Fibre transitions and tapering
A vanishingly small proportion of possible optical fibre structures are uniform along their length. We're interested in studying the rest. Fibre tapering is a convenient and effective way of dramatically varying the nature of a fibre waveguide, while introducing hardly any loss of light. The fibre is placed in a "taper rig", where two motorised stages stretch the fibre while a part of it is heated by a small flame. The heated part therefore narrows to form a waist that is connected to untreated fibre ends by taper transitions. The whole resulting structure is called a tapered fibre, or (more loosely) a taper.
The tapering process.
The taper rig at Bath is home-made and very versatile. Taking advantage of a modification of the "flame brush" technique [T.A.Birks, Y.W.Li: "The shape of fiber tapers", IEEE Journal of Lightwave Technology, 10, April 1992, pp.432-438.], whereby the flame is brushed to and from along the fibre while it is stretched, we can form taper transitions and waists of almost any shape and size, with waist diameters anywhere from the original fibre diameter down to 100 nm, lengths up to 10 cm and negligible (less than 0.02 dB) optical losses. We can also use a carbon-dioxide laser beam as an alternative heat source capable of modifying the fibre on a longitudinal scale of a few 100 µm.
Schematic diagram of the tapering rig.
To give an idea of the sizes involved, the four photos in the following figure are all to the same scale. Top-left is the end of a typical single-mode fibre (Corning SMF-28), with an outer diameter of 125 µm (1/8 mm) and a 9 µm central core. Top-right is a side view of a 2 µm diameter taper waist - that is, the entire fibre has been reduced in diameter by a factor of about 60. It is remarkable that the waveguide can be changed so drastically without losing more than a percent or so of the light travelling from end to end. The lower pictures show human blood cells and human hair: the taper waist is about as big as spider silk.
Tapers - see text for details.
If the taper waist is less than about 20-30 µm in diameter, the light spreads out from the shrinking fibre core to fill the entire fibre as it propagates along the input transition and into the waist. The opposite happens at the output, the light being recaptured by the growing fibre core.
The unusual properties of the glass-air waveguide in the waist of a tapered single-mode fibre (and the ability to get light into it conveniently and with low loss) have inspired some of our key research projects. The large refractive index step between glass and air allows the waist to be an effective waveguide even when the diameter is as small as 1 µm or even less, and also dramatically modifies its dispersion. This gives the waist some extreme nonlinear optical properties. Guidance by the outer boundary of the fibre allows the light to interact with the surrounding medium via the evanescent field.
Tapering of photonic crystal fibres
The tapering process can be applied to PCFs as well as conventional fibres. However, whereas conventional fibres can only do one thing when they are heated and stretched - they just reduce in transverse scale - in a PCF the holes can simultaneously shrink further under the influence of surface tension. The relative extent to which the two processes ((a) drawdown and (b) hole collapse) occur depends on the speed of elongation and the temperature to which the glass is heated. This additional degree of freedom makes the tapering process more difficult to control, but also makes new types of waveguide transition possible.
Tapering can help solve the interfacing problem: how to get the light in and out of PCFs from existing optical systems (eg conventional fibres or free-space optics) with low loss and acceptable stability. This is a major impediment to the uptake of PCFs as a viable technology, and is exemplified by the problem of coupling between highly-nonlinear PCF, with a core of typically about 2 µm diameter, and conventional single-mode fibre with a core diameter of 9 µm. The mode mismatch means that a simple splice between these two fibres will have a loss of more than 10 dB.
Straightforward tapering of a PCF can be used to make PCF sections with submicron cores without much reduction of the air-filling fraction, and also to couple light into them. Indeed, pressurising the air holes while tapering inflates the holes and increase the air-filling fraction while reducing the core size. The ferrule technique is our most versatile, and can be used to form splice-free interfaces between conventional fibres and almost any type of index-guiding PCF, including multicore fibres.