Raman amplification of laser pulses near the threshold for plasma wave breaking

Equations are derived for the amplitudes of counter-propagating laser pulses near the threshold for plasma wave breaking, which allow one to describe laser pulses with durations on the order of the plasma oscillation period. In the quasi-monochromatic approximation, they take the form of conventional threewave equations with an additional nonlinearity for the plasma wave. The amplitudes of the amplified laser pulses estimated using these equations agree with results obtained by solving the complete equations. It is shown that Raman amplification of a weak quasi-monochromatic signal (plasma noise) in rarified plasma is significantly suppressed. At the same time, according to numerical simulations, the amplification of laser pulses with durations on the order of the plasma oscillation period is suppressed insignificantly. This result opens new prospects in the application of Raman compression of laser pulses without additional frequency modulation.