Abstract: A method of fabricating a band-processing optical fibre filter having a center wavelength &lgr;0′, the method comprises (i) radius-reducing a mismatched multi-core optical fibre having a core phase matching wavelength (before radius reduction) of &lgr;0 and a radius (before radius reduction) of a0, to a reduced radius Ra0, where R=&lgr;0′/&lgr;0; and (ii) providing light input and output connections to a section of the radius-reduced multi-core fibre so that input light is launched into one of the cores of the multi-core fibre section and output light emerges from one of the cores of the multi-core fibre section.