Radiation-Balanced Lasers: History, Status, Potential

The review of history and progress on radiation-balanced (athermal) lasers is presented with a special focus on rare earth (RE)-doped lasers. In the majority of lasers, heat generated inside the laser medium is an unavoidable product of the lasing process. Radiation-balanced lasers can provide lasing without detrimental heating of laser medium. This new approach to the design of optically pumped RE-doped solid-state lasers is provided by balancing the spontaneous and stimulated emission within the laser medium. It is based on the principle of anti-Stokes fluorescence cooling of RE-doped low-phonon solids. The theoretical description of the operation of radiation-balanced lasers based on the set of coupled rate equations is presented and discussed. It is shown that, for athermal operation, the value of the pump wavelength of the laser must exceed the value of the mean fluorescence wavelength of the RE laser active ions doped in the laser medium. The improved purity of host crystals and better control of the transverse intensity profile will result in improved performance of the radiation-balanced laser. Recent experimental achievements in the development of radiation-balanced RE-doped bulk lasers, fibre lasers, disk lasers, and microlasers are reviewed and discussed.