Magdalena Sawicka

Modeling of amplified spontaneous emission, heat deposition, and energy extraction in cryogenically cooled multislab Yb 3+ :YAG laser amplifier for the HiLASE Project

A three-dimensional ray-tracing code for determination of amplified spontaneous emission in a multislab laser amplifier is presented. In addition to energy stored in the amplifier, the code also calculates the heat distribution and the amplification of the signal beam. For cryogenically cooled multislab amplifiers with Yb3+:YAG and absorptive Cr4+:YAG cladding, energy storage efficiency greater than 52% and small signal gain of 22  m−1 were obtained. The pump energy converted to heat was found to be 11% in the active volume and 50% in the Cr4+:YAG cladding.

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.

Optimization of Wavefront Distortions and Thermal-Stress Induced Birefringence in a Cryogenically-Cooled Multislab Laser Amplifier

The optimization of the Yb:YAG gain medium and absorbing clad parameters was investigated for efficient heat removal in cryogenically-cooled multislab amplifiers operating in the kilowatt average power range (100 J/10 Hz). The 3-D distributions of temperature, stress, strain, and birefringence were calculated by a finite element analysis. Based on these data, the space-resolved optical path difference and depolarization losses were determined considering eight slabs, two laser heads, and four passes. We have found that a combination of properly designed (doping/width) index matching material and helium cryogenic cooling leads to a quasi-constant transverse temperature distribution in the pump area and a very small axial thermal gradient in the slab. It is shown that the resulting thermally induced phase aberrations, stresses, and average depolarization are rendered insignificant.

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.

Ablative microstructuring with plasma-based XUV lasers and efficient processing of materials by dual action of XUV/NIR–VIS ultrashort pulses

We report on a single-shot micropatterning of an organic polymer achieved by ablation with demagnifying projection using a plasma-based extreme ultraviolet (XUV) laser at 21 nm. A nickel mesh with a period of 100 μ m was 10×demagnified and imprinted on poly(methyl methacrylate) (PMMA) via direct ablation. This first demonstration of single-shot projection, single-step lithography illustrates the great potential of XUV lasers for the direct patterning of materials with a resolution scalable down to the submicrometer domain. In the second part, we present a novel experimental method for improving the efficiency of surface processing of various solids achieved by simultaneous action of XUV, obtained from high-order harmonic generation, and near-infrared (NIR)–VIS laser pulses. The NIR–VIS pulse interacts with free charge carriers produced by the energetic XUV photons, so that its absorption dramatically increases. Laser-induced periodic surface structures were effectively produced using this technique.

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.

HiLASE: a scalable option for Laser Inertial Fusion Energy

The goal of the HiLASE project is to design and optimize parameters for 100 J/ 10 Hz Yb:YAG laser amplifiers that are scalable to the kJ regime. The HiLASE power amplifier design is based on a cryogenic, gas-cooled multi-slab concept. Simulation results of the 10 J pre-amplifier agree very well with experimental measurements. In order to fulfil the very demanding requirements, which include wall-plug efficiency > 12% and repetition rates up to 10 Hz, HiLASE and RAL teams are closely working together and developing the approach described here.

HiLASE: development of fully diode pumped disk lasers with high average power

An overview of Czech national R&D project HiLASE (High average power pulsed LASEr) is presented. The HiLASE project aims at development of pulsed DPSSL for hi-tech industrial applications. HiLASE will be a user oriented facility with several laser systems with output parameters ranging from a few picosecond pulses with energy of 5 mJ to 0.5 J and repetition rate of 1-100 kHz (based on thin disk technology) to systems with 100 J output energy in nanosecond pulses with repetition rate of 10 Hz (based on multi-slab technology).