Influence of thermal change in the phase of an optical beam on its stimulated scattering and wavefront reversal

The approximation of phase changes is used to consider the influence of transient thermal selfinteraction of an optical beam on its stimulated Rayleigh scattering. It is shown that if a selfinduced change in the pump frequency in the length of a nonlinear medium exceeds the width of a spontaneous scattering line, the rise of the intensity of the backscattered light is limited, the characteristic time for the establishment of steady-state stimulated backscattering decreases, and the frequency spectrum of the scattered wave becomes wider. In the case of multimode beams a thermally induced change in the phases may reduce discrimination of the stimulated scattering increment of waves uncorrelated with the pump and, consequently, may suppress wavefront reversal as a result of stimulated scattering. Estimates obtained indicate the possibility that such effects can influence stimulated Brillouin and thermal backscattering of laser pulses of duration in excess of 10^–7 sec in media with absorption coefficients of the order of 10^–2cm^–1.