固体反斯托克斯拉曼激光器的特性研究

Stimulated Raman scattering (SRS) is an efficient method to generate new laser lines. SRS includes Stokes scattering based on down-frequency conversion and anti-Stokes scattering based on up-frequency conversion. In recent years, crystalline Raman lasers based on Stokes scattering have become the focus of researches and have been widely used. However, the output wavelengths of the Raman lasers are still limited for practical applications. So, the study of anti-Stokes lasers is also necessary. So far, anti-Stokes lasers have received much less attention internationally. The conversion efficiencies are also very low (<2% under nanosecond laser pulses). Since the anti-Stokes scattering wavelengths depend on the pumping wavelength and the Raman media, the existing laser wavelengths can be expanded further by utilizing anti-Stokes scattering and changing pump and Raman media in order to meet the needs of different applications. In addition, the heat generated by Stokes scattering can be absorbed by anti-Stokes scattering process. This provides another way of thinking which weakens the thermal effect of the Raman laser. In this project, based on some researches of solid-state anti-Stokes lasers, the efficient approach of improving the conversion efficiency of nanosecond-pulse anti-Stokes laser will be studied in theory and experiment. The collinear generation of anti-Stokes laser will be proved and realized. In brief, this proposal has provided a new idea for laser frequency conversion, and will lay a good basis for the further study.

受激拉曼散射是实现激光频率变换的一种有效手段，包括频率下转换的斯托克斯散射和频率上转换的反斯托克斯散射。近年来，基于斯托克斯散射的固体拉曼激光器成为了研究的热点，然而其输出波长仍然有限，还远不能满足应用需求。因此，固体反斯托克斯激光器的研究有同样的必要性。到目前为止，国际上对固体反斯托克斯激光器的研究仍然比较少，且转换效率均比较低（纳秒脉冲<2%）。由于反斯托克斯光波长由泵浦光和拉曼介质决定，因此通过调整泵浦光，改变拉曼介质可以进一步拓展激光波长。另外，反斯托克斯散射过程会吸收介质中的热量，该效应为减弱拉曼激光器中的热效应提供了另外一种思路。本项目在对固体反斯托克斯激光器进行了一定研究的基础上，将从理论和实验两方面研究提高纳秒脉冲反斯托克斯激光器转换效率的方法，并实现由泵浦光和一阶斯托克斯光共线相互作用获得反斯托克斯激光输出。该研究为激光频率变换提供了一种新的思路，并可为后续研究打下基础。

利用固体拉曼介质的受激拉曼散射进行频率转换是扩展现有光谱范围的一种有效途径，基于频率下转换的反斯托克斯散射可以进一步填补光谱区的空白，本项目研究的固体反斯托克斯激光器是获得较高能量和峰值功率新波段的一种有研究前景的手段。本项目从以下三方面进行了研究：为了解决相位匹配时泵浦光和斯托克斯光光束的空间走离，计算了常用拉曼晶体共线产生反斯托克斯光时的相位匹配角，实验上采用1064nm激光作为泵浦光，YVO4晶体作为拉曼晶体，实现了外腔泵浦的共线反斯托克斯激光器，获得了波长为972nm的反斯托克斯激光的输出，反斯托克斯光的最大脉冲能量为0.3mJ，相应的泵浦光-反斯托克斯光转换效率为0.38%。为了解决以往研究中的泵浦光和斯托克斯光脉冲时间延迟问题，采用斯托克斯光种子注入的方法实现了反斯托克斯激光输出，泵浦光脉冲和斯托克斯脉冲时间上的重合大于90%。理论上，对平面波近似下的内腔式反斯托克斯激光器的速率方程进行了归一化处理，通过数值求解了归一化速率方程组，得到描述内腔式反斯托克斯激光器运行的一组普适理论曲线，分析了复合归一化变量对内腔式反斯托克斯激光器性能的影响.通过实验数据对归一化理论进行了验证，结果表明，从归一化理论出发所估计出的内腔式反斯托克斯激光器的单脉冲能量、脉冲峰值功率和脉冲宽度与实际测量数据相符，证明了所提的归一化速率方程组模型的正确性和可行性。