A review is given of theoretical and experimental investigations of the role of the fanning in the operation of self-pumped four-wave mixing systems. It is shown that in some cases the fanning determines the dynamics of such systems and their final steady states.
Results of recent experimental and theoretical investigations on the role of fanning in the operation of several popular photorefractive self-pumped FWM geometries, including the double phase conjugate mirror (DPCM), the ring oscillator, the semilinear mirror, the system of two coupled DPCMs, and the total internal reflection (TIR) mirror, are presented. The results can be divided into two groups. Geometries of the first group (the DPCM and the ring oscillator) have soft excitation thresholds. Geometries of the second group (the semilinear mirror, the system of the two DPCMs and the TIR mirror) have hard excitation thresholds. Geometries of the first group experience a noticeable influence of fanning only when the level of fanning of a single pumping beam passing through a crystal, is high. Geometries of the second group, such as the semilinear mirror, the system of two DPCMs, and the TIR mirror without fanning, are not self-starting.>
The stability of the steady-state solutions of four-wave mixing in photorefractive crystals has been studied. Two complementary methods have been used: linear stability analysis and time integration of the nonlinear equations. A critical dependence of the stability on the phase of the perturbation has been revealed. The existence of two stable solutions for certain values of some parameters has been found. Also, regimes of unstable behavior in cases with a unique steady-state solution for large values of the pump ratio have been found.
The physical origin of frequency shift and dynamic instability in a photorefractive self-pumped phase conjugator is analyzed by use of four-wave mixing and multi-four-wave mixing. We show that the frequency shift and the instability result from the requirement of the maximal energy transfer between the interacting beams. Two examples of numerical results in a cat self-pumped phase conjugator are given, which are in good agreement with previous experimental results. A simple method is proposed to change an unstable phase-conjugate output to a stable one.