Abstract: A photonic crystal fiber comprising: a core (20) having a refractive index; and a cladding region (10) at least substantially surrounding the core (20) and comprising a bulk material having a second refractive index that is higher that the first refractive index, the bulk material containing an arrangement of elongate, longitudinal holes that comprise hole material of a third refractive index that is lower than the first refractive index; such that an electromagnetic mode guided in the core (20) has an evanescent wave that becomes more closely confined to the vicinity of the core (20) as the wavelength of the electromagnetic mode is increased over a first range of wavelengths.

Abstract: A photonic-crystal fibre comprises a core (20) and a cladding (10) with longitudinal holes. The refractive index of the core is lower than that of the cladding bulk material, e.g. by fluorine doping of the core material. The holes are air-filled cavities extending along the longitudinal direction of the fibre. One property of the fibre is that a guided mode having longer wavelength is more confined in the core than a mode with a shorter wavelength. Such a fibre may have a cut-off wavelength &lgr;c? such that the fibre is unable to guide in its core light of wavelengths shorter than &lgr;c.