Observation of parametric gain suppression in rotational Raman transitions of N_2 and H_2

The ratio of forward Raman conversion thresholds for linearly and circularly polarized laser light has been measured in low-Fresnel-number configurations for the strongest rotational Raman transitions of N(2) and H(2). In both cases the ratio is found to be in the range 2.5-3, corresponding to a threshold-suppression factor of 1.7-2 for the linearly polarized laser light. The observed spatial distributions of the Stokes light also confirm the presence of suppression along the direction of propagation of the incident linearly polarized laser light. Numerical calculations are in good agreement with the experimental results.