Investigation of stimulated Brillouin scattering of vortex laser beams

Abstract

The stimulated Brillouin scattering (SBS) of focused laser beams with screw dislocation of phase front (Gauss-Laguerre vortex modes) has been numerically explored with allowing for transient processes, non-steady-state conditions, hypersound noise and an effect of Stokes field on laser field, i.e. SBS saturation. Calculations as well as an analysis on the basis of perturbation theory under near-threshold SBS conditions have shown that phase conjugation of vortex beams does not take place as a phenomenon because of failure of selection of Stokes mode conjugated to laser one. It occurs since the SBS gain coefficient does not depend on the laser field phase under the near-steady-state conditions, and the Stokes field does not 'feel' how the laser phase helix is twisted. Therefore (for example, in the case of a doughnut mode) the Stokes beam consists of a random superposition of a few modes including conjugated one. For rather wide class of vortex beams (for example, in the case of a laser mode with two rings), a novel phenomenon takes place that can be called as phase transformation at SBS. Its essence is in the fact that in Stokes beam a mode with regular phase front is selected which is orthogonal to laser mode at all. In the near- threshold SBS regime, this selected mode is a principal Gaussian mode. Calculations show that at SBS of vortex laser beam the generation of vortex hypersonic wave takes place in the SBS-medium. Its topological charge coincides with the laser beam charge in view of vortex-free structure of the Stokes beam.