Experimental investigation of high-power single-pass Raman shifters in the ultraviolet with Nd:YAG and KrF lasers

Single-pass Raman cells pumped by either a quadrupled Nd:YAG (266-nm) laser or a KrF excimer laser are studied. The Raman-active gases comprise H2, D2, or CH4, as well as a mixt. of them, with the addn. of He, Ne, or Ar. A parametric study, in which the Stokes conversion efficiency and the beam quality (M2) were measured, was made. The first Stokes efficiency increases and all the Stokes thresholds decrease with an increase in the lens focal length or the M2 parameter of the pump beam. The quality of the Stokes beams deteriorates when the active-gas pressure increases but is improved by the addn. of an inert gas. Laser-induced breakdown is shown to be a factor that limits the conversion efficiency and the quality of the Stokes beams. With a mixt. of D2, H2, and Ar, a 10-15-mJ pulse energy is obtained (depending on the pump M2 parameter) in the first Stokes beam of D2 (289 nm) and H2 (299 nm), with a full-angle divergence of 0.5 mrad (at 86% power).