## 4 High Average Power Short-Pulse Thin Disk Lasers at HiLASE
### Beamline B  
The targeted output power of Beamline B is 500 W at a 1-kHz repetition rate with a pulse width of less than 2 ps. The development is subdivided into the development of the 100-W regenerative amplifier with a linear cavity and the main 500-W regenerative amplifier with a ring cavity. The key limitation of the amplifier designs are the damage threshold values of the optical components. The design of the beamline took these damage threshold limitations into account. The most critical elements are the thin disk and Pockels cell crystals. The typical pump power density of the thin disk is limited to \(\sim\)5 kW/cm\({}^{2}\). The thin disk damage threshold is about 10 J/cm\({}^{2}\) for 10-ns pulses. The square root scaling of the damage threshold fluence in dependence of the pulse width was used to determine the proper combination of the beam width and the stretched pulse width in order to stay below the damage threshold. If needed, the suggested design of the amplifier was optimized to stay with the B-integral below one. Note that the B-integral is mostly influenced by the crystals of the Pockels cell. The laser setup based on the first regenerative amplifier will be described in detail. The second stage amplifier is still under construction. The layout of the amplifier is shown in Figure 4. The front-end is based on a fiber oscillator delivering pulses at a 50-MHz repetition rate and average power up to 2 W. The bandwidth of the oscillator pulse supports the generation of femtosecond pulses (20-nm FWHM bandwidth), but higher orders of dispersion are not well compensated due to a strong self-phase modulation in the fiber oscillator. The stretcher is based on a chirped fiber Bragg grating (CFBG) with 750-ps/nm dispersion and a 2-nm bandwidth. The CFBG stretcher is compact, does not require any alignment and is almost insensitive to vibrations and air fluctuations compared to the standard Martinez-type grating stretcher, which was used previously [25]. The pulse is stretched to 1.5 ns andpreamplified in an ytterbium-doped fiber to 2 nJ of pulse energy. The collimated beam passes the optical isolator and the Faraday rotator and seeds the regenerative amplifier cavity consisting of high reflection mirrors, a Pockels cell, a quarter-wave plate, a thin-film polarizer and an Yb:YAG thin disk gain medium. The Pockels cell includes a 22-mm long BBO (beta barium borate) crystal with a 10 mm \(\times\) 10 mm aperture. The quarter-wave voltage is 10.5 kV, and the high voltage repetition rate of 1 kHz determines the repetition rate of the amplifier. It serves as a pulse picker for incoming pulses, as well. No piezo ringing was observed with the BBO crystal. The 7 atomic % doped Yb:YAG thin disk with a 220-\(\upmu\)m thickness and 8-mm aperture is soldered onto a CuW heat sink. The beveled edge of the thin disk limits the reflection of the amplified spontaneous emission (ASE). The thin disk is pumped by the wavelength-stabilized fiber-coupled laser diodes delivering optical power up to 0.8 kW at the wavelength of 969 nm, _i.e._, the zero-phonon-line pumping is used. The emission wavelength is stabilized by a volume Bragg grating with a linewidth of 0.9 nm, which is less than the 2.8-nm broad absorption line of Yb:YAG. The diameter of the fundamental mode of the beam being amplified is about 80% of the 5.8-mm pump spot diameter on the thin disk. This ratio ensures single transversal mode operation. The laser diodes are driven by a pulsed diode driver with a rise time  
Figure 4: Scheme of Beamline B. Only the first regenerative amplifier of Beamline B, which is now available for experiments, is shown. OSC, oscillator; OI, optical isolator; HR, high reflective mirror; CRC, circulator; CFBG, chirped fiber Bragg grating; FAMP, fiber amplifier; PCTRL, polarization controller; COL, collimator; \(\lambda\)/2, half-wave plate; PBS, polarizing beam splitter; FR, Faraday rotator; TFP, thin film polarizer; \(\lambda\)/4, quarter-wave plate; PC, Pockels cell; TD, thin disk; LD, laser diodes; L, lens; G, diffraction grating.