Influence of Diffraction on the Stimulated Raman Scattering of Laser Radiation in the Middle Atmosphere
The propagation of laser radiation in the middle atmosphere is accompanied by many nonlinear effects, among which the central place is occupied by the phenomenon of stimulated Raman scattering. This effect is caused by buildup of seed thermal vibrations of gas molecules exposed to the incident and scattered waves, which can lead (after excitation of some intensity threshold) to appearance of stimulated Raman scattering. The SRS effect in the atmosphere and its influence on the parameters of the propagating laser beam are studied theoretically and experimentally in numerous papers (see References 3, 23, and bibliography therein). It was found that at near-surface optical paths the SRS effect is masked by many other nonlinear effects, such as the thermal blooming, turbulent broadening of the beam, self-focusing due to the optical Kerr effect, and defocusing at the plasma of optical breakdown of air. As a laser source operates at long high-altitude paths under conditions of rarefied atmosphere, the role of SRS becomes predominant [47—49], and just this effect leads to the power loss of the main radiation owing to the radiation conversion into other frequency ranges  (Stokes and anti-Stokes components) and transformation of the spatial and energy spectrum of the beam [45,51,52].
At the same time, in addition to the nonlinear optical phenomena accompanying the laser energy transfer in the atmosphere, there are always linear effects, in particular, the radiation diffraction at the exit aperture of the laser source and absorption in atmospheric gases and aerosols. If the linear absorption can be often neglected in the case of an airborne laser (according to Reference 48, the mean coefficient of volume absorption of the middle atmosphere in the IR spectral range is Th0-3 km-1), the diffraction effects leading to a change in the transverse profile of the beam intensity should impact quite significantly the energy characteristics of the main radiation under the conditions of SRS. There are two different opinions on the effect of beam diffraction on the value and dynamics of energy exchange between waves at the stimulated scattering. On the one hand, it is believed that the diffraction broadening of the beam radius of the main radiation should decrease the efficiency of amplification of the Stokes components . On the other hand, there are numerical calculations demonstrating the directly contrary role of diffraction as a process amplifying SRS .
In this section, using the oxygen—iodine laser radiation propagating along slant optical paths in the middle atmosphere (at altitudes exceeding 10 km), as an example, this issue is analyzed comprehensively. It is shown that the both opinions are true, and the result (amplification or reduction SRS due to beam diffraction) depends on the beam intensity and the optical path length.