Patent ID: 7174080

Claim:
A guided wave spatial filter for receiving optical input radiation and outputting corresponding filtered output radiation, the filter comprising first and second waveguide sections connected in series, the first and second waveguide sections being: (a) mutually matched relative to fundamental mode radiation for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation; and (b) mutually mismatched for higher-order radiation modes for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation; wherein the first and second waveguide sections comprise rib waveguides, and the first waveguide section is flanked by first regions and the second waveguide section is flanked by second regions, the first regions being more shallowly etched relative to the second regions, and the second regions being etched to a sufficient depth so that said higher-order mode radiation components are substantially unable to propagate in the second regions.