Microstructured (or photonic crystal) fibres are made of fused silica with a periodic array of air holes running down their length. In the type commonly used in optical frequency metrology, the central air hole in this periodic structure is missing. This yields a waveguide structure with a high refractive index defect surrounded by a low refractive index array of air holes. The defect thus acts like the core of a conventional optical fibre and the air holes act as the cladding.

Conventional and microstructured fibres

By varying the dimensions of the air hole structure, it is possible to tailor the waveguide properties so that the zero point of the group velocity dispersion is within the operating spectrum of a mode-locked femtosecond Ti:sapphire laser, at around 800 nm. This means that the short pulse length can be maintained over a significant length of the fibre. Together with the small core diameter of the fibre, this results in strong self-phase modulation or four-wave mixing between the comb lines, and enables comb spectra spanning more than an optical octave to be produced.