## 4 High Average Power Short-Pulse Thin Disk Lasers at HiLASE
### Beamline B
below 9 \(\upmu\)s and fall time bellow 3 \(\upmu\)s, which enabled us to study the effect of pulsed pumping for duty cycles between 20% and 90%. We found the optimum condition of the pulsed pumping to be around 50% of the duty cycle, where the beam quality and optical-to-optical efficiency is improved. The pulsed pumping decreases the large thermal loading present during the continuous-wave pumping. We expect that the ASE is partly suppressed, as well [33]. In the previous setup utilizing a grating stretcher with 500-ps long pulses, the amplification reached saturation after approximately 80 roundtrips (Figure 5a), and the total gain was \(22.5\times 10^{6}\). The gain per roundtrip is about 1.23. We achieved 45 mJ of output energy at the peak pump power of 465 W and extraction efficiency of 19%. The corresponding output beam profile is shown in Figure 5b. The output beam was characterized by a very good beam propagation factor of \(M^{2}=1.24\) and was focused to a 40-\(\upmu\)m spot in diameter (1/e\({}^{2}\)) by a 150-mm focal length lens. The pointing stability was less than 4 \(\upmu\)rad RMS (root mean square). Once the standard Martinez-type grating stretcher was employed, providing 500-ps pulses, the amplified pulse, of about a 1.5-nm bandwidth, was compressed by a dispersion-matched Treacy-type compressor based on two gold-coated reflection gratings with a total transmission of 68%. Though the width of the transform limited pulse is about 1 ps, the measured pulse width was 2.7 ps (assuming a Gaussian pulse shape), and therefore, a further compressor optimization is needed to suppress higher orders of dispersion. Figure 5: **(a)** The oscilloscope trace of the pulse build-up (yellow line) and the output pulse (green line) [25] and **(b)** the beam profile at the output of the regenerative amplifier. In order to reach more than 100 mJ of pulse energy from the first regenerative amplifier, several improvements are planned. The cavity will be redesigned to allow larger mode size on the thin disk, and a second laser head will be added to increase the gain per roundtrip. Using two laser heads instead of one will also reduce the B-integral that is mainly affected by the Pockels cell's crystal. Furthermore, the stretching ratio up to 1.5 ns in a CFBG stretcher will be used to enable safe seeding of the second stage 500-W ring-cavity amplifier. Moreover, the assembly of the 500-W ring-cavity regenerative amplifier has already started. Generally, a multi-pass amplifier is employed for the amplification to a joule-level energy. A multi-pass thin disk amplifier, however, requires many numbers of passes due to its small single-pass gain. Therefore, we are going to develop a regenerative amplifier with a large mode area ring amplifier. Though a more complicated alignment, the ring cavities have several inherent advantages over the linear designs for applications in high energy lasers [38]. The ring cavity can be shorter for the same mode size and enables configurations that are not suitable for the linear cavity, such as the hemispherical resonator. In the linear cavity, the small beam size is on a plane mirror, whereas in the spherical ring cavity (which equals hemispherical), the mirror can be at a distance from the beam waist. Further, the ASE is suppressed and the pulse contrast improved by the use of the ring cavity, because the ASE traveling only in one direction contributes to the pulse background. Neither the input nor the amplified pulse can go back into the previous amplification stage, thus the optical isolator, generally having poor thermal characteristics, is not needed. Assuming the efficiency of the grating-pair compressor as 80%, the amplified pulse must achieve at least 630 mJ in the ring amplifier output to obtain 500 mJ in the compressed pulse. Suppression of the ASE and laser-induced damage of the Yb:YAG thin disk and BBO Pockels cell are the key issues of the ring amplifier.