EXPERIMENTAL INVESTIGATION OF THE SPATIAL STRUCTURE OF THE FIRST STOKES COMPONENT OF STIMULATED RAMAN SCATTERING.

Stimulated Raman scattering (STRS) of single-mode and multimode optical beams has been investigated experimentally. It is shown that under conditions of pumping by a single-mode beam at an excess over the parallel STRS threshold, the Stokes wave divergence increases rapidly and the contribution to the axial intensity of the radiation propagating in the far-field zone is mainly governed by a comparatively small part of a laser pulse at subthreshold power. Under conditions of pumping by a multimode beam, the divergence of a parallel STRS Stokes wave only increases with excess over the threshold for fairly large-scale or, conversely, for essentially small-scale modulation of the laser beam. In the intermediate case, the Stokes wave divergence is similar to that of the pump wave up to a twofold excess over the threshold. It is shown that there is an optimal laser beam power at which the coefficient of conversion of the energy into a Stokes wave with divergence close to the diffraction limit has a maximum.