Waveguide modes with their energy leakage into the substrate while light propagation even along the crystal main axes can exist in proton-exchanged optical waveguides on lithium niobate because of the considerable influence of that process on crystal anisotropic properties. This effect can be used to create planar coupling elements for integrated-optical processors by means of proton exchange in local areas of LiNbO$-3$/:Ti waveguides. Some results both of experimental and theoretical investigations of this effect and of the proton exchange coupling element are presented in this paper.
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