Instabilities of laser beams counterpropagating through a Brillouin-active medium

Counterpropagating laser beams in a Brillouin-active medium are shown to become unstable to the growth of amplitude and phase fluctuations. Slightly above threshold, the nature of the instability is the temporal growth of sidemodes separated from the laser frequency by approximately the Brillouin frequency of the medium. This process leads to sinusoidal oscillations of the intensities of the transmitted waves. At higher input intensities the system can become chaotic; many sidemodes are excited, and the transmitted fields fluctuate wildly. The origin of the Brillouin instability is the combined action of the gain of the standard stimulated Brillouin scattering (SBS) process and of the coupling of the waves due to multiwave mixing mediated by the electrostrictive interaction. The threshold for the instability is at least several times lower than the threshold of the standard SBS process involving a single pump beam.