Beam cleanup, beam combination, and efficient energy extraction in a KrF-pumped Raman amplifier

We have demonstrated 75–85% photon conversion efficiency to a high quality Stokes pulse in a methane Raman amplifier pumped by two poor quality KrF pump pulses propagating in a lightguide. Intensity averaging and combination of poor quality pump beams in a Raman amplifier can be treated using a four-wave mixing model. 1 The model explicitly shows the consequences of finite pump correlation length and illustrates the circumstances under which poor quality pump beams can be used to amplify a high quality Stokes beam without degrading its divergence. Phase-conjugate effects in a Raman amplifier are observed and described. Experimental observations 2 on a double-pass methane Raman amplifier pumped by two poor quality KrF beams propagating in a lightguide demonstrate 75–85% photon conversion efficiency to the Stokes beam with a stage gain of 10–20. The Stokes beam divergence is about 2.5 times the diffraction limit and is not affected by the amplifier.