Stimulated Raman shifting of the Nd:YAG fourth harmonic (266 nm) in H2, HD, and D2

The dependence of the first Stokes stimulated Raman conversion efficiency of fourth-harmonic radiation from a Nd:YAG laser at 266 nm has been studied for the isotopic species H₂, HD, and D₂ as a function of gas pressure and laser energy using a low numerical aperture (~4.5×10^-3) pumping geometry. While the laser energy threshold for first Stokes conversion is seen to vary significantly between the species it has been found that photon conversion efficiencies of at least 50% can be achieved for all of them for laser pump energies at 266 nm ⩽50 mJ/pulse. This study provides a new measurement of the differential cross section for stimulated Raman scattering in HD of 8.1±2.4×10^-29 cm²/sr at 266 nm and at high pressures, and agreement is found with previous measurements of the cross sections for H₂ and D₂. The results have been used to optimize the laser transmitter system for a differential absorption lidar (DIAL) system to measure tropospheric ozone concentration profiles