# Recent Development of High-Energy Short-Pulse Lasers with Cryogenically Cooled Yb:YAG
## 4 Cryogenically Cooled Yb:YAG Lasers
### High-Energy Picosecond Laser  
High-energy short-pulse lasers with high repetition rates can be used to drive ultrafast coherent short wavelength radiation sources [39], drive high repetition frequency high-average power femtosecond lasers, and serve as secondary sources of high-flux high-energy particles [40] and photons [41]. In addition, 100 Hz picosecond Joule lasers have been used to pump soft X-ray lasers [42]. Short-pulse lasers with a repetition frequency of 1 kHz or more and peak power of 1 TW can form a continuous plasma channel in the atmosphere [43], which can be used for laser-induced water condensation [44], remote pollution monitoring [45], light through fog communication [46], laser-induced lightning [47], etc. The above applications make 1 kHz, 1 J kilowatt-level picosecond lasers a common research goal of research institutions in various countries. TRUMPF is oriented to the application of forming plasma channels in the atmosphere, using the Yb:YAG crystal at room temperature as the gain medium, and using the chirped pulse amplification technology to achieve a laser output of 720 mJ, 1 kHz, less than 1 ps in 2020 [48]. The schematic diagram is shown in Figure 9, which consists of a seed laser, a regenerative amplifier, two multipass amplifiers, and a compressor. The seed laser delivers femtosecond pulses with a pulse energy of 100 \(\upmu\)J and a repetition rate of 1 kHz. The seed laser has excellent beam quality of M\({}^{2}\) = 1.15. The pulse is stretched to a duration of 1 ns with a chirped fiber Bragg grating (CFBG). The regenerative amplifier is pumped by laser diodes with 1 kW average pump power. Within multiple resonator roundtrips, the pulse energy is amplified to 240 mJ, while maintaining a beam quality with M\({}^{2}\) < 1.2. The multipass amplifier comprises four industrial thin-disk laser heads. To minimize the thermal load on the disks, the disks are pumped with a duty cycle of 25%, at the pulse repetition rate of 1 kHz. Each disk is pumped with an average power of 2.3 kW. The super-Gaussian-shaped pump spot has a diameter of approximately 12 mm. The cumulated total average pump power is \(>\)9 kW for the four amplifier heads, which is efficiently absorbed in the water-cooled disks, even without optical energy extraction. In the first stage, the pulses are amplified from 240 mJ to 550 mJ using seven passes distributed on the two laser heads. In the second stage, the pulse energy is increased to 800 mJ via four additional disk reflections distributed on the laser heads 3 and 4 while maintaining a beam quality with M\({}^{2}\) < 1.6. The output pulses are compressed by a pair of multilayer dielectric (MLD) reflection gratings, arranged in a Treacy-type configuration under a near-Littrow angle of incidence. The diffraction efficiency of each of the gratings is about 97%, leading to an overall compressor efficiency of \(\sim\)90%. At the full output power of the multipass amplifier (800 W  
Figure 9: (**a–d**) Detailed sketch of the complete laser system. The seed pulses from the oscillator are stretched with a CFBG and first amplified by the regenerative amplifier, employing an industrial thin-disk laser head. The multipass contains 2 amplification stages, as indicated by the green (stage 1) and red (stage 2) areas. In each stage two industrial thin disk laser heads are employed. The grating compressor is set up in a folded Treacy-type near-Littrow configuration. TFP: Thin-film polarizer. HWP: Half-wave plate. BBO PC: BBO-based Pockels cell [48]. uncompressed), 720 W average power was measured after compression, corresponding to a pulse energy of 720 mJ. After compression, the beam quality increased to M2 < 2.1.