Patent ID: 7062124

Claim:
An optical filter comprising (i) a multimode waveguide; and (ii) first and second coupling waveguides which communicate with the multimode waveguide at first and second longitudinal positions therealong respectively; wherein the first and second longitudinal positions and the relative orientations of the waveguides' central longitudinal axes are such that an input optical field distribution ( 140 ; 240 ), being a lowest order transverse mode of the coupling waveguides, introduced into the multimode waveguide at the first longitudinal position is substantially reproduced, by virtue of modal dispersion and inter-modal interference within the multimode waveguide, at the second longitudinal position for optical radiation of a wavelength to be passed by the device in preference to radiation of other wavelengths, and coupled into the second coupling waveguide, the respective intensities of the optical field distributions at the first and second longitudinal positions being substantially equal, characterised in that the coupling waveguides each communicate with the multimode waveguide at a lateral side thereof and in that the central longitudinal axis of each coupling waveguide is inclined to that of the multimode waveguide at the point that coupling waveguide communicates with the multimode waveguide.