0191-9067/85 $3.00 + .00 Copyright ® 1985 SUNSAT Energy Council A SOLAR-PUMPED LASER ON THE SPACE STATION H. ARASHI, Y. OKA and M. ISHIGAME Research Institute for Scientific Measurements Tohoku University 1-1, 2-chome, Katahira Sendai, Japan INTRODUCTION A laser light source is indispensable in space for optical communication, laser propulsion, energy conversion and laser processing, etc. Concentrated solar radiation can be used to optically excite a laser medium. A laser which is directly excited by solar radiation is called a solar-pumped laser. In a solar-pumped laser system, a high conversion efficiency of energy is expected because of direct photon-photon conversion. In addition, an available energy source in space is limited to solar energy. Therefore, a direct conversion of solar radiation into coherent laser light is one of important space technologies. The mission objective is to test the operation of a solar-pumped laser in space. A solar-pumped laser is classified into two types according to the employed laser medium — one a solid-state laser, the other a gas laser. A SOLAR-PUMPED SOLID-STATE LASER A solar-pumped solid-state laser with several hundred W output power is profitable for optical communication in space because the problem of recharging of the laser medium does not occur. We have examined on the Earth a solar-pumped Nd:YAG laser by using a 10 m aperture paraboloidal solar concentrator equipped in the Solar Energy Laboratory of our Institute (1). The experimental setup of our solar-pumped Nd:YAG laser is shown in Fig. 1. The maximum output power of our laser has exceeded 18 W in multi-mode, which is the highest power so far reported on solar- pumped lasers. On the basis of these experimental results, we construct a solar-pumped solidstate laser to be used on the space station. In first mission phase, the paraboloidal solar concentrator, 2 m in aperture, will be mounted on the space station via a two-axis gimballed sun-orientation mechanism as shown in Fig. 2. Using this concentrator, a solar-pumped solid-state laser will be tested to estimate a stability of mode- locked operation which is required for optical communication. The optimization of laser pumping optics will be also examined to increase the conversion efficiency. In this laser system, the laser rod must be placed at the focal point of the concentrator to be efficiently pumped by concentrated solar radiation. Hence, the direction of the laser beam is fixed by the orientation of the concentrator. To remove this restriction, in the second mission phase, a solar radiation concentrated by free-flyer type parabolic mirror will be transferred to the laser pumping
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