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A channel waveguide formed in LiNbO/sub 3/ was fabricated by using distilled water instead of acids for proton exchange and a self-aligned SiO/sub 2/ cap for diffusion. It was demonstrated that the water reaction is an important reaction to cause the index change in proton exchange. >

Channel waveguides fabricated in LiNbO/sub 2/ by proton diffusion with a self-aligned SiO/sub 2/-cladding structure are discussed. Proton diffusion in width and depth directions is amenable to process parameters, so that it provides a simple method for control of mode profiles. Using this method, symmetric depth mode profiles and an aspect ratio of 1.15 at lambda =0.6328 mu m have been achieved for efficient fiber-to-waveguide coupling. >