Enhanced stimulated Raman scattering by suppressing stimulated Brillouin scattering in liquid water

Abstract

Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are two kinds of emblematic inelastic scattering processes resulting from the interaction of high-intensity laser with matter. Generally, competition between SBS and SRS is a common phenomenon in many substances. In liquid or high-pressure gas, if a single longitudinal mode laser is used as a pump source, both SBS and SRS can be excited, but the SBS will become very strong due to higher gain and optical phase conjugation. In comparison, the SRS gain is typically 2 orders of magnitude smaller than the SBS gain so that most of the pump laser energy is spent on the SBS and the SRS is greatly suppressed. To improve the output energy of SRS in liquid medium, a method of suppressing the SBS process by controlling temperature of medium is proposed. The SRS generation system using broadband pulse laser of 532 nm in wavelength as a pumping source is designed, the output energy of forward SRS (FSRS) and backward SBS (BSBS) in water with different temperatures are measured, and the physical mechanisms of the influences of water temperature, pumping linewidth and thermal defocusing on the output energy of SRS are analyzed. The experimental results indicate that by reducing the water temperature, the SBS process can be significantly suppressed, and the beam distortion caused by thermal defocusing effect can be reduced, thus effectively improving the output energy of SRS. Unlike the single longitudinal mode laser, when the pump source is handled in multiple longitudinal modes with a wide linewidth, the gain of FSRS is higher than that of the backward SRS (BSRS). Meanwhile, since the SBS gain coefficient is restricted by the linewidth of the pump laser, the FSRS process is dominant and both backward SBS and BSRS are significantly suppressed. It is necessary to state that none of the influence of backward SRS, self-focusing, optical breakdown and other non-linear effects on the output energy of SRS is considered in this paper, and only the effectiveness of reducing temperature to improve the energy output of forward SRS is verified from the perspective of temperature change. The results are of great significance for the multi-wavelength conversion of SRS in liquid medium.