1.1 J Yb:YAG picosecond laser at 1 kHz repetition rate

Picosecond laser filament-guided electrical discharges in air at 1 kHz repetition rate

Laser-induced filaments have been shown to reduce the voltage necessary to initiate electrical discharges in atmospheric air and guide their propagation over long distances. Here we demonstrate the stable generation of laser filament-guided electrical discharge columns in air initiated by high energy (up to 250 mJ) 1030 nm wavelength laser pulses of 7 ps duration at repetition rates up to 1 kHz and we discuss the processes leading to breakdown. A current proportional to the laser pulse energy is observed to arise as soon as the laser pulse arrives, initiating a high impedance phase of the discharge. Full breakdown, characterized by impedance collapse, occurs 100 ns to several µs later. A record 4.7-fold reduction in breakdown voltage for dc-biased discharges, which remains practically independent of the repetition rate up to 1 kHz, is observed to be primarily caused by a single laser pulse that produces a large (∼80%) density depression. The radial gaps between the filamentary plasma channel and the hollowed electrodes employed are shown to play a significant role in the breakdown dynamics. A rapid increase of 3-4 orders of magnitude in current is observed to follow the formation of localized radial current channels linking the filament to the electrodes. The increased understanding and control of kHz repetition rate filament-guided discharges can aid their use in applications.

Joule-class sub-nanosecond pulses produced by end-pumped direct bonded YAG/sapphire modular amplifier

A Joule-class room-temperature diode-pumped solid-state laser was developed. The energy scaling of the 100 mJ 1064 nm seed pulse was realized by a series of two diode-pumped amplifiers. The gain medium consists in free combinations of Nd:YAG ceramics bonded to sapphire transparent heat sinks, to relax the thermal load induced by the 34 kW pump power. At low repetition rate, parasitic lasing was the main limitation to energy scaling. By choosing a gain module combination producing a step-like gradual doping concentration profile, mitigation of parasitic oscillations was observed, and the system delivered 2.8 J, 800 ps pulses at 2 Hz.

Demonstration of a 1 TW peak power, joule-level ultrashort Tm:YLF laser

We report on the demonstration of a diode-pumped, Tm:YLF-based, chirped pulse amplification laser system operating at λ ≈ 1.9 µm that produces amplified pulse energies exceeding 1.5 J using a single 8-pass power amplifier. The amplified pulses are subsequently compressed to sub-300 fs durations by a diffraction grating pair, producing record >1 TW peak power pulses. To the best of our knowledge, this is the highest peak power demonstrated for any solid-state, near-2 µm laser architecture and illustrates the potential of Tm:YLF for the next generation of high-power, diode-pumped ultrashort lasers.

Flexible tape-drive target system for secondary high-intensity laser-driven sources

We present the development of a flexible tape-drive target system to generate and control secondary high-intensity laser-plasma sources. Its adjustable design permits the generation of relativistic MeV particles and x rays at high-intensity (i.e., ≥1 × 1018 W cm-2) laser facilities, at high repetition rates (>1 Hz). The compact and robust structure shows good mechanical stability and a high target placement accuracy (<4 μm RMS). Its compact and flexible design allows for mounting in both the horizontal and vertical planes, which makes it practical for use in cluttered laser-plasma experimental setups. The design permits ∼170° of access on the laser-driver side and 120° of diagnostic access at the rear. A range of adapted apertures have been designed and tested to be easily implemented to the targetry system. The design and performance testing of the tape-drive system in the context of two experiments performed at the COMET laser facility at the Lawrence Livermore National Laboratory and at the Advanced Lasers and Extreme Photonics (ALEPH) facility at Colorado State University are discussed. Experimental data showing that the designed prototype is also able to both generate and focus high-intensity laser-driven protons at high repetition rates are also presented.

Monolithic tapered Yb-doped fiber chirped pulse amplifier delivering 126 μJ and 207 MW femtosecond laser with near diffraction-limited beam quality

In this work, a high-energy and high peak power chirped pulse amplification system with near diffraction-limited beam quality based on tapered confined-doped fiber (TCF) is experimentally demonstrated. The TCF has a core numerical aperture of 0.07 with core/cladding diameter of 35/250 µm at the thin end and 56/400 μm at the thick end. With a backward-pumping configuration, a maximum single pulse energy of 177.9 μJ at a repetition rate of 504 kHz is realized, corresponding to an average power of 89.7 W. Through partially compensating for the accumulated nonlinear phase during the amplification process via adjusting the high order dispersion of the stretching chirped fiber Bragg grating, the duration of the amplified pulse is compressed to 401 fs with a pulse energy of 126.3 μJ and a peak power of 207 MW, which to the best of our knowledge represents the highest peak power ever reported from a monolithic ultrafast fiber laser. At the highest energy, the polarization extinction ratio and the M2 factor were respectively measured to be ~ 19 dB and 1.20. In addition, the corresponding intensity noise properties as well as the short- and long-term stability were also examined, verifying a stable operation of the system. It is believed that the demonstrated laser source could find important applications in, for example, advanced manufacturing and photomedicine.

Kerr-lens mode-locked femtosecond Yb:CALYO oscillator with more than 20-W average power

We report on the demonstration of a pure Kerr-lens mode-locked Yb:CALYO laser which can directly deliver sub-200 fs pulses with more than 20-W average power. With an incident pump power of 89 W, 153-fs pulses were generated with an average power of 21.5 W at a repetition rate of 77.9 MHz. The corresponding peak power and single pulse energy were 1.6 MW and 0.27 µJ, respectively. The stable operation of the mode-locking was confirmed by very small fluctuations in both spectrum and output power recorded over an hour. Second harmonic generation (SHG) was conducted with 59% conversion efficiency, which indicated that the high-power mode-locking pulses are of good quality. Stable Kerr-lens mode-locking (KLM) with 156-fs pulse duration and 27.2-W average power was also achieved with 109-W pump power. To the best of our knowledge, this is the highest average output power ever reported from a femtosecond mode-locked bulk oscillator.

100-mJ, 100-W cryogenically cooled Yb:YLF laser

We present a diode-pumped Yb:YLF laser system generating 100-mJ sub-ps pulses at a 1-kHz repetition rate (100 W average power) by chirped-pulse amplification. The laser consists of a cryogenically cooled 78 K, regenerative, eight-pass booster amplifier seeded by an all-fiber front end. The output pulses are compressed to 980 fs in a single-grating Treacy compressor with a throughput of 89%. The laser will be applied to multi-cycle THz generation and pumping of high average power parametric amplifiers.

One-joule 500-Hz cryogenic Yb:YAG laser driver of composite thin-disk design

We present results on the development of a cryogenic Yb:YAG multi-pass laser amplifier based on a composite thin-disk design and demonstrate one-joule, diffraction limited, chirped 234-ps pulses with 50% optical-to-optical efficiency. High beam quality was obtained for repetition rates up to 400 Hz. The hardware was disassembled and thoroughly inspected after accumulating 80 hours of use at repetition rates from 100 to 500 Hz and exhibited no signs of damage. This laser driver is now commissioned to a dedicated laboratory where a grating compressor is producing 5.2-ps pulses used in the development of a compact x ray source based on inverse Compton scattering.

Power-scalable thin-disk Ti:sapphire laser amplifier

We experimentally demonstrate a 38-fs chirped-pulse amplified (CPA) Ti:sapphire laser system based on the power-scalable thin-disk scheme with an average output power of 1.45 W at a repetition rate of 1 kHz, corresponding to peak power of 38 GW. The beam profile close to the diffraction limit with a measured M² value of approximately 1.1 is obtained. It demonstrates the potential for an ultra-intense laser with high beam quality compared with the conventional bulk gain amplifier. To the best of our knowledge, this is the first reported Ti:sapphire regenerative amplifier based on the thin-disk approach reaching 1 kHz.

Digital single-photon-avalanche-diode arrays for time-of-flight Thomson scattering diagnostics

The collection optics of Thomson scattering systems for plasma devices are designed with maximum possible étendue to keep the required laser energy low. If the spatial resolution along the laser beam is performed by a time-of-flight method, then the detectors, in addition to a large sensitive area, must offer a high frequency bandwidth. Up until now, only microchannel-plate photomultipliers meet these requirements. Here, we investigate the potential use of digital avalanche photodiode arrays operated in the Geiger mode as alternative detectors. In this mode of operation, each array will serve as a fast, sensitive detector. The use of these detectors will lead to significant improvements of the Thomson scattering diagnostic. Most important of these will be a better spatial resolution, down to about 2 cm without deconvolution. Furthermore, the lifetime of the detectors will be increased; the detectors will cover the whole blue wing of the scattered spectrum when using a single wavelength laser, and this will enable measurements of electron temperature and density profiles at kHz repetition rates.

Demonstration of a side-pumped cross-seeded thin-slab pre-amplifier for high-power Ti:Sa laser systems

We demonstrate a room-temperature Ti:Sapphire (Ti:Sa) amplifier that uses a cross pump-seed geometry (cross-thin-slab) to generate 30-mJ output pulses at 0.5-kHz repetition rate, and 25 mJ at 1 kHz when pumped by 100-mJ, 515-nm pulses from a diode-pumped Yb:YAG laser. The geometry allows to maintain a crystal temperature of ∼30°C using cooling water at 10°C. The amplifier is an attractive solution for use in the first stages of amplification in high peak and high average power chirped pulse amplification laser systems.

Few-cycle Yb laser source at 20 kHz using multidimensional solitary states in hollow-core fibers

We demonstrate ultrashort pulse compression from 300 fs down to 17 fs at a repetition rate of 20 kHz and 160-µJ output pulse energy (3.2 W of average power) using multidimensional solitary states (MDSS) in a 1-meter hollow-core fiber (HCF) filled with N₂O. Under static pressure, thermal limitations at this repetition rate annihilate the MDSS with suppression of spectral broadening. The results obtained in differential pressure configuration mitigate thermal effects and significantly increase the range of repetition rate over which MDSS can be used to compress sub-picosecond laser pulses.

High-power Yb:CALGO regenerative amplifier and 30 fs output via multi-plate compression

The pulse energy and average power are two long-sought parameters of femtosecond lasers. In the fields of nonlinear-optics and strong-field physics, they respectively play the role to unlock the various nonlinear processes and provide enough photon fluxes. In this paper, a high-energy and high-power Yb:CALGO regenerative amplifier with 120 fs pulse width is reported. This high-performance regenerative amplifier can work with high stability in a large tuning range of repetition rates. Varying the repetition rate from 3 to 180 kHz, the maximum output power of 36 W and the pulse energy up to 4.3 mJ, corresponding to a peak power of more than 20 GW are demonstrated. The output beam is near diffraction limited with M²= 1.09 and 1.14 on the horizontal and vertical directions, respectively. In addition, multi-plate compression is employed to achieve 30 fs output with 23 W average power which is attractive for applications such as high-harmonic generation.

Recent Development of High-Energy Short-Pulse Lasers with Cryogenically Cooled Yb:YAG

High-power solid-state lasers are among the hot research directions at the forefront of laser research and have major applications in industrial processing, laser-confined nuclear fusion, and high-energy particle sources. In this paper, the properties of Yb:YAG and Nd:YAG crystals as gain media for high-power solid-state lasers were briefly compared, according to the results of which Yb:YAG crystals are more suitable for high-power applications. Then, the effects of the thermodynamic and spectral properties of Yb:YAG crystals with temperature were analyzed in detail, and it was shown that the laser beams amplified by the cryogenically cooled Yb:YAG crystals could have higher beam quality, higher pump absorption efficiency, lower pump threshold, and higher gain. The change in properties of Yb:YAG crystal at low temperature makes it more suitable as a gain medium for high-power lasers. Subsequently, two types of kilowatt-class lasers using cryogenically cooled Yb:YAG crystals as gain media are introduced—100 J, 10 Hz nanosecond lasers and 1 J, 1 kHz picosecond lasers. Their configuration, main parameters, and typical output results were analyzed. Finally, future directions in the development of cryogenically cooled Yb:YAG lasers are discussed.

Energy scaling beyond the gas ionization threshold with divided-pulse nonlinear compression

We demonstrate how pulse energy in hollow-core fiber can be scaled beyond gas-ionization limitations using divided-pulse nonlinear compression. With one pulse, ionization limits our fiber's output pulse energy to 2.7 mJ at an input of 4 mJ. By dividing the pulse to four low-energy pulses before the fiber, we eliminated the ionization and scaled the pulse energy 2.5× to 6.6 mJ at an input energy of 10 mJ. Larger energy scaling is possible, as our maximum pulse energy has not reached the new gas ionization threshold. Our results motivate applying the method to state-of-the-art systems for large pulse energy scaling without prohibitive system size increases.

Wake dynamics of air filaments generated by high-energy picosecond laser pulses at 1 kHz repetition rate

We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 μm-wavelength Yb:YAG laser at repetition rates up to f=1kHz. Interferograms of the wake generated show that while pulses in a train of repetition rate f=0.1kHz encounter a nearly unperturbed environment, at f=1kHz, a channel with an axial air density hole of ∼20% is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at f=1kHz show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

Gev-Scale Accelerators Driven by Plasma-Modulated Pulses from Kilohertz Lasers

We describe a new approach for driving GeV-scale plasma accelerators with long laser pulses. We show that the temporal phase of a long, high-energy driving laser pulse can be modulated periodically by copropagating it with a low-amplitude plasma wave driven by a short, low-energy seed pulse. Compression of the modulated driver by a dispersive optic generates a train of short pulses suitable for resonantly driving a plasma accelerator. Modulation of the driver occurs via well-controlled linear processes, as confirmed by good agreement between particle-in-cell (PIC) simulations and an analytic model. PIC simulations demonstrate that a 1.7 J, 1 ps driver, and a 140 mJ, 40 fs seed pulse can accelerate electrons to energies of 0.65 GeV in a plasma channel with an axial density of 2.5×10^{17}  cm^{-3}. This work opens a route to high repetition-rate, GeV-scale plasma accelerators driven by thin-disk lasers, which can provide joule-scale, picosecond-duration laser pulses at multikilohertz repetition rates and high wall-plug efficiencies.

Demonstration of a compact, multi-joule, diode-pumped Tm:YLF laser

We report the demonstration of a diode-pumped Tm:YLF laser operating at 1.88 µm that produces pulse energies up to 3.88 J in 20 ns. The compact system consists of a Q-switched cavity-dumped oscillator generating 18 mJ pulses, which are then amplified in a four-pass power amplifier. Energies up to 38.1 J were obtained with long-pulse amplifier operation. These results illustrate the high energy storage and extraction capabilities of diode-pumped Tm:YLF, opening the path to high peak and average power mid-infrared solid-state lasers.

Femtosecond multi-10-mJ pulses at 2 µm wavelength by compression in a hollow-core fiber

High-energy few-ps pulses from a Ho:YLF chirped pulse amplifier operating at a 1 kHz repetition rate are compressed in a two-stage arrangement to sub-90-fs duration. The energy of the compressed pulses is more than 20 mJ at an average power of 20 W. In the first stage, the duration of the 2.8 ps, 40 mJ pulses at 2.05 µm wavelength was reduced to 1.4 ps by using nonlinear propagation in air. Subsequently, the pulses were further compressed to 86 fs after spectral broadening in a 3-m-long Kr-filled stretched flexible hollow-core fiber. The high photon flux, peak power, and excellent beam quality and stability make this light source highly attractive for fs pulse generation in the extreme ultraviolet (XUV) to x-ray spectral range for time-resolved XUV spectroscopy or measurements of structural dynamics in solids.