This PDF file contains the front matter associated with SPIE Proceedings Volume 11173, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

An effective damage mitigation strategy is necessary to operate laser systems at energy densities above the damage growth threshold of their optical components. On the National Ignition Facility, growth of laser-induced damage has conventionally been arrested in situ by employing spatially registered cm-scale “spot blockers” in the laser beam to shadow mm-scale damage sites. Spot blockers come at a cost, however, as they obscure a portion of the laser light delivered to the target and thus require an increase in beam energy to compensate for this loss. This increase adds incremental stress to all optics in the beamline. Most spot blockers assigned to an optic are eliminated as part of the repair process when the optic is removed from NIF. However, defects too wide or deep to repair travel with the optic, along with the need for the blocker, throughout its life. Due to obscuration budgetary constraints, these permanent blockers reduce the optic’s usable lifetime. In this work, we propose an alternative method for mitigating a growing damage site by placing a scattering structure of comparable size to the site upstream to shadow the site. This solution obscures much less of the laser light and increases the lifetime of the optic compared to current mitigation strategies.

The quality of a measurement is a combination of its accuracy and precision. Namely, the ability to repeatedly get the correct result and to agree with results from other’s measurements. This paper develops a theory for the evaluation of the laser damage performance of an optic, a laser damage metric. The theory is formulated as the convolution of the distributions of the underlying physical property, the test procedure and the traceable calibration. This theory is applied to several candidate laser damage metrics. The driving factors on the quality of metrics are isolated and discussed. The paper concludes with a brief discussion of each metric’s driving factor limiting measurement quality and lessons learned for the design of future metrics.

In the semiconductor industry, optical projection lithography is employed for the production of microchips. In this process ultraviolet radiation has been used to exposure of photoresists on silicon wafers. Light sources with shorter wavelength are needed to shrink the chip size due to the diffraction limit. Pulsed excimer lasers have been used since the middle of 1990s instead of mercury lamps. At first KrF lasers (248-nm) were adopted, then ArF lasers (193-nm) have been applied to satisfy tighter leading edge device requirements. Now almost 5,000 excimer lasers for lithography tools are being operated at the world-wide semiconductor fab with stable, its availability up to 99.8%. The latest ArF excimer laser can pulse 15mJ to 20mJ energy with 6-kHz repetition rate, its typical module lifetime which can be replaced is several dozen Billion pulses. The module lifetime are expected to expand to reduce the downtime to replace. Also for precise micromachining applications, ArF hybrid laser consists of all-solid-state DUV light source as a seed laser and excimer laser as amplifier is been developed. The pulse width of this laser is typically sub-nanosecond and its high peak power is another concern for laser optics. Generally, the lifespan of optical elements has been growing to reach 100 Bpls, and its evaluation takes a very long time, typically several years. Comprehensive durability evaluation becomes more efficient by creating accelerated element tests [1]. As an alternative, accelerated lifetime testing with high fluence are helpful approach to screen and select the optics to satisfy the lifetime requirement. We have been developing the accelerated test system to determinate the laser-induced damage threshold of optical surfaces. In this paper, the test system and some results of field-approved optics with 20ns pulse duration will be explained.

Benchmarking optical quality of transparent materials is of vital importance to high power laser optics. Herewith we propose new nondestructive technique suited to quantify atomic impurities, based on spectrometry of laser induced-filament luminescence. Various laser host materials, of different vendors (sapphire, YAG and KGW) were investigated. The intensity and decay times of luminescence indicated significant differences in impurities among samples. To validate the proposed technique, filament induced luminescence results were compared to cathodoluminescence and x-rays luminescence results. Good agreement between sources was found. Finally, the effect of impurities on optical fatigue was evaluated by laser damage testing of the samples.

We report on our recent work on the effect of defects on laser damage initiation and growth in the ultrashort regime. We particularly investigate the mechanisms of energy deposition in nano / micro scale defects in the coatings and their relation to damage initiation and subsequent damage growth using different experimental tools such as in-situ observation of damage developments, pump probe-microscopy, and studies on engineered model defects.

So called “optical fatigue effect” of transparent optical materials is triggered by repetitive laser pulses. It first appears in form of gradual modification of optical properties of the element (change in refractive index or absorption) and eventually leads to formation of catastrophic damage. As this phenomenon can be governed by distinct underlying physical processes it is also sensitive to laser irradiation conditions, intrinsic material and environmental properties, thus it is not always deterministic and therefore hardly predictable. There exist models of optical fatigue that relate absorbed pulse energy, dynamics of lattice deformation, reduced mechanical strength and heat accumulation to predict optical damage, however many quantitative features of such materials as well as scaling laws of irradiation for such models remain unknown. In order to address this issue appropriate set of experimental data is needed. Thus, well known transparent material - fused quartz - was investigated in bulk by using in situ quantitative tool, namely time-resolved digital holographic microscopy. Optical materials response was investigated by optically probing excited material at different time delays. Various dependencies were investigated by changing pump irradiation conditions as a function of incident laser pulses.

The necessity for durable optics for higher laser fluences and intensities grows as new technological advancements allow for increased peak powers of laser systems. This has motivated a substantial effort in the last decades to better understand laser induced damage mechanisms and their mitigation. One major damage mechanism limitation to laser systems at high peak intensities is filamentation in fused silica glass, due to Kerr self-focusing of the light [1], that has been motivating an on-going effort for the last few decades [2]. The past studies had led to a set of simplified rules that allows for the operation of laser system below the onset point for this mechanism to take place, namely what is known as the IL rule (intensity times the collapse distance before filamenting equals some empirical constant) and the Bespalov-Talanov (BT) perturbation growth theory [3-6]. The need to increase the laser beam intensities and optimize the throughput, closer to the point where the optical propagation length in the material is comparable to the predicted filamentation distance, requires revisiting and improving our understanding of the current rule set. This is especially emphasized by the shortcomings of these two highly useful yet under-justified models for the relevant situations of large aperture beams where the contrast perturbations on the beam are the seed for the filamentations (i.e., and not whole beam collapse).

Our work simulated the electron dynamic process based on different models, field-cycle-resolved photoionization theory and Keldysh theory. The central idea for predicting laser-induced damage threshold of few-cycle laser pulse based on the total laser energy coupled with the electron energy transfer in the crystal lattice. With this approach, predictions of the physical model start to converge to the available experimental data of 1-on-1 few-cycle laser damage experiments on the semiconductor (e.g. ZnSe) surface.

We discuss studies of laser damage inside various transparent materials (glasses, polymers, sapphire, diamond) caused by femtosecond lasers at 515, 800, and 1030 nm, with nJ to mJ pulse energies, single-shot to 2 MHz repetition rates, single and 10 ns burst-mode pulses, chirped pulses, and linear, circular, and radial beam polarizations. Experiments have created high-aspect damage features and voids using aberration-controlled focusing, axicon-formed Bessel beams with <1 μm diameter central lobes extending hundreds of microns through the materials, and tightly focused lines (~1 μm × <100 μm). Mechanisms include self-focusing, filamentation, and material expansion/compaction and expulsion.

The laser damage thresholds for gallium nitride and gallium oxide were found after exposing each sample to a femtosecond laser pulse. Threshold fluences were determined for both single pulse and multi pulse exposures. To accurately characterize the excited carrier density criteria in which visible laser damage occurs, we simulated carrier excitation dynamics for the entire laser pulse as it interacts with the target using the Keldysh model. From this a dynamic model of the conduction band carrier concentration was determined. For the measured single shot threshold fluences, the plasma critical density criteria for damage was met.

This year’s competition aimed to survey state-of-the-art near-IR high reflectors. The requirements of the coatings were a minimum reflection of 99.5% at 0 degrees incidence angle light at 1064-nm. The choice of coating materials, design, and deposition method were left to the participants. Laser damage testing was performed at a single testing facility using the ISO standard protocol with a 3-ns pulse length laser system operating at 5 Hz in a multi-longitudinal mode. A double blind test assured sample and submitter anonymity. The damage performance results (LIDT) and sample rankings are compared to last year’s competition results where raster scanning test protocol was involved. In addition, details of the deposition processes, coating materials and substrate cleaning method are also shared. We found that hafnia/silica multilayer coatings deposited by e-beam are the most damage resistant under the test conditions. LIDT differences between testing protocols were up to 38 J/cm², with ISO-reported LIDT results generally higher than those determined by raster scanning.

We compare the distribution of hafnia chemistries as a function of sun and planet position in an ion beam sputtering system. Hafnia film chemistries were investigated both without and with planetary rotation. In the former case, the film thickness, stoichiometries and entrapped argon varied drastically as a function of sun position, with one sun position exhibiting high entrapped argon content. With full planetary rotation used during deposition, the film stoichiometry is nearly ideal with 6% entrapped argon content. It is observed that the center of the planets is an exception, with a slightly metallic stoichiometry and high entrapped argon. Interestingly, all hafnia optical films produced in this study exhibit an inverse relationship between oxygen content and entrapped argon.

Optical glasses, in particular fused silica and BK7, are the most common and used substrates for components manufacturing in laser technology and optics in general. Dielectric coating technologies for those materials are well known and established; both high-reflective and anti-reflective coatings prepared on such substrates demonstrated laser induced damage threshold (LIDT) exceeding tens J·cm^-2 in nanosecond regime. However, LIDT became a major issue in further exploitation of crystalline materials as yttrium aluminum garnet (YAG) crystals, which often serves as a host in laser media and would be used in other components as well. One of the current challenge is the ability to transfer thin film coating technology used on glass to YAG in order to reach the same performance as in the case of fused silica or BK7 counterparts. HR dielectric coatings prepared on fused silica, BK7 and YAG substrates by reactive or ion-assisted e-beam deposition technique were tested on LIDT by s-on-1 method according to the ISO standard recommendations. Results from tests are presented and discussed in following paper.

The pulse duration dependence of single-shot laser-induced damage and ablation of HfO₂/SiO₂-based double- and quadlayer thin films is studied using time-resolved surface microscopy (TRSM) and ex situ imaging down to the few-cycle pulse (FCP) regime. Both samples exhibit a raised, "blister" morphology for a range of fluences between the damage and ablation thresholds. The fluence range associated with blister formation is much larger for FCPs than for 110 fs pulses, and TRSM images at early time-delays show that the density of the laser-generated plasma is much higher for 110 fs pulses for a lower fluence relative to the damage threshold. Also, for high enough fluences the excited electron density exhibits a fast decay down to a significantly high value, which remains even after the onset of mechanical damage of the layers. The pulse duration dependence suggests that as fluence is increased, the increase in absorbed energy is more gradual for FCPs, which points towards inherent differences in the way high intensity FCPs are absorbed in dielectrics relative to longer femtosecond laser pulses.

This work investigates the interaction of 10-ps and 0.6-ps, 1053-nm laser pulses with ≈40 um-diam microparticles located on the surface of a multilayer dielectric mirror. Resulting morphology was examined as a function of fluence and number of pulses (1–10) for four different particle materials (one metal, one glass, and two polymers). Processes of particle ejection, particle-assisted laser-induced–damage, and damage growth are discussed, all of which occur at fluences less than half of the pristine (not contaminated) laser-induced–damage threshold of the mirror.

The laser-induced damage performance of fused silica optics when exposed to 351-nm ns pulses is a limiting factor in the design and operation of most high-energy laser systems. As such, significant effort has been expended in developing laser damage testing protocols and procedures to inform laser system design and operating limits. These tests typically rely on multiple laser exposures for statistical validation. For larger aperture systems testing an area equal to that of the optical components in the system is functionally impossible requiring interrogation of sub-scale witness samples with elevated fluences. In this work, we show that, under the certain circumstances, the laser exposure used to test one location on a sample will generate additional laser-induced damage precursors in regions beyond that exposed to laser light and hence degrade the damage performance observed on subsequent exposures. In addition, we will outline the conditions under which this phenomenon occurs, as well as methods for mitigating or eliminating the effect.

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.

We present a simple and scalable method for the production of optics with incorporated metasurfaces, resulting in durable all-dielectric based meta-optics. The scalability and robustness of this method overcome limitations imposed by current technology when fabricating metasurfaces for high power laser applications, while the simplicity of the fabrication process makes it an exciting technique for metasurface generation. This talk will describe the method, show resultant fabricated metasurfaces and the sensitivity introduced by processing parameters – i.e. control over generated surfaces, and discuss the laser damage performance of these engineered large-scale metasurfaces.

Vacuum ultraviolet (VUV) reflective coatings play an important role in many high-tech fields including cosmic physics, space research, life science and synchrotron radiation. In this research, the emphasis are focused on the aging effect of 135.6nm high reflective coatings. The coatings were deposited by resistive heating method based on Lanthanum fluoride (LaF₃) and Aluminum fluoride (AlF₃). Optical property, surface morphology and roughness, and composition were characterized in different period after deposited. Due to the porous structure and the worse stability of lanthanum fluoride, the content of C and O element increased in LaF₃ thin films during aging process. On one hand, the content of C and O are the hydrocarbon contamination from environment and packing boxes. On the other hand, due to the oxidation of film materials, the fluorides will turn into oxyfluoride, which will increase O content. As a result, there will be an increase of absorption and a decrease of reflectance of the 135.6nm high reflective coating. The surface roughness decreased which led to the reduction of scattering and the rise of reflectivity. There will be LaF₃-AlF₃ mixed layers between interfaces because of interface diffusion, which will further reduce the film performance.

Dichroic coatings have been developed for high transmission at 527 nm and high reflection at 1054 nm for laser operations in the nanosecond pulse regime. The coatings consist of HfO₂ and SiO₂ layers deposited with e-beam evaporation, and laser-induced damage thresholds as high as 12.5 J/cm² were measured at 532 nm with 3.5 ns pulses (22.5 degrees angle of incidence, in S-polarization). However, laser damage measurements at the single wavelength of 532 nm do not adequately characterize the laser damage resistance of these coatings, since they were designed to operate at dual wavelengths simultaneously. This became apparent after one of the coatings damaged prematurely at a lower fluence in the beam train, which inspired further investigations. To gain a more complete understanding of the laser damage resistance, results of a dual-wavelength laser damage test performed at both 532 nm and 1064 nm are presented.

Several sets of fused silica and BK7 windows were anti-reflection (AR) coated for 1030 nm wavelength using ion assisted e-beam deposition under various conditions (substrate temperature, ion-beam energy). Samples were tested for laser-induced damage threshold (LIDT) at 1030 nm, 10 ns with 10 Hz repetition rate in 1000-on-1 mode according to ISO 21254 standard. Measured damage thresholds at normal (0 deg) incidence were compared and discussed.

Cleaning of substrates prior to optical coating is an important step in the manufacturing of high performance optical components. It is well known that the ultra-sonic frequency used during substrate cleaning has a strong influence on the quality of the cleaning process and the number of remaining particles on the surface. Therefore, we have investigated the influence of ultra-sonic frequency during substrate cleaning on the laser resistance of antireflection coatings. For this purpose, a SiO₂ / Ta₂O₅ AR-coating for a normal angle of incidence at 1064 nm was deposited onto fused silica substrates. Prior to deposition, the substrates were cleaned with cleaning processes. The applied ultra-sonic frequencies were 40, 80, 120 and 500 kHz. After deposition the LIDT was measured using a 1064 nm ns-pulsed laser test bench. It turned out that the different ultra-sonic-cleaning processes have a strong influence on the number of remaining particles on the surface of the cleaned samples. The counted number of particles with sizes greater < 83 nm were between 1320 and 12 particles for the different applied ultra-sonic frequencies. In consequence the different cleaned and AR-coated samples show different laser damage behavior. Nevertheless the measured particle density does not totally explain the differences in laser resistance.

As a rule of thumb, laser-induced damage threshold (LIDT) is often reported in terms of a single number, without even mentioning the testing details. However, meaning of reported LIDT numbers could be different depending on the testing protocol used. Such differences are not always obvious to practitioners that are designing or building laser systems (users of LIDT numbers). Furthermore, the properties of laser sources used for LIDT testing could also be very different among various testing laboratories. Thus, in order to exemplify possible effects of LIDT testing details on reported values an experimental study is conducted, where direct comparison of the most popular testing protocols, namely 1-on-1, S-on-1, R-on-1, and Raster Scan, is made. Experiments were organized in such a way that all the tests for the wavelength of interest were done on the same sample (conventional high-reflectivity HR mirror) by using both injection-seeded pulses (single longitudinal mode) as well as non-seeded (multimode) pulses with comparable effective pulse duration. Two sufficiently large dielectric mirrors were tested. Experiments were conducted for fundamental- (1064 nm) and third- (355 nm) harmonic wavelengths of Nd:YAG laser. The LIDTs obtained by using distinct testing protocols as well as pertinent damage morphologies are directly compared and discussed.

The standardization and the comparison of laser-damage testing are essential prerequisites for development and quality control of large optical components used in high-power laser facilities. To this end, the laser-induced–damage thresholds of two different coatings were measured at four laboratories involved in a round-robin experiment. Tests were conducted at 1 m in the subpicosecond range with different configurations in terms of polarization, angle of incidence, and environment (air versus vacuum). In this temporal regime, the damage threshold is known to be deterministic, i.e., the continuous probability distribution transitions from 0 to 1 over a very narrow fluence range. This in turn implies that the damage threshold can be measured very precisely. These traits enable direct comparison of damage-threshold measurements between different facilities, while the difference in the measured values are not accompanied by large statistical uncertainties. In this presentation, the results of this comparative experiment are compiled, illustrating the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of this this round-robin damage-testing effort exhibited significant differences between facilities. The factors to be taken into account when comparing the results obtained with different test facilities are discussed: temporal and spatial profiles, environment, damage detection, samples homogeneity and nonlinear beam propagation.

The damage characteristics of the indium-tin-oxide (ITO) layer and the polyimide (PI) layer, which are two constituent components of a LCD, induced by a high-peak-power laser and a high-average-power laser are investigated. The PI alignment layer is pinned on the ITO film to imitate the structure of the LCD as much as possible in our study. Under the irradiation of the high-peak-power laser, the damage process of the PI/ITO/SUB sample involves thermally induced plastic deformation, followed by cooling when the irradiation fluence is near the LIDT, and rupture when the irradiation fluence is higher. High-average-power laser irradiation results in damaged morphologies of the bulge for the PI/ITO/SUB sample. The temperature distributions induced by the pulsed laser and the high-repetition-rate laser are investigated. The damage is attributed to the intrinsic heat absorption of the ITO films. Under the irritation of the high-peak-power laser, the temperature rises rapidly to a high degree at very short time because of the instant strong absorption in ITO layer, and resulted in vaporization of ITO layer consequently. Subsequently, the vaporized ITO breaks through the surface PI and develops the visible damage. However, under the irritation of high-average-power laser, ITO layer absorbs laser energy, resulting in a slow temperature rise and a small temperature gradient.

The ESA satellite Aeolus was successfully launched into space in August 2018 and measures global wind profiles using the Atmospheric Laser Doppler Instrument (ALADIN). ALADIN features a high-power UV laser source emitting nanosecond pulses at a wavelength of 355 nm. A crucial step in the development of ALADIN was the mitigation of laser-induced contamination (LIC). In this work we assess the opportunity of removing LIC deposits using UV/ozone cleaning with a mercury lamp. We find that UV/ozone cleaning is a very effective tool for removing laser-induced molecular contamination induced by the volatile components of a material mix representative of the ALADIN laser. Furthermore, we show that optical surfaces on which a contamination is removed via UV/ozone cleaning behave similar to pristine optical surfaces with respect to their susceptibility to subsequent LIC as well as laser-induced damage. These results demonstrate that UV/ozone cleaning is a useful and safe way of cleaning optical surfaces after ground-based thermal vacuum/lifetime testing.

"Laser-induced contamination" is a major difficulty for high power photonics instruments in vacuum and in sealed environments. Material outgassing causes molecular contamination on the optical components where the laser irradiation causes photo-fixation and/or polymerization leading to carbonaceous deposits at the location of the laser beam. We studied the morphology of these deposits as function of several parameters of physical and chemical nature. The influence of these parameters on the crater rim height of the "donut"-type deposits are presented and lateral growth of the deposits beyond the laser beam size is observed. The observation of lateral growth beyond the laser beam size indicates an influence of thermal energy input to the deposition process. We hypothesize that this thermal energy is provided by heat conduction from the center of the crater.

The continuous wave laser-induced damage thresholds of Schott chalcogenide glasses, IRG-24, IRG-25, and IRG-26, are measured for a 5s exposure of 1.07 μm light focused to a spot size with 1/e2 diameter of 830 μm, following the International Organization for Standardization standards.

The absorption of fused silica, CaF₂, MgF₂, and sapphire at VUV region (193.4 nm) and IR region (1070 nm) were measured. For this measurement, LID (Laser Induced Deflection) method was used because of its high sensitivity. We report the degradation behavior of materials by comparison of absorption before and after ArF laser irradiation, and also the ultra-minute absorption at IR wavelength. At each wavelength, the absorption of low OH fused silica before the laser irradiation showed the smallest. At ArF wavelength, sapphire and CaF₂ showed higher laser durability than MgF₂ and fused silica. At IR wavelength, VUV-transmissive sapphire showed a lower absorption compared to general sapphire.

Third harmonic generation (THG) in dielectric films with femtosecond laser pulses is used to study properties of dielectric thin films and stacks thereof below and above the 1-on-1 laser damage threshold. Deviations from the ideal cubic relationship between third-harmonic signal and incident fundamental fluence are a result of several fundamental processes. Their relative contributions are assessed by comparing results from LIDT and conversion efficiency measurements as well as beam profile and pump-probe studies.

The typical measurement error reported for laser damage tests is the fluence uncertainty due to inaccuracies in measuring the laser beam energy and its diameter. However, the inherent uncertainty of the testing protocol should also be included in the reported laser damage threshold error bars. Underestimating measurement errors can lead to false conclusions about the impact of process changes on laser damage resistance. In this study, four different laser damage precursor fluence distributions were created from randomly generated numbers and then evaluated using the ISO and raster scan laser damage test protocols to determine a laser damage threshold. Measurement errors are determined for flat top test beams for multiple cases. To add real world relevance, the impact of Gaussian test beams with beam pointing instability was modeled for the lowest accuracy laser damage precursor distribution. The impact of damage test area compared to optic dimension is also examined. The measurement error for the raster scan test ranged from 8% to 24% depending on the test beam spatial profile (flat top or Gaussian) and beam pointing stability. ISO measurement errors ranged from 4% to 250% for a simulated 10 J/cm2 test and was much more sensitive to the laser damage precursor distribution as well as the spatial profile and pointing of the test beam. Both testing protocols poorly predicted the laser damage resistance of large areas with Gaussian precursor laser damage distributions.

In previous years, this committee reported on the need for a US National Laser damage standard, addressing the needs of domestic industry. In 2017, a process was reported that connected the measurement of the active defect density in a small area, a, with the likely density of such defects over a larger area, A. This was presented as the basis of a Type 1, go/no-go test. 2018’s achievement, is development of process starting from a user’s requirements and flowing into test parameters. This year’s report covers the resulting test procedure that implements the test process a useful workable standard.