This PDF file contains the front matter associated with SPIE Proceedings Volume 12577, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

We experimentally demonstrate self-tuning of a white light seeded two-stage optical parametric amplifier using an evolutionary strategy algorithm. Enabled by this approach we demonstrate the automated reproducible adjustment of the lasers working point and achieve highly stable performance of the laser.

This manuscript presents the most recent upgrades made on SYLOS2 laser system in ELI-ALPS1. These modifications on the system were done based on the feedback from the experimental end-stations during the last 3 years. First, the long virtual propagation of the super-Gaussian output beam reduced by modifying the imaging telescope after the amplification stages, which significantly improved the beam profile hindered by the propagation-related diffraction pattern. The optical parametric chirped pulse amplification (OPCPA) stages were equipped with new nonlinear crystals which enabled to reach 42 mJ output energy with <8 fs pulse duration, which is 10 mJ more than the previous configuration. To increase the high-harmonic generation (HHG) efficiency a spatial filter was installed in the laser system which provides a nearly Gaussian beam profile at the output with 80% transmission. The spatial filter is based on a glass cone and is operated under vacuum. Currently, <20 mJ, <8 fs CEP-stable pulses are provided at the user stations after the beam transport system.

The L1-Allegra laser (1 kHz, 50mJ pulse energy, 15 fs pulse duration) developed at ELI-Beamlines in Czechia is already being used for various scientific experiments. The femtosecond synchronization project (referred to as F-SYNC) aims to dramatically improve the experiments with L1-Allegra by providing another 1 kHz beam with arbitrary delay to L1 and femtosecond precision. Therefore, we have developed an independent auxiliary laser system inspired by the design of the L1-Allegra front-end with an output energy of approximately 13mJ at 1 kHz and bandwidth that supports compression to < 16 fs. In essence, F-SYNC consists of a master oscillator, a fiber seed distribution system, a pump laser with a grating compressor, a supercontinuum (SC) seed, and 3 stages of optical parametric chirped pulse amplification (OPCPA).

This study presents a novel way to increase the energy conversion efficiency of optical parametric amplification by eliminating the idler wave from the interaction using consecutive type-I and type-II amplification processes. By using the aforementioned straightforward approach the wavelength tunable narrow-bandwidth amplification with exceptionally high 40% peak pump-to-signal conversion efficiency and 68% peak pump depletion was achieved in the short-pulse regime, while preserving the beam quality factor of less than 1.4. The same optical layout can also serve as an enhanced idler amplification scheme.

We present our recent developments in fiber-based technologies that enable unique high energy and average power ultrafast laser operation regimes. A patented all-fiber front-end incorporates dual-wavelength pulse generation of signal and pump pulses for high intensity OPCPA systems [1]. A newly introduced all-in-fiber active fiber loop (AFL) technology allows to form bursts of ultrashort seed pulses with any desired pulse repetition rate and any number of pulses in a burst with identical intra-burst pulse separation. Furthermore, the AFL introduces the ability to build systems with tunable pulse durations from a few hundred femtoseconds to picoseconds and even up to the nanosecond range. Novel fiber technologies were implemented into a high energy pump laser for a few-cycle mid-IR OPCPA system to produce unprecedented performance and output parameters for high-field science applications.

The Extreme Photonics Applications Centre is an upcoming state-of-the-art research facility under construction at the Rutherford Appleton Laboratory in the United Kingdom. It will deliver petawatt pulses at an unprecedented rate of 10 Hz, made possible by pumping titanium-doped sapphire gain medium with a diode-pumped solid state laser (DPSSL) based on the Central Laser Facility’s cryogenic DiPOLE technology. We will briefly present the DPSSL pump design and its incorporation into the wider PW system, reporting on the most up-todate commissioning status and results of the DPSSL pump laser, which is on target to be fully commissioned by summer 2023.

Dispersion management is among the most challenging aspects of the design and realization of amplified laser systems possessing octave-spanning bandwidth and good compressed pulse quality. Here we demonstrate a new type of device for octave-spanning dispersion management. We combine the paradigm of chirped quasi-phase matching (QPM) for pulse shaping during frequency conversion with the robust, efficient, octave-spanning capability of an adiabatic frequency downconversion device. The result is a simple, monolithic device that can produce an octave-spanning infrared pulse with tailored dispersion – a technique that may be especially convenient for high-energy amplifier chains employing difference frequency generation and/or parametric amplification stages. The technique can also serve as a way to produce pulses of ~10 fs duration throughout the visible to mid-infrared spectrum for hyperspectral ultrafast spectroscopy. Adiabatic frequency conversion employs a slowly changing position-dependent poling frequency in a chirped QPM device to efficiently frequency shift photons over a wide bandwidth with a linear transfer function. In this work, we show that the frequency dependent localized conversion of the process allows tailoring of the total group-delay dispersion (GDD). We have demonstrated a first device with zero GDD, allowing efficient conversion of a few-cycle near-infrared input to a near-single-cycle mid-infrared output of the same duration (~12 fs, with bandwidth spanning 2.0-4.0 microns). We present additional designs for precise custom tailoring of the frequency-dependent group delay.

Both the size of the focal spot and the Rayleigh range of laser beams are increasing with the focal length of a focusing system. When preparing experiments for accelerating electron with lasers, people are considering focal lengths that can range from a few meters up to tens of meters. Telescopic zoom systems made of three spherical mirrors can be designed for such purpose. After a first attempt to design such system based on simple “a priori” parameters, a general algebraic theory has been investigated and shows that there are always solutions with no spherical aberration. When all mirrors are placed off-axis to avoid obscuration of the beam, it is possible to show that there are still solutions that minimize aberrations. When changing the distance between the mirrors, we can obtain a focal excursion of the system while the final focal spot is fixed. Of course, the goal of the study is to find what are the solutions that minimize aberrations for a given numerical aperture over a given zoom range. I have built and tested three zoom systems based on different solutions and I have been able to show that there are simple alignment procedures for generating a fixed focal spot over the zoom range. In this paper, a step-by-step analysis including damage fluence considerations for designing the 3-mirror zoom system will be detailed.

We discuss the use of the thin film compression (TFC) technique for generating an ultrafast X-ray beam having a few femtoseconds in duration which could be appropriate for pump and probe experiment. Our results demonstrate the potential of the technique for the generation of a few fs hard X-ray beam through the LWFA-based betatron process, enabling an extraordinary short X-ray probe for doing time resolved X-ray absorption spectroscopy of ultra-relativistic plasmas.

The L2-DUHA Laser (Dual-beam Ultra-fast High energy OPCPA Amplifier) designed to provide 100 TW-level pulses at 50 Hz is being developed at ELI-beamlines. The front end will provide the seed for 100 TW pulse train and also synchronized multi-mJ, sub 50 fs, 2.2 μm auxiliary output at 2 kHz, both generated via supercontinuum. The near-IR branch centered around 820 nm is amplified in two OPCPA stages and stretched to 1.5 ns. The beam in the mid-IR branch is combined with a 1030 nm beam in DFG to generate a mid-IR beam centered around 2.2 μm, amplified in three OCPA stages and compressed.

High Power Laser’s (HPL) optimal performance is essential for the success of a wide variety of experimental tasks related to light-matter interactions. Traditionally, HPL parameters are optimized in an automated fashion relying on black-box numerical methods. However, these can be demanding in terms of computational resources and usually disregard transient and complex dynamics. Model-free Deep Reinforcement Learning (DRL) offers a promising alternative framework for optimizing HPL performance since it allows to tune the control parameters as a function of system states subject to nonlinear temporal dynamics without requiring an explicit dynamics model of those. Furthermore, DRL aims to find an optimal control policy rather than a static parameter configuration, particularly suitable for dynamic processes involving sequential decision making. This is particularly relevant as laser systems are typically characterized by dynamic rather than static traits. Hence the need for a strategy to choose the control applied based on the current context instead of one single optimal control configuration. This paper investigates the potential of DRL in improving the efficiency and safety of HPL control systems. We apply this technique to optimize the temporal profile of laser pulses in the L1 pump laser hosted at the ELI Beamlines facility. We show how to adapt DRL to the setting of spectral phase control by solely tuning dispersion coefficients of the spectral phase and reaching pulses similar to transform limited with full-width at half-maximum (FWHM) of ∼1.6 ps. The code base of this work, alongside a live demo of the results obtained, is available at github.com/fracapuano/TempoRL.

The aim of this work was to design a compact Q-switched laser generating radiation in the 1.3 μm spectral region. Two active materials, Nd:YAG and Nd:YAP, were used to construct such a compact laser with stable nanosecond pulses which could be used as a laser source of LIDAR for autonomous vehicle control. A constructed laser was pumped longitudinally by a fiber-coupled laser diode (core diameter 400 μm, numerical aperture 0.22) in a pulse regime at a wavelength of around 805nm in the range of repetition frequencies of 10 − 500 Hz. A V:YAG saturable absorber with an initial transmission of 85% was used to achieve the Q-switched regime. The pumping resonator dielectric mirror had a high transmission for pumping radiation and high reflectivity for generated 1.3 μm radiation. The output resonator dielectric mirror reflectivity was 90% @ 1.3 μm. The Nd:YAG/V:YAG laser provided radiation at a wavelength of 1318nm with pulse energy up to 162 μJ, pulse length ∼ 13.5 ns, and pulse peak power up to 12.3 kW. With the Nd:YAP/V:YAG compact laser generating at a wavelength of 1342 nm, a pulse energy of up to 193 μJ, pulse length ∼ 11.8 ns, and pulse peak power up to 16.2 kW, was achieved. Generated pulse energy and peak power decay with increasing pumping frequency was steeper in the case of Nd:YAP/V:YAG laser due to poorer thermal conductivity of Nd:YAP crystal compared to Nd:YAG crystal. On the other hand, the Nd:YAP/V:YAG laser showed better stability of the wavelength and polarization of the output radiation. In the case of both lasers, linearly polarized radiation with TEM00 single-mode spatial profile was generated.

The spectral and laser characteristics of two anti-reflection (AR) coated Cr:ZnSe single crystals 3 mm and 5 mm long were investigated under ~1.7 μm laser diode pumping. The crystals were assembled in a copper holder actively cooled by circulating water. Room temperature absorption and fluorescence spectra were measured, together with the fluorescence lifetime of Cr2+ ions were measured. The influence of the pump pulse durations on the output laser power was investigated. The mean output power of ~2.7 W in CW mode at the wavelength of ~2.4 μm (~24% slope efficiency) was obtained with the 5 mm thick AR coated sample. In this case the M2 beam parameter of ~1.1±0.1 and the beam waist diameter of ~42±4 μm which corresponds to the maximum (in the beam waist) laser power density of ~200 kW/cm2. Furthermore, using a MgF2 birefringent plate, the laser output was tuned from ~2.05 μm up to ~2.65 μm with a spectral linewidth of ~5-10 nm and a Gaussian beam profile. The mean laser output power in the broad mid-infrared range of ~2.1-2.52 μm exceeded 1 W, which corresponds to a power density of at least ~73.5 kW/cm2.

The free-running and acousto-optically Q-switched laser properties of Er:YAP crystal, which is appropriate for generation at 2.8 μm, are presented. The sample of Er:YAP (concentration 5 at.% of Er3+, thickness 9 mm) had plan-parallel polished faces without antireflection coatings. The excitation of Er:YAP crystal was carried out by a laser diode emitting at 973 nm and working in a pulsed regime (pulse duration 5 ms, repetition rate 10 Hz). The laser resonator was hemispherical, 150 mm in length, with a flat pumping mirror (HR @ 2.8 μm) and a spherical output coupler (r = 150 mm, R = 95 % @ 2.5 - 2.9 μm). In the free-running laser regime, the maximum output power of 156 mW and slope efficiency of 17 % with respect to absorbed pumped power were obtained. To obtain a Q-switching laser operation, the acousto-optic germanium element was inserted between the Er:YAP crystal and the output coupler. The shortest pulse duration of 36.2 ns with a repetition rate of 10 Hz and maximum pulse energy of 0.43 μJ were obtained. The emitted laser wavelength of 2.8 μm can be used as a pump source at room-temperature for Fe:ZnSe or in mid-infrared spectroscopy.

The goal of this work was an investigation of Ho:YAP (Ho:YAlO3) crystal as an active medium of resonantly longitudinally pumped multi-watt microchip laser operating at 2.1 ¹m spectral region. Three Ho:YAP crystals,a"-cut, b"-cut, and c"-cut Pbnm, with the same dimensions (7mm long, 3mm in diameter) and Ho-doping concentration (1.06 at.% Ho/Y) were compared. Resonator mirrors were deposited directly on the crystals faces. The output coupler transmission for desired laser emission wavelength range 2.1 µm was 11% and T = 3% @ 1.94 µm. The pumping mirror was highly reflecting at 2.1 µm and T = 89% @ 1.94 µm. Samples were fixed in air-cooled Cu-heatsink and longitudinally pumped by a CW Tm-fibre laser with the maximum output power amplitude of 12W @ 1939.2nm behind a focusing lens (f = 200 mm). The laser output power, emission wavelength, and output beam profile were measured in respect to incident pumping power. All three lasers had similar input-output power characteristics with the laser threshold close to 1.5W and slope efficiencies reaching quantum limit in respect to the incident pumping power. The best result (slope efficiency 79 %, laser threshold 1.54W, max output power 8.2W in an almost diffraction-limited, linearly polarized beam) was reached for microchip laser using b"-cut Ho:YAP crystal. Laser emission wavelength was 2119nm for a"-cut and b"-cut Ho:YAP and 2132nm for c"-cut Ho:YAP-based microchip laser. The designed lasers can serve as compact wavelength converters for laser radiation and could be used to expand capabilities of current Tm-fibre lasers used in medicine and industry preserving the overall system efficiency.

We present a temperature influence (in range from 78 up to 300 K) on the spectroscopic and laser properties of Tm:SrF2 crystal doped with 2 at. % of Tm3+. The sample was grown using the temperature gradient technique in shape of a single-crystal fiber (d= 2 mm, l = 5 mm) with plane-parallel face-polished without any AR coating. The Tm:SrF2 crystal was mounted in a temperature-controlled copper holder of the liquid nitrogen cryostat. The measured absorption and emission spectra remained broad even at low temperature. The fluorescence lifetime was fitted with a double exponential function, and the measured lifetime changed significantly with temperature decrease. The 147 mm long semi-hemispherical laser resonator consisted of a flat pumping mirror (T < 95 % @ 763 nm, HR @ 1750-2100 nm) placed inside the cryostat and a curved output coupler (r=150 mm, R=97.5 % @ 1750-2100 nm) placed outside the cryostat. For longitudinal pumping, a fiber coupled laser diode was used. The diode was operating in the pulse regime (5 ms pulse length, 10 Hz repetition rate) at wavelength 763 nm. At room temperature, the laser emission was achieved at 1949 nm with a high 38 % slope efficiency. With a temperature decrease, the slope efficiency increased, and the laser threshold decreased, and the laser output wavelength shifted.

The technique of chirped pulse amplification has conveniently enabled the development of Joule-class laser facilities delivering terawatt levels of power with a single shot beam on target. In powerful systems like these, beam metrology for pulse charaterization is crucial for maintaining a high level of confidence in generating high intensity laser shots. Optical diagnostics for monitoring beam features such as spectra, beam profile, energy and also measuring dispersion and the compressed pulse duration are necessary. Spatio-temporal couplings such as angular dispersion caused by the presence of chromatic aberrations can distort the pulse energy and duration on target and affect laser matter interactions. A pulsefront- tilt (PFT) diagnostic utilizing a diffractive optic for determining angular dispersion and misalignment at shorter bandwidths is designed and implemented for our laser facility, along-with a homemade single shot auto-correlator for measuring pulse duration after the grating compressor. For real-time monitoring of the laser facility, a smart dashboard protocol using openBIS ELN (Electronics Lab Notebook) is administered to enable regular and uninterrupted saving of raw data which is integrated into the local dashboard for visualization and data analysis.