Nonlinear competing processes in multifrequency Raman generation

Abstract

Ultra-broadband multifrequency Raman generation (UMRG) is one of the most novel nonlinear optical processes to have emerged over the last few years. With H/sub 2/ gas as the Raman medium, we predicted that nearly 50 distinct frequencies of comparable energy may be generated. For UMRG in air, beams containing around 150 waves may be attained. Experimental results that support our overall predictions are appearing in the literature. However, some configurations exhibit competition between distinct Raman resonances. This feature may have serious consequences for many of the proposed applications for UMRG. Here, we present new analyses and simulations that take such nonlinear competing processes into account. An effective gain-length product for a nonparametric parasitic wave is introduced and its characteristics examined. For competing processes generated from background noise or amplified spontaneous emission, an analytical model is derived. Predictions of this model are found to be in good agreement with numerical simulations. The roles played by pump intensity, dispersion, transiency, and the initial level of the competing process are systematically investigated. The efficiency and character of UMRG, with resonant and symmetric pumping, is found to keep such nonlinear competing processes below threshold. UMRG is also found to be robust when the competing signal grows from a strong seed, as could arise from the scattering of a pump beam off an optical element. It is also possible that UMRG lines could parametrically generate a parasitic wave through scattering involving a distinct Raman transition.