Efficient anti-Stokes Raman conversion in collimated beams

We have demonstrated 10% energy-conversion efficiency of 0.5-nsec FWHM laser pulses at 248 to 225 nm by an anti-Stokes process in hydrogen, using collimated (unfocused) laser beams. To accomplish this, we simultaneously seeded the Raman cell with a Stokes pulse that was 2–6% as intense as the pump at the phase-matching angle with respect to the pump beam axis. Under these conditions, a fully transient plane-wave calculation suggests that 44% energy-conversion efficiency is possible. Phase-front imperfections on the pump and Stokes beams are thought to limit the observed conversion efficiency. We present a simple model describing the effect of phase distortion on anti-Stokes production that agrees with our observations. Experimentally observed dynamic effects are in good agreement with theoretical predictions.