Solar Pumped Lasers to Directly Convert Sunlight to Laser Radiation

It has been proposed to use microwaves to beam down power to Earth from space based solar power stations in Geosynchronous Orbit. However the Rayleigh diffraction limited optics requires enormous antennas, typically a few kilometers in diameter, and huge power levels, of 5GW to 10GW. A much lower cost of entry investment (100x lower) is afforded by use of optical or infrared lasers with much shorter wavelength, hence much smaller system size, e.g. 10MW, with one meter size collimator mirror diameter. Since there is no "grid power" in space, most spacecraft today use solar power sources, mostly photovoltaic solar cells. Powering lasers requires high levels of power. PV solar cells are usually less than 25% efficient. This multiplied by a typical 20% “wall plug” efficiency of lasers results in overall a mere 5% efficiency (95% waste heat!). This strongly motivates interest in solar pumping of lasers to greatly increase overall efficiency; theoretical efficiency of about 38% is possible. Other potential benefits of solar pumped lasers would be reduced weight and reduced number of components affording higher reliability (reduced number of failure modes) versus an electrically pumped laser powered from PV cells.