This PDF file contains the front matter associated with SPIE Proceedings Volume 13190, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The year 2024 will mark the 10th anniversary of the commissioning of the Laser Megajoule (LMJ) Inertial Confinement Fusion Laser first bundle of 8 laser beams. About 400 experiments on target were carried out in this period of time. Currently 11 bundles are in operation at half energy and power for experiments delivering up to 330kJ UV on target. The full system will be completed within few years. In this paper, we take the opportunity of the anniversary to look back from the beginning. We detail an historical review of the laser system development carried out at CEA DAM since 1962. We illustrate the different laser systems built and how they evolved as laser technology evolved with some pioneering results from frequency conversion to pulse compression. We then detail the LMJ design and architecture and focus on laser damage management in the different sub-assemblies of the system. We review the performance reached at half/energy and power with half bundles completed. We finally show the laser campaign performed since 2021 to test the energy/power increase on a few numbers of bundles, preparing the LMJ operation at full performance.

This work reports on the 1-on-1 and S-on-1 laser damage behavior of anti-reflection (AR) multilayer dielectric (MLD) coatings synthesized by biased target deposition (BTD) to include mixtures of HfO₂ and SiO₂ and HfO₂ and Al₂O₃ as the high index layer in the 2-layer coating structure. For comparison, HfO₂/SiO₂ AR and HR coatings were also synthesized using ion beam sputtering (IBS) and ion beam assisted evaporation (EBE). The results show that in the BTD ARs the scaling of the 1-on-1 LIDT with the UV band-edge is not significant, unless the content of HfO₂ is less than approximately 20%. The Hf_0.2Si_0.8O_x AR coating 1-on-1 LIDT, 6.1 J/cm², is similar to that measured in AR containing Al₂O₃ as high index layer, 6.9 J/cm². The S-on-1 LIDT of selected ARs shows a decrease of ~10% for S=10 and remains at the same level for up to S=10⁴. This fatigue behavior is also observed in the reference EBE HfO2/SiO2 AR sample. Instead, the IBS reference HfO₂/SiO₂ HR coatings show the S-on-1 LIDT reduces with the increase in pulse number S. These results highlight the dominance of the materials’ properties and the substrate quality on affecting the laser damage behavior of AR coatings for λ=355 nm.

Thin-film layer systems coated by various techniques on the optical component surface are the most common method to finishing laser optics. Anti-reflective thin-film coatings are essential in laser optics to limit unwanted retro-reflections and decrease the reflection-induced losses occurring on boundaries of optical materials and air. Several different technologies are available to prepare laser-quality coatings, when the most common are magnetron sputtering and electron-beam ion-assisted deposition. However, coating materials and deposition parameters may significantly affect both laser resistance and optical quality of the coatings, and the influence of mentioned factors is getting stronger with shorter wavelengths. In following will be disseminated laser damage threshold of anti-reflective coatings prepared by e-beam evaporation with ion assisted deposition and plasma activated reactive magnetron sputtering at wavelength 343 nm in ultra-short pulses regime.

This study discusses absorption in thin film optical filters and its impact on spectral shift and wavefront deformation under high power laser exposure. An experimental set-up has been developed for measuring both absorption and wavefront deformation of optical thin film components. Additionally, a finite element model has been developed to predict optical function spectral shifts and wavefront deformations induced by high power laser exposure. Comparison between theoretical predictions and measured data is used to validate the model and provide valuable insights into the consequences of laser-induced heating on the optical performances of coatings.

Quantized nanolaminates (QNL) are a material system that was developed, produced and characterized by the LZH in 2016 as an alternative optical material. The idea behind it is that, like a normal mixed material, QNLs have a refractive index that is determined by the ratio of the two materials used. However, the electron mobility is severely restricted by the very thin high refractive index material. This results in a higher band gap and a lower absorption edge of the system. Their properties have been demonstrated on ALD and IBS systems. But the complex and slow coating processes meant that only a few iterations could be produced. We have now developed a process on a magnetron sputtering system with a rotating substrate table that makes it possible to produce QNL layers of SiO₂ and Ta₂O₅ at a very high rate of up to 0.8nm/s. This makes it possible to use these nanolaminates economically as a stand-alone material, even in thick and high layer count designs. Because of the process we were able to produce a variety of QNL with different layer thickness and ratio combinations and perform a variety of measurements such as atomic force microscopy (AFM), total scattering (TIS), transmission electron microscopy (TEM) and Laser induced damage threshold (fs-LIDT) to determine their properties. We were able to use the knowledge gained to coat more complex multilayer systems in a range that would otherwise not have been possible with normal Ta₂O₅-SiO₂ coating systems.

High reflector coatings are typically constructed of quarter-wave optical thickness (QWOT) multi-layers of alternating high and low refractive index materials. An absentee half-wave optical thickness (HWOT) low refractive index layer, usually SiO₂, is typically used for the outermost layer of the coating to improve laser damage resistance. Out of plane oblique angle mirrors typically have an additional polarization retardation minimization requirement to maintain the polarization purity of the laser. For monochromatic lasers with a small angular incident range, this can be accomplished with precise layer thickness control for proper angular centration of the coating because of the narrow angular range for low polarization retardation. Reducing the SiO₂ overcoat thickness from HWOT to QWOT significantly increases the spectral bandwidth or angular range for low polarization retardation, however, the standingwave electric field is significantly increased, leading to lower laser resistance. The reflectivity is also reduced leading to the need for extra layers. A three-material hybrid QWOT coating design, with a high fluence medium refractive index material, can be fabricated with simultaneous high laser damage resistance and low phase retardation over a moderately wide spectral or angular range. Finite -difference time-domain simulations explore the impact of different size nodular defects on light intensification in each of the different coating materials for this hybrid design approach. A 2-5x increase in LIDT occurred, depending on polarization, for the two wide low retardation coating, with the best LIDT performance occurring for the narrow low retardation coating.

193nm Argon Fluoride (ArF) Excimer lasers used for photolithography rely on high reflectivity coated mirrors for beam relay, focusing and pulse stretching. These mirrors must provide high reflectivity at moderate peak fluences (~30 mJ/cm²) for >10¹¹ pulses (typically, >1 year in a normal use case) to provide the performance and reliability that semiconductor manufacturers have come to expect. Generally, high intensity Laser Induced Damage Threshold (LIDT) testing has proven unreliable in evaluating coatings with this kind of long-term durability requirement: typical use-case lifetimes and failure modes do not match LIDT predictions. By carefully controlling fluence and peak intensity, we have succeeded in duplicating use-case failure modes with Accelerated Lifetime Damage Testing (ALDT) in tests lasting ~1-2 weeks. This testing has revealed a latent damage mechanism wherein coatings become vulnerable to thermal and environmental damage only after they have been exposed to a significant accumulation of 193nm irradiation. With this learning, two test protocols have been developed to assess coating robustness. First, a thermal cycling test in which a CO₂ laser is used to periodically heat a coated mirror. Results from this test demonstrate no damage for samples that have not been exposed to DUV, but significant delamination (blistering) on samples pre-exposed to DUV, matching field failure observations. Second, samples that are known to be robust to extended atmospheric exposure are exposed to DUV in a purged environment with no apparent damage for several billion pulses. Subsequently, these samples will develop blisters upon exposure to atmosphere.

Over the past two decades, laser technology has made significant strides in scaling peak and average power levels. These advancements span a wide range of laser irradiation techniques, from ultrashort pulses to continuous waves, driving the development of various optical elements and coatings. However, the variability in optics sizes, ranging from optical fibers to meter-sized optics, and the presence of various failure modes pose challenges for laser damage testing. Consequently, there is a pressing need to align relevant laser damage testing standards to ensure the functional performance of optics. In this overview, we discuss recent standards-revision efforts aimed at revising the ISO 21254 family standards. Our primary focus is on improving accuracy and reliability by improving damage criteria, testing procedures, and results analysis methods. These efforts aim to tackle emerging challenges in laser damage testing while ensuring that standards remain compatible with modern technological developments.

Photoluminescence (PL) mapping was utilized to investigate damage in β-Ga₂O₃ epilayers induced by 1064 nm laser pulses. The intensity and position of the intrinsic UV band were determined and plotted as a false-color image. Two types of damage were identified: circular damage and damage cracks. Circular damage shows lower UV PL intensity than the surrounding material with color centers in a “halo” around the damaged region. Damage cracks are aligned with the a and c axes and show higher PL intensity than undamaged material. Defects in the as-grown material were revealed by shifts in the UV band energy.

We present a novel damage detection method for use in high-repetition rate, high pulse energy laser systems. Synthetic dark field imaging involves numerical beam propagation and high-pass spatial filtering of the laser beam profile to simultaneously detect damage and identify the optic responsible within the optical chain. This technique utilises the main beam path and has the capability to monitor damage in situ, across numerous optics with a single image of the beam profile. Synthetic dark field imaging will be trialled in the new 10 Hz repetition rate, 1 PW laser driver currently being commissioned at the Extreme Photonics Applications Centre (EPAC) at the Rutherford Appleton Laboratory, UK.

This year’s laser damage competition involves short pulse laser damage on high reflectors tuned for near normal incidence and >99.5% reflectance for 1030 nm. All laser damage testing was done by Lidaris Ltd. via a near Gaussian laser beam from a commercial laser system (Yb:KGW, Kerr lens mode-lock) operating at 500 kHz repetition rate with 200-fs pulse duration (FWHM). All testing was done in a similar fashion to the ISO 21254-1 and 21254-2 S-on-1 standards, yielding data on laser damage for 10x number of shots, where x = 0, 1, 2, 3, 4, 5, 6. Laser-matter interaction either leads to material removal and ablation or a more subtle coating admittance change. The former is referred to as catastrophic damage while the latter is referred to as color change. The choice of coating materials, design, and deposition method were left to the participants. A double-blind test assured sample and submitter anonymity. The damage performance results (LIDT), sample rankings, details of the deposition processes, coating materials and substrate cleaning methods are shared. These results are compared both to the nanosecond 1053-nm laser damage testing on high reflectors from the 2018 competition as well as the nanosecond-femtosecond damage testing study from the 2020-2021 years. All samples exhibited a fatiguing effect in the laser damage performance at high number of shots, but this was particularly noticeable for the color change damage type. We found that ion beam sputtered HfO₂/SiO₂ multilayer coatings of approximately 30 total layers did the best for the short pulse regime. This is in sharp contrast to the 1053-nm nanosecond study, which has demonstrated that electron beam deposited HfO₂/SiO₂ high reflectors are the clear winners.

Laser based nuclear fusion requires enormous laser powers which are taxing on optical elements in the beam path and often lead to laser damage. A key factor in reducing risk of laser damage is minimizing bubbles, especially small bubbles and other inhomogeneities in the optical material. Over the decades, Heraeus Conamic has fine-tuned its fused silica manufacturing processes to improve material quality with respect to bubble size and quantity. In synthetic fused silica bubble quality can differ significantly depending on the respective process. Heraeus Conamic’s patented homogenization process results a in a reduced bubble content compared to the produced raw material. Heraeus Conamic’s homogenization process manages these microbubbles. These incremental but important recent fused silica material quality improvement enable higher laser power and more frequent shot of the laser systems in inertial fusion research and upcoming commercial systems.

Cones are machined into the surfaces of final fused silica NIF optics to remove laser induced damage. Applied to the input surface they are also to prevent the growth of exit surface damage by generating a shadow over the damage. As a result, cones with different sizes, depths, and shapes are being deployed in greater numbers. The expanding waves from input surface cones produce an intensification pattern at the exit surface from interference between the expanding annular wave from the cone walls and the incident beam. The probability of damage from this intensification will increase if two or more expanding waves overlap. The likelihood of overlap will increase with increasing use of these cones. It is impractical to survey all possible overlap situations through costly damage tests. We will explore alternative diagnostic and analytical tools to predict the probability of damage from the expanding waves of input cones with different degrees of overlap and at the higher NIF energies anticipated in the future.

The National Ignition Facility (NIF), the world’s most energetic laser system and the first to demonstrate fusion ignition, routinely operates at energies that can damage its final optics. To enable sustained operation, NIF recycles optics by ablating fractured material associated with damage, leaving behind a benign, cone-shaped void. These mitigation cones range in depth and diameter and typically are applied using the smallest effective cone for one damage site. However, when multiple damage sites are closely situated, various combinations of larger and smaller cones can be used to repair the region, and the number of options grows exponentially. Standard brute force approaches that iterate through each of these possibilities are thus computationally impractical beyond a relatively low threshold. To overcome these limitations, we introduce Combinatorial Optimization for OPtic Repair (COOPR), a novel combinatorial optimization framework to solve the problem of cone placement given any configuration of damage sites. Using tools from the seemingly unrelated literature on facility location problems in urban planning, we formulate and solve mixed-integer linear programs that identify optimal cone configurations for damage mitigation with respect to a multi-objective cost function. We show that even for optics with hundreds of clustered damage sites, COOPR finds more effective cone placements faster than existing approaches, thus enabling a more efficient optic mitigation cycle with a reduced need for human intervention.

Precursors generated by fused silica during high-energy laser exposure are known to increase damage initiations on nearby fused silica surfaces, on which they are thought to adsorb. These precursors are of concern to increasing the laser energy in the final optics of the Lawrence Livermore National Laboratory (LLNL) National Ignition Facility (NIF). In this work, precursors were generated by exposing a fused silica sample with a single 3ω, 5 ns, 35 mm-diameter laser shot at approximately 12 J/cm² average fluence in the presence of identical, witness samples not exposed to laser light in a precision cleaned test chamber. Five tests were conducted, one for each of five pressures: 760, 350, 10, 2.5, and 10^-5 torr, using the LLNL Optical Sciences Laser Laboratory. The witness samples were then damage tested with a single laser shot: 3ω, 5 ns, 10 mm-diameter, and 26 J/cm² average fluence to evaluate the effect of the different environmental pressures. The results of this experiment show that laser exposures in ambient pressure above 350 torr resulted in less observed laser-induced damage presumably due to suppression of precursor generation or inhibition of precursor transport. A detailed analysis of the fluence, damage and correlation to environmental pressure will be shown in this work. These results inform potential damage-reduction solutions for NIF final optics for future operation at higher power and energy and may be relevant to other high energy UV fused silica-based laser optical systems.

Our research focuses on designing metallic coatings to create broadband all-reflective phase retarders that generate circularly polarized (CP) light for the MTW-OPAL Laser System while ensuring the desired polarization state on the target. This all-reflective phase retarder can function as a phase retarder when used in an out-of-plane configuration, whereas it acts as a normal mirror set in an in-plane configuration. If the polarization of the beam is not purely s- or p-polarized, however, mirrors will in general introduce retardance, and therefore compensators or polarization-independent mirror pairs are needed to ensure the desired polarization at the target plane.

High-power laser systems use hundreds to thousands of large optical components to amplify, filter, transport, sometimes compress and/or frequency-convert, and focus laser beams. Most of these optics are dioptric optical components: mirrors, lenses, windows, laser slabs, crystals, ... Apart from the iconic example of the compression gratings used in the chirped pulse amplification, the use of diffractive optics and in particular transmission gratings is relatively limited. Here we detail the development we carried out to use transmission grating for beam steering and focusing laser beams of the Megajoule laser (LMJ). We describe our early attempts, the first prototype, and the performances finally reached to equip the 176 laser beams of the LMJ. We follow this path by extending the implementation of transmission gratings for beam steering and focusing to the manipulation of the polarization state of highly energetic laser beams. We detail the design and performance of nanostructured silica for achieving linear-to-circular polarization conversion. This full-silica meta-optics acts as a quarter waveplate operating in the UV frequency range at the wavelength of 351 nm. In addition to its effect on polarization, we show how this meta-optics can be used to push back the Kerr filamentation threshold occurring in components of these high-power lasers.

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.

The contamination of the mirrors used in vacuum at different laser facilities is one of the factors that lead to damage to the mirrors. To mitigate this risk, we propose a non-invasive method based on broadband source interferometry to investigate the contamination quantitatively. By monitoring the phase, group delay, and group delay dispersion of the mirrors, we were able to qualify the contamination as a function of coating design, deposition method, and contamination time in the vacuum chamber.

We report the current observations of the carbonization problem in ELI-NP laser beamlines. All the beamlines of 0.1 to 10 PW had the carbonization of the beam transport mirrors at fs pulse in vacuum. We tested 3 different cleaning methods to remove this carbonization, namely the dip cleaning in the water-alcohol mixture, oxygen plasma cleaner, and pump down with silica gel. The dip cleaning was applied to 1 PW mirror to remove the contamination before the carbonization was observed. For the first time for large mirrors. The oxygen plasma can remove surface carbon, but it is not enough to remove the carbon inside the coating. Silica gel provides some slow pumping of the vacuum and some increase of low-mass gasses due to the increased surfaces. The method to identify the pass-way of the contamination into the vacuum chamber was proposed.

The production of clean energy is nowadays a very important and critical topic and Laser Fusion is one of the possibilities to achieve this goal. In order to improve the efficiency of this technology, one of the problematics is the increase in laser energy in the optical systems, and therefore the need for optical coatings able to sustain higher fluence. In this study, the contribution of the substrate surface quality on the optical performance and Laser Induced Damage Threshold (LIDT) will be investigated. Mirror coatings for a wavelength of 532nm will be coated using an Ion Beam Sputtering System on different types of glass substrates and the optical properties like scattering, absorption and Laser Damage will be presented and discussed.

In this work we showcase the formulation of an anti-reflective (AR), UV/ozone-curable polysilsesquioxane or glass resin (GR) system with a high laser damage threshold suitable for Third-Harmonic Generator (THG) optics. Potassium Dideuterium Phosphate (DKDP) single crystals are used for these non-linear optics at the National Ignition Facility (NIF). However, DKDP is thermally sensitive making it crucial to avoid elevated thermal conditions, therefore UV/ozone-curing glass resins are the optimal choice. The commercial GR, utilized in this study was specifically engineered to undergo complete curing during UV processing, while also offering the flexibility of a low thermal cure at temperatures below 100 °C. UV/ozone exposure resulted in a significant loss of methyl silane peaks and a thermally cured network formed a total reduction of R-OH & Si-OH peaks. Laser damage testing confirmed equivalent damage-resistance for each network based on their curing condition. The newly formulated GR is a highly shelf-stable solution that allows peak optical performance on large-high-power laser systems where unique optics and operating conditions exist.

Quantized nanolaminates (QNL) are a new type of metamaterials proposed only recently. The basic properties of QNL single layers have been investigated for various material combinations and deposition techniques. Based on these results the hypothesis was put forward that, thanks to the blueshift of the absorption edge, multilayer interference filters composed of QNL-SiO₂ will lead to an increased laser damage threshold in the femtosecond regime compared to standard coatings of the same material combination. In our work we will show a comparison of mirrors with and without QNL designed for the wavelength of 1030nm. For these coatings both standard Ta₂O₅ and SiO₂-Ta₂O₅ QNL were used as high and SiO₂ as low refractive index material. Mirrors consisting of Ta2₂O₅ and SiO₂ without QNL were also deposited for reference. The designs used were either quarter-wave designs or designs aiming at reducing the electric field. A magnetron sputter system with a rotating table was used for depositing the multilayer designs. The design of the tool allows to deposit a Ta₂O₅/SiO₂ layer pair at every rotation of the table, which results in a QNL deposition rate higher than the rate for the individual materials. In order to accurately terminate the layers at the design thicknesses, broadband optical monitoring was used. Subsequently, the coatings were investigated by spectrophotometry and femtosecond laser induced damage threshold (LIDT) measurements at 1030nm. These measurements showed that samples with QNL exhibit an improved damage threshold compared to standard high-low mirrors as well as to a commercial ion beam coated fs-mirror. Furthermore, it is shown that the designs with optimized electric field exhibit higher LIDT values than their standard λ/4 design counterparts.

We have developed a new accelerated laser-induced damage testing (ALDT) protocol to evaluate the sensitivity of deep-ultraviolet (DUV) excimer laser optics to trace contaminants added to the purge gas environment. This testing reveals a strong sensitivity of optics damage mode and lifetime to added contaminants with ppm to sub-ppm level concentrations.

To eliminate parasitic light oscillations (amplified spontaneous emission (ASE)), high energy laser systems often utilize polymeric materials to adhere optically absorptive cladding materials to laser gain media (glass or crystal). Most of these polymeric optical adhesives are index matched to lower index gain media, with limitations on achieving refractive indices (RI) greater than 1.60 with low color, while also maintaining mechanical compliance to serve as adhesives for cladding applications. Through formulation of novel polymer crosslinkable networks, we have developed a suitable adhesive candidate with refractive indices above 1.64 at 532 nm while also exhibiting Shore A hardness between 50-90, with high optical clarity, low color, and minimal haze for films ~1-2 mm thick (84 %Transmission at 532 nm). This enables mapping a process space to allow future customizable adhesive formulations for building a variety of laser systems with tunable refractive index matching for various gain and cladding media. We will describe the development and properties of these materials.

The study investigates Laser-Induced Contamination (LIC) effects on beam quality, focusing on sub-picosecond UV lasers, crucial in industrial applications. By analyzing LIC at 343 nm, 480 fs and 30 W, the research aims to understand its impact on beam parameters like M² factor. A simulation and experimental setup are employed to study LIC dynamics and its consequences on beam propagation. Through theoretical simulations and practical observations, the study seeks provide insights into LIC’s influence on laser beam quality.

The TF7 Task Force of the US Optics and Electro-Optics Standards Council is responsible for laser damage standards and has proposed and is developing a Type 1 standard – that is, a “Go/No-Go” test to determine whether an optic, when exposed to specified laser irradiation, will likely have more or fewer damage sites according to requirements based on what constitutes system failure as determined by the user. The mathematical framework for this proposed standard assumed top-hat laser beams that probe a portion of the total area of an optic. This paper reports on some practical aspects and measurement uncertainties of laser damage test protocols and looks at pros and cons of using Gaussian beams in the proposed “Go/No-Go” tests.

The laser-induced damage threshold (LIDT) is a critical parameter affecting the performance of high-power laser systems. In LIDT testing, laser type, operating conditions, and measurement parameters frequently require adjustment according to specific testing needs, making the development of a versatile system with rapid parameter-switching capabilities essential. Building upon previous automated LIDT measurement systems, we have developed a desktop, integrated, and convenient LIDT measurement instrument based on international standards. This desktop design enables flexible switching between nanosecond pulsed and continuous-wave lasers, allowing for standardized, fully automated testing while accommodating multiple measurement protocols. This system is capable of detecting minute defects in tested components and assessing the stability of damage growth points. Additionally, integrated control software developed in LabVIEW facilitates highly accurate, synchronized acquisition and storage of multiple beam parameters in real time, correlating parameters of each laser pulse, damage images, and data processing. The system also provide s a range of integrated functionalities, including sample angle adjustment, front and rear surface identification, damage site reinspection, and automated report generation.