Effect of acoustical streaming on SBS wavefront reversal

Abstract

The fluid equations governing the medium response to stimulated Brillouin scattering (SBS) are solved to second order in the quasi steady-state approximation, and coupled with Maxwell's equations to obtain the scattered radiation fluence profiles. Large scale fluid motion, or acoustical streaming, is predicted as a second-order, time-dependent effect. Since this streaming motion causes phase shifts in the optical grating, the gain of the scattered signal is reduced. 'The differential equations governing the large-scale motion is integrated numerically to obtain the streaming phase shifts. These phase shifts along with the acoustical amplitudes (obtained in the quasi steady-state approximation) are then inserted into Maxwell's equations in the slowly varying envelope approximation to obtain the intensity profile of the scattered radiation, assuming a speckle.inhomogeneous field configuration and a Gaussian pump wave profile. The intensity profile of the scattered pulse is then integrated over the pulse time to obtain the scattered pulse fluence. Initially, scattering is the most intense near the center of the beam, and therefore streaming phase shifts grow most rapidly in this region. As the pulse evolves, this grewth in phase shift causes more reduction in gain near the center of the beam than in the wings. Therefore, it is predicted that the intensity of the scattered wave profile is suppressed near the center. The calculations and plots we obtained showing this effect are qualitatively consistent with some experimental results obtained by Dolgopolov et. al.