A guided wave optical duplexer is described, in which there is provided a four-port duplexer with the property that light of a first wavelength and polarization input to Port 1 is substantially routed to Port 2 (the balance to Port 4) while light of a second wavelength and polarization input to Port 2 is substantially routed to Port 3. This invention concerns the provision of means whereby light of the first kind reflected from an interface beyond Port 2, may be prevented from reaching Port 3. The technique used involves interferometry of light reflected from Port 2 with that from the complementary output at Port 4, requiring only the provision of a 90 DEG phase-shift between these outputs at the reflective interface. This is achieved either by angling the interface or by using curved waveguides.
