As mentioned in the article of D. Liang and J. Almeida published in the Applied Optics, the idea of direct converting the broad-band non-coherent solar radiation into coherent and narrow-band laser radiation is almost as old as the laser itself. Because mankind uses only a negligible part of the energy that our planet receives from the sun, solar energy remains to be the largest source of renewable energy. On the other hand, any optically pumped laser material can also be used as a solar laser. If lasers are needed in remote off-grid locations with a plenty of sunlight, a solar-pumped laser design seems to be a natural choice. Among the potential applications of solar lasers are earth, ocean, and atmospheric sensing; detecting, illuminating, and tracking hard targets in space; deep space communications. Another encouraging application for high power solar-pumped lasers was recently proposed by Japanese scientists. T. Funatsu with colleagues from the Tokyo Institute of Technology introduced a fossil-fuel-free energy cycle where energy is produced by a chemical reaction of magnesium with water. Hydrogen and heat energy from this reaction can be used for engines and fuel cells, while magnesium oxide, the reaction product, is reduced back to magnesium by a solar-pumped laser. MgO deoxidization (at temperature of ~ 4000 K) can be easily obtained by high power laser radiation focused into a small spot.

Among the diversity of lasers, the solid lasers seem to be most attractive for solar pumping because of their inherent high energy density, compactness, relatively low pumping threshold, and potential efficiency of solar-to-laser power conversion. However, the compactness of solid-state lasers makes them sensitive to thermal loading during the pump process: thermal distortions and thermal lensing in the solid-state laser element impose a limit to the power conversion efficiency and quality of laser beam.

D. Liang and J. Almeida proposed a new concept in this paper, which is to apply thin-disk laser technology to solar-pumped lasers, which initially was developed for diode-pumped laser systems with high output power. Due to the small volume-to-surface area ratio, the gain medium in the thin-disk design can be cooled very efficiently. The optimal laser parameters for this system were modeled through ZEMAX© and LASCAD© numerical analysis, which resulted in a significant enhancement of brightness figure of merit - 180 times higher - compare to the previous record. This can improve noticeably the efficiency of solar-pumped laser systems and could be regarded as the next important step for an access to a convenient supply of renewable and clean energy.

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