2μm人眼安全波段太阳光抽运激光器的理论研究

太阳光直接抽运激光器在空间光通信、遥感等领域有着重要的潜在应用,但是一直以来人们对太阳光抽运激光器的研究局限于以掺Nd3+粒子为增益介质的1μm波段.通过对现有固体激光工作物质的吸收谱进行分析,发现掺Tm3+离子在太阳辐射较强的可见光波段具有强的吸收峰,使2μm人眼安全波段实现太阳光直接抽运激光输出成为可能.本文对Tm:YAG和Tm:YAP两种常见晶体的吸收谱与太阳光谱匹配度进行了分析计算,得出两种材料用于太阳光抽运激光器的阈值抽运功率密度分别为1.14和1.434 kW/cm3.选择与抽运阈值功率密度低的Tm:YAG晶体作为增益介质,使用TracePro软件建立太阳光抽运激光器的二级抽运模型,并对模型进行优化,得到了锥形腔窗口与菲涅耳透镜的最佳距离、晶体棒的最佳长度以及锥形腔最佳锥度.本文的工作为实现太阳光直接抽运2μm激光输出做了理论上的准备.

Theoretical study of eye-safe 2μm laser directly pumped by sunlight

Solar energy has become one of the new types of energy sources for humanity in the future due to its abundant recourse, clean use and huge reserve. Solar-pumped laser has potential applications in free space optical communications, remote sensing and other fields. However, the research on solar-pumped laser is limited to 1 μm band with neodymium-doped material as a gain medium. To expand the output wavelength range of solar-pumped solid-state lasers, thereby expanding their application fields is one of the goals pursued by researchers in the field. According to the analysis of the absorption spectra of existing solid laser materials, we find that the thulium-doped crystals also have strong absorption peaks in visible light band where solar radiation is strong. Therefore, it is possible that solar-pumped laser could also generate output at 2 μm eye-safe wavelength. In this paper, the absorption spectrum and spectral matching of two common laser crystals -Tm:YAG and Tm:YAP with solar spectrum are analysed and calculated. According to the quasithree-level transition rate equation of thulium ion and the model of solar-pumped laser system, we obtain the theoretical threshold pump power densities of these two crystals to be 1.14 kW/cm^3 and 1.434 kW/cm^3,respectively. We choose the Tm:YAG crystal with lower threshold pump power density as the gain medium and built a two-stage pumping model with TracePro software. In our model, Fresnel lens is the primary solar light concentrator, and a conical cavity with diffusion reflection surface is used as a secondary concentrator to couple the solar energy to laser crystal. Laser setup parameters such as the distance between the Fresnel lens and the window of conic cavity, length of crystal, taper of conic cavity are optimized with the model. The work in this paper offers a valuable reference for future experimental research of 2 μm solar-pumped laser. Finally, we point out the challenge of the future work. Special attention needs to be paid to the huge thermal effect caused by a large amount of sunlight shining on the Tm:YAG crystal. We could find a new kind of diffuse reflection coolants or use thermally bonded crystals to mitigate thermal effects. It will be the focus of future work.