High-energy laser systems are limited by the onset and subsequent growth of damage on constituent optics. This has been extensively studied for optics comprised of fused silica, but less so for other common optical materials. There are very few materials as well characterized as fused silica and, in this work, we explore the growth characteristics of other widely used optical materials with a range of physical parameters, namely sapphire, potassium dihydrogen phosphate, calcium fluoride, and compare them to fused silica. Since current understanding is that material fracture must be present before the fluences used in ns laser systems might cause a surface flaw to grow, we have chosen to study flaws on the exit surfaces created with a Vickers indenter. A range of indenter forces were selected that would produce flaw sizes typical of those that have been seen in laser created damage. Samples with arrays of indents were tested in the in the Optical Science Laser (OSL), a master oscillator power amplifier system, with a front-end pulse shaping capability able to deliver relevant fluences with a large area beam. Samples were tested in vacuum at 351 nm and at atmosphere at 1053 nm with a single shot fired every 45 minutes exposing multiple sites simultaneously. High resolution images of each flaw were taken after every shot to document changes. Additional tests at 1064 nm were conducted of individual sites at a 60 Hz rep rate in the Gigashot Optical Laser Damage (GOLD) system. The probability of growth at 3ω at 5.5 J/cm2 is near 100% for both calcium fluoride and fused silica about 50% for the other materials. The growth rates at 3ω at from 5 to 8 J/cm2 are comparable for all but potassium dihydrogen phosphate which are better than five times lower. At 1ω all the materials had about a factor of five increase in the threshold for growth.
We report an investigation on the response to laser exposure of a protective capping layer of 1ω (1053 nm) high-reflector
mirror coatings, in the presence of differently shaped Ti particles. We consider two candidate capping layer materials,
namely SiO2 and Al2O3. They are coated over multiple silica-hafnia multilayer coatings. Each sample is exposed to a
single oblique (45°) shot of a 1053 nm laser beam (p polarization, fluence ~ 10 J/cm2, pulse length 14 ns), in the
presence of spherically or irregularly shaped Ti particles on the surface. We observe that the two capping layers show
markedly different responses. For spherically shaped particles, the Al2O3 cap layer exhibits severe damage, with the
capping layer becoming completely delaminated at the particle locations. In contrast, the SiO2 capping layer is only
mildly modified by a shallow depression, likely due to plasma erosion. For irregularly shaped Ti filings, the Al2O3
capping layer displays minimal to no damage while the SiO2 capping layer is significantly damaged. In the case of the
spherical particles, we attribute the different response of the capping layer to the large difference in thermal expansion
coefficient of the materials, with that of the Al2O3 about 15 times greater than that of the SiO2 layer. For the irregularly
shaped filings, we attribute the difference in damage response to the large difference in mechanical toughness between
the two materials, with that of the Al2O3 being about 10 times stronger than that of the SiO2.
The National Ignition Facility has recently achieved the milestone of delivering over
1.8 MJ and 500 TW of 351 nm laser energy and power on target, which required average fluences up to 9 J/cm2 (3 ns equivalent) in the final optics system. Commercial fused silica laser-grade UV optics typically have a maximum
operating threshold of 5 J/cm2. We have developed an optics recycling process which enables NIF to operate above the laser damage initiation and growth thresholds. We previously reported a method to mitigate laser damage with laser
ablation of the damage site to leave benign cone shaped pits. We have since developed a production facility with four
mitigation systems capable of performing the mitigation protocols on full-sized (430 mm) optics in volume production.
We have successfully repaired over 700 NIF optics (unique serial numbers), some of which have been recycled as many
as 11 times. We describe the mitigation systems, the optics recycle loop process, and optics recycle production data.
Surface modification of fused silica windows caused by the laser ablation of surface-bound microparticles is
investigated. Using optical and electron microscopies between laser pulses, we detail the ablation, fragmentation
and dispersal of 2-150 μm diameter particles of various materials. Following complete ablation and ejection of all
debris material, surface pitting was found to be highly dependent on material type and particle size. Subsequent
light propagation modeling based on pit morphology indicates up to ~4x intensification. Understanding this class of
non-local, debris-generated damage is argued to be important for effective design of high-power optical windows
and debris-mitigation strategies.
J. Adams, J. Jarboe, C. Carr, M. Feit, R. Hackel, J. Halpin, J. Honig, L. Lane, R. Luthi, J. Peterson, D. Ravizza, F. Ravizza, A. Rubenchik, W. Sell, J. Vickers, T. Weiland, T. Wennberg, D. Willard, M. Yeoman
Previous work [1] has shown the optimum pulse length range for laser-conditioning tripler-cut DKDP with 355 nm (3ω)
light lies between 200 ps and 900 ps for damage initiated at 3 ns. A 3ω, 500 ps (500 ps) table-top laser system has been
built at Lawrence Livermore National Laboratory (LLNL) [2] to take advantage of this optimal conditioning pulse length
range. This study evaluates parameters important for practically utilizing this laser as a raster-scan conditioning laser
and for determining the effectiveness of various conditioning protocols. Damage density vs. test fluence (ρ(Φ) was
measured for unconditioned and 500-ps laser-conditioned (conditioned) DKDP with 3ω, 3 ns (3 ns) test pulses. We find
a 2.5X improvement in fluence in the 3 ns ρ(Φ) after conditioning with 500 ps pulses to 5 J/cm2. We further determine
that the rate of improvement in ρ(Φ)decreases at the higher conditioning fluences (i.e. 3.5 - 5 J/cm2). Single-shot
damage threshold experiments at 500 ps were used to determine the starting fluence for our 500 ps conditioning ramps.
We find 0%, 70%, and 100% single-shot damage probability fluences of 4, 4.5, and 5 J/cm2, respectively at 500 ps. Bulk
damage size distributions created at 3 ns are presented for unconditioned and conditioned DKDP. The range of
diameters of bulk damage sites (pinpoints) in unconditioned DKDP is found to be 4.6 ± 4.4 µm in agreement with
previous results. Also, we observe no apparent difference in the bulk damage size distributions between unconditioned
and conditioned DKDP for testing at 3 ns.
Optics damage under high-intensity illumination may be the direct result of laser light interaction with a contaminant on the surface. Contaminants of interest are small particles of the materials of construction of large laser systems and include aluminum, various absorbing glasses, and fused silica. In addition, once a damage site occurs and begins to grow, the ejecta from the growing damage site create contamination on nearby optic surfaces and may initiate damage on these surfaces via a process we call "fratricide."
We report on a number of experiments that we have performed on fused silica optics that were deliberately contaminated with materials of interest. The experiments were done using 527-nm light as well as 351-nm light. We have found that many of the contaminant particles are removed by the interaction with the laser and the likelihood of removal and/or damage is a function of both fluence and contaminant size. We have developed an empirical model for damage initiation in the presence of contaminants.
Prior to the recent commissioning of the first National Ignition Facility (NIF) beamline, full-scale laser-amplifier-glass cleanliness experiments are performed. Aerosol measurements and obscuration data acquired using a modified flatbed scanner compare favorably to historical large-scale lasers and indicate that NIF is the cleanest large-scale laser built to date.
With the first four of its eventual 192 beams now executing shots and generating more than 100 kJ of laser energy at its primary wavelength of 1.06 µm, the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is already the world's largest and most energetic laser. The optical system performance requirements that are in place for NIF are derived from the goals of the missions it is designed to serve. These missions include inertial confinement fusion (ICF) research and the study of matter at extreme energy densities and pressures. These mission requirements have led to a design strategy for achieving high-quality focusable energy and power from the laser and to specifications on optics that are important for an ICF laser. The design of NIF utilizes a multipass architecture with a single large amplifier type that provides high gain, high extraction efficiency, and high packing density. We have taken a systems engineering approach to the practical implementation of this design that specifies the wavefront parameters of individual optics to achieve the desired cumulative performance of the laser beamline. This paper provides a detailed look at the causes and effects of performance degradation in large laser systems and how NIF has been designed to overcome these effects. We also present results of spot size performance measurements that have validated many of the early design decisions that have been incorporated in the NIF laser architecture.
A large aperture, kJ-class, multi-wavelength Nd-glass laser system has been constructed at Lawrence Livermore National Lab which has unique capabilities for studying a wide variety of optical phenomena. The master-oscillator, power-amplifier (MOPA) configuration of this "Optical Sciences Laser" (OSL) produces 1053 nm radiation with shaped pulse lengths which are variable from 0.1 - 100 ns. The output can be frequency doubled or tripled with high conversion efficiency with a resultant 100 cm2 high quality output beam. This facility can accommodate prototype hardware for large-scale inertial confinement fusion lasers allowing for investigation of integrated system issues such as optical lifetime at high fluence, optics contamination, compatibility of non-optical materials, and laser diagnostics.
With the first four of its eventual 192 beams now executing shots, the National Ignition Facility (NIF) at the Lawrence
Livermore National Laboratory is already the world's largest and most energetic laser. The optical system performance
requirements that are in place for NIF are derived from the goals of the missions it is designed to serve. These missions
include inertial confinement fusion (ICF) research and the study of matter at extreme energy densities and pressures.
These mission requirements have led to a design strategy for achieving high quality focusable energy and power from
the laser and to specifications on optics that are important for an ICF laser. The design of NIF utilizes a multipass
architecture with a single large amplifier type that provides high gain, high extraction efficiency and high packing
density. We have taken a systems engineering approach to the practical implementation of this design that specifies the
wavefront parameters of individual optics in order to achieve the desired cumulative performance of the laser beamline.
This presentation provides a detailed look at the causes and effects of performance degradation in large laser systems
and how NIF has been designed to overcome these effects. We will also present results of spot size performance
measurements that have validated many of the early design decisions that have been incorporated in the NIF laser
architecture.
A novel technique to multiplex high energy laser pulses from a Nd:YLF laser into an array of fibers at energies near the bulk damage limit of fused silica is presented with the object of delivering N equal high energy laser pulses with a minimal time dispersion. Characteristics of the multiple fiber system, diffractive grating splitter, and spatial mode structure of the laser to minimize fiber damage are presented along with preliminary results in scaling the system to larger fiber numbers (N approximately equals 200) with a high energy 10-Joule Nd:Glass laser system. Fiber array alignment techniques and morphologies of fiber damage will also be presented and discussed.
A 1-J laser was designed to launch light down 16, multi-mode fibers (400-μm-core dia.). A diffractive-optic splitter was designed in collaboration with Digital Optics Corporation1 (DOC), and was delivered by DOC. Using this splitter, the energy injected into each fiber varied <1%. The spatial profile out of each fiber was such that there were no "hot spots," a flyer could successfully be launched and a PETN pellet could be initiated. Preliminary designs of the system were driven by system efficiency where a pristine TEM00 laser beam would be required. The laser is a master oscillator, power amplifier (MOPA) consisting of a 4-mm-dia. Nd:YLF rod in the stable, q-switched oscillator and a 9.5-mm-dia. Nd:YLF rod in the double-passed amplifier. Using a TEM00 oscillator beam resulted in excellent transmission efficiencies through the fibers at lower energies but proved to be quite unreliable at higher energies, causing premature fiber damage, flyer plate rupture, stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS). Upon further investigation, it was found that both temporal and spatial beam formatting of the laser were required to successfully initiate the PETN. Results from the single-mode experiments, including fiber damage, SRS and SBS losses, will be presented. In addition, results showing the improvement that can be obtained by proper laser beam formatting will also be presented.
A compact, 2-J Master Oscillator, Power Amplifier (MOPA) laser system was designed and built to support a multiple- fiber injection experiment. The system was built in a breadboard configuration to support a risk-reduction/proof- of-concept effort. A common design approach for MOPA systems is to utilize a single-mode oscillator as the input source to the amplifier. However, to optimize this system for fiber injection, a multi-mode oscillator was chosen. A stable, multi-mode, 1053-nm, Nd:YLF laser oscillator was designed and built. A plano/concave resonator was utilized, with a 4.0-mm diameter Nd:YLF laser rod, pumped in a dual flashlamp, diffuse, close-coupled pump cavity. A lithium niobate (LiNbO3) Q-switch crystal was used in a quarter- wave scheme. This pump cavity design did not use any active cooling and was ideal for low duty cycle applications requiring no more than one shot every 60 seconds. The oscillator output was amplified using a neodymium-doped phosphate glass laser rod in a four-pass configuration. Two Joules of output energy with an output pulsewidth of 12 ns were obtained. The 9.53-mm diameter Nd:Glass amplifier rod was pumped in a dual flashlamp, diffuse, close-coupled pump cavity. Output energy, pulsewidth, far-field beam divergence and intensity profile results will be presented for a Schott LG750 amplifier rod.
A diode-pumped Nd:YAG laser for use as a driver for a soft x-ray projection lithography system is described. The laser will output 0.5 to 1 J per pulse with about 5 ns pulse width at up to 1.5 kHz repetition frequency. The design employs microchannel-cooled diode laser arrays for optical pumping, zigzag slab energy storage, and a single frequency oscillator injected regenerative amplifier cavity using phase conjugator beam correction for near diffraction limited beam quality. The design and initial results of this laser's activation experiments are presented.
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