Quasi-collinear and partially degenerate four-wave mixing: an alternative explanation of the phase-conjugation property of backward stimulated scattering

A quasi-collinear and partially degenerate four-wave mixing model is proposed to explain the optical phase-conjugation property of various types of stimulated backscattering. According to this model, after passing through a phase-disturbed medium or an aberration plate, the input pump beam can be resolved into two portions: a stronger undisturbed regular portion and a weaker phase-disturbed irregular portion. These two portions interfere with each other and create a volume holographic grating in the pumped region of the scattering medium. Only the stronger undisturbed portion of the pump field can excite an initial backward stimulated scattering beam with a regular wavefront. When the latter ~as a reading beam! passes through the induced holographic grating, a diffracted wave will be created and then amplified together with the reading beam. A rigorous mathematical analysis shows that under certain conditions the combination of these two portions ~the reading wave and the diffracted wave! of the backward stimulated scattering can be an approximate phase-conjugate field of the input pump field. The major theoretical conclusions are basically supported by the experimental results based on a specially designed two-beam interference setup. © 1999 American Institute of Physics. @S1063-7761~99!00502-8#