Stimulated Rotational Raman Scattering in the Atmosphere

Theoretical and experimental information in the literature is used to calculate the gain coefficient for stimulated rotational Raman scattering by atmospheric N₂ and O₂. The dependence on laser wavelength and polarization, as well as on the pressure and altitude is discussed. It is pointed out that because of pressure broadening, the gain coefficient is independent of altitude up to an altitude of 40-50 km, where the Raman transition becomes Doppler broadened. The 1 percent conversion threshold for vertical propagation from the ground up is calculated for various transitions and laser beam characteristics. The highest-gain transition, the S(8) transition of N₂, is shown to have a 1 percent conversion intensity threshold of ~1.2 MW/cm2 for linearly polarized light at a wavelength of 400 nm.