Experimental investigation of the spatial structure of the first Stokes component of stimulated Raman scattering

An experimental investigation was made of stimulated Raman scattering (STRS) of single-mode and multimode optical beams. It was found 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 also shown that as the scale of the transverse modulation of the laser beam decreases, the relative importance of the four-photon interaction decreases. As a result, the relative intensity of the anti-Stokes radiation decreases and that of the Stokes component increases. In conclusion, data are presented on the amplification of an approximately single-mode Stokes wave in the field of a focused multimode pump beam. 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.