Evaluation of Nd:YAG-pumped Raman shifter as a broad-spectrum light source

We have examined the utility of a gas-filled, Nd:YAG-laser-pumped Raman shifter as a possible broad-spectrum light source. Six to nine new output frequencies with pulse energies above 1 µJ are produced when a pure-hydrogen or pure-methane Raman shifter is pumped with 40 mJ of secondharmonic, 20 mJ of third-harmonic, or 11 mJ of fourth-harmonic pump pulse energy. Optimum output occurs at pressures of approximately 10 atm for the pure-gas experiments. We also report the output frequencies and pulse energies of a mixed hydrogen-methane Raman shifter pumped by 20 mJ of the third harmonic of the laser for various proportions of the two gases at pressures up to nearly 20 atm. Depending on composition and pressure, over a dozen new output lines with pulse energies over 1 µJ can be produced. We discuss the nonlinear processes involved, the optimum operating conditions, and the suitability of the source for our application of groundwater monitoring.