单纵模Nd∶YAG激光抽运CH_4中强后向受激拉曼散射

用单纵模Nd∶YAG二倍频激光波长532 nm,线宽Δνp<100 MHz,脉宽(半峰全宽)6.5 ns]抽运CH4气体,观察到很强的后向一级斯托克斯(BS1)受激拉曼散射,这与前人采用脉宽30 ns的单纵模抽运激光得到的绝大部分为后向受激布里渊散射(SBS)完全不同,其原因是脉宽6.5 ns与本实验条件下CH4的受激布里渊散射声子寿命接近,受激布里渊散射处于瞬态。理论计算表明,这时的受激布里渊散射瞬态增益系数已略小于后向一级斯托克斯的增益系数,而被其竞争抑制。当脉冲重复频率为2 Hz,抽运能量为95 mJ时,在1.1 MPa CH4中,后向一级斯托克斯的量子转换效率高达73%,其时间波形出现张弛振荡,脉宽被压窄到1.2 ns,从而使后向一级斯托克斯峰值功率达到了抽运激光功率的2.7倍,而且其光束质量要大大优于抽运激光的光束质量。用编制的准二维计算机模型程序相当好地再现了实验中后向一级斯托克斯的时间波形张弛振荡。

Generation of Intense Backward Stimulated Raman Scattering in CH4 Pumped by a Single Longitudinal Mode Nd∶YAG Laser

Highly intense backward first-Stokes (BS1) in CH4 is generated by using a single longitudinal mode second-harmonic Nd∶YAG laser 532 nm, linewidth Δνp <100 MHz， pulse-width (FWHM) 6.5 ns] as the pump source, which is radically different from previous reports where the output from a longer pulse-width (30 ns) pump laser is mainly stimulated Brillouin scattering (SBS). This is because that in authors' experiments the pulse-width is comparable with the SBS phonon lifetime that results in a transient SBS process. According to authors' calculation, the gain coefficient for the transient SBS in the present case is already somewhat lower than that of BS1, and SBS is suppressed by BS1. Under a CH4 pressure of 1.1 MPa, a pump laser repetition rate 2 Hz and laser pulse energy 95 mJ, the quantum conversion efficiency of BS1 is as high as 73%. Due to the relaxation oscillation, the BS1 pulses are narrowed to about 1.2 ns. As a result, the BS1 peak power turns out to be 2.7 times that of the pump. Furthermore, the beam quality of BS1 is much better than that of the pump. The relaxation oscillation in the experimental BS1 temporal waveform is well reproduced by a calculation using a quasi-two dimensional computer program.