This PDF file contains the front matter associated with SPIE Proceedings Volume 10805, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

This talk will cover my personal recollections about the early years of lasers and the laser damage situation in the 60s: the persons involved, the lasers used in damage experiments, the companies involved, funding agencies involved, personal interactions in those days, the nature of the papers in 1968, and the unique style of management and conduct of the first and later meetings by Art Guenther and Alex Glass, a co-invited speaker with me at this 50th anniversary meeting.

This is a reminiscence of where things stood in the laser damage problem at the time when the laser reached its 10th birthday. All of the different types of lasers that we know today had been demonstrated and those that could reach high power were shown to destroy the optics used in the resonators or to manipulate the high power beams. If high power lasers were to be useful the problem of laser damage had to be solved but first it had to be understood.

Fire in an operating room is the most dangerous situation for patient and staff. Besides electrosurgical devices and endoscopic light sources, even surgical lasers can be ignition sources for drapes, gowns and tracheal tubes. This risk was identified very early and several ISO standards for laser proof materials have been published. The medical beam delivery system itself, however, was out of focus. Due to the increasing market on the one hand and necessity for cost reduction in health care on the other hand fibres have come into the market with a risk of self-ignition of the core or cladding material. Furthermore with reinvention of fibre-applicator-systems for contact application or integrated diffusor systems they have an increased risk for self-ignition due to high absorption. So it is important to perform quality requirements for companies suppliers and hospitals. At this time there is no existing work or standard to this topic. This project elaborates reproducible test parameters for medical beam delivery systems. Because the problem of ignition and damage due to laser transmission is not limited only to medical devices but even e.g. in communication systems and fiber laser system the work was started in close cooperation with WG1 SC9 to avoid any duplication. The presented draft follows the structure, terminology and test procedure the existing standards for surgical drapes ISO11810 and Endotracheal tubes ISO11990 to avoid inconsistency in these standards

State Optical Institute (SOI) named after S.I. Vavilov was the major Federal research institute in the USSR responsible for research and development of optical materials, optical components and optical systems for wide range of applications. Creation of first high power pulsed lasers in ruby and Nd doped glasses in middle sixties resulted in damage of optical components used in those lasers. Therefore complex researches were triggered at SOI in several directions. The main attention was paid the role of different defects on damage of different materials. The leading research group in both theory and experiment was headed by Dr. Alexey Bonch-Bruevich while development of technology of materials with high resistance to laser radiation was managed by Dr. Gury Petrovskii. This presentation will provide a survey of results in theory and experimental study of mechanisms of laser induced damage.

The thin film damage competition series at the Boulder Damage Symposium provides an opportunity to observe general trends in laser damage behavior between different coating types (high reflector, anti-reflector, Polarizer, and Fabry-Perot filter), wavelength ranges (193 – 1064 nm), and pulse length ranges (40 fs – 18 ns). Additionally, the impact of deposition process, coating material, cleaning process, and layer count can be studied within a single year or more broadly across the history of this competition. Although there are instances where participants attempted to isolate a single variable to better understand it’s impact on laser resistance, this series of competitions isolates the variable of the damage testing service and protocol for a wide variety of participants. In total 275 samples from 58 different participants have been tested at four different laser damage testing facilities over the last ten years. Hafnia was clearly the best high refractive index material except for UV applications; although a wide range of high refractive index materials performed well. The best deposition process varied significantly between the different competitions, so it was much more strongly dependent on the coating type, wavelength, and pulse duration. For 1064 nm coatings with nanosecond scale pulse lengths, e-beam coatings tended to be the best performers. For short pulse length NIR mirrors and nanosecond pulse length UV mirrors, densified coating processes which all involved sputtering of the target material were the best performers. For UV AR coatings and excimer mirrors, both tested at nanosecond pulse lengths, they tended to favor very low energetic deposition methods yielding soft coatings such as sol gel dip coating for the AR and resistive heating of fluorides for the excimer mirrors. Finally cleaning method and layer count have had a less obvious correlation with laser resistance over the history of this thin film damage competition.

An adapted quality management is a major prerequisite for the reliable production of optical components and coatings. Especially laser applications impose highest demands on modern optical systems and can be considered as a major pacemaker for the development of advanced quality management strategies. Therefore, most of the standardized concepts for the determination of the properties necessary for a comprehensive quality control, as for example losses and transfer functions as well as laser-induced damage thresholds or defect densities of optical surfaces, are based on laser systems. This contribution is intended to offer a brief review on the present status of optics characterization and the related standards often applied in the production of laser components. A selection of International Standards will be presented with focus on the determination of laser induced damage thresholds (ISO 21254), optical absorption (ISO 11551), and Total Scattering (ISO 13696). The corresponding measurement methods will be described and discussed before the background of recent developments in laser technology. Finally, some aspects of the future development and projects of international standardization activities will be discussed.

In previous years, this committee reported on the need for a US National Laser damage standard, addressing the needs of domestic industry. [1] Last year, 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. The main issue as reported last year is that the proper flow of a standard is to start with the required properties of the larger area and design a robust test. The process presented in 2017 [2] is hard to implement in a way convenient for the non-expert user, which is nearly all. The main thrust of the work in 2018, is developing and evaluating options for implementing a useful workable standard. [1] “Periodic Review of ISO 21254: US National Committee Proposal for Revision”, Jonathan W. Arenberg, Donna J. Howland, Christopher Wren Carr, Michael D. Thomas, John C. Bellum, Trey Robinson and Jason Yager, Presented at SPIE Laser Damage, Boulder CO, 2016 [2] "U.S. National Committee proposed revision to the ISO Laser Damage Standard”, Jonathan W. Arenberg, Donna Howland, Michael Thomas, Trey Turner, John Bellum, Ella Field, C. Wren Carr, Gary Shaffer, Matthew Brophy, Allen Krisiloff, Proc. SPIE 10447, Laser-Induced Damage in Optical Materials 2017, 104471E (21 November 2017)

Substrate-transferred crystalline coatings have emerged as a groundbreaking new concept in optical interference coatings. Building upon our initial demonstration of this technology in 2013, we have recently realized significant improvements in the optical performance of these novel single-crystal GaAs/AlGaAs multilayers. In the near-infrared, for center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter + absorption) to less than 5 ppm, with the direct measurement of sub-ppm optical absorption in these films, enabling the realization of a cavity finesse exceeding 600,000 at the telecom-relevant wavelength range near 1550 nm. In this presentation we outline preliminary measurements of the laser-induced damage threshold (LIDT) of these novel semiconductor-based interference coatings. For pulsed excitation (ns pulse durations at 1064 nm), the narrow bandgap of the constituent mirror materials limits the LIDT to 3-5 J/cm2. Under these conditions, laser damage is driven by two-photon absorption (TPA) in the semiconductor multilayer, primarily the high-refractive-index GaAs films. Note that improved performance may be realized for illumination wavelengths >1740 nm, where TPA is eliminated. For continuous-wave (CW) illumination, the high thermal conductivity (~30 Wm-1K-1) and low intrinsic absorption yield the potential for excellent performance. Here we present preliminary CW damage measurements for a 10-ppm transmission quarter-wave GaAs/AlGaAs Bragg mirror transferred to super-polished fused silica, with only a 1.4 K temperature rise for an intensity of ~1.5 MW/cm2. Further efforts will continue to push the limits of the structure with the aim of determining the maximum CW intensity that such mirrors can tolerate.

Yttrium aluminum garnet (YAG) crystals are one of the most important materials for active media in solid-state laser technology. Reach for higher energies brings more stress into crystals thin film coatings field, where methods used in the past are not sufficient anymore. Laser induced damage threshold (LIDT) became a major issue in further exploitation of YAG crystals as required extraction fluencies exceed tens J·cm^-2 in nanosecond regime. Consequently, improved coating techniques based on e-beam deposition were introduced in order to improve damage resistance of active media. Thin films prepared on YAG crystals either by reactive or ion-assisted e-beam deposition technique were tested on LIDT by son- 1 method according to the ISO standards recommendations and results are presented in following paper.

The lifetime of optics, especially windows, has grown to reach 100 Bpls, and its evaluation lasts for several years at least. In elementary testing (short term), focusing on each damage phenomenon must be established. The degradation of calcium fluoride windows used as laser chamber windows in ArF excimer lasers (193-nm wavelength, 30-ns pulse width, 10-mJ output energy, ~80-mJ/cm2, 6-kHz and several dozen billion pulses) is analyzed. The results of analysis such as TEM-EDX, Nomarski-type differential interference contrast (DIC) microscope, AFM, etc. is shown. The damage mechanism can be estimated from these results. Comprehensive durability evaluation becomes more efficient by creating accelerated element tests (short term).

The x-ray free electron laser facility at SLAC National Accelerator Laboratory (named as LCLS) will be upgraded to LCLS-II in the near future. The high repetition rate light source makes the x-ray optics or components exposed to trillions of pulses over years of operation. Material fatigue properties of x-ray optics are essentially important for their lift-time prediction, optics optimization and opto-mechanical design. In this work, the fatigue properties of typical x-ray optics materials such as single-crystal silicon are experimentally measured by using laser pulses. The laser source can have an average power of 50 W at wavelength of 1.03 μm and repetition rate of 0.928 MHz with pulse duration of ~230 fs. The SHG crystal is used to generate 515 nm laser beam for the test to get an equivalent absorption length to soft x-rays. The maximum single-pulse energy is more than 16 μJ. The numbers of pulses that the optics can survive are measured for different pulse energies (fluences). The definition of the damage of x-ray optics is the significant reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold.

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.

Reductions of UV transmission in silica-based multimode fibers, with low-OH or high-OH synthetic silica core, due to optically active UV defects will be shown using pulsed 355 nm (3rd harmonics) and, for the first time, 213 nm (5th harmonics) Nd-YAG lasers. A new experimental set-up with nearly simultaneous laser damage and spectral analysis is proposed and realized, with the main aim that the laser-induced damage can be measured for wavelengths from 190 up to 1000 nm without movement of the fiber under test. In addition, the damaging UV lasers can be easily changed and aligned. For the two wavelengths, the fibers’ UV transmission is quite different. At 355 nm, the low attenuation level lead to a nearly constant intensity along the fibers, within approx. 10 m. Therefore, the almost constant twophoton absorption is responsible for a homogeneous axial distribution of optically active UV defects, well known below 280 nm wavelengths. At 213 nm, these defects can be generated by one photon alone. However, the defect concentration depends on the axial position and is significantly higher than the wellknown values during D2-lamp irradiation.

High-power, ultrashort laser-induced periodic surface structures (also referred as ripples), which has been observed on metals, dielectrics and semiconductors surface, could be generated and deliberately modulated by controlling the incident laser pulse. The periodicity, orientation and structure are the typical parameters in the study this near/subwavelength structures. The formation mechanism of LIPSS is still under investigation, and the current formation mechanisms on LIPSS include classical surface scattering model, self-organization, second/third harmonic generation, excitation of surface plasma polaritons, coulomb explosion, and cavitation instability and so on. In our work, 1-on-1 and N-on-1 laser-induced damage experiments were conducted on ZnSe substrate by using 170 fs laser and few-cycle laser to verifying the structure dependence with polarization, periodicity, and laser-induced damage threshold. Damage mechanism based on phenomenon was proposed.

The need for optics that can sustain higher laser fluences and intensities grows as new technological advancements allow laser systems to operate at increased in peak power. This has motivated a substantial effort in recent decades to study laser-induced damage mechanisms and their mitigations. One well known laser-induced damage mechanism is filamentation in fused silica glass, due to Kerr self-focusing of the light [1]. The study of filamentation has been an ongoing effort for the last few decades [2] as it turned out to be a major limitation to laser systems at high peak intensities. Past studies have led to a set of simplified rules that allows for the operation of laser system below the onset point of filamentation to occur, namely what is known as the “IL rule” (intensity times the length before filamenting equals some empirical constant) and the Bespalov-Talanov (BT) perturbation growth theory [3-8]. The necessity 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.

In order to correlate laser damaging fluence with the pertinent theoretical considerations, there were many attempts in the past to establish reliable damage predicting criterion. Such criterion then could be used to estimate laser fluence that triggers the damage process in various optical materials. For example, reaching of materials critical property such as - temperature (melting point), - thermoelastic stress, - electron density are good examples. On the other hand, however, it is already clear that damage mechanism is irradiation condition (wavelengths, pulse duration) and material property dependent. There are no physical restrictions of causing damage by reaching critical stress without critical electron density and vice versa. Accordingly, total absorbed energy or absorbed energy density is likely more suited candidate of universal damage criteria as a common denominator for all critical processes. To our best knowledge, it was never estimated experimentally in the vicinity of the damaging fluence of optical materials. In this study, we present a novel approach based on pump- probe digital holographic microscopy that enables quantitative assessment of absorbed energy during the damage process in transparent dielectric media. By using this method, a case study is conducted in fused silica glass with sharply focused infrared laser pulses at 1030 nm central wavelength and 450 fs pulse duration. By doing so we were able to estimate energy fraction of the incident pulse that is needed to trigger optical damage.

Investigations of reduced laser-induced damage thresholds in dielectric coatings when tested under vacuum show that film stress is one of the contributors to this effect. Also, there are a number of different approaches to ensure wavefront integrity of surfaces that high stress films have been applied to. To not only identify optimum wavefront integrity but also optimum laser-induced damage threshold, a specific set of samples has been designed, coated and tested. This paper reports on these results, based on films produced in an ion beam sputtering process.

This competition aimed to survey state-of-the-art near-IR high reflectors. The requirements of the coatings are 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 raster scan method 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. In addition to the laser damage resistance results, details of the deposition processes, cleaning method, coating materials and layer count are also shared. We found that hafnia/silica multilayer coatings deposited by e-beam are the most damage resistant under the test conditions.

The decrease of laser-induced damage threshold (LIDT) when exposed with high number of laser pulses is a well-known phenomenon in dielectrics. In the femtosecond regime this fatigue is usually attributed to the accumulation of laser-induced lattice defects. Little is known about the accumulation mechanisms in oxides used for dielectric coatings. In this work, S-on-1 laser-induced damage threshold test was combined with time-resolved digital holography in order to investigate laser-induced lattice defects in Nb₂O₅ single layer. The results provided insights into the current understanding of accumulation of laser-induced defects.

Completing our suite of deposition equipment, we are developing a new Ion Beam Sputtering (IBS) System with different substrate configurations: the High Throughput version (HT) and the High Precision version (HP). The HT version enables the coating of 4 planets of up to 350mm diameter substrates, whereas the HP version allows coating of substrates up to 600mm diameter in a single planet configuration. The IBS system is configured with a Bühler proprietary Optical Monitoring System for layer termination, a large 22cm RF sputtering source, and a LION plasma source for assist. In this presentation the optical performance of this IBS coatings, including LIDT, absorption, total loss and residual coating stress, will be discussed and compared to the other available deposition techniques, such as Plasma Assisted Reactive Magnetron Sputtering, and Plasma Ion Assisted deposition (PIAD). Preliminary results of a 1064nm mirror show less than 5ppm absorption, reflectivity’s of 99.997%, and no visible damage in CW LIDT testing up to 10MW/cm². Pulsed laser damage testing is in process and will be reported. These results will be compared to the coatings being done using PARMS and Evaporation.

A kind of HfO2/SiO2 355nm and 1064nm high-reflective (HR) coatings were deposited by electron beam evaporation. Laser-induced damage of the coatings were tested by 355nm-7ns pulses, 355nm-1ns pulses and 1064nm-30ps pulses in 1-on-1 mode. All tests were carried out with S-polarized and P-polarized pulses in two angles of incidences (AOIs) of 30° and 50°. Damage morphologies and cross-sectional profiles were characterized using scanning electron microscope (SEM) and focused ion beam (FIB), respectively. It is shown that the typical morphologies in all the tests were μm-sized pits. In the 1064nm-30ps tests, the damage pits appeared mostly as 3-4μm ripple-like pits with a density of 13000-25000 mm^-2, accompanied by a few tiny pits around. The ripple-like pits were all conical pits with a cylindrical cavity at the bottom, the depth of which was around 1μm. The tiny pits were all cylindrical shaped with a depth of about 400nm. In the 355nm-7ns tests, most of the pits were flat-pits scaled from 4 to 18μm with a density of 500-900 mm^-2, few with a bulge in the bottom. The depth of the pits increased with their size, which can be 2.5μm for the largest ones. In the 355nm-1ns tests, pits appeared to be similar with those in 355nm-7ns tests, with a smaller size of 4-6 μm, The damage pits were preliminary inferred to be formed because of the material removal induced by the thermal stress.

We had developed a unique porous thin films by a special coating method1. In this technique, two dielectric materials A and B having different refractive indices nA and nB ,where nA＞nB are simultaneously deposited in vacuum on a substrate such as fused silica or optical glasses. Then the coated surface is processed in ultra-pure water which preferentially dissolves the material B. These processes result in a porous thin film which has gradient refractive index and has the antireflection (AR) property over broad bandwidth. The porous coating obtained by this method cannot apply depositing a multilayered dielectric thin film. We have developed a novel method. The present technique, a dielectric material D and a plastic P are simultaneously deposited in vacuum on a heated-substrate such as fused silica, ceramic or optical glasses. Then the coated surface forms an adaptively mixed thin film ( AMTF ) with dielectric material and plastic. In this coating process, plastics partially evaporate due to the heated-substrate. The refractive index of the coated AMTF mainly decided by the mixing ratio of the dielectric material and plastic. In our samples the damage threshold was confirmed to be 115 J/cm2 at 10 ns and λ=1064 nm. The band width of AMTF with MgF2 and Teflon (AMTF: MgF2 ) was confirmed to cover from 200 to 8000 nm. This AMTF: MgF2 can be applicable not only to AR thin film, but to a high reflectance mirror and polarizer in various high intensity laser syetems. 1K.Yoshida, H.Yoshida, Y.Kato, and C.Yamanaka, Appl.Phy.Lett.47,911(1985)

The demonstration of a Yb:YAG chirped pulse amplification laser producing 1 J, 5 ps pulses at 500 Hz repetition rate [1] and recently 1 J pulses at 1 kHz repetition rate [2] relied on efficient thermal management and high performance multilayer dielectric coatings on the laser amplifier active mirrors. In the active mirror configuration, the Yb:YAG amplifier crystals use HfO2/SiO2 multilayer dielectric anti-reflection (AR) and high reflection (HR) coatings. The Joule-level amplifier is operated in vacuum and at liquid nitrogen boiling temperature (77 K) with 1030 nm, 220 ps duration laser pulses making four reflections from each HR coating and 8 passes through each AR coating. The LIDT performance of these coatings is crucial to the future scaling of these amplifiers. In this work we describe results of an investigation of the laser induced damage threshold (LIDT) of Yb:YAG active mirror laser amplifier disks at atmospheric, vacuum and cryogenic temperature conditions. The measurements were conducted for 220 ps pulses, the typical pulse duration of stretched pulses we are using to implement kW-class average power CPA laser amplifiers [1,2]. We measured the 1-on-1 (single-shot) and 3000-on-1 LIDT on Yb:YAG crystals with and without the coatings. The results show that the LIDT for single shot damage occurs near 20 J/cm2, and 100% damage probability occurs near 29 J/cm2 for either the uncoated or coated Yb:YAG crystal at atmospheric conditions. Similar results were obtained in the vacuum and cryogenic temperatures tests. This leads to the conclusion that the Yb:YAG material itself, and not the coatings, is the limiting factor in the LIDT. This work was performed under the auspices of the U.S. Department of Energy, Office of High Energy Physics, Accelerator Stewardship Program under Award DE-SC0016136. References [1] C. Baumgarten, M. Pedicone, H. Bravo, H. Wang, L. Yin, C. S. Menoni, J. J. Rocca, and B. A. Reagan, Optics Letters 41, 3339 (2016). [2] Brendan A Reagan, Cory Baumgarten, Elzbieta Jankowska, Han Chi, Herman Bravo, Kristian Dehne, Michael Pedicone, Liang Yin, Hanchen Wang, Carmen S Menoni, Jorge J Rocca, High Power Laser Science and Engineering 6, e11 (2018).

Contaminants can severely limit the efficiency, laser damage threshold, and strength of photonic crystal fiber-based lasers. Such contamination can occur due to environmental exposure during the pulling or stacking of rods and tubes or improper handling and storage of these glass components. A preform made by the “stack and draw” process is susceptible to incorporating surface contaminants into the bulk laser glass. We have adapted cleaning and handling protocols originally developed for processing large fused silica optics for the National Ignition Facility. The etch cleaning process reported here mimics the “AMP” or “Advanced Mitigation Process” developed for NIF optics that see high fluence 351nm light. In addition, all cleaning, fixturing and assembly processes used to prep a stack for pulling into a fiber are done in a Class 100 cleanroom. Glass rods (1-3mm in diameter and 10” long) are assembled into a Teflon fixture that only contacts the rods at each end. The loaded fixture receives 120kHz ultrasonic cleaning in 10% sodium hydroxide at 45C and 3% Brulin 1696 detergent at 55C. Parts are thoroughly rinsed using ultrasonicated ultrapure water and spray rinses. A 200nm etch in buffered hydrofluoric acid (6:1 BOE diluted 2:1 in DI water) is followed by additional ultasonicated (120kHz-270kHz) ultrapure water and spray rinse. Finally, the components are allowed to fully dry inside the Teflon frame. The rods are cleaned, stacked, and assembled into a fused silica tube. The preform stack is then returned to a non-cleanroom area to be pulled into fiber using standard telecom fiber-based draw tower equipment and without clean air filters around the draw area. Four fibers were made to test independently the damage threshold and the background loss, two Yb core active fibers and two silica core (F clad) fibers. One of each was cleaned with the AMP process, and one of each with a methanol wipe cleaning process. The active fiber was coated with a dual acrylate coating, first with a low-index inner coating to provide a pump cladding, and then with a relatively hard coating to protect the relatively soft primary coating. The active fibers were pumped at 980nm in a double Fresnel cavity configuration and the power increased until the fiber was damaged up to 1kW. The passive fiber background loss was measured using a standard cut-back technique. Replacing the former methanol wipe clean process with this aqueous cleaning process improved the 1060nm damage threshold of a fiber laser by >30x to above the kW level in the laboratory and reduced the background attenuation by >18x. Early indications are that the acid etching also makes the tensile strength of the fiber consistently high. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

The National Ignition Facility (NIF) uses an in-situ system called the Final Optics Damage Inspection (FODI) system to monitor the extent of damage on installed optical components. Among this system's uses is to alert operators when damage sites on a Grating Debris Shield (GDS) require repair (≈300 microns) and triggers the removal of the damaged optic. FODI, which can reliably detect damage sites larger than 50 microns, records the size and location of observed sub-critical damage observed on the optic, so each of these sites can be repaired before the optic is next installed. However, by only identifying, and hence repairing sites larger than ≈50 microns, optics are left with numerous smaller sites, some fraction of which resume growing when the host optic is reinstalled. This work presents a method of identifying and repairing damage sites below the FODI detection limit that have a significant probability of growth. High resolution images are collected of all likely damage candidates on each optic, and a machine learning based automated classification algorithm is used to determine if each candidate is a damage site or something benign (particle, previously repaired site, etc.). Any damage site greater than 20 microns is flagged for subsequent repair. By repairing these smaller sites, recycled optics had a 40% increased lifetime on the NIF.

Non-diffracting surface relief grating structures combined with high refractive index films were designed as high efficiency, narrow-band, polarization selective high reflectors for the near infrared wavelength region. Such nanostructure polarizers (NSP) have the potential for increased laser damage resistance due to reduced absorption and the ability to create arbitrary refractive index layers with fewer defects and reduced electric field enhancement. Three NSP designs based on gratings in fused silica combined with tantala and magnesium fluoride films, were prototyped and characterized for efficiency, surface absorption and pulsed laser damage resistance at a wavelength of 1064nm. Most NSP prototypes exhibited <99.7% reflectivity for linearly polarized illumination over a several nm bandwidth with high transmission of the orthogonal polarization leading to extinction ratios greater than 300:1 for the best performers. NSP prototype performance was worse than predicted by the design models due to the imprecise replication of the fused silica grating surface in the film layers resulting from the deposition system configuration. Surface absorption measurements showed the expected low absorption in the 4 ppm range for film layers deposited on non-structured control substrates, but voids and growth defects revealed through scanning electron microscopy in the same films deposited over gratings, likely caused an observed 5 fold increase in NSP prototype surface absorption. Initial 1064nm wavelength, 6.2ns pulsed laser damage testing also showed a reduced damage resistance for NSP prototypes compared to the films deposited on non-structured control substrates. Follow-on work to eliminate the film defects for NSP designs is underway.

Absorbing defects such as fractures and contaminants are a leading cause of surface damage in nanosecond pulsed lasers. Etching such defects has proven to be a powerful technique for increasing the laser damage threshold of fused silica, but to date no etching process has been reported for potassium dihydrogen phosphate (KH₂PO₄ or KDP) or its deuterated analog (DKDP). We show that physical dissolution in water is a viable strategy for etching DKDP surfaces but surface-redeposited byproducts can serve as laser damage precursors. We use a water-in-oil microemulsion to etch engineered surface fractures in DKDP. Etching widens surface fractures laterally and decreases their optical activity, as measured by photoluminescence. The removal of 1 μm of the surface of a DKDP crystal increases the laser damage threshold (λ = 355 nm, 7 ns) of the engineered surface fractures by 2-4 J/cm2 (15-30%).

UV laser damage is still the key issue of high power nanosecond laser systems. The operation performance of the final optics in SGII-UP facility is first reviewed. Based on a high power laser prototype, laser-induced damage of large aperture final optics at 351nm was experimentally studied, including damage initiation, growth and morphologies. The near filed of 351nm laser beam was precisely measured with a high resolution by using the precision diagnostics system (PDS) to study the effects of laser modulation and propagation on laser damage. The damage behaviors were comprehensively analyzed and the main contributors to laser damage were discussed. The development perspective of final optics system for high power laser system is briefly introduced.

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.

The Orion facility is used mainly for studying high energy density physics. It has ten “long” pulse UV laser beams, two “short” pulse Petawatt beams and ancillary beams for probing of the experimental conditions. Occasionally experimental teams are requested to perform dedicated shots of a given configuration to obtain information on laser target fragments and their effects on surfaces within the target chamber. After these test shots witness plate surfaces were inspected by optical microscopy so that palliative measures could be taken or modifications to the experiment undertaken to minimize any deleterious effects on laser optics or diagnostic instrumentation.

In this paper, we studied the onset and further development phases of LIC deposits. A slight antireflective effect was consistently observed at the onset of the deposition process in our experimental conditions. The increase of the laser transmission signal could thus be used as a signature to predict the initiation of a LIC deposit in the absence of in situ fluorescence monitoring which is even more sensitive. Using a monochromatic microscope, we showed that the analyzed deposit behaved like an interference coating and, due to its nanometric porosity shown by atomic force microscope images, its effective refractive index can be lower than that of the substrate. Such a layer could thus have slightly antireflective properties as they are observed during the first phase of the LIC deposit evolution.

Particles generated from laser-induced damage during operation of fusion class lasers can be a source of damage precursors for neighboring optics. Such particles have been identified on the National Ignition Facility as ejecta from 1) laser damage on absorbing glass protecting the installation hardware from known stray focusing reflections in the optical path and 2) bulk damage in the borosilicate target debris shield that grows and erupts on the input surface. The dependence of the particle generation and damage initiation rate of this newly recognized damage source on laser shot parameters is not yet known, making it difficult to predict how this source would affect facility optic lifetime for projected laser operation. In this work, we measure the 351-nm fluence-dependence on the size distribution of glass particles generated by ejection from absorbing and borosilicate shield glasses onto a neighboring fused silica window exit surface. In addition, we track the fates of these ejecta and measure their probabilities of removal, damage initiation, and damage growth upon subsequent laser exposure. Thousands of particles can be ejected and deposited onto the exit surface of the fused silica window following a single pulse. Damage initiation following exposure of large borosilicate particles was observed above a fluence of 6 J/cm². A laser-driven strategy to remove particles before a high fluence pulse is explored.

Previous studies have shown that nanometer scale defects can lead to the formation of submicrometer craters, if located in coatings with a relatively small thickness. Due to the small size, such damages are challenging to detect in the online and offline damage detection and may therefore lead to an overestimation of the LIDT for the tested optical component. However, the influence of these nanopits on the optical properties and the impact on the initiation of catastrophic damage was not investigated in detail in the past. In order to study the correlation between nanopits, optical properties and catastrophic damage, samples with an AR-coating were fabricated by means of ion beam sputtering (IBS) and tested for their laser resistance by LIDT raster scans in the nanosecond regime at 355 nm. The generation and morphology changes of the nanopits were monitored for different pulse numbers and in dependence of the starting fluence. In addition to the inspection with an optical microscope in differential interference contrast (DIC) mode as prescribed by ISO 21254, alternative inspection methods, for example, dark field microscopy and scanning electron microscopy (SEM), were used to detect the nanopits. The damage test revealed that nanopits occur rarely in standard AR-coatings and possess only a small relevance for the LIDT. The typical damage morphology observed consisted of micrometer-sized pits which exhibited a stable size over a large fluence range and no growth after repeated irradiation.

In this work we study the effect of laser conditioning on laser-induced damage of ion-beam sputtered, anti-reflection coated laser optics with ns-pulsed laser radiation at a wavelength of 355 nm. With respect to applications in aerospace, measurements were performed under high vacuum. At laser fluences below 20 J/cm², laser-induced damage appears as pin-point damage (small explosion pits with sizes in the range of 1 μm), sometimes referred to as "grey haze". We find that ramped laser conditioning is an effective tool to reduce not only the abundance but also the average size of pin-point damage at laser fluences exceeding the optic’s unconditioned laser-induced damage threshold. We discuss our results in the context of the small absorber model for damage crater formation.

Several sets of polished substrates were manufactured from monocrystalline yttrium-aluminium-garnet (YAG) grown by the Czochralski method. Samples were coated by both narrow-band and broad-band dielectric anti-reflection (AR) thin-film system prepared using either reactive or ion-assisted e-beam deposition technology and tested for laser-induced damage threshold (LIDT) at 1030 nm 10 ns in s-on-1 mode according to the ISO 21254 standard. Measured damage thresholds at normal (0 deg) incidence were compared for different thin-film designs and coating technology.

The laser damage thresholds of optical coatings can degrade over time due to a variety of factors, including contamination and aging. Optical coatings deposited using electron beam evaporation are particularly susceptible to degradation due to their porous structure. In a previous study, the laser damage thresholds of optical coatings were reduced by roughly a factor of two from 2013 to 2017. The coatings in question were high reflectors for 1054 nm that contained SiO₂ and HfO₂ and/or TiO₂ layers, and they were stored in sealed PETG containers in a class 100 cleanroom with temperature control. At the time, it was not certain whether contamination or thin film aging effects were responsible for the reduced laser damage thresholds. Therefore, to better understand the role of contamination, the coatings were recleaned and the laser damage thresholds were measured again in 2018. The results indicate that contamination played the most dominant role in reducing the laser damage thresholds of these optical coatings, even though they were stored in an environment that was presumed to be clean.

Atomic layer deposition (ALD) enables coating complex shaped substrates with excellent uniformity along the surface of the optic. Recently developed nanoporous SiO2 layers have been applied as single layer antireflection coatings on fused silica substrates at both 1064 nm and 532 nm wavelengths. The LIDT in the nanosecond regime at both 1064 nm and 532 nm of these nanoporous SiO2 coatings as well as the bare substrates were investigated. The stability of the coatings with respect to LIDT has been evaluated under normal atmospheric conditions, dry air with relative humidity < 10% and nitrogen atmosphere. The multiple pulse damage characteristic for 5000 shots showed in all cases no significant pulse dependence. At 532 nm wavelength, the 0%-LIDT value is between 60 J/cm² and 70 J/cm², which is comparable to the values measured on uncoated substrates (80 J/cm²). In case of 1064 nm the 0%-LIDT is only between 40 J/cm² and 50 J/cm2 (uncoated substrate: 100 J/cm²) which is attributed to generated defects during the fabrication process.

We have studied laser induced material modification in a frequency tripling mirror (FTM) consisting of alternating hafnia and silica layers. The third-harmonic signal generated by a train of femtosecond laser pulses (791 nm, 55 fs, 110 MHz) drops over time until it reaches about 20% of the initial value. From the observed changes in reflection and transmission of the mirror a refractive index change of 0.07 was estimated, which occurs in the layer with the highest field enhancement. This index change triggers a drop in the field enhancement, which reduces the efficiency of nonlinear optical processes. The estimated value of ▵n allowed us to explain the 80% reduction in conversion efficiency and as well as an observed decrease in two-photon absorption.

Particles, which contaminate the substrate during thin film deposition, are prone to cause irremovable defects and demand special attention in the field of high precision laser optics as they can lead to localized absorption and laser damage. In this contribution, we present a camera based fully in-vacuum device which continuously monitors the coating surface of a transparent test substrate under dark field illumination. We show the possibilities of this setup regarding the sensitivity to small particles (diameter 1 μm). As the first operational test of the particle monitor, singe layers of HfO₂ are grown on fused silica. By analyzing the evolution of the scattering intensity of the particles, we derive their position in the substrate-coating-system. Therefore, this in situ particle detection concept can deliver data on which process step is responsible for particle generation in multilayer films and aims to be a tool to minimize coating defects.

High-energy laser systems are limited by the onset and subsequent growth of damage on constituent optics. This has been extensively studied for optics comprised of fused silica, but less so for other common optical materials. There are very few materials as well characterized as fused silica and, in this work, we explore the growth characteristics of other widely used optical materials with a range of physical parameters, namely sapphire, potassium dihydrogen phosphate, calcium fluoride, and compare them to fused silica. Since current understanding is that material fracture must be present before the fluences used in ns laser systems might cause a surface flaw to grow, we have chosen to study flaws on the exit surfaces created with a Vickers indenter. A range of indenter forces were selected that would produce flaw sizes typical of those that have been seen in laser created damage. Samples with arrays of indents were tested in the in the Optical Science Laser (OSL), a master oscillator power amplifier system, with a front-end pulse shaping capability able to deliver relevant fluences with a large area beam. Samples were tested in vacuum at 351 nm and at atmosphere at 1053 nm with a single shot fired every 45 minutes exposing multiple sites simultaneously. High resolution images of each flaw were taken after every shot to document changes. Additional tests at 1064 nm were conducted of individual sites at a 60 Hz rep rate in the Gigashot Optical Laser Damage (GOLD) system. The probability of growth at 3ω at 5.5 J/cm² is near 100% for both calcium fluoride and fused silica about 50% for the other materials. The growth rates at 3ω at from 5 to 8 J/cm² are comparable for all but potassium dihydrogen phosphate which are better than five times lower. At 1ω all the materials had about a factor of five increase in the threshold for growth.

We report on the laser damage performance of the DKDP third-harmonic frequency conversion crystals (THG) on the National Ignition Facility (NIF) since its operations began in 2009. An in-situ damage inspection system is used to monitor and track status of final optics in the UV section of the laser over time. Most THG optics last between 2 and 5 years on NIF before damage grows large enough that we have to exchange them. The critical damage size is related to our recycle strategy. About 10% of optics have lasted 10+ years which show we are not even close to the inherent limitation for this optic type. The short life THG optics are all limited by a relatively few number of flaws. Here we describe our efforts to understand these flaws so we can manage and, eventually, eliminate them.

The exploitation of nonlinear effects in multi-layer thin films allows for optics with novel functions, such as all- optical switching and frequency conversion. In this contribution, an improved interferometric setup for the measurement of the nonlinear refractive index in dielectric substrates and deposited single layers is presented. The setup is based on the wave front deformation caused by the self-focusing in the measured samples. Additionally, measurement results for a highly nonlinear material, indium-tin-oxide (ITO) are presented with respect to the materials power handling capabilities and compared to values from other materials.

Excimer lasers are the commonly used light sources for photo-lithography industries; one challenge is to minimize the production interruption by providing a reliable source of DUV (193 nm) photons. This requires CaF₂ optics with high transmission and optical uniformity at 193 nm over a long lifetime. However, CaF₂ crystal can possess defects due to mis-orientation and dislocations in the sub-grain boundaries, which can act as an absorption or scattering center leading to transmission loss and thermal stress induced birefringence in the CaF₂ crystal. In addition, the CaF₂ surface can suffer from different mechanical stress due to cleaning and finishing processes leading to formation of surface imperfections/fractures known as sub-surface damage. This sub-surface damage layer is prone to damage under high fluence 193 nm exposure and can lead to fluorine escape from the CaF₂ lattice. Protective coatings for CaF₂ optics have been developed to prevent surface fluorine depletion from 193 nm exposure. However, these protective coatings can also develop defects due to imperfections in the coating fabrication process and/or photochemical reaction initiated by 193 nm photons in presence of traces of oxygen, water vapor or carbon dioxide. Therefore, to provide uninterrupted 193 nm photons a robust protective coating is required to extend the lifetime of CaF₂ optics. Generally, field learnings for high fluence protective coating can require from 6 months to a year of normal operation and thus validation of protective coatings based on field data alone can hinder the adoption of improved technology. To expedite this selection process, an accelerated 193 nm exposure setup was built to test high fluence protective coatings from different suppliers at various elevated fluences for a short period of time (~2-3 weeks). This setup was successful in screening the best high fluence protective coating under highly accelerated 193 nm exposure. Additionally, based on the relative performance of the protective coatings under accelerated conditions and use case in-laser fluence conditions, the lifetime for the high fluence protective coatings were estimated for the use case scenario.

Nd:YAG lasers are easy to operate, and are used in material processing applications. Because they are high-power lasers, optical materials with high laser durability are required for their optical systems. One of the commonly used materials for high-power lasers is silica glass. The silica glass has not only high laser durability but also high transmittance from the UV region to the IR region. In this study, we evaluated the laser durability by measuring laser-induced bulk damage threshold (LIDT) of silica glasses at 1064 nm and 213 nm, as the one of the key indexes for laser durability. We obtained following results. First, at 1-on-1 LIDT measurements, the LIDTs of samples were almost the same at each wavelength. On the other hand, the LIDT results were different depending on the sample at 213 nm as well as at 355 nm and 266 nm. The more hydroxyl concentration the silica glass had, the lower laser durability the silica glass had. However the difference of LIDT results at 1064 nm was also small at 10000-on-1 LIDT. Second, the silica glass which includes chlorine had lower durability even if its hydroxyl concentration was very low. Finally, hydrogen concentration dependency of the durability was varied by the hydroxyl content. If the content of hydroxyl was ≤10 ppm, LIDT became lower as hydrogen concentration increased. On the other hand, if it was equal to 30 ppm, LIDT became higher as hydrogen concentration increased. Based on these results, we can improve laser durability of silica glass at 213 nm by either reducing hydroxyl and chlorine or controlling hydrogen concentration. We can also select an appropriate silica glass in accordance to use.

The “Bivoj” 10 J, 10 ns, 10 Hz, Yb:YAG (1030 nm) diode-pumped solid state laser (DPSSL) at the HiLASE Centre was used to investigate the laser-induced damage of optical glasses with different refractive index (BK7, SF8, FS, LIBA2000). The samples were polished using a combination of methods and cleaned in ultrasonic bath or with ion beams. Sample surface was characterized using white-light interferometry (WLI) and laser confocal microscopy (LCM). For the laser-induced damage threshold (LIDT), an S-on-1 procedure was selected, the testing taking place in accordance with the ISO 21254 standard. Due to the high energy per pulse of the “Bivoj” system we were capable of using beams with more than 500 μm diameter (using a long focusing mirror) and thus, including different surface defect in the LIDT measurement. The damage of the glasses was usually observed on the rear side (ballistic damage) due to constructive interference, however we manage to see on few samples front damage also. Values above 50 J/cm² were common for all tested samples.

Interest in qualitative analysis of damage morphology of laser-induced damage test sites has increased in recent years. Such analysis can potentially provide valuable information about underlying damage mechanisms and can be used for separation of different damage modes. However, morphological analyses are currently performed manually and only on a few test sites at a time. In this work, a novel computational approach to the analysis of damaged test sites is presented. Image processing algorithms were applied to images of test sites in order to identify damaged test sites and extract features of damage morphology. Unsupervised machine learning was performed to automatically cluster damaged test sites. It was shown that ZrO₂ single layer’s laser-induced damage can be separated into well defined clusters. The clusters were grouped to distinct catastrophic and color-change modes. Characteristic damage curves of different damage modes were investigated to reveal different fatigue behavior.

Investigating the properties of manufactured polymer optical fibers is essential to determine proper areas of application. Using pulsed laser radiation, especially with respect to laser activity in optical fibers, the maximum acceptable transmittable energy without inducing damage is of particular interest. Therefore, this work is related to laser-induced damage in polymer optical fibers at a wavelength of 532 nm and a pulse duration of 7.3 ns. In particular, the influence of the coupling condition on the transmittable pulse energy and the damage behavior applying an R-on-1 test procedure are analyzed in this study. The obtained results give information about the long-term behavior and will be used to optimize the manufacturing process.

In this work the influence of non-uniformity effects on the spectral transmission properties of broad-band dielectric optical coatings was examined. Recently, it was observed that in modern complex dielectric coatings significant spectral, resonantlike errors of the reflected wavefront can occur at specific wavelengths, which are induced by lateral coating nonuniformities [1]. For a detailed investigation of this effect, a setup was developed for monitoring the spectrally dependent wavefront error, utilizing a broad-band monochromatized plasma lamp (spectral range from 400 – 900nm) as light source and a high sensitivity Hartmann-Shack wavefront sensor for detection of reflected or transmitted wavefronts. In addition, a method for absolute and relative calibration of the measured wavefront error is presented. Two broadband dielectric beam splitters (#1 and #2) deposited by magnetron sputtering (high reflectance 400 – 900 nm, high transmittance 920 – 2300 nm) with different coating specifications were analyzed. It could be shown, that for an optimized design the spectral wavefront error can be significantly reduced compared to a standard beam splitter design [2].

The beam quality information of 3ω laser is very important to analyze the damage problem in the final optics assembly in the high power laser system. Sometimes there is a hot image whose intensity is several times of the average intensity in the near field. It can easily damage the optical components. However, it is very hard to detect the hot image online due to its small spatial size in a large aperture near field, which is usually about 100μm or smaller. Here we propose a method to detect and evaluate the possibility of the hot image in the online experiment. It is based on the near field information of multiple resolutions. The hot image intensity can be deduced from the near filed modulation variations if there is any hot image in the near field. The method could give some references to the online near filed beam quality measurement and system damage diagnosis in the final optics assembly.

The defect densities controlling the LIDT of three HfO₂ films with different underlying interfaces were measured using STEREO-LID. This technique measures the actual damage fluence during a 1-on-1 test. The films were tested with pulses of ~10 ns duration at 1064 nm. The 30-nm HfO₂ films were prepared by ion-beam sputtering: the first was deposited directly on a fused silica substrate; the second was deposited after first laying down a half-wave buffer layer of SiO₂; the third was deposited on a half-wave SiO₂ buffer with a gradual transition to HfO₂. The buffer layer reduces the density of defects triggering damage at low fluence by more than a factor of two, but the gradual interface slightly adds to the defect density. The implications of these results are compared to the damage behavior of a thicker (quarter-wave) HfO₂ film.