Theoretical Explanation Of Spectral Super-Broadening Of Backward Stimulated Raman Scattering In Liquids

Spectral super-broadening (could be up to >= 450 cm(-1)) has been recently observed for backward stimulated Raman scattering (SRS) in several liquids under the pump conditions using 532 nm and similar to 11 ns pulses from a frequency-doubled and Pockels Q-switched Nd:YAG laser device working in multi-longitudinal modes. A theoretical model of Raman resonance-enhanced refractive-index change and the cross-phase modulation (XPM) between the pump sub-pulse and the SRS sub-pulse is presented. Based on the experimental fact that the pump pulse consists o f successive sub-pulses of <= 50 ps duration, the pump-induced phase change experienced by a forward and a backward SRS sub-pulse is calculated separately as a function of the peak intensity of the pump sub-pulse. For an individual forward SRS sub-pulse, it can only interact with one co-propagating pump sub-pulse; thus, the XPM induced spectral broadening remains in a limited spectral broadening range. However, for each individual backward SRS sub-pulse, it can sequentially interact with multiple counter-propagating pump sub-pulses, which results in a much larger broadening range of more than several hundreds of cm(-1), depending on how many pump sub-pulses could interact with a single backward SRS sub-pulse within the Raman liquid. By choosing appropriate numerical simulation parameters, the calculated distribution of spectral broadening can be in good agreement with the experimental result. (C) 2020 Optical Society of America