Chapter 2: Threshold for Raman conversion of FEL laser pulses in the atmosphere: Final report

Transmission of a high intensity laser beam through the atmosphere can be affected by a variety of stimulated and parametric nonlinear processes. As a result, gain at other wavelengths will be generated along the propagation path of the beam. But the conversion of the original laser energy into other wavelengths is not the only result. Unless special measures are taken, the wavefront of the Raman shifted radiation will be determined by the turbulence in the atmosphere and by the aspect ratio of the gain medium. These processes will lead to beam divergences which can be significantly greater than the diffraction limit. As a result of these nonlinear processes, it will not be possible to efficiently propagate the beam unless proper measures are taken. Developing a quantitative model is especially important for determining the propagation characteristics of FEL laser pulses through the atmosphere. The objective of this research report is to present such a time dependent model for stimulated Raman conversion of short laser pulse propagating in the atmosphere and to calculate the threshold laser pulse energy and intensity for stimulated rotational Raman scattering as a function of wavelength, pulse width and beam diameter. 12 refs., 10 figs.