HIGH POWERED DIFFRACTION LIMITED RAMAN LASER

Abstract : Problems associated with the realization of a high energy, highly coherent Raman laser have been investigated. Since the discovery that the self- focusing of pump radiation and scattered radiation is present in most laboratory Raman lasers and tends to spoil the coherence of the output, the nature and mechanisms of this self-trapping have been the primary objects of study in order that it might be prevented. Time resolved photographic studies have been made which show that the small filaments of self-trapped light in the pump beam persist for only a small fraction of the beam pulse duration and are sufficiently intense to resolve the apparent discrepancy between the average intensity and observed Raman gains. Time integrated photographic studies and photometric studies showed that only purely linearly polarized light preserves its polarization after self-trapping in CS2 and nitrobenzene. Attempts to compare measured nonlinear indices for both circular and linear polarizations with those calculated for various mechanisms were frustrated by this instability in the propagation of circularly polarized light. The instability was explained qualitatively and indicates that electrostriction is not a dominant mechanism. A classical statistical mechanical calculation of the nonlinear index of a liquid (with no macroscopic density changes) showed that the local redistribution of molecules is generally important. This effect will cause perfectly symmetric molecules to have a significant nonlinear index. It is therefore concluded that it is very unlikely that any room temperature liquid will perform as a high energy Raman laser material without beam deterioration due to self-focusing.