介质FC-770受激布里渊散射脉冲压缩Stokes线宽特性研究(特邀)

Research on the Stokes linewidth characteristics of the pulse compressionby stimulated Brillouin scattering in medium FC-770 (invited)

布里渊散射是指入射到介质的泵浦光束与介质内的弹性声波声子发生相互作用而产生的三阶非线性光散射现象。在受激布里渊散射(SBS)脉冲压缩过程中,时域脉宽压缩研究较为广泛,但压缩过程中频域线宽变化与介质粘度、折射率等特性密切相关,因此文中研究聚焦单池结构下SBS脉冲压缩过程中介质FC-770产生的Stokes线宽变化影响因素,发现随着泵浦能量的增加,Stokes线宽先迅速增加后逐渐压窄至400 MHz左右,而随着产生池前透镜焦距的逐渐减小,Stokes线宽迅速压窄,且随能量变化线宽变化范围减小,最后探究了入射激光线宽对Stokes线宽变化的影响因素,得出介质FC-770产生的Stokes线宽值与入射激光线宽呈正比例关系,入射激光线宽值由280 MHz变化到450 MHz左右时,输出的Stokes线宽值由500 MHz变化到680 MHz左右。

Objective Brillouin scattering is a third-order nonlinear light scattering phenomenon resulting from the interaction of a pump beam incident on a medium with elastic acoustic phonons within the medium.The stimulated Brillouin scattering(SBS)technique is widely used for its high-energy conversion efficiency,small Brillouin frequency shift,and phase conjugation.Currently,the SBS technique has been widely used in Brillouin spectroscopy,pulse width compression,and beam combination,in which the frequency shift and linewidth are two essential parameters of Brillouin scattering.The frequency shift and linewidth have been successfully applied in Brillouin spectroscopy to differentiate the information about medium type,concentration,and temperature characteristics. Previous studies mainly focus on measuring time-domain pulse width in SBS pulse-widthcompression. The study of frequency-domain linewidth variation is less involved. In contrast, the linewidthvariation characteristics are closely related to the information about medium viscosity, temperature, and refractiveindex, so it is of great significance to investigate the linewidth variation characteristics during SBS compression.Methods The experiment probes the linewidth change of the medium FC-770 during SBS compression byfocusing a single-cell setup, which is mainly composed of a generator and a long focusing lens and ischaracterized by controllable incident energy and a simple structure. The linewidth change incident to the secondstagegenerator is controlled by the compact two-cell setup, which has high-energy conversion efficiency, and thetime-domain waveform of the incident pulse generated by the compact two-cell setup can maintain a better shapeto avoid the effect of the time-domain waveform on the linewidth change. Due to the low laser repetitionfrequency during SBS compression, the Fabry-Pérot (F-P) combined with the COMS beam profilers (CBP) isselected to measure Stokes linewidth at low repetition frequency. The interference scattering plot is obtained byprocessing the interference circle acquired by CBP, and the Stokes intrinsic linewidth value is obtained after nonlinearfitting.Results and Discussion During the SBS compression, pump energy, lens focusing parameters, and incidentlaser linewidth influence the Stokes linewidth output. With the increase of the pump energy, the energyreflectivity increases rapidly, and the Stokes linewidth first increases rapidly and then gradually narrows. Whenthe pump energy is 35 mJ, the Stokes linewidth can be narrowed to about 400 MHz (Fig.5). As the focal length ofthe lens increases, the energy density at the focal point decreases, but the increase in focal length of the lensincreases the effective interaction length of the pump light with the back-transported Stokes light, and the Stokeslinewidth increases rapidly (Fig.6). By taking a compact two-cell setup to control the linewidth incident to thegenerator, it can be seen that as the value of the linewidth incident to the second-stage generator graduallyincreases, the value of the output Stokes linewidth gradually increases, and when the value of the linewidthincident to the second-stage generator varies from 280 MHz to about 450 MHz, the value of the output Stokeslinewidth varies from 500 MHz to about 680 MHz (Fig.8).Conclusions During the SBS compression process, the variation of the Stokes linewidth of the medium FC-770output shows a tendency of first increasing and then rapidly narrowing with the increase of the pump powerdensity. As the lens's focal length in front of the generator decreases, the Stokes linewidth becomes narrower.When the focal length of the lens is small, the Stokes linewidth variation is less effective by the pump energy. Theinput laser linewidth is controlled by secondary compression, and the Stokes output linewidth gradually broadensas the incident linewidth value increases. From the experiments, it can be seen that the linewidth variation in theSBS compression process has a specific law, and the frequency domain linewidth variation contains richinformation about the medium properties, so the study of Stokes linewidth variation in the SBS compressionprocess is of great significance for the study of medium properties.