tion efficiency for a 5900 K black body reaches 11% with Nd:YAG and 26% with codoped Nd:Cr:GSGG matrix 5. The experiments carried out with Nd:YAG have given promising results (1.5% overall efficiency, emitted power > 60 W) but this is still insufficient, due to the thermal effect of the pumping process and the accuracy of solar pumped laser medium coupling. Today the sun-pumped laser appears very promising, in particular due to the availability of a new type of solid-state laser medium. For example, the Alexandrite or Ti:Al2O3 crystals have an absorption band well centered on the sun spectrum, a high quantum efficiency and a thermal conductivity better than most other materials. Within this context solid state solar pumped lasers promise higher efficiency. Note that the use of a Ti:Al2O3 crystal might be questionable because such sources require high concentration collector. Finally even though little work has been done on solar pumped lasers, their development for power transmission in space seems very promising. Compared to other candidates, their advantages include: • the photon-photon conversion process, which allows the use of reflectors which are lighter and smaller than solar panels; • a direct conversion which eliminates additional energy consumption through intermediate processes; • overall efficiency fairly competitive with other laser candidates (sun-laser efficiency ratio: 5 to 10%); • a less complex and more rugged technology; • an emission band (0.7 -1 pm) well matched to the quantum efficiency curve of Si or GaAs cells. In conclusion, sun-pumped laser research and development phases should focus on the following points: • optimization of the conversion efficiency of sunlight to laser light; • thermal issues; • geometrical amplifier design able to increase conversion efficiency and lessen thermal effects; • effective heat removal; • elimination of useless solar flux; • research to find the best crystal.
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