<p>An optical filter coupler for coupling optical signals at a selected wavelength comprises a first optical waveguide (3) and a second optical waveguide (4) closely spaced in parallel with each other for allowing light propagation in one of the waveguides to couple into the other waveguide. The first optical waveguide (3) consists of a first material having a first wavelength-dispersive refractive index and the second optical waveguide (4) consists of a second material having a second wavelength-dispersive refractive index which is different from the first wavelength-dispersive refractive index. The first material and the second material have a common refractive index at the selected frequency.</p>
