Fanning and photorefractive self-pumped four-wave mixing geometries

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.<
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