STIMULATED BRILLOUIN BACKSCATTER OF A SHORT-PULSE LASER

Stimulated Brillouin backscattering (SBBS) from a short-pulse laser, where the pulse length is short compared to the plasma length, is found to be qualitatively different than in the long pulse regime, where the pulse length is long compared to the plasma length. After an initial transient of order the laser pulse length transit time, the instability reaches a steady state in the variables x'=x-V(g)t, t'=t, where V-g is the pulse group velocity. In contrast, SBBS in a long pulse can be absolutely unstable and grows indefinitely, or until nonlinearities intervene. The motion of the laser pulse induces Doppler related effects that substantially modify the backscattered spectrum at higher intensities, where the instability is strongly coupled (i.e., has a growth rate large compared to the ion acoustic frequency). (C) 1995 American Institute of Physics.