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.