An optical device comprising an optical waveguide (13) on a substrate (11) which over part of its length forms one of a stack (15) of coupled optical waveguide sections of the same length, the stack being of a length such that a light wave of predetermined frequency propagating along the waveguide (13) to enter the stack (15) at one end thereof produces light waves at the other end of the stack which are substantially in phase adjacent each waveguide section. The device finds particular application for coupling light from a substrate waveguide into another waveguide, e.g. an optical fibre (27), off the substrate (11).