On the reconstruction of parameters of quasi-Gaussian pump beams during transient SBS

The radii and radii of curvature of Stokes stimulated Brillouin scattering (SBS) beams are measured by the method of moments for smooth nearly Gaussian focused pump beams with the propagation ratio M²_σp≤1.2. It is shown that in the case of sufficiently deeply focused pump radiation, the propagation ratio M²_σS of Stokes beams near the threshold of the transient SBS is smaller than M²_σp and approaches it with increasing the pump pulse energy. It is also found that the radii of Stokes beams at the output from a nonlinear medium are smaller than the radii of pump beams, while the radii of wave-front curvature are close (in modulus) to the radii of wave-front curvature for pump beams.