This PDF file contains the front matter associated with SPIE Proceedings Volume 12726, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The first demonstration of fusion in the laboratory required the National Ignition Facility (NIF) to leverage its novel approach to laser-induced damage; managing laser-induced damage rather than avoiding it. Accepting regular damage to the laser’s final optics allows the NIF to deliver about 300% higher energy on its targets. This mode of operation is made viable by a unique optics maintenance strategy and a variety of damage management and mitigation. We will review the prominent damage mitigation and management technologies which are classified in three categories: Damage Initiation Prevention, Damage Management, and Damage Repair. Examples of these technologies are the Fused Silica Debris Shield, AI driven damage detection, and the CO2 mitigation cone, respectively. We introduce a simple empirical model which both quantifies individual and combined impacts of the technologies on NIF’s optics usage.

Lawrence Livermore National Security’s (LLNS) National Ignition Facility (NIF) requires over 600 large crystals of KH₂PO₄ (KDP) and KH_0.6D_1.4PO₄ (DKDP) for use in polarization rotation and frequency conversion assemblies. There are near term plans to increase the energy on NIF to 2.2 MJ and up to 3 MJ in future years. Managing optics’ damage is one critical aspect of achieving these aggressive goals. Frequency conversion crystals are being examined for readiness to meet the goals for higher energy operations. All aspects of the crystal fabrication process are being studied including growth, processing, laser conditioning, and mitigation to strengthen the crystals against damage. THG crystals have an approximately 10× lower exchange rate than fused silica optics, so unlike NIF fused silica optics which are micromachined to mitigate surface laser damage between recycle loops, THG crystals are refinished to remove surface laser damage. Given the long time to grow NIF size DKDP crystals (3 years) and the impact of THG thickness on frequency conversion, a good understanding of the evolution of the inventory is necessary. The number of damage sites that require laser blocking dictates the exchange rate of THG crystals. Understanding the difference between bulk and surface damage, is important to limit the number of blocked sites. Also, in preparation for higher power operations on NIF, the current KDP polarization rotators on half of the NIF beams are being exchanged with DKDP due to an ~5% reduction in absorption at 1053 nm thus reducing beam contrast in the NIF amplifiers.

The power available in many industrial high-power 1 µm laser-cutting systems now exceeds 10 kW. To take advantage of that power, systems must be fast and nimble in order to accurately trace toolpaths at high speed. These power levels drive focusing head designs to be compact and lightweight. The best way to achieve this, in general, is with an all-transmissive design consisting of lenses and windows. The industry-standard material for lenses is fused silica, which has extremely low absorption. However, should the lens become contaminated, the poor thermal conductivity of fused silica traps much of the absorbed power inside the optic, raising its temperature and index of refraction, which causes the focus of the system to shift back toward the focusing head. At some point, the amount of focus shift becomes unsuitable for material processing and the focusing head must be replaced or refurbished with clean optics. Sapphire is an alternative lens material that until recently was only used in windows due to optical fabrication challenges in the polishing of aspheric surfaces. At Coherent Corp., we have overcome many of those challenges and currently offer sapphire aspheres. The key advantage of sapphire over fused silica is its relatively high thermal conductivity. This enables any absorbed power to quickly reach the edge of a sapphire optic where water-cooled mounts can remove the heat and keep the lens temperature low. Accounting for all the property differences between the two materials, sapphire should result in less focus shift than a comparable fused silica optic with the same absorbed power. The purpose of this study is to measure and compare the focus shift properties of the two materials in a controlled setting at high and low levels of absorbed power. Sapphire was also tested for laser-induced-damage threshold (LIDT) by Spica Technologies Inc.

In many laser applications, there is a higher and higher demand for more efficient coatings with reduced losses, in terms of absorption and scattering as those are contributing factors to diverse laser damage regimes. Ion Beam Sputtering (IBS) is a known technique to provide such high optical quality thin films. Indeed, it allows to achieve high density layers with low absorption and scattering. In this work, various coatings were developed using Bühler IBS technology. Then, total losses were measured using Cavity Ring Down, absorption using Laser Induced Deflection or Laser thermography, and Total Integrated Scatter using dedicated scatterometers. A correlation between the effect of the chosen deposition method and parameters and the measurement performances were made with the aim of a better understanding of the level and the origin of losses in the coatings. Finally, highly reflecting mirror coatings for 1064 nm wavelength were fabricated with different designs and deposition parameters. The results of the different measurements of absorption, scattering and total losses using different equipment are presented and discussed.

This year’s competition proposed to survey the state-of-the-art broadband, near-IR multilayer dielectric (MLD) mirrors designed for ultra-short, pulsed laser applications. The requirements for the coatings were a minimum reflection of 99.5% at 45-degree incidence angle for S-polarization from 830 nm to 1010 nm and group delay dispersion (GDD) < ± 50 fs². The participants in this effort selected the coating materials, coating design, and deposition method. Samples were damage tested at a single testing facility to enable direct comparison among the participants using a 25 ± 5 fs OPCPA laser system operating at 5 Hz. A double blind test assured sample and submitter anonymity. The damage performance results, sample rankings, details of the deposition processes, coating materials and substrate cleaning methods are shared here. We found that multilayer coatings using tantala and/or hafnia as high index materials were top performers within several coating deposition groups. Specifically, dense coatings by ion-beam sputtering (IBS), magnetron sputtering (MS), and electron-beam ion assisted deposition (e-beam IAD) exhibited highest damage initiation onset (LIDT) while e-beam coatings were low performers. In addition, damage growth onset (LDGT) was also examined and the results are reported here for all samples as this performance metric plays an important role in establishing the safe operational conditions for larger aperture, ultrashort pulsed lasers. Lastly, not all coating samples in the survey met the GDD requirements stated above and associated measurements are discussed in the context of the present and past competitions focused on similar broadband, near-IR MLD coatings.

Multilayer dielectric (MLD) gratings provide high diffraction efficiency and a high damage threshold. They represent the main solution to compressing a high-power laser beam. However, the laser resistance of MLD gratings limits the power of such facilities. The community devoted a lot of resources to increasing the damage threshold of those components. Today, it is well known that the etching profile plays a key role in the electric field distribution and consequently the laser resistance. In this paper, we focused our optimization on the multilayer dielectric stack to increase the laser-induced damage threshold (LIDT). We numerically and experimentally demonstrated the impact of the MLD stack on the electric field distribution and the LIDT. We manufactured two sets of three samples with identical etching profiles. The calculated electric field intensities were in good agreement with the measured LIDTs. These results demonstrated how to further optimize grating designs through the dielectric stack.

Laser-induced damage threshold (LIDT) tests were performed at 1064 nm and 20 ns. Nodule defects were identified as the LIDT-limiting factor. The results suggest that the scale of the nodules is associated with the size of defects residing on the aluminum substrate surface. 3D finite-difference time-domain (FDTD) simulation was employed to calculate the electric field intensity (EFI) enhancement at the nodular defects with a seed diameter ranging from 0.35 μm to 2.5 μm. A direct linkage between the EFI enhancement and laser-induced damage morphology was established. Additional LIDT tests were conducted on surface modified aluminum substrate by using Corning aluminum process (CAP). The surface modification led to a 10x increase of the LIDT. Finally, LIDT of the multiband mirrors was predicted based on the absorption-driven damage and defect-driven damage. The results suggested that a combination of the CAP-modified Al6061 and low defect deposition process of the dielectric enhanced layers lead to high laser durability.

We experimentally study single-shot laser ablation of GaSb, GaAs, GaP and GaN, for laser pulse durations ranging from 200 fs to 20 ps. We find that the laser ablation threshold fluence of GaSb is almost independent of pulse duration, whereas the ablation threshold for GaN depends strongly on pulse duration. More generally we find that the larger the bandgap, the stronger the dependence of pulse duration. This is expected, as intrinsic laser absorption is mainly linear when the bandgap is small compared to the photon energy, whereas a larger bandgap requires strong field ionization. Thus a larger bandgap leads to a stronger influence of the peak intensity of the pulse and therefore a stronger dependence on the pulse duration, when compared to smaller bandgaps.

The Matter in Extreme Conditions Upgrade (MEC-U) project is a major upgrade to the MEC instrument of the LINAC Coherent Light Source (LCLS) X-ray free electron laser (XFEL) user facility at SLAC National Accelerator Laboratory. The envisioned MEC upgrade will significantly enhance the capabilities of the pump laser sources in current MEC experimental station, boosting the energy of the nanosecond shock driver from 100 J to the kJ level, and increasing the power and repetition rate of the short pulse laser from 25 TW at 5 Hz to 1 PW at 10 Hz rate. Building such high energy/power pump laser systems presents challenges to minimize and mitigate against laser-induced optical damage. As part of the system design, we have identified the optics at high-risk to damage and we have designed the laser systems to mitigate against these damage risks to ensure sustained facility operation.

For most of the laser applications is optics equipped with antireflective coatings must. Therefore, laser damage resistance and stability at high energies of used components is a key performance limiting factor at the large portion of the user cases. In UV region, issue of laser damage is particularly enhanced as many optical materials tends to degrade at longer exposure and any contamination may accelerate that. In the following paper will be disseminated laser damage performance of selected commercially available optical windows equipped with AR coatings, designed for high-power lasers in UV region. Damage threshold measured with mm-size laser beam will be compared and influence of the long exposure to ultrashort pulses will be considered.

Cones machined into the surface of the final fused silica optics on the NIF have been used to remove laser induced damage from exposure to high fluence 351 nm laser light. When applied to the input surface of an optic, a shadow is created on the exit surface due to the divergence of the laser light by the cone walls. In recent years input surface cones have been utilized to shadow exit surface damage and thus arrest its continued growth. The expanding waves from the cone walls interfere with the incident beam to create a high fluence intensification at the exit surface. This intensification has the characteristic periodic spatial variation on a scale of the order of the 351 nm wavelength. The question arises as to how the damage density probability, ρ(Φ), is affected by this variation as compared to a uniform fluence. Does it follow the local periodic variation, or is it averaged over that variation. We consider both cases, how it can be predicted by direct measurement of the intensification as opposed to costly damage tests, and how we might measure the effect directly.

In general, there are just a few numbers of available dielectric materials suitable to produce laser optics. Such binary dielectric films are limited by fixed optical properties e.g. index of refraction and intrinsic laser induced damage threshold (LIDT). Optical properties of dielectric layers need to be manipulated precisely for further improvement. One approach is given by the deposition of ternary composites. The other approach is well known for crystalline materials and takes benefit of quantized effects. In the last years it could be shown that such effects can be utilized in amorphous dielectric layers as well. As a major benefit quantized nanolaminates enable the possibility to keep the index of refraction high with improved optical band gap. This proceeding gives an overview about key experiments of manufactured dielectric quantized nanolaminate samples and its advantages compared to ternary composites.

Quantizing nanolaminates are an interesting alternative to classical coating materials with greater independence of refractive index and the optical bandgap energy. This leads to more flexibility and considerable potential to increase the laser-induced damage threshold in the ultra-short pulse regime. The following study presents and compares the design choices, characterization, and LIDT testing of different quantizing nanolaminates for the ultraviolet spectral range to classical coating materials.

Estimation of error bars on laser damage threshold measurements using only the fluence inaccuracy of the test laser unfortunately does not cover the full range of systematic errors in the laser damage protocol. A poor understanding of the actual measurement errors can lead to inaccurate conclusions or inadequate safety factors when designing laser systems. This Monte Carlo analysis focuses on improving the understanding of typical systematic errors of the ISO and raster scanning laser damage test protocols through modeling of the impact of fluence variations, laser beam pointing, and laser beam shape. The impact of increased test area and the magnitude of fluence increments between testing sites is also explored in an attempt to reduce systematic errors. Raster scanning tends to have significantly lower systematic errors than the ISO test. However, the raster scan protocol cannot measure unconditioned laser damage thresholds and has microscopy resolution limitations for pre and post irradiation inspection.

Laser-induced damage (LID) tests were conducted on CaF₂ optics at 193 nm using ISO S-on-1 method with the S varying from a standard 200 to 10³, 10⁴, and 10⁵ shots/site and fluences ranging from 0.1 J/cm² to 4.0 J/cm². Using a flat-top beam profile and a beam footprint of 250 μm × 250 μm, absorption-derived LID was observed on the standard 200-on-1 test. Defect-initiated LID was detected by increasing the pulse count with a reduced fluence. The absorption-driven LID was attribute to subsurface damage and two-photon absorption. The former was eliminated by using a FemtoFinish polishing process. The latter was experimentally determined by using laser calorimetric measurement. Improved crystal bulk and surface finishing quality were confirmed by X-ray diffraction and laser calorimetric measurement. Accelerated lifetime damage test (ALDT) was further conducted with an increased pulse count up to 10⁶ shots/site. The results confirm an enhanced lifespan prediction of the demanding laser optics.

With numerous manufacturers providing different laser-induced damage threshold (LIDT) values in the nanosecond regime, a simple ranking based on numbers alone may not provide a clear picture of the best choice. Variations in testing procedures, albeit following the ISO 21254 standard, further complicate the selection process. By employing a comprehensive 1-on-1 test procedure, it becomes possible to observe various parameters that influence LIDT values. When sharing test results within the community, adhering to good practices and meticulous attention to the error budget and its contributors are crucial. Above all, laser optics users must comprehend the intricacies of laser-induced damage testing and seek detailed information instead of relying solely on numerical comparisons. This study explores the challenges and considerations in selecting and testing laser optics, emphasizing the importance of a comprehensive approach.

Surface damage of silica optics routinely limits the operation of high energy laser systems. Initiated damage can grow upon additional laser pulses, eventually requiring optic removal/replacement. However, when damage is first initiated, small damage sites grow in a stochastic manner, readily parameterized by the size of the damage site, surface of residence, and the fluence and pulse duration of subsequent laser exposures. The National Ignition Facility (NIF), which exposes ~100 m2 of fused silica optics surface to high-energy-nanosecond-laser light on every full system shot, provides an ideal platform to study the growth behavior of laser-induced damage. High-resolution microscopy of individual damage sites is captured as part of the standard NIF recycling loop. However, not all damage sites are repaired depending on the age and quality of the host optic leaving many thousands sub-50-micron damages sites to resume growth after being imaged. By measuring such sites each time an optic is removed for recycling, high-resolution microscopy becomes available for many thousands of sites before and after exposure to various shot sequences on the NIF laser. Using this observed growth, a multi-shot description was fit to predict the likelihood of exit surface damage site growth under exposure from 3ω, nanosecond regime pulses for shot sequence lengths between 1-100 laser exposures. This provides a basis for accurately predicting when a recycled optic will require additional repair.

Nonlinear absorption is mainly governed by mechanisms involving excitation processes of electrons. Typically, two phenomena are considered when discussing nonlinear absorption; the multiphoton absorption where multiple photons interact directly with a single electron, and tunnel ionization, where the high electric field results in a shifting of the bandgap allowing an electron to tunnel into the conduction band. Electrons in the conduction band can be accelerated through the absorption of further photons until they obtain enough energy to excite further electrons to the conduction band, leading to runaway absorption and finally damage of the sample. By laser calorimetric measurement of the nonlinear absorption, it is expected that the laser damage threshold can be predicted without damaging the optic. Before accurate predictions can be made, the process must be thoroughly characterized and understood. The nonlinear behavior of the absorption was demonstrated with potential increases in absorption of an order of magnitude. Initial results show a noticeable impact of contaminants, though a nonlinear response is still observed.

Metasurfaces exhibit great potential to redefine limitations inhibiting high power laser optics. Some areas of expected improvement include throughput improvement with enhanced design flexibility, mitigation of filamentation damage by enabling thinner optics, and reduction in system complexity and price. Metasurface utilize engineered surface ‘layer’ with thickness on the order of the design wavelength, which consists of an array of sub-wavelength elements. Our methodology is based on scalable generation of sacrificial metal nanoparticle mask followed by directional etching to pattern the glass. The end-result all-glass metasurface has high laser damage durability, mechanical robustness, design flexibility and controllability of the metasurface features, and the ability to craft antireflective layers and basic optical elements. Recent advancements have been made resulting in ultra-broadband antireflective layers, induced birefringence in the glass for waveplates, and refined optical elements.

Free space communications between ground stations and geostationary satellites offer high-speed and secure data transmission, but compensating for atmospheric absorption poses a significant challenge. The need for high levels of beam power to overcome atmospheric losses calls for optics that can withstand strong flux, especially in the continuous wave (CW) regime. In this context, the absorption of optics at 1.5 µm is a critical parameter that must be accurately measured and understood to develop efficient and reliable photonic systems. This study focuses on the absorption of optical coatings at 1570 nm. Lock-In Thermography (LIT) has been developed to measure the total absorption of the coatings with high sensitivity under 1 ppm. A modulated 100 W CW laser is used to induce heating into the coating stack, and the resulting rise in internal temperature is measured with a thermal camera. The LIT experimental setup offers a non-destructive and non-contact measurement technique, making it ideal for assessing the absorption of delicate thin-film coatings. To understand the photo induced effects in a stack of thin film layers subjected to high-power laser heating, a finite element model is developed using COMSOL. The model simulates the index and thickness variations of each layer and predicts the shift in optical function resulting from photo induced effects. The results offer valuable insights into the impact of laser-induced heating on the optical properties of the coatings and provide guidelines for designing robust and reliable photonic systems.

We explore the laser-damage behavior of gallium alloy-based liquid metal mirrors for their potential to provide higherdamage- threshold performance. One of the key advantages of using liquid metal mirrors is the self-healing potential following perturbations arising from exposure to high-power laser pulses. In this work, key performance metrics, such as reflectivity and the laser-damage initiation mechanism and initiation threshold, were investigated using fused-silica cells filled with three different Ga liquid metal alloys. The results suggest that irreversible modification (damage) under 355- nm, 6-ns pulses are associated with the formation of gallium oxide, taking place at a fluence significantly higher than that for damage initiation in conventional metal mirrors. This exploratory work is the first of its kind and highlights the strong performance of gallium alloy metal mirrors.

To mitigate risks due to laser-induced contamination (LIC) for the LISA space mission, we have carried out an extensive LIC test campaign, including a series of short duration tests with different test parameters, as well as a long-duration test. Those previous experimental results as well as theoretical considerations indicate that LIC might be less of a concern for the LISA mission. A remaining concern is whether LIC could occur in the presence of metallic particles on optical surfaces and whether a higher pressure does have an impact. Our ongoing research thus aims at testing for a possible deposit formation in a combined LIC and metallic particulate contamination test. Therefore, a HR optics is contaminated with metallic (aluminum) particles, mounted in the sample holder and tested within a similar test setup used for previous tests. The test is performed at a pressure similar to the actual expected pressure of 10^-5 mbar at the optical bench during the mission (previous tests at 10^-8 mbar). These tests do not indicate that LIC is a concern and metallic particulate contaminants seem not to accelerate or trigger LIC in this laser regime.

We present a temporally and spatially resolved photoluminescence (PL) measurement technique developed to rapidly characterize fused silica damage sites and determine their propensity to grow under subsequent laser irradiation. A diffusional model is used to describe the observed PL dynamics and correlation to the local damage morphologies. We believe that our measurement and analysis approach can allow rapid identification of growth-prone damage sites, providing a pathway to fast, non-destructive predictions of laser-induced damage growth and enable selective damage site mitigation which will greatly reduce the time required to recycle NIF’s optics.

A novel instrumentation design and operation to study ultrafast laser damage with atomic scale characterization by scanning tunneling microscopy is described here. The STM system operates in an ultrahigh vacuum chamber, fitted with an in situ objective allowing for tight focusing of laser excitation onto the sample. A combination of in situ and ex situ laser machining is used to define fiducial registry markers that help identify the overlap region for the optical excitation and STM scanning, aided by simultaneous confocal imaging and far-field cameras. We report initial measurements of laser damage on silicon in UHV with 10 pulses (77 fs compressed width, 0.67J/cm² peak fluence) from a 1030nm Yb:KGW laser. STM imaging of damage sites show several characteristic regions with sharply defined boundaries determined by underlying damage thresholds, including an ablation crater and the beginnings of periodic surface structures.

Many damage morphologies are possible following irradiation of materials with an intense laser beam. We present ultrafast time-resolved microscopic imaging of materials immediately following laser-matter interaction. In the case of transparent, brittle materials such imaging allows monitoring of multiple resultant processes such as shockwaves, induced stress, crack, and others. We present an optical system for such imaging and examples of several studies completed. Imaging shockwaves produced from impulsive absorption of intense light provides insight into shockwave speed and intensity. Imaging of crack evolution over time allows for understanding of the driving forces behind crack initiation and growth. Time-resolved imaging of visible emissions provides a timescale of radiative relaxation mechanisms and images the extension of cracks during and following bursts of multiple ultrafast pulses helps to understand the enhancement of crack growth under pulse bursts.