In the paraxial limit the photorefractive response of a cubic gyrotropic crystal with applied square-wave electric field is considered for the symmetric incidence on a crystal of two plane light waves with approximately equal intensity. The theoretical description is based on the analysis of the nonlinear equation for the time-average value of the space-charge field and coupled-wave equations for the amplitudes of the eigenwaves of a crystal. It takes into account the additional light waves diffracted in the second orders. The numerical solution of the self-consistent task is obtained for the diagonal electro-optic configuration of interaction in Bi_(12)TiO_(20) crystal under external field with amplitude l0kV/cm.
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