ISO standards are periodically reviewed to ensure their relevance to the best industry practices. Significant advancements have been observed in laser source development and related technologies over the last two decades. These advancements encompass new irradiation regimes, ranging from ultrashort pulses to kW-class continuous wave irradiation, with substantially increased peak- and average laser power levels. This new reality also necessitates the adaptation of pertinent laser damage testing standards. As high-power laser applications introduce optical elements with unique failure mechanisms and size constraints, there is a growing need for the introduction of alternative testing methods. In this paper, we provide a brief overview of recent standardization efforts undertaken by ISO TC 172 SC 9 WG 1 for the revision of the ISO 21254 series standards - ”Lasers and laser-related equipment — Test methods for laser-induced damage threshold”. Specifically, we discuss the need for the extension of ’classical’ damage criteria, the introduction of alternative test procedures, and possible improvements in interrogation methods and analysis. The overarching goal of this paper is to promote transparency in the standardization process and inspire discussion, ultimately leading to the enhancement of accuracy and reliability in laser damage testing.
Recent studies suggest that fatigue effect in dielectric optical coatings is possibly associated with the presence of strong nonlinear absorption, however, up to now there was only indirect evidence for such hypothesis. One of the reasons for that was a technical rigor to characterize nonlinear absorption losses in optical coatings and a lack of pertinent experimental data. Recent advancement of common-path interferometry and LIDT testing allows us to overcome such limitations. In this study we examine nonlinear response and fatigue effect in single- and multilayer dielectric coatings below single shot damage threshold. Although there is no quantitative model that could predict fatigue from absorptance, we found an interesting correlation between nonlinear absorption and fatigue effect under comparable experimental conditions. These results help us to understand the mechanism of fatigue in optical coatings and possibly make more durable femtosecond optics.
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.
Anti-reflective (AR) coatings are frequently used for imaging and laser-related applications, however, their highest possible output power is limited by laser-induced damage threshold (LIDT). LIDT data for AR-coated nonlinear crystals is very limited, especially when it comes to pulse temporal scaling laws. Therefore, we investigated four identical lithium triobate (LBO) crystals deposited with the AR@1064+532 nm coating on one side and the AR@355 nm coating on the opposite side. Numerous tests were conducted following 1-on-1 and S-on-1 testing protocols at UV as well as IR wavelengths in the pulse duration range of 50 fs - 9 ns. To interpret experimental findings, we analyze empirical pulse temporal scaling laws of LIDT for distinct failure modes. Furthermore, we also considered numerical analysis based on rate equations.
Absorptance is often considered a static feature of an optical element that is determined via standardized measurement procedures. Although such measurements are often performed using optical instruments with low light intensity, in high power laser applications irradiation conditions are considerably different. Optics might become unstable due to highly intense light: optical properties change in a nonlinear way and might eventually lead to laser-induced damage. To study these effects we employed the common-path interferometry technique in combination with a high energy and high average power laser source, operating at 1 MHz repetition rate and delivering 10 ps pulses at 355 nm wavelength. We investigated an anti-reflective (AR@355 nm) coating deposited using ion beam sputtering on a lithium triborate (LBO) crystal. Our preliminary results indicate both strong nonlinear absorptance and fatigue near the damaging fluence, however, damage events were not directly related to the critical absorptance level. An attempt is made to predict the lifetime of an AR coated optics by establishing a numerical model of nonlinear absorption.
This year’s competition is a continuation of the 2020 thin film damage competition conducted with nanosecond (ns) laser pulses and aimed to survey state-of-the-art visible high reflectors in the femtosecond (fs) pulse length regime. The requirements for the coatings were a minimum reflection of 99.5% at 0 degrees incidence angle at 532 nm. The fs-laser source available for testing in 2021 delivered pulses at 515 nm; as a result, to accommodate both ns- and fs-damage tests on the same samples, the latter tests were conducted at an incidence angle of 25 degrees to re-center the reflection spectral band of the coatings, i.e., a blue shift from 532 nm to 515 nm. The choice of coating materials, design, and deposition method were left to the participants. Laser damage testing of samples received in 2020 and new sample submissions in 2021 was per- formed at a single testing facility using the S-on-1 standardized test protocol with a 200-fs pulse length laser system operating at 5 kHz in a single-longitudinal mode. A double-blind test assured sample and submitter anonymity. The damage performance results (LIDT), sample rankings, details of the deposition processes, coating materials and substrate cleaning methods are shared. These experiments enabled not only direct comparison among the participants but also the performance contrast in two different pulse length regimes. We found that different deposition methods and coating materials yield comparably high laser resistance in the fs-pulse regime. Namely, hafnia/silica, hafnia/alumina/silica or tantala/alumina/silica multilayer coatings were the most damage resistant under exposure to fs-laser pulses within the coating deposition groups. In contrast, hafnia/silica coatings by e-beam were identified as best performers in the ns-pulse regime.
The laser-induced damage threshold (LIDT) of anti-reflective (AR) coatings is a limitation when it comes to the maximum laser power. Furthermore, LIDT data for AR coatings on nonlinear crystals are rather scarce in scientific literature, mainly available for ns pulse durations. Lately, however, ps and fs pulse durations are also becoming important. Thus, in this work, four LBO crystals were IBS coated on both sides with two distinct coatings, namely, AR@1064 nm and AR@355 nm. These coatings were then LIDT tested for both UV and IR wavelengths with ns, ps, and fs pulses.
When discussing laser-induced damage phenomena, the main parameter of interest is the laser-induced damage threshold (LIDT). Since LIDT is a function of irradiation time (or number of pulses), its characterization is of critical importance when designing reliable industrial or medical laser systems or even planning long-term space missions involving high power lasers. Within laser-induced damage community, decrease of LIDT with increase in irradiation time (the so-called fatigue effect) is often estimated by using the S-on-1 test procedure described in the ISO 21254-2 standard. However, due to measurement limitations, S-on-1 tests are usually carried out for relatively small numbers of pulses, therefore additional extrapolation methods must be used in order to predict lifetime of optical components. The simple extrapolation procedure provided by the ISO 21254-2 standard no longer meets the demands of the community as it ignores existence of multiple failure modes and data censoring resulting from different damage detection techniques, therefore a new approach for LIDT extrapolation is much needed.
In our previous work [1], we explored application of accelerated lifetime testing (ALT) techniques on S-on-1 test data. ALT approach treats S-on-1 experiments as sets of lifetime (or time to failure) distributions at different fluence levels instead of damage probability curves at different numbers of pulses. We have shown that this approach, combined with Bayesian inference and Markov chain Monte Carlo (MCMC) sampling, is well suited for extrapolation and uncertainty evaluation of S-on-1 experiments. However, proper application of these methods require empirical knowledge about lifetime distributions for different material types and irradiation parameters. Therefore, in this work an attempt is made to experimentally characterize lifetime distributions of highly reflective dielectric and metallic laser optics at both nanosecond and femtosecond pulse durations. UV and IR wavelengths as well as influence of irradiation fluence and polarization state are also considered. The results of this study provided insights into lifetime distributions of laser-induced damage and helped to shape guidelines for extrapolating S-on-1 experiments to longer irradiations.
[1] L. Smalakys, A. Melninkaitis, Predicting lifetime of optical components with Bayesian inference, Opt. Express 29, 903-915 (2021).
The standardization and comparison of laser-damage protocols and results 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 in a round-robin experiment involving five well-equipped damage testing facilities. Investigations were conducted at the wavelength of 1 μm in the sub picosecond pulse duration 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 characteristics enable direct comparison of damage-threshold measurements between different facilities, with the difference in the measured values indicating systematic errors or other parameters that were not previously appreciated. The results of this work illustrate the challenges associated with accurately determining the damage threshold in the short-pulse regime. Specifically, the results of 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, sample homogeneity, and nonlinear beam propagation.
This year’s competition aimed to survey state-of-the-art visible high reflectors in the nanosecond pulse length regime. The requirements for the coatings were a minimum reflection of 99.5% at 0 degrees incidence angle light at 532-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 scanning test protocol with a 6-ns pulse length laser system operating at 100 Hz in a single-longitudinal mode. A double blind test assured sample and submitter anonymity. The damage performance results (LIDT), sample rankings, details of the deposition processes, coating materials and substrate cleaning methods are shared. We found that hafnia/silica multilayer coatings deposited by all five deposition methods surveyed were the most damage resistant within their group under these test conditions. In addition, hafnia/alumina/silica designs deposited performed second best.
In the previous year’s work, we have shown that reporting laser-induced damage threshold (LIDT) as a single number might be misleading without mentioning the testing details. Furthermore, we also demonstrated that the longitudinal mode structure of the laser pulse could also lead to some discrepancies in LIDT estimates. A direct comparison of most popular testing protocols, namely 1-on-1, S-on-1, R-on-1, and Raster Scan was made for dielectric mirror coatings. In this year's study, we extended our work to new experiments on transmissive optics, namely uncoated fused silica and AR coatings. As in the previous study we used similar irradiation conditions: both single longitudinal mode injection seeded pulses as well as non-seeded multimode pulses with comparable effective pulse durations. Samples were tested using both first (1064 nm) and third (355 nm) harmonics of nanosecond Nd: YAG laser. In addition to experimental work, we also added a theoretical part that includes the results of pertinent Monte Carlo simulations. The experimental results and simulations are directly compared and discussed.
Quality standards are the basis for establishing a common ground between both research institutions and commercial organizations that exchange quantitative information about product features and related issues. In the rapidly changing world, the field of photonics is at the forefront of innovation: research, development, and manufacturing of new types of optics and lasers take place daily. In this context, new important aspects and limitations of the current damage testing approach become apparent. Accordingly, the ISO family of standards (ISO 21254) related to laser-induced damage threshold testing is reviewed and updated regularly. Various inputs into laser-induced damage testing were collected over the years by Lithuanian laser-induced damage community, which is a long-time practitioner of the current and previous revisions of ISO standards. Within this presentation, an attempt is made to contribute to the tremendous work that has already been done. Two types of efforts are made. First of all, we seek to identify weak points, edge cases and areas that leave some space for various misinterpretations and systematic errors. Secondly, we propose possible solutions (with corresponding Monte Carlo simulations for validation) that are currently in practice at laboratories of Lidaris UAB and Laser Research Center of Vilnius University. The suggested inputs are mostly related to definitions of damage criteria, test procedures, analysis of results, selection of appropriate testing protocols and their parameters as well as data representation. By no means, our observations seek to diminish the value of present approaches. As imperfect as they are, current standards were the only appropriate reference for a long time and proved to be adequate in most cases. We believe that the idea of a single protocol that would address all the issues of the community is flawed (considering the wide gamut of optics and lasers), therefore different testing protocols should be viewed as tools to address different problems. We also hope that some of our quantitative inputs could bring more clarity to preconceptions about laser-induced damage testing and significantly improve current approaches.
Virtually all optical materials degrade over time when they are used in high average power or intensity optical systems. Extrapolation of optical components’ lifetime is crucial in such applications in order to avoid downtime or project failure. Measurements of the laser-induced damage threshold (LIDT) fatigue are usually done using the so-called S-on-1 test described in the ISO 21254-2 standard. The standard, however, suggests only rudimentary techniques for extrapolating LIDT, which are rarely used in practice, therefore the goal of this work is to provide a framework for analyzing LIDT fatigue data using well established methods of Bayesian statistics. Numerical S-on-1 experiments (assuming constant fatigue) were performed for cases of online detection, interval detection and offline detection. Appropriate lifetime distributions were determined and used to fit simulated data taking into consideration data censoring. Credible intervals of lifetime predictions were determined using Markov chain Monte Carlo (MCMC) technique and compared with results from multiple experiments. Finally, the Bayesian lifetime analysis method was compared with technique described in the ISO standard for cases of low and high defect densities.
Laser-induced damage threshold decreases when an optic is exposed to multiple laser pulses. In the femtosecond regime, the fatigue of dielectric materials is attributed to incubation of laser-induced lattice states. The goal of this study is to investigate sub-critical damage in a quantitative manner so that the lifetime of single-layer dielectric coatings could be predicted for femtosecond applications. Sub-critical damage was found to consist of localized nanogrooves (100 nm wide, 1-2 um long) oriented perpendicularly to laser polarization. Simple fatigue model was used to simulate fatigue curves which were in good agreement with experimental S-on-1 data for the catastrophic damage.
Nanoimprint lithography (NIL) is a technique suitable for the mass production of micro-optical elements using a mould. One drawback, however, is that the materials used in NIL have low laser-induced damage threshold (LIDT). Here, we present our results in the development of a series of high-LIDT organic-inorganic hybrid materials, and their application in NIL using moulds made by multiphoton lithography.
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.
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.
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.
Laser systems dedicated to space missions require durable and well-characterized optics, that could ensure long-term operation under high average output power. If any of optical elements in such system experiences light-induced fatigue, the performance of whole laser system suffers. Thus, any delayed failure of the optical element would also endanger the entire space mission as repairing optic in the orbit is rather complicated. Up to now, the ability to predict optic’s longevity required by space programs was difficult and expensive, because of limited experimental data and lack of validated prediction models and methods. In order to address this problem, Lidaris and ESA joined forces for a two years cooperation project ESPRESSO. The overall aim of the project is to carry out research and development work required for essential preparation of reliable longevity qualification procedure to evaluate high power laser optics with intended use in space applications. Main elements of chosen methodology for optics lifetime prediction is reported in this paper. The essence of the proposed method lies in online video detection, failure mode (damage mechanism) separation and subsequent search of appropriate extrapolation models and methods. Main experimental findings confirm the suitability of the suggested approach for prediction of laser-induced damage threshold (LIDT) from a limited set of data to the extrapolated higher number of incident laser pulses.
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.
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.
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.
A special dielectric edge filter extremely sensitive to any change in refractive indices, layer thicknesses and angle of incidence has been investigated using holographic pump-probe measurements at different intensity values. Different physical processes overlapping in time were found to occur, namely the Kerr effect, free- electron generation and their subsequent trapping. A numerical model was used to reproduce the experimental results and decouple these processes.
This paper summarizes our results of S-on-1 testing carried out over the last few years. Our experimental data sets were
taken with nanosecond laser pulse durations. An attempt was made to use the same scaling laws with femtosecond pulse
widths but it was not successful. The conclusion was made: there is no single model than can universally applied to all
kinds of survivability curves. We present this summary with a particular goal of making recommendations to those
involved in the periodic review of ISO 21254. A preliminary review of models, describing damage threshold evolution
with respect to incident laser pulses, is made.
Laser induced damage of optical coatings has been one of the most important targets during many decades of intensive research. Different techniques were used and explored with the aim to increase the resistance of multilayer systems to laser pulses. In this work, LIDT results of different “base” structures made by ion beam sputtering of Al2O3, SiO2 and their mixtures are presented, and further enhancement possibilities are discussed by applying additional layer structure using higher bandgap material – fluorides and glancing angle deposited SiO2.
Band-gap and refractive index are known as fundamental properties determining intrinsic optical resistance of multilayer dielectric coatings. By considering this fact we propose novel approach to manufacturing of interference thin films, based on artificial nano-structures of modulated porosity embedded in high band-gap matrix. Next generation all-silica mirrors were prepared by GLancing Angle Deposition (GLAD) using electron beam evaporation. High reflectivity (HR) was achieved by tailoring the porosity of highly resistant silica material during the thin film deposition process. Furthermore, the proposed approach was also demonstrated to work well in case of anti-reflection (AR) coatings. Conventional HR HfO2 and SiO2 as well as AR Al2O3 and SiO2 multilayers produced by Ion Beam Sputtering (IBS) were used as reference coatings. Damage performance of experimental coatings was also analyzed. All-silica based GLAD approach resulted in significant improvement of intrinsic laser damage resistance properties if compared to conventional coatings. Besides laser damage testing, other characteristics of experimental coatings are analyzed and discussed – reflectance, surface roughness and optical scattering. We believe that reported concept can be expanded to virtually any design of thin film coatings thus opening a new way of next generation highly resistant thin films well suited for high power and UV laser applications.
Laser resistance of optical elements is one of the major topics in photonics. Various routes have been taken to improve optical coatings, including, but not limited by, materials engineering and optimisation of electric field distribution in multilayers. During the decades of research, it was found, that high band-gap materials, such as silica, are highly resistant to laser light. Unfortunately, only the production of anti-reflection coatings of all-silica materials are presented to this day. A novel route will be presented in materials engineering, capable to manufacture high reflection optical elements using only SiO2 material and GLancing Angle Deposition (GLAD) method. The technique involves the deposition of columnar structure and tailoring the refractive index of silica material throughout the coating thickness. A numerous analysis indicate the superior properties of GLAD coatings when compared with standard methods for Bragg mirrors production. Several groups of optical components are presented including anti-reflection coatings and Bragg mirrors. Structural and optical characterisation of the method have been performed and compared with standard methods. All researches indicate the possibility of new generation coatings for high power laser systems.
Time resolved digital holography (TRDH) is a versatile tool that provides valuable insights into the dynamics of femtosecond damage initiation by providing spatiotemporal information of excited material. However, interpreting of TRDH data in thin film dielectric coatings is rather complicated without appropriate theoretical models that are able to correctly describe underlying nature of damage formation. Therefore, a model based on finite difference time domain (FDTD) method with complete Keldysh theory for nonlinear ionization of atoms and multiple rate equation (MRE) method for conduction band electrons was developed. The model was used to reproduce both temporal and spatial characteristics of TRDH experiment performed on Ta2O5 dielectric coating. Fitted material parameters were then applied to indirectly estimate LIDT of the coating.
Time-resolved investigations of laser-matter interaction processes in dielectric coatings and bulk silica leading to laserinduced damage were performed with high temporal and spatial resolution. Distinct excitation geometries were used to study different aspects of laser matter interaction. Samples were irradiated at the pump fluence levels below and above their single shot laser-induced damage thresholds. The obtained results provide new insights about the sequence of interdependent processes. The fundamental differences between the so called 1-on-1 and S-on-1 damage morphologies are observed and discussed. New data of numerical simulations revealing the nonlinear properties of optical thin films are presented. Increased visibility in time resolved damage detection as well as observation of coherent oscillations in measured signals are introduced and discussed.
Damage induced by nanosecond laser in optical materials can often be attributed to the presence of laser damage precursor in the material. The presence of these precursors within dielectric optics can be successfully described by so called distributed defect ensembles. The physical parameters of these precursor presence models can be deduced by fitting experimental laser damage probability data. For a degenerate defect ensemble these parameters are the precursor threshold and the precursor density in the sample. To deduce precursor densities correctly it is essential to consider the real shape of laser beam that often deviates from Gaussian or hat-top models. To address these issues we discuss a new fitting procedure that minimizes significant errors in the deduced model parameters using experimental beam profile images. We suggest two methods: Defining a Gaussian replacement beam or using a numerical approximation of the surface over threshold (SOT) of the real beam. Both methods are discussed at the example of a degenerate damage precursor population but apply to any type of damage precursor population.
Presented study addresses the nano-size defects acting as damage precursors in nanosecond laser pulse irradiation regime. Defects embedded within the surface of glass are investigated in terms of defect ensembles. Damage frequency method and raster scan procedure are directly compared on the set of two samples: uncoated fused silica substrates and SiO2 monolayer films. The extracted defect ensembles appear to be different from each other. The limitations of compared methods such as pulse-to-pulse variation of laser intensity and sample contamination induced by laser ablation were identified as the main causes of observed differences.
Laser-induced damage is defined as any permanent laser-induced change in the characteristics of a sample. This change can be observed by many different inspection techniques, with different sensitivity, depending on the intended objectives and available techniques. The damage threshold definition and measurement are therefore very subjective and related to the detection method. The choice and implementation of a damage test system is then a critical issue on any experiment. In this work we present some implementation of detection techniques for laser damage metrology in the sub-picosecond regime. Different damage testing methods that have been applied will be discussed in view of their potential applications for testing functional optical components or to study physical process in the femtosecond regime, particularly the role of defects: optical microscopy, phase microscopy and time-resolved microscopy.
Nanosecond laser - induced damage threshold (LIDT) of dielectric coatings is limited by absorption of nanometer sized defects inherent to optics manufacturing process. Herewith theoretical and experimental efforts were made in order to characterize internal damage thresholds of defects introduced during substrate polishing and coating deposition processes. For this purpose LIDT testing was performed under UV (355 nm, 4.8 ns) irradiation on three different types of samples by varying irradiation conditions such angle of incident (0°, 45°, 56°) and polarization (s, p). Experimentally obtained damage probability curves were analyzed numerically by employing model considering relative electric field distributions and randomly distributed defect ensembles attributed to distinct manufacturing processes. An attempt is made to identify the layers with the weakest optical resistance.
Nodular defects tend to limit laser-induced damage threshold (LIDT) of multilayer dielectric coatings frequently used for laser applications. Cross-sections of localized damage morphologies correlate well with light intensifi- cation patterns caused by defect geometries. In vast majority of studies electric field enhancement in nodular defects was investigated for infrared spectral region. In this work theoretical analysis has been extended for IR - UV range. Light intensification in HfO2/SiO2 multilayer mirror coating was studied numerically. The analysis of obtained results indicates that phenomena is very sensitive to almost every investigated parameter. It was also found that field enhancement effect can be reached within distinct material layers (either of low or high refractive index). The discussion and insights complementing existing knowledge on nodular defects were made.
We characterize laser-induced damage threshold (LIDT) in transparent photopolymers by a sub-ps laser pulses of 515 nm wavelength representing case of high light intensities. Five different photopolymers (SZ2080, OrmoComp, SU-8, PDMS and PMMA) widely used in the laser lithography are investigated. The relationship of the damage threshold and optical band-gap energy of the polymers indicating possible damage mechanism is considered. Incubation model validating damage threshold dependence on the number of laser pulses is studied as well. The obtained characteristic values of LIDT reveal potential of photopolymers and their possible applications in high power laser systems.
In the determination of the laser-induced damage threshold (LIDT) of optical coatings former Round-Robin experiments
stand as the empirical foundation for the development of the International Standard as it is known today. In 1983 and
1997 such experiments were conducted at the fundamental wavelength of the Nd:YAG laser under atmospheric
conditions settling the International Standard as it is known today.
To cope with the growing demand of LIDT testing for satellite missions, existing test methods have to be extended to
deal with operation in space-like environments. This requires LIDT measurements performed under customized vacuum
conditions to validate the laser resistance capability and estimate the life time of optical components. To foster the
quality of measurements in such environments the need for an inter-laboratory comparison in vacuum conditions
emerged.
In this study influence of temporal effects are investigated within a context of laser-induced damage threshold
(LIDT) measurements. 1-on-1 LIDT testing has been performed with laser operating in single- and multilongitudinal
mode regimes. Four fused silica samples were chosen for investigation. Qualitative differences in
the damage morphology and damage probability curve have been observed. Analysis of these phenomena was
performed by employing Monte Carlo simulations representing the statistical interaction between laser irradiation
and randomly distributed damage precursors. The results and findings of this study are reported and discussed
in detail.
In the past years the usage of mixed oxides coatings lead to an important improvement of laser damage threshold and
quality of optical elements. In this study influence of post treatment procedure - ex-situ annealing - is examined in terms
of quality, optical constants and laser induced damage threshold (LIDT) of mixed HfO2 and SiO2 coatings. Monolayer
thin films containing different fractions of HfO2 are deposited with ion beam sputtering technology (IBS.) All samples
are post annealed at different temperatures and optimal regimes are defined. Refractive index and absorption coefficient
dispersion is evaluated from transmission spectra measurements. Surface roughness of all samples is characterized
before and after deposition and annealing, using atomic force microscopy (AFM). Microstructural changes are identified
from changes in surface topography. Further, optical resistance was characterized by 5.7 ns duration pulses for 355 nm
wavelength laser radiation, performing 1-on-1 sample exposure tests with high resolution micro-focusing approach for
monolayer samples and S-on-1 tests for multilayer reflectors. Morphology of damaged sites was analyzed through
optical microscopy. Finally, conclusions about annealing effect for mixed HfO2 and SiO2 monolayer and multilayer
coatings are made.
In this work we report an experimental investigation of subsurface damage (SSD) in conventionally polished fused silica
(FS) substrates which are widely used in laser applications and directly influence performances of optical elements.
Two procedures were developed: 1 - acid etching and 2 - superpolishing. Additionally, surface roughness and total
integrated scattering (TIS) measurements were performed to find correlation between the main surface properties and
laser induced damage threshold (LIDT) as circumstantial evidence of elimination of SSD.
Different durations of acid etching have been used to study LIDT of FS substrates. These experiments revealed that the
optimal etching time is ~1 min. for a given acid concentration. Laser induced damage threshold of etched and SiO2 layer
coated FS samples increased ~3 times, while of the ones that were not coated - 4 times. It has been revealed that for nonetched
surface a single nano- to micro-scale absorbing defect ensemble most likely associated with polishing particles
within Beilby layer was dominant, while damage morphology in ~1 min etched FS sample had no point defects
observed.
More than 5 times lower roughness value (RMS) was obtained by superpolishing procedure using colloidal silica
abrasive particles. LIDT of such superpolished fussed silica substrates was also strongly increased and compared with
conventional CeO2 abrasive polishing.
An imaging of strongly excited thin film dielectric coating is done by the means of femtosecond time-resolved off-axis
digital holography (TRDH). Ta2O5 single layer coating have been investigated at different time moments in transmission
mode. The evolving damage process was recorded in series of microscopic amplitude and phase contrast images.
Different processes were found to occur and namely: Kerr effect, free-electron generation, ultrafast lattice heating and
shock wave generation. The trends in electronic contribution are qualitatively reproduced by the theoretical model while
the other effects require additional studies.
Mixed metal oxide coatings of ZrO2 / SiO2 , and Nb2 O5 / SiO2 as well as films of pure SiO2 , ZrO2 and Nb2 O5 have been
prepared by the Ion Beam Sputtering (IBS) technique and characterized on their physical properties. The Laser-Induced
Damage Thresholds (LIDT) of these samples have been measured at 1064 nm with the 1-on-1 mode in the nanosecond
regime. The optical resistance results obtained from laser damage probability curves indicate a decrease of the LIDT in
both sets of the mixtures when the content of the high index material is increased. By comparing the LIDT to the
bandgap values (Eg) that have been measured, a dependence of LIDT to Eg is evidenced. Following these results,
comparisons are made with the case of 500fs LIDT that have been measured on the same samples. It is found that the
same behavior is observed on both cases (ie LIDT dependence with Eg). Discussions are then conducted on the possible
physical mechanisms to explain the results and it is found that in the case of the nanosecond regime, the LIDT
dependence on Eg can be explained by a critical temperature reached during the laser damage process that exhibit a
dependence on the band-gap of the material. The influence of the film thermal conductivity on the values of the critical
temperature is studied.
In this study the applicability of commonly used Damage Frequency Method (DFM) is addressed in the context
of Laser-Induced Damage Threshold (LIDT) testing. A simplified computer model representing the statistical
interaction between laser irradiation and randomly distributed damage precursors is applied for Monte Carlo
experiments. The reproducibility of LIDT predicted from DFM is examined under both idealized and realistic
laser irradiation conditions by performing numerical 1-on-1 tests. A widely accepted linear fitting resulted in
systematic errors when estimating LIDT and its error bars. For the same purpose a Bayesian approach was
proposed. A novel concept of parametric regression based on varying kernel and maximum likelihood fitting
technique is introduced and studied. Such approach exhibited clear advantages over conventional linear fitting
and led to more reproducible LIDT evaluation. Furthermore LIDT error bars are obtained as a natural outcome
of parametric fitting which exhibit realistic values. The proposed improvements are of practical importance in
LIDT metrology.
In this paper new laser-induced damage threshold testing system operating in broad range of pulse repetition
rates (from 0.02 Hz up to 200 kHz) is introduced. The system is capable to test either bare or coated optical
components, used for high average and peak power femtosecond laser applications. Pulses of tunable duration
(300 - 5000 fs) from diode pumped Yb:KGW solid state laser are employed at fundamental wavelength (1030 nm)
and its II-IV harmonics (515 nm, 343 nm and 258 nm). Thanks to advanced adaptive damage detection technique
so called S-on-1 tests are performed with single shot resolution. The capabilities of the system were characterized
and demonstrated on niobia and zirconia - single layer dielectric coatings at different repetition rates.
Despite the growing improvement in optical polishing and deposition technologies optical resistance of the laser
components used for high-power UV applications remains insufficient in many cases. In this study influence of different
fused silica substrate preparation, post treatment processing and deposition techniques are examined in terms of surface
roughness, optical scattering and laser damage performance. The conventional techniques of polishing, etching, and
finally surface cleaning of substrates have been investigated. Further, a part of samples were also coated with SiO2
monolayer by Ion Beam Sputtering (IBS) technique. Surface quality was characterized prior to and after the treatment
and deposition processes by the means of total integrated scattering (TIS) and atomic force microscopy (AFM). The
experimental results of surface roughness measurements exhibited a good correlation between AFM and TIS methods.
Further optical resistance was characterized with 10 ns duration pulses for 355 nm wavelength laser radiation performing
1-on-1 sample exposure test with high resolution micro-focusing approach. A dominating damage precursor ensembles
produced during manufacturing processes were identified and directly compared. Finally, the conclusions about the
quality influencing factors of investigated processes were drawn.
Group III nitrides are wide band-gap semiconductors which are commonly used in high power and high frequency
electronics and optoelectronics. A rapid development of GaN/InGaN devices is in progress however many technological
improvements are still demanded. One of them is a convenient formation of electrical contacts attached to appropriate
layers. Currently a selective etching step of GaN and InGaN layers is performed by using quite expensive methods such
as plasma, chemical-lithographic or electron beam exposure. However, very little research has been done towards
investigation of an alternative selective laser etching possibility. Therefore in this work we study optical resistance and
damage morphology of thin film GaN and InxGa1-xN layers grown on sapphire substrates in the femtosecond regime.
Laser induced damage threshold (LIDT) tests were carried out in both S-on-1 and 1-on-1 regimes by exposing samples
from front (deposited) and rear (substrate) sides. For optical resistance testing a femtosecond Yb:KGW laser combined
with harmonic generator covering near IR spectrum to visible and UV was used. Experimental results of optical
resistance dependence on band-gap in InxGa1-xN layers with different indium concentration (X up to 22%) are presented.
Also detailed morphology study for different laser wavelengths is performed and discussed.
We report on the laser damage resistance of thin films prepared by Ion Beam Sputtering. The samples are fused silica
substrates coated with single layer films of pure oxides (SiO2, Nb2O5, ZrO2, HfO2, Ta2O5, Al2O3, Sc2O3) and oxide mixtures with various ratios (Nb2O5/SiO2, ZrO2/SiO2, HfO2/SiO2, Ta2O5/SiO2, Al2O3/SiO2 and Sc2O3/SiO2). For this study the LIDT of more than 60 different samples have measured at 1030nm with pulse durations of 500fs with single pulse irradiation. The results are expressed and compared in terms of LIDT as a function of the measured band gap
energy and refractive index. For simple oxide materials a linear evolution of the LIDT with bandgap is observed, with the exception of Sc2O3 material where a very high damage threshold is observed, compared to other high index materials. In the case of mixtures, a more complex behavior is evidenced.
We report on the fabrication of the minimized conventional microoptical components out of the hybrid organic-
inorganic SZ2080 and SG4060 photoresins using laser direct writing technique. An ascending laser focus multiscan
approach is introduced as a method for the structuring of 2D nanolines. The diameters and heights of the
nanolines are comparable to the ones written with the electron beam lithography. Using our proposed laser
direct writing approach one can write 3D microstructures with the 2D nanofeatures in a single step procedure.
As demonstration of this technology, microlenses with 1D, 2D and circular transmission gratings were fabricated.
Additionally, for the rst time, ISO certied laser-induced damage testing was applied to determine the optical
breakdown threshold of the SZ2080 photoresin used for the laser direct writing.
We report an experimental investigation in the laser-induced damage threshold (LIDT) of optical coatings materials. The
samples are single layers of Al2O3, Nb2O5, HfO2, SiO2, Ta2O5, ZrO2 deposited through different deposition techniques
(evaporation or sputtering with/without ion assistance) and mixtures of Al2O3/SiO2, Nb2O5/SiO2, HfO2/SiO2, Ta2O5/SiO2 and ZrO2/SiO2 on silica substrates. The LIDT is measured at 1030nm, 500fs in single shot mode. The results are
expressed and compared in term of LIDT as a function of bandgap and LIDT as a function of refractive index.
In the ultra short laser pulse regime, the damage process is driven by the interaction of the laser pulse with the electronic
structure of the material. The way of excitations in dielectric materials is dominated by multi photon and avalanche
ionization processes. Often, the complete theoretical description is limited by the lack of knowledge of the precise
material properties. Usually, LIDT measurement data are only available for pure materials (e.g. TiO2, Ta2O5 or SiO2).
The development of composite materials opens the way to vary material properties, continuously. Additionally, all
material changes are based on the same chemical elements in different compositions.
The paper compares measurement results of the University of New Mexico and Vilnius University performed on the
same set of TixSi1-xO2-mixtures to calculations based on Keldysh theory. When applying simple approximations for the
physical properties of the mixture, the theoretical description agrees well with the measurement results.
Single shot LIDT of single layer coatings of different deposited materials (SiO2, HfO2, Ta2O5 and Nb2O5) have been
studied. We report dependence of the damage threshold with different operational and material parameters (pulse
duration, nature of the deposited material, deposition process or thickness of the layer). For interpretation a model
dedicated to optical coatings and based on the conduction band electron rate equation is used. The simulations are
compared to experiments. The theoretical approach is in good accordance to the experimental data.
In this study, we report on our recent progress in research of single layer mixed zirconia-silica and niobia-silica
composite coatings prepared by Ion Beam Sputtering technique. All coatings of the same optical thickness
were characterized in terms of reflection/transmission spectrometry, X-ray diffraction, atomic force and optical
microscopy, optical back-scattering and optical resistance (laser-induced damage threshold - LIDT) in subpicosecond
mode. The optical resistance, TIS and LIDT results reveal clear dependence on high refractive index
material content in composite coating and its crystalline structure. The results are interpreted and discussed
by the means of different models available in literature.
Digital holography (DH) technique allows obtaining quantitative amplitude- and phase-contrast images. However,
there are only several attempts to use advantages of digital holography in time resolved manner. Herewith
we demonstrate that combination of conventional off-axis DH and ultrashort probing laser pulses results in
versatile tool suitable for ultra-fast phenomena studies. We demonstrate its applications in spatial-temporal
characterization of ionizing pre-damage propagation of femtosecond pulses in transparent material as well
as characterization of post-damage ablation processes in metals. Phase-and amplitude-contrast images are
obtained in single-shot mode with the best to our knowledge temporal resolution of ≈ 30 fs.
Laser-Induced Damage Threshold (LIDT) measurements are typically performed in order to characterize the optical
resistance of laser components. However the sensitivity of online damage detection techniques is often limiting factor for
the accuracy and reproducibility of so called S-on-1 and 1-on-1 measurements. In fact the sensitivity of damage detection
has the biggest impact to precision of these experiments. In this paper we describe the idea of making improvements on
scattered light registration based damage detection. It was learned, that scattered light intensity is linearly proportional to
incident energy of laser pulses while material is not damaged. However in case of induced damage the linear
proportionality becomes nonlinear: this feature is used in order to detect optically induced surface and bulk changes in
material. According to the base of those theoretical considerations the adaptive scattering detector was proposed and
made up. It was put into practice by projecting it on discrete element schematics. The improved sensitivity of laserinduced
damage detection was reached. This technique helps to avoid degradation of damaged site and pollution of
surrounding area due to laser ablation during the S-on-1 tests since it allows blocking the repetitive irradiation
immediately when damage appears. Numerous tests were made, that shows, that adaptive scattering detector can
precisely detect damages in their initiation state, independently from material ability of light scattering. Calibration of
this detector can be automated, therefore the influence of human factor is minimized. This fact opens up the possibility to
run whole damage threshold measurement procedure automatically.
Many investigations in laser induced damage thresholds (LIDT) have shown a very deterministic behavior for the ultra
short pulse regime. From the current understanding, femtosecond laser damage is driven by electronic photon-matter
interaction. This process can be theoretically described by photo-, and avalanche-ionization and the respective relaxation
processes. On the basis of a wavelength dependency of the laser-induced damage threshold of titania, a dominant
influence of multi-photon absorption (MPI) on the damage behavior could be demonstrated. This particular
characteristic could be observed in a stepwise increase of the LIDT in the transition ranges of the orders of multi-photon
absorption.
This paper presents an analysis of the wavelength dependence of femtosecond LIDT with different theoretic models and
a comparison of simulated data with the measured wavelength dependence of titania. Both, the wavelength position and
the quantitative change of the laser power resistance in the range of the n to the n+1 photon absorption are calculated in
a theoretical analysis. Additionally, the influence of different microstructures of titania on the quantized MPIcharacteristic
is investigated.
Determination of absolute Laser-Induced Damage Threshold (LIDT) value from experimentally obtained statistical data
is still very important metrological problem in terms of accuracy and repeatability. In fact experimentally estimated
LIDTs are always affected by many factors such as (temporal, energy, pointing and beam shape) stability of laser pulses
used for damage testing as well as properties (homogeneity and limited size) of the sample to be tested. These problems
are especially important in case of small aperture limited samples when testing with nanosecond pulses where the
mechanism of damage is usually driven by defects. Several known experimental techniques or its modifications (for
example raster scan) are typically applied for LIDT estimation in 1-on-1 mode namely Damage Frequency Method
(DFM) and General Binary Search Technique (GBST). Almost all methods lead to the same value of LIDT under ideal
experimental conditions and sufficiently large number of interrogated test sites. The goal of this numerical study is to
analyze the performance (accuracy and repeatability) of above mentioned algorithms with respect to damage limiting
surface defect density under non-ideal experimental conditions: energetic instabilities of laser radiation and aperture
limited to max. 300 sites. Herewith we also introduce "moving average" measurement concept. The conclusions are
drawn about the precision of all above mentioned methods.
Experimental investigation of the laser conditioning efficiency by nanosecond pulses at 266 and 355 nm in high
reflectivity mirrors used in optical parametric oscillators (OPOs) is present in this report. The high reflection coatings
were deposited on the fused silica substrates. The materials used for e-beam coating deposition were ZrO2 and SiO2.
Laser conditioning was investigated as function of number of pulses, wavelength and conditioning protocol. Ramped-fluence
pre-exposure was used as a method to explore optimal improvement to the damage performance at 266 and
355 nm. No conditioning effect was observed using nanosecond pulses at 266 nm, but the mirror conditioning with
355 nm pulses increased the laser-induced damage threshold (LIDT) by 2.5-3 times. The experimental results support the
concept that the laser conditioning effect observed in high quality optical thin films is associated with intrinsic electronic
defects in the films.
Laser-induced damage threshold determination as a function of the number of incident pulses on a specific optic is a
classic problem in laser damage studies. There are several models of the fundamental mechanisms explaining the fatigue
laser damage behavior including temperature accumulation and changes of electronic or chemical material structure.
Herewith we discuss the effects of unstable laser radiation on
S-on-1 laser-induced damage probability. Numerical
simulations of S-on-1 measurements for specific cases of defect densities, spot sizes and beam jitters are performed. It is
demonstrated that the statistical effects of "pseudo-accumulation" reasoned by unstable laser radiation in transparent
dielectrics containing nanometer sized defects leads to accumulation-like behavior. The magnitudes of the random beam
walking and the energy fluctuations are directly related to the damage probability. Experimental results are also
introduced to illustrate the theoretical results.
Femtosecond laser micromachining of grooves in the SiO2 coated crystal silicon is investigated using 300 fs laser pulses
at a center wavelength of 1030 nm. A novel chirped pulse amplified femtosecond Yb:KGW laser source (Pharos, Light
Conversion, Lithuania) with high pulse repetition rate of 1- 350 kHz and high average power up to 8 W is employed. The
ablation depth of grooves as a function of pulse repetition rate, number of passes over the same groove, and the light
polarization relative to the cutting direction is investigated. Different scanning algorithms as well as influence of the focal
plane height relative to the sample are investigated.
S-on-1 laser-induced damage threshold (LIDT) dependence on the pulse duration at two different wavelengths was
experimentally investigated in metallic and dielectric laser mirrors. LIDT's of high-reflective dielectric coatings made of
alternating λ/4 layers of TiO2/SiO2 and Ta2O5/SiO2 and those of protected metallic Au and Ag coatings were tested at
800 nm and 400 nm wavelengths with Ti:Sapphire laser pulses of 46 fs, 130 fs and 1.8 ps duration. S-on-1 measurements were performed according to international ISO 11254-2 standard using 10000 pulses/per site and compared with 1-on-1 measurements.
Various investigations show that damage threshold of optical coatings by intense ultrashort laser pulses is closely related
to the intensity of electric field at layer interfaces. LIDT measurements of high reflectance optical coatings using
femtosecond pulses at 800 nm wavelength are presented. ZrO2, HfO2 and Ta2O5 as high refractive index materials for two sets of experiments were chosen. Two different coating designs were investigated: standard quarter-wavelength design with SiO2 overcoat and modified "E-field" non quarter-wavelength design with suppressed electric field. Damage sites were studied using optical and AFM microscopes. Relation between electric field distribution and damage
morphology was observed. The results demonstrate, that suppressing electric field at layer interfaces enables to increase
LIDT for high reflectance coatings almost twice if compared to standard quarter-wavelength design when using
ultrashort laser pulses. However electric field distribution is sensitive to variations in thicknesses of outer layers, so
deposition process should be precisely controlled to get improvement in LIDT of coatings.
A modified IBS-process was used to create mixtures of oxide coating materials. The process allows to manufacture new
designs, whereas the important optical and electronic properties of the material can be varied in a wide range. Especially
for ultra short pulse applications, higher damage thresholds can be achieved.
In this paper, LIDT measurements of mixed and pure single layers are presented. The coatings were investigated at
different wavelengths and in a wide pulse duration range. The results of the measurements confirm the empirical law of
the linear LIDT dependency on the absorption gap. Based on this empirical law, the Refractive Index StEps Down
(RISED) concept was developed. From the data of the single layer measurements, an optimization of RISED optical
components in the fs-regime will lead to even higher damage thresholds. Particularly, for high reflecting mirrors the
damage threshold could be doubled for different dielectric coating materials. Additionally, the paper presents a
theoretical analysis of the stack LIDT on the basis of the single layer properties.
This investigation was aimed at optimization of optical properties, stability and radiation resistance of optical coatings
deposited using the standard vacuum coating plant equipped with the ion source for ion assisted deposition. There are
some reports showing that porous dielectric coatings are more resistant to intense laser radiation, however they have
smaller environmental stability than denser coatings, which are more sensitive to laser radiation. The influence of
important technological parameters (deposition rate, substrate temperature, energy of ions) on optical properties and
radiation resistance of high reflection dielectric coatings based on Nb2O5/SiO2 and Ta2O5/SiO2 in VIS spectral region is
presented.
A quest for higher laser powers is one of the main driving forces in development of laser technology. Unfortunately all
laser components have some limit to the intensity of optical radiation that can be applied on them - the so-called laser-induced
damage threshold (LIDT). To enable further power scaling of laser devices, novel highly resistant optical
components have to be developed. Such components are laser crystals, mirrors, fibers and other components typically
coated with periodic dielectric layers made using e-beam, sputtering or sol-gel technologies. The production materials
and methods of all the mentioned optics are under constant development, which requires a reliable quality test to provide
the feedback to the manufacturing process; one of such tests are the measurements of LIDT. LIDT measurement
procedure using repetitive laser pulses, as described in ISO 11254-2 standard, is time- and human resource consuming, if
performed without automation. We developed an automated station for the measurements of LIDT that greatly reduces
the required human resources and allows fast data collection. In this presentation, we briefly describe the main
components of this automated LIDT test station. Furthermore we present the comparison of the latest results obtained on
LIDT measurements of ZrO2/SiO2, Nb2O5/SiO2, Ta2O5/SiO2 and TiO2/SiO2 periodic high reflecting dielectric layers
performed using repetitive nanosecond laser pulses.
Polymethyl methacrylate (PMMA) is a versatile polymeric material that is well suited for fabrication of many commercial
optical components: lenses, fibers, windows, phase waveplates and others. Our focus is achromatic zero-order waveplates
made of anisotropic PMMA which can be used to modify the state of polarization of electromagnetic radiation. Such
waveplates have a broad range of application in devices where polarized radiation is used. For example, when tunable lasers
are used or when spectropolarimetric measurements are performed, one needs an achromatic waveplate providing a specific
retardation in a wide wavelength range. Herewith anisotropic properties of PMMA subjected to one-axis stretching are
analyzed and the technology for manufacturing such achromatic and super-achromatic, one-axis-stretched PMMA
waveplates is described. This technology excludes any mechanical processing of waveplate component surfaces. Technical
characteristics of achromatic and super-achromatic waveplates manufactured of PMMA including results of laser-induced
damage threshold (LIDT) measurements are discussed below.
In recent years, there has been a growing interest in further development of sol-gel method which can produce ceramics
and glasses using chemical precursors at relative low-temperatures. The applications for sol-gel derived products are
numerous. Department of General and Inorganic Chemistry with Laser Research Center of Vilnius University and
Institute of Physics continues an ongoing research effort on the synthesis, deposition and characterization of porous solgel.
Our target is highly optically resistant anti-reflective (AR) coatings for general optics and nonlinear optical crystals.
In order to produce AR coatings a silica (SiO2) sol-gel has been dip coated on the set of fused silica substrates. The
optical properties and structure of AR-coatings deposited from hydrolysed tetraethylorthosilicate (TEOS) sol were
characterized in detail in this study. The influence of different parameters on the formation of colloidal silica
antireflective coatings by dip-coating technique has been investigated. All samples were characterized performing,
transmission electron microscopy, UV-visible spectroscopy, atomic force microscopy, ellipsometric, total scattering and
laser-induced damage threshold measurements. Herewith we present our recent results on synthesis of sol-gel solvents,
coating fabrication and characterization of their optical properties.
High power laser systems are one of the most rapidly growing areas in the development of laser technology. This also
leads towards higher requirements for environmental stability of optical components and their resistance to laser
radiation. There are some reports showing that porous dielectric coatings are more resistant to intense laser radiation,
however they have smaller environmental stability than denser coatings, which are more sensitive to laser radiation.
The influence of important technological parameters (deposition rate, substrate temperature, energy of ions) on optical
and microstructural properties of high reflection dielectric coatings based on Nb2O5/SiO2, and Ta2O5/SiO2 in VIS spectral
region is presented.
Furthermore the LIDT measurements using repetitive nanosecond laser pulses of Nb2O5/SiO2 and Ta2O5/SiO2 high
reflecting optical coatings based on ISO 11254-2 standard are presented.
We report on laser-induced damage threshold (LIDT) and UV-laser excited defect formation measurements in large
aperture KDP crystals developed as doublers and triplers for mega-Joule laser. Measurements of LIDT were performed
according to the ISO 11254-2 standard for repetitive pulses with duration ~ 4 ns and repetition rate of 10 Hz. The results
for different laser wavelengths (1064, 532 and 355 nm) and polarizations are presented. The largest LIDT was observed
for 532 nm pulses and the 1064 nm wavelength had a strong dependence on laser polarization. The LIDT values at 532
nm and 355 nm also depended on the crystal cutting angle, which is different for doublers and triplers. A comparison of
LIDT with earlier reported crystal absorptance at different wavelengths is also performed.
The UV-laser induced defect formation was investigated by the means of pump-probe technique. The excitation was
performed with a single pulse of ns Nd:YAG laser (355 or 266 nm wavelength) and probing with another Nd:YVO4 laser
system (532 nm) operating at 1kHz. This gave us a temporal resolution of 1ms. The transient absorption of defect states
relaxed non-exponentially and fully disappeared in ~10 s. A comparison is made between crystal grown by distinct
growth methods and between different laser polarizations. An influence of laser conditioning on UV induced defect state
formation is also revealed.
All laser components can withstand a limited intensity of optical radiation and the measurement of laser-induced damage thresholds (LIDT) is required. In the case of repetitive pulses the LIDT measurements should be performed according ISO 11254-2 standard. This procedure is time consuming and puts a high requirement on human resources. In order to speed up the LIDT measurements with a minimal human resource we developed the automated station for LIDT measurements according ISO-11254-2 standard. In this paper we overview the main parts of this automated station and present the results of LIDT measurements with repetitive pulses. In order to control the LIDT measurements, software based on LabView programming package was created. The LIDT software controls experimental sample positioning in X and Y directions, laser pulse energy attenuation and shutter. It also automates damage detection and performs statistical analysis. The program recognizes damage by detecting scattered light from damaged surface. The input of sample and laser beam technical parameters is required to start the measurements. The minimal distance between test sites on the sample surface is calculated automatically, and the surface area is divided in to a hexagonal matrix. The program also chooses the laser pulse train energy for each test site. The program also allows fast damage inspection by translating the sample under the Nomarski microscope. After completion of measurement and damage inspection, the program automatically generates the measurement report.
Lithium triborate, LiB3O5 (LBO) is a popular nonlinear optical crystal typically used for frequency conversion. For power scaling of laser radiation in transparency range of LBO crystals with maintaining low reflection losses is very important to reach high optical resistance of anti-reflective dielectric coatings. The measurements of laser-induced damage thresholds (LIDT) of AR coated LBO used for second and third harmonic generation of Nd:YAG lasers were performed at 1064, 532, and 355 nm wavelengths for ~4 ns pulses. Two types of coatings where tested; a dual peak anti-reflection at 1064 nm and 532 nm, three peak anti-reflection at 1064 nm, 532 nm, and 355 nm. Ion beam sputtering and magnetron sputtering technologies were used for coatings deposition. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. The obtained LIDT were in range of 5-21 J/cm2.
An influence of substrate temperature and working gas in coating plant during evaporation process on the laser-induced damage threshold (LIDT) of high reflection dielectric coatings was experimentally investigated. Also a LIDT comparison of ion assisted deposition (IAD) and conventional electron-beam evaporation (non-IAD) coatings fabricated under the same substrate temperature (300 °C) was performed. A set of different type high reflection mirrors were tested for LIDT at 532 nm for 3.4 ns pulses: one type of non-IAD and six types of IAD evaporated at different substrate temperatures and different working gases. All coatings were made on BK7 glass substrates from ZrO2 and SiO2. The computer controlled test station for LIDT measurements according to the requirements of current ISO 11254-2 standard was used. All measurements were performed at 10 Hz pulse repetition rate (S-on-1 test). The tests were performed at fixed spot size. Strong LIDT dependence on substrate temperature of was observed.
Nonlinear absorption measurements at 800 nm and 400 nm in single wavelength high reflection (HR) dielectric mirrors were performed, according to the ISO 11551 standard by pulse, gradient and exponential absorption evaluation methods, using pulsed, diode pumped femtosecond laser system with pulse duration ~130 fs. Pulsed laser output at 1kHz repetition rate had 1 W and 0.36 W average power at 800 nm and 400 nm, respectively. The HR mirrors were made of ZrO2 and SiO2 layers. The beam was focused into the mirror, and changing the beam power by step attenuator, it was possible to evaluate nonlinear absorption at different intensities up to intensity close to damage threshold. The nonlinear absorptance for 400 nm pulses at the femtosecond pulse intensity 0.8 TW/cm2 was 0.48 % and ~20 times exceeded the nonlinear absorptance for the 800 nm pulses.
High density, improved adhesion and environmental stability are the main features of dielectric optical coatings produced using ion-assisted deposition (IAD) technology. However, investigations of resistance of IAD coatings to intensive laser radiation show controversial results. A series of experiments were done to examine the influence of ion gun operation on the transmittance of fused silica substrates. It was shown that operation of ion source introduced extinction in UV spectral range. Optical properties of single hafnia layers and multilayer dielectric mirrors deposited using conventional e-beam evaporation and different modes of IAD were investigated. Microstructural analysis using X-ray diffraction (XRD) measurements and AFM scanning of coated areas was carried out. Single hafnia layers deposited using high energy ion assistance had more amorphous structure with smaller crystallites of monoclinic phase. High reflection UV mirrors deposited using high energy ion assistance had slightly higher mean refractive indices of hafnia, higher extinction than conventional e-beam deposition, but demonstrated slightly higher laser induced damage threshold (LIDT) values measured at 355 nm. Deposition using the lowest energy ions produced the most porous coatings with the best LIDT of 7.7 J/cm2.
We report the measurements of the linear and non linear absorption at 1064, 532 and 355 nm in samples of KDP crystals fabricated with the rapid growth process developed for NIF and LMJ high power lasers. Measurements were performed according to the ISO11551 standard by the "pulse" or "gradient" calorimetric method using a pulsed, diodepumped, Q-switched Nd:YAG laser. Time resolved spectroscopy method was used for the investigation of defects formation, responsible for the non linear absorption at 355 nm.
An investigation of laser-induced damage thresholds (LIDT) of antireflection coated lithium triborates used for second and third harmonic generation were performed at 1064, 532, and 355 nm wavelengths for ~4 ns pulses. Two types of coatings were tested: an anti-reflection coatings with dual peak at 1064 nm and 532 nm and anti-reflection coatings with three peaks at 1064 nm, 532 nm, and 355 nm, respectively. Ion-beam sputtering and magnetron sputtering technologies where used for coatings deposition. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. The obtained LIDT were in range of 3-15 J/cm2.
The ion assisted thin film deposition (IAD) method has been used extensively for more than two decades, but questions about possibility of improving of the laser-induced damage threshold (LIDT) by this method compared with the conventional electron-beam evaporation (non-IAD) method are still not fully answered. A more complete understanding of different factors that can influence laser-induced damage threshold is necessary for continued development of multilayer dielectric coatings optimized for high-power laser applications. To clarify these factors we performed comparison of LIDT for IAD and non-IAD coatings in nanosecond and femtosecond pulse ranges. High reflectance mirrors at 800 nm and 532 nm were tested. Mirror coatings were made of ZrO2 and SiO2. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. Two lasers were used for the measurements: Nd:YAG (λ = 532 nm, τ = 5 ns) and Ti:Sapphire (λ = 800 nm, τ = 130 fs). Measurements at 800 nm and 532 nm were performed at 1-kHz and 10 Hz pulse repetition rate respectively (S-on-1 test). The damage morphology of coatings was characterized by Nomarski microscopy and relation of LIDT with coating parameters was analyzed.
A comparison of laser induced damage thresholds (LIDT) of ion assisted deposition (IAD) and standard electron beam deposition dielectric coatings on BK7 glass with different surface roughness was performed. Five types of high reflectance mirrors at 800 nm and two types of high reflectance mirrors at 1064 nm were tested. Mirror coatings were made of ZrO2 and SiO2. Automated LIDT measurements were performed according to the requirements of current ISO 11254-2 standard. Two lasers were used for the measurements: Nd:YAG (l = 1064 nm, t = 13 ns) and Ti:Sapphire (l = 800 nm, t = 130 fs ). All measurements were performed at 1-kHz pulse repetition rate (S-on-1 test). A fixed spot size was used for each laser. For 1064 nm it was ~ 70 um and for 800 nm ~ 500 um. The damage morphology and structure of coatings were characterized by an atomic force microscopy (AFM), Nomarski microscopy and X-ray diffraction (XRD).
In comparison to studies at longer pulse regimes, investigations of laser induced damage threshold indicate a contrary behavior on the fs-scale for the dielectric coatings. In general, experiments reveal an electronic cause of the damage. The strong correlation of theoretical calculations with experimental data of laser induced damage thresholds for quartz verifies this assumption. Consequently, the characteristic function of the wavelength dependence of the damage threshold differs in this range from the classical behavior. The quantized structure of the electronic transition leads to a typical step function of the LIDT in dependence on the band gap energy of the materials. Hence, the step should be observed between energy levels from n to n+1 electron ionization. In detail, the probability for the transition of the electron from the valence band to the conduction band changes abruptly.
In an international cooperation with the University of Vilnius the wavelength dependence of the LIDT was investigated for dielectric coatings of TixSi1-xO2 as a function of the stoichiometry. The measurements were performed for a wavelength range from 600 to 800 nm and at a pulse duration of 130 fs by using an OPA laser system. The step from two photon to three photon ionization was measured. The assumption of the mentioned behavior of the fs-damage was proven. For different concentrations of silicon and titanium in the oxide, the electronic structure of the material changes. The experiments have shown an increasing gap energy and LIDT for a high content of silica.
We report the measurements of the linear absorption at 1064, 532, and 355 nm and non linear absorption at 355 nm in samples of KDP crystals fabricated with the rapid growth process developed for NIF and LMJ high power lasers.
We report on the development and use of coherent spectrophotometers specialized for the unusual requirements of characterizing nonlinear optical materials and multilayer dielectric coatings used in laser systems. A large dynamic range is required to measure the linear properties of transmission, reflection and absorption and nonlinear properties of laser-induced damage threshold and nonlinear frequency conversion. Optical parametric oscillators generate coherent radiation that is widely tunable with instantaneous powers that can range from milliwatts to megawatts and are well matched to this application. As particular example a laser spectrophotometer based on optical parametric oscillators and a diode-pumped, Q-switched Nd:YAG laser and suitable for optical characterization in the spectral range 420-4500 nm is described. Measurements include reflectance and transmittance, absorption, scattering and laser-induced damage thresholds. Possibilities of a system based on a 130-fs Ti:sapphire laser and optical parametric generators are also discussed.
We report polarization dependent measurements of absorptance of some crystal performed according to the ISO11551 standard by the 'pulse' or 'gradient' calorimetric method at 1064 and 532 nm using a pulsed, diode-pumped, Q-switched YAG:Nd laser.
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