Improvements in or Relating to Photonic Crystal Fibres

A photonic crystal fibre comprising a bulk material having an arrangement of longitudinal holes, the fibre including a cladding region and a guiding core (280), the cladding region having an effective-refractive-index which varies across the fibre's cross section, 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.; A photonic crystal fibre comprises a bulk material having an arrangement of longitudinal holes, the fibre including a cladding region and a guiding core (280), the cladding region having an effective-refractive-index which varies across the fibre's cross section. The core may confine a first mode while the refractive index gradient of the cladding may confine a pump beam. The hole structure has been chosen to provide the guiding core and/or the refractive index gradient.