Summary
Team:
Dr. Wei Ding (CPPM), Dr. Steve Andrews (Dept. of Physics), Dr. Stefan Maier (CPPM)
Fiber tapering produces an ideal transition from a buried waveguide of the un-tapered fiber to an open waveguide of micron-scale taper waist, with low excess loss and conservation of cylindrical symmetry. Exploiting this convenient access to the evanescent field of the guided wave, we try to introduce longitudinal structures along fiber links. Two motivations for this research work are:
1. Applications of fiber tapers are widely enriched: Longitudinal structures allow control over the properties of light propagating along the fiber. Longitudinal mode coupling, not only the transverse evanescent coupling, dramatically widens the applications of fiber tapers;
2. Planar processing techniques are introduced: This way, versatile structures, rather than only UV-induced index modulations, can be formed along the fiber. The ranges of index contrast and structure geometry are broadened.
However, the challenge of this research is manifest: how to transplant planar processing techniques, which are usually compatible with large-sized planar wafers, to the curved surface of a micron-scale fiber taper? The following examples represent our explorations in this direction.
Fibre tapers decorated with Au surface Bragg gratings
Using interference lithography and metal deposition/lift-off techniques, periodic Au strips with a period of ~0.5µm have been produced on the top side of a ~10µm-diameter fiber taper, as schematically shown in Fig. 1a. Fig. 1b is the corresponding scanning electron micrograph. The high dielectric contrast between the grating material (Re {εAu} ~ -60 in near infrared) and the environment results in a strong mode coupling even with a small field overlap. This is verified by the measured transmission spectrum as shown in Fig. 1c, where the dips on the right represent the couplings to the backward guided modes, and the dip on the left is induced by the radiation mode coupling.
Figure 1
When this Au grating fiber taper is immersed into a high index medium, the transmission dips move towards longer wavelengths, as shown in Fig. 2a and 2b. The evolution of the resonant wavelengths of various mode couplings with external medium index are plotted in Fig. 2c. The high sensitivity of the radiation mode coupling to external medium index suggests the application of this device in refractometric sensing. Another merit of this refractometric sensing is its wide working range (εEx ~ [1, 1.42]).
Figure 2
Fibre tapers with surface corrugation Bragg gratings
Using interference lithography and reactive ion etching techniques, we also produced corrugation Bragg gratings on the surface of fiber tapers as shown in Fig. 3a and Fig. 3b. Although the cylindrically asymmetric gratings in the taper waist can couple the forward fundamental mode to backward high-order modes, these components are filtered into highly lossy cladding modes in the un-tapered fiber via an adiabatic taper transition. The reflection spectrum in Fig, 3c illustrates this function. Therefore, this grating fiber taper can be used as a reflection-based fiber filter, which works in a semi single-mode model.
Figure 3
In order to enrich the spectral property of this grating fiber taper, we keep the grating period constant and vary the taper diameter profile. This method is much easier in fabrication than to write a period-variable grating along a fiber. Fig. 4 shows the SEM measured taper diameter profiles (a and b) and corresponding measured and calculated reflection spectra. (c and d). The manipulation to the spectral characteristics of a fiber filter device can be obtained by controlling the shape of the fiber taper using the mature fiber tapering technique.
Figure 4
