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During the past few years major advances have been made, both in performance and theoretical understanding, of frequency converters based on the Raman effect. The impetus for the effort has come largely from the NAVY/DARPA interest in advanced blue-green communication systems, but other applications such as isotope separation and remote sensing have contributed as well. Parallel efforts exist in development of backward Raman pulse compressors for ICF applications, but these will not be discussed in this paper.

We have carried out experimental and theoretical investigations of beam quality improvement in forward Raman amplifiers. In appropriate conditions, efficient conversion of laser beams of poor spatial quality into Stokes beams with nearly diffraction-limited wave fronts can be realized. The results were obtained using a configuration with parallel copropagating pump and Stokes beams. A discharge-pumped XeCl laser with an output of 0.5 J in a 60-ns pulse was used to carry out the experiments. In the present experiments, a near diffraction-limited low-energy Stokes beam was generated in a superfluorescent oscillator. The first Stokes order output of this oscillator was injected with the pump beam in a Raman amplifier with a Fresnel number as high as 150. The wave front of the pump was distorted using aberrators of variable strength and spatial frequency content. In conditions of amplifier Fresnel number and pump beam distortion where phase distortions on the pump did not lead to significant intensity distortions in the amplifier, the Stokes beam quality was not affected by the saturated amplification process. Pulse shapes and phase front interferograms were obtained for the pump and Stokes. The results are in good agreement with 3-D wave-optics code simulations.