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 Rama n shifter as a possible broad-spectrum light source. Six to nine new o utput frequencies with pulse energies above 1 muJ are produced when a pure-hydrogen or pure-methane Raman shifter is pumped with 40 mJ of se cond-harmonic, 20 mJ of third-harmonic, or 11 mJ of fourth-harmonic pu mp pulse energy. Optimum output occurs at pressures of approximately 1 0 atm for the pure-gas experiments. We also report the output frequenc ies and pulse energies of a mixed hydrogen-methane Raman shifter pumpe d 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 compos ition and pressure, over a dozen new output lines with pulse energies over 1 muJ can be produced. We discuss the nonlinear processes involve d, the optimum operating conditions, and the suitability of the source for our application of groundwater monitoring.