# Recent Development of High-Energy Short-Pulse Lasers with Cryogenically Cooled Yb:YAG
## 4 Cryogenically Cooled Yb:YAG Lasers
### High-Energy Picosecond Laser
The advantage of Yb:YAG crystal operating at room temperature is that the emission spectral width is wide and small pulse width can be achieved, but its thermodynamic properties make it difficult to manage thermal effects at room temperature, resulting in poor beam quality, and its spectral properties lead to lower pump absorption efficiency, lower gain, and lower energy extraction efficiency, and it is difficult to obtain larger pulse energy. The University of Colorado reported in 2016 that an active-mirror configuration laser using a cryogenically cooled Yb:YAG crystal as a gain medium produced a 1 J, 500 Hz, 3.8 ps laser output [49] and obtained a 1 J, 1 kHz laser output before compression in 2018 [50]. In 2019, DESY in Germany reported the use of a cryogenically cooled Yb:YAG laser to generate a 1 J, 500 Hz, 20 ns laser output, and predicted that its pulse width could be compressed to 5 ps [51]. In 2020, the University of Colorado in the United States used diode-pumped chirped pulse amplification technology, using a cryogenically cooled Yb:YAG crystal as the gain medium, and obtained a laser output of 1.1 J, 1 kHz, and 4.5 ps [52], which is the highest single-pulse energy achieved by a kilohertz picosecond laser to date. The system configuration is shown in Figure 10, which consists of a front end and two cryogenically cooled Yb:YAG amplifiers. The front end consists of a mode-locked oscillator, a pulse stretcher, and a regenerative amplifier. The Yb:KYW oscillator outputs a 10 nJ, 55 MHz, 200 fs pulse, which is stretched to 270 ps via a 1740 lines/mm grating while adjusting the stretcher to select a spectral bandwidth that matches the peak gain region of the subsequent Yb:YAG crystal. The regenerative amplifier works at room temperature, using a Yb:YAG crystal with a thickness of 0.5 mm and an atomic doping concentration of 7% as the gain medium, and is cooled by  
Figure 10: Schematic diagram of the diode-pumped high energy CPA laser system: Regen, regenerative amplifier; SF, spatial filter; PC, Pockels cells; FCLD, fiber-coupled laser diodes; SA, serrated aperture; TFF, thin-film polarizer; P, periscope [52]. [Reprinted/Adapted] with permission from [ref #] © The Optical Society. OSA requires a special citation format. a water-cooled copper heat sink. A fiber-coupled laser diode was used as the pump source, the pump power was 60 W, the pump wavelength was 969 nm, the pump pulse width was 500 us, and the pump light diameter was 1 mm. The resonator design enables the seed light to pass through the gain medium four times per round trip, amplifying the pulse energy from about 1 nJ to 2 mJ, and the Pockels cell selects a pulse repetition frequency of 1 kHz. After output from the regenerative amplifier, the beam passes through a spatial filter, a Pockels cell, and a pair of orthogonal polarizers to improve beam quality and pulse contrast and achieve feedback isolation. The preamplifier operates at low temperature, using two 5 mm thick Yb:YAG crystals with 2% atomic doping concentration as the gain medium. Both crystals are placed in a vacuum chamber to avoid condensation forming on the crystals, and the back is cooled by liquid nitrogen. Each crystal is pumped by a fiber-coupled laser diode, the pump power is 400 W, the pump wavelength is 940 nm, the pump pulse width is 450 us, and the pump light diameter is 4 mm. Through optical path design, the laser is amplified five times in the first crystal (one amplification is defined as being reflected by the reflective layer and passing through the gain medium twice) and then amplified twice in the second crystal. The pulse energy was amplified from millijoules to 100 mJ. The laser output from the preamplifier passes through a sawtooth aperture with a diameter of 8 mm to achieve a flat-top spot distribution, and the feedback is isolated by a Faraday rotator and a pair of orthogonal polarizers. The main amplifier also operates at a low temperature and consists of two bonded crystals placed in a vacuum chamber.