Jakub Novák

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.

1444-nm Q-switched pulse generator based on Nd:YAG/V:YAG microchip laser

Q-switched microchip laser emitting radiation at eye-safe wavelength 1444 nm was designed and realized. This laser was based on composite crystal which consists of 4 mm long Nd:YAG active medium diffusion bonded with 1 mm long V:YAG saturable absorber. The diameter of the composite crystal was 5 mm. The initial transmission of the V:YAG part was T0 = 94% @ 1440 nm. The microchip resonator consists of dielectric mirrors, directly deposited onto the composite crystal surfaces. These mirrors were specially designed to ensure desired emission at 1444 nm and to prevent parasitic lasing at other Nd3+ transmissions. The output coupler with reflectivity 94% for the generated wavelength 1444 nm was placed on the V3+-doped part. The laser was operating under pulsed pumping for the duty-cycle up to 50%. With increasing value of mean pumping power a strong decrease of generated pulse length was observed. The shortest generated pulses were 4.2 ns long (FWHM). Stable pulses with energy 34 μJ were generated with repetition rate up to 1.5 kHz. Corresponding pulse peak power was 8.2 kW. The wavelength of linearly polarized TEM00 laser mode was fixed to 1444 nm.

Thin disk amplifier-based 40 mJ, 1 kHz, picosecond laser at 515 nm.

We report on a frequency-doubled picosecond Yb:YAG thin disk regenerative amplifier, developed as a pump laser for a kilohertz repetition rate OPCPA. At a repetition rate of 1 kHz, the compressed output of the regenerative amplifier has a pulse duration of 1.2 ps and pulse energy of 90 mJ with energy stability of σ < 0.8% and M2 < 1.2. The pulses are frequency doubled in an LBO crystal yielding 42 mJ at 515 nm.

ELI-Beamlines laser systems: status and design options

We present an overview of the projected and/or implemented laser systems for ELI-Beamlines. The ELI-Beamlines facility will be a high-energy, high repetition-rate laser pillar of the ELI (Extreme Light Infrastructure) project. The facility will make available high-brightness multi-TW ultrashort laser pulses at kHz repetition rate, PW 10 Hz repetition rate laser pulses, and kilojoule nanosecond laser pulses that will be used for generation of 10 PW, and potentially higher, peak power. These systems will allow meeting user requirements for cutting-edge laser resources for programmatic research in generation and applications of high-intensity X-ray sources, in electron and proton/ion acceleration, and in dense plasma and high-field frontier physics.

Broadband OPCPA system with 11 mJ output at 1 kHz, compressible to 12 fs

We report on a broadband OPCPA system, pumped at 515 nm by frequency doubled Yb:YAG thin disk lasers. The system delivers 11.3 mJ pulses at a central wavelength of 800 nm with a spatial beam quality of M2 = 1.25 and > 25% pump-to-signal conversion efficiency. The broadband pulses were demonstrated to be compressible to 12 fs using a chirped mirror compressor.

Pulse synchronization system for picosecond pulse-pumped OPCPA with femtosecond-level relative timing jitter

A simple and compact scheme for synchronization of the pump and signal pulses for short-pulse OPCPA is demonstrated. Relative timing jitter of 17 fs RMS is achieved (1% of the pump pulse duration) and the system remains locked for hours. The scheme uses a balanced optical cross correlator to detect relative delays between the pump and signal pulses and can be operated with just 10s of μJ of pump energy and pJ-level signal energies.

Thin disk picosecond pump laser for jitter stabilized kHz OPCPA

We report on the initial performance of the first ELI-Beamlines high repetition rate, thin disk-based OPCPA pump laser. The laser is designed to produce a pulse train with pulse energies of 10-30 mJ at a 1 kHz repetition rate and is intended to be used as a pump source for an OPCPA amplifier. While the preliminary tests and analysis show that these target energies are well within the capabilities of the equipment available, the output energies of the current design are limited by self-phase modulation. We discuss the sources of this modulation and a new amplifier design to reduce these nonlinear effects. The efficiency of the second harmonic conversion of the thin disk amplifier output is measured to be higher than 65% and scaling to higher energies is discussed.

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.

Influence of temperature on Nd:YAG/V:YAG compact laser generation at 1444 nm

Compact Q-switched diode-pumped laser, emitting radiation at eye-safe wavelength 1444 nm, was studied. This laser was based on composite crystal (diameter 5mm) consisting of 4mm long Nd:YAG active medium diffusion bonded with 1mm long V:YAG saturable absorber (initial transmission @ 1444nm 94 %). The laser resonator mirrors were directly deposited onto the composite crystal surfaces. These mirrors were designed to ensure emission at 1444nm and to prevent parasitic lasing at other Nd3+ transmissions. The pump mirror (R < 10% for pump radiation @ 808 nm, R < 2% @ 1064 nm, R < 15% @ 1330 nm, HR @ 1444 nm) was placed on the Nd3+-doped YAG part. The output coupler with reflectivity 94% for the generated wavelength 1444nm was placed on the V3+-doped part (R < 5% @ 1064 nm, R < 15% @ 1330 nm). Temperature dependence of giant pulse energy and length was studied independently on pumping pulses duty cycle. It was found that for constant duty cycle 1% and for crystal holder temperature rise from 8.2 up to 43.2 °C the pulse width dropped from 31 to 5.1 ns and pulse energy rose from 17 to 57 μJ. This represents a pulse peak power increase from 0.54 up to 11kW. From a mathematical model of passively Q-switched laser it follows that this behaviour can be explained by temperature caused increase of ground-state absorption and ground-state to excited-state absorption ratio (FOM) of V:YAG saturable absorber at wavelength 1444nm in case if FOM ~ 1.

Multi-channel, fiber-based seed pulse distribution system for femtosecond-level synchronized chirped pulse amplifiers

We report on the design and performance of a fiber-based, multi-channel laser amplifier seed pulse distribution system. The device is designed to condition and distribute low energy laser pulses from a mode-locked oscillator to multiple, highly synchronized, high energy amplifiers integrated into a laser beamline. Critical functions such as temporal pulse stretching well beyond 100 ps/nm, pulse picking, and fine control over the pulse delay up to 300 ps are all performed in fiber eliminating the need for bulky and expensive grating stretchers, Pockels cells, and delay lines. These functions are characterized and the system as a whole is demonstrated by seeding two high energy amplifiers in the laser beamline. The design of this system allows for complete computer control of all functions, including tuning of dispersion, and is entirely hands-free. The performance of this device and its subsystems will be relevant to those developing lasers where reliability, size, and cost are key concerns in addition to performance; this includes those developing large-scale laser systems similar to ours and also those developing table-top experiments and commercial systems.