Counter-propagating four-wave mixing on phase--amplitude holographic gratings in a cubic photorefractive crystal

system of coupled-wave equations is presented, which can be used to find the components of the vector amplitudes of linearly polarised light waves during four-wave mixing on phase-amplitude holographic gratings in a cubic photorefractive crystal. When deriving the coupling equations, account is taken of linear electro-optical, photoelastic, and inverse piezoelectric effects, as well as of natural optical activity, circular dichroism, and absorption in the crystal. Regularities of energy exchange during four-wave mixing in a (111)-cut InP:Fe crystal are analysed. It is shown that the best agreement between the theoretically calculated dependences of the intensities of the p- and s-polarised components of the conjugated light wave on the orientation angle and the known results of experimental studies is achieved under the assumption that several holographic gratings with a phase--amplitude structure are formed in the crystal. The dependence of the intensity of the conjugated wave on the orientation angle and azimuth of the linear polarisation of the pump wave is theoretically studied, and it is shown that it can be substantially increased by choosing the optimal values of these parameters.