STIMULATED BRILLOUIN BACKSCATTER OF A SHORT-PULSE LASER

Stimulated Brillouin backscattering (SBBS) from a short-pulse laser, w here the pulse length is short compared to the plasma length, is found to be qualitatively different than in the long pulse regime, where th e pulse length is long compared to the plasma length. After an initial transient of order the laser pulse length transit time, the instabili ty 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 i ntervene. The motion of the laser pulse induces Doppler related effect s that substantially modify the backscattered spectrum at higher inten sities, 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.