Abstract

Competition between stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) is common in many substances. Because the steady state Brillouin gain is often larger than the steady state Raman gain, particularly at higher densities of the medium, the SRS process usually dominates only in a regime in which transient effects reduce the SBS gain significantly below its steady state value. The objective of the present work was to determine the extent to which the nature of the SBS/SRS competition in methane gas could be affected not by adjusting the temporal characteristics of the pump laser, but by adding a second gas such as helium. Our experimental results show that the SBS gain reduction can be substantial and is manifested in two ways. First, the SBS threshold systematically increases by as much as a factor of 2 with increasing helium partial pressure. Second, SRS becomes the dominant process at the highest pump energies studied. The data are consistent with a transient gain model in which the effect of the helium is to increase the Brillouin linewidth. This broadening most likely results from increases in any of several transport coefficients (the viscosity, thermal conductivity, and the mass diffusion constant) or the enhancement of relaxation effects.

© 1989 Optical Society of America