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

Abstract

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 quasi-three-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<sup>3</sup> and 1.434 kW/cm<sup>3</sup>, 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.