Abstract
Spectral super-broadening (could be up to ${\ge} 450\;{{\rm cm}^{- 1}}$) has been recently observed for backward stimulated Raman scattering (SRS) in several liquids under the pump conditions using 532 nm and ${\sim}{11}\;{\rm ns}$ 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 of successive sub-pulses of ${\le} 50\;{\rm ps}$ 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 ${{\rm cm}^{- 1}}$, 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.
© 2020 Optical Society of America
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