Stimulated Raman scattering with a Gaussian pump beam inH2gas

Raman scattering of the Q(1) line in ${\mathrm{H}}_2$ gas was investigated from the spontaneous to the stimulated scattering region using a frequency-doubled Q-switched Nd:YAG laser (where YAG denotes yttrium aluminum garnet). We measured the Raman Stokes energy as a function of the laser power and the radial distribution of the Stokes intensity for the unfocused and the focused Gaussian laser beam. There is good agreement between the experiments and the results of a nonorthogonal mode theory in the region of stimulated Raman scattering. For cell lengths short compared to the Rayleigh range of the laser beam or for Raman cells far from the beam waist, stimulated Raman scattering is described in a good approximation by an adapted plane-wave theory. In the region of spontaneous Raman scattering, the measured dependence on cell length, solid angle, and focusing conditions is not in accordance with the nonorthogonal mode theory, but agrees with the standard theory of spontaneous Raman scattering.