Photonic crystal fibre (PCF) is an optical fibre (OF), whose shell has the structure of a two-dimensional photonic crystal.

Such structures open up new opportunities for controlling the dispersion properties of fibres in a wide range and the degree of localisisation of electromagnetic radiation in the guided wave modes.

In most cases, PCFs are made of glass or quartz with air-filled holes. The holes may be partially filled with other gases or liquids, including liquid crystals. Less common are PCFs made of two different kinds of glass with considerably differing refractive indices.

Sometimes the term “photonic crystal fibre” is used in a broader sense to refer to almost all types of fibres with a complex shell structure, including microstructured and nanostructured fibres, Bragg fibres, and holey fibres.

By the physical mechanism of light confinement within the fibre core all PCFs can be divided into two broad classes.

The first class includes PCFs, in which the light is localisised in the core due to the mirror reflection from the shell having photonic bandgaps (PBG). It is particularly important that the PCF core with PBGs may be hollow, thus allowing an increase in the introduced radiation power by several orders, as well as reducing losses and nonlinear effects.

The mechanism of light confinement in the PCFs of the second class is quite traditional for the optical fibre, i.e. based on total internal reflection. However, they use a new principle of managing the refractive index of the shell depending on its structure. The possibility of controlling the refractive index of the shell provides for creating the so-called unlimited single-mode fibre. They have only one mode propagating at any wavelength. Another feature of PCFs is that the single-mode operation can be implemented in large core diameter fibres.

PCFs with air holes are manufactured by high temperature drawing from a preform made of round or hexagonal hollow tubes. The holes may be filled with different substances to control the PCF properties. Less common method includes drilling holes in a preform produced using one of the traditional technologies for optical fibre preforms.

The photonic-crystal fibres can overcome the limitations characteristic of standard optical fibres and waveguides. There are PCFs with many unusual properties, such as:

- PCFs in which single-mode light propagation has no spectral limitations;

- PCF with PBG, supporting the waveguide mode of light propagation in the air core;

- PCFs with large or, vice versa, very small effective mode area;

- super-nonlinear PCFs;

- PCFs maintaining polarisation, with very strong anisotropy;

- PCFs with zero dispersion at any wavelength in the visible and near infra-red ranges.

One of the most important practical applications of PCFs is the creation of supercontinuum generators with their use*). A very promising application for PCFs is light wave length conversion for the creation of optical signal processing devices to be used for the transfer of high-power optical radiation and for many other tasks.

The future of PCFs will largely depend on the development of production technologies; specifically, by achievements in reducing attenuation and increasing the mechanical strength. Another important question is the manufacturing cost reduction of PCFs.