Abstract

Very high intensity laser beams used in laser-fusion experiments are subject to rotational Raman conversion, which generates a Stokes beam—initially by spontaneous Raman scattering and subsequently by stimulated Raman amplification. At the typical irradiance level of gigawatts per square centimeter, the laser may be significantly depleted over several tens of meters. Accurate calculation of the degree of Raman conversion is essential for the design of the device but requires detailed treatment of the combined effects of aberration, spontaneous noise, nonlinear amplification, and diffraction. A wave optics calculation is required but is very difficult because the Fresnel numbers are very high (in the range 1,000 to 10,000). The transient behavior needs four-dimensional calculations. Inevitably aberrations in the upstream part of the beam lead to some fluctuations in intensity. Even very weak hot spots in the laser, because of the amplifications of about exp(30), generate strong hot spots in the Stokes beam. We show these hot spots typically dominate the process and lead to very significant spatial coherence of the Stokes beam and a concomitant decrease in the modeling difficulties.

© 1992 Optical Society of America