# Recent Development of High-Energy Short-Pulse Lasers with Cryogenically Cooled Yb:YAG
## 4 Cryogenically Cooled Yb:YAG Lasers
### High-Energy Nanosecond Laser  
Since the beginning of the new century, high-energy nanosecond solid-state lasers with a repetition frequency of more than 10 Hz and pulse energy of more than 10 J have developed vigorously [21]. There is an urgent need in the fields of laser-confined nuclear fusion [22], high-energy physics research [23], and X-ray sources [24]. Such laser systems have been built in many countries, such as the United Kingdom (DiPOLE [25]), France (LUCIA [26]), the United States (Mercury [27], HAPLS [28], etc. ), Japan (CcAMA [29], HALNA [30], Hamamatsu [31], etc.). In China, Tsinghua University [32], Chinese Academy of Engineering Physics [33], Chinese Academy of Sciences Institute of Optoelectronics [34], and other institutions have relevant research results. Systems such as Hamamatsu and CcAMA in Japan and DiPOLE in the United Kingdom use cryogenically cooled Yb:YAG crystals as the gain media of the main amplifier. In 2017, the DiPOLE system in the United Kingdom reported the highest index of high-energy nanosecond lasers so far [25] (105 J, 10 Hz, 10 ns), also the first kilowatt-class system. Hamamatsu of Japan reported a 117 J, 0.05 Hz laser output in 2019 [35]; the system structure is shown in Figure 6[31]. The front end of the system outputs 5 J, 40 ns, 0.05 Hz seed light, which is amplified by the preamplification stage and the main amplifying stage to amplify the pulse energy to 28 J and 117 J, respectively. The preamplification stage is an active mirror configuration, the cooling temperature is 100 K, and the injection pump energy exceeds 80 J. The main amplifying stage is two double-pass amplification modules with a lamination configuration, each module is composed of six pieces of Yb:YAG crystals with a diameter of 120 mm and a thickness of 10 mm. The gain medium is cooled with helium gas, the helium gas pressure is adjustable from 1 atm to 5 atm, and the temperature is adjustable from 150 K to 300 K. A 7 mm thick Cr\({}^{4+}\):YAG crystal is bonded around the Yb:YAG crystal to suppress the ASE effect, and gradient doping with atomic doping concentrations of 0.5%, 0.7%, and 1.0% is used to homogenize the absorption. When 895 J of pump energy was injected, pulse energy of 117 J was obtained, and the optical efficiency was 12.5%. Its pump source has a 10 Hz output capability, so its next goal is to increase the repetition frequency to 10 Hz. Osaka University reported in 2021 that its CcAMA system achieved a 9.3 J, 33.3 Hz laser output [29]. The system consists of a seed source, a preamplification stage, and a main amplifying stage. The structure of the main amplifying stage is shown in Figure 7. The single, longitudinal-mode fiber seed source used has an output power of 5 mW. The preamplification stage consists of a regenerative amplifier and a rod-shaped multipass amplifier. The temperature of the cooling medium is 77 K, the output energy is 285 mJ, and the repetition frequency is 1-100 Hz. The main amplifying stage adopts a lamination configuration, and the gain medium is composed of four Yb:YAG crystals with a diameter of 45 mm and a thickness of 7 mm. The crystal is surrounded by 7.5 mm thick Cr\({}^{4+}\):YAG  
Figure 6: Schematic diagram of the Hamamatsu system [31]. and is pumped by two LD arrays with a peak power of 50 kW. The laser passes through the gain medium twice for amplification (one amplification is defined here as being reflected twice through the gain medium by the reflective layer). Osaka University focused on the analysis of the heat dissipation process, pointing out that active mirror configuration is not suitable for high repetition frequency operation, and improved the cooling method of the main amplifying stage by adding a molybdenum film between the crystal and the heat sink to reduce stress. The heat sink is flow-cooled using 78 K liquid nitrogen. The system achieved 10 J, 20 Hz stable operation for more than one minute, but when the repetition rate increased to 33.3 Hz, the pulse energy decreased, and the average energy for 20 s was 9.3 J.