# Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE
## 1 Introduction
### Perla A and Perla B Platforms
% doping concentration, and an 8-mm clear aperture; however, the first regenerative amplifier uses only 4 mm of the diameter of the aperture. Prospectively, the output could be boosted by adding a booster amplifier equipped by large size thin disks with an energy extraction of 1.5 J or 3 J per burst, depending on the disk size and pump module available [32]. Pulse compression of the pulse burst is realized by an multi-layer dielectric (MLD)-based grating compressor. Reaching sub-1-ps pulses is expected, as it was already demonstrated on the 45-mJ pulses. A demonstration of the multipass amplifier at full power is planned in 2018. The Perla A is a new platform based on cryogenically-cooled disk laser technology. Laser operation of quasi-three-level Yb:YAG gain media at low temperature brings many advantages such as higher thermal conductivity, which is almost 4\(\times\) higher at 100 K for 1 at. % Yb-doping then the room temperature thermal conductivity, lower thermo-optical coefficient (at 100 K, approximately one eighth of its original value at room temperature), and lower thermal expansion coefficient (at 100 K, one third of the room temperature value) [33]. These properties significantly improve the laser cooling efficiency and reduce effects like thermal lensing, which usually prevent lasing at high power and room temperature operation. On the other hand, low temperature operation shrinks the emission bandwidth of Yb:YAG more than six times to approximately 1.5 nm (FWHM) [34], and the achievable pulse length is longer than for room temperature operation. It normally approaches 10 ps [5]. Development of the cryogenic Perla A platform has started this year and the system is still under design. The platform front-end will consist of an Yb-doped fiber oscillator providing \(\upmu\)J pulses, and Yb:KYW bulk, room-temperature-operated preamplifier reaching output pulse energy of 10-20 mJ at 1 kHz repetition rate, due to the prospective upgrade of the whole system to 1 kHz. Yb:KYW was chosen because of the good overlap of its peak wavelength and the wavelength of cryogenically-cooled Yb:YAG, whose peak emission shifts to shorter wavelengths as the temperature decreases. The pulses from the Yb:KYW amplifier will be seeded to a cryogenically-cooled Yb:YAG single slab four-pass amplifier with ceramic Yb:YAG gain medium with saturable absorber cladding and will boost the pulse to 100 mJ. Final, power amplification is planned by a new design of a liquid nitrogen-cooled Yb:YAG thin-disk laser, which is still under technical consideration. The expected pulse energy is about 1.2 J in a diffraction-limited beam and 100 Hz repetition rate with a potential of a 1-kHz upgrade. Due to emission bandwidth shrinking at low temperatures, bandwidth-limited pulse length prolongs and will probably reach 10 ps after compression. Behavior of the single slab gain medium in a closed-loop He cryostat for the Perla A preamplifier was already studied at low temperatures, both experimentally and theoretically, in Reference [35]. Commissioning of the system at full power is scheduled by the end of 2019.