frequency noise between spectral components of the comb, relative to a fixed optical frequency near the 1.3 micron carrier of the Cr: forsterite laser. The correlated frequency fluctuations are shown to arise from amplitude noise on the pump laser. We also report a preliminary comparison of excess noise that occurs during supercontinuum generation in both highly nonlinear fiber and an extruded glass microstructured fiber. (c) 2007 Optical Society of America.

Enhanced tunability and linewidth sharpening of plasmon resonances in hybridized metallic ring/disk nanocavities
F. Hao, P. Nordlander, M.T. Burnett & S.A. Maier
Phys. Rev. B 76 (2007) 245417
Using the finite-difference time-domain method, we investigate the plasmonic mode spectrum of a metallic nanostructure consisting of a concentric arrangement of a solid disk and a surrounding ring. We demonstrate that the energies of the plasmon modes depend sensitively on the structural parameters of the disk and the ring. We show that the nature of the plasmon modes can be understood simply as a hybridization of individual disk and ring plasmons. This interaction results in a redshifted bonding plasmon resonance of significantly narrower linewidth and larger electromagnetic field enhancements than the parent plasmons. This highly tunable nanostructure has significant potential as a substrate for surface enhanced spectroscopies.

Generation and photonic guidance of multi-octave optical-frequency combs
F. Couny, F. Benabid, P.J. Roberts, P.S. Light & M.G. Raymer
Science 318 (2007) 1118-1121
Ultrabroad coherent comb-like optical spectra spanning several octaves are a chief ingredient in the emerging field of attoscience. We demonstrate generation and guidance of a three-octave spectral comb, spanning wavelengths from 325 to 2300 nanometers, in a hydrogen-filled hollow-core photonic crystal fiber. The waveguidance results not from a photonic band gap but from the inhibited coupling between the core and cladding modes. The spectrum consists of up to 45 high-order Stokes and anti-Stokes lines and is generated by driving the confined gas with a single, moderately powerful (10-kilowatt) infrared laser, producing 12-nanosecond-duration pulses. This represents a reduction by six orders of magnitude in the required laser powers over previous equivalent techniques and opens up a robust and much simplified route to synthesizing attosecond pulses.

Enhanced four-wave mixing and parametric oscillation in photonic crystal fibre
K. Cook, C. Xiong & W.J. Wadsworth
J. Opt. A-Pure Appl. Opt. 9 (2007) 1095-1099
We demonstrate efficient four- wave mixing in photonic crystal fibres with nanosecond, 1047 nm pump pulses. The incorporation of a ring cavity configuration allows a parametric oscillator to be formed, generating up to 31 mW of average power at 790 nm with a conversion efficiency of 15%.

Theory of radiation trapping by the accelerating solitons in optical fibers
A.V. Gorbach & D.V. Skryabin
Phys. Rev. A 76 (2007) 053803
We present a theory describing trapping of the normally dispersive radiation by the Raman solitons in optical fibers. Frequency of the radiation component is continuously blueshifting, while the soliton is redshifting. Underlying physics of the trapping effect is in the existence of the inertial gravitylike force acting on light in the accelerating frame of reference. We present analytical calculations of the rate of the opposing frequency shifts of the soliton and trapped radiation and find it to be greater than the rate of the redshift of the bare Raman soliton. Our findings are essential for understanding of the continuous shift of the high-frequency edge of the supercontinuum spectra generated in photonic crystal fibers toward higher frequencies.

Light trapping in gravity-like potentials and expansion of supercontinuum spectra in photonic-crystal fibres
A.V. Gorbach & D.V. Skryabin
Nat. Photonics 1 (2007) 653-657
Femtosecond pulses of light propagating along photonic-crystal fibres can generate a broad optical supercontinuum(1,2). This striking discovery has applications ranging from spectroscopy and metrology(3) to telecommunication(4) and medicine(5,6). Among the physical principles underlying supercontinuum generation are soliton emission(7), a variety of four-wave mixing processes(8-11), Raman-induced soliton self-frequency shift(12,13), and dispersive wave generation mediated by solitons(7,13,14). Although all of the above effects contribute to supercontinuum generation, none of them can explain the generation of blue and violet light from infrared femtosecond pump pulses. In this work we argue that the most profound role in the shaping of the short-wavelength edge of the continuum is played by the effect of radiation trapping in a gravity-like potential created by accelerating solitons. The underlying physics of this effect has a straightforward analogy with the inertial forces acting on an observer moving with a constant acceleration.

Bouncing of a dispersive pulse on an accelerating soliton and stepwise frequency conversion in optical fibers
A.V. Gorbach & D.V. Skryabin
Opt. Express 15 (2007) 14560-14565
We demonstrate that a short pulse with spectrum in the range of normal group velocity dispersion can experience periodic reflections on a refractive index maximum created by a co-propagating with it soliton, providing the latter is continuously decelerated by the intrapulse Raman scattering. After each reflection the intensity profile and phase of the pulse are almost perfectly reconstructed, while its frequency is stepwise converted. This phenomenon has direct analogy with the effect of 'quantum bouncing' known for cold atoms. (C) 2007 Optical Society of America.

Subwatt threshold cw raman fiber-gas laser based on H-2-filled hollow-core photonic crystal fiber
F. Couny, F. Benabid & P.S. Light
Phys. Rev. Lett. 99 (2007) 143903
We report on what is, to our knowledge, the first cw pumped Raman fiber-gas laser based on a hollow-core photonic crystal fiber filled with hydrogen. The high efficiency of the gas-laser interaction inside the fiber allows operation in a single-pass configuration. The transmitted spectrum exhibits 99.99% of the output light at the Stokes wavelength and a pump power threshold as low as 2.25 W. The study of the Stokes emission evolution with pressure shows that highly efficient Raman amplification is still possible even at atmospheric pressure. The addition of fiber Bragg gratings to the system, creating a cavity at the Stokes wavelength, reduces the Raman threshold power below 600 mW.

Effective index method for all-solid photonic bandgap fibres
Y.F. Li, C.Y. Wang, T.A. Birks & D.M. Bird
J. Opt. A: Pure Appl. Opt. 9 (2007) 858-861
An effective index method is proposed for all-solid photonic bandgap fibres, where the effective cladding index is taken to be that of the lower edge of the bandgaps. The weak-guidance approximation allows the effective indices of both the fibre cladding and the defect modes to be calculated analytically. Comparison with a vectorial plane wave method shows that good accuracy is obtained.

Hollow-core photonic crystal fibres for delivery and compression of ultrashort optical pulses
J.C. Knight, F. Gerome & W.J. Wadsworth
Opt. Quantum Electron. 39 (2007) 1047-1056
We describe the use of tapered hollow-core photonic crystal fibres for delivery and compression of ultrashort optical pulses. We demonstrate delivery of transform-limited pulses with less than 100 fs pulse length and above 50 nJ energy through 8 m of fibre, in a single transverse mode.

Symposium on microstructured and nanostructured optical fibers in ICMAT 2007 - Preface
P. Shum, J. Knight, J. Love & M. Yan
Opt. Quantum Electron. 39 (2007) 947-948

Optical pulse compression in dispersion decreasing photonic crystal fiber
J.C. Travers, J.M. Stone, A.B. Rulkov, B.A. Cumberland, A.K. George, S.V. Popov, J.C. Knight & J.R. Taylor
Opt. Express 15 (2007) 13203-13211
Improvements to tapered photonic crystal fiber (PCF) fabrication have allowed us to make up to 50 m long PCF tapers with loss as low as 30 dB/km. We discuss the design constraints for tapered PCFs used for adiabatic soliton compression and demonstrate over 15 times compression of pulses from over 830 fs to 55 fs duration at a wavelength of 1.06 mu m, an order of magnitude improvement over previous results. (C) 2007 Optical Society of America.

Nonclassical interference and entanglement generation using a photonic crystal fiber pair photon source
J. Fulconis, O. Alibart, J.L. O'Brien, W.J. Wadsworth & J.G. Rarity
Phys. Rev. Lett. 99 (2007) 120501
We demonstrate two key components for optical quantum information processing: a bright source of heralded single photons; and a bright source of entangled photon pairs. A pair of pump photons produces a correlated pair of photons at widely spaced wavelengths (583 nm and 900 nm), via a chi((3)) four-wave mixing process. We demonstrate nonclassical interference between heralded photons from independent sources with a visibility of 95% (after correction for background), and an entangled photon pair source, with a fidelity of 89% with a Bell state.

Mid-infrared methane detection in a photonic bandgap fiber using a broadband optical parametric oscillator
L. Kornaszewski, N. Gayraud, J.M. Stone, W.N. MacPherson, A.K. George, J.C. Knight, D.P. Hand & D.T. Reid
Opt. Express 15 (2007) 11219-11224
We demonstrate methane sensing using a photonic bandgap fiber-based gas cell and broadband idler pulses from a periodically-poled lithium niobate femtosecond optical parametric oscillator. The hollow core of the fiber was filled with a methane: nitrogen mixture, and Fourier transform spectroscopy was used to measure transmission spectra in the 3.15-3.35 mu m methane absorption region. The method has applications in gas sensing for remote or hazardous environments and potentially at very low concentrations. (c) 2007 Optical Society of America.

Surface corrugation Bragg gratings on optical fiber tapers created via plasma etch postprocessing
W. Ding, S.R. Andrews & S.A. Maier
Opt. Lett. 32 (2007) 2499-2501
We describe a reflection-based fiber filter fabricated by plasma etching a surface corrugation Bragg grating on a tapered single-mode fiber. The taper waist with the grating forms the functional part of the filter, and the adiabatic taper transition removes unwanted higher-order modes. The spectral response is controlled by varying the taper diameter while maintaining a constant grating period. Reflection spectra have been investigated theoretically and experimentally and found to be in good agreement. (c) 2007 Optical Society of America.

Quantum interference with photon pairs using two micro-structured fibres
J. Fulconis, O. Alibart, W.J. Wadsworth & J.G. Rarity
New J. Phys. 9 (2007) 276
We demonstrate a quantum interference experiment between two photons coming from non-degenerate pairs created by four-wave mixing in two separated micro-structured fibres. When the two heralded photons are made indistinguishable a 95% visibility is demonstrated.

Photonic crystal fibers and fiber lasers (Invited)
J.C. Knight
J. Opt. Soc. Am. B-Opt. Phys. 24 (2007) 1661-1668
Photonic crystal fibers and fiber lasers have been two of the most rapidly evolving areas of optics and photonics over the past few years. Recent developments in the field of photonic crystal fibers are enabling new ways to generate, transform, and deliver light, and have significant implications for fiber laser design and applications. We review the ways in which photonic crystal fibers are affecting and may further affect the development and deployment of fiber laser technology. (c) 2007 Optical Society of America.

Reduction of fresnel back-reflection at splice interface between hollow core PCF and single-mode fiber
F. Couny, F. Benabid & P.S. Light
IEEE Photonics Technol. Lett. 19 (2007) 1020-1022
Optical feedback from the Fresnel reflection at the air-silica interface of a splice between a hollow core photonic crystal fiber (HC-PCF) and a conventional fiber can cause laser instability or damage. The reported solution consists of the preparation of angle cleaved fibers, spliced together using a filament fusion splicer. A reduction of 44 dB of the return loss is observed. The method will considerably reduce potential damage such as fiber fusion in case of a fiber laser. The method is also dernonstrated to decrease the Fabry-Wrot effect in setups such as saturable absorption when the spliced HC-PCF is used As a gas cell device.

Cascaded generation of multiply charged optical vortices and spatiotemporal helical beams in a Raman medium
A.V. Gorbach & D.V. Skryabin
Phys. Rev. Lett. 98 (2007) 243601
Using an example of a Raman active medium we describe how a common nonlinear process of four-wave mixing can be used to induce strong coupling between the spatial and temporal degrees of freedom in optical waves. This coupling produces several unexpected effects. Amongst those are cascaded excitation of multiply charged optical vortices, spatial focusing in a nonlinearly defocusing medium, and generation of helically shaped spatiotemporal optical solitons.

Internal excitation and superfocusing of surface plasmon polaritons on a silver-coated optical fiber tip
W. Ding, S.R. Andrews & S.A. Maier
Phys. Rev. A 75 (2007) 063822
We have theoretically studied the conversion of radially polarized waveguide modes of a tapered optical fiber into surface plasmon polaritons (SPPs) propagating at the outer surface of an apertureless silver-coated optical tip. Optimization of this process is important in exploiting SPP superfocusing in scanning near-field optical microscopy without the need for external illumination. Our approach is based on analyzing the evolution of the local modal index as a function of the fiber radius. The influence of mode projection, intermodal coupling, and metal dissipation are treated analytically, while a numerical finite integration technique is used to model radiation coupling. The results identify and quantify the mode conversion processes that need to be taken into account. We estimate that at least 10% of the modal energy in an uncoated fiber taper can be fed into the superfocusing mode at a silver-coated tip.

Delivery of sub-100fs pulses through 8m of hollow-core fiber using soliton compression
F. Gerome, K. Cook, A.K. George, W.J. Wadsworth & J.C. Knight
Opt. Express 15 (2007) 7126-7131
We report soliton compression in a tapered hollow-core photonic bandgap fiber. We compress unchirped 195fs input pulses at 800 nm wavelength to less than 100fs after single-mode propagation through 8m of fiber, at pulse energies of around 50nJ.

Electromagnetically induced transparency in Rb-filled coated hollow-core photonic crystal fiber
P.S. Light, F. Benabid, F. Couny, M. Maric & A.N. Luiten
Opt. Lett. 32 (2007) 1323-1325
We report the observation of lambda-configuration electromagnetically induced transparency as well as optical pumping in rubidium-filled kagome-structure hollow-coated-core photonic crystal fiber. We show that a polydimethylsiloxane coating of the fiber core reduces the linewidth of the transparency below that which could be expected for an uncoated fiber. The measured 6 MHz linewidth was dominated by optical broadening. (C) 2007 Optical Society of America

Clear for launch
S. Maier
Nat. Phys. 3 (2007) 301-303

Coupling in dual-core photonic bandgap fibers: theory and experiment
Z. Wang, T. Taru, T.A. Birks & J.C. Knight
Opt. Express 15 (2007) 4795-4803
We have theoretically and experimentally investigated dual-core photonic bandgap fibers (PBGFs), which consist of a cladding with an array of high-index rods and two cores formed by omitting two nearby rods. We find novel features in their coupling characteristics such as maxima and minima in coupling length, complete decoupling of the cores, and an inversion of the usual ordering of supermodes so that the odd supermode has the higher propagation constant. This behavior is understood by considering the field distribution in the rods between the cores. (c) 2007 Optical Society of America

Initial dynamics of supercontinuum generation in highly nonlinear photonic crystal fiber
J.T. Moeser, N.A. Wolchover, J.C. Knight & E.G. Omenetto
Opt. Lett. 32 (2007) 952-954
We present a theoretical and experimental analysis of supercontinuum generation in very short lengths of high-nonlinearity photonic crystal fibers. The Raman response function for Schott SF6 glass is presented for what is believed to be the first time and used for numerical modeling of pulse propagation. Simulation and experiments are in excellent agreement and demonstrate the rapid transition to regimes of spectral complexity due to higher-order nonlinear effects. (c) 2007 Optical Society of America.

935 nm Nd3+ fibre laser incorporating tapered photonic bandgap fibre filter
A. Wang, W. Ding, T.A. Birks & J.C. Knight
Electron. Lett. 43 (2007) 327-329
An 8 cm length of photonic bandgap fibre was tapered in the middle to narrow the transmission windows. The tapered fibre was incorporated into a ncodymium fibre laser system, in which it suppressed the strong gain peaks at 907 and 1080 nm and ensured lasing at 935 nm.

Wavelength-independent all-fiber mode converters
K. Lai, S.G. Leon-Saval, A. Witkowska, W.J. Wadsworth & T.A. Birks
Opt. Lett. 32 (2007) 328-330
We have used two different photonic crystal fiber (PCF) techniques to make all-fiber mode converters. An LP01 to LP11 mode converter was made by the ferrule technique on a drawing tower, and an LP01 to LP02 mode converter was made by controlled hole inflation of an existing PCF on a tapering rig. Both devices rely on adiabatic propagation rather than resonant coupling; so high extinction was achieved across a wide wavelength range. (c) 2007 Optical Society of America.

High nonlinearity glass photonic crystal nanowires
N.A. Wolchover, F. Luan, A.K. George, J.C. Knight & F.G. Omenetto
Opt. Express 15 (2007) 829-833
Soft glass photonic crystal fibers (PCFs) have been fabricated for the first time with the stack and draw process. The same SF6-PCFs have been successfully tapered using a brush flame method. The transverse structure of the PCF does not collapse in the tapering process and core dimensions of the fabricated photonic nanowire has been measured to be 400 nm in diameter. Supercontinuum radiation in excess of one octave has been generated in both the untapered and tapered PCF and, in the latter case, pulse energy thresholds of 65 picojoules at a pump wavelength of 1550 nm were observed. (c) 2007 Optical Society of America.

Field enhancement within an optical fibre with a subwavelength air core
G.S. Wiederhecker, C.M.B. Cordeiro, F. Couny, F. Benabid, S.A. Maier, J.C. Knight, C.H.B. Cruz & H.L. Fragnito
Nat. Photonics 1 (2007) 115-118
Tightly confined light enables a variety of applications ranging from nonlinear light management to atomic manipulation. Photonic-crystal fibres (PCFs) can provide strong guidance in very small cores while simultaneously offering long interaction lengths(1). However, light confinement in waveguides is usually ultimately limited by diffraction(2,3), which tends to spread light away from the waveguiding core, despite its higher refractive index. It was recently demonstrated that such spreading fields can be trapped by a nanometre-scale slot inside a strongly guiding silicon-on-insulator (SOI) waveguide(4,5). In this letter we demonstrate the concentration of optical energy within a subwavelength-scale air hole running down the length of a PCF core. The core resembles a submicrometre-diameter tube with a bore diameter of 200 nm or less. The high intensity in an air hole, coupled with long interaction lengths, promises a new class of experiments in light-matter interaction and nonlinear fibre optics.

Enhanced SRS in H-2 filled hollow core photonic crystal fibre by use of fibre Bragg grating
F. Couny, F. Benabid & O. Carraz
J. Opt. A: Pure Appl. Opt. 9 (2007) 156-159
An all-fibre hydrogen Raman-laser cavity using fibre Bragg gratings and an H-2-filled HCPCF-based gas cell is reported for the first time to our knowledge. An enhancement of stimulated Raman scattering is observed when the cavity is introduced. This includes a reduction of the threshold on all generated Raman lines and an enhancement of power conversion to the second Stokes of ortho-H-2 by more than 30 dB. These results confirm the feasibility of an all-fibre CW Raman gas laser.

Identification of Bloch-modes in hollow-core photonic crystal fiber cladding
F. Couny, F. Benabid, P.J. Roberts, M.T. Burnett & S.A. Maier
Opt. Express 15 (2007) 325-338
We report on the experimental visualization of the cladding Bloch-modes of a hollow-core photonic crystal fiber. Both spectral and spatial field information is extracted using the approach, which is based on measurement of the near-field and Fresnel-zone that results after propagation over a short length of fiber. A detailed study of the modes near the edges of the band gap shows that it is formed by the influence of three types of resonator: the glass interstitial apex, the silica strut which joins the neighboring apexes, and the air hole. The cladding electromagnetic field which survives the propagation is found to be spatially coherent and to contain contributions from just a few types of cladding mode. (c) 2007 Optical Society of America.

Fresnel zone imaging of Bloch-modes from a Hollow-Core Photonic Crystal Fiber Cladding
F. Couny, F. Benabid, P.J. Roberts & P.S. Light
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 (2007) 937-938
A Fresnel zone imaging technique at the output of a short length of hollow core photonic crystal fiber allows the identification of the photonic crystal cladding Bloch-modes. The experimental results show excellent agreement with theory. (C) 2007 Optical Society of America

Sub-Wavelength Intensity Profiles and Field Enhancement within an Optical Fiber
G.S. Wiederhecker, C.M.B. Cordeiro, F. Couny, F. Benabid, S.A. Maier, J.C. Knight, C.H.B. Cruz & H.L. Fragnito
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 (2007) 939-940
We demonstrate concentration of optical energy within a sub-wavelength air hole running the length of an optical fiber. The fiber core resembles a tiny tube with a bore diameter of 200 rim or less. (C) 2007 Optical Society of America

Large Pitch Kagome-Structured Hollow-Core PCF
F. Couny, F. Benabid & P.S. Light
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 LA English (2007) 929-930
A new type of hollow-core-PCF based on large pitch (similar to 12 mu m) kagome-lattice cladding is reported. The fiber exhibits broad visible and IR transmission bands with low loss, low chromatic dispersion and high core-light confinement. (C) 2007 Optical Society of America

Identification of the Band-Edge Cladding Modes of a Hollow-Core Photonic Crystal Fibre
F. Couny, F. Benabid, P.J. Roberts, M.T. Burnett & S.A. Maier
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 (2007) 1577-1578
The cladding-modes adjacent the photonic bandgap of a hollow-core photonic crystal fiber are identified. The results show that the bandgap is due to three types of resonators: the glass-apex, the silica-strut, and the air hole. (C) 2007 Optical Society of America PU IEEE; 345 E 47TH ST, NEW YORK, NY 10017 USA

Control of the Transient Regime of Stimulated Raman Scattering in Hollow-Core Photonic Crystal Fiber.
F. Couny, F. Benabid & P.S. Light
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 (2007) 1029-1030
Detailed experimental results on the evolution of stimulated Raman scattering amplification regimes in H-2 filled hollow-core PCF are reported. The role of the fiber length and the gas pressure is highlighted. (C) 2007 Optical Society of America PU IEEE; 345 E 47TH ST, NEW YORK, NY 10017 USA

Photonic crystal fibre tapers and devices.
T.A. Birks
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 (2007) 1654-1655
Tapering (heat treatment after fabrication) can radically change the properties of photonic crystal fibres over centimetre lengths. Such transitions give useful fibre devices, including low-loss interfaces to dissimilar fibres, waveguides and other optical systems. (C)2007 Optical Society of America PU IEEE; 345 E 47TH ST, NEW YORK, NY 10017 USA

Low Insertion-Loss (1.8 dB) and Vacuum-Pressure All-Fiber Acetylene Cell Based on Hollow-Core PCF
P.S. Light, F. Couny & F. Benabid
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 (2007) 1500-1501
A novel hollow-core PCF acetylene-cell fabrication technique based on helium-diffusion through silica is reported. The gas cell combines low insertion loss (1.8 dB) and low pressure (0.001 mbar). Electromagnetically induced transparency was used to determine the final acetylene-pressure. (c) 2007 Optical Society of America. PU IEEE; 345 E 47TH ST, NEW YORK, NY 10017 USA

Experimental Study Comparing EIT in V and Lambda Schemes in Acetylene-Filled HC-PCF.
P.S. Light, F. Benabid, F. Couny, G. Pearce & D.M. Bird
2007 CONFERENCE ON LASERS & ELECTRO-OPTICS/QUANTUM ELECTRONICS ANDLASER SCIENCE CONFERENCE (CLEO/QELS 2007), VOLS 1-5 (2007) 101-102
We report detailed experimental and theoretical results comparing electromagnetically-induced-transparency obtained in V and Lambda configurations in acetylene-filled-hollow-core PCF. For the same experimental conditions, the EIT in V-scheme shows a stronger peak but larger linewidth. (C)2007 Optical Society of America

2006 Publications

Hollow-core photonic bandgap fibre: new light guidance for new science and technology
F. Benabid
Philos. Trans. R. Soc. A-Math. Phys. Eng. Sci. 364 (2006) 3439-3462
We review the progress made on the fabrication and applications of hollow-core photonic crystal fibres (HC-PCFs). The mechanism of the light guidance in these fibres is described along with their dispersion properties. We review the HC-PCF fabrication, the different results achieved in the fields of laser-induced particle guidance, low-threshold stimulated Raman scattering in hydrogen (vibrational and rotational), laser frequency metrology and quantum optics. Finally, we show the different new prospects opened up by these fibres.

Large-pitch kagome-structured hollow-core photonic crystal fiber
F. Couny, F. Benabid & P.S. Light
Opt. Lett. 31 (2006) 3574-3576
We report the fabrication and characterization of a new type of hollow-core photonic crystal fiber based on large-pitch (similar to 12 mu m) kagome lattice cladding. The optical characteristics of the 19-cell, 7-cell, and single-cell core defect fibers include broad optical transmission bands covering the visible and near-IR parts of the spectrum with relatively low loss and low chromatic dispersion, no detectable surface modes and high confinement of light in the core. Various applications of such a novel fiber are also discussed, including gas sensing, quantum optics, and high harmonic generation. (c) 2006 Optical Society of America.

Photonic sensing based on variation of propagation properties of photonic crystal fibres
J.H. Rothwell, D.A. Flavin, W.N. MacPherson, J.D.C. Jones, J.C. Knight & P.S. Russell
Opt. Express 14 (2006) 12445-12450
We report on a low-coherence interferometric scheme for the measurement of the strain and temperature dependences of group delay and dispersion in short, index-guiding, 'endlessly-single-mode' photonic crystal fibre elements in the 840 nm and 1550 nm regions. Based on the measurements, we propose two schemes for simultaneous strain and temperature measurement using a single unmodified PCF element, without a requirement for any compensating components, and we project the measurement accuracies of these schemes.

Modulational instability of discrete solitons in coupled waveguides with group velocity dispersion
A.V. Yulin, D.V. Skryabin & A.G. Vladimirov
Opt. Express 14 (2006) 12347-12352
We study temporal modulational instability of spatial discrete solitons in waveguide arrays with group velocity dispersion (GVD). For normal GVD we report existence of the strong 'neck'-type instability specific for the discrete solitons. For anomalous GVD the instability leads to formation of the mixed discrete-continuous spatio-temporal quasi-solitons. Feasibility of experimental observation of these effects in the arrays of silicon-on-insulator waveguides is discussed. (c) 2006 Optical Society of America.

T-rays show promise for chemical analysis
S. Maier
Trac-Trends Anal. Chem. 25 (2006) V-V

Localized states in a triangular set of linearly coupled complex Ginzburg-Landau equations
A. Sigler, B.A. Malomed & D.V. Skryabin
Phys. Rev. E 74 (2006) 066604
We introduce a pattern-formation model based on a symmetric system of three linearly coupled cubic-quintic complex Ginzburg-Landau equations, which form a triangular configuration. This is the simplest model of a multicore fiber laser. We identify stability regions for various types of localized patterns possible in this setting, which include stationary and breathing triangular vortices.

Spectral-discrete solitons and localization in frequency space
A.V. Gorbach & D.V. Skryabin
Opt. Lett. 31 (2006) 3309-3311
We report families of discrete optical solitons in frequency space, or spectral-discrete solitons existing in a dispersive Raman medium, where individual sidebands are coupled by coherence. The associated time-domain patterns correspond either to trains of ultrashort pulses or to weakly modulated waves. We describe the physics behind the spectral localization and study soliton bifurcations, stability, and dynamics. (c) 2006 Optical Society of America.

Slowing down of solitons by intrapulse Raman scattering in fibers with frequency cutoff
A.V. Yulin & D.V. Skryabin
Opt. Lett. 31 (2006) 3092-3094
A method for transforming fast solitans into slow ones in bandgap fibers is proposed. The approach is based on the deceleration of the solitons by intrapulse Raman scattering, which can be achieved for fiber modes having a cutoff frequency. We develop a comprehensive theory for the soliton slowdown and elucidate how the fiber losses introduce a fundamental minimum for the soliton velocity. (c) 2006 Optical Society of America.

Plasmonics: The promise of highly integrated optical devices
S.A. Maier
IEEE J. Sel. Top. Quantum Electron. 12 (2006) 1671-1677
This paper reviews the fundamentals of surface plasmon polariton (SPP) excitations, sustained by interfaces between metallic and insulating media, with a focus on applications in waveguiding of electromagnetic waves at visible and near-infrared frequencies. The large wavevectors accessible via SPP oscillations allow for significantly reduced wavelengths and thus increased confinement of the propagating modes, promising a subwavelength photonic infrastructure suitable for integration on Si-based photonic chips. Various geometries such as single interfaces, multilayer structures, and nanoparticle ensembles are discussed and their properties assessed in terms of light confinement and energy attenuation of the guided modes. Metal/insulator/metal (MIM) and certain forms of nanoparticle waveguides promise novel avenues for light confinement, guiding, and coupling, which could provide the basis for subwavelength photonic devices.

Plasmonics: Metal nanostructures for subwavelength photonic devices
S.A. Maier
IEEE J. Sel. Top. Quantum Electron. 12 (2006) 1214-1220
This paper reviews recent progress toward the creation of a nanophotonic framework for confining and guiding electromagnetic energy at visible and near-infrared frequencies using surface plasmon excitations sustained by metallic nanostructures. Prominent geometries such as metallinsulator/metal-gap waveguides are assessed in terms of light confinement and the energy attenuation of the guided modes, as well as metal-nanoparticle waveguides that guide light via near-field coupling of localized particle or gap plasmon modes for deep subwavelength confinement. The effective mode volume concept of dielectric optics is then applied to plasmonic nanoresonators, which allows a comparison with established dielectric microcavities and demonstrates the deep subwavelength confinement achievable in metallic nanocavities. Lastly, a solution to the coupling problem of surface plasmon-polariton modes to the outside world is presented in the form of a fiber-accessible metal-nanoparticle plasmon waveguide with experimentally demonstrated power transfer up to 75 % at lambda = 1. 5 mu m.

Near-field optical imaging with a CdSe single nanocrystal-based active tip
Y. Sonnefraud, N. Chevalier, J.F. Motte, S. Huant, P. Reiss, J. Bleuse, F. Chandezon, M.T. Burnett, W. Ding & S.A. Maier
Opt. Express 14 (2006) 10596-10602
We report near-field scanning optical imaging with an active tip made of a single fluorescent CdSe nanocrystal attached at the apex of an optical tip. Although the images are acquired only partially because of the random blinking of the semiconductor particle, our work validates the use of such tips in ultra-high spatial resolution optical microscopy.

All solid photonic bandgap fiber based on an array of oriented rectangular high index rods
A. Wang, G.J. Pearce, F. Luan, D.M. Bird, T.A. Birks & J.C. Knight
Opt. Express 14 (2006) 10844-10850
We report the fabrication, characterization and modeling of an all-solid photonic bandgap fiber (PBGF) based on an array of oriented rectangular rods. Observed near-field patterns of cladding modes clearly identify the cut-off rod modes at the bandgap edges. The bend losses in this fiber depend on the bend direction, and can be understood by the directional coupling properties of the different rod modes and the modeled density of cladding states. (c) 2006 Optical Society of America.

Terahertz surface plasmon-polariton propagation and focusing on periodically corrugated metal wires
S.A. Maier, S.R. Andrews, L. Martin-Moreno & F.J. Garcia-Vidal
Phys. Rev. Lett. 97 (2006) 176805
In this Letter, we show how the dispersion relation of surface plasmon polaritons (SPPs) propagating along a perfectly conducting wire can be tailored by corrugating its surface with a periodic array of radial grooves. In this way, highly localized SPPs can be sustained in the terahertz region of the electromagnetic spectrum. Importantly, the propagation characteristics of these spoof SPPs can be controlled by the surface geometry, opening the way to important applications such as energy concentration on cylindrical wires and superfocusing using conical structures.

Four-wave mixing of solitons with radiation and quasi-nondispersive wave packets at the short-wavelength edge of a supercontinuum
A.V. Gorbach, D.V. Skryabin, J.M. Stone & J.C. Knight
Opt. Express 14 (2006) 9854-9863
We apply the recently developed theory of frequency generation by mixing of solitons and dispersive waves [Phys. Rev. E 72, 016619 (2005)] to explain the observed formation, quasi-trapping and frequency shift of the spectral peaks at the blue edge of supercontinua generated in silica-core photonic crystal fibers. (c) 2006 Optical Society of America.

Energy exchange between colliding solitons in photonic crystal fibers
F. Luan, D.V. Skryabin, A.V. Yulin & J.C. Knight
Opt. Express 14 (2006) 9844-9853
Collisions of femtosecond solitons in silica core photonic crystal fibers are investigated experimentally and theoretically. Clear spectral signatures of the significant energy exchange between the interacting pulses are reported. Two primary and competing effects causing energy exchange are interpulse Raman scattering, which is insensitive to the phase difference of the colliding solitons, and the phase sensitive interaction via the Kerr nonlinearity. (c) 2006 Optical Society of America.

Approximate band structure calculation for photonic bandgap fibres
T.A. Birks, G.J. Pearce & D.M. Bird
Opt. Express 14 (2006) 9483-9490
An approximate method for finding the band structure of simple photonic bandgap fibres is presented. Our simple model is an isolated high-index rod in a circular unit cell with two alternative boundary conditions. Band plots calculated this way are found to correspond closely to calculations using an accurate numerical method. (c) 2006 Optical Society of America

Raman solitons with group velocity dispersion
D.V. Skryabin & A.V. Yulin
Phys. Rev. E 74 (2006) 046616
We consider the coupled propagation of the pump and Stokes waves in a Raman active medium accounting for the group velocity walk off and group velocity dispersion. Interplay of the Raman coherence and the dispersion can lead to the formation of a complete band gap in the spectrum of linear waves consisting of the two consecutive subgaps located at different frequencies. Using an approximate analytic technique, we find exponentially localized solitons residing in the complete gap, and find algebraic solitons when the gap is closed. Feasibility of observation of these structures in hollow fibers is discussed.

Measuring beam quality of hollow core photonic crystal fibers
J.D. Shephard, P.J. Roberts, J.D.C. Jones, J.C. Knight & D.P. Hand
J. Lightwave Technol. 24 (2006) 3761-3769
In this paper, the authors measure the quality of the delivered beam from hollow core photonic crystal fibers (HC-PCFs). The M-2 parameter is determined, and the near- to far-field transition is examined. The influence on these properties due to the presence of a core surround mode is evaluated. The applicability of the International Standards Organization 11146:1999 standard for M-2 measurement of the beam quality of HC-PCFs is discussed. Because they are dependent on the measurement parameters, such as choice of aperturing scheme and the axis of measurement, M-2 values could vary from 1.32 to 3.17 for the same output beam. This highlights the need for careful consideration when measuring and describing the beam quality delivered by these novel photonic fibers.

All-fiber anamorphic core-shape transitions
A. Witkowska, K. Lai, S.G. Leon-Saval, W.J. Wadsworth & T.A. Birks
Opt. Lett. 31 (2006) 2672-2674
We demonstrate low-loss anamorphic transitions between circular and rectangular fiber cores with aspect ratios up to 5:1, and hence improved coupling from a diode laser by using only a spherical lens. Differential hole pressurization and localized heating of a stock photonic crystal fiber inflates the holes at different rates. Some holes are plugged in the fiber end face where pressure is applied, so they remain at ambient pressure. Alternatively, holes of different sizes expand at different rates because the effective pressure due to surface tension differs. (c) 2006 Optical Society of America.

Quantum lattice solitons in ultracold bosons near the Feshbach resonance
K.V. Krutitsky & D.V. Skryabin
J. Phys. B-At. Mol. Opt. Phys. 39 (2006) 3507-3517
Quantum lattice solitons in a system of two ultracold bosons near Feshbach resonance are investigated. It is shown that their binding energy, effective mass and spatial width can be manipulated varying the detuning from the Feshbach resonance. In the case of attractive atomic interactions, the molecule creation stabilizes the solitons. In the case of repulsive interactions, the molecule creation leads to the possibility of existence of bright solitons in some interval of detunings. Due to quantum fluctuations the distance between the atoms is a random quantity with the standard deviation larger than the mean value.

Third-harmonic generation by Raman-shifted solitons in a photonic-crystal fiber
E.E. Serebryannikov, A.B. Fedotov, A.M. Zheltikov, A.A. Ivanov, M.V. Alfimov, V.I. Beloglazov, N.B. Skibina, D.V. Skryabin, A.V. Yulin & J.C. Knight
J. Opt. Soc. Am. B-Opt. Phys. 23 (2006) 1975-1980
Raman-shifted solitons in a photonic-crystal fiber can serve as a pump field for phase-matched third-harmonic generation in a higher-order guided mode of the same fiber. Phase matching for this soliton-dispersive-wave mixing process differs in its physics and in its formal notation from the conventional phase matching for third-harmonic generation with a dispersive pump. (c) 2006 Optical Society of America.

Modal coupling in fiber tapers decorated with metallic surface gratings
W. Ding, S.R. Andrews, T.A. Birks & S.A. Maier
Opt. Lett. 31 (2006) 2556-2558
An interference-based scheme for fabricating periodic metal gratings on one side of the uniform waist of optical fiber tapers has been developed. Optical characterization of a 5 mm long, 511 nm period gold grating fiber taper with a 10 mu m diameter reveals backward coupling to both guided and radiation modes that is explained by using an analytical mode-coupling analysis. A refractometer based on this grating taper has a high and constant sensitivity over a large refractive index operating range of 1 to 1.41. (c) 2006 Optical Society of America.

Low optical insertion-loss and vacuum-pressure all-fiber acetylene cell based on hollow-core photonic crystal fiber
P.S. Light, F. Couny & F. Benabid
Opt. Lett. 31 (2006) 2538-2540
We report a novel and easy-to-implement hollow-core photonic crystal fiber cell fabrication technique based on helium diffusion through silica. The formed gas cells combine low optical insertion loss (1.8 dB) and vacuum acetylene pressure (mu bar regime). The estimates of the final gas pressure, using both Voigt interpolation and electromagnetically induced transparency, show a good match with the initial fitting pressure. (c) 2006 Optical Society of America.

Design of low-loss and highly birefringent hollow-core photonic crystal fiber
P.J. Roberts, D.P. Williams, H. Sabert, B.J. Mangan, D.M. Bird, T.A. Birks, J.C. Knight & P.S. Russell
Opt. Express 14 (2006) 7329-7341
A practical hollow-core photonic crystal fiber design suitable for attaining low-loss propagation is analyzed. The geometry involves a number of localized elliptical features positioned on the glass ring that surrounds the air core and separates the core and cladding regions. The size of each feature is tuned so that the composite core-surround geometry is antiresonant within the cladding band gap, thus minimizing the guided mode field intensity both within the fiber material and at material/air interfaces. A birefringent design, which involves a 2-fold symmetric arrangement of the features on the core-surround ring, gives rise to wavelength ranges where the effective index difference between the polarization modes is larger than 10(-4). At such high birefringence levels, one of the polarization modes retains favorable field exclusion characteristics, thus enabling low-loss propagation of this polarization channel. (c) 2006 Optical Society of America.

Solid photonic bandgap fibres and applications
J.C. Knight, F. Luan, G.J. Pearce, A. Wang, T.A. Birks & D.M. Bird
Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Brief Commun. Rev. Pap. 45 (2006) 6059-6063
We report on the development of optical fibres which guide light in a solid core using a photonic bandgap effect. The photonic bandgap cladding consists of a two-dimensional array of isolated high-index regions in a lower-index matrix, with a relatively low index contrast. The core is one or more unit cells of the matrix material without the inclusions. The frequency bands for. photonic bandgap guidance can be predicted by considering the cut-off frequencies of the guided modes of the high-index rods in the cladding using the weakly-guiding approximation. We demonstrate the basic properties of such fibres and their use as a wavelength-selective element in a fibre laser cavity.

Polarization instability of solitons in photonic crystal fibers
F. Luan, A.V. Yulin, J.C. Knight & D.V. Skryabin
Opt. Express 14 (2006) 6550-6556
We present experimental and numerical results demonstrating the existence of polarization instability of femtosecond solitons in small core photonic crystal fibers. The frequency of the solitons in our setup shifts rapidly with propagation along the dispersion curve due to the Raman effect. This process can cause initially stable solitons to become unstable, or vice versa depending on the parameters. (c) 2006 Optical Society of America.

Electromagnetically induced transparency and saturable absorption in all-fiber devices based on (C2H2)-C-12-filled hollow-core photonic crystal fiber
F. Couny, P.S. Light, F. Benabid & P.S. Russell
Opt. Commun. 263 (2006) 28-31
Electromagnetically induced and saturable absorption transparencies are observed in several lines of the v(1) + v(3) acetylene absorption band using two novel all-fiber devices based on a hermetically sealed hollow-core-PCF filled with acetylene at sub-millibar pressure. The method for scaling the hollow-core-PCF at vacuum pressure is described and the devices' flexibility is demonstrated by the generation of electromagnetically induced transparency at cryogenic temperature. (c) 2006 Elsevier B.V. All rights reserved.

An improved photonic bandgap fiber based on an array of rings
J.M. Stone, G.J. Pearce, F. Luan, T.A. Birks, J.C. Knight, A.K. George & D.M. Bird
Opt. Express 14 (2006) 6291-6296
We describe the modeling, fabrication and characterization of a silica-core photonic bandgap fiber based on a 2-d array of raised-index cladding rings. The use of rings to form the cladding is shown to re-order the cladding modes in such a way as to broaden the photonic band gaps and reduce bend sensitivity. We compare the performance of the ring fiber with that of a similar fiber made using solid rods. (c) 2006 Optical Society of America.

Enhanced visible continuum generation from a microchip 1064nm laser
C. Xiong, A. Witkowska, S.G. Leon-Saval, T.A. Birks & W.J. Wadsworth
Opt. Express 14 (2006) 6188-6193
We demonstrate a cascaded nonlinear process using pump conversion to 742 nm by four-wave mixing in the normal dispersion regime then continuum generation by modulation instability to generate bright single-mode visible continuum with an average power up to -20 dBm/nm, from a compact 1064 nm infrared source in a monolithic single-mode photonic crystal fibre with a tapered section in one end.

Low-loss singlemode fibre 1 x 2 Y-junction
N. Healy, E. McDaid, D.F. Murphy, C.D. Hussey & T.A. Birks
Electron. Lett. 42 (2006) 740-742
A new method for the fabrication of a singlemode fibre wavelength independent Y-junction is described; through asymmetrically tapering a pre-assembled Y-junction precursor, the splice point is drawn into the waist region of the device and low-loss is ensured. The lowest loss and flattest wavelength response for such a device is reported.

Terahertz pulse propagation using plasmon-polariton-like surface modes on structured conductive surfaces
S.A. Maier & S.R. Andrews
Appl. Phys. Lett. 88 (2006) 251120
The dispersion relation and confinement of plasmon-polariton-like terahertz surface modes sustained by a perfectly conducting planar surface patterned with a regular lattice of holes are studied using full-field three-dimensional numerical simulations. Edge coupling and subsequent surface guiding with subwavelength vertical confinement of the frequency components below the band edge of the surface mode is demonstrated for a broadband pulse with 0.5 THz <= f <= 2 THz. Additionally, the constraints imposed by lattice constant and hole size on the creation of defect waveguides offering lateral confinement are discussed. (c) 2006 American Institute of Physics.

Zero-dispersion wavelength decreasing photonic crystal fibers for ultraviolet-extended supercontinuum generation
A. Kudlinski, A.K. George, J.C. Knight, J.C. Travers, A.B. Rulkov, S.V. Popov & J.R. Taylor
Opt. Express 14 (2006) 5715-5722
We report the fabrication of photonic crystal fibers with a continuously-decreasing zero-dispersion wavelength along their length. These tapered fibers are designed to extend the generation of supercontinuum spectra from the visible into the ultraviolet. We report on their performance when pumped with both nanosecond and picosecond sources at 1.064 mu m. The supercontinuum spectra have a spectral width ( measured at the 10 dB points) extending from 0.372 mu m to beyond 1.75 mu m. In an optimal configuration a flat ( 3 dB) spectrum from 395 to 850 nm, with a minimum spectral power density of 2 mW/nm was achieved, with a total continuum output power of 3.5 W. We believe that the shortest wavelengths were generated by cascaded four-wave mixing: the continuous decrease of the zero dispersion wavelength along the fiber length enables the phase-matching condition to be satisfied for a wide range of wavelengths into the ultraviolet, while simultaneously increasing the nonlinear coefficient of the fiber. (c) 2006 Optical Society of America.

Bend loss in all-solid bandgap fibres
T.A. Birks, F. Luan, G.J. Pearce, A. Wang, J.C. Knight & D.M. Bird
Opt. Express 14 (2006) 5688-5698
Experimental measurements of all-solid photonic bandgap fibres with an array of high-index rods in a low-index background revealed an unexpected variation of bend loss across different bandgaps. This behaviour was confirmed by calculations of photonic band structure, and explained with reference to the differing field distributions of the modes of the cladding rods. Our understanding was confirmed by further experiments, leading to proposals for the improvement of these fibres. (c) 2006 Optical Society of America.

Phase-sensitive scattering of a continuous wave on a soliton
A. Efimov, A.J. Taylor, A.V. Yulin, D.V. Skryabin & J.C. Knight
Opt. Lett. 31 (2006) 1624-1626
Using cross-correlation frequency-resolved optical gating, we observe the phase-sensitive resonance in the interaction of a soliton with a continuous wave in a photonic crystal fiber. This interaction strongly depends on the difference in the phase velocities of the orthogonally polarized fiber modes and leads to generation of a new spectral peak. The spectral and temporal structure of this signal is revealed in our measurements, which are supported by analytical theory and numerical simulations. (c) 2006 Optical Society of America.

Optical frequency measurement using chirped-mirror-dispersion-controlled mode-locked TiAl2O3 laser
K. Sugiyama, F.L. Hong, J. Ishikawa, A. Onae, T. Ikegami, S.N. Slyusarev, K. Minoshima, H. Matsumoto, H. Inaba, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Brief Commun. Rev. Pap. 45 (2006) 5051-5062
An optical frequency measurement system based on an octave-spanning optical frequency comb generated by a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser and a photonic-crystal fiber is developed. All of the modes-of the octave-spanning optical frequency comb are frequency-stabilized to a microwave frequency standard, where the carrier-envelope offset frequency is phase-locked with self-referencing of the comb. We investigate the methods of controlling carrier-envelope offset frequency in a chirped-mirror-dispersion-controlled mode-locked laser. The rotation of a pair of chirped mirrors is useful for setting the bias of carrier-envelope offset frequency. Although our mode-locked laser has a low pulse-repetition frequency of 150 MHz, a high signal-to-noise ratio in beats results in the direct measurement of beat frequency with a laser to be measured using a frequency counter, and enables us to phase lock carrier-envelope offset frequency merely by using a mixer analogously without the need for a prescaler, with a servo bandwidth at approximately 500 kHz. The uncertainty of our optical frequency measurement system, besides the uncertainty of microwave reference frequency, is 4 x 10(-14), and is limited by the uncertainty of the rf synthesizer used for phase locking and by that of the beat frequency measurement. Frequency measurements of an iodine-stabilized frequency-doubled Nd:YAG laser at 532 nm, an iodine-stabilized He-Ne laser at 633 nm and a rubidium two-photon-absorption stabilized extended-cavity laser diode at 778 nm are conducted. The results contributed to the revision of the practical realization of the metre adopted by the International Conference on Weights and Measures (CIPM) in 2001.

Stimulated Brillouin scattering from multi-GHz-guided acoustic phonons in nanostructured photonic crystal fibres
P. Dainese, P.S.J. Russell, N. Joly, J.C. Knight, G.S. Wiederhecker, H.L. Fragnito, V. Laude & A. Khelif
Nat. Phys. 2 (2006) 388-392
Wavelength-scale periodic microstructuring dramatically alters the optical properties of materials. An example is glass photonic crystal fibre(1) ( PCF), which guides light by means of a lattice of hollow micro/nanochannels running axially along its length. In this letter, we explore stimulated Brillouin scattering in PCFs with subwavelength-scale solid silica glass cores. The large refractive-index difference between air and glass allows much tighter confinement of light than is possible in all-solid single-mode glass optical fibres made using conventional techniques. When the silica-air PCF has a core diameter of around 70% of the vacuum wavelength of the launched laser light, we find that the spontaneous Brillouin signal develops a highly unusual multi-peaked spectrum with Stokes frequency shifts in the 10-GHz range. We attribute these peaks to several families of guided acoustic modes each with different proportions of longitudinal and shear strain, strongly localized to the core(2,3). At the same time, the threshold power for stimulated Brillouin scattering(4) increases fivefold. The results show that Brillouin scattering is strongly affected by nanoscale microstructuring, opening new opportunities for controlling light-sound interactions in optical fibres.

Spectrally smooth supercontinuum from 350 nm to 3 mu m in sub-centimeter lengths of soft-glass photonic crystal fibers.
F.G. Omenetto, N.A. Wolchover, M.R. Wehner, M. Ross, A. Efimov, A.J. Taylor, V.V.R.K. Kumar, A.K. George, J.C. Knight, N.Y. Joly & P.S.J. Russell
Opt. Express 14 (2006) 4928-4934
The conversion of light fields in photonic crystal fibers ( PCFs) capitalizes on the dramatic enhancement of several optical nonlinearities. We present here spectrally smooth, highly broadband supercontinuum radiation in a short piece of high-nonlinearity soft-glass PCF. This supercontinuum spans several optical octaves, with a spectral range extending from 350 nm to beyond 3000 nm. The selection of an appropriate propagation-length determines the spectral quality of the supercontinuum generated. Experimentally, we clearly identify two regimes of nonlinear pulse transformation: when the fiber length is much shorter than the dispersion length, soliton propagation is not important and a symmetric supercontinuum spectrum arises from almost pure self-phase modulation. For longer fiber lengths the supercontinuum is formed by the breakup of multiple Raman-shifting solitons. In both regions very broad supercontinuum radiation is produced. (c) 2006 Optical Society of America.

Photon pair generation using four-wave mixing in a microstructured fibre: theory versus experiment
O. Alibart, J. Fulconis, G.K.L. Wong, S.G. Murdoch, W.J. Wadsworth & J.G. Rarity
New J. Phys. 8 (2006) 67
We develop a theoretical analysis of four-wave mixing used to generate photon pairs useful for quantum information processing. The analysis applies to a single mode microstructured fibre pumped by an ultra-short coherent pulse in the normal dispersion region. Given the values of the optical propagation constant inside the fibre, we can estimate the created number of photon pairs per pulse, their central wavelength and their respective bandwidth. We use the experimental results from a picosecond source of correlated photon pairs using a micro-structured fibre to validate the model. The fibre is pumped in the normal dispersion regime at 708 nm and phase matching is satisfied for widely spaced parametric wavelengths of 586 and 894 nm. We measure the number of photons per pulse using a loss-independent coincidence scheme and compare the results with the theoretical expectation. We show a good agreement between the theoretical expectations and the experimental results for various fibre lengths and pump powers.

Polarization modulation instability in photonic crystal fibers
R.J. Kruhlak, G.K.L. Wong, J.S.Y. Chen, S.G. Murdoch, R. Leonhardt, J.D. Harvey, N.Y. Joly & J.C. Knight
Opt. Lett. 31 (2006) 1379-1381
Polarization modulation instability (PMI) in birefringent photonic crystal fibers has been observed in the normal dispersion regime with a frequency shift of 64 THz between the generated frequencies and the pump frequency. The generated sidebands are orthogonally polarized to the pump. From the observed PMI frequency shift and the measured dispersion, we determined the phase birefringence to be 5.3 X 10(-5) at a pump wavelength of 647.1 nm. This birefiringence was used to estimate the PMI gain as a function of pump wavelength. Four-wave mixing experiments in both the normal and the anomalous dispersion regimes generated PMI frequency shifts that show good agreement with the predicted values over a 70 THz range. These results could lead to amplifiers and oscillators based on PMI. (c) 2006 Optical Society of America

Three-level neodymium fiber laser incorporating photonic bandgap fiber
A. Wang, A.K. George & J.C. Knight
Opt. Lett. 31 (2006) 1388-1390
We report a neodymium fiber laser incorporating an all-solid photonic bandgap fiber to suppress the four-level laser transition F-4(3/2)-I-4(11/2). We demonstrate lasing at 907 nm on the three-level transition F-4(3/2)-I-4(9/2) when pumping at 808 nm. The maximum slope efficiency obtained was 32% with a threshold pump power of 70 mW. (c) 2006 Optical Society of America

Localized polaritons and second-harmonic generation in a resonant medium with quadratic nonlinearity
D.V. Skryabin, A.V. Yulin & A.I. Maimistov
Phys. Rev. Lett. 96 (2006) 163904
We derive model equations for the propagation of ultrashort pulses in materials with resonant linear and quadratic nonlinear responses and find approximate soliton solutions describing all-bright and dark-bright polaritons. We report the specific phase matching condition for efficient 2nd harmonic generation, which involves detuning from the resonance. We also demonstrate that the 2nd harmonic emission by the polaritonic pulses can lead to reduction of their group velocity, having zero as a theoretical limit. Our analytical results are supported by numerical simulations.

Experimental demonstration of the frequency shift of bandgaps in photonic crystal fibers due to refractive index scaling
G. Antonopoulos, F. Benabid, T.A. Birks, D.M. Bird, J.C. Knight & P.S.J. Russell
Opt. Express 14 (2006) 3000-3006
Experimental demonstration of the frequency shift of photonic bandgaps due to refractive index scaling using D2O-filled hollow-core photonic crystal fibers is presented. The results confirm a simple scaling law for bandgaps in fibers in which the low-index medium is varied. (c) 2006 Optical Society of America.

Cross-phase modulation instability in photonic crystal fibers
J.S.Y. Chen, G.K.L. Wong, S.G. Murdoch, R.J. Kruhlak, R. Leonhardt, J.D. Harvey, N.Y. Joly & J.C. Knight
Opt. Lett. 31 (2006) 873-875
We report on the observation of cross-phase modulation instability in a highly nonlinear photonic crystal fiber. In such fibers the presence of higher orders of dispersion results in a complex phase-matching curve. We are able to observe this behavior experimentally and obtain excellent agreement between the measured and predicted shifts. (c) 2006 Optical Society of America.

Novel methods pair holey fibers with conventional fibers
T. Birks, S. Leon-Saval, A. Witkowska, K. Lai & W. Wadsworth
Laser Focus World 42 (2006) 70-73
With their small mode-field diameters, some holey fibers are difficult to couple to standard single-mode fibers. Innovative techniques, such as heating and pressurizing to reshape fiber geometry, then tapering make low-loss, coupling possible.

Nontopological Raman-Kerr self-induced transparency solitons in photonic crystal fibers
D.V. Skryabin, A.V. Yulin & F. Biancalana
Phys. Rev. E 73 (2006) 045603
Using analytical and numerical methods we demonstrate that the pulse propagation equations accounting for the quantum description of the Raman transition and for the optical Kerr nonlinearity have a multiparameter family of nontopological solitary wave solutions. We study properties of these solitons and report the transition from stable to unstable regimes of propagation. We also discuss the feasibility of observation of these structures in gas filled hollow-core photonic crystal fibers.

Plasmonic field enhancement and SERS in the effective mode volume picture
S.A. Maier
Opt. Express 14 (2006) 1957-1964
The controlled creation of nanometric electromagnetic field confinement via surface plasmon polariton excitations in metal/insulator/metal heterostructures is described via the concept of an effective electromagnetic mode volume V-eff. Extensively used for the description of dielectric microcavities, its extension to plasmonics provides a convenient figure of merit and allows comparisons with dielectric counterparts. Using a one-dimensional analytical model and three-dimensional finite-difference time-domain simulations, it is shown that plasmonic cavities with nanometric dielectric gaps indeed allow for physical as well as effective mode volumes well below the diffraction limit in the gap material, despite significant energy penetration into the metal. In this picture, matter-plasmon interactions can be quantified in terms of quality factor Q and V-eff, enabling a resonant cavity description of surface enhanced Raman scattering. (c) 2006 Optical Society of America.

Liquid-crystal-based in-fiber tunable spectral structures
Z. Zalevsky, F. Luan, W.J. Wadsworth, S.L. Saval & T.A. Birks
Opt. Eng. 45 (2006) 035005
We present new approaches for realizing enhanced tunable filters based on filling special fibers with nematic or cholesteric liquid crystal. One of the suggested approaches is to realize high-resolution longitudinal structure of electrodes using printed circuit board technology and then fill tapered capillaries with the liquid crystal, thus obtaining the desired spectral response at low voltage and with low insertion loss. Another approach is to fill special photonic crystal fibers with liquid crystal and obtain spectral tunability by generating phase differences between light paths. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

Supercontinuum generation system for optical coherence tomography based on tapered photonic crystal fibre
G. Humbert, W.J. Wadsworth, S.G. Leon-Saval, J.C. Knight, T.A. Birks, P.S.J. Russell, M.J. Lederer, D. Kopf, K. Wiesauer, E.I. Breuer & D. Stifter
Opt. Express 14 (2006) 1596-1603
We report smooth and broad continuum generation using a compact femtosecond Ti:Sapphire laser as a pump source and a tapered photonic crystal fibre as a nonlinear element. Spectral output is optimised for use in optical coherence tomography, providing a maximum longitudinal resolution of 1.5 mu m in free space at 809 nm centre wavelength without use of additional spectral filtering. (c) 2006 Optical Society of America.

Gain-assisted propagation of electromagnetic energy in subwavelength surface plasmon polariton gap waveguides
S.A. Maier
Opt. Commun. 258 (2006) 295-299
The propagation of electromagnetic energy via coupled surface plasmon polariton modes in a metal-insulator-metal heterostructure is analyzed analytically for a core material exhibiting optical gain. It is shown that a sufficiently large gain can completely compensate for the absorption losses due to energy dissipation in the metallic boundaries, enabling long-range transport with a confinement below the diffraction limit for on-chip switching and sensing applications. For a free-space wavelength of 1500 nm, lossless propagation in a gold-semiconductor-gold waveguide with a core size of 50 nm is predicted for a gain coefficient gamma = 4830 cm(-1) comparable to that of semiconductor gain media. The gain requirements decrease with the use of low-index nanocrystal-doped glasses or polymers as core materials. (c) 2005 Elsevier B.V. All rights reserved.

Hybrid photonic crystal fiber
S.A. Cerqueira, F. Luan, C.M.B. Cordeiro, A.K. George & J.C. Knight
Opt. Express 14 (2006) 926-931
We present a hybrid photonic crystal fiber in which a guided mode is confined simultaneously by modified total internal reflection from an array of air holes and antiresonant reflection from a line of high-index inclusions. Experimental results demonstrate that this fiber shares properties of both index-guided and photonic bandgap structures. (c) 2006 Optical Society of America.

Bragg localized structures in a passive cavity with transverse modulation of the refractive index and the pump
A.G. Vladimirov, D.V. Skryabin, G. Kozyreff, P. Mandel & M. Tlidi
Opt. Express 14 (2006) 1-6
We consider a passive optical cavity containing a photonic crystal and a purely absorptive two-level medium. The cavity is driven by a superposition of two coherent beams forming a periodically modulated pump. Using a coupled mode reduction and direct numerical modeling of the full system we demonstrate the existence of bistability between uniformly periodic states, modulational instabilities and localized structures of light. All are found to exist within the conduction band of the photonic material. Moreover, contrary to similar previously found intra-band structures, we show that these localized structures can be truly stationary states. (c) 2006 Optical Society of America.

Silica-clad neodymium-doped lanthanum phosphate fibers and fiber lasers
R.A. Martin & J.C. Knight
IEEE Photonics Technol. Lett. 18 (2006) 574-576
We have fabricated a neodymium-doped phosphate glass fiber with a silica cladding and used it to form a fiber laser. Phosphate and silicate glasses have considerably different glass transition temperatures and softening points making it hard to draw a fiber from these two glasses. A bulk phosphate glass of composition (Nd2O3)(0.011)(La2O3)(0.259)(P2O5)(0.725)(Al2O3)(0.005) was prepared and the resultant material was transparent, free from bubbles and visibly homogeneous. The bulk phosphate glass was drawn to a fiber while being jacketed with silica and the resultant structure was of good optical quality, free from air bubbles and major defects. The attenuation at a wavelength of 1.06 mu m was 0.05 dB/cm and the refractive index of the core and cladding at the pump wavelength of 488 nm was 1.56 and 1.46, respectively. The fibers were mechanically strong enough to allow for ease of handling and could be spliced to conventional silica fiber. The fibers were used to demonstrate lasing at the F-4(3/2) - I-4(11/2) (1.06 mu m) transition. Our work demonstrates the potential to form silica clad optical fibers with phosphate cores doped with very high levels of rare-earth ions (27-mol % rare-earth oxide).

Effective mode volume of nanoscale plasmon cavities
S. Maier
Opt. Quantum Electron. 38 (2006) 257-267
The controlled squeezing of electromagnetic energy into nanometric volumes via surface plasmon-polariton excitations in plasmonic nanoresonators is analyzed using the concept of an effective electromagnetic mode volume V-eff, while taking careful account of the plasmon-polariton dispersion and the electromagnetic energy stored in the metal. Together with the quality factor Q of the cavity resonance, this enables a comparison with dielectric optical cavities, where V-eff is limited by diffraction. For a Fabry-Perot type planar metallic cavity, a one-dimensional analytic model as well as a three-dimensional finite-difference time-domain simulation reveal that V-eff is not bounded by diffraction, and that Q/V-eff increases for decreasing cavity size. In this picture, matter-plasmon interactions can be quantified in terms of Q and V-eff, and a resonant cavity model for the enhancement of spontaneous Raman scattering is presented.

2005 Publications

Stokes amplification regimes in quasi-cw pumped hydrogen-filled hollow-core photonic crystal fiber
F. Benabid, G. Antonopoulos, J.C. Knight & P.S. Russell
Phys. Rev. Lett. 95 (2005) 213903
Pure rotational stimulated Raman scattering spectra containing nine strong spectral components were generated from a similar to 11 m long hollow-core photonic crystal fiber filled with hydrogen and pumped with nanosecond pulses having energies around 100-300 nJ. Observation of both transient and steady-state scattering threshold behavior is reported. Passage from the transient to the steady state is observed with a pulse as long as 14 ns. Convenient analytical expressions for energy and power threshold are deduced for the present configuration.

Interaction of an optical soliton with a dispersive wave
A. Efimov, A.V. Yulin, D.V. Skryabin, J.C. Knight, N. Joly, F.G. Omenetto, A.J. Taylor & P. Russell
Phys. Rev. Lett. 95 (2005) 213902
Scattering of a dispersive wave by optical solitons is studied experimentally in photonic crystal fibers in cases when the soliton and the dispersive wave have either identical or orthogonal polarization states. Observations of new resonant frequencies are reported. The experimental results are compared to numerical simulations and predictions from the recently derived wave vector matching conditions.

Continuous-wave tunable optical parametric generation in a photonic-crystal fiber
G.K.L. Wong, A.Y.H. Chen, S.G. Murdoch, R. Leonhardt, J.D. Harvey, N.Y. Joly, J.C. Knight, W.J. Wadsworth & P.S. Russell
J. Opt. Soc. Am. B-Opt. Phys. 22 (2005) 2505-2511
Continuous-wave and quasi-cw operation of tunable optical parametric generation has been demonstrated in a photonic-crystal fiber. The frequency shift of the generated sidebands, which arise from modulation instability, depends strongly on the detuning of the pump from the fiber's zero-dispersion wavelength. Over 30 nm of sideband tunability has been demonstrated using a 300 mW cw pump, and over 185 nm of tunability using a 1.6 W quasi-cw pump. Continuous wave and quasi-cw pumps eliminate the detrimental effects of pump-sideband walk-off. In the absence of walk-off it is the fluctuations in the index profile of the photonic-crystal fiber along its length that limit the tunable sideband range. (c) 2005 Optical Society of America.

Realizing low loss air core photonic crystal fibers by exploiting an antiresonant core surround
P.J. Roberts, D.P. Williams, B.J. Mangan, H. Sabert, F. Couny, W.J. Wadsworth, T.A. Birks, J.C. Knight & P.S.J. Russell
Opt. Express 13 (2005) 8277-8285
The modal properties of an air core photonic crystal fiber which incorporates an anti-resonant feature within the region that marks the transition between the air core and the crystal cladding are numerically calculated. The field intensity at the glass/air interfaces is shown to be reduced by a factor of approximately three compared to a fiber with more conventional core surround geometry. The reduced interface field intensity comes at the expense of an increased number of unwanted core interface modes within the band gap. When the interface field intensity is associated with modal propagation loss, the findings are in accord with recent measurements on fabricated fibers which incorporate a similar antiresonant feature. (c) 2005 Optical Society of America.

Loss in solid-core photonic crystal fibers due to interface roughness scattering
P.J. Roberts, F. Couny, H. Sabert, B.J. Mangan, T.A. Birks, J.C. Knight & P.S. Russell
Opt. Express 13 (2005) 7779-7793
The loss resulting from roughness scattering at hole interfaces within solid core photonic crystal fibers is theoretically analyzed and compared with measurements on fabricated fibers. It is found that a model roughness spectrum corresponding to frozen in capillary waves gives results in reasonably good agreement with experiments on small core fibers. In particular, the roughness scattering loss is shown to be only weakly dependent on wavelength. Agreement at a larger core size requires a long length-scale cut-off to be introduced to the roughness spectrum. Due to the long range nature of the roughness correlations, the scattering is non Rayleigh in character and cannot be interpreted in terms of a local photon density of states. (c) 2005 Optical Society of America.

Multimode fiber devices with single-mode performance
S.G. Leon-Saval, T.A. Birks, J. Bland-Hawthorn & M. Englund
Opt. Lett. 30 (2005) 2545-2547
A taper transition can couple light between a multimode fiber and several single-mode fibers. If the number of single-mode fibers matches the number of spatial modes in the multimode fiber, the transition can have low loss in both directions. This enables the high performance of single-mode fiber devices to be attained in multimode fibers. We report an experimental proof of concept by using photonic crystal fiber techniques to make the transitions, demonstrating a multimode fiber filter with the transmission spectrum of a single-mode fiber grating. (c) 2005 Optical Society of America.

High brightness single mode source of correlated photon pairs using a photonic crystal fiber
J. Fulconis, O. Alibart, W.J. Wadsworth, P.S. Russell & J.G. Rarity
Opt. Express 13 (2005) 7572-7582
We demonstrate a picosecond source of correlated photon pairs using a micro-structured fibre with zero dispersion around 715 nm wavelength. The fibre is pumped in the normal dispersion regime at similar to 708 nm and phase matching is satisfied for widely spaced parametric wavelengths. Here we generate up to 107 photon pairs per second in the fibre at wavelengths of 587 nm and 897 nm, while on collecting this light in single-mode-fibre-coupled Silicon avalanche diode photon counting detectors, we detect similar to 3.2x10(5) coincidences per second at pump power 0.5 mW. (c) 2005 Optical Society of America.

Linear and nonlinear guidance in an ultralow loss planar glass membrane
N.Y. Joly, T.A. Birks, A. Yulin, J.C. Knight & P.S.J. Russell
Opt. Lett. 30 (2005) 2469-2471
We describe the fabrication and characterization of a free-standing silica glass membrane waveguide formed using fiber fabrication processes. The membrane has a thickness of 0.6 mu m and a width of 60 mu m and is many meters long. The optical attenuation is measured as 0.4 dB/m. Such attenuation outperforms that of conventional planar waveguides by several orders of magnitude. (c) 2005 Optical Society of America.

Single-mode mid-IR guidance in a hollow-core photonic crystal fiber
J.D. Shephard, W.N. MacPherson, R.R.J. Maier, J.D.C. Jones, D.P. Hand, M. Mohebbi, A.K. George, P.J. Roberts & J.C. Knight
Opt. Express 13 (2005) 7139-7144
We report, for the first time, bandgap guidance above 3 m in a silica based air-core photonic crystal fiber. The peak of the bandgap is at 3.14 m with a typical attenuation of similar to 2.6 dB m(-1). By further optimization of the structure, modeling suggests that a loss below 1 dB m(-1) should be achievable, greatly extending the useful operating range of silica-based single-mode fibers. Such fibers have many potential applications in the mid-IR, offering an alternative to fluoride, tellurite or chalcogenide glass based optical fibers for chemical and biological sensing applications. (c) 2005 Optical Society of America

Hollow-core PCF for guidance in the mid to far infra-red
G.J. Pearce, J.M. Pottage, D.M. Bird, P.J. Roberts, J.C. Knight & P.S.J. Russell
Opt. Express 13 (2005) 6937-6946
A major limitation to attaining low-loss single-mode guidance in hollow core photonic crystal fibre (PCF) is surface guided modes that are trapped in the core surround. This is particularly severe when high index (n > 2) glasses are used. By modelling a structure that has the characteristic features of a realistic fibre we show that, by tuning the thickness of the core wall, the influence of these 'surface' modes can be minimised. For a refractive index of 2.4 we predict power-in-air fractions of over 95% over a fractional bandwidth of similar to 5%, peaking at over 98%. The designs are appropriate for mid- to far-IR PCFs for which suitable glasses (e.g., tellurites and chalcogenides) have high refractive indices. (c) 2005 Optical Society of America.

Hole inflation and tapering of stock photonic crystal fibres
W.J. Wadsworth, A. Witkowska, S.G. Leon-Saval & T.A. Birks
Opt. Express 13 (2005) 6541-6549
We report controlled hole expansion in photonic crystal fibres (PCFs) by heating the fibre while the holes were pressurised. This was done by post-processing an existing fibre, not during fibre fabrication. Small holes in an endlessly single-mode (ESM) PCF were inflated to become large holes. The large-hole PCF was then tapered to produce a "cobweb" PCF with a small highly-nonlinear core, interfaced to the ESM PCF at both ends by gradual transitions. The loss was less than 0.4 dB in the complete structure, which was used to demonstrate supercontinuum generation when pumped with a fs Ti:sapphire laser. (c) 2005 Optical Society of America.

Highly birefringent lamellar core fiber
A. Wang, A.K. George, J.F. Liu & J.C. Knight
Opt. Express 13 (2005) 5988-5993
We report a polarization-maintaining fiber in which the birefringence is due to artificially introduced anisotropy in the core material. The beat length was measured by direct observation at three different wavelengths, giving a shortest result of 85 mu m at a wavelength of 543 nm. The measured phase-index birefringence is about one third of that expected, which is explained by diffusion between the core layers, which are each less than 200 nm thick. By taking account of this diffusion, we can accurately model the experimental beat length and differential group delay over a wide wavelength range. (C) 2005 Optical Society of America.

Engineering the dispersion of tapered fibers for supercontinuum generation with a 1064 nm pump laser
C.M.B. Cordeiro, W.J. Wadsworth, T.A. Birks & P.S.J. Russell
Opt. Lett. 30 (2005) 1980-1982
We experimentally demonstrate dispersion tailoring of tapered fibers by immersing them in fluids. We obtain a 1200 nm wide supercontinuum in a train of two tapered fibers pumped by a low-cost diode-pumped Nd:YAG laser with a pulse width of 0.6 ns and an output power of 40 mW, where one tapered fiber is immersed in heavy water. To our knowledge, this is the widest spectrum ever generated with a system of such simplicity. (c) 2005 Optical Society of America.

Electromagnetically-induced transparency grid in acetylene-filled hollow-core PCF
F. Benabid, P.S. Light, F. Couny & P.S. Russell
Opt. Express 13 (2005) 5694-5703
We report a set of experimental observations on electromagnetically induced transparency in acetylene filled hollow-core photonic crystal fiber, involving both Lambda-type and V-type interactions over several lines of the R-branch of the nu(1)+nu(3) ro-vibrational overtone band. Transparency as high as similar to 70% was achieved. A theoretical account of the sources of decoherence shows that collisions with the inner wall of the fiber core and laser frequency-jitter dominate the coherence decay. (C) 2005 Optical Society of America.

Improved hollow-core photonic crystal fiber design for delivery of nanosecond pulses in laser micromachining applications
J.D. Shephard, F. Couny, P.S. Russell, J.D.C. Jones, J.C. Knight & D.P. Hand
Appl. Optics 44 (2005) 4582-4588
We report the delivery of high-energy nanosecond pulses (similar to 65 ns pulse width) from a high-repetition-rate (up to 100 kHz) Q-switched Nd:YAG laser through the fundamental mode of a hollow-core photonic crystal fiber (HC-PCF) at 1064 nm. The guided mode in the HC-PCF has a low overlap with the glass, allowing delivery of pulses with energies above those attainable with other fibers. Energies greater than 0.5 mJ were delivered in a single spatial mode through the hollow-core fiber, providing the pulse energy and high beam quality required for micromachining of metals. Practical micromachining of a metal sheet by fiber delivery has been demonstrated. (c) 2005 Optical Society of America.

Splice-free interfacing of photonic crystal fibers
S.G. Leon-Saval, T.A. Birks, N.Y. Joly, A.K. George, W.J. Wadsworth, G. Kakarantzas & P.S.J. Russell
Opt. Lett. 30 (2005) 1629-1631
We report a new method for making low-loss interfaces between conventional single-mode fibers and photonic crystal fibers (PCFs). Adapted from the fabrication of PCF preforms from stacked tubes and rods, this method avoids the need for splicing and is versatile enough to interface to virtually any type of index-guiding silica PCF. We illustrate the method by forming interfaces to two problematic types of PCF, highly nonlinear and multicore. In particular, we believe this to be the first method capable of individually coupling light into and out of all the cores of a fiber with multiple closely spaced cores, without input or output cross talk. (c) 2005 Optical Society of America.

Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures
S.A. Maier & H.A. Atwater
J. Appl. Phys. 98 (2005) 011101
We review the basic physics of surface-plasmon excitations occurring at metal/dielectric interfaces with special emphasis on the possibility of using such excitations for the localization of electromagnetic energy in one, two, and three dimensions, in a context of applications in sensing and waveguiding for functional photonic devices. Localized plasmon resonances occurring in metallic nanoparticles are discussed both for single particles and particle ensembles, focusing on the generation of confined light fields enabling enhancement of Raman-scattering and nonlinear processes. We then survey the basic properties of interface plasmons propagating along flat boundaries of thin metallic films, with applications for waveguiding along patterned films, stripes, and nanowires. Interactions between plasmonic structures and optically active media are also discussed. (c) 2005 American Institute of Physics.

Theory of generation of new frequencies by mixing of solitons and dispersive waves in optical fibers
D.V. Skryabin & A.V. Yulin
Phys. Rev. E 72 (2005) 016619
We develop a theory of the generation of new spectral components in optical fibers pumped with a solitonic pulse and a weak continuous wave (cw). We derive the wave number matching conditions for the above process and present an analytical method of finding the amplitudes of the generated waves. We discuss related effects of the depletion of the cw pump and spectral recoil on the soliton. We also point out examples of the generation of supercontinuum spectra in fibers, where mixing between solitons and dispersive waves plays an important role.

Intermediate asymptotic evolution and photonic bandgap fiber compression of optical similaritons around 1550 nm
C. Billet, J.M. Dudley, N. Joly & J.C. Knight
Opt. Express 13 (2005) 3236-3241
We report the complete characterization of the self-similar scaling of parabolic pulse similaritons in an optical fiber amplifier. High dynamic range frequency resolved optical gating allows the direct observation of the evolution of a hyperbolic secant-like input pulse to an asymptotic amplifier similariton, and reveals the presence of intermediate asymptotic wings about the parabolic pulse core. These results are used to optimize additional self-similar propagation in highly-nonlinear fiber and subsequent compression in hollow-core photonic bandgap fiber. (C) 2005 Optical Society of America.

Dissipative localized structures of light in photonic crystal films
A.V. Yulin, D.V. Skryabin & P.S.J. Russell
Opt. Express 13 (2005) 3529-3534
We introduce simple model equations describing the dynamics of light in thin photonic crystal films with Kerr nonlinearity. We report modulational instabilities and bright and dark localized structures of light that exist in this system in the proximity of Fano resonances. (C) 2005 Optical Society of America.

Adaptive curvilinear coordinates in a plane-wave solution of Maxwell's equations in photonic crystals
G.J. Pearce, T.D. Hedley & D.M. Bird
Phys. Rev. B 71 (2005) 195108
A method is described to compute the modes propagating at a given frequency in dielectric systems that are periodic in two dimensions and uniform in the third dimension, using a plane-wave basis expressed in a system of generalized curvilinear coordinates. The coordinates are adapted to the structure under consideration by increasing the effective plane-wave cutoff in the vicinity of the interfaces between dielectrics, where the electromagnetic fields vary most rapidly. The favorable efficiency and convergence properties of the method are shown by comparison with the conventional plane-wave formulation of Maxwell's equations. Although the method is developed to study propagation in photonic crystal fibers, it is also applicable more generally to plane-wave modal solutions of structured dielectrics.

Guidance properties of low-contrast photonic bandgap fibres
A. Argyros, T.A. Birks, S.G. Leon-Saval, C.M.B. Cordeiro & P.S. Russell
Opt. Express 13 (2005) 2503-2511
We investigate the guidance properties of low-contrast photonic band gap fibres. As predicted by the antiresonant reflecting optical waveguide (ARROW) picture, band gaps were observed between wavelengths where modes of the high-index rods in the cladding are cutoff. At these wavelengths, leakage from the core by coupling to higher-order modes of the rods was observed directly. The low index contrast allowed for bend loss to be investigated; unlike in index-guiding fibres, anomalous "centripetal" light leakage through the inside of the bend can occur. (C) 2005 Optical Society of America.

Selective mode excitation in hollow-core photonic crystal fiber
A.D. Galea, F. Couny, S. Coupland, P.J. Roberts, H. Sabert, J.C. Knight, T.A. Birks & P.S.J. Russell
Opt. Lett. 30 (2005) 717-719
Modes are selectively excited by launching light through the cladding from the side into a hollow-core photonic crystal fiber. Measuring the total output power at the end of the fiber as a function of the angle of incidence of the exciting laser beam provides a powerful diagnostic for characterizing the cladding bandgap. Furthermore, various types of modes on either side of the bandgap are excited individually, and their near-field images are obtained. (c) 2005 Optical Society of America.

Widely tunable optical parametric generation in a photonic crystal fiber
A.Y.H. Chen, G.K.L. Wong, S.G. Murdoch, R. Leonhardt, J.D. Harvey, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Opt. Lett. 30 (2005) 762-764
We report on the observation of widely tunable optical parametric generation in a photonic crystal fiber. The frequency shift of the generated sidebands that arise from modulational instability is strongly dependent on the detuning of the pump from the fiber's zero-dispersion wavelength. We are able to demonstrate experimentally more than 450 nm of sideband tunability as we tune the pump wavelength over 10 nm. Excellent agreement has been found between the experimentally measured and theoretically predicted shifts. (c) 2005 Optical Society of America.

Competition between spectral splitting and Raman frequency shift in negative-dispersion slope photonic crystal fiber
N.Y. Joly, F.G. Omenetto, A. Efimov, A.J. Taylor, J.C. Knight & P.S. Russell
Opt. Commun. 248 (2005) 281-285
We report on the nonlinear behavior of high air-filling fraction solid-core photonic crystal fibers pumped with ultra short pulses in the vicinity of a negative-slope zero-crossing of the group velocity dispersion. We observed dramatically different behavior when the pump wavelength lies in the normal or the anomalous dispersion range. When pumping at the zero-dispersion wavelength the combined effects of spectral splitting, self-phase modulation and soliton self frequency shift result a "comma"-shape of the power-dependant spectra. This spectral feature is explained using a simple model. (c) 2004 Elsevier B.V. All rights reserved.

Compact, stable and efficient all-fibre gas cells using hollow-core photonic crystal fibres
F. Benabid, F. Couny, J.C. Knight, T.A. Birks & P.S. Russell
Nature 434 (2005) 488-491
Gas-phase materials are used in a variety of laser-based applications - for example, in high- precision frequency measurement(1,2), quantum optics and nonlinear optics(3,4). Their full potential has however not been realized because of the lack of a suitable technology for creating gas cells that can guide light over long lengths in a single transverse mode while still offering a high level of integration in a practical and compact set-up or device. As a result, solid-phase materials are still often favoured, even when their performance compares unfavourably with gas-phase systems. Here we report the development of all-fibre gas cells that meet these challenges. Our structures are based on gas-filled hollow-core photonic crystal fibres, in which we have recently demonstrated substantially enhanced stimulated Raman scattering(5,6), and which exhibit high performance, excellent long-term pressure stability and ease of use. To illustrate the practical potential of these structures, we report two different devices: a hydrogen-filled cell for efficient generation of rotational Raman scattering using only quasi-continuous-wave laser pulses; and acetylene-filled cells, which we use for absolute frequency-locking of diode lasers with very high signal-to-noise ratios. The stable performance of these compact gas-phase devices could permit, for example, gas-phase laser devices incorporated in a 'credit card' or even in a laser pointer.

Resonant radiation and collapse of ultrashort pulses in planar waveguides
A.V. Yulin, D.V. Skryabin & P.S.J. Russell
Opt. Lett. 30 (2005) 525-527
We study the spatiotemporal. dynamics of ultrashort pulses close to a point of zero group-velocity dispersion in planar waveguides with focusing nonlinearities. We find that the process of pulse collapse enhances the emission of so-called resonant radiation, providing an efficient mechanism of energy transfer from solitonic to dispersive waves and leading to suppression of the collapse. (C) 2005 Optical Society of America.

Finite-element analysis and experimental results for a microstructured fiber with enhanced hydrostatic pressure sensitivity
W.N. MacPherson, E.J. Rigg, J.D.C. Jones, V.V.R.K. Kumar, J.C. Knight & P.S. Russell
J. Lightwave Technol. 23 (2005) 1227-1231
In this paper, we report measurements for a microstructured fiber which displays an enhanced sensitivity to pressure. The pressure response of a length of this fiber is compared with SMF28 and with a finite element model of the fibers, which confirm the enhanced sensitivity.

Experimental demonstration of fiber-accessible metal nanoparticle plasmon waveguides for planar energy guiding and sensing
S.A. Maier, M.D. Friedman, P.E. Barclay & O. Painter
Appl. Phys. Lett. 86 (2005) 071103
Experimental evidence of mode-selective evanescent power coupling at telecommunication frequencies with efficiencies up to 75% from a tapered optical fiber to a metal nanoparticle plasmon waveguide is presented. The waveguide consists of a two-dimensional square lattice of lithographically defined Au nanoparticles on an optically thin silicon membrane. The dispersion and attenuation properties of the waveguide are analyzed using the fiber taper. The high efficiency of power transfer into these waveguides solves the coupling problem between conventional optics and plasmonic devices and could lead to the development of highly efficient plasmonic sensors and optical switches. (C) 2005 American Institute of Physics.

Visualizing the photonic band gap in hollow core photonic crystal fibers
F. Couny, H. Sabert, P.J. Roberts, D.P. Williams, A. Tomlinson, B.J. Mangan, L. Farr, J.C. Knight, T.A. Birks & P.S. Russell
Opt. Express 13 (2005) 558-563
The light radiated from the guided mode of a hollow core photonic crystal fiber into free space is measured as a function of angle and wavelength. This enables the direct experimental visualization of the photonic band gap and the identification of localized modes of the core region. (C) 2005 Optical Society of America.

Photonic crystal fiber source of correlated photon pairs
J.G. Rarity, J. Fulconis, J. Duligall, W.J. Wadsworth & P.S. Russell
Opt. Express 13 (2005) 534-544
We generate correlated photon pairs at 839 nm and 1392 nm from a single-mode photonic crystal fiber pumped in the normal dispersion regime. This compact, bright, tunable, single-mode source of pair-photons will have wide application in quantum communications. (C) 2005 Optical Society of America.

Photonic bandgap with an index step of one percent
A. Argyros, T.A. Birks, S.G. Leon-Saval, C.M.B. Cordeiro, F. Luan & P.S.J. Russell
Opt. Express 13 (2005) 309-314
Early work suggested that very large refractive index contrasts would be needed to create photonic bandgaps in two or three dimensionally periodic photonic crystals. It was then shown that in two-dimensionally periodic structures ( such as photonic crystal fibres) a non-zero wavevector component in the axial direction permits photonic bandgaps for much smaller index contrasts. Here we experimentally demonstrate a photonic bandgap fibre made from two glasses with a relative index step of only 1%. (C) 2005 Optical Society of America.

Ultimate low loss of hollow-core photonic crystal fibres
P.J. Roberts, F. Couny, H. Sabert, B.J. Mangan, D.P. Williams, L. Farr, M.W. Mason, A. Tomlinson, T.A. Birks, J.C. Knight & P.S.J. Russell
Opt. Express 13 (2005) 236-244
Hollow-core photonic crystal fibres have excited interest as potential ultra-low loss telecommunications fibres because light propagates mainly in air instead of solid glass. We propose that the ultimate limit to the attenuation of such fibres is determined by surface roughness due to frozen-in capillary waves. This is confirmed by measurements of the surface roughness in a HC-PCF, the angular distribution of the power scattered out of the core, and the wavelength dependence of the minimum loss of fibres drawn to different scales. (C) 2005 Optical Society of America.

Plasmonics - Towards subwavelength optical devices
S.A. Maier
Curr. Nanosci. 1 (2005) 17-23
Aspects of the optical properties of metallic nanostructures sustaining surface plasinon-polaritors are reviewed, focusing on differences with dielectric counterparts for the creation of functional nanoscale optical cevices such as waveguides and resonators. The ability of plasmonic structures to confine light to volumes significantly smaller than the diffraction limit of light is discussed and a number of important applications are presented, with an emphasis on structures consisting of metallic nanoparticles. The important challenge of efficient excitation of surface plasmon-polaritons in a micro-optic framework is also addressed. A unified description of both plasmonic and dielectric photonic structures using quality factor and effective mode volume as figures of merit should allow for a clear choice of materials system for micro- and nano-optic devices.

2004 Publications

Time-spectrally-resolved ultrafast nonlinear dynamics in small-core photonic crystal fibers: Experiment and modelling
A. Efimov, A.J. Taylor, F.G. Omenetto, A.V. Yulin, N.Y. Joly, F. Biancalana, D.V. Skryabin, J.C. Knight & P.S. Russell
Opt. Express 12 (2004) 6498-6507
Nonlinear dynamics of ultrashort optical pulses in the vicinity of the second zero-dispersion point of a small-core photonic crystal fiber is visualized and studied using cross-correlation frequency-resolved optical gating. New spectral features observed in the experiments match well with recent theoretical predictions of the generation of new frequencies via mixing of solitons and dispersive waves. Power-as well as length-dependent dynamics is obtained showing strong interaction between solitons and dispersive waves, soliton-soliton interaction, soliton stabilization against Raman self-frequency shift and Cherenkov continuum generation. (C) 2004 Optical Society of America.

All-solid photonic bandgap fiber
F. Luan, A.K. George, T.D. Hedley, G.J. Pearce, D.M. Bird, J.C. Knight & P.S.J. Russell
Opt. Lett. 29 (2004) 2369-2371
We describe the design and fabrication of a photonic bandgap fiber formed with two different glasses. As in a hollow-core fiber, light is guided in a low-index core region because of the antiresonances of the high-index strands in the fiber cladding. The structure described represents an ideal bandgap fiber that exhibits no interface modes and guides over the full width of multiple bandgaps. (C) 2004 Optical Society of America.

Four-wave mixing of linear waves and solitons in fibers with higher-order dispersion
A.V. Yulin, D.V. Skryabin & P.S.J. Russell
Opt. Lett. 29 (2004) 2411-2413
We derive phase-matching conditions for four-wave mixing between solitons and linear waves in optical fibers with arbitrary dispersion and demonstrate resonant excitation of new spectral components via this process. (C) 2004 Optical Society of America.

Coupled core-surface solitons in photonic crystal fibers
D.V. Skryabin
Opt. Express 12 (2004) 4841-4846
We predict existence and study properties of the coupled core-surface solitons in hollow-core photonic crystal fibers. These solitons exist in the spectral proximity of the avoided crossings of the propagation constants of the modes guided in the air core and at the interface between the core and photonic crystal cladding. (C) 2004 Optical Society of America.

Effective Kerr nonlinearity and two-color solitons in photonic band-gap fibers filled with a Raman active gas
D.V. Skryabin, F. Biancalana, D.M. Bird & F. Benabid
Phys. Rev. Lett. 93 (2004) 143907
We predict a strong effective Kerr nonlinearity in hollow-core photonic crystal fibers filled with a Raman active gas which exceeds the intrinsic Kerr nonlinearity by 2 orders of magnitude. Two-color bright-bright and dark-bright solitons supported by this nonlinearity are found and the feasibility of their experimental observation is demonstrated.

Ultrahigh efficiency laser wavelength conversion in a gas-filled hollow core photonic crystal fiber by pure stimulated rotational Raman scattering in molecular hydrogen
F. Benabid, G. Bouwmans, J.C. Knight, P. St Russell & F. Couny
Phys. Rev. Lett. 93 (2004) 123903
We report on the generation of pure rotational stimulated Raman scattering in a hydrogen gas hollow-core photonic crystal fiber. Using the special properties of this low-loss fiber, the normally dominant vibrational stimulated Raman scattering is suppressed, permitting pure conversion to the rotational Stokes frequency in a single-pass configuration pumped by a microchip laser. We report 92% quantum conversion efficiency (40 nJ pulses in 2.9 m fiber) and threshold energies (3 nJ in 35 m) more than 1x10(6) times lower than previously reported. The control of the output spectral components by varying only the pump polarization is also shown. The results point to a new generation of highly engineerable and compact laser sources.

Low-threshold supercontinuum generation from an extruded SF6 PCF using a compact Cr4+:YAG laser
V.L. Kalashnikov, E. Sorokin, M. Cronin-Golomb, S. Naumov, I.T. Sorokina, V.V. Ravi Kanth Kumar & A.K. George
Appl. Phys. B 79 (2004) 591-596
The modal properties of an extruded SF6 photonic crystal fiber (PCF) have been analyzed on the basis of a full-vector model. The favorable dispersion properties (the zero-dispersion wavelength for a PCF with 4.5-µmcore size lies at approximately 1.5 µm) and the large nonlinearity figure of up to 280W-1 km-1 may provide more than an octave of spectral broadening with launched pulse energies below 200 pJ. In agreementwith simulations, experiments have demonstrated the low-energy spectral broadening under excitation by 60–70-fs pulses from a compact mode-locked Cr4+:YAG laser operating at a 100-MHz repetition rate. Besides the spectral broadening due to the soliton-fission mechanism, the rich mode structure of the fiber allows third-order harmonic generation and four-wave mixing, which extend the supercontinuum spectrumdown to the visible-wavelength range.(C) Springer-Verlag 2004.

Dispersion and refractive index measurement for Ge, B-Ge doped and photonic crystal fibre following irradiation at MGy levels
W.N. MacPherson, R.R.J. Maier, J.S. Barton, J.D.C. Jones, A.F. Fernandez, B. Brichard, F. Berghmans, J.C. Knight, P.S.J. Russell & L. Farr
Meas. Sci. Technol. 15 (2004) 1659-1664
We have investigated the effects of ionizing radiation-induced changes on dispersion and refractive index in photonic crystal fibre, boron co-doped photosensitive fibre and standard SMF-28 fibre up to a level of 7 MGy of (CO)-C-60 gamma-radiation. We have been unable to observe any changes within an experimental error of similar to1% for the dispersion value and < 1.5 x 10(-3) for the refractive index.

Theory of the soliton self-frequency shift compensation by the resonant radiation in photonic crystal fibers
F. Biancalana, D.V. Skryabin & A.V. Yulin
Phys. Rev. E 70 (2004) 016615
We develop a theory of the soliton self-frequency shift compensation by the resonant radiation recently observed in photonic crystal fibers. Our approach is based on the calculation of the soliton plus radiation solution of the generalized nonlinear Schrodinger (GNLS) equation and on subsequent use of the adiabatic theory leading to a system of equations governing evolution of the soliton parameters in the presence of the Raman effect and radiation. Our theoretical results are found to be in good agreement with direct numerical modeling of the GNLS equation.

Supercontinuum generation in submicron fibre waveguides
S.G. Leon-Saval, T.A. Birks, W.J. Wadsworth, P.S.J. Russell & M.W. Mason
Opt. Express 12 (2004) 2864-2869
Submicron-diameter tapered fibres and photonic crystal fibre cores, both of which are silica-air waveguides with low dispersion at 532 nm, were made using a conventional tapering process. In just cm of either waveguide, ns pulses from a low-power 532-nm microchip laser generated a single-mode supercontinuum broad enough to fill the visible spectrum without spreading far beyond it. (C) 2004 Optical Society of America.

Hollow core photonic crystal fibers for beam delivery
G. Humbert, J.C. Knight, G. Bouwmans, P.S. Russell, D.P. Williams, P.J. Roberts & B.J. Mangan
Opt. Express 12 (2004) 1477-1484
Hollow-core photonic crystal fibers have unusual properties which make them ideally suited to delivery of laser beams. We describe the properties of fibers with different core designs, and the observed effects of anti-crossings with interface modes. We conclude that 7-unit-cell cores are currently most suitable for transmission of femtosecond and sub-picosecond pulses, whereas larger cores (e. g. 19-cell cores) are better for delivering nanosecond pulsed and continuous-wave beams.

Vector modulational instabilities in ultra-small core optical fibres
F. Biancalana & D.V. Skryabin
J. Opt. A-Pure Appl. Opt. 6 (2004) 301-306
We present a detailed analysis of vector modulational instabilities in ultra-small core optical fibres. The existence of new instability peaks emerging due to a strong waveguide contribution to the dispersion characteristic of these fibres is reported and their properties are analysed for the cases of low and high birefringence.

Low-loss deposition of solgel-derived silica films on tapered fibers
G. Kakarantzas, S.G. Leon-Saval, T.A. Birks & P.S.J. Russell
Opt. Lett. 29 (2004) 694-696
Films of porous silica are deposited on the uniform waists of tapered fibers in minutes by a modified solgel dip coating method, inducing less than 0.2 dB of loss. The coated tapers are an ideal platform for realizing all-fiber devices that exploit evanescent-field interactions with the deposited porous film. As an example we demonstrate structural long-period gratings in which a periodic index variation in the film arises from the porosity variation produced by spatially varying exposure of the waist to a scanned CO2 laser beam. The long period grating is insensitive to temperature up to 800degreesC. (C) 2004 Optical Society of America.

Femtosecond soliton pulse delivery at 800nm wavelength in hollow-core photonic bandgap fibers
F. Luan, J.C. Knight, P.S. Russell, S. Campbell, D. Xiao, D.T. Reid, B.J. Mangan, D.P. Williams & P.J. Roberts
Opt. Express 12 (2004) 835-840
We describe delivery of femtosecond solitons at 800nm wavelength over five meters of hollow-core photonic bandgap fiber. The output pulses had a length of less than 300fs and an output pulse energy of around 65nJ, and were almost bandwidth limited. Numerical modeling shows that the nonlinear phase shift is determined by both the nonlinearity of air and by the overlap of the guided mode with the glass. (C) 2004 Optical Society of America.

Doppler doubts
P. Russell
Phys. World 17 (2004) 20-20

Very high numerical aperture fibers
W.J. Wadsworth, R.M. Percival, G. Bouwmans, J.C. Knight, T.A. Birks, T.D. Hedley & P.S. Russell
IEEE Photonics Technol. Lett. 16 (2004) 843-845
Air-silica microstructured fibers are designed and fabricated to yield numerical aperture (NA) greater than 0.9. A model is presented which accurately predicts the structural parameters required in order to realize high NAs. Such fibers have application in lasers and laser-induced-fluorescence systems.

High energy nanosecond laser pulses delivered single-mode through hollow-core PBG fibers
J.D. Shephard, J.D.C. Jones, D.P. Hand, G. Bouwmans, J.C. Knight, P.S. Russell & B.J. Mangan
Opt. Express 12 (2004) 717-723
We report on the development of hollow-core photonic bandgap fibers for the delivery of high energy pulses for precision micro-machining applications. Short pulses of (65ns pulse width) and energies of the order of 0.37 mJ have been delivered in a single spatial mode through hollow-core photonic bandgap fibers at 1064nm using a high repetition rate (15kHz) Nd:YAG laser. The ultimate laser-induced damage threshold and practical limitations of current hollow-core fibers for the delivery of short optical pulses are discussed. (C) 2004 Optical Society of America.

Modeling the propagation of light in photonic crystal fibers
J. Arriaga, J.C. Knight & P.S.J. Russell
Physica D 189 (2004) 100-106
Using the plane wave expansion method, we have solved Maxwell's equations for the propagation of electromagnetic waves in photonic crystal fibers. Taking the cross section of the photonic crystal fiber as a big unit cell, and generating an artificial infinite two-dimensional periodic system, the plane wave expansion allows us to study the properties of the core in exactly the same way as defects in photonic crystals. (C) 2003 Elsevier B.V. All rights reserved.

Supercontinuum and four-wave mixing with Q-switched pulses in endlessly single-mode photonic crystal fibres
W.J. Wadsworth, N. Joly, J.C. Knight, T.A. Birks, F. Biancalana & P.S.J. Russell
Opt. Express 12 (2004) 299-309
Photonic crystal fibres exhibiting endlessly single-mode operation and dispersion zero in the range 1040 to 1100 nm are demonstrated. A sub-ns pump source at 1064 nm generates a parametric output at 732 nm with an efficiency of 35%, or parametric gain of 55 dB at 1315 nm. A broad, flat supercontinuum extending from 500 nm to beyond 1750 nm is also demonstrated using the same pump source. (C) 2004 Optical Society of America.

Scaling laws and vector effects in bandgap-guiding fibres
T.A. Birks, D.M. Bird, T.D. Hedley, J.M. Pottage & P.S. Russell
Opt. Express 12 (2004) 69-74
Scaling laws for photonic bandgaps in photonic crystal fibres are described. Although only strictly valid for small refractive index contrast, they successfully identify corresponding features in structures with large index contrast. Furthermore, deviations from the scaling laws distinguish features that are vector phenomena unique to electromagnetic waves from those that would be expected for generic scalar waves. (C) 2003 Optical Society of America.

Simple optical profiling of complex guiding structures
A. Efimov, A.J. Taylor, F.G. Omenetto, J.C. Knight, W.J. Wadsworth & P.S. Russell
Appl. Optics 43 (2004) 29-32
Working with complex guiding structures such as holey fibers requires coupling light into the input face of the structure. We use a simple in vivo technique to determine the scale, morphology, and orientation of the input cleave of the fiber without resorting to separate and more complex methods like optical imaging or scanning electron microscopy. Further, after obtaining the transverse scan of the fiber tip one can precisely position the focal spot anywhere relative to the fiber structure. (C) 2004 Optical Society of America.

2003 Publications

Transition radiation by matter-wave solitons in optical lattices
A.V. Yulin, D.V. Skryabin & P.S.J. Russell
Phys. Rev. Lett. 91 (2003) 260402
We demonstrate that matter-wave solitary pulses formed from Bose condensed atoms moving inside optical lattices continuously radiate dispersive matter waves with prescribed momentum. Our analytical results for the radiation parameters and the soliton decay rate are found to be in excellent agreement with numerical modeling performed for experimentally relevant parameters.

Ultrahigh resolution real time OCT imaging using a compact femtosecond Nd : Glass laser and nonlinear fiber
S. Bourquin, A.D. Aguirre, I. Hartl, P. Hsiung, T.H. Ko, J.G. Fujimoto, T.A. Birks, W.J. Wadsworth, U. Bunting & D. Kopf
Opt. Express 11 (2003) 3290-3297
Ultrahigh resolution, real time OCT imaging is demonstrated using a compact femtosecond Nd:Glass laser that is spectrally broadened in a high numerical aperture single mode fiber. A reflective grating phase delay scanner enables broad bandwidth, high-speed group delay scanning. We demonstrate in vivo, ultrahigh resolution, real time OCT imaging at 1 mum center wavelength with <5 mu m axial resolution in free space (<4 mum in tissue). The light source is robust, portable, and well suited for in vivo imaging studies.

Supercontinuum generation with 200 pJ laser pulses in an extruded SF6 fiber at 1560 nm
H. Hundertmark, D. Kracht, D. Wandt, C. Fallnich, V.V.R.K. Kumar, A.K. George, J.C. Knight & P.S. Russell
Opt. Express 11 (2003) 3196-3201
We report an all-fiber diode-pumped Erbium-fiber oscillator-amplifier system as a source for supercontinuum generation in a photonic crystal fiber at 1560 nm. The passively mode-locked oscillator-amplifier system provides linearly polarized output pulses of 60 fs and an average output power of 59 mW at a repetition rate of 59.1 MHz. The laser pulses were launched into an extruded SF6-fiber for generation of an ultra-broadband supercontinuum. The evolution of the supercontinuum as a function of launched pulse energy was investigated. With pulse energies of about 200 pJ we observed a more than octave-spanning supercontinuum from 400 nm to beyond 1750 nm. (C) 2003 Optical Society of America.

Scalar modulation instability in the normal dispersion regime by use of a photonic crystal fiber
J.D. Harvey, R. Leonhardt, S. Coen, G.K.L. Wong, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Opt. Lett. 28 (2003) 2225-2227
Modulation instability at high frequencies ha's been demonstrated in the normal dispersion regime by use of a photonic crystal fiber. This fiber-optic parametric generator provides efficient conversion of red pump light into blue and near-infrared light. (C) 2003 Optical Society of America.

Robust photonic band gaps for hollow core guidance in PCF made from high index glass
J.M. Pottage, D.M. Bird, T.D. Hedley, T.A. Birks, J.C. Knight, P.S. Russell & P.J. Roberts
Opt. Express 11 (2003) 2854-2861
We report a new type of photonic bandgap that becomes substantial at remarkably low air-filling fractions (similar to 60%) in triangular-lattice photonic crystal fibres (PCF) made from high index glass (n greater than or similar to 2.0). The ratio of inter-hole spacing to wavelength makes these new structures ideal for the experimental realisation of hollow-core PCF in the mid/far-infrared, where suitable glasses (e.g., tellurites and chalcogenides) tend to have high refractive indices. (C) 2003 Optical Society of America.

Phase-matched third harmonic generation in microstructured fibers
A. Efimov, A.J. Taylor, F.G. Omenetto, J.C. Knight, W.J. Wadsworth & P.S. Russell
Opt. Express 11 (2003) 2567-2576
Strong guiding provided by the high-delta microstructured fibers allows for efficient intermodally phase-matched harmonic generation with femtosecond pumping at telecom wavelengths. Visible harmonics are generated in a number of distinct transverse modes of the structure. We present a detailed experimental and theoretical study of the third harmonic generation in such fibers including phase-matching wavelengths, far-field intensity distributions and polarization dependence. Good agreement between the theory and experiment is achieved. (C) 2003 Optical Society of America.

Tellurite photonic crystal fiber
V.V.R.K. Kumar, A.K. George, J.C. Knight & P.S. Russell
Opt. Express 11 (2003) 2641-2645
We report the fabrication of a Tellurite photonic crystal fiber, and demonstrate its waveguiding properties. The measured minimum loss is 2.3 dB/m at a wavelength of 1055 nm. The fiber supports several modes, but in practice just the fundamental mode can be used. We have observed strong stimulated Raman scattering in a fiber with an effective area A(eff)= 21.2 mum(2), using sub-ns, similar to 1 muJ pump pulses at 1064 nm. (C) 2003 Optical Society of America.

Sonic band gaps in PCF preforms: enhancing the interaction of sound and light
P.S. Russell, E. Marin, A. Diez, S. Guenneau & A.B. Movchan
Opt. Express 11 (2003) 2555-2560
We study the localisation and control of high frequency sound in a dual-core square-lattice photonic crystal fibre preform. The coupled states of two neighboring acoustic resonances are probed using an interferometric set up, and experimental evidence is obtained for odd and even symmetry trapped states. Full numerical solutions of the acoustic wave equation show the existence of a two-dimensional sonic band gap, and numerical modelling of the strain field at the defects gives results that agree well with the experimental observations. The results suggest that sonic band gaps can be used to manipulate sound with great precision and enhance its interaction with light. (C) 2003 Optical Society of America.

Four-wave mixing instabilities in photonic-crystal and tapered fibers
F. Biancalana, D.V. Skryabin & P.S. Russell
Phys. Rev. E 68 (2003) 046603
Four-wave mixing instabilities are theoretically studied for continuous wave propagation in ultrasmall core photonic-crystal and tapered fibers. The waveguide, or geometrical, contribution to the overall dispersion of these structures is much stronger than in conventional fibers. This leads to the appearance of unstable frequency bands that are qualitatively and quantitatively different from those seen in conventional fibers. The four-wave mixing theory developed here is based on the full wave equation, which allows rigorous study of the unstable bands even when the detunings are of the order of the pump frequency itself. Solutions obtained using the generalized nonlinear Schrodinger equation, which is an approximate version of the full wave equation, reveal that it suffers from several deficiencies when used to describe four-wave mixing processes.

Photonic band gaps deliver the goods
J. Knight
Phys. World 16 (2003) 26-27

Soliton self-frequency shift cancellation in photonic crystal fibers
D.V. Skryabin, F. Luan, J.C. Knight & P.S. Russell
Science 301 (2003) 1705-1708
We report the cancellation of the soliton self-frequency shift in a silica-core photonic crystal fiber with a negative dispersion slope. Numerical and experimental results show that stabilization of the soliton wavelength is accompanied by exponential amplification of the red-shifted Cherenkov radiation emitted by the soliton. The spectral recoil from the radiation acts on the soliton to compensate for the Raman frequency shift. This phenomenon may find applications in the development of a family of optical parametric amplifiers.

Maximization of supercontinua in photonic crystal fibers by using double pulses and polarization effects
V.L. Kalashnikov, P. Dombi, T. Fuji, W.J. Wadsworth, J.C. Knight, P.S.J. Russell, R.S. Windeler & A. Apolonski
Appl. Phys. B-Lasers Opt. 77 (2003) 319-324
We show with sub-20 fs pulses both experimentally and theoretically two techniques to control the width and polarization of spectral supercontinua generated in photonic crystal fibers. The first exploits double pulses which interact inside the fiber nonelastically due to stimulated Raman scattering and thus make the supercontinuum wider, whereas the peak intensity can be kept at a reasonably moderate level to prevent fiber damage. The second approach includes polarization manipulations which allow a desirable polarization state to be obtained in a definite spectral range and the spectral shape and width to be varied.

Enhanced visualization of choroidal vessels using ultrahigh resolution ophthalmic OCT at 1050 nm
B. Povazay, K. Bizheva, B. Hermann, A. Unterhuber, H. Sattmann, A.F. Fercher, W. Drexler, C. Schubert, P.K. Ahnelt, M. Mei, R. Holzwarth, W.J. Wadsworth, J.C. Knight & P.S. Russel
Opt. Express 11 (2003) 1980-1986
In this article the ability of ultrahigh resolution ophthalmic optical coherence tomography (OCT) to image small choroidal blood vessels below the highly reflective and absorbing retinal pigment epithelium is demonstrated for the first time. A new light source (lambda(c) = 1050 nm, Deltalambda = 165 nm, P-out = 10 mW), based on a photonic crystal fiber pumped by a compact, self-starting Ti:Al2O3 laser has therefore been developed. Ex-vivo ultrahigh resolution OCT images of freshly excised pig retinas acquired with this light source demonstrate enhanced penetration into the choroid and better visualization of choroidal vessels as compared to tomograms acquired with a state-of-the art Ti:Al2O3 laser (Femtolasers Compact Pro, lambda(c) = 780 nm, Deltalambda = 160 nm, P-out = 400 mW), normally used in clinical studies for in vivo ultrahigh resolution ophthalmic OCT imaging. These results were also compared with retinal tomograms acquired with a novel, spectrally broadened fiber laser ( MenloSystems, lambda(c) = 1350 nm, Deltalambda = 470 nm, P-out = 4 mW) permitting even greater penetration in the choroid. Due to high water absorption at longer wavelengths retinal OCT imaging at similar to1300 nm may find applications in animal ophthalmic studies. Detection and follow-up of choroidal neovascularization improves early diagnosis of many retinal pathologies, e.g. age-related macular degeneration or diabetic retinopathy and can aid development of novel therapy approaches. (C) 2003 Optical Society of America.

Photonic crystal fibres
J.C. Knight
Nature 424 (2003) 847-851
Photonic crystal fibres have wavelength-scale morphological microstructure running down their length. This structure enables light to be controlled within the fibre in ways not previously possible or even imaginable. Our understanding of what an optical fibre is and what it does is changing because of the development of this new technology, and a broad range of applications based on these principles is being developed.

High-power Er : Yb fiber laser with very high numerical aperture pump-cladding waveguide
G. Bouwmans, R.M. Percival, W.J. Wadsworth, J.C. Knight & P.S.J. Russell
Appl. Phys. Lett. 83 (2003) 817-818
One watt of laser power at 1535 nm has been generated from an Er:Yb codoped double-clad fiber structure, in which the inner cladding was suspended in air by a large number of thin silica webs. The acceptance numerical aperture (NA) of the inner cladding has been measured over a range of wavelengths, for lengths from 1 up to 100 m. For fiber lengths of similar to40 m, the NA at the pump diode wavelength was 0.88. (C) 2003 American Institute of Physics.

Two-core photonic crystal fibre for Doppler difference velocimetry
W.N. MacPherson, J.D.C. Jones, B.J. Mangan, J.C. Knight & P.S. Russell
Opt. Commun. 223 (2003) 375-380
We present the first application of a two-core photonic crystal fibre for fringe projection for use as a Doppler difference velocimeter. The two-core fibre projects a series of fringes when both cores are simultaneously illuminated with 633 nm light. Light scattered from moving particles is coupled into a separate multi-mode collection fibre and detected using a silicon detector and amplifier. The scattered signal is processed in the frequency domain to obtain the particle velocity component perpendicular to the fringes. (C) 2003 Published by Elsevier B.V.

Transformation and control of ultra-short pulses in dispersion-engineered photonic crystal fibres
W.H. Reeves, D.V. Skryabin, F. Biancalana, J.C. Knight, P.S. Russell, F.G. Omenetto, A. Efimov & A.J. Taylor
Nature 424 (2003) 511-515
Photonic crystal fibres (PCFs) offer greatly enhanced design freedom compared to standard optical fibres. For example, they allow precise control of the chromatic dispersion (CD) profile-the frequency dependence of propagation speed-over a broad wavelength range. This permits studies of nonlinear pulse propagation in previously inaccessible parameter regimes. Here we report on spectral broadening of 100-fs pulses in PCFs with anomalously flat CD profiles. Maps of the spectral and spatio-temporal behaviour as a function of power show that dramatic conversion (to both longer and shorter wavelengths) can occur in remarkably short lengths of fibre, depending on the magnitude and shape of the CD profile. Because the PCFs used are single-mode at all wavelengths, the light always emerges in a fundamental guided mode. Excellent agreement is obtained between the experimental results and numerical solutions of the nonlinear wave equation, indicating that the underlying processes can be reliably modelled. These results show how, through appropriate choice of CD, nonlinearities can be efficiently harnessed to generate laser light at new wavelengths.

Enhanced two-photon biosensing with double-clad photonic crystal fibers
M.T. Myaing, J.Y. Ye, T.B. Norris, T. Thomas, J.R. Baker, W.J. Wadsworth, G. Bouwmans, J.C. Knight & P.S.J. Russell
Opt. Lett. 28 (2003) 1224-1226
A double-clad photonic crystal fiber was used to improve detection efficiency over a standard single-mode fiber in a two-photon fluorescence detection scheme in which the dye was excited and the fluorescence was detected back through the same fiber. (C) 2003 Optical Society of America.

Properties of a hollow-core photonic bandgap fiber at 850 nm wavelength
G. Bouwmans, F. Luan, J.C. Knight, P.S.J. Russell, L. Farr, B.J. Mangan & H. Sabert
Opt. Express 11 (2003) 1613-1620
We describe a hollow-core photonic bandgap fiber designed for use in the 850 nm wavelength region. The fiber has a minimum attenuation of 180dB/km at 847nm wavelength. The low-loss mode has a quasi-Gaussian intensity profile. The group-velocity dispersion of this mode passes through zero around 830nm, and is anomalous for longer wavelengths. The polarization beat length varies from 4 mm to 13 mm across the band gap. We expect this fiber to be useful for delivery of high-energy ultrashort optical pulses. (C) 2003 Optical Society of America.

Generation of a spectrally asymmetric third harmonic with unamplified 30-fs Cr : forsterite laser pulses in a tapered fiber
D.A. Akimov, A.A. Ivanov, A.N. Naumov, O.A. Kolevatova, M.V. Alfimov, T.A. Birks, W.J. Wadsworth, P.S.J. Russell, A.A. Podshivalov & A.M. Zheltikov
Appl. Phys. B-Lasers Opt. 76 (2003) 515-519
Unamplified subnanojoule 30-fs pulses of Cr:forsterite laser radiation are shown to generate the third-harmonic signal in a tapered fiber. Analysis of the dispersion properties of a tapered fiber indicates the possibility of phase matching third-harmonic generation in higher-order waveguide modes. When the pump and third-harmonic pulses are group-velocity-mismatched, optimal phase matching is achieved for frequency components spectrally offset from the central frequency of the pump pulse, which gives rise to an asymmetry in the spectrum of the third harmonic.

Modelling photonic crystal fibres
J. Arriaga, J.C. Knight & P.S. Russell
Physica E 17 (2003) 440-442
One of the potential applications of photonic crystals is the so-called photonic crystal fibres. These systems can be constructed, for example, using a long thread of silica glass with a periodic array of airholes running down its length. If the central hole is absent, we generate a high-index "defect" in the repeating structure which acts like the core of an optical fibre. We study the propagation of light in these fibres. We solve the Maxwell equations using the plane wave expansion and the super cell method. (C) 2002 Elsevier Science B.V. All rights reserved.

Frequency conversion of femtosecond Cr : forsterite-laser pulses in a tapered fibre
D.A. Akimov, A.A. Ivanov, M.V. Alfimov, A.B. Fedotov, T.A. Birks, W.J. Wadsworth, P.S. Russell, O.A. Kolevatova, S.O. Konorov, A.A. Podshivalov, A. Petrov, D.A. Sidorov-Biryukov & A.M. Zheltikov
Quantum Electron. 33 (2003) 317-320
Efficient frequency conversion of femtosecond pulses of a Cr : forsterite laser in tapered fibres is experimentally demonstrated. Unamplified 30-fs Cr : forsterite-laser pulses are used to generate the third harmonic in the phase-matching regime. Propagation of 75-fs, 10-200-nJ Cr : forsterite-laser pulses through a tapered fibre in the range of anomalous dispersion is accompanied by multiply phase-matched cascaded four-wave mixing, giving rise to a manifold of new spectral components in the visible range and substantial spectral broadening within the wavelength range from 1300 to 1600 nm.

Structural rocking filters in highly birefringent photonic crystal fiber
G. Kakarantzas, A. Ortigosa-Blanch, T.A. Birks, P.S. Russell, L. Farr, F. Couny & B.J. Mangan
Opt. Lett. 28 (2003) 158-160
We report what we believe is the first example of efficient rocking filter formation in polarization-maintaining photonic crystal fiber. Very high coupling efficiencies (as much as -23.5-dB suppression of the input polarization) and loss of <0.02 dB were achieved for fibers as short as 11 mm. The filters, which we prepared by periodic mechanical twisting and heating with a scanned CO2 laser beam, are highly compact, and they are expected to be temperature stable. (C) 2003 Optical Society of America.

Out-of-gap Bose-Einstein solitons in optical lattices
A.V. Yulin & D.V. Skryabin
Phys. Rev. A 67 (2003) 023611
We study the existence and the spectral and collisional properties of bright and dark solitons in a Bose-Einstein condensate (BEC) in optical lattices. Both types of these solitons are located on the background of a matter wave of finite amplitude with characteristic frequency lying outside the forbidden gap. Thus we term our solitons out-of-gap solitons. We discuss feasibility of practical preparation of the background wave to observe these solitons and make connections with recent experiments studying BEC in optical lattices. We prove that the dark out-of-gap solitons are robust against perturbations and numerically verify the possibility of their excitation using a phase imprinting technique.

Photonic crystal fibers
P. Russell
Science 299 (2003) 358-362

Polarization dependent harmonic generation in microstructured fibers
F.G. Omenetto, A. Efimov, A.J. Taylor, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Opt. Express 11 (2003) 61-67
We report on the control of visible harmonic generation in microstructured fiber through the polarization state of the fundamental radiation. By coupling lambda=1.55 mum fermosecond pulses that have the same peak power into a short length (Z=20 cm) of high-Delta microstructured fiber, we observe the generation of distinct visible spectral components in the visible at the output of the fiber in dependence of the input pulse's polarization state. (C) 2003 Optical Society of America.

High power air-clad photonic crystal fibre laser
W.J. Wadsworth, R.M. Percival, G. Bouwmans, J.C. Knight & P.S.J. Russel
Opt. Express 11 (2003) 48-53
An Ytterbium-doped photonic crystal fibre laser is demonstrated with a 100 mum(2) core area and single transverse mode with an output efficiency of 30 %. Double-clad PCF laser structures are demonstrated with pump cladding NA greater than 0.8 and output power up to 3.9 W. Such lasers are potentially scalable to high power. (C) 2003 Optical Society of America.

Doubly phase-matched cascaded parametric wave mixing of ultrashort laser pulses
D.A. Akimov, M.V. Alfimov, A.A. Ivanov, A.B. Fedotov, T.A. Birks, W.J. Wadsworth, P.S. Russell, S.O. Konorov, O.A. Kolevatova, A.A. Podshivalov & A.M. Zheltikov
Jetp Lett. 77 (2003) 7-11
Doubly phase-matched cascaded parametric wave mixing of femtosecond laser pulses in tapered fibers is experimentally demonstrated. Fibers with an appropriately tailored dispersion profile allow simultaneous phase matching for two types of nonlinear-optical processes-third-harmonic generation and parametric four-wave mixing. Doubly phase-matched cascaded parametric interactions of ultrashort light pulses give rise to a manifold of new spectral components, expanding substantially the capabilities of the available laser sources of ultrashort pulses. (C) 2003 MAIK "Nauka/Interperiodica".

2002 Publications

Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation
V.V.R.K. Kumar, A.K. George, W.H. Reeves, J.C. Knight, P.S. Russell, F.G. Omenetto & A.J. Taylor
Opt. Express 10 (2002) 1520-1525
We report the fabrication and properties of soft glass photonic crystal fibers (PCF's) for supercontinuum generation. The fibers have zero or anomalous group velocity dispersion at wavelengths around 1550 nm, and approximately an order of magnitude higher nonlinearity than attainable in comparable silica fibers. We demonstrate the generation of an ultrabroad supercontinuum spanning at least 350 nm to 2200 nm using a 1550 nm ultrafast pump source. (C) 2002 Optical Society of America.

Optical solitons due to quadratic nonlinearities: from basic physics to futuristic applications
A.V. Buryak, P. Di Trapani, D.V. Skryabin & S. Trillo
Phys. Rep.-Rev. Sec. Phys. Lett. 370 (2002) 63-235
We present an overview of nonlinear phenomena related to optical quadratic solitons-intrinsically multicomponent localized states of light, which can exist in media without inversion symmetry at the molecular level. Starting with presentation of a few derivation schemes of basic equations describing three-wave parametric wave mixing in diffractive and/or dispersive quadratic media, we discuss their continuous wave solutions and modulational instability phenomena, and then move to the classification and stability analysis of the parametric solitary waves. Not limiting ourselves to the simplest spatial and temporal quadratic solitons we also overview results related to the spatio-temporal solitons (light bullets), higher order quadratic solitons, solitons due to competing nonlinearities, dark solitons, gap solitons, cavity solitons and vortices. Special attention is paid to a comprehensive discussion of the recent experimental demonstrations of the parametric solitons including their interactions and switching. We also discuss connections of quadratic solitons with other types of solitons in optics and their interdisciplinary significance. (C) 2002 Elsevier Science B.V. All rights reserved.

Pulse breaking and supercontinuum generation with 200-fs pump pulses in photonic crystal fibers
A. Ortigosa-Blanch, J.C. Knight & P.S.J. Russell
J. Opt. Soc. Am. B-Opt. Phys. 19 (2002) 2567-2572
We have carried out a detailed experimental study of the behavior of 200-fs pulses in highly nonlinear photonic crystal fiber to elucidate the mechanisms for supercontinuum generation. To avoid unwanted polarization effects, our experiments were performed using polarization-maintaining fiber. The experimental evidence shows that, as in conventional fibers, Raman scattering leads to the breakup of higher-order solitons, which is accompanied by the generation of radiation at shorter wavelengths than the pump, leading eventually to an ultrabroad supercontinuum. (C) 2002 Optical Society of America.

Stability of spiralling solitary waves in Hamiltonian systems
D.V. Skryabin, J.M. McSloy & W.J. Firth
Phys. Rev. E 66 (2002) 055602
We present a rigorous criterion for stability of spiralling solitary structures in Hamiltonian systems incorporating the angular momentum integral and demonstrate its applicability to the spiralling of two mutually incoherent optical beams propagating in a photorefractive material.

Particle levitation and guidance in hollow-core photonic crystal fiber
F. Benabid, J.C. Knight & P.S. Russell
Opt. Express 10 (2002) 1195-1203
We report the guidance of dry micron-sized dielectric particles in hollow core photonic crystal fiber. The particles were levitated in air and then coupled to the air-core of the fiber using an Argon ion laser beam operating at a wavelength of 514 nm, The diameter of the hollow core of the fiber is 20 mum. A laser power of 80 mW was sufficient to levitate a 5 mum diameter polystyrene sphere and guide it through a similar to150 nun long hollow-core crystal photonic fiber. The speed of the guided particle was measured to be around 1 cm/s. (C) 2002 Optical Society of America.

Submicrometer axial resolution optical coherence tomography
B. Povazay, K. Bizheva, A. Unterhuber, B. Hermann, H. Sattmann, A.F. Fercher, W. Drexler, A. Apolonski, W.J. Wadsworth, J.C. Knight, P.S.J. Russell, M. Vetterlein & E. Scherzer
Opt. Lett. 27 (2002) 1800-1802
Optical coherence tomography (OCT) with unprecedented submicrometer axial resolution achieved by use of a photonic crystal fiber in combination with a compact sub-10-fs Ti:sapphire laser (Femtolasers Produktions) is demonstrated for what the authors believe is the first time. The emission spectrum ranges from 550 to 950 nm (lambda(c) = 725 nm, P-out = 27 mW), resulting in a free-space axial OCT resolution of similar to0.75 mum, corresponding to similar to0.5 mum in biological tissue. Submicrometer-resolution OCT is demonstrated in vitro on human colorectal adenocarcinoma cells HT-29. This novel light source has great potential for development of spectroscopic OCT because its spectrum covers the absorption bands of several biological chromophores. (C) 2002 Optical Society of America.

Stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber
F. Benabid, J.C. Knight, G. Antonopoulos & P.S.J. Russell
Science 298 (2002) 399-402
We report on stimulated Raman scattering in an approximately 1-meter-long hollow-core photonic crystal fiber filled with hydrogen gas under pressure. Light was guided and confined in the 15-micrometer-diameter hollow core by two-dimensional photonic bandgap. Using pulsed laser source (pulse duration, 6 nanoseconds; wavelength, 532 nanometers), the threshold for Stokes (longer wavelength) generation was observed at pulse energies as low as 800 +/- nanojoules, followed by coherent anti-Stokes (shorter wavelength) generation threshold at 3.4 +/- 0.7 microjoules. The pump-to-Stokes conversion efficiency was 30 +/- 3% at pulse energy of only 4.5 microjoules. These energies are almost two orders of magnitude lower than any other reported energy, moving gas-based nonlinear optics to previously inaccessible parameter regimes of high intensity and long interaction length.

Polarization dynamics of Bragg solitons
A.V. Yulin, D.V. Skryabin & W.J. Firth
Phys. Rev. E 66 (2002) 046603
The families of vectorial Bragg solitons existing in transversely periodic media and their stability properties are studied in detail. Two qualitatively distinct types of polarization instabilities have been found. One leads to the significant radiation transfer into nonsolitonic forms, while the other mainly redistributes energy between two soliton components.

Long-wavelength continuum generation about the second dispersion zero of a tapered fiber
J.M. Harbold, F.O. Ilday, F.W. Wise, T.A. Birks, W.J. Wadsworth & Z. Chen
Opt. Lett. 27 (2002) 1558-1560
A dispersion zero near 1300 nm of a narrow-diameter tapered fiber is used to generate broadband, near-infrared light. When femtosecond pulses at 1260 nm with 750 pJ of energy are launched in proximity to the second zero-dispersion wavelength, a continuum spanning 1000-1700 nm is produced. (C) 2002 Optical Society of America.

Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source
W.J. Wadsworth, A. Ortigosa-Blanch, J.C. Knight, T.A. Birks, T.P.M. Man & P.S. Russell
J. Opt. Soc. Am. B-Opt. Phys. 19 (2002) 2148-2155
Broadband continua extending from 400 to 1600 nm are generated in photonic crystal fibers and in tapered conventional optical fibers. The continuum is generated in the fundamental fiber mode. Femtosecond pulses from an unamplified Ti:sapphire laser with energies up to 4 nJ are used, and the resultant spectra from several photonic crystal fibers and taper structures are compared and analyzed. (C) 2002 Optical Society of America.

Spectral shaping of supercontinuum in a cobweb photonic-crystal fiber with sub-20-fs pulses
A. Apolonski, B. Povazay, A. Unterhuber, W. Drexler, W.J. Wadsworth, J.C. Knight & P.S. Russell
J. Opt. Soc. Am. B-Opt. Phys. 19 (2002) 2165-2170
Multiple approaches to generate a smooth, powerful, and stable supercontinuum in cobweb photonic-crystal fibers were undertaken by use of 18-fs pulses. These approaches include utilization of incident pulses with various chirp, power, and polarization states, as well as fibers with different lengths and core sizes. For long fibers (tens of centimeters) the supercontinuum contains a finely modulated structure that can be smoothed when the oscillator is in a regime of relaxation oscillations. Short fibers provide a supercontinuum free of gaps. By optimization of these parameters supercontinua exceeding one octave with modulations of less than 10 dB have been generated. (C) 2002 Optical Society of America.

Holey fiber concept spawns optical-fiber renaissance
P. Russell
Laser Focus World 38 (2002) 77-+
Analysis of photonic-crystal fiber has occupied theoreticians worldwide and the list of potential applications grows monthly.

Vortex induced rotation of clusters of localized states in the complex Ginzburg-Landau equation
D.V. Skryabin & A.G. Vladimirov
Phys. Rev. Lett. 89 (2002) 044101
We report existence of a qualitatively distinct class of spiral waves in the two-dimensional cubic-quintic complex Ginzburg-Landau equation. These are stable clusters of localized states rotating around a central vortex core emerging due to interference of the tails of the individual states involved. We also develop an asymptotic theory allowing calculation of the angular frequency and stability analysis of the rotating clusters.

Demonstration of ultra-flattened dispersion in photonic crystal fibers
W.H. Reeves, J.C. Knight, P.S.J. Russell & P.J. Roberts
Opt. Express 10 (2002) 609-613
We demonstrate photonic crystal fibers with ultra-flattened, near zero dispersion. Two micro-structured fibers showing dispersion of 0+/-0.6 ps/nm.km from 1.24 mum-1.44 mum wavelength and 0+/-1.2 ps/nm.km over 1 mum-1. 6 mum wavelength have been measured. (C) 2002 Optical Society of America.

Experimental investigations of the influence of a tapered fibre on the stability of the intermode frequency of highly stable femtosecond pulses
S.N. Bagayev, A.M. Goncharenko, V.F. Zakharyash, V.M. Klementyev, D.B. Kolker, S.A. Kuznetsov, Y.A. Matyugin, V.S. Pivtsov, S.V. Chepurov, T.A. Birks, W.J. Wadsworth & P.S.J. Russell
Quantum Electron. 32 (2002) 639-640
The experimental procedure and the results of high-precision measurements of the intermode frequency of a femtosecond Ti: sapphire laser at the output of a tapered fibre are described. In the process of mode transformation, the stability of the intermode frequency is shown to degrade roughly by a factor of 2 for short averaging time intervals (10 s) and by a factor of 1.1 for long averaging time intervals (1000s).

Structural long-period gratings in photonic crystal fibers
G. Kakarantzas, T.A. Birks & P.S. Russell
Opt. Lett. 27 (2002) 1013-1015
We report what is believed to be the first example of structural long-period gratings written in pure silica photonic crystal fibers (PCFs). The gratings are realized by periodic collapse of the holes of the PCF by heat treatment with a CO2 laser. The resulting periodic hole-size perturbation produces core-to-cladding-mode conversion. These results can lead to a new family of structural all-fiber devices that use the unique properties of PCFs. (C) 2002 Optical Society of America.

Absolute frequency measurement of an I-2 stabilized Nd : YAG optical frequency standard
G.D. Rovera, F. Ducos, J.J. Zondy, O. Acef, J.P. Wallerand, J.C. Knight & P.S. Russell
Meas. Sci. Technol. 13 (2002) 918-922
The frequency of an 1, stabilized Nd:YAG optical frequency standard, developed at the BNM-INM as a national standard for the 'mise en pratique' of the definition of the metre, has been measured with a frequency measurement chain based on a femtosecond laser. A frequency (in-)stability of 2.5 x 10(-13)tau(-1/2) has been measured in the optical domain against a hydrogen maser. The measured (in-)stability decreases to 1 x 10(-14) Without evidence of flicker floor.

Experimental evidence for supercontinuum generation by fission of higher-order solitons in photonic fibers
J. Herrmann, U. Griebner, N. Zhavoronkov, A. Husakou, D. Nickel, J.C. Knight, W.J. Wadsworth, P.S.J. Russell & G. Korn
Phys. Rev. Lett. 88 (2002) 173901
We report on an experimental study of supercontinuum generation in photonic crystal fibers with low-intensity femtosecond pulses, which provides evidence for a novel spectral broadening mechanism. The observed results agree with our theoretical calculations carried out without making the slowly varying envelope approximation. Peculiarities of the measured spectra and their theoretical explanation demonstrate that the reason for the white-light generation in photonic crystal fibers is fission of higher-order solitons into redshifted fundamental solitons and blueshifted nonsolitonic radiation.

Soliton self-frequency shift effects in photonic crystal fibre
D.T. Reid, I.G. Cormack, W.J. Wadsworth, J.C. Knight & P.S.J. Russell
J. Mod. Opt. 49 (2002) 757-767
Using a femtosecond Ti:sapphire laser operating at a wavelength of 810 nm we have demonstrated infrared generation in photonic crystal fibre at distinct wavelengths which can be attributed to the soliton self-frequency shift effect. The maximum observed shift produced spectra centred at 1260 nm and the frequency-shifted light accounted for up to 80% of the fibre output power. We show that the shifts can be explained by the dispersion properties of the fundamental and higher-order waveguide modes of the fibre.

Applied optics: New ways to guide light
J.C. Knight & P.S. Russett
Science 296 (2002) 276-277

Two-octave spectral broadening of subnanojoule Cr : forsterite femtosecond laser pulses in tapered fibers
D.A. Akimov, A.A. Ivanov, M.V. Alfimov, S.N. Bagayev, T.A. Birks, W.J. Wadsworth, P.S. Russell, A.B. Fedotov, V.S. Pivtsov, A.A. Podshivalov & A.M. Zheltikov
Appl. Phys. B-Lasers Opt. 74 (2002) 307-311
Spectral broadening of femtosecond Cr:forsterite laser pulses is enhanced due to the use of tapered fibers, Supercontinuum generation with unamplified subnanojoule femtosecond Cr:forsterite laser pulses is observed for the first time. With 40-fs 0.6-nJ pulses of 1.25-mum Cr:forsterite laser radiation coupled into a tapered fiber having a taper waist diameter of about 2 mum and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber in the regime of anomalous group-velocity dispersion. This result opens the way for the creation of compact femtosecond Cr:forsterite laser plus tapered fiber systems for optical metrology and biomedical applications.

Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers
S. Coen, A.H.L. Chau, R. Leonhardt, J.D. Harvey, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
J. Opt. Soc. Am. B-Opt. Phys. 19 (2002) 753-764
Supercontinuum generation is investigated experimentally and numerically in a highly nonlinear index-guiding photonic crystal optical fiber in a regime in which self-phase modulation of the pump wave makes a negligible contribution to spectral broadening. An ultrabroadband octave-spanning white-light continuum is generated with 60-ps pump pulses of subkilowatt peak power. The primary mechanism of spectral broadening is identified as the combined action of stimulated Raman scattering and parametric four-wave mixing. The observation of a strong anti-Stokes Raman component reveals the importance of the coupling between stimulated Raman scattering and parametric four-wave mixing in highly nonlinear photonic crystal fibers and also indicates that non-phase-matched processes contribute to the continuum. Additionally, the pump input polarization affects the generated continuum through the influence of polarization modulational instability. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate the importance of index-guiding photonic crystal fibers for the design of picosecond and nanosecond supercontinuum light sources. (C) 2002 Optical Society of America.

Two-dimensional clusters of solitary structures in driven optical cavities
A.G. Vladimirov, J.M. McSloy, D.V. Skryabin & W.J. Firth
Phys. Rev. E 65 (2002) 046606
Using analytical and numerical approaches we study clusters of the two-dimensional localized structures of light excited in the externally driven optical cavities. Stability and instability properties of clusters of two, three, and four structures are analyzed in detail. We develop a technique for calculation of the expression for the interaction potential through modified Bessel functions that has applicability going beyond the model under consideration. Qualitative differences between stability properties of triangular and square structures are found that emphasize the role of diagonal interactions in the latter.

Energy of the soliton internal modes and broken symmetries in nonlinear optics
D.V. Skryabin
J. Opt. Soc. Am. B-Opt. Phys. 19 (2002) 529-536
Applications of the concepts of the energy and, in particular, of the negative energy of the internal modes are described in the context of nonlinear optics. It is shown that the transformation of a soliton propagating in a bulk into solitons propagating in a coherently pumped cavity is followed by emergence of the negative-energy modes. These modes are directly associated with a locking of the soliton phase to the phase of the coherent pump, i.e., with a breaking of the phase symmetry. Energy of the Vakhitov-Kolokolov modes is also calculated. This energy is positive, providing that conditions of the Vakhitov-Kolokolov stability criterion are met. Mutual annihilation of the positive- and the negative-energy modes leads to oscillatory (Hopf) instability of the cavity solitons. (C) 2002 Optical Society of America.

Observation of soliton self-frequency shift in photonic crystal fibre
I.G. Cormack, D.T. Reid, W.J. Wadsworth, J.C. Knight & P.S.J. Russell
Electron. Lett. 38 (2002) 167-169
Results from a study in which Raman-shifted outputs were observed from photonic crystal fibres pumped by 800 nm femtosecond pulses are reported. Under optimum conditions. a spectral shift to wavelengths of 1260 nm was recorded.

Internal oscillations of solitons in two-dimensional NLS equation with nonlocal nonlinearity
N.N. Rosanov, A.G. Vladimirov, D.V. Skryabin & W.J. Firth
Phys. Lett. A 293 (2002) 45-49
We develop asymptotic theory to find small eigenfrequencies of two-dimensional bright solitons in the nonlinear Schrodinger equation with weak nonlocality. (C) 2002 Elsevier Science B.V. All rights reserved.

Microstructured glass for optical fibers
J.C. Knight & R.S.J. Russell
Glass Sci. Technol. 75 (2002) 54-62
A new optical fiber technology based on the use of microstructure glass is described. The fibers made using this technology offer new optical physics as well as the possibility of superior performance.

2001 Publications

Femtosecond frequency combs stabilized with a He-Ne/CH4 laser: Toward a femtosecond optical clock
S.N. Bagayev, A.K. Dmitriyev, S.V. Chepurov, A.S. Dychkov, V.M. Klementyev, D.B. Kolker, S.A. Kuznetsov, Y.A. Matyugin, M.V. Okhapkin, V.S. Pivtsov, M.N. Skvortsov, V.F. Zakharyash, T.A. Birks, W.J. Wadsworth, P.S. Russell & A.M. Zheltikov
Laser Phys. 11 (2001) 1270-1282
A concept of high-precision optical frequency measurements involving the use of femtosecond frequency combs with a bandwidths exceeding the frequency of a reference laser source is discussed. This relation between the frequency of the reference laser source and the bandwidth of the frequency comb allows the difference frequency of two modes in a femtosecond frequency comb to be phase-locked to the reference laser. The radio-frequency interval between two adjacent modes in the femtosecond frequency comb is also stabilized in this case, allowing high-precision frequency measurements to be performed in a very simple way. A laser system allowing this principle of high-precision frequency measurements to be implemented includes a mode-locked Ti:sapphire laser, a methane-stabilized He-Ne laser, and a tapered fiber. Special measures have been taken to improve stability characteristics of the laser system. The process of spectral broadening of femtosecond pulse trains in holey and tapered fibers is considered in terms of the Zakharov equation as a result of four-wave mixing of phase-locked equidistant spectral components. The main physical factors influencing the phase distribution and coherence properties of broadly spanning frequency combs produced with the use of this technique are discussed. Phase shifts and spectral distortions arising due to dispersion effects, modulation instabilities, and shock waves of pulse envelopes are explored, and general recipes to reduce the influence of these effects by means of fiber optics are discussed.

Perturbation theory for domain walls in the parametric Ginzburg-Landau equation
D.V. Skryabin, A. Yulin, D. Michaelis, W.J. Firth, G.L. Oppo, U. Peschel & F. Lederer
Phys. Rev. E 64 (2001) 056618
We demonstrate that in the parametrically driven Ginzburg-Landau equation arbitrarily small nongradient corrections lead to qualitative differences in the dynamical properties of domain walls in the vicinity of the transition from rest to motion. These differences originate from singular rotation of the eigenvector governing the transition. We present analytical results on the stability of Ising walls, deriving explicit expressions for the critical eigenvalue responsible for the transition from rest to motion. We then develop a weakly nonlinear theory to characterize the singular character of the transition and analyze the dynamical effects of spatial inhomogeneities.

Nonlinear dynamics of higher-order solitons near the oscillatory instability threshold
K.Y. Kolossovski, A.V. Buryak, D.V. Skryabin & R.A. Sammut
Phys. Rev. E 64 (2001) 056612
Nonlinear theory describing the dynamics of solitons in the vicinity of oscillatory instability threshold with a low frequency offset is developed, The theory is tested on the example of parametric degenerate four-wave mixing. All major predictions of our theory are in agreement with the results of direct numerical modeling. This includes the position of oscillatory instability threshold, instability rates. and various instability development scenarios.

Energy of internal modes of nonlinear waves and complex frequencies due to symmetry breaking
D.V. Skryabin
Phys. Rev. E 64 (2001) 055601
Considering a class of Hamiltonian systems, it is demonstrated that energy of the internal modes with real frequencies supported by nonlinear waves and appearing due to perturbations breaking a continuous symmetry has its sign determined by the symmetry itself, independently of the nature of the perturbations. In particular. it is shown that negative energy modes emerge as a result of the breaking of the phase symmetry in the perturbed nonlinear-Schrodinger equation. An expression for energy of the Vakhitov-Kolokolov internal modes is also derived. Comparative analysis of the energy signs of the internal modes in these two cases explains the ubiquity of instabilities with complex frequencies of solitary and continuous waves in systems with broken phase symmetry.

Fundamental-mode cutoff in a photonic crystal fiber with a depressed-index core
B.J. Mangan, J. Arriaga, T.A. Birks, J.C. Knight & P.S. Russell
Opt. Lett. 26 (2001) 1469-1471
We report a photonic crystal fiber with a depressed-index core doped with fluorine. The effective index of the cladding matches that of the core at an antiguiding wavelength, below which the fiber does not guide light at all. (C) 2001 Optical Society of America.

1-D acoustic cavity in optical fibers using two acoustic Bragg gratings
A. Diez, G. Kakarantzas, T.A. Birks & P.S. Russell
IEEE Photonics Technol. Lett. 13 (2001) 975-977
We report the fabrication and characterization of a one-dimensional acoustic cavity made on a tapered single-mode optical fiber with two acoustic mirrors. The mirrors are acoustic Bragg gratings formed by modulating the fiber diameter periodically. The cavity exhibits resonances with quality factors of around 300. Higher Q factors are achievable by increasing the number of periods in the gratings. A simple transfer matrix model successfully simulates the acoustic properties of the cavity. This type of acoustic cavity is of interest for its potential applications in acoustooptic components and the development of devices based on stimulated Brillouin scattering.

White-light frequency comb generation with a diode-pumped Cr : LiSAF laser
R. Holzwarth, M. Zimmermann, T. Udem, T.W. Hansch, P. Russbuldt, K. Gabel, R. Poprawe, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Opt. Lett. 26 (2001) 1376-1378
We have created a broad spectrum spanning more than an optical octave by launching femtosecond pulses from a battery operated Cr:LiSAF laser into a photonic crystal fiber. Despite the massive broadening in the fiber, the comb structure of the spectrum is preserved, and this frequency comb is perfectly suited for applications in optical frequency metrology. (C) 2001 Optical Society of America.

Absolute frequency measurement of iodine lines with a femtosecond optical synthesizer
R. Holzwarth, A.Y. Nevsky, M. Zimmermann, T. Udem, T.W. Hansch, J. Von Zanthier, H. Walther, J.C. Knight, W.J. Wadsworth, P.S.J. Russell, M.N. Skvortsov & S.N. Bagayev
Appl. Phys. B-Lasers Opt. 73 (2001) 269-271
We have used a single laser femtosecond (.)optical frequency synthesizer together with a widely tunable Nd:YAG laser to measure the absolute frequency of several absorption lines in molecular iodine around 532 nm. The use of two different repetition frequencies allows us to determine the number of modes used for the frequency measurement unambiguously. The lines also provide data for the determination of improved ro-vibrational constants of the iodine molecule.

White-light supercontinuum generation with 60-ps pump pulses in a photonic crystal fiber
S. Coen, A.H.L. Chan, R. Leonhardt, J.D. Harvey, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Opt. Lett. 26 (2001) 1356-1358
The generation of a spatially single-mode white-light supercontinuum. has been observed in a photonic crystal fiber pumped with 60-ps pulses of subkilowatt peak power. The spectral broadening is identified as being due to the combined action of stimulated Raman scattering and parametric four-wave-mixing generation, with a negligible contribution from the self-phase modulation of the pump pulses. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate that ultrafast femtosecond pulses are not needed for efficient supercontinuum. generation in photonic crystal fibers. (C) 2001 Optical Society of America.

Strain-induced phase-matching and tunability of acoustic gratings in fibers
A. Diez, G. Kakarantzas, T.A. Birks & P.S. Russell
Appl. Phys. Lett. 79 (2001) 1390-1392
We study the effect of strain on the properties of a one-dimensional acoustic grating: a tapered optical fiber with a periodically varying diameter. Its spectral response has a strong frequency stop band for flexural waves. Changing the tension of the fiber changes the wave velocity, and so modifies the acoustic response of the grating. The resulting tunability of the stop band over a wide frequency range is demonstrated experimentally. Furthermore, the stop band can almost disappear completely due to a strain-induced phase-matching phenomenon. Such structures are suitable as frequency-selective acoustic mirrors in acousto-optic fiber devices. (C) 2001 American Institute of Physics.

Miniature all-fiber devices based on CO2 laser micro structuring of tapered fibers
G. Kakarantzas, T.E. Dimmick, T.A. Birks, R. Le Roux & P.S.J. Russell
Opt. Lett. 26 (2001) 1137-1139
A focused carbon dioxide laser beam is used to microstructure fibers that have already been narrowed by conventional fiber tapering. We describe three new miniature devices made with this technique: a fused fiber microcoupler with an interaction length of 200 mum, a long-period grating made from a periodic chain of microtapers, and a new type of prolate whispering-gallery mode microcavity. (C) 2001 Optical Society of America.

Simultaneous generation of spectrally distinct third harmonics in a photonic crystal fiber
F.G. Omenetto, A.J. Taylor, M.D. Moores, J. Arriaga, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Opt. Lett. 26 (2001) 1158-1160
By coupling femtosecond pulses at lambda = 1.55 mum in a short length (Z = 95 cm) of photonic crystal fiber, we observe the simultaneous generation of two visible radiation components. Frequency-resolved optical gating experiments combined with analysis and modal simulations suggest that the mechanism for their generation is third-harmonic conversion of the fundamental pulse and its split Raman self-shifted component. (C) 2001 Optical Society of America.

Remotely addressed optical fibre curvature sensor using multicore photonic crystal fibre
W.N. MacPherson, M.J. Gander, R. McBride, J.D.C. Jones, P.M. Blanchard, J.G. Burnett, A.H. Greenaway, B. Mangan, T.A. Birks, J.C. Knight & P.S. Russell
Opt. Commun. 193 (2001) 97-104
We demonstrate an all-fibre curvature sensor that uses two-core photonic crystal fibre (PCF) as the sensing element. The PCF acts as a two-beam interferometer in which phase difference is a function of curvature in the plane containing the cores. A broadband source illuminates both cores, and the spectrum at a single point in the far-field interferogram is recorded. Applying a three-wavelength phase recovery algorithm to the data provides an unambiguous measurement of the interferometer phase, and hence curvature. (C) 2001 Published by Elsevier Science B.V.

Photonic crystal fibres: An endless variety
T.A. Birks, J.C. Knight, B.J. Mangan & P.S. Russell
IEICE Trans. Electron. E84C (2001) 585-592
A photonic crystal fibre has an array of microscopic air holes running along its length. The periodicity of the array is broken by a deliberate "defect" that acts as a waveguide core. Light is confined to this core by the holes. Although some designs of photonic crystal fibre guide light by total internal reflection and so can be considered analogues of conventional optical fibres, their properties can be strikingly different. Other designs guide light by photonic bandgap, confinement and represent a totally new type of fibre.

Photonic crystal fibres: An endless variety
T.A. Birks, J.C. Knight, B.J. Mangan & P.S.J. Russell
IEICE Trans. Commun. E84B (2001) 1211-1218
A photonic crystal fibre has an array of microscopic air holes running along its length. The periodicity of the array is broken by a deliberate "defect" that acts as a wave-guide core. Light is confined to this core by the holes. Although some designs of photonic crystal fibre guide light by total internal reflection and so can be considered analogues of conventional optical fibres, their properties can be strikingly different. Other designs guide light by photonic bandgap confinement and represent a totally new type of fibre.

Room-temperature photoluminescence from erbium-doped multilayer porous silicon microcavity
Y. Zhou, P.A. Snow & P.S. Russell
Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 81 (2001) 40-42
A multilayer microcavity was electrochemically etched on a silicon substrate and doped with erbium. Thermal processing under a nitrogen atmosphere optically activated the erbium-ions. Photo-pumping yielded room temperature emission around 1.54 mum from the erbium-doped samples with the emitted light strongly modified by the microcavity structure. Emission spectra with a peak at 1.536 mum had a full width at half maximum of similar to 6 nm. (C) 2001 Elsevier Science B.V, All rights reserved.

Vertical-cavity surface-emitting resonances in photonic crystal films
J.M. Pottage, E. Silvestre & P.S. Russell
J. Opt. Soc. Am. A-Opt. Image Sci. Vis. 18 (2001) 442-447
It is shown that thin films of dielectric, etched through with a suitably chosen lattice of holes, can support surface-emitting vertical resonances with very-high-cavity Q factors (similar to 10(5) in the case of AlxGaAs1-x on oxidized AlyGaAs1-y). A Bloch-wave expansion is used to develop a complete vector-field analysis of these resonances and to reveal their underlying physics. Since they do not require multilayer mirrors, such resonators are a practical and simple replacement for conventional vertical-cavity surface-emitting laser structures. Other applications include wavelength-division-multiplexing components and highly sensitive gas detectors. (C) 2001 Optical Society of America OCIS codes: 310.6860, 230.3990, 230.5750, 250.7260, 230.3120.

Spectral superbroadening of subnanojoule Cr : Forsterite femtosecond laser pulses in a tapered fiber
D.A. Akimov, A.A. Ivanov, M.V. Alfimov, S.N. Bagayev, T.A. Birks, W.J. Wadsworth, P.S.J. Russell, A.B. Fedotov, V.S. Pivtsov, A.A. Podshivalov & A.M. Zheltikov
Jetp Lett. 74 (2001) 460-463
Spectral superbroadening of subnanojoule femtosecond Cr:forsterite laser pulses is demonstrated for the first time in experiments with a tapered fiber. Coupling 40-fs 0.6-nJ pulses of 1.25-mum Cr:forsterite laser radiation into a tapered fiber with a taper waist diameter of about 2 mum and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber. These experimental results open new horizons for the creation of compact femtosecond systems based on Cr:forsterite lasers and tapered fibers for optical metrology and biomedical applications. (C) 2001 MAIK "Nauka / Interperiodica".

2000 Publications

The effect of thermal processing on multilayer porous silicon microcavity
Y. Zhou, P.A. Snow & P.S. Russell
Phys. Status Solidi A-Appl. Res. 182 (2000) 319-324
The effect of thermal processing, that gave limited oxidation from annealing under N-2 atmosphere. on multilayer porous silicon (P-Si) microcavities has been investigated. The reflectivity spectra were mca;nl-cd before and after thermal processing. The observation of multilayer samples by scanning electron microscopy has shown unexpected expansion and compression of P-Si layers. It is found that thermal processing not only changed the effective refractive indices of the individual porous layers but also roughened their respective interfaces.

Strong modification of photoluminescence in erbium-doped porous silicon microcavities
Y. Zhou, P.A. Snow & P.S.J. Russell
Appl. Phys. Lett. 77 (2000) 2440-2442
A microcavity composed of porous silicon multilayer mirrors was electrochemically etched and doped with erbium. Measurements of the reflectivity and photoluminescence spectra are presented. Thermal processing under a nitrogen atmosphere optically activated the erbium ions. Photopumping yielded room temperature emission around 1.54 mu m from the erbium-doped samples with the emitted light strongly modified by the microcavity structure. Emission spectra with a peak at 1.536 mu m had a full width at half maximum of similar to 6 nm. (C) 2000 American Institute of Physics. [S0003-6951(00)00442-3].

Excitation of cladding modes in photonic crystal fibers by flexural acoustic waves
A. Diez, T.A. Birks, W.H. Reeves, B.J. Mangan & P.S. Russell
Opt. Lett. 25 (2000) 1499-1501
We report the excitation by flexural acoustic waves of an individual cladding mode in a single-mode photonic crystal fiber. The propagation constant and the field distributions of the mode have been investigated by use of this technique. The results give the basis for developing a family of acousto-optic devices based on photonic crystal fibers. (C) 2000 Optical Society of America. OCIS codes: 060.0060, 060.2340, 060.2270.

Supercontinuum generation in tapered fibers
T.A. Birks, W.J. Wadsworth & P.S. Russell
Opt. Lett. 25 (2000) 1415-1417
Supercontinuum light with a spectrum more than two octaves broad (370-1545 nm at the 20-dB level) was generated in a standard telecommunications fiber by femtosecond pulses from an unamplified Ti:sapphire laser. The fiber had been tapered to a diameter of similar to 2 mu m over a 90-mm length. The pulse energy was 3.9 nJ (average power, 300 mW). This source of high-intensity single-mode white light should find widespread applications in frequency metrology and spectroscopy, especially since no unconventional fibers are needed. (C) 2000 Optical Society of America.

Highly birefringent photonic crystal fibers
A. Ortigosa-Blanch, J.C. Knight, W.J. Wadsworth, J. Arriaga, B.J. Mangan, T.A. Birks & P.S.J. Russell
Opt. Lett. 25 (2000) 1325-1327
We report a strongly anisotropic photonic crystal fiber. Twofold rotational symmetry was introduced into a single-mode fiber structure by creation of a regular array of airholes of two sizes disposed about a pure-silica core. Based on spectral measurements of the polarization mode beating, we estimate that the fiber has a beat length of approximately 0.4 mm at a wavelength of 1540 nm, in good agreement with the results of modeling. (C) 2000 Optical Society of America.

Optical frequency synthesizer for precision spectroscopy
R. Holzwarth, T. Udem, T.W. Hansch, J.C. Knight, W.J. Wadsworth & P.S.J. Russell
Phys. Rev. Lett. 85 (2000) 2264-2267
We have used the frequency comb generated by a femtosecond mode-locked laser and broadened to more than an optical octave in a photonic crystal fiber to realize a frequency chain that links a 10 MHz radio frequency reference phase-coherently in one step to the optical region. By comparison with a similar frequency chain we set an upper limit for the uncertainty of this new approach to 5.1 X 10(-16). This opens the door for measurement and synthesis of virtually any optical frequency and is ready to revolutionize frequency metrology.

Compact all-fiber acoustooptic tunable filters with small bandwidth-length product
T.E. Dimmick, G. Kakarantzas, T.A. Birks, A. Diez & P.S. Russell
IEEE Photonics Technol. Lett. 12 (2000) 1210-1212
All-fiber acoustooptic tunable filters (AOTFs) with bandwidth as small as 2.8 nm were fabricated from highly uniform, tapered optical fiber. These filters have typical excess loss of 0.02 dB, greater than 20 dB extinction, are polarization insensitive, and have the smallest bandwidth-length products reported to date.

Yb3+-doped photonic crystal fibre laser
W.J. Wadsworth, J.C. Knight, W.H. Reeves, P.S. Russell & J. Arriaga
Electron. Lett. 36 (2000) 1452-1454
An ytterbium doped core has been incorporated into a photonic crystal fibre (PCF) with anomalous group velocity dispersion at all wavelengths longer than 730nm. Laser action is observed for the first time in such a fibre. This demonstration of lasing in PCF is a vital step towards realising the huge potential of PCF for fibre laser applications.

Design of thin-film photonic crystal waveguides
E. Silvestre, J.M. Pottage, P.S. Russell & P.J. Roberts
Appl. Phys. Lett. 77 (2000) 942-944
We present numerical designs for single-mode leak-free photonic crystal waveguides exhibiting strongly anisotropic spatial and temporal dispersion. These structures may be produced quite simply by drilling regular arrays of holes into thin films of high refractive index, and permit the realization of highly compact optical elements and wavelength division multiplexing devices. (C) 2000 American Institute of Physics. [S0003-6951(00)03933-4].

Experimental study of dual-core photonic crystal fibre
B.J. Mangan, J.C. Knight, T.A. Birks, P.S. Russell & A.H. Greenaway
Electron. Lett. 36 (2000) 1358-1359
The authors report the fabrication and characterisation of photonic crystal ('holey') fibres in which multiple cores are created by filling in selected air-holes with glass. The guidance mechanism is modified total internal reflection. Using a tunable diode laser, the inter-core coupling is investigated as a function of air-hole spacing.

Acousto-optic superlattice modulation in fiber Bragg gratings
P.S. Russell & W.F. Liu
J. Opt. Soc. Am. A-Opt. Image Sci. Vis. 17 (2000) 1421-1429
The superposition of a long-period grating and a fiber Bragg grating, which we call an optical superlattice, causes high-efficiency narrow-band reflections to be induced on either side of the Bragg wavelength. This effect was recently observed experimentally in a fiber-based acousto-optic superlattice modulator. We develop in detail the theory of optical superlattices in fiber Bragg gratings,treating both the acousto-optic and the fixed-grating cases. Applications include reconfigurable wavelength division multiplexers, fiber lasers and sensors, tunable filters, modulators, and frequency shifters. (C) 2000 Optical Society of America [S0740-3232(00)01606-9].

High strain-induced wavelength tunability in tapered fibre acousto-optic filters
A. Diez, G. Kakarantzas, T.A. Birks & P.S. Russell
Electron. Lett. 36 (2000) 1187-1188
A study into the effect of the strain on a tapered fibre acoustooptic filter is presented. The centre wavelengths of the resonances are dramatically affected by strain: > 500 nm tunability range has been obtained for a maximum applied strain of 1.3 x 10(-2).

Anomalous dispersion in photonic crystal fiber
J.C. Knight, J. Arriaga, T.A. Birks, A. Ortigosa-Blanch, W.J. Wadsworth & P.S. Russell
IEEE Photonics Technol. Lett. 12 (2000) 807-809
We describe the measured group-velocity dispersion characteristics of several air-silica photonic crystal fibers with anomalous group-velocity dispersion at visible and near-infrared wavelengths. The values measured over a broad spectral range are compared to those predicted for an isolated strand of silica surrounded by air. We demonstrate a strictly single-mode fiber which has zero dispersion at a wavelength of 700 nm, These fibers are significant for the generation of solitons and supercontinua using ultrashort pulse sources.

Acoustic stop-bands in periodically microtapered optical fibers
A. Diez, G. Kakarantzas, T.A. Birks & P.S. Russell
Appl. Phys. Lett. 76 (2000) 3481-3483
A one-dimensional grating microstructure was fabricated on a tapered optical fiber and its acoustic properties experimentally studied. The structure consisted of a number of concatenated and equally spaced microtapers, fabricated using a focused CO2 laser. Acoustic characterization of the device showed a strong frequency stop-band for flexural waves. Such structures are suitable as frequency-selective acoustic mirrors in acousto-optic fiber devices. (C) 2000 American Institute of Physics. [S0003-6951(00)01423-6].

Two-dimensional bend sensing with a single, multi-core optical fibre
P.M. Blanchard, J.G. Burnett, G.R.G. Erry, A.H. Greenaway, P. Harrison, B. Mangan, J.C. Knight, P.S. Russell, M.J. Gander, R. McBride & J.D.C. Jones
Smart Mater. Struct. 9 (2000) 132-140
Measurement of two-dimensional bending in a structural element using intrinsic optical fibre strain gauges would normally require three sensors to be attached to, or embedded within, the structure. The same measurement can now be made using a single multi-core optical fibre, reducing deployment cost and increasing practicality. Fabrication of a novel three-core photonic crystal fibre is described. The ability of the fibre sensor to measure bend in two dimensions is demonstrated in the laboratory using interferometric interrogation at a single wavelength. Deployment of the sensor to measure the deformation of a bridge undergoing loading trials is described.

High-resolution measurement of the fiber diameter variations using whispering gallery modes and no optical alignment
T.A. Birks, J.C. Knight & T.E. Dimmick
IEEE Photonics Technol. Lett. 12 (2000) 182-183
Light is coupled from the guided mode along one tapered optical fiber into the whispering gallery modes around the circumference of another, Small diameter variations in the second fiber are measured by tracking, at different points along it, the wavelengths at which these modes exist. The measurement does not require any optical alignment, and its resolution can be 1 part in 10 000 or better.

Direct measurement of optical phase in the near field
P.L. Phillips, J.C. Knight, J.M. Pottage, G. Kakarantzas & P.S.J. Russell
Appl. Phys. Lett. 76 (2000) 541-543
To fully characterize photonic crystal guided wave optical devices, one needs to measure the spatial variation of both the phase and amplitude of the electromagnetic field. In this work, we simultaneously measure the intensity and phase in the near field of both propagating and evanescent fields by incorporating a scanning near-field optical microscope into one arm of a Mach-Zehnder interferometer. We demonstrate the technique by imaging the phase fronts of an evanescent wave formed by total internal reflection and by measuring the phase variation in the LP11 mode in an overmoded optical fiber. (C) 2000 American Institute of Physics. [S0003-6951(00)00405-8].

Soliton effects in photonic crystal fibres at 850 nm
W.J. Wadsworth, J.C. Knight, A. Ortigosa-Blanch, J. Arriaga, E. Silvestre & P.S.J. Russell
Electron. Lett. 36 (2000) 53-55
Soliton effects are observed at 850nm in a pure silica photonic crystal fibre with group velocity dispersion (GVD) characteristics unattainable in conventional fibre. Zero GVD is obtained at 740nm.

Light emission from highly reflective porous silicon multilayer structures
E.K. Squire, P.A. Snow, P.S.J. Russell, L.T. Canham, A.J. Simons, C.L. Reeves & D.J. Wallis
J. Porous Mat. 7 (2000) 209-213
Porous silicon photoluminescence and electroluminescence can be controlled by periodically modulating the material porosity to form high quality multilayer stacks and microcavities. Important issues not yet fully addressed are (a) the precise role played by this microstructuring, given that the luminescence is distributed throughout the entire structure and that the low porosity layers are highly absorbing at short wavelengths, and (b) whether the quality of such microcavities could be sufficient to support lasing. Using both experimental and theoretical techniques, the emission and reflection properties of different porous silicon single and multilayer structures have been investigated in order to understand further and exploit the nature of light propagation within them.

1999 Publications

Kilohertz pulse repetition frequency slab Ti : sapphire lasers with high average power (10 W)
W.J. Wadsworth, D.W. Coutts & C.E. Webb
Appl. Optics 38 (1999) 6904-6911
High-average-power broadband 780-nm slab Ti:sapphire lasers, pumped by a kilohertz pulse repetition frequency copper vapor laser (CVL), were demonstrated. These lasers are designed for damage-free power scaling when pumped by CVL's configured for maximum output power (of order 100 W) but with poor beam quality (M-2 similar to 300). A simple Brewster-angled slab laser side pumped by a CVL produced 10-W average power (1.25-mJ pulses at 8 kHz) with 4.2-ns FWHM pulse duration at an absolute efficiency of 15% (68-W pump power). Thermal lensing in the Brewster slab laser resulted in multitransverse mode output, and pump absorption was limited to 72% by the maximum doping level for commercially available Ti:sapphire (0.25%). A Slab laser with a multiply folded zigzag path was therefore designed and implemented that produced high-beam-quality (TEM00-mode) output when operated with cryogenic cooling and provided a longer absorption path for the pump. Excessive scattering of the Ti:sapphire beam at the crystal surfaces limited the efficiency of operation for the zigzag laser, but fluorescence diagnostic techniques, gain measurement, and modeling suggest that efficient power extraction (>15 W TEM00 >23% efficiency) from this laser would be possible for crystals with an optical quality surface polish. (C) 1999 Optical Society of America. OCIS codes: 140.3580, 140.3600, 140.3590, 140.6810.

Carbon dioxide laser fabrication of fused-fiber couplers and tapers
T.E. Dimmick, G. Kakarantzas, T.A. Birks & P.S. Russell
Appl. Optics 38 (1999) 6845-6848
We report the development of a fiber taper and fused-fiber coupler fabrication rig that uses a scanning, focused, CO2 laser beam as the heat source. As a result of the pointlike heat source and the versatility associated with scanning, tapers of any transition shape and uniform taper waist can be produced. Tapers with both a linear shape and an exponential transition shape were measured. The taper waist uniformity was measured and shown to be better than +/- 1.2%. The rig was also used to make fused-fiber couplers. Couplers with excess loss below -0.1 dB were routinely produced. (C) 1999 Optical Society of America OCIS codes: 060.2310, 060.1810, 060.2340, 220.0220, 220.4610.

Distribution of spontaneous emission from an Er3+-doped photonic crystal fiber
R.F. Cregan, J.C. Knight, P.S. Russell & P.J. Roberts
J. Lightwave Technol. 17 (1999) 2138-2141
This paper studies the emission of 532-nm light through the photonic crystal cladding of an optically pumped Er3+-doped silica/air photonic crystal fiber. The angular emission pattern is compared with the results of a multiple scattering simulation. Good agreement is obtained.

Hamiltonian optics of nonuniform photonic crystals
P.S. Russell & T.A. Birks
J. Lightwave Technol. 17 (1999) 1982-1988
The passage of light through slowly varying nonuniform photonic crystal structures is treated using Hamiltonian optics. The approach allows the inverse design of complex and highly compact optical elements as well as providing insight and an appealing physical picture. It can also be used to treat the trapping and deflection of light at distributed defects in one- and two-dimensional (2-D) crystals, and to study the frequency dependent time delay in chirped one-dimensional (1-D) fiber Bragg gratings (FBG's).

Localized function method for modeling defect modes in 2-D photonic crystals
D. Mogilevtsev, T.A. Birks & P.S. Russell
J. Lightwave Technol. 17 (1999) 2078-2081
We present a novel, general, and numerically efficient treatment of electromagnetic modes localized at defects in two-dimensional (2-D) photonic crystals. The method represents the fields in terms of orthogonal functions localized at the defect and is a fully vector treatment.

Single-mode photonic band gap guidance of light in air
R.F. Cregan, B.J. Mangan, J.C. Knight, T.A. Birks, P.S. Russell, P.J. Roberts & D.C. Allan
Science 285 (1999) 1537-1539
The confinement of light within a hollow core;(a Large air hole) in a silica-air photonic crystal fiber is demonstrated. Only certain wavelength bands are confined and guided down the fiber, each band corresponding to the presence of a full two-dimensional band gap in the photonic crystal cladding. Single-mode vacuum waveguides have a multitude of potential applications from ultrahigh-power transmission to the guiding of cold atoms.

Vapor sensing using the optical properties of porous silicon Bragg mirrors
P.A. Snow, E.K. Squire, P.S.J. Russell & L.T. Canham
J. Appl. Phys. 86 (1999) 1781-1784
Large wavelength shifts have been measured in the reflectivity spectra of Bragg mirrors etched in porous silicon after exposure of the mirrors to vapor from organic solvents. The shift in the Bragg wavelength of the mirror arises from refractive index changes, induced by capillary condensation of the vapor in the mesoporous silicon, in the layers of the mirrors. Modeling of the reflectivity changes shows that the layer liquid volume fraction occurring in the measurements was 0.29 for acetone and 0.33 for chlorobenzene. Time-resolved measurements show that condensation occurs on the time scale of tens of seconds. (C) 1999 American Institute of Physics. [S0021-8979(99)03416-7].

Measurement of the wavelength dependence of beam divergence for photonic crystal fiber
M.J. Gander, R. McBride, J.D.C. Jones, T.A. Birks, J.C. Knight, P.S. Russell, P.M. Blanchard, J.G. Burnett & A.H. Greenaway
Opt. Lett. 24 (1999) 1017-1019
We report measurements of the wavelength dependence of beam divergence for single-mode photonic crystal fiber. These measurements confirm predictions of strongly wavelength-dependent beam divergence, consistent with the effective-index model for the photonic crystal cladding material.

Dispersion compensation using single-material fibers
T.A. Birks, D. Mogilevtsev, J.C. Knight & P.S. Russell
IEEE Photonics Technol. Lett. 11 (1999) 674-676
The properties of photonic crystal fibers with large air holes can be modeled by a silica rod in air. Such approximate calculations show that the dispersion of photonic crystal fibers could exceed -2000 ps/nm/km, or they could compensate (to within +/-0.2%) the dispersion of 35 times their length of standard fiber over a 100-nm range.

Fluorescence and absorption spectra of rare earth metaphosphate glasses
T. Brennan, J.C. Knight & G.A. Saunders
Phys. Chem. Glasses 40 (1999) 113-121
The rare earth elements (R) form a series of highly stable glasses with compositions in the vicinity of (R2O3)(0.25)(P2O5)(0.75) which include the most magnetic oxide glasses yet found and in consequence show a wide variety of fundamentally interesting and useful magneto-optical properties. To provide detailed comparative information of the optical properties of the series of rare earth doped metaphosphate glasses needed for device applications the absorption and fluorescence spectra of glasses modified with Pr Nd, Sm, Eu, Tb, Dy, Ho and Er have been determined over the wavelength range 400-1750 nm. By comparing the measured transition wavelengths with those found in the optical spectra known for other hosts of rare earth ions, it is possible to account for virtually all the observed optical features. To understand further the magnetic and magneto-optic properties of these glasses, it is necessary to extend knowledge of the valence states of rare earth ions in this type of host. The optical spectra show that the 4f transitions assigned to the peaks correspond to the trivalent state alone for all of the ions.

Near-field optical microscopy of thin photonic crystal films
P.L. Phillips, J.C. Knight, B.J. Mangan, P.S.J. Russell, M.D.B. Charlton & G.J. Parker
J. Appl. Phys. 85 (1999) 6337-6342
Near-field optical microscopy is used to image the light propagating in a Si3N4-on-silica film with a strip of deeply etched submicron holes in the center. Fringe patterns created by interference between incident and diffracted guided modes are observed in the smooth waveguide surrounding the large air-hole photonic film. Observation of the fields in the photonic crystal regions indicates that out-of-plane scattering at the smooth/periodic transitions and leakage caused by grating coupling play major roles. The effects of different air-hole sizes and lattice pitches are also explored. (C) 1999 American Institute of Physics. [S0021-8979(99)01809-5].

Designing a photonic crystal fibre with flattened chromatic dispersion
A. Ferrando, E. Silvestre, J.J. Miret, J.A. Monsoriu, M.V. Andres & P.S. Russell
Electron. Lett. 35 (1999) 325-327
Using a full-vector modal method, the authors have identified a region of nearly zero flattened chromatic dispersion in a specially designed photonic crystal fibre. The approach permits an accurate control of the dispersion features of these fibres in terms of their structural parameters.

Full photonic bandgaps and spontaneous emission control in 1D multilayer dielectric structures
P.S.J. Russell, S. Tredwell & P.J. Roberts
Opt. Commun. 160 (1999) 66-71
It is shown that a layer of low refractive index, sandwiched between two multilayer stacks made from commonly used. dielectric materials, can be designed to be completely free of all modes within a band of frequencies, i.e., to support a species of full photonic bandgap. In a slightly thicker layer, only one mode is supported. For in-plane electric field polarisation and a radiating dipole placed in the centre of the layer, the spontaneous emission coupling ratio is beta(SE) = 0.96, very high for such a simple structure. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Experimental measurement of group velocity dispersion in photonic crystal fibre
M.J. Gander, R. McBride, J.D.C. Jones, D. Mogilevtsev, T.A. Birks, J.C. Knight & P.S. Russell
Electron. Lett. 35 (1999) 63-64
Thr authors report measurements of group velocity dispersion in photonic crystal fibre using low coherence techniques. The results confirm theoretical predictions that photonic crystal fibre, unlike conventional step-index fibre can exhibit anomalous waveguide dispersion while remaining singlemode, This allows the: design of singlemode fibres with zero dispersion points at wavelengths much shorter than is possible in standard fibre.

Photonic crystals as optical fibres - physics and applications
J.C. Knight, T.A. Birks, R.F. Cregan, P.S.J. Russell & J.P. de Sandro
Opt. Mater. 11 (1999) 143-151
We describe the fabrication, characterisation and possible applications of a new type of optical material - a 2-dimensional photonic crystal made of silica and air. This macroporous silica, which is made using the technology of optical fire fabrication, has properties which differ remarkably from those of conventional materials. We describe how such a material can be used to form waveguides with new and unusual properties, and present some results to illustrate this. Such photonic crystal waveguides could be significant in the design of new lasers and amplifiers. (C) 1999 Elsevier Science B.V. All rights reserved.

1998 Publications

Analysis and design of an endlessly single-mode finned dielectric waveguide
E. Silvestre, P.S. Russell, T.A. Birks & J.C. Knight
J. Opt. Soc. Am. A-Opt. Image Sci. Vis. 15 (1998) 3067-3075
We report what is to our knowledge a novel dielectric waveguiding structure consisting of periodic arrays of parallel planar fins placed on both sides of and perpendicular to a central planar core. This structure is analyzed, and the behavior of its bound modes is investigated. This finned guide exhibits many of the same features as a recently reported all-silica photonic crystal fiber. For example, it can be designed to support only the fundamental transverse bound mode over the entire electromagnetic frequency spectrum. This new class of endlessly single-mode waveguide may prove to have important applications in many areas of optoelectronics. (C) 1998 Optical Society of America [S0740-3232(98)00712-1].

Light emission from porous silicon single and multiple cavities
E.K. Squire, P.A. Snow, P.S. Russell, L.T. Canham, A.J. Simons & C.L. Reeves
J. Lumines. 80 (1998) 125-128
Experimental and theoretical techniques are used to examine the effects of microstructuring on the optical properties of multilayer, single and multiple microcavity structures fabricated from porous silicon. Measurements of the reflectivity and photoluminescence spectra of three multilayer samples are presented. The results are modelled using a transfer matrix technique including a negative absorption term to represent the effect of spontaneous emission which gives luminescence. The emitted light is strongly controlled by the optical modes of the structures and very good agreement is observed between theory and experiment. (C) 1999 Elsevier Science B.V. All rights reserved.

Photonic band cap guidance in optical fibers
J.C. Knight, J. Broeng, T.A. Birks & P.S.J. Russel
Science 282 (1998) 1476-1478
A fundamentally different type of optical waveguide structure is demonstrated, in which light is confined to the vicinity of a Low-index region by a two-dimensional photonic band gap crystal, The waveguide consists of an extra air hole in an otherwise regular honeycomb pattern of holes running down the length of a fine silica glass fiber. Optical fibers based on this waveguide mechanism support guided modes with extraordinary properties.

Highly increased photonic band gaps in silica/air structures
J. Broeng, S.E. Barkou, A. Bjarklev, J.C. Knight, T.A. Birks & P.S. Russell
Opt. Commun. 156 (1998) 240-244
We explore the possibilities of achieving larger out-of-plane band gaps in two-dimensional silica/air photonic crystals with hexagonal symmetry. By modification of the basic hexagonal unit-cell, we demonstrate a new photonic crystal structure, for which the size of the band gaps is increased several times compared to those of simple hexagonal structures. The new structure allows design of silica/air photonic crystals exhibiting complete out-of-plane two-dimensional photonic band gaps for realistic fabrication parameters. We propose a new design of photonic crystal fibers based on the modified hexagonal structure. The advantages of the new design over recently fabricated photonic crystal fibers are discussed, and a calculation of a guided mode, localized at a low-index core region, is presented. (C) 1998 Elsevier Science B.V. All rights reserved.

Group-velocity dispersion in photonic crystal fibers
D. Mogilevtsev, T.A. Birks & P.S. Russell
Opt. Lett. 23 (1998) 1662-1664
The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 mu m when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 mu m. (C) 1998 Optical Society of America.

100% efficient narrow-band acoustooptic tunable reflector using fiber Bragg grating
W.F. Liu, P.S. Russell & L. Dong
J. Lightwave Technol. 16 (1998) 2006-2009
This paper reports a 100% efficient Bragg-grating-based acoustooptic superlattice modulator in a fiber whose diameter is reduced by HF etching. The acoustically induced reflections, which appear on both sides of the Bragg stop-band, are tunable by altering the acoustic frequency, and have bandwidths corresponding to weak versions of the permanent Bragg grating, These unique properties may lead to important applications in, e,g,, reconfigurable wavelength division multiplexing, Q-switching, amplitude modulation, and frequency shifting.

Optimized light emission from layered porous silicon structures
E.K. Squire, P.S. Russell & P.A. Snow
Appl. Optics 37 (1998) 7107-7111
We discuss in detail the physics of light emission from porous silicon microcavities formed by periodically modulating the porosity to produce multilayered structures. Changing the porosity alters not only the refractive index and absorption but also the luminescence, resulting in a complex interplay of effects that has not yet been addressed in the Literature as far as we know. A transfer matrix model is developed that accounts for the dispersion of the refractive index, absorption, and photoluminescence. A multilayer porous silicon mirror is found to emit light almost as well as a conventional distributed feedback microcavity system with a mid-stop-band resonant state. (C) 1998 Optical Society of America.

Large mode area photonic crystal fibre
J.C. Knight, T.A. Birks, R.F. Cregan, P.S. Russell & J.P. de Sandro
Electron. Lett. 34 (1998) 1347-1348
The authors report the realisation of a new design for a large mode area monomode optical fibre. This photonic crystal fibre will guide only a single mode, no matter how large the Fibre diameter. provided the shape is kept constant. This is demonstrated with a fibre which has a core diameter equal to approximately 50 free-space wavelengths.

Properties of photonic crystal fiber and the effective index model
J.C. Knight, T.A. Birks, P.S.J. Russell & J.P. de Sandro
J. Opt. Soc. Am. A-Opt. Image Sci. Vis. 15 (1998) 748-752
We report the waveguiding properties of a new type of low-loss optical waveguide. The photonic crystal fiber can be engineered to support only the fundamental guided mode at every wavelength within the transparency window of silica. Experimentally, a robust single mode has been observed over a wavelength range from 337 nm to beyond 1550 nm (restricted only by available wavelength sources). By studying the number of guided modes for fibers with different parameters and the use of an effective index model. we are able to quantify the requirements for monomode operation. The requirements are independent of the scale of the fiber for sufficiently short wavelengths. Further support for the predictions of the effective index model is given by the variation of the spot size with wavelength. (C) 1998 Optical Society of America.

Bragg scattering from an obliquely illuminated photonic crystal fiber
J.C. Knight, T.A. Birks, P.S.J. Russell & J.G. Rarity
Appl. Optics 37 (1998) 449-452
Scattering at visible frequencies from a two-dimensional silica/air photonic crystal material in the form of a fine fiber reveals the hexagonal crystal structure of the material. Oblique illumination allows the observation of first-order Bragg conditions even for a crystal structure with a pitch several times the wavelength of light. These scattering measurements demonstrate the feasibility of a low-loss waveguide based on photonic bandgap effects. (C) 1998 Optical Society of America.

Silica/air photonic crystal fibres
P.S.J. Russell, T.A. Birks, J.C. Knight, R.F. Cregan, B. Mangan & J.P. De Sandro
Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 37 (1998) 45-48
We describe the fabrication, characterisation and applications of silica/air photonic crystal fibres with microscopic arrays of air capillaries running along their length.

1997 Publications

Focused acoustic wave acousto-optic device using a planar domain-inverted lithium niobate transducer
A. Lawrow, C.N. Pannell, M. Negoita, P.S. Russell & J. Webjorn
Opt. Commun. 144 (1997) 161-164
An acousto-optic device operating in the Raman-Nath regime has been built and tested to investigate the potential benefits of using planar domain-inverted lithium niobate transducers to generate complex acoustic fields in acousto-optic (AO) devices. To our knowledge, this is the first example of a domain-inverted LiNbO3 transducer used to produce a focused acoustic field in an AO application. (C) 1997 Elsevier Science B.V.

Acousto-optic superlattice modulator using a fiber Bragg grating
W.F. Liu, P.S.J. Russell & L. Dong
Opt. Lett. 22 (1997) 1515-1517
A modulator is reported in which an extensional acoustic wave is launched along a fiber Bragg grating. The acousto-optic superlattice effect causes an enhancement in reflectivity within a narrow spectral region on both sides of the Bragg wavelength. For a fixed acoustic propagation direction, the Doppler shift can be either positive or negative, depending on whether the wavelength of the incident light lies above or below the Bragg condition. The device can function as a Bragg cell and a tunable filter. (C) 1997 Optical Society of America.

Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper
J.C. Knight, G. Cheung, F. Jacques & T.A. Birks
Opt. Lett. 22 (1997) 1129-1131
We show that high-Q whispering-gallery modes in fused-silica microspheres can be efficiently excited by an optical fiber taper. By adjusting the taper diameter to match the propagation constant of the mode in the taper with that of the resonant mode of interest, one can couple more than 90% of the light into the sphere. This represents a significant improvement in excitation efficiency compared with other methods and is, we believe, the most efficient excitation of a high-Q microcavity resonance by a monomode optical fiber yet demonstrated. (C) 1997 Optical Society of America.

Glass fiber poling and applications
P.G. Kazansky, P.S. Russell & H. Takebe
J. Lightwave Technol. 15 (1997) 1484-1493
Recent developments in the application of poled optical fibers to second harmonic generation and electrooptic Light modulation are reviewed.

Special issue on fiber gratings, photosensitivity, and poling - Foreword
K.O. Hill, P.S. Russell, G. Meltz & A.M. Vengsarkar
J. Lightwave Technol. 15 (1997) 1261-1262

Endlessly single-mode photonic crystal fiber
T.A. Birks, J.C. Knight & P.S. Russell
Opt. Lett. 22 (1997) 961-963
We made an all-silica optical fiber by embedding a central core in a two-dimensional photonic crystal with a micrometer-spaced hexagonal array of air holes. An effective-index model confirms that such a fiber can be single mode for any wavelength. Its useful single-mode range within the transparency window of silica, although wide, is ultimately bounded by a bend-loss edge at short wavelengths as well as at long wavelengths. (C) 1997 Optical Society of America.

The acousto-optic effect in single-mode fiber tapers and couplers (vol 14, pg 2519, 1996)
T.A. Birks, P.S.J. Russell & D.O. Culverhouse
J. Lightwave Technol. 15 (1997) 731-731

Nonlinear dynamics of a backward quasi-phase-matched second-harmonic generator
G. DAlessandro, P.S. Russell & A.A. Wheeler
Phys. Rev. A 55 (1997) 3211-3218
We study the stability of the interaction between a pump wave and a counter propagating second-harmonic wave phase-matched in a periodically poled chi((2)) material using both analytical and numerical methods. In contrast to the more usual copropagating case, backward phase-matched frequency doubling displays richer and more complex behavior, owing to the presence of built-in feedback.

3x3 all-fiber routing switch
D.O. Culverhouse, T.A. Birks, S.G. Farwell & P.S.J. Russell
IEEE Photonics Technol. Lett. 9 (1997) 333-335
An all-fiber acoustooptic switch has been constructed from a 3 x 3 null fused coupler. The maximum drive power required is 4 mW, the switching time is 100 mu s and the insertion loss is less than 0.5 dB and has a cross talk of 20 dB. The switch shares the advantages of an earlier 2 x 2 switch but with more fiber ports, and should permit the design of compound routing arrays with fewer switching elements per channel.

1996 Publications

40-MHz all-fiber acoustooptic frequency shifter
D.O. Culverhouse, T.A. Birks, S.G. Farwell, J. Ward & P.S. Russell
IEEE Photonics Technol. Lett. 8 (1996) 1636-1637
We report the first demonstration of a 40 MHz four-port alt-fiber frequency shifter operating at a wavelength of 1.55 mu m. The device has a high-conversion efficiency and low-drive power consumption, and is made from standard telecommunications fiber.

The acousto-optic effect in single-mode fiber tapers and couplers
T.A. Birks, P.S.J. Russell & D.O. Culverhouse
J. Lightwave Technol. 14 (1996) 2519-2529
All-fiber acousto-optic devices based on the null fused taper coupler have been successfully demonstrated as frequency shifters, variable splitters, switches and tunable filters, In this paper, the interaction upon which these devices are based has been extensively analyzed under a set of approximations that are valid in most cases, Simple analytical expressions for the important properties are derived, which provide a set of design rules for such devices.

Cylindrical metal-coated optical fibre devices for filters and sensors
A. Diez, M.V. Andres, D.O. Culverhouse & T.A. Birks
Electron. Lett. 32 (1996) 1390-1392
Novel fibre-optic components suitable for sensor applications and wavelength filters are reported. The devices consist of a tapered fibre whose uniform waist has been coated with a thin layer of gold. The operation principle is the resonant excitation of a surface plasma mode of the metal film.

Photonic crystals: Molding the flow of light - Joannopoulos,JD, Meade,RD, Winn,JN
P.S.J. Russell
Nature 381 (1996) 290-290