This PDF file contains the front matter associated with SPIE Proceedings Volume 10447, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The primary sources of damage on the National Ignition Facility (NIF) Grating Debris Shield (GDS) are attributed to two independent types of laser-induced particulates. The first comes from the eruptions of bulk damage in a disposable debris shield downstream of the GDS. The second particle source comes from stray light focusing on absorbing glass armor at higher than expected fluences. We show that the composition of the particles is secondary to the energetics of their delivery, such that particles from either source are essentially benign if they arrive at the GDS with low temperatures and velocities.

Typically, plasma physics targets are of millimeter or sub millimeter dimensions and use an irradiance of ~10¹⁶ W/cm² in nanosecond [“long”] pulses or ~10²¹ W/cm² for “short” [~500fs] pulse lengths. These conditions lead to target and target mount materials being raised to temperatures that cause changes from the solid state into liquid, gaseous and plasma conditions. Matter from the altered states are then subsequently ejected from the originally solid target location and are distributed in space with a variety of masses and velocities and form layers or regions of contamination, some of which may be deposited on sensitive laser or X ray optical surfaces. If low energy densities are used then there is insufficient energy to change the state of the target and a plume of solid fragments is emitted by the target.

A study of the continuous wave (CW) laser induced damage threshold (LiDT) of fused silica and yttrium aluminum garnet (YAG) optics was conducted to further illustrate the enhanced survivability within high power laser systems of an anti-reflection (AR) treatment consisting of randomly distributed surface relief nanostructures (RAR). A series of three CW LiDT tests using the 1070nm wavelength, 16 KW fiber laser test bed at Penn State Electro-Optic Center (PSEOC) were designed and completed, with improvements in the testing protocol, areal coverage, and maximum exposure intensities implemented between test cycles. Initial results for accumulated power, stationary site exposures of RAR nano-textured optics showed no damage and low surface temperatures similar to the control optics with no AR treatment. In contrast, optics with thin-film AR coatings showed high surface temperatures consistent with absorption by the film layers. Surface discriminating absorption measurements made using the Photothermal Common-path Interferometry (PCI) method, showed zero added surface absorption for the RAR nanotextured optics, and absorption levels in the 2-5 part per million range for thin-film AR coated optics. In addition, the surface absorption of thin-film AR coatings was also found to have localized absorption spikes that are likely pre-cursors for damage. Subsequent CW LiDT testing protocol included raster scanning an increased intensity focused beam over the test optic surface where it was found that thin-film AR coated optics damaged at intensities in the 2 to 5 MW/cm² range with surface temperatures over 250C during the long-duration exposures. Significantly, none of the 10 RAR nano-textured fused silica optics tested could be damaged up to the maximum system intensity of 15.5 MW/cm², and surface temperatures remained low. YAG optics tested during the final cycle exhibited a similar result with RAR nano-textured surfaces surviving intensities over 3 times higher than thin-film AR coated surfaces. This result was correlated with PCI measurements that also show zero-added surface absorption for the RAR nano-textured YAG optics.

High average-power, nanosecond-duration, laser pulses induce damage on uncoated optics, due in part to an enhanced localized field at the exit surface of the components. Similarly, anti-reflection (AR) thin-film coated optics have similar field enhancement regions, due to multiple boundaries, and experience laser induced damage on both entry and exit interfaces. Sub-wavelength anti-reflection randomly structured surfaces (rARSS) have been shown to have a higher laser-induced damage threshold than traditional AR coatings. Previously published work detailed laser-induced damage on rARSS on a single surface of optical quality, planar, fused silica substrates; optimized for maximum transmission (99.5%) at 1064 nm. The present study explores the introduction of rARSS to both sides of the substrate. Laser-induced damage was systematically created and measured at contiguous locations along the substrate, using 1064 nm wavelength, 6-10 ns duration pulses. Laser output was focused to increase incident irradiance at the initial interface. Incident fluence was directly controlled by Q-switching the laser to create fluence values at, and above, damage thresholds for both entry and exit sides. It was determined that double-sided rARSS substrates have a higher damage threshold than thin-film AR coatings, while they have a lower damage threshold than entrance-only and exit-only sided rARSS (previous study), as well as, lower damage threshold than plain, optical quality, uncoated, fused silica. Damage on the exit-side of the substrate was ballistic in nature, showing surface cracks and outward-oriented debris craters. Contrastingly, damage on the entry-side of the substrate was thermally-induced local-densification of random structures with a latent footprint.

The growth of damage sites from micrometric to millimetric scales under high energy laser system conditions have herein been investigated. In this realm, a saturation of the surface growth followed by the rapid expansion of radial cracks has been observed. This observation contrasts with the previously reported exponential behavior1 (for pulse durations above 2 ns) and linear behavior (for pulse durations below 2 ns). The observation of the longitudinal damage structure coupled with fractal analysis has shown that these shifts in growth behavior seem to be correlated with changes in the damage morphology.

The laser-induced damage threshold of fused-silica samples processed via magnetorheological finishing is investigated for polishing compounds depending on the type of abrasive material and the post-polishing surface roughness. The effectiveness of laser conditioning is examined using a ramped pre-exposure with the same 351-nm, 3-ns Gaussian pulses. Finally, we examine chemical etching of the surface and correlate the resulting damage threshold to the etching protocol. A combination of etching and laser conditioning is found to improve the damage threshold by a factor of ~3, while maintaining <1-nm surface roughness.

Fatigue effects in fused silica have been largely studied in the past years, as this phenomenon is directly linked to the lifetime of high power photonic materials. Indeed, in the UV regime, we observe a decrease of the LIDT when the number of laser shots increases and this has been attributed to laser-induced material modifications. Under 266 nm laser irradiation, with nanosecond pulses of constant fluence, we observed that the photoluminescence is modified until damage occurs. High-OH fused silicas like Suprasil, “UV fused silica” or Herasil® show NBOHC (Non-Bridging Oxygen Hole Center) luminescence at 664 nm (1.87 eV) whereas low-OH fused silica like Infrasil shows ODC (Oxygen- Deficient Center) luminescence at 404 nm (3.07 eV). We found that the laser-induced density of NBOHCs increased until bulk damage occurred while the ODC’s density decreased. We propose a new representation of the experimental Son- 1 breakdown data which allows predicting the occurrence of material breakdown consuming fewer sample surface and saving time compared to the classic representation Nd (Number of shots before damage) versus F (Fluence). The link between LIF and the modifications leading to breakdown is however modified if a break is used during the irradiation.

Laser induced damage in the final optics assembly is one of the bottleneck problems in high power laser systems for the inertial confinement fusion. Defects on the optical elements can cause optical intensity intensification and therefore damage the optical elements in the downstream. However, only single defect is considered for most cases. In this paper, physical models are established to study enhancement of light intensity related to distribution of defects in the final optics assembly. Results show that, when the distance of two localized defects reduces to a certain distance, there will be a stronger light intensity intensification duo to the interference effect. What’s more, it will be much more serious when the nonlinear effect is taken into consideration. Meanwhile, the interaction of two kinds of different defects are also studied, i.e., the periodic defect and the localized defect. The optical field will be enhanced to a certain extent at the overlapped area. Thus, we can see that single defect may not cause optical damage. But when there are more than one defect with a certain distribution, light field may be further enhanced, thus damaging the optical element. As a conclusion, the distribution of defects also needs strict constraints. The results could give some references to the mitigation of damage caused by defects in the final optics assembly.

Understanding the warm dense matter (WDM) state is of fundamental importance in the modeling of femtosec- ond laser damage because laser electron coupling and subsequent electron lattice coupling can rapidly increase the material temperature at the laser focal region to on the order of an eV, producing WDM not well de- scribed by standard liquid and solid equations of state. We have developed a simulation approach based on the particle-in-cell (PIC) method capable of modeling the formation of warm dense matter via an ultrashort pulse on a mesoscopic scale by utilizing two temperature interionic potentials. The dynamics are simulated via two sequential stages, the first of which models the femtosecond laser-target interaction directly by solving Maxwell’s equations and the Lorentz force law, along with a sophisticated scheme for properly modeling the short range collisionality of particles. The second simultaneously models electron diffusion and electron-ion relaxation via the two temperature model and material ablation using the PIC pair potential model. Our simulation enables us to calculate the temporal and spatial dynamics of particles over the entirety of the laser affected material and to determine a crater profile which can be used to compare to experimental results.

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.

The operating fluence of traditional zigzag slab laser amplifier is limited by the interfacial laser damage between the laser crystal and SiO₂ film at the total internal reflection surface. A novel coating solution for the total internal reflection surface was proposed to increase the laser damage resistance. A HfO₂ layer with optimal thickness was inserted between the laser crystal and SiO₂ film to manipulate electric-field intensity at the vulnerable crystal-film interface to be zero. The laser damage resistance of this new coating for the total internal reflection surface is about two times higher than that of traditional zigzag slab laser amplifier.

An investigation into the potential for increased laser damage resistance was made for a random distribution and regular array of nanometer scale surface relief structures integrated with dielectric thin-films to create 355nm wavelength selective high reflectors (HR). First, Random Anti-Reflection (RAR) nanostructures were fabricated in a thick silica cap layer deposited on top of a conventional 30-layer HR stack designed as 45 degree turning mirrors. Surface absorption scans of these RAR nano-texture enhanced HR stacks showed a slight decrease in absorption with no impact on performance, however standardized pulsed laser damage threshold testing found no improvement in damage resistance over non-textured silica-cap HR stacks. Second, polarization and wavelength selective nanostructure resonant (NSR) array filters designed to be embedded within thick high damage resistance silica layers were modeled using rigorous coupled wave analysis. Prototypes were fabricated of one NSR design consisting of a low-aspect ratio grating defined in a fused silica substrate with a single thin layer of hafnium oxide over-coat. The performance of NSR prototypes was limited due to multiple fabrication and testing issues. Initial 355nm wavelength, 5ns pulse, s-on-1 laser damage testing yielded a damage threshold in the 3 to 4 J/cm² range, comparable to that obtained for the multi-layer HR stacks. Despite these modest early results, it appears that with further fabrication improvements, nano-structure array resonators embedded within silica layers could yield significant increases in the laser damage resistance of HR optics.

This competition aimed to survey state-of-the-art UV high reflectors. The requirements of the coatings are a minimum reflection of 99.5% at 45 degrees incidence angle for P-polarized light at 355-nm. The choice of coating materials, design, and deposition method were left to the participants. Laser damage testing was performed at a single testing facility using the raster scan method with a 5-ns pulse length laser system operating at 10 Hz in a single longitudinal mode. A double blind test assured sample and submitter anonymity. In addition to the laser damage resistance results, details of the deposition processes, cleaning method, coating materials and layer count are also shared.

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.

The effect of protective layer on the picosecond laser-induced damage behaviors of HfO₂/SiO₂ high-reflective (HR) coatings are explored. Two kinds of 1064nm HR coatings with and without protective layer are deposited by electron beam evaporation. Laser-induced damage tests are conducted with 1064nm, 30ps S-polarized and P-polarized pulses with different angle of incidence (AOI) to make the electric fields intensity in the HR coatings discrepantly. Damage morphology and cross section of damage sites were characterized by scanning electron microscope (SEM) and focused ion beam (FIB), respectively. It is found that SiO2 protective layer have a certain degree of improvement on laser induced damage threshold (LIDT) for every AOIs. The onset damage initiated very near to the Max peak of e-field, after which forms ripple-like pits. The damage morphology presents as layer delamination at high fluence. The Laser damage resistance is correspond with the maximum E-intensity in the coating stacks.

Ultra-short laser applications require high quality dielectric optics. The natural dispersion of light needs to be matched by dielectric components. However such dispersive components are very challenging for the deposition process and are characterized by high field intensities inside the layer stack. Such layers are expected to diminish the possible laser induced damage thresholds (LIDTs) because of their low optical gap value for suitable high refractive index materials. This paper reports about the manufacturing of amorphous nanolaminates to tune the optical gap. Such sequences are substituted into a conventional high reflective mirror to decrease the electric field of binary Tantala layers by 30 % which correlates to an improvement in LIDT of almost 16%.

Optical coatings deposited using electron beam evaporation are subject to aging effects that change the spectral characteristics of the optical coating. The aim of this study was to determine whether aging effects can also negatively impact the laser damage resistance of an optical coating. Maintaining high resistance to laser damage is particularly important for the performance of high fluence laser systems. In 2013, we deposited different high reflection coatings for 1054 nm containing HfO₂/TiO₂/SiO₂ layers. For this study, we re-measured the laser damage thresholds of these coatings at 3.5 ns to determine if aging effects cause the laser damage threshold to decline, and to compare whether HfO₂ or TiO₂ is superior in terms of long-term laser damage resistance.

The MegaJoule Laser (LMJ) for inertial confinement fusion experiments is currently in operation at CEA-CESTA in France. All the lenses are coated by an antireflective (AR) layer to optimize the light power transmission. This AR layer is manufactured by sol-gel process, a soft chemical process, associated with a liquid phase coating technique to realize thin film of metal oxide. These optical components are hardened into ammoniac vapors in order to mechanically reinforce the AR coating and to make them more handling. This hardening induces a thickness reduction of the layer so an increase of the stiffness and sometimes a crazing of the layer. As these optical components undergo a high-power laser beam, so, it is important to verify if the AR properties (optical and mechanical) influence the value of the threshold laser damage. A series of coated samples have been manufactured having variable elastic moduli to discuss this point. In that purpose, a homemade Laser Induced Damage Threshold (LIDT) setup has been developed to test the layers under laser flux. We describe the used methods and different results are given. Preliminary results obtained on several coated samples with variable elastic moduli are presented. We show that whatever are the elastic stiffness of the AR coating, an overall decrease of the threshold appears with no noticeable effect of the mechanical properties of the AR coatings. Some possible explanations are given.

Laser damage measurements with multiple pulses at constant fluence (S-on-1 measurements) are of high practical importance for design and validation of high power photonic instruments. Using nanosecond lasers, it has been recognized long ago that single pulse laser damage is linked to fabrication related defects. Models describing the laser damage probability as the probability of encounter between the high fluence region of the laser beam and the fabrication related defects are thus widely used to analyze the measurements. Nanosecond S-on-1 tests often reveal the “fatigue effect”, i.e. a decrease of the laser damage threshold with increasing pulse number. Most authors attribute this effect to cumulative material modifications operated by the first pulses. In this paper we discuss the different situations that are observed upon nanosecond S-on-1 measurements of several different materials using different wavelengths and speak in particular about the defects involved in the laser damage mechanism. These defects may be fabrication-related or laser-induced, stable or evolutive, cumulative or of short lifetime. We will show that the type of defect that is dominating an S-on-1 experiment depends on the wavelength and the material under test and give examples from measurements of nonlinear optical crystals, fused silica and oxide mixture coatings.

So-called hybrid mirrors, consisting of broadband metallic surface coated with dielectric reflector designed for specific wavelength, becoming more important with progressing development of broadband mid-IR sources realized using parametric down conversion system. Multiple pulse nanosecond laser induced damage on such mirrors was tested by method s-on-1, where s stands for various numbers of pulses. We show difference in damage threshold between common protected silver mirrors and hybrid silver mirrors prepared by PVD technique and their variants prepared by IAD. Keywords: LIDT,

The laser damage performance of optical components is often limited by the presence of sparse defects rather than intrinsic material properties. In this regime, it is costly to perform destructive laser damage testing over areas large enough to make high-confidence statements of damage likelihood in non-tested parts or regions. Instead, one may record non-destructively the sizes and locations of all defects over a much larger area. It is also straightforward to do selective laser damage testing centered on defects (and defect-free sites) in a subregion. This latter measurement will yield a table that quantifies damage probability as a function of fluence and defect size. Combining the complete defect map and the damage probability table allows laser damage prediction at every location over the whole area of interest. In this paper large-area defect mapping of real-world coated optics is combined with previously established damage probability tables. The defect-driven contribution is shown to enhance the predictive power of the simulations as judged by standard damage testing.

Photothermal technology provides sensitive detection of the optical absorption in bulk materials and coatings. To obtain the absolute absorption numbers it requires a proper calibration. In this work a new, “proxy pump” calibration approach is described. The proxy pump has a wavelength at which the material exhibits high enough optical absorption to be evaluated via direct loss measurements. The pump beam is shaped to have the same spot size as the main pump at which the optical absorption of the material is to be determined. Once the thermal field in the material has the same profile both with the proxy and main pump, the sample is self-referenced. Consecutive tests with proxy and main pumps provide absolute absorption numbers. LBO crystals are notorious objects for which the photothermal response is not easy to calibrate since the material has very low absorption in the UV, visible and near IR. In order to calibrate these materials using the above approach we used 2.3 nm laser as a proxy pump. At this wavelength LBO absorbs more than 15 % per cm length. For crystals oriented in XY and YZ planes the photothermal response was found to be 3 times weaker than for Schott glass NG12 with the same amount of absorbed power. With this correction, NG12 glass that has absorption more than 45 %/cm in the wide range of wavelengths can be used as a reference calibration material for LBO crystals.

This paper reports on the fundamental idea behind a US National Committee, The Optics and Electro-Optics Standards Council (OEOSC) Task Force (TF) 7, proposal for a so-called Type 1 laser damage test procedure. A Type 1 test is designed to give a simple binary, pass or fail, result. Such tests are intended for the transactional type of damage testing typical of acceptance and quality control testing. As such is it intended for bulk of certification of optics for the ability to survive a given fluence, useful for manufacturers of optics and their customers, the system builders. At the root of the proposed method is the probability that an optic of area A will have R or less damage occurrences with a user specified probability P at test fluence Φ. This assessment is made by a survey of area and the observation of n events. The paper presents the derivation of probability of N or less damage sites on A given n events observed in area a. The paper concludes with the remaining steps to development of a useful test procedure based on the idea presented.

Spatio-TEmporally REsolved Optical Laser-Induced Damage, or STEREO-LID, is a novel laser damage technique which measures the actual fluence (and intensity) at which damage occurs in a single pulse. This is accomplished by measuring the initiation time during the pulse and the initiation position within the beam profile. This technique has been demonstrated in the measurement of the defect distribution of single films and surfaces. In this paper, we demonstrate the technique to characterize laser-induced damage in high reflectors by single 8.3 ns pulses at 1064 nm. The high reflectors were quarter-wave stack of HfO₂/SiO₂ on a fused silica substrate. Applying STEREO-LID to high reflectors required a change in geometry as the technique previously used light transmitted through the optic. Instead, the initiation time is identified by a disruption of reflection and scattered light. The initiation position is determined by imaging the backscattered laser light. The STEREO-LID results are compared to the traditional 1-on-1 damage test to show that it is superior at detecting critical fluence limiting defects.

Damage-test data are scarce for liquid crystalline (LC) materials at 1-ns pulse lengths and nonexistent at shorter pulselengths. Here we describe the methodology to develop a comprehensive database of damage performance for typical nematic LC’s for a wide range of pulse lengths at 1053 nm. This series of nematic LC materials investigates the effect of a varying degree of π-electron delocalization. Obtaining damage-threshold measurements is of fundamental interest for the consideration of LC materials for applications in short-pulse laser systems.

Micro-machining has been regarded as the most promising method to mitigate the laser damage growth on KDP/DKDP crystal surfaces. In this work, the near-field and far-field light modulations caused by three kinds of typical mitigation contours (spherical, Gaussian and conical) were theoretically investigated and compared to determine the optimal contours for achieving the minimum light intensification. Then, based on Computer Aided Manufacturing (CAM), a specific machining flow combining layer milling (rough repairing) and spiral milling (fine repairing) was developed to repair the surface damage with high efficiency and surface quality. Finally, the morphology, transmittance and laser damage resistance of the repaired KDP surfaces were tested. The theoretical and experimental results indicate that the conical mitigation contours mostly possess the best repaired surface quality and optical performance. The developed combined rough and fine machining flow could be applied as a practical repairing flow to mitigate the laser-induced surface damage growth of KDP crystal optics.

We present the results of comprehensive bulk absorption studies on numerous LBO crystals at 1070nm, 532nm and 355nm using the sandwich-LID measurement concept. One aim of the studies is answering the question whether there is a correlation between bulk absorption coefficients at the different applied wavelengths. Further, intensity dependent absorption measurements at 355nm are performed to investigate the nonlinear absorption in the LBO crystals. From the results it is verified, that at least to a certain extent the nonlinear absorption is related to impurities or defects, i.e. to sequential three photon absorption in addition to a potential intrinsic 3-photon absorption process.

This paper introduces an analysis of the uncertainty of the determination of the areal density of defects. Estimating the density of defects is a fundamental aspect of the study of laser induced damage. The proper accounting for the uncertainty in this measurement is also a seminal matter. This paper introduces two means of estimation of estimating the uncertainty, the so-called frequentist and Bayesian approaches. The paper concludes with a comparison of the results from the two methods.

This paper presents a method to make an estimation of the low fluence edge of the defect distribution, ρ(φ), from data collected from ISO 21254 standard damage frequency measurements. The cumulative probability of damage is formulated and the defect distribution is formulated as an expansion of an orthonormal set. This formulation allows the explicit isolation of the terms pertaining to the defect distribution and the intensity distribution. When the intensity distribution is specified, the resulting system is a polynomial in nature. Solutions for the systems relevant for Gaussian and are presented.

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.

This research work was focused on the laser peening surface process in a metallic Ti-6Al-4V biomaterial. The Ti-6Al- 4V samples were surface treated at different laser conditions varying parameters such as pulse density and wave length. Laser peening induced effects were evaluated by synchrotron radiation X-ray diffraction (SR-XRD) to determine the residual stress state; scanning electron microscopy (SEM) to assess microstructural changes and thermoelectric testing (TEP) to sense the subtle material variations such as local texture, increased dislocation density, hardening and residual stresses degree. The TEP measurements demonstrate that the non-contact technique is very sensitive to the compressive residual stresses with increasing the severity of the laser treatment parameters, while the TEP contact results are closely related to grain size, cracks, anisotropy, and work hardening.

A millimetric aperture Nd:glass laser system has been designed and constructed at the CEA-CESTA. Its aim is to best mimic the laser conditions that can be found in inertial confinement fusion facilities. It is therefore used to study the main phenomena that prevents these lasers to work at their maximum power: the laser induced damage of the optical components. The combination of temporal and spatial modulators provides, every minute, a 6 J, 7 mm, 351 nm homogeneous beam at the fused silica sample location. This proceeding illustrates the capacity of the facility over two experiments: the study of damage initiation and the growth of laser damage sites on fused silica, up to millimetric scales

An established method for precise determination of optical absorption is the so called laser calorimetry. According to ISO 11551¹ laser calorimetry is preferred to other photothermal test methods, because of its capability to deliver absolute calibration. Many optical materials have low heat conductivity, which can affect the calibration significantly. The timeand spatial dependent temperature profile in a sample of materials with low heat conductivity requires accurate temperature measurement strategies to determine material-independent and absolutely calibrated absorption values. For thin cylindrical samples, ISO 11551 provides a strategy to compensate heat conductivity effects. The optimal temperature sensor position, where accordingly calibrated measurement results² can be obtained, is simply based on the symmetric sample geometry. For thick geometries an additional temperature distribution along propagation direction of the heating beam must be considered. The current version of ISO 11551 does not provide a sophisticated solution for this problem, because the heating scheme of a sample is usually unknown. Therefore, a reliable calibration procedure can only be applied to samples of well-known absorption properties of surfaces and bulk material. Utilizing such kind of specifically prepared reference samples in combination with Finite Element Method (FEM) calculations, a general measurement and data evaluation concept based on laser calorimetry is presented, that allows deriving absolutely calibrated absorption measurement results for rectangular sample geometries.

A photothermal absorption measurement system was set up, deploying a Hartmann-Shack wavefront sensor with extreme sensitivity to accomplish spatially resolved monitoring of thermally induced wavefront distortions. Photothermal absorption measurements in the near-infrared and deep ultra-violet spectral range are performed for the characterization of optical materials, utilizing a Yb fiber laser (λ = 1070 nm) and an excimer laser (193nm, 248nm) to induce thermal load. Wavefront deformations as low as 50pm (rms) can be registered, allowing for a rapid assessment of material quality. Absolute calibration of the absorption data is achieved by comparison with a thermal calculation. The method accomplishes not only to measure absorptances of plane optical elements, but also wavefront deformations and focal shifts in lenses as well as in complex optical systems, such as e.g. F-Theta objectives used in industrial high power laser applications. Along with a description of the technique we present results from absorption measurements on coated and uncoated optics at various laser wavelengths ranging from deep UV to near IR.

The Laser MégaJoule (LMJ) is a French high power laser that requires thousands of large optical components. For all those optics, scratches, digs and other defects are severely specified. Indeed, diffraction of the laser beam by such defects can lead to dangerous “hot spots” on downstream optics. With the help of a near-field measurement setup, we make quantitative comparison between simulated and measured near-fields of reference objects (such as circular phase steps). This leads to a better understanding which parameters impact the diffracted field. In this paper, we proposed to study two types of reference objects: phase disks and phase rings. We actually made these objects by CO2 laser ablation. The experimental setup to observe the diffracted intensity by these objects will be described and calibrated. Comparisons between simulations and measurements of the light propagation through these objects show that we are able to predict the light behavior based on complete phase measurement of these objects.

LiNbO₃ and LiTaO₃ are frequently used in second harmonic conversion of continuous-wave light from the infrared to the visible regions. Optical damage of LiNbO₃-type crystals is a crucial issue in the high-average-power laser systems. The optical damage by a light-induced heating is investigated. We have proposed a light-induced heating by the accumulated long-lived states like polarons or self-trapped excitons. In addition to the long-lived states, point defects as color centers are created by a radiation. A new model includes these states. As a results, it is shown that a creating rate of the color center becomes a important parameter on the determination of critical power.

This paper describes and presents results from a model created in MATLAB® to calculate and display the time dependent temperature profile on a target aimpoint as it is being engaged by a high energy laser (HEL) beam. The model uses public domain information namely physics equations of heat conduction and phase changes and material properties such as thermal conductivity/diffusivity, latent heat, specific heat, melting and evaporation points as well as user input material type and thickness. The user also provides time varying characteristics of the HEL beam on the aimpoint, including beam size and intensity distribution (in Watts per centimeter square). The model calculates the temperature distribution at and around the aimpoint and also shows the phase changes of the aimpoint with the material first melting and then evaporating. User programmable features (selecting materials and thickness, erosion rates for melting) make the model highly versatile. The objective is to bridge the divide between remaining faithful to theoretical formulations such as the partial differential equations of heat conduction and at the same time serving practical concerns of the model user who needs to rapidly evaluate HEL thermal effects. One possible use of the tool is to assess lethality values of different aimpoints without costly (as well as often dangerous and destructive) experiments.

Low absorption, low scattering and high-density HfO₂ coatings lead to an important improvement of high power laser systems. In order to suppress the crystallization of HfO2 coatings fabricated with ion assisted deposition (IAD), we used double electron-beam (EB) coevaporation with ion beam assisting method to fabricate HfO₂-SiO₂ mixtures from two independent material sources. Crystallization following the different mixture ratios was investigated. Several prototypes were designed, featuring different HfO₂-SiO₂ ratios with similar physical thickness (500nm). The samples were deposited on fused silica and silicon substrates. X-ray diffraction patterns show that the degree of crystallization gradually fades away with increasing SiO₂ contents and when SiO₂ component reach 18%, the mixture film becomes almost amorphous. To decrease the high absorption originating from IAD method, thermal annealing in air at progressive temperatures was subsequently performed. It was found that post-annealing treatment at 600°C could eliminate the absorption at 1064nm. However, high temperature annealing would induce the crystallization of these initial amorphous coatings. In order to suppress the crystallization further to obtain amorphous structure even after 600°C annealing, the 25% SiO₂ sample was fabricated and it successfully obtain low roughness and low absorption equivalent to the bare substrate. Finally, a 1064 nm HR coating using SiO2 and mixed film of 25% SiO₂ concentration was prepared and annealed to prove its practical application in low loss and high LIDT optical elements.

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.

Several sets of mirror samples with multilayer system Ta2O5/SiO2 on silver metal layer were manufactured using either PVD or IAD coating technology. Both BK7 and fused silica substrates were used for preparation of samples. Laserinduced- damage-threshold (LIDT) of metal-dielectric mirrors was tested using a laser apparatus working at 1030 nm wavelength, in ns and ps pulse length domains in S-on-1 test mode. The measured damage threshold values at 45 deg angle of incidence and P-polarization were compared for different pulse length, substrate materials and coating technology.

Based on the z-scan method, an interferometric set-up for measuring the optical Kerr-effect was engineered and optimized. Utilizing a Mach-Zehnder configuration, the wave front deformation caused by the Kerr induced selffocusing is monitored. Fitting this deformation to a theoretical approach basing on a beam propagation model, the nonlinear refractive index is obtained. The procedure can be applied to measure the nonlinear refractive index of both, the substrate material as well as the deposited dielectric layer on top of the substrate. The nonlinear refractive index of a layer specially deposited for this purpose as well as for several substrate materials was measured and the results presented.

In this investigation the influence of the local environment on the laser damage threshold of anti-reflective coatings is reported. For this purpose, HfO₂ / SiO₂ anti-reflective coatings were deposited on fused silica substrates using an ionbeam sputter system. Laser damage threshold measurements were performed using two test procedures, S-on-1 and Ron- 1, at 355 nm for temperatures ranging from room temperature up to 250 °C and in different atmospheres. The two test procedures had comparable LIDT results with a possible pre-conditioning effect evidenced by a broadening of the transition range of the R-on-1 measured samples. It was found that samples measured in normal atmospheric air showed superior laser resistance compared to samples measured under nitrogen purge or in dry air. Samples measured in normal atmospheric air also showed a temperature dependence with an improved laser resistance at 25 °C. No temperature dependence was observed for samples measured under nitrogen purge or in dry air. In this paper, literature showing similar effects is reviewed and the influence of a water epilayer on the coating as a possible cause for the observed results is discussed.

We describe an optical measurement technique based on Carrier Frequency Interferometry (CFI) that allows to measure wavefront deformation of coated optics with high accuracy. The sensitivity of the method is considerably greater than phase shifting interferometry. The limits of validity of the Stoney equation in calculating stress in thin films is assessed using CFI. The evolution of stress in Ta2O5 films deposited by ion beam sputtering is also evaluated.

Today the laser induced damage threshold (LIDT) is one of the most important properties of laser components. As laser systems reach higher and higher optical power densities, optical components with improved LIDT values are highly required and their characterization has gained an ever-increasing importance. Many optical components rely on multilayer coating sequences of dielectric materials. Therefore, several design strategies such as the rugate and RISED concept have been applied for highly reflecting layer systems for the design wavelength of 1030 nm and an angle of incidence of 44°. The investigations were based on IBS and PIAD deposition methods using HfO₂ and SiO₂ for high respectively low refractive index material. A movable zone target was used for IBS in order to allow for direct material mixing to obtain for the rugate and mixed design continuous variations in the refractive index. The IBS designs have been tested for the LIDT at 560 fs and s-polarization and compared to those produced by PIAD. The designs were also tested in cw operation. The highest values for LIDT in pulsed conditions have been found for mixed IBS and PIAD designs with average values up to 2 J/cm² at 1-on-1 test. The best results at 10⁵ -on-1 test have been achieved by rugate designs with a LIDT of > 1.3 J/cm² . In CW operation, the samples could be subjected to power densities of up to 1 MW/cm² without the notification of any damage.