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

Abstract

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.