A. Kritcher, D. Schlossberg, C. Weber, C. Young, E. Dewald, A. Zylstra, O. Hurricane, A. Allen, B. Bachmann, K. Baker, S. Baxamusa, T. Braun, G. Brunton, D. Callahan, D. Casey, T. Chapman, C. Choate, D. Clark, J.-M. Di Nicola, L. Divol, M. Edwards, S. Haan, T. Fehrenbach, S. Hayes, D. Hinkel, M. Hohenberger, K. Humbird, O. Jones, E. Kur, B. Kustowski, C. Kong, O. Landen, D. Larson, X. Lepro-Chavez, J. Lindl, B. MacGowan, S. Maclaren, M. Marinak, M. Millot, A. Nikroo, R. Nora, A. Pak, P. Patel, J. Ralph, M. Ratledge, M. Rubery, S. Sepke, M. Stadermann, D. Strozzi, T. Suratwala, R. Tommasini, R. Town, B. Woodworth, B. Van Wonterghem, C. Wild
An exciting use of high powered lasers is to inertially confine fusion plasmas in the laboratory. This presentation describes the first design to achieve controlled fusion target gain exceeding one using high powered lasers in the inertial confinement fusion approach and recent experimental results on the NIF (National Ignition Facility). In these experiments, laser beams incident on the inside of a cylindrical can (Hohlraum) generates an intense x-ray radiation bath that is used to spherically implode pellets containing Deuterium and Tritium. On Dec 5th 2022, the imploded pellet generated more fusion energy (3.15 MJ) than laser energy incident on the target (2.05 MJ), reaching a milestone for the field that was more than six decades in the making. Follow on experiments in this platform using 2.2 MJ of laser energy have generated >5 MJ and >2x target gain.
On December 5, 2022, Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility (NIF) made history, demonstrating fusion ignition for the first time in a laboratory setting. A review of the major large optic technologies over the past several decades is presented that have enabled the National Ignition Facility laser to both routinely operate >2MJ and achieve fusion ignition.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to 351nm light or 3w. It was built to access the extreme high energy density conditions needed to support the nation’s nuclear stockpile in the absence of further underground nuclear tests, including studying Inertial Confinement Fusion (ICF) and ignition in the laboratory.
Over the last year, important results have been obtained demonstrated a fusion yield of 1.35MJ with 1.9MJ of laser energy (and 440 TW power) injected in the target, bringing the NIF to the threshold of ignition [2-3]. As the yield curve near ignition is steep, the laser performance team has focused on providing improved power accuracy and precision (better shot-to-shot reproducibility) with a high-fidelity pulse shaping system (HiFiPS), and also on extending the NIF operating power and energy space by 15% to 2.2MJ and 500TW.
High-average-power solid-state lasers are often severely limited by thermally induced stress birefringence, leading to depolarization losses and damage issues. Depolarization losses can be mitigated by selecting appropriate polarization optics to compensate the space-variant stress birefringence distribution. While some small-aperture, low-fluence technologies are available, no solution so far can provide a combination of high fluence and large aperture with a high degree of control of the beam quality. In this work, we demonstrate the use of magnetorheological finishing (MRF) technology to carve a prescribed thickness profile in a quartz waveplate to achieve precise space-variant polarization control. An arbitrary distorted polarization distribution can be converted into a uniform linearly polarized beam using two freeform MRF crystal optics with their crystal axes offset by 45 degrees. This technology is readily scalable to high-fluence, large-aperture applications, potentially enabling new regimes of laser operation.
The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a
192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to
351nm light or 3w. It has been designed to support the study of Inertial Confinement Fusion (ICF)
and High Energy Density Physics (HEDP). The NIF Precision Diagnostic System (PDS) was reactivated and new
diagnostic packages were designed and fielded that offer a more comprehensive suite
of high-resolution measurements. The current NIF laser performance will be presented as well as the preliminary results obtained with the various laser experimental campaigns using the new diagnostic tool suites.
The final optics in the National Ignition Facility (NIF) are protected from target debris by sacrificial (disposable) debris shields (DDS) comprised of 3-mm thick Borofloat. While relatively inexpensive, Borofloat has been found to have bulk inclusions which, under UV illumination, damage, grow, and occasional erupt though the surface of the DDS. We have shown previously that debris generated from Input Surface Bulk Eruptions (ISBE) are a significant source of damage on NIF. Inclusion-free fused silica debris shield (FSDS) have been installed in between the DDS and the final optics on some NIF beam lines to test their efficacy in mitigating damage initiation. We will show results of the damage performance of the FSDS and its role in protecting the final optics. These results will help in our economic analysis of the potential benefits of using FSDS to protect NIF final optics.
Loose abrasive grinding was performed on a wide range of optical workpiece materials [single crystals of Al2O3 (sapphire), SiC, Y3Al5O12 (YAG), CaF2, and LiB3O5 (LBO); a SiO2-Al2O3-P2O5-Li2O glass-ceramic (Zerodur); and glasses of SiO2 : TiO2 (ULE), SiO2 (fused silica), and P2O5-Al2O3-K2O-BaO (phosphate)]. Using the magneto rheological finishing (MRF) taper wedge technique (where a wedge was polished on each of the ground workpieces and the resulting samples were appropriately chemically etched), the subsurface mechanical damage (SSD) characteristics were measured. The SSD depth for most of the workpiece materials was found to scale as E11/2 / H1, where E1 is the elastic modulus and H1 is the hardness of the workpiece. This material scaling is the same as that for the growth of lateral cracks, suggesting that lateral cracks are a dominant source for SSD rather than radial/median cracks, as previously proposed. Utilizing the SSD depth data from both this study and others, semiempirical relationships have been formulated, which allows for estimating the SSD depth as a function of workpiece material and important grinding parameters (such as abrasive size and applied pressure).
The National Ignition Facility (NIF) regularly operates at fluences above the onset of laser-induced optics damage. To do so, it is necessary to routinely recycle the NIF final optics, which involves removing an optic from a beamline, inspecting and repairing the laser-induced damage sites, and re-installing the optic. The inspection and repair takes place in our Optics Mitigation Facility (OMF), consisting of four identical processing stations for performing the repair protocols. Until recently, OMF has been a labor-intensive facility, requiring 10 skilled operators over two shifts to meet the throughput requirements. Here we report on the implementation of an automated control system—informed by machine learning— that significantly improves the throughput capability for recycling of NIF optics while reducing staffing requirements. Performance metrics for mid-2018 show that approximately 85% of all damage sites can be automatically inspected and repaired without any required operator input. Computer keystrokes have been reduced from about 6000 per optic to under 300.
Operating the National Ignition Facility (NIF) near its power and energy performance limits has revealed a new damage initiation mechanism in the final UV optics. The typical damage event involves the last three optics in the NIF beamline: the final focusing lens, the grating debris shield, and the target debris shield. It occurs on high power shots from intensifications from small phase defects (pits) on the exit surface of the focusing lens that travel through the grating debris shield before reflecting off the AR-coated target debris shield about 75 cm downstream, then propagate back upstream and damage the input surface of the grating debris shield optic which is 15 cm downstream of the focusing lens. Ray tracing has firmly established the direct relationship between the phase defects on the final focusing lens and the damage on grating debris via the reflection from the target debris shield. In some cases, bulk filamentary damage is also observed in the 1-cm thick fused silica grating debris shield. It is not fully understood at this point how there can be enough energy from the reflected beam to cause damage where the forward-going beam did not. It does not appear that interaction between the forward-going beam and the backward-going reflected beam is necessary for damage to occur. It does appear necessary that the target debris shield be previously exposed to laser shots and/or target debris. Furthermore, there is no evidence of damage imparted to the target debris shield or the final focusing lens. We will describe all the conditions under which we have (and have not) observed these relatively rare events, and the steps we have taken to mitigate their occurrence, including identification and elimination of the source phase defects.
Additive manufacturing offers new routes to lightweight optics inaccessible by conventional methods by providing a broader range of reconciled functionality, form factor, and cost. Predictive lattice design combined with the ability to 3D print complex structures allows for the creation of low-density metamaterials with high global and local stiffness and tunable response to static and dynamic loading. This capacity provides a path to fabrication of lightweight optical supports with tuned geometries and mechanical properties. Our approach involves the simulation and optimization of lightweight lattices for anticipated stresses due to polishing and mounting loads via adaptive mesh refinement. The designed lattices are 3D printed using large area projection microstereolithography (LAPuSL), coated with a metallic plating to improve mechanical properties, and bonded to a thin (1.25 mm) fused silica substrate. We demonstrate that this lightweight assembly can be polished to a desired flatness using convergent polishing, and subsequently treated with a reflective coating.
*This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 within the LDRD program. LLNL-ABS-738806.
The primary sources of damage on the National Ignition Facility (NIF) Grating Debris Shield (GDS) are attributed to
two independent types of laser-induced particulates. The first comes from the eruptions of bulk damage in a
disposable debris shield downstream of the GDS. The second particle source comes from stray light focusing on
absorbing glass armor at higher than expected fluences. We show that the composition of the particles is
secondary to the energetics of their delivery, such that particles from either source are essentially benign if they
arrive at the GDS with low temperatures and velocities.
C. Haefner, A. Bayramian, S. Betts, R. Bopp, S. Buck, J. Cupal, M. Drouin, A. Erlandson, J. Horáček, J. Horner, J. Jarboe, K. Kasl, D. Kim, E. Koh, L. Koubíková, W. Maranville, C. Marshall, D. Mason, J. Menapace, P. Miller, P. Mazurek, A. Naylon, J. Novák, D. Peceli, P. Rosso, K. Schaffers, E. Sistrunk, D. Smith, T. Spinka, J. Stanley, R. Steele, C. Stolz, T. Suratwala, S. Telford, J. Thoma, D. VanBlarcom, J. Weiss, P. Wegner
Large laser systems that deliver optical pulses with peak powers exceeding one Petawatt (PW) have been constructed at dozens of research facilities worldwide and have fostered research in High-Energy-Density (HED) Science, High-Field and nonlinear physics [1]. Furthermore, the high intensities exceeding 1018W/cm2 allow for efficiently driving secondary sources that inherit some of the properties of the laser pulse, e.g. pulse duration, spatial and/or divergence characteristics. In the intervening decades since that first PW laser, single-shot proof-of-principle experiments have been successful in demonstrating new high-intensity laser-matter interactions and subsequent secondary particle and photon sources. These secondary sources include generation and acceleration of charged-particle (electron, proton, ion) and neutron beams, and x-ray and gamma-ray sources, generation of radioisotopes for positron emission tomography (PET), targeted cancer therapy, medical imaging, and the transmutation of radioactive waste [2, 3]. Each of these promising applications requires lasers with peak power of hundreds of terawatt (TW) to petawatt (PW) and with average power of tens to hundreds of kW to achieve the required secondary source flux.
Overview of progress in construction and testing of the laser systems of ELI-Beamlines, accomplished since 2015, is presented. Good progress has been achieved in construction of all four lasers based largely on the technology of diode-pumped solid state lasers (DPSSL). The first part of the L1 laser, designed to provide 200 mJ <15 fs pulses at 1 kHz repetition rate, is up and running. The L2 is a development line employing a 10 J / 10 Hz cryogenic gas-cooled pump laser which has recently been equipped with an advanced cryogenic engine. Operation of the L3-HAPLS system, using a gas-cooled DPSSL pump laser and a Ti:sapphire broadband amplifier, was recently demonstrated at 16 J / 28 fs, at 3.33 Hz rep rate. Finally, the 5 Hz OPCPA front end of the L4 kJ laser is up running and amplification in the Nd:glass large-aperture power amplifiers was demonstrated.
Modeling of laser-induced optics damage has been introduced to benchmark existing optic usage at the National Ignition
Facility (NIF) which includes the number of optics exchanged for damage repair. NIF has pioneered an optics recycle
strategy to allow it to run the laser at capacity since fully commissioned in 2009 while keeping the cost of optics usage
manageable. We will show how the damage model is being used to evaluate strategies to streamline our optics loop
efficiency, as we strive to increase the laser shot rate without increasing operating costs.
Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will
review important optical damage precursors in silica up to UV fluences as high as 45J/cm2 (3ns) along with studies of
the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface
damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of
a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily
responsible for optic damage below about 10J/cm2; they can be mitigated by an optimized oxide etch processes. At
fluences above about 10J/cm2, precipitates of trace impurities are responsible for damage; they can be mitigated by
eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica
damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating
fluences these optics see.
A challenging aspect of preparing cryogenic targets for National Ignition Facility (NIF) ignition experiments is growing a single crystal layer (~ 70 m thick) of solid frozen deuterium-tritium (DT) fuel on the inner surface of a spherical hollow plastic capsule 2 mm in diameter. For the most critical fusion experiments, the layer must be smooth, having uniform thickness, and largely free of isolated defects (e.g. grooves). A single target layer typically takes up to 18 hours to form. X-ray images on 3 orthogonal axes are used to monitor the growth of the crystal and evaluate the quality of the layer. While these methods provide a good indicator of target layer condition, new metrics are currently being developed to take advantage of other properties in the x-ray image, which may give earlier indications of target quality. These properties include symmetry of texture, seed formation, and eigenimage analysis. We describe the approach and associated image processing to evaluate and classify these metrics, whose goal is to improve overall layer production and better quantify the quality of the layer during its growth.
Previous studies have identified two significant precursors of laser damage on fused silica surfaces at fluences <35 J/cm2: photoactive impurities from polishing and surface fractures. We evaluate isothermal heating as a means of remediating the defect structure associated with surface fractures. Vickers indentations are applied to silica surfaces at loads between 0.5 and 10 N, creating fracture networks. The indentations are characterized before and following thermal annealing under various time and temperature conditions using confocal time-resolved photo-luminescence (CTP) imaging, and R/1 damage testing with 3-ns, 355-nm laser pulses. Improvements in the damage thresholds with reductions in CTP intensity are observed at temperatures well below the glass transition temperature (Tg). The damage threshold on 0.5-N indentations improves from <8 to >35 J/cm2 after annealing at approximately 750°C. Larger fracture networks require longer or higher temperature treatment to achieve similar results. At an annealing temperature >1100°C, optical microscopy indicates morphological changes in some of the fractures surrounding the indentations, although remnants of the original fractures are still observed. We demonstrate the potential of using isothermal annealing to improve the laser damage resistance of silica optics, and provide a means of further understanding the physics of optical damage and mitigation.
A system of customized spatial light modulators has been installed onto the front end of the laser system at the National
Ignition Facility (NIF). The devices are capable of shaping the beam profile at a low-fluence relay plane upstream of the
amplifier chain. Their primary function is to introduce "blocker" obscurations at programmed locations within the beam
profile. These obscurations are positioned to shadow small, isolated flaws on downstream optical components that might
otherwise limit the system operating energy. The modulators were designed to enable a drop-in retrofit of each of the 48
existing Pre Amplifier Modules (PAMs) without compromising their original performance specifications. This was
accomplished by use of transmissive Optically Addressable Light Valves (OALV) based on a Bismuth Silicon Oxide
photoconductive layer in series with a twisted nematic liquid crystal (LC) layer. These Programmable Spatial Shaper
packages in combination with a flaw inspection system and optic registration strategy have provided a robust approach
for extending the operational lifetime of high fluence laser optics on NIF.
Customized spatial light modulators have been designed and fabricated for use as precision beam shaping devices in
fusion class laser systems. By inserting this device in a low-fluence relay plane upstream of the amplifier chain,
"blocker" obscurations can be programmed into the beam profile to shadow small isolated flaws on downstream optical
components that might otherwise limit the system operating energy. In this two stage system, 1920 × 1080 bitmap
images are first imprinted on incoherent, 470 nm address beams via pixelated liquid crystal on silicon (LCoS)
modulators. To realize defined masking functions with smooth apodized shapes and no pixelization artifacts, address
beam images are projected onto custom fabricated
optically-addressable light valves. Each valve consists of a large,
single pixel liquid cell in series with a photoconductive Bismuth silicon Oxide (BSO) crystal. The BSO crystal enables
bright and dark regions of the address image to locally control the voltage supplied to the liquid crystal layer which in
turn modulates the amplitude of the coherent beams at 1053 nm. Valves as large as 24 mm × 36 mm have been
fabricated with low wavefront distortion (<0.5 waves) and antireflection coatings for high transmission (>90%) and
etalon suppression to avoid spectral and temporal ripple. This device in combination with a flaw inspection system and
optic registration strategy represents a new approach for extending the operational lifetime of high fluence laser optics.
Current methods for the manufacture of optical components inevitably leaves a variety of sub-surface imperfections
including scratches of varying lengths and widths on even the finest finishes. It has recently been determined that these
finishing imperfections are responsible for the majority of laser-induced damage for fluences typically used in ICF class
lasers. We have developed methods of engineering subscale parts with a distribution of scratches mimicking those found
on full scale fused silica parts. This much higher density of scratches provides a platform to measure low damage
initiation probabilities sufficient to describe damage on large scale optics. In this work, damage probability per unit
scratch length was characterized as a function of initial scratch width and post fabrication processing including acidbased
etch mitigation processes. The susceptibility of damage initiation density along scratches was found to be strongly
affected by the post etching material removal and initial scratch width. We have developed an automated processing
procedure to document the damage initiations per width and per length of theses scratches. We show here how these
tools can be employed to provide predictions of the performance of full size optics in laser systems operating at 351 nm.
In addition we use these tools to measure the growth rate of a damage site initiated along a scratch and compare this to
the growth measured on an isolated damage site.
Using high-sensitivity confocal time-resolved photoluminescence (CTP) techniques, we report an ultra-fast
photoluminescence (40ps-5ns) from impurity-free surface flaws on fused silica, including polished, indented or
fractured surfaces of fused silica, and from laser-heated evaporation pits. This fast photoluminescence (PL) is not
associated with slower point defect PL in silica which has characteristic decay times longer than 5ns. Fast PL is
excited by the single photon absorption of sub-band gap light, and is especially bright in fractures. Regions which
exhibit fast PL are strongly absorptive well below the band gap, as evidenced by a propensity to damage with 3.5eV
ns-scale laser pulses, making CTP a powerful non-destructive diagnostic for laser damage in silica. The use of CTP
to provide insights into the nature of damage precursors and to help develop and evaluate new damage mitigation
strategies will be presented.
Fluoride-based wet chemical etching of fused silica optical components is useful to open up surface fractures for
diagnostic purposes, to create surface topology, and as a possible mitigation technique to remove damaged material. To
optimize the usefulness of etching, it is important to understand how the morphology of etched features changes as a
function of the amount of material removed. In this study, we present two geometric etch models that describe the
surface topology evolution as a function of the amount etched. The first model, referred to as the finite-difference etch
model, represents the surface as an array of points in space where at each time-step the points move normal to the local
surface. The second model, referred to as the surface area-volume model, more globally describes the surface evolution
relating the volume of material removed to the exposed surface area. These etch models predict growth and coalescence
of surface fractures such as those observed on scratches and ground surfaces. For typical surface fractures, simulations
show that the transverse growth of the cracks at long etch times scales with the square root of etch time or the net
material removed in agreement with experiment. The finite-difference etch model has also been applied to more complex
structures such as the etching of a CO2 laser-mitigated laser damage site. The results indicate that etching has little
effect on the initial morphology of this site implying little change in downstream scatter and modulation characteristics
upon exposure to subsequent high fluence laser light. In the second part of the study, the geometric etch model is
expanded to include fluid dynamics and mass transport. This later model serves as a foundation for understanding
related processes such as the possibility of redeposition of etch reaction products during the etching, rinsing or drying
processes.
There is a longstanding, and largely unexplained, correlation between the laser damage susceptibility
of optical components and both the surface quality of the optics, and the presence of near surface
fractures in an optic. In the present work, a combination of acid leaching, acid etching, and confocal
time resolved photoluminescence (CTP) microscopy has been used to study laser damage initiation
at indentation sites. The combination of localized polishing and variations in indentation loads
allows one to isolate and characterize the laser damage susceptibility of densified, plastically flowed
and fractured fused silica. The present results suggest that: 1) laser damage initiation and growth are
strongly correlated with fracture surfaces, while densified and plastically flowed material is
relatively benign, and 2) fracture events result in the formation of an electronically defect rich
surface layer which promotes energy transfer from the optical beam to the glass matrix.
We have developed an experimental technique that combines magnetorheological finishing (MRF) and microscopy to examine fractures and/or artifacts in optical materials. The technique can be readily used to provide access to, and interrogation of, a selected segment of a fracture or object that extends beneath the surface. Depth slicing, or cross-sectioning at selected intervals, further allows the observation and measurement of the three-dimensional nature of the sites and the generation of volumetric representations that can be used to quantify shape and depth, and to understand how they were created, how they interact with surrounding material, and how they may be eliminated or mitigated.
Managing subsurface damage during the shaping process and removing subsurface damage during the polishing process is essential in the production of low damage density optical components, such as those required for use on high peak power lasers. Removal of subsurface damage, during the polishing process, requires polishing to a depth which is greater than the depth of the residual cracks present following the shaping process. To successfully manage, and ultimately remove subsurface damage, understanding the distribution and character of fractures in the subsurface region introduced during fabrication process is important. We have characterized the depth and morphology of subsurface fractures present following fixed abrasive and loose abrasive grinding processes. At shallow depths lateral cracks and an overlapping series of trailing indentation fractures were found to be present. At greater depths, subsurface damage consists of a series of trailing indentation fractures. The area density of trailing fractures changes as a function of depth, however the length and shape of individual cracks remain nearly constant for a given grinding process. We have developed and applied a model to interpret the depth and crack length distributions of subsurface surface damage in terms of key variables including abrasive size and load.
Understanding the behavior of fractures and subsurface damage in the processes used during optic fabrication plays a key role in determining the final quality of the optical surface finish. During the early stages of surface preparation, brittle grinding processes induce fractures at or near an optical surface whose range can extend from depths of a few μm to hundreds of μm depending upon the process and tooling being employed. Controlling the occurrence, structure, and propagation of these sites during subsequent grinding and polishing operations is highly desirable if one wishes to obtain high-quality surfaces that are free of such artifacts. Over the past year, our team has made significant strides in developing a diagnostic technique that combines magnetorheological finishing (MRF) and scanning optical microscopy to measure and characterize subsurface damage in optical materials. The technique takes advantage of the unique nature of MRF to polish a prescribed large-area wedge into the optical surface without propagating existing damage or introducing new damage. The polished wedge is then analyzed to quantify subsurface damage as a function of depth from the original surface. Large-area measurement using scanning optical microscopy provides for improved accuracy and reliability over methods such as the COM ball-dimple technique. Examples of the technique's use will be presented that illustrate the behavior of subsurface damage in fused silica that arises during a variety of intermediate optical fabrication process steps.
The Optical Sciences Laser (OSL) Upgrade facility, described in last year's proceedings, is a kJ-class, large aperture (100cm2) laser system that can accommodate prototype optical components for large-scale inertial confinement fusion lasers. High-energy operation of such lasers is often limited by damage to the optical components. Recent experiments on the OSL Upgrade facility using fused silica components at 4 J/cm2 (351-nm, 3-ns) have created output surface and bulk damage sites that have been correlated to phase objects in the bulk of the material. Optical Path Difference (OPD) measurements of the phase defects indicate the probability of laser-induced damage is strongly dependent on OPD.
The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized facility containing a 192-beam, 1.8-Megajoule, 500-Terawatt, ultraviolet laser system together with a 10-meter diameter target chamber with room for nearly 100 experimental diagnostics. Each beam line requires three different large-aperture optics made from single crystal potassium dihydrogen phosphate (KDP). KDP is used in the plasma electrode pockels cell (PEPC) and frequency doubling crystals, while deuterated KDP (DKDP) crystals are used for frequency tripling. Methods for reproducible growth of single crystals of KDP that meet all material requirements have been developed that enable us to meet the optics demands of the NIF. Once material properties are met, fabrication of high aspect ratio single crystal optics (42 × 42 × 1 cm) to meet laser performance specifications is the next challenge. More than 20% of the required final crystal optics have been fabricated and meet the stringent requirements of the NIF system. This manuscript summarizes the challenges and successes in the production of these large single-crystal optics.
The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized high-energy (1.8 megajoule) / high-peak power (500 terawatt) laser system, which will utilize over 3000 meter-size Nd-doped metaphosphate glasses as its gain media. The current production status, the selection criteria of individual slabs for specific beam line locations, and some recent technical advances are reviewed. The glass blanks are manufactured by a novel continuous glass melting process, and the finished slabs are then prepared by epoxy bonding a Cu-doped phosphate glass edge cladding and by advanced finishing techniques. To date, nearly 3400 slab equivalents have been melted, 2600 have been rough-cut to blanks, 1200 have been finished, and 144 have been installed in NIF. A set of selection rules, which are designed to optimize laser performance (e.g., maintain gain balance between beam lines and minimize beam walkoff) and to maximize glass lifetime with respect to Pt damage site growth, have been established for assigning individual slabs to specific beam line locations. Recent technical advances for amplifier slab production, which include: 1) minimizing surface pitting (hazing) after final finishing; 2) minimizing humidity-induced surface degradation (weathering) upon storage and use; and 3) preventing mounting-induced surface fractures upon installation, have contributed in improving the laser glass quality.
Rates of dehydroxylation of two Nd-doped metaphosphate laser glasses (LG-770 and LHG-8) are measured and modeled. Glass melts ranging in size from 100 g to 2.8 kg were bubbled with O2 containing various H2O partial pressures (PH(subscript 2O)) and with O2/Cl2 mixtures at temperatures ranging from 925 - 1300 degree(s)C. The OH content in the glass was measured by monitoring the OH absorption at 3.333 micrometers at various bubbling times. The OH removal by inert gas bubbling (e.g. O2 bubbling) is governed by the transport (diffusion) of OH to the glass liquid/vapor interface and by the chemical equilibrium between OH at the surface and H2O in the gas phase. The equilibrium OH content in glass melts bubbled with O2 containing different PH(subscript 2O) varies as PH(subscript 2O)1/2.
Laser-induced damage on the tensile side of vacuum-barrier fused silica optics can result in catastrophic fracture. This fracture can lead to two possible modes of failure: a benign failure resulting in a slow air leak into the vacuum chamber or an implosion. In previous work, we measured fracture in round vacuum windows and lenses and proposed a 'fail-safe' design that would insure the benign failure mode by fracturing into only two parts, thus eliminating the possibility of implosion. In this paper we extend the previous work to include square vacuum-barrier windows and lenses.
The NIF and LMJ laser systems require about 3380 and 4752 Nd-doped laser glass slabs, respectively. Continuous laser glass melting and forming will be used for the first time to manufacture these slabs. Two vendors have been chosen to produce the glass: Hoya Corporation and Schott Glass Technologies. The laser glass melting systems that each of these two vendors have designed, built and tested are arguably the most advanced in the world. Production of the laser glass will begin on a pilot scale in the fall of 1998.
A series of optically transparent SiO2: polydimethylsiloxane (PDMS) polyceram monoliths have been synthesized by two-step acid/base sol-gel processes. Two different processing routes are discussed and compared; one synthetic route (Route 1) utilizes lower water content, shorter reflux times, and faster drying conditions compared to the other synthetic route (Route 2). The Route 1 polycerams were all essentially non-porous at all PDMS contents examined (20 - 80 volume % PDMS). In contrast, the porosity of the Route 2 polycerams varied dramatically as a function of PDMS content. The surface area and pore volume for a 0% PDMS Route 2 polyceram were 573 m2/gm and 0.59 cm3/gm, respectively; the surface area and pore volume decreased with increasing PDMS content. The amount of porosity within the polycerams is proposed to be controlled by the relative rates of condensation and evaporation during processing and by the amount of PDMS trapped in the pores. This idea is supported by the differences in the drying behavior with processing and by the structural information obtained by magic angle spinning solid-state 29Si NMR of the polyceram monoliths. Quantitative evaluation of the 29Si NMR and porosity data are utilized to formulate structural models of these polycerams. The structural models are then specifically used to describe the effect of porosity on the photostabilization of a laser dye doped within these polyceram monoliths.
The use of solid-state dye laser for commercial applications has been limited largely by the poor photostability of the gain medium. Techniques are examined to improve the photostability of Coumarin and Pyrromethene-BF2 567 (PM- 567) laser dyes within xerogel and Polyceram hosts synthesized by sol-gel processing. The photochemical mechanisms by which laser dyes degrade are discussed and determined specifically for PM-567. PM-567 was determined to degrade both by photo-oxidation and acid degradation. Techniques for improving photostability are described from a molecular engineering perspective. These techniques include: covalently attaching the laser dye to the host; controlling the chemical environment of the dye; increasing dye caging by increasing the SiO2 content; removing porosity from the host; and incorporating additives such as hindered amine light stabilizers to minimize photodegradation.
Wet chemical processing of ceramics, glasses and inorganic-organic hybrids in the form of films has a large number of both proven and potential optical applications. The present review focuses on progress since 1990 in the areas of ferroelectric films, electrochromic and photochromic films, planar waveguides, and NLO films. Where appropriate, advances are illustrated by results obtained in our laboratories.
A silylated Coumarin 4 (derCoum) laser dye has been incorporated over a large range of concentrations in sol-gel silica composites. Optically transparent films of derCoum and Coumarin 4 (Coum) doped silica were obtained; and their absorption and fluorescence spectra and fluorescence efficiency were measured. Dye extraction was investigated as a function of sol-gel processing conditions. Dye extraction results indicated that prehydrolysis of the derCoum and full hydrolysis of TMOS resulted in films from which the dye could not be extracted, suggesting that all the dye is bonded within the sol-gel matrix. The silylated dye films showed higher fluorescence efficiency at all concentrations with respect to the normal dye film.
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