Thermodynamics of radiation-balanced lasing

Athermal lasers dispose of their waste heat in the form of spontaneous fluorescence (i.e., by laser cooling) to avoid warming the medium. The thermodynamics of this process is discussed both qualitatively and quantitatively from the point of view of the first and second laws. The steady-state optical dynamics of an ytterbium-doped KGd(WO4)(2) fiber is analyzed as a model radiation-balanced solid-state laser. A Carnot efficiency for all-optical amplification is derived in terms of the energy and entropy transported by the pump, fluorescence, and laser beams. This efficiency is compared with the performance of the model system. (C) 2003 Optical Society of America.