The threshold for transient stimulated rotational Raman scattering in the atmosphere

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Abstract
A model is presented for the calculation of the threshold for the stimulated rotational Raman scattering of short laser pulses in the atmosphere. The coupled field equations for transient stimulated Raman scattering are modified to account for the variation of the atmospheric density and the variation of the Raman linewidth with altitude. The laser field is assumed to be circularly polarised, in which case there is no parametric coupling of the Stokes and anti-Stokes fields. An effective spontaneous source is defined and the coupled field equations are solved numerically, to obtain the threshold intensity and the threshold energy flux for a laser beam propagating vertically through the atmosphere. These threshold values are calculated as a function of the laser wavelength, pulse width and the beam cross-sectional area.