单纵模Nd∶YAG激光抽运H_2-He混合气体中的强后向受激拉曼散射

Generation of Intense Backward Stimulated Raman Scattering in H_2-He Mixture Pumped by Single Longitudinal Mode Nd∶YAG Laser

用单纵模Nd∶YAG二倍频激光(波长532 nm,线宽0.003 cm-1,脉宽(半峰全宽,FWHM)6.5 ns)抽运H2/He气体,观察到很强的后向一阶斯托克斯(BS1)受激拉曼散射。在H2的分压分别为1.0 MPa和1.5 MPa的H2/He(体积比)(3/7)混合气体中,当抽运能量为92 mJ时,后向一阶斯托克斯光的量子转换效率高达69%左右,而在纯H2中后向一阶斯托克斯光的量子转换效率分别只有15%和18%。这是因为加入He增强了后向拉曼散射多普勒线宽的Dicke压窄效应,使后向一阶斯托克斯光的拉曼增益系数与前向一阶斯托克斯(FS1)光的拉曼增益系数的比率提高,而且因为后向一阶斯托克斯光与抽运光在反方向传播,它可以提取大部分抽运光的能量,并且脉冲被压窄到1.1 ns,使后向一阶斯托克斯光峰值功率达到了抽运光的2.6倍。从激光光斑的强度分布可以观察到后向一阶斯托克斯光呈现为抽运光的相位共轭波。

Very intense backward first-Stokes （BS1） scattering in H2/He gas mixture is generated by using a single longitudinal mode second-harmonic Nd： YAG laser （532 nm, linewidth 0. 003 cm^-1 , pulse-width （FWHM） 6.5 ns） as the pump source. Under a pump energy 92 mJ, the quantum conversion efficiency of BS1 is as high as 69% in H2/He （3/7） gas mixture when the Hz partial pressures are 1.0 MPa and 1.5 MPa respectively. Under the same H2 partial pressures, but no He, the respective values are only 15 % and 18%. This is because that addition of He enhances Dicke narrowing effect for Doppler broadening of backward Raman scattering, which increases the ratio of gain coefficient of BS1 to that of forward first-Stokes （FS1）. Furthermore, because BS1 propagates counter the pump beam, it can abstract most parts of the pump energy and its pulses are narrowed to 1. 1 ns, so its output power becomes 2.6 times that of the pump. In addition, the luminance distribution of laser spot has shown that in H2-He mixture BS1 represents a wave-front-reversed replica of the pump beam.