This year’s laser damage competition involves short pulse laser damage on high reflectors tuned for near normal incidence and >99.5% reflectance for 1030 nm. All laser damage testing was done by Lidaris Ltd. via a near Gaussian laser beam from a commercial laser system (Yb:KGW, Kerr lens mode-lock) operating at 500 kHz repetition rate with 200-fs pulse duration (FWHM). All testing was done in a similar fashion to the ISO 21254-1 and 21254-2 S-on-1 standards, yielding data on laser damage for 10x number of shots, where x = 0, 1, 2, 3, 4, 5, 6. Laser-matter interaction either leads to material removal and ablation or a more subtle coating admittance change. The former is referred to as catastrophic damage while the latter is referred to as color change. The choice of coating materials, design, and deposition method were left to the participants. A double-blind test assured sample and submitter anonymity. The damage performance results (LIDT), sample rankings, details of the deposition processes, coating materials and substrate cleaning methods are shared. These results are compared both to the nanosecond 1053-nm laser damage testing on high reflectors from the 2018 competition as well as the nanosecond-femtosecond damage testing study from the 2020-2021 years. All samples exhibited a fatiguing effect in the laser damage performance at high number of shots, but this was particularly noticeable for the color change damage type. We found that ion beam sputtered HfO2/SiO2 multilayer coatings of approximately 30 total layers did the best for the short pulse regime. This is in sharp contrast to the 1053-nm nanosecond study, which has demonstrated that electron beam deposited HfO2/SiO2 high reflectors are the clear winners.
Over the past two decades, laser technology has made significant strides in scaling peak and average power levels. These advancements span a wide range of laser irradiation techniques, from ultrashort pulses to continuous waves, driving the development of various optical elements and coatings. However, the variability in optics sizes, ranging from optical fibers to meter-sized optics, and the presence of various failure modes pose challenges for laser damage testing. Consequently, there is a pressing need to align relevant laser damage testing standards to ensure the functional performance of optics. In this overview, we discuss recent standards-revision efforts aimed at revising the ISO 21254 family standards. Our primary focus is on improving accuracy and reliability by improving damage criteria, testing procedures, and results analysis methods. These efforts aim to tackle emerging challenges in laser damage testing while ensuring that standards remain compatible with modern technological developments.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Operation of high fluence pulsed laser systems in space imposes various risks to optical components involved. Volatile
organic components are omnipresent in vacuum vessels housing space-borne laser systems and can be the source for
selective contamination of optics. Laser systems may respond very sensitively to absorption increases of their multiple
optical surfaces leading to inacceptable transmission losses and system degradation. In the recent past, thorough and
long term laser tests, performed at the optics qualification laboratories at DLR and at ESTEC using space relevant and
model substances, have revealed the onset, the built-up, and the later stages of the deposition process. It was found that
these deposits tend to accumulate preferably on the laser footprint area of the optic. Observed thicknesses are on the
order of several tens of nanometers, which can be sufficient to induce noticeable absorption. Sensitive techniques for insitu
and ex-situ monitoring of these molecular contaminative effects under vacuum conditions were developed and are
applied successfully. They are summarized in this paper, along with the phenomena, which are significant for the
appearance of deposits. In addition, adverse conditions, which are favorable for provoking deposits, are communicated.
Finally, mitigative and preventive methods are discussed.
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.
Particle Image Velocimetry (PIV) is a well known measurement protocol for analyzing the dynamic behavior of fluids in
liquid or gaseous phases (granulate analysis is also possible). With respect to the demands of the measurement accuracy,
a high fluence at the observation zone is required. Presently, this can only be realized by using very precisely aligned
equipment and high power laser pulses. For industrial applications a simpler set-up is needed. Thus the research project
is aimed at the development of a portable endoscopic-based solution which requires the guidance of laser light through
optical fibers. The realization of such an optical fiber system is a technical challenge since the high instantaneous
energies, which exist in the laser pulse, can cause irreversible damage to the optical fiber. Consequently, the main goal is
the determination of the maximum fluences, that different fiber core bulk materials can tolerate, and the comparison of
these results with the maximum achievable fluence when transmitting light through optical fibers. A simple theoretical
modeling tool for the approximation of the power handling capability was developed. Based on this theoretical analysis,
Laser Zentrum Hannover examined the impacts that laser pulses and fiber materials have on light incoupling and
guidance. An experimental set-up was developed to investigate the laser light resistance of different fiber bulk materials
as well as the fibers themselves. This paper introduces the measurement set-up and the results of LIDT measurements of
several fiber core materials. Furthermore, the fiber measurement set-up, achievable fluences, transmission efficiencies as
well as the typical fiber damage behavior are presented.
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