Patent ID: 6990282

Claim:
A photonic crystal fibre comprising a bulk material having an arrangement of longitudinal holes, the fibre including a cladding region and a guiding core, the cladding region having an effective-refractive-index which varies across the fibre's cross-section, in which the fibre is capable of supporting a fundamental transverse mode, which is substantially confined to the guiding core, and at least one other transverse mode, which is less confined to the guiding core than the fundamental mode, wherein, in use, the effective-refractive-index variation causes light propagating in the fibre to have a different transverse profile from the profile that it would have if the cladding region were of a substantially uniform effective refractive index, and causes the other mode and/or a superposition of modes to have a different profile from the profile it or they would have if the cladding region were of a substantially uniform refractive index.