A combination interaction in a field of oncoming light waves

It is theoretically and experimentally shown that, in a four-wave combination interaction in a field of oncoming excitation beams, a regime of unstable reversal exists in the wave front of the weak signal wave and that the reflectivity of this wave increases exponentially with time. In the absence of this signal wave, this instability leads to a spontaneous generation of radiation in the reversing mirror. With an additional reflecting surface, spontaneous generation is also possible in the region of stability in the reversing mirror, provided the condition of self-excitation in the optical resonator formed by this mirror and the reflecting surface is met. The self-excitation of radiation between the reversing mirror and the reflecting surface is observed experimentally; here, the threshold of generation depends on the skewness of the reflecting surface and on the optical losses.