New phenomena at stimulated Brillouin scattering of Laguerre-Gaussian laser modes: theory, calculation and experiments

摘要

New phenomenon predicted earlier - failure of phase conjugation at the stimulated Brillouin scattering (SBS) of focused laser beams with screw dislocation of phase front (Laguerre-Gaussian vortex modes) - has been experimentally confirmed 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 amplification coefficient does not depend on the laser field phase, and the Stokes field does not "feel" how the laser phase helix is twisted. Therefore the amplification coefficients of a Stokes conjugated mode and an analogous mode of opposite helicity are equal to each other. For example, in the case of a doughnut laser LG_0~1 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 LG_1~1 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 close to a principal Gaussian mode. At SBS of the vortex laser beam the generation of the vortex hypersonic wave takes place in the SBS-medium so that the topologic charge in the system "Stokes wave+hypersonic wave" is equal to the charge of incident laser wave.