This PDF file contains the front matter associated with SPIE Proceedings Volume 9237, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

The cleanliness of optical surfaces is of great concern as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) project transitions from installation to operation at full power. More particulates than expected were observed on and near the core optics as a result of assembly and installation work, prompting a re-evaluation of longheld contamination control practices. Even low particulate levels can potentially damage the fused silica optics and reduce overall interferometer sensitivity. These risks are mitigated from a combination of the following approaches: quantifying the extent of the contamination, identifying its sources, improving practices to reduce the generation of particulates, introducing a non-contact in-situ cleaning technique for suspended optics in air, qualifying cleanliness levels against induced damage, and developing methods for remotely measuring and cleaning suspended optics under vacuum. While significant progress has been made in understanding and mitigating contamination, and thus, protecting the optics from losses and damage, there is still more work to be done to reach ultimate performance requirements.

Low-loss and high damage threshold mirrors are needed for laser interferometer-type gravitational-wave detectors. Collaborative development with Japanese company of SIGMA KOKI CO., LTD., National astronomical observatory of Japan and Institute for Laser Science, University of Electro-Communications was made for this purpose. As a result, high reflectivity mirror of 99.99% for 1064nm has both low-scattering loss of less than 10ppm and high-damage threshold of over 400 J/cm². Such mirrors can be applied for high finesse cavity of more than 10000 with high laser input power of over 10 Watts. The mirror will offer great benefit for various precise measurements with high power lasers.

When lasers are used to produce high temperature, high density plasmas from solid targets it is inevitable that the targets are turned into a variety of products [gas, liquid, solid, sub-atomic particles and electromagnetic radiation] that are distributed around the surfaces of the vacuum chamber used to field such experiments. These by products are produced in plumes of debris and shrapnel that depend on the irradiation conditions, target materials and target geometry. We have monitored the distribution of such plumes by witness plates and used microscopy, photography and spectrophotometry to determine the physical state of material in the plumes and the spatial distribution from various target geometries. The impact of this material on the operations of laser optics and plasma physics diagnostics is discussed.

It is believed that surface and subsurface defects formed during standard grinding and polishing processes are mainly responsible for laser induced damage in fused silica. The correlation between the laser damage susceptibility and absorption property of these defects has not been totally understood. In this paper, we present the characterization of surface and subsurface defects of fused silica by measuring their absorption properties based on a photothermal technique at 355 nm. The photothermal microscopic imaging reveals that the surface/subsurface absorption defects in fused silica can be identified. In addition, a 3D photothermal imaging of a laser damage site on the silica is also obtained. Our results demonstrate that photothermal microscopy is a powerful tool for defect characterization of optical materials for high power laser applications.

Laser induced damage of optical components is often a limiting factor for the development of high power lasers. Indeed, for many years, organic contamination is identified as a factor decreasing the laser induced damage threshold of optical surfaces, limiting the use of high fluencies. Also, for the development of its laser facilities, Laser MégaJoule and PETawatt Aquitaine Laser, the Commissariat à l’Energie Atomique et aux Energies Alternatives investigates the influence of organic contamination on the performances of the optical components. Actually, although great care is provided on the cleanliness of the optics, organic volatile compounds outgassed from surrounding materials can be adsorbed by the sensitive surfaces during its timelife. Thus, for this study, performances of clean and contaminated multilayer dielectric mirrors are compared. Contamination is intentionally realized either by controlled protocols or by exposing optics inside the laser facilities. Qualification and quantification of the organic contamination is realized by automated thermal desorption and gas chromatography coupled with mass spectrometry. Laser induced damage threshold of clean and contaminated mirrors are then investigated by 1053 nm laser at 670 fs.

In optical coating production the generation of particles and defects is always an undesirable side effect and cannot be completely avoided in the handling steps of the optical components. Particles and defects on the substrates and in the functional coatings lead to scattering and absorption, which may cause a lower damage threshold for components of high power laser application. In this study, results of a long term investigation in the quality and the state of the cleanliness of multilayer systems produced by different deposition techniques are presented. Coated samples of different coating processes are investigated with the help of a Fast Total Scatter scanning system. Adapted data reduction algorithms for the determination of the particle sizes derived from the scattering measurements are developed and applied to the measurement results. On this basis, the density distribution of particle contamination on the samples is evaluated for selected coating runs over a long term period. The calculated statistics of the samples are related to the corresponding production conditions of individual coating plants to extract specific effects of the process environment.

We report on the experimental study of optical breakdown induced in multilayer thin-films by ultrashort pulses at kHz and MHz repetition rates, while keeping all other parameters similar. The investigated samples were coatings composed of TiO₂, Ta₂O₅, HfO₂, or Al₂O₃ as high-index material and SiO₂ as low-index material. We compared the distinct band gap dependencies obtained in the two regimes.

We report on the laser-induced damage threshold at 500fs of optical films made by Magnetron Sputtering and submitted to single and multiple irradiations at different harmonics of an Ytterbium laser (1030nm, 515nm and 343nm). Single layers of SiO₂, HfO₂, and Nb₂O₅ as bare fused silica samples are under investigation.

Dielectrics as single layers and broadband high-reflective stacks were deposited by electron beam deposition processes compatible with 1-meter class optics. After being physically and optically characterized, samples were irradiated with several ultrafast lasers (KYW:Yb 500fs, Ti:Sa 40fs and Ti:Sa 11fs) with single and multi-pulses. The setups of the test platforms, laser-induced damage threshold investigations of intrinsic materials, dielectric multilayers and hybrid metal/dielectric multilayers and electric field intensity distributions are described.

We have observed and studied a nonlinear response of dispersive dielectric multilayer mirrors (DM). It was found that the structure of the mirror itself causes strong enhancement of the electric field inside the multilayer stack consequently triggering strong two-photon absorption (2PA). We have developed a mathematical model, that allows estimation of the coefficient of the 2PA, β, subsequent prediction and to some extent tuning of the strength of the nonlinear response of any multilayer coating.

In an effort to develop next generation UV frequency conversion systems, several steps have to be considered. One aspect crucial for the final conversion stage is a durable coating which shows high resistance for all incident wavelengths. In the regular case, two wavelengths are involved in the generation of the fourth harmonic of the Nd:YAG laser. For a conversion process involving the wavelengths 532nm and 266nm, model AR-coating designs have been developed and tested including SiO₂, Al2O₃, and HfO₂ deposited in an IBS process. During the testing, procedures have been applied that involve both wavelengths at the same time. As in the application, the exit surface is exposed to visible and UV laser radiation, a qualifying test needs to account for these conditions as well.

Narrow-bandwidth Fabry-Perot transmission filters are used in telecommunications, fiber lasers¹, and for diode pumped alkali lasers (DPAL)². Because of their interference properties, extremely high standing-wave electric fields occur at peak transmission. For this study, the filters met a minimum transmission of 90% and were spectrally centered within an angle tuning range of 10-30 degrees. A blind laser damage test assured sample and submitter anonymity. The participants selected the coating materials, design, spectral bandwidth, cleaning method, and deposition method. Laser damage testing was performed at a wavelength of 1064 nm using a raster scan method on a single testing facility to enable a direct comparison among the participants. Pulse length scaling relationships were explored by laser damage testing at a 3.5-ns and 18-ns pulse length. The results show that the spectral bandwidth had the strongest relationship to the laser damage threshold. Other parameters such as deposition processes, cleaning method, coating materials, and layer count were also explored.

Ion beam sputtering (IBS) is a deposition technique being well known for resulting in very dense and damage resistant coatings due to high kinetic energies of the sputtered atoms. While different layers are deposited homogeneously, abrupt interfaces between the materials are the most susceptible part of the stack. Therefore we aim for an improvement of the laser damage threshold by sputtering material mixtures. Using a target with high- and low-index material next to each other, arbitrary refractive indices can be realized by adjusting the target axis. Our material system of choice is HfO₂- SiO₂, already yielding good results with non-rugate coatings. A comparison in terms of laser damage threshold between these designs and varying refractive index coatings will be shown.

The laser damage resistance of coatings for high power laser systems depends greatly on the surface quality of substrate. In this work, experimental approaches with theoretical simulation were employed to understand the coupling effect of subsurface defect and coating on the laser resistance of coating. 1064 nm anti-reflection coating was deposited by E-beam deposition on fused silica. Substrate with and without micro-scale pits were fabricated precisely by femtosecond laser processing. Experimental results indicate that impurities induced in the finishing process shifted to the substrate surface and aggregated during the heating process. Theoretical simulation result shows that the coupling effect of the aggregated impurities and coating are mainly responsible for the low LIDT of E-beam deposition coating.

Near-ultraviolet absorption in hafnium oxide and silica oxide thin-film pairs in a configuration strongly departing from the regular quarter-wave–thickness approach has been studied with the goal of separating film and interfacial contributions to absorption and pulsed laser damage. For this purpose, we manufactured a model HfO₂ SiO₂ thin-film coating containing seven HfO₂ layers separated by narrow SiO₂ layers and a single-layer HfO₂ film in one coating run. The two coatings were characterized by a one-wave total optical thickness for the HfO₂ material and similar E-field peak intensity inside the film. Absorption in the electron-beam–deposited films was measured using photothermal heterodyne imaging. By comparing absorption for the seven-layer and single-layer films, one can estimate the partial HfO₂ SiO₂ interface contribution. Relevance of obtained data to the thin-film pulsed-laser damage was verified by conducting 351-nm, nanosecond-laser–damage measurements and damage-morphology characterization using atomic force microscopy.

Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will review important optical damage precursors in silica up to UV fluences as high as 45J/cm² (3ns) along with studies of the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily responsible for optic damage below about 10J/cm²; they can be mitigated by an optimized oxide etch processes. At fluences above about 10J/cm², precipitates of trace impurities are responsible for damage; they can be mitigated by eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating fluences these optics see.

Laser solid interaction with < 100 fs pulses from 800 - 3600 nm wavelength was studied at 45 degree angle of incidence with s- and p-polarized light. Mid-IR femtosecond laser damage thresholds of Si, Ge, and ZnSe were determined. Obtained results were compared against existing Two Temperature Model for laser damage which was shown to fail at longer wavelengths. Experimental evidence suggests that electron dynamics at the interaction vacuum-solid interface contribute to lowering of laser damage threshold at longer wavelengths.

A model simulating transient optical properties during laser damage in the bulk of KDP/DKDP crystals is presented. The model was developed and tested using as a benchmark its ability to reproduce the well-documented damage initiation behaviors but most importantly, the salient behavior of the wavelength dependence of the damage threshold. The model involves two phases. During phase I, the model assumes a moderate localized initial absorption that is strongly enhanced during the laser pulse via excited state absorption and thermally driven generation of additional point defects in the surrounding material. The model suggests that during a fraction of the pulse duration, the host material around the defect cluster is transformed into a strong absorber that leads to significant increase of the local temperature. During phase II, the model suggests that the excitation pathway consists mainly of one photon absorption events within a quasicontinuum of short-lived vibronic defect states spanning the band gap that was generated after the initial localized heating of the material due to thermal quenching of the excited state lifetimes. The width of the transition (steps) between different number of photons is governed by the instantaneous temperature, which was estimated using the experimental data. The model also suggests that the critical physical parameter prior to initiation of breakdown is the conduction band electron density. This model, employing very few free parameters, for the first time is able to quantitatively reproduce the wavelength dependence of the damage initiation threshold, and thus provides important insight into the physical processes involved.

We present the first fundamental simulation method for the determination of crater morphology from femtosecond-pulse laser damage. To this end we have adapted the Particle-In-Cell (PIC) method for use in the study of laser damage, and developed the first implementation of a pair-potential for PIC codes. We discuss how the PIC method is a complementary approach to modeling laser damage, bridging the gap between fully ab-initio molecular dynamics approaches and empirical models. We demonstrate our method by modeling a femtosecond-pulse laser incident on a flat copper slab, for a range of intensities.

Comprehensive modeling of laser-induced damage in optics for the National Ignition Facility (NIF) has been performed on fused silica wedge focus lenses with a metric that compares the modeled damage performance to online inspections. The results indicate that damage models are successful in tracking the performance of the fused silica final optics when properly accounting for various optical finishes and mitigation processes. This validates the consistency of the damage models and allows us to further monitor and evaluate different system parameters that potentially can affect optics performance.

This paper focuses on the three main effects that can induce wave-front distortion due to thermal lensing in laser gain media: 1) thermo-optic (dn/dT); 2) stress-optic; and 3) surface deformation (e.g., “end-bulging” of a laser rod). Considering the simple case of a side-pumped cylindrical rod which is air- or water-cooled along its length, the internal temperature distribution has long been known to assume a simple parabolic profile. Resulting from this are two induced refractive index variations due to thermo-optic and stress-optic effects that also assume a parabolic profile, but generally not of the same magnitude, nor even of the same sign. Finally, a small deformation on the rod ends can induce a small additional lensing contribution. We had two goals in this study: a) use finite-element simulations to verify the existing analytical expressions due to Koechner1 and Foster and Osterink; and b) apply them to glasses from the SCHOTT laser glass portfolio. The first goal was a reaction to more recent work by Chenais et al. who claimed Koechner made an error in his analysis with regard to thermal stress, throwing into doubt conclusions within studies since 1970 which made use of his equations. However, our re-analysis of their derivations, coupled with our FE modeling, confirmed that the Koechner and Foster and Osterink treatments are correct, and that Chenais et al. made mistakes in their derivation of the thermally-induced strain. Finally, for a nominal laser rod geometry, we compared the thermally-induced optical distortions in LG-680, LG-750, LG-760, LG-770, APG-1, and APG-2. While LG-750, -760, and -770 undergo considerable thermo-optic lensing, their stress-optic lensing is nearly of the same magnitude but of opposite sign, leading to a small total thermal lensing signature.

Femtosecond laser-induced damage threshold (LIDT) measurements for different optical components are well studied for a set of laser pulse repetition rates spanning the range between 1 Hz and 1 kHz. Recent years saw the advent of high-repetition-rate femtosecond systems with relatively high pulse energy. Therefore investigation of LIDT in the MHz region is essential. We performed several comparative femtosecond LIDT measurements on typically used ultrafast optical elements with different Ti:Sapphire laser systems having substantially different pulse repetition rates (a 1 kHz regenerative amplifier and a 4.3 MHz long-cavity oscillator) and found a substantially lower MHz LIDT threshhold.

Dispersive and composite nanomaterials based on multi-walled and single-walled carbon nanotubes and its conjugates with dye of zinc phthalocyanine were produced. The composition and the structure of dispersive and composite materials were investigated using analytical methods of spectroscopy and microscopy. Nonlinear characteristic of nanomaterials of limiters by direct nonlinear scanning and Z-scan method were investigated. Studies suggest the possibility of using such nanomaterials in laser intensity limiters. Proposed threshold model characterizing limiters of powerful laser radiation takes into account the threshold nature of nonlinear interaction of irradiation with the nonlinear material. Threshold effect of nonlinear interaction of laser irradiation with several nonlinear material based on multi-walled and single-walled carbon nanotubes was experimentally found. It was shown that threshold model fit better with experimental data of Z-scan.

A database of spectral and temperature-dependent emissivities was created for painted Al-alloy laser-damage-testing targets for the purpose of improving the uncertainty to which temperature on the front and back target surfaces may be estimated during laser-damage testing. Previous temperature estimates had been made by fitting an assumed gray-body radiance curve to the calibrated spectral radiance data collected from the back surface using a Telops Imaging Fourier Transform Spectrometer (IFTS). In this work, temperature-dependent spectral emissivity measurements of the samples were made from room temperature to 500 °C using a Surface Optics Corp. SOC-100 Hemispherical Directional Reflectometer (HDR) with Nicolet FTS. Of particular interest was a high-temperature matte-black enamel paint used to coat the rear surfaces of the Al-alloy samples. The paint had been assumed to have a spectrally flat and temperatureinvariant emissivity. However, the data collected using the HDR showed both spectral variation and temperature dependence. The uncertainty in back-surface temperature estimation during laser-damage testing made using the measured emissivities was improved from greater than +10 °C to less than +5 °C for IFTS pixels away from the laser burn-through hole, where temperatures never exceeded those used in the SOC-100 HDR measurements. At beam center, where temperatures exceeded those used in the SOC-100 HDR, uncertainty in temperature estimates grew beyond those made assuming gray-body emissivity. Accurate temperature estimations during laser-damage testing are useful in informing a predictive model for future high-energy-laser weapon applications.

A well-known method for the determination of the threshold fluence for laser-induced damage (LIDT) and ablation with single laser pulses is the measurement of the ablated crater area as a function of incident fluence and extrapolation to zero crater area. In this paper, we introduce a general incubation model for a large class of materials, for example dielectric materials and metals, and we compare it to experimental data. We conclude that multi-pulse LIDT measurements can also be based on crater size measurements similar to 1-on-1 damage tests if the incubation does not involve coupling between spatial and temporal processes.

The morphology of laser-induced damage sites at the exit surface of fused silica is tightly correlated to the mode composition of the nanosecond laser pulses at 1064 nm. In the single longitudinal mode (SLM) configuration, a molten and fractured central zone is surrounded by a funnel-shaped surface modification. Ring patterns surround the damage sites when these are initiated by multiple longitudinal modes (MLM) laser pulses. In this last mode configuration, the pulses temporal profiles as well as the damage ring patterns differ from pulse to pulse. The appearance chronology of the rings is found to be closely related to the temporal shape of the laser pulses. This supports that the damage morphology originates from the coupling of a laser-supported detonation wave propagating in air with an ablation mechanism in silica. In our experiments, the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube root of the laser intensity, in good agreement with theory.

Potassium dihydrogen phosphate (KDP) is commonly used for frequency conversion and optical switching applications in many high-power laser systems. Such applications require high damage threshold of KDP crystals. Damage behavior of KDP has been investigated for many years, and the results show that intrinsic or extrinsic defects are responsible for highly localized absorption in KDP materials, and that in turn will cause the laser damage. In this paper, we studied the absorption properties of KDP crystals at wavelengths of 355 nm by using a three-dimensional (3D) photothermal microscope. Several 3D images of the bulk defects were obtained. The results indicated that both surface defects and bulk defects can be determined and analyzed using the 3-D photothermal microscope. Our results indicate that 3D photothermal microscopy is a powerful tool for defect characterization of optical materials for high power laser applications.

This paper introduces a method to determine the uncertainty in the model parameters for a laser damage measurement analyzed via maximum likelihood methods. The behavior of the model parameters with the number of sites (search length) is examined using repeated measurements of a virtual optic. This virtual optic is represented by a probability curve that is a cumulative Gaussian. The damage model is a 2 parameter Weibull distribution. Measurements of various lengths are used to establish convergence of the model parameters. It is shown that increasing search length decreases the variance in the parameters and that the parameters are basically independent.

Standard techniques for characterizing laser damage are ill-suited to the regime in which sparse defects form the dominant damage mechanism. Previous work on this problem using REO’s automated laser damage threshold test system has included linking damage events in HfO₂/SiO₂ high reflector coatings with visible pre-existing defects, and using a probability per defect based on size and local fluence to generate predictions of damage events in subsequent coating runs. However, in all this work the test sites were always in a predefined array, and the association of defects with damage events was done only after the fact. In an effort to make this process both more efficient and less susceptible to uncertainties, we have now developed an adaptive test strategy that puts defect identification and analysis into the loop. A map of defect locations and sizes on a test surface is compiled, and a set of test sites and corresponding fluences based on that map is then generated. With defects of interest now centered on the damaging beam, the problem of higher-order spatial variation in the beam profile is greatly reduced. Test sites in zones with no detectable defects are also included. This technique allows for the test regimen to be tailored to the specific surface under consideration. We report on characterization of a variety of coating materials and designs with this adaptive method.

In this paper we will discuss the challenges of performing comparable laser damage testing as well as a detailed analysis of the measurements conducted on the samples for this harmonisation activity. The goal of the activity is to enlarge the test capacities within ESA’s EarthExplorer space program, especially for the missions ADM Aeolus [5] and EarthCare [6], both having as main payload instruments containing high energy diode pumped nanosecond lasers. Four samples have been compared with the S-on-1 method according to ISO21254-2, two AR1064/0° windows and two silicon wafers leading to an agreement of better than 20% concerning the S-on-1 damage threshold.

We investigate quantitative phase imaging as a measurement method for laser damage detection and analysis of laser induced modification of optical materials. Experiments have been conducted with a wavefront sensor based on lateral shearing interferometry technique associated to a high magnification optical microscope. The system has been used for in situ observation of optical thin films and bulk samples irradiated by 500fs pulses. It is shown that the technique realizes high sensitivity, convenient use and can provide quantitative information on the refractive index or surface modification of the samples under test.

Laser induced damage threshold is a key parameter for all components in high-power laser system, establishing limits of maximum achievable energy, and consequently average power. To provide reliable and stable laser sources, desirable both in academic and industrial area, involved components has to be tested and meet certain quality criteria. To provide such laser source is the goal of HiLASE project, where the development of scalable kW-class laser delivering ps pulses in kHz repetition rate is taking place. Broadband, high damage threshold mirrors are one of the key components for future installations and their development is carried out synergistically with laser system and LIDT station.

When an optical coating is damaged, deposited incorrectly, or is otherwise unsuitable, the conventional method to restore the optic often entails repolishing the optic surface, which can incur a large cost and long lead time. We propose three alternative options to repolishing, including (i) burying the unsuitable coating under another optical coating, (ii) using ion milling to etch the unsuitable coating completely from the optic surface, and then recoating the optic, and (iii) using ion milling to etch through a number of unsuitable layers, leaving the rest of the coating intact, and then recoating the layers that were etched. Repairs were made on test optics with dielectric mirror coatings according to the above three options. The mirror coatings to be repaired were quarter wave stacks of HfO₂ and SiO₂ layers for high reflection at 1054 nm at 45° incidence in P-polarization. One of the coating layers was purposely deposited incorrectly as Hf metal instead of HfO₂ to evaluate the ability of each repair method to restore the coating’s high laser-induced damage threshold (LIDT) of 64 J/cm₂. The repaired coating with the highest resistance to laser-induced damage was achieved using repair method (ii) with an LIDT of 49 - 61 J/cm².

This work focuses on the optimization of a high reflector design for operation at 1 μm wavelength to achieve a high laser damage threshold when tested at pulse durations of 0.19 and 4 ns. Different designs that modify the standing wave electric field distribution of a quarter wave Ta₂O₅/SiO₂ multilayer dielectric coating are considered. It is found that the addition of an extra SiO₂ quarter wave to reduce the peak electric field in the coating, increases the 50% damage probability by over 100% at both pulse durations.

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.

The surface damage morphologies produced by continuous-wave laser irradiation of coated optics were measured and analyzed. A few laser damage morphologies were observed to be bull’s-eye patterns. It is noted that the bull’s-eye pattern has some similarities to Bessel distributions of the form found in solutions of basic heat transfer or surface acoustic wave problems, which may indicate a relationship. If these morphologies are truly thermal phenomenon, then the Bessel ring diameter would be a function of thermal diffusivity. This might indicate that the ring diameter could be used to assess the resistance of a film to laser damage.

Yb³⁺ doped YAG is one of the most promising materials for high energy, high repetition rate laser systems producing nanosecond pulses. YAG as the host medium offers good thermo-mechanical and thermo-optical properties and, if it is used in ceramic form, it can be produced in large sizes with laser-grade optical properties. Large sized, laser-grade gain media are pivotal for the development of high energy kJ-class laser systems. Much effort has been devoted to the development of advanced polishing and coating techniques in order to produce optical materials able to withstand high fluence levels at different environmental conditions. In this paper, we present experimental results for 1 on 1 laser induced damage threshold (LIDT) tests in the nanosecond regime following ISO standards on anti-reflective coated ceramic Yb:YAG samples. Experimental results show that, generally, Ion Beam Sputtering (IBS) coatings perform better than Ion Assisted Deposition (IAD) coatings on low roughness substrates, while IAD and IBS coatings deposited on substrates characterised by higher surface roughness values offer a comparable performance. Performance of IBS coatings improves as substrate roughness decreases, whereas performance of IAD coatings improves as substrate roughness increases. No clear correlation has been observed between LIDT values and temperature or pressure. However, an inspection of damage sites allowed to conclude that both temperature and pressure have an impact on damage morphology.

Using 9 fs UV laser pulses, laser induced ultrafast dynamics in fused silica is investigated in the present study. The free carrier dynamics under few-cycle UV laser excitation far below laser damage threshold in the fused silica was studied. After laser excitation, free carrier in the conductive band is found to turn into self-trapped excitons within about 300fs. It is possible that the trapped exciton will result in the incubation effect under the condition of ultrafast high-frequency pulsed UV laser exposure.

A single pulse of a nanosecond laser was tightly focused in the bulk of transparent materials (soda lime glass, borosilicate glass, fused silica , sapphire and Gorilla Glass) to a beam spot diameter of ~ 2.1μm. A value of the total energy absorbed in the materials was measured with corrections for the transmitted, scattered and reflected components of the incident energy. It was found that 3-11% of the incident radiation was scattered but the total absorption still achieved a very high level of up to 88%. Absorptance dependence on the incident fluence was reasonably approximated by the sigmoidal Hill function. Here we suggest using this analytical description to identify empirical intrinsic laser-induced breakdown threshold (LIBT). Optical damage threshold (ODT) was identified by optical inspection. The results for some materials suggest significantly lower breakdown threshold than that reported earlier for more loosely focused beams. A study of the damage area morphology with a scanning electron microscope (SEM) and a high resolution transmission microscope (HRTEM) revealed existence of the shock waves-affected area with a localized nano-crystallization. Spectroscopic study of the light emission accompanying breakdown showed typical quasi-continuum emission with temperature as high as 8917K (0.8 eV).

In the femtosecond regime laser damage thresholds are often determined by the electric field distribution within the optical component. Commercially available ultra-short pulse laser systems provide ever increasing output powers in fundamental and harmonic wavelengths. Therefore, an increasing demand for frequency conversion or multiwavelengths optics with high damage thresholds for both, fundamental and second harmonic wavelengths is given. These optics are under increased strain and face even more design difficulties. Also, the electric field distribution is of higher complexity and favors multi-photon excitation of high efficiencies. We investigate the LIDT of dichroic high reflecting mirrors under simultaneous exposure to fundamental and second harmonic radiation. As laser source we use a Ti:Sa system delivering sub 200 fs pulses at 780nm/390nm. A delay-line was incorporated to ensure temporal overlap of the 2 pulses in the test plane. Further, the LIDT of a single layer of Ta₂O₅ under irradiation with fundamental and second harmonic radiation is calculated and results are compared with our experiment.

Localized CO₂ laser heating of silica glass has demonstrated its ability to mitigate surface damage on optics used for high power laser applications. The parameters for this process such as the power, the beam size and the exposition time are however critical and some fundamental studies on the silica behavior under CO₂ laser irradiation are required to develop the processes. It is necessary for instance to understand the silica transformation, the material ejection and the thermo-mechanical stresses induced by the laser heating and subsequent cooling. A thermo-mechanical model based on finite-element method has been used to calculate the temperature of silica heated by CO₂ laser irradiation and the residual stress after cooling of the samples. The model, as the different parameters used for calculations, are detailed in this paper and the numerical results are compared to different dedicated experimental studies.

Laser induced periodic surface structures (LIPSS) were generated via interaction of multiple 90 femtosecond 1900 - 3600 nm mid IR laser pulses (3 -10,000) on single crystal Ge targets. For specific laser parameters, both low and high frequency LIPSS are found together, which are oriented perpendicular to each other. Study of polarization dependence of LIPSS revealed that orientation and symmetry of interaction could be controlled by rotating polarization of laser pulses. Low frequency LIPSS formation was consistent with surface plasmon coupling of laser pulses with excited Ge.

In order to improve laser damage resistance of the Final Optics Assembly (FOA), simulation analysis have been done for 1ω, 2ω and 3ω laser beam considering ghost images to the 4th order. The panels of ground glass scatter ghost laser around the FOA walls and the panels of architectural glass absorb the 1^th order energy. The appearance of smoothing fused silica surface defect and the effect of wiping off etching contamination are researched on HF-based etching processes under ultrasonic. Now, 18 shots were executed using 310x310mm laser with 3ns pulse width. During the experiment, the third harmonic laser terminal output energy is 1500J~3500J, and the maximum laser energy flux is about 4J/cm². This presentation addresses the optical configuration of the FOA, the simulation analysis of ghost, the way of ground glasses absorbing energy and the result of laser damage resistance of fused silica on HF-based etching processes under ultrasonic.

In this paper, we present the progress in the development of an automated measurement instrument for optical thin film coatings. Based on the laser-induced surface thermal lensing effect, the instrument shows a measurement sensitivity of absorbance down to 10 ppb, and it provides user-friendly operation of the whole absorption measurement process. Compared with a typical bench-top system the instrument requires little special skills from the operators and is therefore more reliable and reproducible. The specific applications of this instrument include measuring weak absorption, detecting local absorption defects, and monitoring laser-coating-interaction dynamics. The measurement results show that such a high sensitive automated instrument is an effective diagnostic tool for the optimization of optical thin film coatings with desired optical properties.

Having been of special interest in thin film technology for a long time already, mixtures of coating materials are still in the focus of research aiming for highest performance in high power as well as in ultra-short pulse laser applications. On the one hand, coating material combinations allow customizing the coating for a certain application by modifying advantageously the refractive index or the band gap energy. On the other hand, that technology is essential in the production of Rugate-filters, using gradually varied refractive index profiles. Therefore, it is of special interest to get insight into the composition of such mixed layers, not only in terms of refractive index and absorption coefficient, but also to evaluate the fractions of materials involved for gaining a better understanding, and therefore to reach highest possible reproducibility for production of such kind of thin films. In this work, single layers of binary mixtures of aluminum oxide, aluminum fluoride, and silicon dioxide are studied with respect to their composition using extreme ultraviolet reflectometry (EUV-R). As the penetration depth of EUV radiation is only a few tens of nanometers under grazing incidence, this non-invasive measurement technique is sensitive to the near surface composition of the film. Therefore it allows investigating the layer material independently of the substrate on which it was deposited. Using specific absorption edges of the involved materials in the EUV spectrum, an empirical correlation between EUV response and mixture ratio is developed and compared to the deep ultraviolet (VUV) absorption edges of the mixture materials.

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.

In this contribution we present a technology for deposition and testing of interference coatings for optical components designed to operate as in power pulsed lasers. We have designed and are in the process of building a testing apparatus which serves as an addition to our existing optical coating production facility. This allows us to prepare a coating which can then tested and the results might be used to optimize it. The test samples are placed in a vacuum chamber, cooled down to approximately 120K and illuminated by a pulsed laser to determine laser damage threshold of the coatings under conditions similar to real life operation. Optical microscopy and spectrophotometer measurements are going to be used for coating investigation after the conducted experiments.

Photo-thermal deflection (PD) method is one of the typical pump-probe techniques for absorption evaluation. In this study, the PD method was used for detection of a signature of laser-induced damage prior to critical destruction. A setup for the PD technique was incorporated into the nanosecond damage testing system and the absorption of some optics was measured in combination with N-on-1 damage test. The absorption decreased with pulse-laser irradiation at first. Then, the absorption increased at some point. Finally, the laser damage was caused and the absorption decreased. The relaxation times of the absorption were also evaluated to reveal the reason for the variation. The measured results indicated that the decrease came from the disposal of initial contamination and the effect might be considered as laser conditioning. On the other hand, the increase of the absorption might be attributed to generation of laser-induced defects (self-trapped excitons).

In this paper, we report on a continuing multi-year empirical investigation into the nature of the laser survivability curve. The laser survivability curve is the onset threshold as a function of shot number. This empirical investigation is motivated by the desire to design a universal procedure for the measurement of the so-called S on 1 damage threshold. In this year’s paper we investigate the usefulness of scaling the fluence with shot number. First the scaling process is defined and applied to a result from our experimental archives. The probability of damage curve for a single shot test is extrapolated to 10⁴ shots. The scaled result is shown to be very close the observed results providing a basis for extrapolation to very large values of n.

This paper reports on the analysis of laser damage measurements made on an entire lot of approximately identically processed and coated samples. Each sample’s test data is analyzed to determine its probability of damage curve, pi(φ). The probability of damage curves are further processed to derive the defect distribution, fi(φ), for each sample. The individual fi(φ) are then examined to determine if they are likely to have come from a single parent distribution, f(φ), which represents the performance of the manufacturing process.

In the last decades, the resistance to high-power laser flux was largely improved in most of optical components insofar as 1-on-1 measurements are concerned. Another challenge lies in improving their resistance to multiple laser shots for highpower laser applications. Indeed, in multi-pulse irradiation, a decrease of the laser-induced damage threshold with increasing number of pulse was observed in various optical materials as in glasses, crystals, and thin-films. This effect, commonly denominated "fatigue" effect, is a limiting factor in many applications where optics have to be long-lifetime, as for example for space applications. Representing the laser damage probability as a function of pulse number for a given fluence allows to distinguish statistical pseudo-fatigue and fatigue which is due to cumulative material modifications. Investigating on the fatigue effects in the bulk of synthetic fused silica (Suprasil 1®) for different wavelengths, we evidenced that the fatigue effect was due to statistical pseudo-fatigue when irradiated at 1064 nm while the fatigue effect at 355 nm came from cumulative material modifications. The current work is dedicated a more detailed study of fatigue effects in Suprasil 1®, testing the influence of the beam size on the fatigue effects. Moreover, an estimation of the lifetime of the created defects is performed using a destructive technique.

The surface induced flow in micro groove has attracted much attention because it can be used as a passive power source in microfluidic devices. In recent years, some researchers have used the surface induced force to deliver liquids in micro groove. The flow speed should be controlled accurately in order to displace liquids with desirable volumes. In this paper, we investigated the feasibility to control the flow speed by constructing microstructure on surface groove using femtosecond laser. Firstly, we used femtosecond laser to fabricate different microstructures on alumina surfaces with different laser scanning speed. It was found that the flow speed in groove increased when the femtosecond laser scanning speed decreased. And the liquid spread distance was linear to the square of spread time. Then we investigated the dynamics of the liquid flow which was influenced by the surface chemical composition property. Some metal materials with different surface energies were sputtered on the irradiated surface. The coated metal film can also change the liquid spread speed in groove. This work provides a method to obtain the expected controllable spread speed by constructing the microstructure using femtosecond laser.

We present the development of a laser damage and ablation test-bench able to accommodate ultrashort pulses down to 10 fs pulse duration. The laser test-bench is operated in air and we demonstrate its capability to accurately measure the damage and ablation characteristics of optical materials, like fused silica, irradiated by single ultrashort pulses of < 15 fs pulse duration. The careful characterization of beam propagation allows us to ascertain the precise retrieval of laserinduced damage and ablation threshold fluences as well as to identify the energetic regime yielding to beam filamentation.

An accurate evaluation method with an amplified spontaneous emission (ASE) as the irradiation source has been developed for testing thin-film damage threshold. The partial coherence of the ASE source results in a very smooth beam profile in the near-field and a uniform intensity distribution of the focal spot in the far-field. ASE is generated by an Nd: glass rod amplifier in SG-II high power laser facility, with pulse duration of 9 ns and spectral width (FWHM) of 1 nm. The damage threshold of the TiO₂ high reflection film is 14.4J/cm² using ASE as the irradiation source, about twice of 7.4 J/cm² that tested by a laser source with the same pulse duration and central wavelength. The damage area induced by ASE is small with small-scale desquamation and a few pits, corresponding to the defect distribution of samples. Large area desquamation is observed in the area damaged by laser, as the main reason that the non-uniformity of the laser light. The ASE damage threshold leads to more accurate evaluations of the samples damage probability by reducing the influence of hot spots in the irradiation beam. Furthermore, the ASE source has a great potential in the detection of the defect distribution of the optical elements.

Contamination plays a major role in lifetime of vacuum optics. Several efforts have been made to derive suitable models for lifetime prediction in laser-induced contamination related optical breakdown. But the broad spectrum of potential contaminants present in the various applications with their very specific contamination mechanisms complicates the derivation of universal optics degradation models. As one possible contamination initiation process, the impact of optical absorption on the laser-induced contamination and resulting optical breakdown is studied in this work. A set of specifically prepared samples using nanometer sized gold particles embedded in dense IBS anti-reflecting coatings is exposed to radiation of 355nm in low pressure naphthalene atmosphere. Even though the artificial defects are not in direct contact with the contaminant, their influence on the long-term optics performance in dependence on the particle concentration in the coating is evident. In the presence of naphthalene, the artificial nano-defects cause a significantly accelerated degradation compared to reference samples without those defects or in absence of the contaminant. For this specific type of contaminant, a correlation of the optical absorption and long-term durability is derived.

Operating high power space-based laser systems in the visible and UV range is problematic due to laser-induced contamination (LIC). In this paper LIC growth on high-reflective (HR) coated optics is investigated for UV irradiation of 355 nm with naphthalene as contamination material in the range of 10-5 mbar. The investigated HR optics were coated by different processes: electron beam deposition (EBD), magnetron sputtering (MS) or ion beam sputtering (IBS). In-situ observation of contamination induced damage was performed using a long distance microscope. Additionally the onset and evolution of deposit formation and contamination induced damage of optical samples was observed by in-situ laserinduced fluorescence and reflection monitoring. Ex-situ characterization of deposits and damage morphology was performed by differential interference contrast and fluorescence microscopy. It was found that contamination induced a drastic reduction of laser damage threshold compared to values obtained without contamination. Contamination deposit and damage formation was strongest on IBS followed by MS and smallest on EBD.

Laser-induced-damage-threshold of two types of metal-dielectric mirrors was tested using a laser apparatus working at 800 nm wavelength with 1 ps pulse length at 1 kHz repetition rate and in 106-on-1 test mode. Four sets of mirror samples with different layer system designs using a multilayer Ta₂O₅/SiO₂ coating on silver or gold metal layer were manufactured. Both BK7 and fused silica substrate materials were used for manufacturing of samples. The measured damage thresholds at 45 deg incidence and P-polarization were compared with computed properties of layer system and used materials.

A setup was developed at Laser-Laboratorium that allows determination of LIDT data under simultaneous 1064nm, 532nm and 355nm irradiation (3l), in order to investigate the influence of combined irradiation of specific elements with multiple wavelengths. Utilizing a fully characterized test laser beam and a well defined sample environment, 10.000-on- 1 LIDT data of high-reflecting mirrors (AOI 45°) in vacuum were determined both at multiple wavelengths (3l) and at 355nm alone. In addition, a test optics (HR355, 45°, ambient conditions) was irradiated for 150 million pulses at (3l) at a fixed fluence of 2 J/cm² as a certification test.

We describe the cleaning processes, treatment methods, facilities, and cleanliness verification techniques developed to achieve and maintain the demanding cleanliness requirements for both hardware and optics used in the National Ignition Facility (NIF).

Surface damage and surface contamination of optics has long been a source of problems for laser, lithography and other industries. Nano-sized surface defects may present significant performance issues in optical materials for deep UV and EUV applications. The effects of nanometer sized surface damage (scratches, pits, and organics) on the surface of optics made of traditional materials and new more exotic materials is a limiting factor to high end performance. Angstrom level smoothing of materials such as calcium fluoride, spinel, zinc sulfide, BK7 and others presents a unique set of challenges. Exogenesis Corporation, using its proprietary Accelerated Neutral Atom Beam (ANAB) technology, is able to remove nano-scale surface damage and contamination and leaves many material surfaces with roughness typically around one angstrom. This process technology has been demonstrated on nonlinear crystals, and various other high-end optical materials. This paper describes the ANAB technology and summarizes smoothing results for various materials that have been processed with ANAB. All surface measurement data for the paper was produced via AFM analysis. Exogenesis Corporation’s ANAB processing technology is a new and unique surface modification technique that has demonstrated to be highly effective at correcting nano-scale surface defects. ANAB is a non-contact vacuum process comprised of an intense beam of accelerated, electrically neutral gas atoms with average energies of a few tens of electron volts. The ANAB process does not apply normal forces associated with traditional polishing techniques. ANAB efficiently removes surface contaminants, nano-scale scratches, bumps and other asperities under low energy physical sputtering conditions as the removal action proceeds. ANAB may be used to remove a precisely controlled, uniform thickness of material without any increase of surface roughness, regardless of the total amount of material removed. The ANAB process does not involve the use of slurries or other polishing compounds and therefore does not require any post process cleaning. ANAB can be integrated as an in-situ surface preparation method for other process steps in the uninterrupted fabrication of optical devices.