Multifield, circularly polarized seeded stimulated Raman scattering

Abstract

Multifield Raman seeding may be applied to stimulated rotational Raman scattering and is promising for beam-quality control when the Raman gain-length product is large (50). The laser is seeded by two copropagating seed beams, e.g., 1st and 2nd Stokes. The rotational Raman polarization selection rules dictate that each beam must be circularly polarized with alternate heliocities. Because gain suppression is then absent, the Raman gain for seed amplification is high and discriminates well against noise (spontaneous Raman scattering). A parametric process coupling any four distinct waves recursively generates arbitrarily high-order Stokes waves. Because the beam quality of parametrically generated waves should be determined by that of the incident beams, good-quality conversion can occur at very high gain–length products. Code predictions support this conclusion under the following conditions: (1) seeds intense enough to discriminate against initiation by noise, (2) reasonably good beam quality and smooth edges of the incident beams, and (3) good spatial and temporal overlap between the incident beams. In a two-seed experiment on the S(1) transition of H2, it was found that a goodbeam-quality beam core was converted and amplified, but a poor-quality background was also amplified. The Raman code indicates that the observed mixture between good and bad beam quality arises from competition between noise and seeds, which is exacerbated by imperfect temporal overlap between the laser and seed pulses.