Stimulated Brillouin scattering gain variation and transient effects in a CH_4 : He binary gas mixture

Abstract

Experimental measurements are presented that show that the stimulated Brillouin scattering gain of methane, and hence the nature of the competition between stimulated Brillouin scattering and stimulated Raman scattering, can be systematically varied by the addition of helium. The gain variation is explained in terms of a transient gain model in which the dominant effect of the helium is to increase the Brillouin linewidth (acoustic attenuation). Broadening mechanisms are discussed. At methane and helium partial pressures of 19 and 41.8 kTorr (25 and 55 atm), respectively, backward stimulated Raman scattering dominates stimulated Brillouin scattering when the 532-nm pump energy exceeds 160 mJ.