1 W of w laser output has been obtained at room temperature from a sun-pumped, neodymium-doped YAG crystal. The water-cooled laser rod was pumped with a modified Cassegrain sun-tracking telescope consisting of a 61-cm diam paraboloidal primary mirror collector, a water-cooled hyperbolic-cylindric secondary mirror and a hemicircular cylindric tertiary mirror. The cylindrical image volume was coincident with 24 mm of the 3-mm by 30-mm YAG rod. The spike-free output was obtained for hours at a time with a late October sun at a 42° North latitude. Using the same primary mirror and near-unity numerical aperture refractors, approximately 1.25 W were obtained in 7 msec pulses from an uncooled, sun-pumped, neodymium-doped, glass rod. Further refinements in the telescope and the laser crystal, and a space environment, should allow 1 W of laser output to be generated by using a 30-cm diam collector.
C. G. Young, "A Sun-Pumped cw One-Watt Laser," Appl. Opt. 5, 993-997 (1966)