Properties of dark solitons under SBS in focused beams

Using the method of four-wave probing of the waist of the laser beam focused into the bulk of a short active medium ($L\ll\tau c$, where $L$ is the length of the active medium, $\tau$ is the pulse duration, and $c$ is the speed of light), we have studied the dynamics of the behaviour of a dark soliton, appearing upon a jump of the input Stokes signal phase by about $\pi$ under SBS. The computer simulation has shown that when spontaneous noises with the gain increment $\Gamma$, exceeding the self-reflection threshold by 2 – 3 times, are generated, the dark soliton propagates along the interaction region for the time $t\approx T_2\Gamma_{th}/2$, where $T_2$ is the the lifetime of acoustic phonons, and $\Gamma_{th}$ =25 – 30 is the stationary threshold gain increment.