The nonlinear coupling characteristics of the nonparallel coupler due to photorefraction is investigated using coupled mode theory with a step-like refractive index profile. The calculated results show no significant degradation of the performance of the device when less than 3×104W cm-2optical intensity is coupled into the waveguide. The optically induced drift effects depend on the structural geometry and the initial light intensity. Means of reducing the crosstalk induced by the photorefractive damage are also presented. Numerical results are given to illustrate the nonlinear coupling characteristics of various structures and to establish useful guidelines for the design of nonparallel directional couplers
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