Abstract

We present theory and experiments to show that the four-wave mixing efficiency in a photorefractive BaTiO₃ crystal can, in some conditions, be enhanced by using optical beams of slightly different (~ 1-Hz) frequencies to write a moving grating. This enhancement is an intrinsic property of the four-wave mixing process and is a result of the finite response time of the material to changes in the writing intensity pattern. The optimum frequency shift scales as I^0.6, where I is the incident optical intensity. Theory and experiments show the effects of any curvature mismatch between the pumping beams, of the relative beam intensities, and of the magnitude of the nonlinear coupling strength. We believe that our results explain the frequency shifts observed in a self-pumped phase conjugator^1,2 and obviate the need to invoke a photovoltaic field or a resonator condition, as previously suggested.

© 1985 Optical Society of America