Observation of parametric gain suppression in stimulated rotational Raman scattering over long air paths

The authors report the first observation of parametric gain suppression of stimulated rotational Raman scattering (SRRS) in atmospheric N/sub 2/ in propagation experiments over long air paths with a large-aperture large-Fresnel-number beam. They previously reported our observations of small-scale beam breakup in the near field and high-divergence speckle in the farfield in strong SRRS conditions. In their most recent data this speckle is absent in a far-field zone surrounding the forward direction and appears in a narrow cone peaked at an angle of ≥300 μrad. They believe this is a manifestation of Raman gain suppression. Suppression of the exponential gain in stimulated Raman scattering in low-dispersion media by competition with four-wave parametric mixing of pump, Stokes, and anti-Stokes waves was first recognized by Shen and Bloembergen. When the anti-Stokes wave is phase matched for generation in the near-forward direction, excitation of an optical phonon wave is prevented by the upconversion of pump photons to anti-Stokes photons. Stimulated growth of the Stokes wave is thus suppressed because the phonon excitation is damped