Research On Brillouin And Raman Scattering In Water Pumped By A Nd: YAG Laser

Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) aretwo kinds of well-known inelastic scattering processes resulting from the interaction ofhigh power laser with matter, and have been subject to extensive theoretical andexperimental investigations over the past decades. SRS competing with SBS forpumping laser energy, the result of competition depends on the media and the incidentlaser. For multi-longitudinal, the wideband stimulated Brillouins cattering (WSBS) andforward stimulated Raman scattering (FSRS) always in a competitive relation in water.When the interaction length is relatively shorter or pump energy is lower, the WSBSwill be dominant. With the increase of the interaction length and pump energy and theappearance of optical breakdown, the FSRS will be dominant. In this paper, on the basisof theoretical analysis, a multi-longitudinal mode laser was used as the pump source, therelationship among the WSBS, FSRS and optical breakdown have been studed in detail.We found that, the competition between WSBS and FSRS is an alternate process, whichone dominated depends on the linewidth and energy of pump laser, focal length andoptical breakdown.In our experiments, water with the attenuation coefficient of about0.06/m was usedas the medium. The experimental results show that, during the pumping process, WSBSand FSRS are still in a competitive state WSBS without amplification effect on theFSRS and they are in the same order of magnitude), and the linewidth, pump laserenergy, focal length and optical breakdown decide that which kind of scattering processoccupy the leading edge.Finally, the optical suppression effect of second-order SBS excited by a singlelongitudinal mode laser has been reaearched according to the output energy offirst-order SBS and the pulse compression effect, and meanwhile, a new approach tomeasure the threshold of SBS based on the output energy characteristic of stimulatedBrillouin scattering optical limiting is proposed. The experimental results show that,when the medium length is less than the real gain length (RGL), the threshold of SBS isnot a constant, and it increases with the decrease of medium length.