## 4 High Average Power Short-Pulse Thin Disk Lasers at HiLASE
### Beamline C  
Beamline C fills the gap between the high power low energy oscillators operating at a repetition rate of tens of MHz and the high energy laser systems. Therefore, the repetition rate is 100 kHz, and the planned pulse energy is 5 mJ. The damage threshold limitations for the Beamline C design were based on the same considerations as for Beamline B, described in the previous section. Figure 6 shows the scheme of the already operating laser system with the first amplifier. Seed pulses are generated by a fiber oscillator delivering pulses at a 50-MHz repetition rate. The flat top-like pulse spectrum of a 20-nm bandwidth (FWHM) supports the generation of pulses of about a 150-fs pulse width after compression. This is partly done by a chirped fiber Bragg grating with a 6-ps/nm dispersion and a 20-nm bandwidth. The average power of the seed is then increased in a 15-cm Yb-doped fiber amplifier to 200 mW, corresponding to 4 nJ of the pulse energy. As the front-end is based on non-polarizing fibers, the polarization state is corrected by a polarization controller to linear polarization before entering the regenerative amplifier. The proper collimation optics ensures mode matching between the regenerative amplifier cavity and the seed beam. The pulse stretcher consisting of a reflective chirped volume Bragg grating (CVBG) of 62-ps/nm second order dispersion (2.2-nm bandwidth) increases the pulse width to 150 ps at the input of the regenerative amplifier cavity. The CVBG represents a very compact, robust and easy-to-align solution for pulse stretching/compression devices. The CVBG pulse stretcher is identical to the CVBG compressor, which should ensure proper dispersion compensation of the output pulse. Note that the fiber preamplifiers and stretcher select only the central part of the oscillator pulse bandwidth. The first regenerative amplifier is based on the Yb:YAG thin disk of a 7 atomic % of Yb doping concentration, 220-\(\upmu\)m thickness, 8-mm free aperture and soldered to a copper-tungsten (CuW) heat sink. The thin disk is pumped by wavelength-stabilized laser diodes operating at a wavelength of 969 nm and continuous-wave output power up to 500 W. The pump beam is imaged onto the thin disk to a spot of 2.8 mm in diameter. Pumping at the 969-nm wavelength reduces the generated heat and consequently the thin disk temperature, as described in Section 3. The ASE formation is limited by the relatively small pump spot diameter and beveled edge of the thin disk. The number of roundtrips of the stretched pulse being amplified in the cavity is governed by the switching time of the Pockels cell with two BBO crystals of 8 mm \(\times\) 8 mm \(\times\) 25 mm in size. On the one hand, the BBO crystal shows a low piezo electric ringing, but on the other hand, a high switching voltage is needed. Therefore, a series of two BBO crystals was employed in order to halve the voltage. The quarter wave operation of the Pockels cell requires a 3.9-kV voltage applied to each crystal. The number of roundtrips was set to 90 and is limited by the maximum switching time of the Pockels cell driver. The pulse build-up evolution in the cavity of the regenerative amplifier is shown in Figure 7a. The pulse build-up evolution is not fully saturated due to the low gain per roundtrip of about 1.15 for the maximum output, which corresponds to the total gain of about 2.4 \(\times\) 10\({}^{5}\). The dependence of the uncompressed output pulse power and the optical-to-optical conversion efficiency on the pump power is shown in Figure 7b. The maximum  
Figure 6: Scheme of the 100-kHz Beamline C. The first regenerative amplifier is shown only. CVBG, chirped volume Bragg grating; other abbreviations are the same as in Figure 4[39]. conversion efficiency of 27% was reached for the output power of 55 W. For higher output powers, the efficiency decreases.