# Radiation-Balanced Lasers: History, Status, Potential
## 5 Conclusions  
The presented work gives an overview of the latest achievements in a very promising and intensively developing area of laser physics known as radiation-balanced or athermal lasers. It is shown that the radiation balance can be maintained using optical refrigeration based on anti-Stokes fluorescence in the system of RE ions doped in a low-phonon host material. Optical refrigeration is the only solid-state technology that can cool to cryogenic  
Figure 19: (**a**) The core of the fibre is Yb\({}^{3+}\)-doped glass. The inner region of the cladding (yellow) is composed of glass and YLF nanocrystals, and the outer region of the cladding (blue) is made of the same glass as the inner cladding region, but with no Yb\({}^{3+}\):YLF. (**b**) The fibre cross section [60].  
Figure 20: Schematics of (**a**) a radiation-balanced microlaser and (**b**) a 3D composite laser refrigeration microsphere [63].  
temperatures. A basic model describing the operation of radiation-balanced lasers has been presented and discussed. The analytic expressions describing radiation-balanced laser operation were obtained using the set of coupled rate equations. The requirements for radiation balance predict that the characteristics of a radiation-balanced laser are affected by cavity parameters and fundamental material properties. The pump wavelength is the most sensitive parameter. To attain optical refrigeration in the laser, the pump wavelength must exceed the mean fluorescence wavelength.  
At the present time, athermal lasers follow four main designs: radiation-balanced bulk and fibre lasers, radiation-balanced disk lasers, and athermal microlasers. All four designs considered in this review are very promising for different applications. Anti-Stokes cooling of RE-doped bulk, fibre, and disk lasers have been experimentally demonstrated. Both experiments and calculations indicate that improving the purity of host crystals will result into improving athermal laser performance. The development of new RE-doped low-phonon materials can facilitate the realization of athermal lasers and accelerate their commercialisation. Recently, [PERSON] et al. [64; 65] proposed to use Yb\({}^{3+}\)-doped yttrium aluminum perovskite (YAP) as an active medium of radiation-balanced lasers. YAP has properties that are similar to those of YAG, but it possesses a phonon energy \(\left(620\text{ cm}^{-1}\right)\) that is lower than that of YAG \(\left(865\text{ cm}^{-1}\right)\). This lower phonon energy is a source of higher quantum efficiency. It has been shown that the small-signal gain of athermal laser materials utilizing an \(f-f\) transition of RE ions can be controlled by engineering the surrounding crystal fields of the RE dopant.  
The further average power scaling requires better control of the beam intensity profile. Recently, athermal laser operation with mode-mismatched Gaussian and super-Gaussian beams was analyzed by [PERSON] and [PERSON] [66]. As one can see in [66], in a disk geometry, beam-area scaling to high-power operations can be accompanied by large transverse temperature gradients. These undesirable gradients can be countered by pump beam shaping and/or employing longer gain media.  
Considering four main designs of radiation-balanced lasers, I believe that athermal disk lasers will be the first commercially available radiation-balanced lasers in the coming years. Indeed, the disk shape of a gain medium provides significant freedom in the control of pump power distribution, which is a key element for athermal operation of the laser.  
This research received no external funding.  
Not applicable.  
The author declares no conflict of interest.