Gain, Frequency Shift, and Angular Distribution of Stimulated Raman Radiations under Multimode Excitation

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Abstract
A theoretical explanation of anomalous gain, large frequency shift, and variable angular distribution in the stimulated Raman radiation from liquids under multimode excitation is presented. The laser output under multimode oscillation is treated as a series of very sharp pulses which come out at the recurrence frequency of axial mode separation. The anomalous Stokes gain and its apparent independence of the mode structure are explained when the relaxation time of the molecule is between 10-9 and 10-8s. The intensity-dependent index of refraction of the medium produces phase shifts in the Raman radiation. This is shown to produce considerable frequency shifts and changes in the angular distribution of the Raman radiations which are emitted from a filamentary region under multimode excitation.