A two-mode photonic crystal fiber includes a core of a substantially transparent core material. The core material has a core refractive index and a length, and has a core diameter. The fiber also includes a cladding region surrounding the length of core material. The cladding region has a first substantially transparent cladding material, having a first refractive index. The first substantially transparent cladding material has embedded along its length a substantially periodic array of holes, having a diameter, d, and being spaced apart by a pitch, .LAMBDA.. The holes have a second refractive index, which is less than the first refractive index. The dimensions of the hole diameter, d, and the pitch, .LAMBDA., co-operate to give two modes propagation within the photonic crystal fiber independent of input radiation wavelength for any value of the pitch, .LAMBDA., for a substantially fixed d/.LAMBDA. ratio within a range of approximately 0.45 0.65.