Observation of vortex structure in stimulated Raman scattering beam profile

The beam profile of stimulated Raman scattering (SRS) in a Kerr medium (nitrobenzene) pumped by multimode radiation with regular structure having nearly four-fold azimuthal symmetry was studied. The SRS was excited near the threshold of generation by nanosecond pulses of laser radiation at wavelength 530 nm. The profile of output pump beam had a uniform intensity distribution, whereas the SRS beam profile showed kaleidoscopic change from shot to shot, while the energies of input pulses were kept stable. The circularly distributed speckle pattern, as well as interference fringe structure in the profile of the beam of SRS I Stokes component was observed. The interference fringes showed the number of points with origination and vanishing of fringes. Such behavior, which is the vortex signature, allows to suppose that SRS waves, generated from quantum noise, carry the screw dislocations. The origin of dislocations is analogous to one described fora speckle field (28) and discussed for SRS in (15).