Photonic crystal fibres: An endless variety

A photonic crystal fibre has an array of microscopic air holes running alon g its length. The periodicity of the array is broken by a deliberate "defec t" that acts as a waveguide core. Light is confined to this core by the hol es. Although some designs of photonic crystal fibre guide light by total in ternal reflection and so can be considered analogues of conventional optica l fibres, their properties can be strikingly different. Other designs guide light by photonic bandgap, confinement and represent a totally new type of fibre.