# Radiation-Balanced Lasers: History, Status, Potential
## 4 Development of Radiation-Balanced Lasers
### Athermal Disk Lasers  
In 2000, an Yb\({}^{3+}\):KGW laser disk was edge-pumped with radially focused laser diode bars (Figure 9a) [34]. It generated up to 490 W. This was the first system with heat loading below the laser quantum defect. Figure 6: Athermal laser: (**a**) Stable Yb\({}^{3+}\):YAG resonator. (**b**) Ytterbium fibre laser pump array propagation shown without the target laser rod [34]. Figure 7: Yb\({}^{3+}\):YAG rod inside the laser. This image was captured after 10 s of lasing at 70 W with an ambient temperature of 22 °C. The color scale corresponds to \(\mp\) 5 °C as shown on right. Fluorescence from the laser rod heats the mount for the pump injection mirror on left [33]. Figure 8: Radiation-balanced laser gain element constructed from a tapered 110 mm 3% Yb\({}^{3+}\):YAG slab fusion-bonded to a 2 mm \(\times\) 20 mm sapphire plate. The sapphire plate provides a refractive index mismatch of 0.07. This allows for guiding of the 1030 nm pump lasers while significantly reducing fluorescent trapping. The gold-plated mount and fused silica pins were designed to avoid radiative heating of the fixture [34]. Figure 9: Yb\({}^{3+}\):KGW disk lasers. (**a**) Edge-pumped Yb\({}^{3+}\):KGW disk laser with a laser quantum defect of 4.8%. (**b**) Thin-disk athermal laser system. \(\mathrm{I_{p}}^{(1)}\), \(\mathrm{I_{p}}^{(2)}\), \(\mathrm{I_{p}}^{(3)}\), \(\mathrm{I_{p}}^{(4)}\), and \(\mathrm{I_{p}}^{(5)}\) are the pump intensities, \(\mathrm{I_{L}}^{\mathrm{out}}\) is the intensity of the output laser signal. The violet arrows around the disks illustrate anti-Stokes fluorescence [35]. In 2014, a thin-disk athermal laser system was proposed and theoretically analysed [35]. This scheme, consisting of a sequence of thin disks, can provide high flexibility, which is key for athermal laser operation (Figure 9b). Yb\({}^{3+}\):KGW laser disks were considered in [35]. The sequence of thin disks provides precise control of the pump intensity on each disk. It allows the control of the pump intensity almost locally; that is, almost at each point along all laser gain medium. In addition, the generic design of the laser module allows easy scaling to higher powers. The concentration of the ions, as well as the thickness, can be changed from one disk to another. In 2019, the radiation-balanced Yb\({}^{3+}\):YAG disk laser was experimentally demonstrated in an intracavity pumping geometry (Figure 10) [36]. An optically pumped vertical-external-cavity surface-emitting laser (VECSEL) was used to enhance the pump absorption. The broad tunability and good beam quality of VECSELs provided additional parametrical freedom (Figure 10). The thermal images of the mounted laser disk at room temperature after 30 min at radiation-balanced condition are presented in Figure 11. Darker shades represent lower temperatures. Recorded thermal images of the unpumped laser disk at room temperatures, and under pump, are shown in Figures 11a and 11b, respectively. Figure 11c illustrates the temperature dynamics of the radiation-balanced process. The temperature profile of the laser disk was monitored with a thermal camera (Nanocore 640, L3 Communications Corporation, Garland, TX, USA) with 0.05 K resolution as the VECSEL power was varied. As one can see in Figure 11c, net cooling takes place from the very beginning, that is, as soon as the pump diode is turned on. Indeed, the VECSEL and the fluorescence emission process start immediately. The lasing process with the Yb\({}^{3+}\):YAG disk takes some time to initiate. It warms up the laser disk. About 10% power fluctuation was observed in the first 5 min of the experiment. The radiation balancing condition was realized with 57 W of incident pump power at 808 nm, or 32 W of absorbed power. About 1 W output power radiation balance was obtained. A temperature difference of \(\sim\)2 K was observed between the centre and the edge of the disk of 4 mm \(\times\) 5 mm cross section as a result of  
Figure 10: Schematic of the intracavity-pumped radiation-balanced disk laser setup.