It is shown theoretically and experimentally that a weak signal light amplification is possible in thin photorefractive crystals with a local nonlinear response due to photovoltaic charge transport.The origin of the power exchange is that four wave interaction of the input beams and reflection beam at the rear will form two sets of bending gratings with certain spatial phase shift.When the grating written by the reflection wave and its diffracted wave is read by the input pump beam under off Bragg condition ,the weak signal beam achieves coherent amplification.Theoretical and experimental results are compared with each other.
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