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. Solar-pumped solid-state lasers are promising for many applications. Among the potential applications of solar lasers are Earth, ocean, and atmospheric sensing; laser beaming; deep space communications; and energy support in space. A solar laser has also a large potential for many terrestrial applications, e.g., high temperature materials processing, magnesium–hydrogen energy cycle and so on. Solar-pumped lasers are natural candidates for applications where sunlight is plentiful and other forms of energy sources are scarce. As solar energy is the main continuous energy source in space, this technology becomes particularly attractive for space-based applications. Compared with electrically powered lasers, solar-pumped lasers benefit from simplicity and reliability because of the complete elimination of the electrical power generation and conditioning equipment. Since the report of the first sun-pumped solid-state laser, several pumping schemes have been proposed for enhancing solar-laser performance. Although the most efficient solar-laser systems have end-pumping approaches, the thermal loading effects caused by nonuniform distribution of absorbed pump light in these pumping configurations negatively affect their efficiencies. The side-pumping configuration can present higher laser beam quality as it allows uniform absorption distribution along the laser rod axis and spreads the absorbed power within the laser medium, reducing the associated thermal loading problems. Here, we report a review of research carried out on Nd:YAG solar-lasers with continuous wave emission, using either end-pumping or side-pumping techniques with a special focus on the thermal loading effects on the solar-lasers performance. In the end-pumping configuration, record-high multimode collection efficiency of 32.1  W  /  m2 is attained by pumping a 6-mm diameter, 95-mm length, Nd:YAG/YAG rod with 1.03  m2 area Fresnel lens. However, large beam quality factors of (Mx2=My2=61) have been associated with this approach, resulting in poor beam quality. In fundamental-mode operating, TEM00-mode, with this pumping technique, the highest collection efficiency is 7.9  W  /  m2 by pumping a 4-mm diameter, 35-mm length, Nd:YAG rod with 1.18-m2 area parabolic mirror with the lowest beam quality factors of 1.2. In the side-pumping configuration, the multimode collection efficiency was quite low; in contrast, the beam quality factors are much reduced. Low beam quality factors of (Mx2=8.9) and My2=9.6) are obtained by pumping a 4-mm diameter, 30-mm length, Nd:YAG rod with 2.88-m2 area parabolic mirror with only 9.6  W  /  m2 multimode collection efficiency. The highest multi-mode collection efficiency achieved with this pumping method is 11.7  W  /  m2 by pumping a 7-mm diameter, 30-mm length, Cr:Nd:YAG rod with 2.88-m2 area parabolic mirror. In the TEM00-mode regime, using the side-pumping method, the maximum collection efficiency is 4.0  W  /  m2 by pumping a 4-mm diameter, 25-mm length, Nd:YAG rod with 1.13-m2 area parabolic mirror. Beam quality factors <1.05 are reached by pumping a thin and long Nd:YAG rod (3-mm diameter and 50-mm length), with 1.18-m2 area parabolic mirror. More importantly, a TEM00-mode solar-laser with 1.7% laser power stability is produced, being significantly more stable than the previous TEM00-mode solar-lasers.

Thermal loading effects on Nd:YAG solar-laser performance in end-pumping and side-pumping configurations: a review