Positive feedback effect in four-wave mixing in photorefractive crystals with absorption

Using a two-level model of optical transitions, the positive feedback effect observed in the presence of absorption in four-wave mixing in photorefractive crystals has been determined analytically. It is shown that with positive feedback, as the intensity of readout beam increases, the values of diffraction efficiency and intensity of amplified beam also increase. An analysis of the holographic grating amplitude shows that the effective thickness of the grating decreases in this case, but its amplitude increases. The dependences obtained, very important from the practical point of view, show that the optimal thickness of the crystal is different for diffraction efficiency and intensity of the amplified beam, and decreases with increasing readout beam intensity.