The invention concerns a guided wave
spatial filter (300) for receiving input
radiation and outputting corresponding filtered output
radiation, the filter comprising first and second
waveguide sections (30, L3, L5) connected in series, the sections: (a) mutually matched for transmitting fundamental mode
radiation components present in the input radiation therethrough to provide the output radiation; and (b) mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation. The
spatial filter (300) is implemented using
rib waveguides (30) for the sections with associated relatively deeply and relatively shallowly etched structures (20, 310, 320) for imparting to the sections their
radiation mode filtration characteristics. The
spatial filter according to the invention can be incorporated into optical splitters and optical modulators to enhance their performance and desensitise them to higher-order mode radiation injected thereinto.