Spectrum of a pulse compressed in SBS in CCl4 to nanosecond and subnanosecond durations

The change in the spectral shape and the Stokes frequency shift of radiation compressed in SBS is studied experimentally as a function of the incident laser pulse intensity. It is shown that when the duration of the reflected laser pulse is reduced down to the subnanosecond one, its spectrum is monotonically displaced with increasing the pump intensity, which leads to a decrease in the Stokes shift. Then, the spectrum splits and acquires a double-humped shape, which is explained by the presence of phase modulation in the vicinity of the SBS resonance.