An overview of the Czech national R&D project HiLASE (High average power pulsed laser) is presented. The project focuses on the development of advanced high repetition rate, diode pumped solid state laser (DPSSL) systems with energies in the range from mJ to 100 J and repetition rates in the range from 10 Hz to 100 kHz. Some applications of these lasers in research and hi-tech industry are also presented.

An overview of the latest developments of kilowatt-level diode pumped solid state lasers for advanced applications at the HiLASE Centre is presented. An overview of subcontracted and in-house-developed laser beamlines is presented. The aim of development is to build kW-class beamlines delivering picosecond pulses between 1-and 100-kHz repetition rates and high-energy nanosecond pulses at 10 Hz. The picosecond beamlines are based on Yb:YAG thin-disk amplifiers and chirped pulse amplification. The current status of the beamlines' performance is reported. The advantages of zero-phonon line and pulsed OPEN ACCESS Appl. Sci. 2015, 5638 pumping are demonstrated with respect to efficiency, thin disk temperature and beam quality. New diagnostics methods supporting the high average power lasers' development, such as the high-resolution spectroscopy of Yb-doped materials, in situ thin disk deformation measurements, single-shot M 2 measurement, realization of wavefront correction by a deformable mirror and the laser performance of a new mixed garnet ceramics, are described. The energetic, thermal and fluid-mechanical numerical modeling for the optimization of the multi-slab amplifiers is also described.

A quantitative comparison of conventional absorption line (940 nm) pumping and zero phonon line (ZPL) (969 nm) pumping of a Yb:YAG thin disk laser is reported. Characteristics of an output beam profile, surface temperature, and deformation of a thin disk under the different pump wavelengths are evaluated. We found that a nonlinear phonon relaxation (NPR) of the excited state in Yb:YAG, which induces nonlinear temperature rise and large aspheric deformation, did not appear in the case of a ZPL pumped Yb:YAG thin disk. This means that the advantage of ZPL pumping is not only the reduction of quantum defect but also the suppression of NPR. The latter effect is more important for high power lasers.

We demonstrate an optimization method of beam quality and optical-to-optical (O-O) efficiency by using pulsed pumping. By changing the pulse duration and the peak intensity of pump pulse at the repetition rate of 1 kHz, the beam quality and O-O efficiency of the Yb:YAG thin-disk regenerative amplifier can be improved. We applied this method to the regenerative amplifier under the pumping wavelength of both 940 and 969 nm, and found that the method was effective in both pumping wavelengths. Although a Yb:YAG thin disk soldered on a copper tungsten heat sink, which has poor thermal properties compared with a thin disk mounted on a diamond substrate, was applied as a gain media, we obtained 45 mJ output with 19.3% O-O efficiency and nearly diffraction-limited beam.

Multi-beam micro- and nano-machining of material surfaces has been getting more important because of its great potential to increase production speed of large size laser induced periodic surface structures (LIPSS). Fast and cheap production of engineered surfaces structures can bring unique properties of surfaces like tailored wettability, friction, antibacterial properties, etc., to mass-production with consequence in, for example, energy and costs savings. However, tailoring of long-term stable interference patterns from ultrashort laser pulses requires an extremely stable laser system with nearly diffraction-limited output beams. HiLASE Centre developed such a thin-disk-based Yb:YAG sub-picosecond laser platform, PERLA, providing average output power up to 0.5 kW with 2nd and 4th harmonic generation extensions and demonstrated its potential for direct laser interference patterning (DLIP). In this paper, we focus on details of the thin-disk PERLA laser.