The advent of high energy, high peak power laser systems through chirped pulse amplification (CPA) in broadband solid-state gain media has opened new avenues into High Energy Density, High Field and Material Science research. There are ongoing efforts at numerous institutions in Europe, USA, and China that are striving to achieve output powers up to 200 PW. One main limitation of total laser energy output is the damage threshold and physical size of diffraction gratings. For the 10 PW (1.5 kJ, 150 fs) ELI-Beamline L4 Aton laser, we have developed a new class of meter-sized, multilayer dielectric (MLD) gratings based a low-dispersion design of 1136 lines/mm for a Littrow out-of-plane compressor design operating at 1060 nm. This new class of MLD gratings allows for approximately 4X more total energy on grating compared to the present state of the art. Fabrication of a 850 mm wide x 700 mm tall grating resulted in 98.7% efficiency with 0.3% uniformity at 1060 nm.
Novel architectures of Petawatt-class, high peak power laser systems that allow operating at high repetition rates are opening a new arena of commercial applications of secondary sources and discovery science. The natural path to higher average power is the reduction of the total heat load induced and generated in the laser gain medium and eliminating other inefficiencies with the goal to turn more energy into laser photons while maintaining good beam quality. However, the laser architecture must be tailored to the specific application and laser parameters such as wavelength, peak power and intensity, pulse length, and shot rate must be optimized. We have developed a number of different concepts tailored to secondary source generation that minimize inefficiencies and maximize the average power. The Scalable Highaverage- power Advanced Radiographic Capability (SHARC) and the Big Aperture Thulium (BAT) laser are examples of two such high average power laser concepts; SHARC is designed for production of ion beams and x-rays, and exploration of high energy density physics at 1.5 kW average power, and BAT is envisioned for driving laser-based electron accelerators at 300 kW average power.
Petawatt laser applications, such as laser plasma acceleration, EUV generation, neutron generation, and materials processing are average-power limited. However, the highest average-power petawatt-class laser to date has an average power of less than 1 kW. Scaling Petawatt-class lasers beyond 10 kW of average power requires a paradigm shift in laser design. To date, average power scaling has been accomplished by increasing the repetition rate of single-shot lasers, in which each shot represents a complete pump/extraction cycle. We propose an alternative scheme, multipulse extraction, in which the gain medium is pumped continuously and the upper state population is extracted over many pulses. This method has two primary benefits: First, because efficient extraction is not necessary in a single pulse, the extraction fluence (and hence the B-integral) can be much lower than in a single pulse design. Second, there isn’t a need to pump within a single inverse lifetime, and therefore less expensive, less complex, and more efficient CW pump sources can be used. Multipulse extraction requires that the gain material have an inverse lifetime significantly less than the desired repetition rate. The design and optimization two multipulse extraction amplifiers, a 10 kHz-100 fs-30J amplifier and a 200 Hz-240 fs-240 J amplifier, will be presented. These point designs have applications in laser plasma acceleration and neutron generation, respectively
New control techniques are required to utilize the full potential of next generation high-energy high-repetition-rate pulses lasers while ensuring their safe operation. During automated optimization of an experiment, the control system is required to identify and reject unsafe laser configurations proposed by the optimizer. Using conventional physics codes render impossible when applied to a high energy laser system with 1ms or less time between shots, and also including laser fluctuations and drift. To mitigate this, we are using a deep Bayesian neural network to map the laser’s input power spectrum to its output power spectrum and demonstrate the speed of this approach. The Bayesian neural network can provide an estimate of its own uncertainty as a function of wavelength. A recently developed algorithm enables the uncertainty to be calculated inexpensively using multiple dropout layers inserted into the model. The uncertainty estimates are used by an active learning algorithm to improve the accuracy of the model and intelligently explore the input domain.
A developed formalism1 for analyzing the power scaling of diffraction limited fiber lasers and amplifiers is applied to a
wider range of materials. Limits considered include thermal rupture, thermal lensing, melting of the core, stimulated
Raman scattering, stimulated Brillouin scattering, optical damage, bend induced limits on core diameter and limits to
coupling of pump diode light into the fiber. For conventional fiber lasers based upon silica, the single aperture,
diffraction limited power limit was found to be 36.6kW. This is a hard upper limit that results from an interaction of the
stimulated Raman scattering with thermal lensing. This result is dependent only upon physical constants of the material
and is independent of the core diameter or fiber length. Other materials will have different results both in terms of
ultimate power out and which of the many limits is the determining factor in the results. Materials considered include
silica doped with Tm and Er, YAG and YAG based ceramics and Yb doped phosphate glass. Pros and cons of the
various materials and their current state of development will be assessed. In particular the impact of excess background
loss on laser efficiency is discussed.
Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National Ignition Facility (NIF) places stringent requirements on the control of each of the beam's pointing and overall wavefront quality. One quad of the NIF beams, four beam pairs, will be utilized for these experiments and hydrodynamic and particle-in-cell simulations indicate that for the fast ignition experiments, these beams will be required to deliver 50% (4.0 kJ) of their total energy (7.96 kJ) within a 40-µm-diam spot at the end of a fast ignition cone target. This requirement implies a stringent pointing and overall phase conjugation error budget on the adaptive optics system used to correct these beam lines. The overall encircled energy requirement is more readily met by phasing of the beams in pairs but still requires high Strehl ratios and root-mean-square tip/tilt errors of approximately 1 µrad. To accomplish this task we have designed an interferometric adaptive optics system capable of beam pointing, high Strehl ratio, and beam phasing with a single pixilated microelectromechanical systems deformable mirror and interferometric wavefront sensor. We present the design of a testbed used to evaluate the performance of this wavefront sensor along with simulations of its expected performance level.
Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National
Ignition Facility (NIF) places stringent requirements on the control of each of the beam's pointing and overall wavefront
quality. One quad of the NIF beams, 4 beam pairs, will be utilized for these experiments and hydrodynamic and
particle-in-cell simulations indicate that for the fast ignition experiments, these beams will be required to deliver
50%(4.0 kJ) of their total energy(7.96 kJ) within a 40 μm diameter spot at the end of a fast ignition cone target. This
requirement implies a stringent pointing and overall phase conjugation error budget on the adaptive optics system used
to correct these beam lines. The overall encircled energy requirement is more readily met by phasing of the beams in
pairs but still requires high Strehl ratios, Sr, and RMS tip/tilt errors of approximately one μrad. To accomplish this task
we have designed an interferometric adaptive optics system capable of beam pointing, high Strehl ratio and beam
phasing with a single pixilated MEMS deformable mirror and interferometric wave-front sensor. We present the design
of a testbed used to evaluate the performance of this wave-front sensor below along with simulations of its expected
performance level.
We are developing an all fiber laser system optimized for providing input pulses for short pulse (1-10ps), high energy (~1kJ) glass laser systems. Fiber lasers are ideal solutions for these systems as they are highly reliable and enable long term stable operation. The design requirements for this application are very different than those commonly seen in fiber lasers. High-energy lasers often have low repetition rates (as low as one pulse every few hours), and thus high average power and efficiency are of little practical value. What is of high value is pulse energy, high signal to noise ratio (expressed as pre-pulse contrast), good beam quality, consistent output parameters and timing. Our system focuses on optimizing these
parameters. Our prototype system consists of a mode-locked fiber laser, a compressed pulse fiber amplifier, a "pulse cleaner", a chirped fiber Bragg grating, pulse selectors, a transport fiber system and a large mode area fiber amplifier. We will review the system and present theoretical and experimental studies of critical aspects, in particular the requirement for high pre-pulse contrast.
Femtosecond ablation has several distinct advantages: the threshold energy fluence for the onset of damage and ablation is orders of magnitude less than for traditional nanosecond laser machining, and by virtue of the rapid material removal of approximately an optical penetration depth per pulse, femtosecond machined cuts can be cleaner and more precise than those made with traditional nanosecond or longer pulse lasers. However, in many materials of interest, especially metals, this limits ablation rates to 10-100 nm/pulse. We present the results of using multiple pulse bursts to significantly increase the per-burst ablation rate compared to a single pulse with the same integrated energy, while keeping the peak intensity of each individual pulse below the air ionization limit. Femtosecond ablation with pulses centered at 800-nm having integrated energy of up to 30 mJ per pulse incident upon thin gold films was measured via resonance frequency shifts in a gold-electrode-coated quartz-crystal oscillator. Measurements were performed using Michelson-interferometer-based burst generators, with up to 2 ns pulse separations, as well as pulse shaping by programmable acousto-optic dispersive filter (Dazzler from FastLite) with up to 2 ps pulse separations.
Femtosecond ablation of both absorbing and transparent materials has several distinct advantages: the threshold energy fluence for the onset of damage and ablation is orders of magnitude less than for traditional nanosecond laser machining, and by virtue of the rapid material removal of approximately an optical penetration depth per pulse, femtosecond machined cuts can be cleaner and more precise than those made with traditional nanosecond or longer pulse lasers. However, in many materials of interest, especially metals, this limits ablation rates to 10 - 100 nm/pulse. We will present the results of using multiple pulse bursts to significantly increase the per-burst ablation rate compared to a single pulse with the same integrated energy, while keeping the peak intensity of each individual pulse below the air ionization limit. Femtosecond ablation using 850-nm single and eight-pulse 30-ns duration bursts with 4-mJ integrated energy was seen to yield a five-fold increase in the copper ablation rate in ambient air.
We report on the application of femtosecond x-ray scattering to experimental studies of the photo-induced, structural phaser transition in VO2. The transition between the two crystalline phases of the material occurs, for sufficiently intense excitation, within 500 fs.
Recent advances in femtosecond laser plasma x-rays sources have resulted in several experiments to explore the dynamics of physical and chemical processes on the femtosecond time scale. We present our most recent progresses on the development of an intense broadband x-ray source in the multi-keV range, for application to time-resolved EXAFFS experiments. Experiments have been realized with two different CPA laser systems having different pulse durations and characteristics. X-ray emissions in the 5KeV range generated form solid targets with the INRS Nd:Glass laser and the UCSD Ti:Sapphire laser have been characterized through high resolution and time resolved x-ray spectroscopy. The application of this source to time resolved EXAFS measurements with a sub-picosecond time resolution will also be discussed.
Large-aperture biased photoconductive emitters which can generate high-power narrow-band terahertz (THz) radiation are developed. These emitters avoid saturation at high fluence excitation and achieve enhanced peak power spectral density by employing a thick layer of short lifetime low- temperature-grown GaAs (LT-GaAs) photoconductor and multiple-pulse excitation. THz waveforms are calculated from the saturation theory of large-aperture photoconductors, and a comparison is made between the theory and the measurement. A direct comparison of the multiple-phase saturation properties of terahertz emission from semi-insulating GaAs and LT-GaAs emitters with different carrier lifetimes reveals a strong dependence of the multiple pulse saturation properties of terahertz emission on the carrier lifetime. In particular, the data demonstrate that saturation is avoided only when the interpulse spacing is longer than the carrier lifetime.
Using ultrafast x-ray diffraction from a laser-plasma x-ray source, we have observed coherent photon generation and propagation in bulk(111)-GaAs, (111)-Ge, and thin(111)-Ge- on-Si films. At higher optical pump fluences, ultrafast melting of Ge films is observed.
We have constructed a self-starting Kerr-lens mode-locked Ti:sapphire laser pumped synchronously by a mode-locked, LBO-doubled Nd:YAG laser. Using a home-built beam- pointing stabilizer, beam wander of the 532 nm pump is reduced by a factor of 25, thus enabling long term operation of TEM00 nearly transform limited pulses with amplitude, repetition rate, and pulsewidth fluctuations comparable to or better than those of Ar-pumped Ti3+:Al2O3 lasers. Interferometric autocorrelation using second harmonic in reflection from GaAs yielded a pulse width of 30 fs, limited by the dispersion of wide bandwidth cavity optics that permit tunability from 0.7 to 1.0 micrometers .
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