Spatial evolution of laser beam profiles in an SBS amplifier.

We have performed an experimental and theoretical analysis of the modification of the transverse structure of a probe beam on propagation through an SBS amplifier. The theoretical analysis is based on performing a mode decomposition of the incident fields and determining how each mode amplitude is modified as a result of the nonlinear coupling. For the case of input beams with Gaussian intensity profiles and in the limit of negligible pump depletion, we find that the input probe profile is preserved during amplification only when the probe spot size is smaller than that of the pump in the focal region.