Abstract

We have performed an experimental and theoretical analysis of how a probe beam is modified on propagation through an SBS gain medium for various input beam parameters and total gains through the SBS medium. For the case of an undepleted lowest-order Gaussian pump beam, we found that when the probe spot size is smaller than the pump spot size in the focal region, the input probe profile is essentially preserved during amplification. However, if the probe is larger than the pump, the beam is modified as it propagates through the medium and in the limit of high gain takes on the mode structure of the pump. To model these effects mathematically, we decompose the probe wave into Laguerre-Gaussian modes and solve numerically the equations describing the spatial evolution of the individual modes. The result of the nonlinear coupling is to transfer energy from the pump beam to the probe and to redistribute the energy among the probe modes. In some cases, the transmitted probe shows a pronounced ring structure.

© 1987 Optical Society of America