# Radiation-Balanced Lasers: History, Status, Potential
###### Abstract  
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.  
radiation-balanced lasers; athermal lasers; solid-state lasers; laser cooling of solids; rare earth-doped materials +
Footnote †: journal: Applied Sciences  
and stimulated Raman scattering (SRS), which result in depletion of the amplified signal in ion-doped lasers.  
In 1972, [PERSON] and [PERSON] [10] proposed a new approach to the development of solid-state lasers. It is based on the idea of propagating laser beams in a direction that averages the temperature gradients in the gain medium [11]. The laser beam is injected into the slab so that it will allow the beam to make multiple total internal reflections from the polished sides as it propagates down the slab.  
In 1991, [PERSON] et al. proposed to power a scale Yb\({}^{3+}\):YAG laser using a thin-disk laser design. The first thin-disk laser was demonstrated in 1994 [12; 13]. The core concept of the thin-disk lasers is the use of a thin, disk-shaped active medium that is cooled through one of the flat faces of the disk. The cooled face is used as a mirror of the resonator. This face cooling minimizes the transversal temperature gradient, as well as the phase distortion transversal to the direction of the beam propagation, providing a laser beam of outstanding quality.  
In 1999, [PERSON] [14] proposed a radiation-balanced (athermal) RE-doped bulk laser, which operates _without_ internal heating. In this laser, all photons generated in the laser cycle are annihilated with the cooling cycle, that is, the heat generated from stimulated emission is offset by cooling from anti-Stokes emission. Athermal lasers, which are free from all thermal effects, provide a tremendous potential for an increase in the output power, maintaining a high-quality output laser beam. Beginning in 1999, several kinds of radiation-balanced lasers, including fibre lasers, disk lasers, and microlasers, have been proposed and developed. Some of these lasers have been discussed in [15], while others are considered in this review. Thermal lensing arising in traditional lasers from the quantum-defect heating is briefly discussed in Section 2. Section 3 is devoted to the theory of radiation-balanced lasing. Recent achievements in the development of radiation-balanced RE-doped bulk lasers, fibre lasers, disk lasers, and microlasers are presented in Section 4.