Pawel Sikocinski

Design of high-energy-class cryogenically cooled Yb3+∶YAG multislab laser system with low wavefront distortion

Abstract. Detailed modeling results of 100 J class laser systems with respect to the output energy, beam propagation, nonlinear phase accumulation, wavefront aberrations, and adaptive optics performance obtained in MIRÓ and MATLAB codes are presented here. The laser system is based on a cryogenically cooled Yb3+∶YAG multislab amplifier with two identical amplifier heads and operates at 10 Hz repetition rate with an average power above 1 kW.

Spectroscopic characterization of various Yb3+ doped laser materials at cryogenic temperatures for the development of high energy class diode pumped solid state lasers

Precise values of absorption, emission and gain cross-sections of Yb:YAG, Yb:LuAG, Yb:CaF2 and Yb:FP15-glass at cryogenic temperatures are presented. To obtain the emission cross-sections two theoretical approaches were used. The first is the McCumber or reciprocity method (RM) which is based on the absorption spectra. The second is the Fuchtbauer-Ladenburg (FL) method using fluorescence spectra. From the results of cross-sections one can expect significant impact on laser performance on these materials especially in the case of high energy class diode pumped solid state lasers.

Design and optimization of an adaptive optics system for a high-average-power multi-slab laser (HiLASE)

We report numerical and experimental results obtained with an optical setup that simulates the heating and cooling processes expected in a multi-slab high-average-power laser head. We have tested the performance of an adaptive optics system consisting of a photo-controlled deformable mirror (PCDM) and a Shack-Hartmann wavefront sensor for the effective correction of the generated wavefront aberrations. The performance of the adaptive optics system is characterized for different layouts of the actuator array and for different configurations of the heating mechanisms. The numerical results are benchmarked using a PCDM, which allowed us to experimentally compare the performances of different deformable mirrors.

HiLASE cryogenically-cooled diode-pumped laser prototype for inertial fusion energy

We present the design parameters of a diode-pumped 100J-class multi-slab Yb:YAG laser at 10 Hz scalable to the kJ regime. Results of detailed energetics and thermo-optical modeling confirm the viability of cryogenic helium-gas cooling approach to drastically reduce thermally-induced distortions in the laser slabs. In addition, a comparison of spectral measurements from laser-diode stacks and Yb:YAG crystals validates the feasibility of highly efficient diode-pumped solid-state lasers at cryogenic temperatures.

Simulation of performance of wavefront correction using deformable mirror in high-average-power laser systems

We present calculation of aberration compensation in high average power multi-slab laser by a deformable mirror. The calculations were compared with a simple experiment. For the calculations we have developed a code that works with a square-shaped piston driven push/pull deformable mirror with continuous facesheet and allows optimization of mirror parameters and actuator array geometry. We have corrected the calculated output wavefront of Hilase 10 J multi-slab laser system in MIRÓ by using several actuator array geometries. The numerical results were benchmarked on an experimental model of multi-slab chamber using a membrane deformable mirror.

Progress in kW-class picosecond thin-disk lasers development at the HiLASE

High average power picosecond Yb:YAG thin-disk lasers are being developed at Hilase. A compact 1 mJ/100 kHz and 4 mJ/100 kHz zero-phonon-line-pumped regenerative amplifiers PERLA C with a CVBG compressor provide <2 ps long pulses in a nearly diffraction-limited beam. The output was successfully converted to 2nd and 4th harmonic frequency with high conversion efficiency. High energy, QCW-pumped beamline PERLA B is operated at 45mJ/1kHz in fundamental spatial mode and pulse length < 2ps. Its second stage amplifier is being assembled and 1.8 J was extracted. The latest development status of all thin-disk beamlines at the Hilase center is reported.

High energy picosecond light source based on cryogenically conduction cooled Yb-doped laser amplifier

We are developing one joule energy and one picosecond pulse duration laser system at the repetition rate of 120 Hz based on Yb-doped solid-state materials. The amplified output from the thin disk regenerative amplifier is amplified by a cryogenically conduction cooled single slab amplifier. In this work, we also present a new measurement method of a gain distribution insensitive to mode matching. One of the advantages of this method is a fact that it provides real dimensions of the gain distribution. Knowledge about it allows one to find the optimum spatial mode matching to maximize the output energy.

Characterization of diode-laser stacks for high-energy-class solid state lasers

In this work, we present a comparative study of high power diode stacks produced by world’s leading manufacturers such as DILAS, Jenoptik, and Quantel. The diode-laser stacks are characterized by central wavelength around 939 nm, duty cycle of 1 %, and maximum repetition rate of 10 Hz. The characterization includes peak power, electrical-to-optical efficiency, central wavelength and full width at half maximum (FWHM) as a function of diode current and cooling temperature. A cross-check of measurements performed at HiLASE-IoP and Ferdinand-Braun-Institut (FBH) shows very good agreement between the results. Our study reveals also the presence of discontinuities in the spectra of two diode stacks. We consider the results presented here a valuable tool to optimize pump sources for ultra-high average power lasers, including laser fusion facilities.

Comparative design study of 100 J cryogenically cooled Yb:YAG multi-slab amplifiers operating at 10 Hz

We present a comparison of two conceptual layouts of 100 J class laser systems for HiLASE and ELI Beamlines projects with respect to the output energy, beam propagation and system performance obtained from MIRO code. Both systems are based on cryogenically cooled Yb:YAG multi-slabs amplifiers technology and operate at 10 Hz repetition rate with average power above 1 kW. First concept of the laser system consists of separate 10 J preamplifier and 100 J power amplifiers, while the second consists of a main amplifier with two identical amplifier heads delivering 100 J.

Wavefront aberration measurement in a cryogenically cooled Yb:YAG slab using a wavefront sensor

We investigated wavefront aberrations in a cryogenically cooled Yb:YAG slab with a wavefront sensor using a probe beam technique under non-lasing condition. To analyze the pump-induced phase aberrations created in the crystal, the measured wavefronts were fitted with orthonormal Zernike polynomials. The Yb:YAG crystal of 2 mm thickness, 10 mm diameter, and 3 at.% doping concentration was mounted in a copper holder in a closed-loop pulse tube cryostat with cooling capacity of 12 W at 100 K. The gain medium was single-end pumped by a fiber-coupled laser diode at pumping intensity of ~6.5 kW/cm2 with a maximum repetition rate of 100 Hz, pulse duration of 1 ms, and pump spot diameter of 2.5 mm. The time resolved measurement revealed that defocus, which was the main wavefront aberration, represents not only a thermal lensing effect but also an electronic lensing effect. The thermally induced defocus is more dominant at high repetition rate than the electronically induced defocus. We also measured wavefront aberrations of amplified beams in a cryogenically cooled Yb:YAG slab. A room temperature operated thin-disk regenerative amplifier was used as a seed laser. The seed beam was amplified in the cryogenically cooled crystal at 160 K in a double pass configuration. The wavefront measurement was conducted at semi-saturated conditions, at three different repetition rates: 10 Hz, 20 Hz and 40 Hz, and at five different pump intensities in the range between 6.5 kW/cm2 and 14.8 kW/cm2. Under lasing condition, only defocus aberration were induced. Due to opposite signs of the defocus aberration of the seed beam and pumped induced in the Yb:YAG crystal, wavefront of the amplified beam had smaller PtV (Peak to Valley) and RMS values than the seed beam.