Superradiant Linear Raman Amplification in Plasma Using a Chirped Pump Pulse

A theoretical and numerical investigation of small-signal Raman backscattering from a chirped pump pulse in plasma shows that an ultrashort probe pulse will grow superradiantly, i.e., with an amplitude that scales with the propagation length while contracting self-similarly. These features are commonly associated with the nonlinear stages of Raman amplification in the pump depletion and Compton regimes. We show that the superradiant scaling results in very broad-bandwidth amplification due to gain distributed in frequency as well as spatially. Since different frequencies excite the plasma at different positions, wave breaking is avoided, and prepulses and pedestals are substantially suppressed. Linear chirped pulse amplification in plasma could provide a very broad-bandwidth alternative to solid state laser amplifiers, potentially usable for optical pulses a few cycles in duration