An efficient double-pass Raman amplifier with pump intensity averaging in a light guide

Experimental observations on a double-pass methane Raman amplifier pumped by a KrF laser demonstrate efficient photon extraction (75-85 percent) of multiple pump pulses and high stage gain (10-20) with simultaneous intensity gain or "pulse compression" of about 2. The Stokes beam divergence is about 2.5 times the diffraction limit and is unchanged in the amplifier to within the precision of the experiment. The pump pulses are of poor spatial quality and propagate through the amplifier in a light guide. A simple one-dimensional theoretical model fits the results of the experiment. Analysis using the model permits a projection of the performance of very large final Raman amplifier-compressor stages for fusion laser systems. These systems, with somewhat better controlled pulse shapes than those used in the experiment, can deliver average intensity gains of 2-3 at a net energy conversion efficiency (all losses included) of about two thirds and a stage gain of 30.