Beam cleanup and low-distortion amplification in efficient high-gain hydrogen Raman amplifiers

We report the results of a theoretical and experimental investigation of two important properties of Raman converters. The first property is Raman beam cleanup, which refers to the generation of high-quality Stokes beams in a Raman amplifier pumped by laser beams of poor spatial quality. The second is nearly distortion-free amplification of aberrated Stokes beams in a Raman amplifier pumped by good-quality laser beams. A parallel beam geometry was used with collinear pump and Stokes beams. In both cases, excellent energy-conversion efficiencies into the first Stokes order, of the order of 60%, were demonstrated at high amplifier gains of 100 to 1000. The spatial characteristics of the amplified beams were found to be in good agreement with model predictions.