Possibility of effective amplification of optical signals in four-wave resonant parametric processes

A theoretical analysis is made of the possibility of the amplification of optical signals in four-wave parametric processes involving two-photon (second-order) resonances. An expression is obtained for the gain in the case of spatial phase matching between the pump, signal, and idler waves. Conditions for achieving a high gain (of the order of 10³–10⁵) are determined.