# Radiation-Balanced Lasers: History, Status, Potential
## 4 Development of Radiation-Balanced Lasers
### Athermal Fibre Lasers  
As previously noted, in radiation-balanced lasers and amplifiers, the radiated energy increases only linearly with the length of the active medium. This requires a long active medium, which is undesirable. To overcome the problem, lasers and amplifiers with a cooler made from RE ions and integrated in the body of the device have been proposed and theoretically investigated by [PERSON] and [PERSON] [39; 40; 41].  
As one can see in Figure 13a, the properly distributed Yb\({}^{3+}\) ions pumped at the wavelength 1015 nm with a cooling pump power \(P_{p}{}^{cool}\) work as an integrated cooler, compensating for the heat generated inside the active medium due to the quantum defect in the Raman lasers or amplifiers. In this scheme, the lasing action is separated from the cooling process. Such separation of actions provides flexibility in the laser design. It permits one to maintain the exponential growth of the amplified signal along the length of the laser medium.  
Figure 13b illustrates an athermal Yb\({}^{3+}\)-doped fibre laser, in which the heating is caused by Stokes-shifted stimulated emission generated during the lasing process. In this scheme, the lasing process takes place in the Yb\({}^{3+}\)-doped fibre core. The cooling process takes place in the Tm\({}^{3+}\)-doped fibre cladding pumped at a wavelength of 1900 nm, that  
Figure 12: Schematic of laser cavity and multipass pumping for (**a**) the Yb\({}^{3+}\):YLF laser disk and for (**b**) the Yb\({}^{3+}\):LLF laser disk [38].  
Figure 13: Fibre lasers with an integrated cooler. (**a**) Athermal Raman fibre laser. Two curves in the core illustrate the distribution of Yb\({}^{3+}\) ions providing cooling. (**b**) Athermal Yb\({}^{3+}\)-doped fibre laser with the Tm\({}^{3+}\)-doped cladding. Two curves in the cladding illustrate the distribution of Tm\({}^{3+}\) ions providing cooling. The dashed arrows illustrate anti-Stokes fluorescence [41].  
is, in the long-wavelength tail of the Tm\({}^{3+}\) absorption spectrum. Here, the Tm\({}^{3+}\) ions play the role of an integrated cooler. The distribution of the RE ions in the integrated cooler is properly arranged in both schemes to provide complete compensation for the heat in the case of fixed laser pump power, \(P_{p}\). The dashed arrows in Figure 13 illustrate anti-Stokes fluorescence. A deviation in the laser input pump power causes a small deviation in complete compensation of the temperature along the length of the laser medium. However, this deviation in the temperature is small in comparison with the peak temperature change without cooling. It can be partially compensated for by a change in the cooling pump power, \(P_{p}{}^{cool}\).  
The temperature distribution inside a double-cladding optical fibre laser or amplifier was theoretically investigated by [PERSON] [42]. It has been shown that heat generation in the cladding due to the parasitic absorption of the high-power pump must be taken into account in modern high-power fibre lasers and amplifiers, where the quantum defect is lowered, or when the amplifier operates in a nearly radiation-balanced regime, or for radiation-balanced lasers. The developed approach permits one to interpolate between the case where the quantum defect heating is dominant and when the parasitic absorption heating is comparable in size or is the dominant source of heating.