A prism coupler for coupling light into and out of an optical waveguide and characterized by the temperature dependency being eliminated. This is accomplished by a skillful selection of the prism angle and the prism material of a single prism or by combining two prisms of different materials whose indices of refraction and prism angles are selected to compensate for the effects of temperature on both the prism and the waveguide. Preferably, the waveguide is doped in a substrate of LiNbO.sub.3 and the prism utilizes a compound prism having a first prism consisting of LiNbO.sub.3 and a second prism of polystyrene.