Abstract

The spectral super-broadening of backward stimulated Raman scattering (SRS) in four liquid samples (${\rm CCl}_4$, dimethyl sulfoxide, acetone, and ${\rm CH}_2{\rm Cl}_2$) have been observed under the pump conditions of using 532-nm and ${\sim}11 {\text -} {\rm ns}$ pulsed laser beams from a frequency-doubled and multilongitudinal mode Pockels Q-switched Nd:YAG laser device. Under the same pump conditions, in acetone the observed maximum broadening range was ${\ge}450\;{\rm cm}^{-1}$ for the backward SRS, while it was ${\le}45\;{\rm cm}^{-1}$ for the forward SRS. The physical origin of this observed effect is essentially related to the detailed temporal structure of the 532-nm pump pulse that consists of a series of subpulse of ${\sim}{50}\;{\rm ps}$ width, owing to the randomly beating effect among a large number of longitudinal modes of the Pockels Q-switched laser source. Each backward SRS subpulse undergoes multiple cross-phase modulation (XPM) by interacting with a certain number of forward pump subpulses, and consequently manifests a super-broadened spectral distribution. In contrast, each forward SRS subpulse interacts only with a single pump subpulse and therefore experiences a much limited XPM influence, resulting in a much smaller spectral broadening. Furthermore, when the 532-nm pump beam was from a single-longitudinal mode-seeded Nd:YAG laser, there was no beating-effect-induced subpulse structure of the pump pulse, and thereby no noticeable spectral broadening for both backward and forward SRS could be observed.

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