# Radiation-Balanced Lasers: History, Status, Potential
## 4 Development of Radiation-Balanced Lasers
### Athermal Fibre Lasers
#### 4.3.1 Athermal ZBLAN Fibre Lasers  
An ytterbium-doped ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) optical fibre was experimentally investigated to extract its laser cooling related parameters [43]. It has been shown that the Yb\({}^{3+}\):ZBLAN fibre is a viable laser-cooling medium for radiation-balanced lasers and amplifiers. A comparison between a conventional Yb\({}^{3+}\):ZBLAN fibre laser pumped at 975 nm wavelength and an athermal Yb\({}^{3+}\):ZBLAN fibre laser pumped at 1030 nm was made [43]. They are presented in Figure 14. The output laser signal in both lasers was assumed to be 3 W in power at the wavelength 1070 nm. Both systems were optimized for maximum efficiency. As one can see in Figure 14, the temperature rise in both designs is comparable, while the athermal laser required a 20-fold larger pump power, making the athermal laser design totally unpractical. The authors of [43] attributed the problem to the relatively large value of the background absorption of the pump. They considered the problem of the background absorption in [44]. Following their simulations, a 10-fold reduction in the background absorption can reduce the heating in the athermal laser design significantly, rendering the athermal laser viable. These results are presented in Figure 14.  
In 2021, the first observation of anti-Stokes cooling of Yb\({}^{3+}\):ZBLAN fibres, in which both the core and the cladding were doped with Yb\({}^{3+}\) to increase the number of Yb\({}^{3+}\) ions contributing to cooling, was announced [45]. It is worth mentioning that laser cooling of the core-doped Yb\({}^{3+}\):ZBLAN fibres was demonstrated previously in a vacuum [46] and at atmospheric pressure [47; 48]. The main goal of [45] was to provide an experimental validation of the concept of cooling a fibre more aggressively by doping its cladding with a cooling ion and pumping the cladding, which was proposed by [PERSON] and [PERSON] in [39]. The performances of three fibre designs were compared. One of these fibres was a conventional large-core multimode fibre. Two others were cladding-doped fibres. One of these two cladding-doped fibres had a double-D shape, and the other had an octagonal cladding. Both shapes were designed to induce greater mode mixing (Figure 15). The pump light from a 1025.5 nm fibre-pigtailed semiconductor laser was launched into the Yb\({}^{3+}\):ZBLAN fibre under test. The temperature change of a fibre was measured with a custom slow-light FBG sensor with a sub-mK resolution [49]. The conventional multimode fibre (Figure 15a) with relatively poor mode mixing due to its symmetrical core was used to illustrate the importance of mode mixing. The maximum measured temperature change observed in this fibre under 1025.5 nm pumping was \(-0.65\) K for 584 mW of pump power at the location of the sensor. In order to improve mode mixing, a second fibre with a double-D cladding doped with Yb\({}^{3+}\) was fabricated (Figure 15b). The largest temperature change recorded for this fibre was \(-78\) mK for a pump power of 240 mW. The efficiency (temperature change divided by pump power) was higher (\(\sim\)3 times) than in the conventional multimode fibre. The third Yb\({}^{3+}\)-ZBLAN fibre, which had an octagonal cladding, was designed specifically to improve the mode filling of the cladding (Figure 15c). This fibre design permits the mode to fill a noticeably larger portion of the cladding. The temperature change of \(-1.3\) K was reached with the maximum available pump power of 3 W. To improve this value, the fibre was pumped with the pump power of 15 W at the wavelength 1040 nm. The temperature change of \(-2\) K was reached. The fibre with a double-D cladding (Figure 15b) did not perform as well as the fibre with an octagonal inner cladding (Figure 15c), largely due to limited mode mixing, which  
Figure 14: (**a**) Propagation of the forward pump (FW pump), backward pump (BW pump), forward signal (FW signal), backward signal (BW signal), and temperature rise along the fibre for a conventional fibre laser pumped at 975 nm.