2. Laser Research for Power Transmission 2.1. Direct Solar-Pumped Lasers Solar-pumped lasers convert sunlight directly into beamed laser power. Numerous laser media have been investigated, and a representative sample is shown in Table I. Of special interest are the organic-iodide gas laser and both solid state and liquid Nd3+ systems. The organic-iodide laser operates by the cycle shown in Fig. 1. Approximately 99% of the UV-induced photodissociation leads to excited atomic iodine, and after participating in the lasing, the iodine atom recombines with an organic radical to form the ground state with high probability [1], The laser emission is at 1.3 //m in the near infrared. This laser has an acceptable quantum efficiency of approximately 20% [2], The only visible or ultraviolet absorption of this medium is the photodissociation absorption band in the ultraviolet at a wavelength slightly shorter than 300 nm. Thus, while this lasant is not strongly heated by concentrated sunlight, it absorbs only on the order of 1% of the incident solar energy. These values imply that the solar-pumped organic-iodide laser will at best have a solar-to-laser-power efficiency of 0.5%. To effectively transmit useful amounts of power, it will be necessary to emit high average power with the beam quality necessary for projection to a small distant receiver. Sustained high average power requires effective mechanisms for removing heat from a laser medium. A flowing-gas solar-pumped laser provides a natural heat removal system, since the hot gas is removed and replaced by cool lasant. Solar- pumped solid-state lasers with Nd3+ as the lasant are much more sensitive to heat. Neodymium in yttrium aluminum garnet (YAG) or yttrium lanthanum fluoride (YLF) offer the highest ratio of host thermal conductivity to host absorption of solar energy. In such hosts, Nd3+ absorbs greater than 10% of the incident sunlight and, with the proper geometry and cooling, could provide continuous power at an acceptable operating temperature. A geometry for Nd in a glass host which offers the possibility of quasi-static
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