Spatial structure of the Stokes radiation in stimulated Brillouin backscattering of light beams

It was found experimentally that the transverse structure of the Stokes radiation field in stimulated Brillouin scattering changed from spatially coherent to spatially noncoherent when the diameter of the single-mode pump beam was increased. The experimental results obtained just above the stimulated scattering threshold were in good agreement with a theoretical model describing stimulated scattering as the amplification of spontaneous noise in a medium with a gain profile induced by a given pumping. The experimental results obtained under nonlinear conditions when the threshold was exceeded significantly were explained by assuming that the scattering region was split into two and in one of them a Strokes wave was formed under the same conditions as near the threshold whereas in the other the Stokes wave increased in power to a value comparable with the pump power because of the transfer of energy from the pump beam.