## 4 High Average Power Short-Pulse Thin Disk Lasers at HiLASE
### Beamline C
The efficiency decrease for the pump power over 200 W is caused by the thin disk temperature increase, which results in higher thin disk mechanical deformations, causing the mode mismatch and beam aberrations. Furthermore, the higher thin disk temperature leads to a lower single-pass gain. The efficiency is 24% for the maximum uncompressed output power of 90 W in the high quality beam (as can be seen in the inset of Figure 7b). Gain narrowing is low due to the good match between the seed spectrum and gain medium bandwidth (Figure 8a). The amplified beam is compressed by the 8 mm \(\times\) 8 mm \(\times\) 13 mm CVBG placed in a water-cooled mount. The compressor CVBG has the same properties as the stretcher CVBG, but the beam enters the grating from the opposite direction, and thus, the stretcher dispersion should be well compensated. Contrary to our expectations, the compressed pulse of 1.9 ps (FWHM) suffers from the higher orders of dispersion, as indicated by the wings of the autocorrelation trace in Figure 8b. Probably the pre-pulse or post-pulse are present and stem from the fiber-based laser oscillator, the pulse of which is not transform limited and is affected by self-phase modulation in the fibers. The efficiency of the compressor was almost 90%, and we did not observe beam degradation at the high load of the compressor. The use of the CVBG compressor is quite unique among the high average power laser systems. Figure 7: **(a)** The oscilloscope trace of the pulse build-up in the first regenerative amplifier of Beamline C; **(b)** the dependencies of the average output power and optical-to-optical (opt.-to-opt.) efficiency on the pump power; the inset shows the beam profile at the 80-W output [39]. Several improvements are planned to enhance the regenerative amplifier performance. First, a semiconductor optical amplifier and another fiber amplifier will be added into the front-end to increase the pulse energy and to lower the pulse repetition rate at the regenerative amplifier input. This will enable a better energy extraction in the regenerative amplifier. If we use a diamond-bonded thin disk, higher output powers and better beam quality can be reached due to a lower thermally-induced distortion of the thin disk. The proper compensation of dispersion, done, e.g., by thermal tuning of the chirped fiber Bragg grating in the fiber front-end [40], will enable reaching a smooth pulse without side pulses. Further development of the 100-kHz beamline will focus on the final regenerative amplifier with a ring cavity. The ring cavity is used in order to avoid the usage of a Faraday isolator that is hardly available for such a high average power. The second regenerative amplifier will be seeded by the stretched pulses of the first amplifier and aims for 500 W of the average output power at the 100-kHz repetition rate. To decrease the probability of the optics' damage, the stretching to 500 ps will be used. The Yb:YAG thin disk bonded to a diamond substrate is used due to a better thermal management. The fiber-coupled 969-nm wavelength-stabilized laser diodes of a power up to 2.5 kW pump the thin disk. The continuous-wave operation of the power up to 565 W in the fundamental spatial mode at a 1.2-kW pumping (Figure 9) was already achieved from the assembled cavity. It promises a similar output power in the pulsed regime after the Pockels cell is implemented into the ring cavity. The compactness of the compressor will be ensured by a 49 mm-long double-pass CVBG with a dispersion of 220 ps/nm. Figure 8: **(a)** The pulse spectra at different parts of the laser system of Beamline C; **(b)** the autocorrelation trace of the compressed pulse [39].