# Recent Development of High-Energy Short-Pulse Lasers with Cryogenically Cooled Yb:YAG
## 4 Cryogenically Cooled Yb:YAG Lasers
### High-Energy Nanosecond Laser
After calculation, when the device ran at 100 Hz, the working temperature was 116 K. In order to achieve 10 J, 100 Hz laser output, it is necessary to further improve the cooling performance to reduce the working temperature to below 100 K.  
The DiPOLE system in the United Kingdom reported a laser output of 105 J, 10 Hz, 10 ns in 2017 [25], setting a world record for the average power of nanosecond lasers, which has not been broken so far. The configuration of the DiPOLE system is shown in Figure 8. It consists of a front end, a 10 J main amplifying stage, and a 100 J main amplifying stage. The 10 J main amplifying stage was reported in 2015 and can achieve a 10.8 J, 10 Hz laser output [36]. The front end is composed of a fiber seed source, regenerative amplifier, and multipass amplifier [37]. The output of the continuous oscillator is chopped by an acousto-optic modulator into a 10 kHz, 150 ns pulse output, which is amplified by a multistage fiber and modulated by an electro-optic modulator, to output a 7 nJ, 10 ns pulse seed. In the regenerative amplifier, a Pockels cell is used to select a pulse with a repetition rate of 10 Hz from the 10 kHz repetition rate pulse. After about 170 round trips in the cavity, the pulse energy is amplified to 4.5 mJ. A Gaussian beam with a diameter of 2 mm is passed through a beam shaper, a sawtooth aperture, and a spatial filter. The beam is adjusted to a square super-Gaussian beam with a side length of 8 mm, and the pulse energy is reduced. The multipass amplifier uses a 1.8 kW pump source, the beam passes through the gain medium  
Figure 7: System layout of a 10 J class Yb:YAG cryogenically cooled active-mirror amplifier. DFB, distributed feedback; EOM, electro-optic moderator; BE, beam expander; SA, serrated aperture; TFP, thin-film polarizer [29]. [Reprinted/Adapted] with permission from [ref #] © The Optical Society. OSA requires a special citation format. 8 times, and the pulse energy is amplified to 100 mJ. After feedback isolation and shaping, the light beam becomes a square light spot with a side length of 22 mm. The 10 J main amplifying stage adopts a lamination configuration [36], consisting of fpir bonded crystals with a diameter of 55 mm and a thickness of 5 mm. The space between sheets is 1.5 mm, and 150 K helium gas is charged at 35 m\({}^{3}\)/h for cooling. The center of the crystal is a Yb:YAG crystal with a diameter of 45 mm, and the periphery is surrounded by a 5 mm wide Cr\({}^{4+}\):YAG crystal to suppress the ASE effect. Gradient doping is used, with atomic doping concentrations of 1.1% and 2.0% from the two sides to the middle, respectively, to homogenize the pump absorption. The crystal is pumped by two 940 nm LD arrays with a peak power of 29 kW with a typical pump pulse width of 1 ms. Under the injection of 48 J pump energy, 10.8 J pulse energy can be obtained, and the optical efficiency is 22.5%. The polarization state of the output laser is adjusted to match the pump light of the 100 J main amplifier stage. After feedback isolation and beam expansion, the side length of the light spot becomes 75 mm to ensure that the output flux is less than 2 J/cm\({}^{2}\). The 100 J main amplifying stage also adopts a lamination configuration [37], which is composed of six bonded crystals with an interval of 2 mm and 8.5 mm thick. Gradient doping is also used, and the atomic doping concentrations from the two sides to the middle are 0.4%, 0.6%, and 1.0%, respectively. Two pump sources are used, with a peak power of 250 kW to pump, the pump pulse width is 1.2 ms, and the pump spot is a square spot with a side length of 79 mm. Its cooling system has a cooling capacity of 6 kW, and the rated operating state of helium gas is a mass flow rate of 135 \(\pm\) 5 g/s, a temperature of 150 \(\pm\) 0.5 K, and a pressure of 10 bar. Under the condition of pre-stage injection pulse energy of 6 J and pump energy of 465 J, the output of 105 J, 10 Hz, and 10 ns was achieved.