# Radiation-Balanced Lasers: History, Status, Potential
## 4 Development of Radiation-Balanced Lasers
### Athermal Fibre Lasers
#### 4.3.2 Athermal Silica Fibre Lasers  
Most commercial fibre lasers are made of silica, due to silica's low loss and durability. Laser powers as high as 10 kW have been achieved with a silica fibre laser [50]. However, crystals and fluoride-based samples are widely used hosts for laser cooling with anti-Stokes fluorescence. Relative to silica, fluorides have fairly high loss and brittleness, that is, they can undergo sudden structural failure instead of plastic deformation. As a result, the highest power achieved with a ZBLAN laser is only 50 W [51]. In 2019, laser cooling with anti-Stokes fluorescence was demonstrated in two separate silica samples: a silica fibre [52] and a bulk silica preform [53]. These results were breakthroughs for development of silica-based athermal lasers. In [54], several influential factors on laser cooling with anti-Stokes fluorescence in silica fibres were investigated. The cooling performances of six Yb\({}^{3+}\)-doped silica fibres with various dopant and codopant concentrations, OH\({}^{-}\) loss, and core dimensions were compared. It was shown that silica can accommodate unexpectedly high Yb\({}^{3+}\) concentrations without suffering from quenching, in spite of its high phonon energy. Cooling a fibre with high 2.52 wt.% Yb\({}^{3+}\) concentration was enabled by reducing the absorptive loss below 5 dB/km and codoping the fibre with 2.0 wt.% Al, leading to a record-breaking critical quenching concentration, 30% higher than the previous reported. A temperature change of \(-\)70 mK was observed at atmospheric pressure for 170 mW/m of pump power absorbed at 1040 nm. In 2021, the first radiation-balanced silica fibre amplifier was demonstrated. An Yb\({}^{3+}\)-doped silica fibre served as an active medium [55]. The core diameter of the silica fibre was 21 \(\upmu\)m. Its numerical aperture was 0.13. The Yb\({}^{3+}\) concentration was 2.52 wt.%. The fibre was codoped with 2.00 wt.% Al to reduce concentration quenching. The wavelength of the pump was 1040 nm and the signal wavelength was 1064 nm. The mean fluorescence wavelength of Yb\({}^{3+}\) was 1003.9 nm and the radiative and quenching lifetimes were 765 \(\upmu\)s and 38 ms, respectively. The Yb\({}^{3+}\) critical quenching concentration was 21.0 wt.%. The absorptive loss of the fibre was 18 dB/km. The temporal trace of the temperature change in this fibre amplifier is illustrated in Figure 16. At time \(t=0\) s, the pump was turned on. The temperature of the fibre decreased \(\sim\)130 mK below room temperature. After 15 s, the seed signal was turned on. The temperature of the fibre increased, reaching a new steady-state value of \(\sim\)110 mK below room temperature. The temperature change of the fibre amplifier was defined as the difference between the average temperature in the first 5 s, when both the pump and signal were off, and the average temperature in the last 5 s after both had been turned on. Each measurement was repeated three times and averaged. Measured temperature change versus position along the fibre amplifier is presented in Figure 17. The solid curves in Figure 17 are simulation results obtained using the model of a radiation-balanced fibre laser described in [56]. In [55], it was shown that light can be coherently amplified with a gain approaching 20 dB and generating no net internal heating. with a 1020 nm continuous wave laser focused onto a crystal at a range of irradiances up to 1 MW/cm\({}^{2}\). The pump laser at the wavelength 1020 nm excites electrons from energy level E\({}_{4}\) to energy level E\({}_{5}\). After thermalization accompanied by phonon absorption in the \({}^{3}\)F\({}_{5/2}\) manifold, electrons undergo transition to the \({}^{3}\)F\({}_{7/2}\) manifold, and the fluorescence spectrum arises (Figure 18b). The fluorescence spectra were recorded with a spectrometer (Ocean Optics, NIR512, Midland, ON, Canada). A 1000 nm short-pass filter (Thorlabs, FESH1000, Newton, MA, USA) was used to filter the laser line.