Photorefractive amplifier-converter and coherent oscillator with nonexponential gain

For some parametric interactions with identically zero exponential gain for the signal wave the intensity of the idler wave can grow as a second power of the propagation coordinate. Such an amplification is revealed for the p arametric mixing of four copropagating waves in BaTiO3; two of them are ord inarily polarized and the two others are extraordinarily polarized. This mi xing is used to build up a coherent oscillator. A reasonable qualitative ag reement of the experimental results with the calculated data is demonstrate d.