Forward Raman gain suppression by StokesantiStokes coupling
Abstract
We present a simple planewave model of Raman gain suppression for offaxis Stokes and antiStokes beams propagating in a finitediameter pump beam. Gain per unit length increases off axis as the Stokes becomes mismatched to the antiStokes. The total path length with gain decreases off axis due to the finite beam. The angles of maximum total gain and of maximum Stokes intensity over threshold is a function of the gain over a distance of one wavelength. This model is applied to the case of stimulated rotational Raman scattering of freeelectron laser pulses of N sub 2 in the atmosphere. The StokesantiStokes coupling does not significantly extend the propagation range of steadystate pulses above threshold. StokesantiStokes coupling causes significant gain suppression for phasematched rays propagating near the beam axis, but, for interesting beam geometries and intensities over the steadystate threshold, the weakly suppressed offaxis rays have much higher gain and reach threshold in a short distance. Including diffraction of transient Raman effects in the analysis might make the gain suppression a little more effective.
 Publication:

NASA STI/Recon Technical Report N
 Pub Date:
 January 1989
 Bibcode:
 1989STIN...8929745M
 Keywords:

 Amplification;
 Beams (Radiation);
 Free Electron Lasers;
 Wave Propagation;
 Mathematical Models;
 Raman Spectra;
 Lasers and Masers