# Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE
## 1 Introduction
### Perla Picosecond Thin-Disk Laser Platform  
The development of compact thin-disk-based kW-class thin-disk lasers started at HiLASE several years ago. In-house developed thin-disk platforms [28] at HiLASE recently received the name Perla. Figure 1: Actual scheme and target parameters of thin-disk based beamlines and frequency conversion stages, which are operated or under construction at the HiLASE Center in 2017. CVBG: chirped volume Bragg grating-based; OPG: optical parametric generation; OPA: optical parametric amplification. Several thin-disk Perla platforms with various pulse parameters exist (Figure 1). The most progressive is the continued construction of the high repetition rate platform Perla C, which is important for the majority of industrial picosecond laser applications including applications in lithography. The Perla C system based on a regenerative amplifier aims for 1 kW of average power at a variable repetition rate 50-1000 kHz [29; 30]. Nowadays, the system can be operated up to 500 W.  
The design of the Perla C is based on a common CPA (Chirped Pulse Amplification) method. A low pulse energy (nJ) Yb-doped fiber oscillator producing several picosecond-long chirped pulses with a 20-nm bandwidth (full width at half maximum (FWHM)) is followed by fiber preamplifiers, two stage thin-disk regenerative amplifiers, and a highly dispersive chirped volume Bragg grating-based (CVBG) pulse compressor (chirp rate 220 ps nm\({}^{-1}\), full bandwidth 4 nm). The design specification of this system is targeted to 5 mJ of pulse energy at 100 kHz repetition rate; however, upon requirement, the system was modified for 50 kHz operation up to 500 W of average power. Prospectively, the adjustability of the repetition rate from 50 kHz to 1 MHz at 0.5 kW of average power should be enabled by the installation of a new high-voltage Pockels cell driver. The research trial also aimed to upgrade the system to the level of 1-kW of average power by keeping the repetition rate unchanged, i.e., 100 kHz. Thin disks used in the regenerative amplifiers are diamond-bonded Yb:YAG single crystals (220 um thick, 7 at. % doping) continuously pumped at the zero-phonon-line wavelength (968.8 nm) [24; 25]. The optical setup of the first-stage regenerative amplifier [31] includes a standing-wave cavity, thin-disk laser head with a pump spot diameter of 2.8 mm, a 500 W continuous wave (CW) pump diode laser, and a dual beta barium borate (BBO) crystal (each 8 \(\times\) 8 \(\times\) 25 mm\({}^{3}\)) and a Pockels cell to enable the fast switching on and off of the system. The maximum output pulse energy from this first regenerative amplifier before compression is 1.2 mJ (average output power of 120 W) at a pump power of 430 W in a nearly diffraction-limited beam (M\({}^{2}\)\(\approx\) 1.4). Optical-to-optical efficiency therefore reaches 29%. A maximum efficiency of 33% is reached at 1 mJ pulse energy, i.e., 100 W of average power. Compressed pulses were obtained with the CVBG compressor (net-efficiency 85%) and the duration of 1.3 ps. The second-stage amplifier is seeded by a fraction of the uncompressed output of the first stage (\(\approx\)20 W, 200 uJ). The thin disk is pumped by a CW, 968.8-nm fiber-coupled diode laser on a pump spot with a diameter of 5.2 mm. The disk is located in a ring cavity with two V-passes through the disk per one roundtrip, and the footprint area of the amplifier is only 100 \(\times\) 60 cm\({}^{2}\). A dual Pockels cell equipped by BBO crystals with dimensions of 10 \(\times\) 10 \(\times\) 25 mm\({}^{3}\) for each of the crystals placed in in-house developed holders is operated at a half-wave voltage of 10-kV and a repetition rate of 50 or 100 kHz. A maximum CW output power of 565 W was obtained at 1.21 kW pump power, and the optical-to-optical efficiency was 47%. In seeded operation with the input pulse energy of 0.2 mJ, obtained pulses were 4.5 mJ at 100 kHz (450 W average output power) with an extraction efficiency of 43%, as shown in Figure 2a.