Multilayer mirrors are fluence-limited by nodular defects. Such defects originate from the deposition source, inadequate cleaning, transport, pump down, heating, shedding from rotating hardware, etc. These overcoated inclusions behave as micro-lenses resulting in light intensification within the multilayer structure. To minimize the impact of these defects, a planarization process has been developed to reduce geometric-induced light intensification. By exploiting the angledependent etching rate of materials, a deposit-and-etch process reduces nodular defect height and diameter. Planarized defects demonstrate a greater than 20x increase in laser resistance at a wavelength of 1064 nm and pulse length of 10 ns. Process parameters were explored such as planarization efficiency of the coating materials, discrete versus continuous etching, thick planarization layers for substrate defects, and etching throughout the multilayer to planarize coating defects.

_{Hafnium oxide is the most frequently used high-index material in multilayer thin-film coatings for high-power laser applications ranging from near-infrared to near-ultraviolet. Absorption in this high-index material is also known to be responsible for nanosecond-pulse laser-damage initiation in multilayers. In this work, modification of the near-ultraviolet absorption of HfO2 monolayer films subjected to irradiation by continuous-wave (cw) 355-nm or 351-nm laser light focused to produce power densities of the order of ~100 kW/cm² is studied. Up to a 70% reduction in absorption is found in the areas subjected to irradiation. Temporal behavior of absorption is characterized by a rapid initial drop on the few-tens-of-seconds time scale, followed by a longer-term decline to a steady-state level. Absorption maps generated by photothermal heterodyne imaging confirm the permanent character of the observed effect. Nanosecond-pulse, 351-nm and 600-fs, 1053-nm laser-damage tests performed on these cw laser-irradiated areas confirm reduction of absorption by measuring up to 25% higher damage thresholds. We discuss possible mechanisms responsible for near-ultraviolet absorption annealing and damage-threshold improvement resulting from irradiation by near-ultraviolet cw laser light.}

In this study multilayer thin film optical coatings, which are indispensable parts of optical systems are investigated from a heat transfer point of view. Laser irradiation induced temperature distribution on a multilayer coating stack is obtained by discretizing the heat diffusion equation using the finite volume method. In order to obtain mathematical representation of the energy flow and Electric Field Intensity (EFI) through the stack, Maxwell equations are solved by using the commercial software MacLeod®. Laser energy, which is absorbed by the multilayer stack in terms of heat, is calculated as a function of space and time by using the computed EFI, coating materials' optical properties and Gaussian laser beam parameters. Computed heat load is used in the finite volume solver ANSYS FLUENT® through a user defined function. Temperature distribution on a 19 layer HR multilayer coating stack irradiated by 1064 nm laser beam are obtained for both quarter wave and non-quarter wave designed configurations. Results of numerical simulations show that maximum temperature rise is seen in the first high index layer for quarter wave design (QWD). In addition to that, high temperatures are also seen in film/film interfaces, which is associated to both EFI distribution on the stack and wide differences in material properties between high and low index film layers. Non-quarter wave design (NQWD) is seen to be successful in decreasing temperatures at high index layers and at film/film interfaces. But it also changes the EFI distribution inside the multilayer stack, increasing absorbed laser energy and resulting in higher temperatures at modified low index layers.

The laser induced damage threshold (LIDT) and damage morphology of the monolayer coating are easily influenced by the finish condition of the substrate, which makes it difficult to compare the LIDT of different coating materials. In order to eliminate the influence of defect and sub-defect on the substrate, HfO₂, Sc₂O₃, Y₂O₃, Al₂O₃ and SiO₂ monolayer coatings were prepared on 1064 nm HfO₂/SiO₂ high reflection coatings, using conventional e-beam deposition. The LIDT, as well as the damage morphology after laser irradiation at wavelength of 1064 nm, was measured and compared with that of the monolayer coating deposited on BK7 glass substrate.

This Brewster angle thin film polarizer damage competition is a continuation of last year’s test with “P” polarization results published in 2012 and “S” polarization results in this study. This competition allows a direct laser resistance comparison between polarizations because the samples were laser damage tested under identical conditions. The requirements of the coatings are a minimum transmission of 95% at “P” polarization and minimum reflection of 99% at “S” polarization at 1064 nm and 56.4 degrees angle of incidence. The choice of coating materials, design, and deposition method were left to the participant. Laser damage testing was performed according to the ISO 11254 standard utilizing a 1064 nm wavelength laser with a 10 ns pulse length operating at 20 Hz. A double blind test assured sample and submitter anonymity. In addition to the laser resistance results, details of deposition processes, coating materials and layer count, and spectral results are shared.

General precursors and growth model of Laser Induced Damage (LID) have been the focus of research in fused silica material, such as polishing residues, fractures, and contaminations. Assuming the absorption due to trapped material and mechanical strength is the same across the surfaces, various studies have shown that the LID could be minimized by reducing the light field intensification of the layers upon the laser strikes. By revisiting the definition of non-ionising radiation damage, this paper presents the modelling work and simulation of light intensification of laser induced damage condition. Our contribution is to predict the LID growth that take into various factors, specifically on the light intensification problem. The light intensification problem is a function of the inter-layer or intra-layer micro-optical properties, such as transmittance and absorption coefficient of the material at micro- or sub-micro-meter range. The proposed model will first estimate the light propagation that convoluted with the multiply scattering light and subsequently the field intensification within the nodule dimension. This will allow us to evaluate the geometrical factor of the nodule effect over the intensification. The result show that the light intensification is higher whenever the backscattering and multiple scattering components are higher due to its interference with the incoming wave within its coherency.

To cover the preparatory test issues of upcoming ESA space laser missions, in joined effort amongst various laboratories, an adaptation of existing laser damage test benches has been performed. Conventional S-on-1 tests were extended with raster scanning procedures. Various aspects of characteristic damage curve issues are discussed. Sensitive surface analysis like time-of-flight SIMS is used to identify potential low density low damage threshold precursors. The inter-correlation of flight module testing and preceding single component testing is demonstrated. Finally, the successful execution of a flight module endurance test with more than 200 Mio. shots is detailed.

In the determination of the laser-induced damage threshold (LIDT) of optical coatings former Round-Robin experiments stand as the empirical foundation for the development of the International Standard as it is known today. In 1983 and 1997 such experiments were conducted at the fundamental wavelength of the Nd:YAG laser under atmospheric conditions settling the International Standard as it is known today. To cope with the growing demand of LIDT testing for satellite missions, existing test methods have to be extended to deal with operation in space-like environments. This requires LIDT measurements performed under customized vacuum conditions to validate the laser resistance capability and estimate the life time of optical components. To foster the quality of measurements in such environments the need for an inter-laboratory comparison in vacuum conditions emerged.

Laser damage testing is widely utilized by laser and laser system builders to ensure the reliability of their products. When damage is due primarily to sparse defects, the relatively limited data sets acquired under typical testing protocols tend to imply that laser damage probabilities go to zero below some reported damage threshold. However, this is rarely an accurate picture of the actual damage characteristics of the sample set. This study attempts to establish a correlation between observed coating defects and laser damage (from a 1064 nm laser in the nanosecond regime), utilizing a large sample size from a single coating run, together with the actual fluence levels present at the defect sites. This correlation is then used to predict damage for optics coated under different circumstances. Results indicate that it might be possible to develop an alternate methodology for determining damage characteristics, based on observed defects, which is both more reliable and less time-consuming than traditional laser damage testing.

Bulk laser damage variability in deuterated potassium di-hydrogen phosphate (DKDP) crystals is well known and makes online conditioning of multiple-beam laser systems difficult to optimize. By using an empirical model, called Absorption Distribution Model (ADM), we were able to map the damage variability of the crystals (boule to boule as well as within the same boule) in terms of defect populations using a damage probability test. Furthermore, a relationship on defect density and the relative damage behavior of a boule based on its late growth behavior have been found and has been used successfully to predict laser damage/conditioning using a damage probability test only.

Anti-reflecting (AR) surface relief nano-textures have been integrated with fused silica diffraction gratings to demonstrate the potential of stable diffractive 3ω beam samplers with increased energy to target at the National Ignition Facility (NIF). TelAztec’s AR texturing process was used to etch Random-type AR (RAR) microstructures in sub-scale NIF Grating Debris Shields consisting of large pitch, shallow line gratings. This superposition yielded the desired ~3.5% increase in zero-order transmission uniformly over the full aperture without compromising the grating function. Another fused silica window fabricated with RAR nano-textures in both faces for a 3ω (351nm) transmission of 99.5%, was subjected to capillary condensation tests to evaluate the resistance of the RAR texture to the adsorption of organic compounds. It was found that for a one day exposure time to a surrogate suite of organic contaminants, the RAR textured fused silica surfaces adsorbed less than one fourth the amount of organic contaminants found on a NIF baseline hardened sol-gel AR coated optic. In two additional exposure cycles, further RAR process refinement reduced the amount of adsorbed organics to a level nearly 200 times below the current NIF baseline. Significantly, the 3ω transmission of the RAR textured window remained unchanged after all three exposure cycles, whereas the sol-gel coated windows showed losses up to 4.9% for the highest contaminant concentration. Large beam pulsed laser damage testing of RAR textured fused silica windows was conducted with the Optical Sciences Laser (OSL) at NIF. The RAR sample damage resistance was found to be equivalent to the current NIF baseline - even after multiple aggressive chemical cleaning cycles. Lastly, a series of RAR textured and sol-gel AR coated windows were subjected to commercial 3ω pulsed laser damage testing at Quantel. The results indicate an average RAR damage threshold of 26 J/cm², a level about 80% of the two NIF fused silica samples tested, two times higher than the best performing ion beam sputtered thin-film AR coatings reported in the literature, and six times higher than catalog high power laser window coating specifications.

3D Meta-Optics are optical components that are based on the engineering of the electromagnetic fields in 3D dielectric structures. The results of which will provide a class of transformational optical components that can be integrated at all levels throughout a High Energy Laser system. This paper will address a number of optical components based on 2D and 3D micro and nano-scale structures and their performance when exposed to high power lasers. Specifically, results will be presented for 1550 nm and 2000 nm spectral bands and power densities greater than100 kW/cm².

Femtosecond laser pulse irradiation of inorganic glasses allows a selective modification of the optical properties with very high precision. This results in the possibility for the production of three-dimensional functional optical elements in the interior of glass materials, such as optical data storage, waveguide writing, etc. The influence of the chemical glass composition to the response upon ultrashort laser irradiation has not been studied systematically. For that, simple silicabased model glasses composed of systematically varying alkaline- and earth-alkaline components were prepared, irradiated on the surface and in the volume with single fs-laser pulses (~130 fs, 800 nm), and were subsequently analyzed by means of micro-Raman spectroscopy and quantitative phase contrast microscopy in order to account for changes in the glass structure and for alterations of the optical refractive index, respectively. The Raman spectroscopic studies of the laser-irradiated spots revealed no change in the average binding configuration (the so called Q-structure), but local changes of bond-angles and bond-lengths within the glass structure structure. Those changes are explained by structural relaxation of the glass network due to densification caused by a transient laser-induced plasma generation and the following shock wave and other thermal phenomena. Glasses with a low amount of network modifiers show changes in the Si-O network while glasses with a high amount of network modifiers react primarily via variation of the nonbridging oxygen ions. The results are discussed in terms of possible structural response mechanisms and conclusions are outlined regarding glass compositions with technical suitability for fs-laser modifications.

Various methods and procedures have been developed so far to test laser induced optical damage. The question naturally arises, that what are the respective sensitivities of these diverse methods. To make a suitable comparison, both the processing of the measured primary signal has to be at least similar to the various methods, and one needs to establish a proper damage criterion, which has to be universally applicable for every method. We defined damage criteria based on the probability density distribution of the obtained detector signals. This was determined by the kernel density estimation procedure. We have tested the entire evaluation procedure in four well-known detection techniques: direct observation of the sample by optical microscopy; monitoring of the change in the light scattering power of the target surface and the detection of the generated photoacoustic waves both in the bulk of the sample and in the surrounding air.

This paper continues an investigation into the applicability of maximum likelihood methods to the problem of laser damage threshold measurement. This year’s investigation applies maximum likelihood methods to an archival set of data and compares two models of damage probability. The Type 1 model has a defined threshold, below which the probability of damage is identically zero, the Type 2 model has finite probability of damage at all fluences. The models are applied to an archival set of data and shown to have similar predictions for the damage threshold, the models differ on the probability of damage at higher fluences. This difference is traced to the fluence selection methods, which in the case of this data set were meant to focus more on the area around threshold. The paper concludes with some next steps in the development of maximum likelihood methods as a laser damage threshold technique.

In this paper, we present laser damage threshold testing performed on Un-Coated Fused Silica (SiO₂) substrates after multiple laser pulse irradiation. We will outline our methods of testing and observation of laser damage. Using carefully prepared 1” optical flats with 0.25” thickness, we observe competition between laser damage on the surface and in the bulk of the optic. Damage in the bulk is observed at the level of approximately 40-50 J/cm2 when irradiated with 1,000- 3,000 shots per site. Damage appears initially on the back surface of the substrate (without visible damage to the front/focused surface) and propagates slowly in time through the bulk towards the front of the optic. We believe this is due to self-focusing of the laser beam in the bulk material. An understanding of surface damage threshold has important consequences for applications, such as LIDAR, laser machining, and the lifetime of optical components. This work was done within the laser mission testing for NASA’s Ice, Cloud, and land Elevation Satellite-II (ICESat-II) program at Goddard Space Flight Center in Greenbelt, MD.

Since the late 1990s staff at national laboratories have been studying the effects of high energy focussed laser beams [>100J] on a variety of plasma physics targets to understand the disassembly of targets and their effects on target chamber surfaces. Target geometries have included metal foils, polymer foils, metal cylinders or cones, gas bags, metal wires and complex geometries of combinations of the above. The post shot target remnants have been studied by both optical microscopy and scanning electron microscopy. The morphology of exposed targets indicated phase changes and other physical phenomena [shock, spall, crater formation and material ejection]. Pre and post weighing of the targets has been used to determine mass lost from the target. Initially most of the material distribution analysis was performed by catching target by-products with glass or silica witness plates. Spatial and image analysis of micrographs has been used to measure angular distributions of material and its form. Spectrophotometry of the exposed witness plates in the UVVis- NIR region allowed transmission spectra to be determined and the reduction of transmittance at the laser wavelengths of interest. It also allowed estimation of average debris thickness. Shrapnel size and velocity has been studied by capturing fragments in silica aerogels. One unexpected aspect of studying the witness plates was the identification of secondary emissions from solid surfaces close to the irradiated target, this showed that the near environment of the target is also important in determining overall material distributions. We have been fortunate to find interested collaborators at other UK, European and US laboratories that have brought considerable insight into target disassembly processes and palliative measures.

In this paper some basic investigations about laser-induced contamination are reported. As contamination materials pure aromatic hydrocarbons (naphthalene and anthracene) were used. A particular focus of the tests was to investigate the impact of laser-induced contamination on damage threshold. Onset and evolution of deposit formation and damage were observed in-situ by laser-induced fluorescence and transmission monitoring. As optical samples uncoated fused silica substrates and AR and HR coated optics with different coating morphology, depending on coating process (e-beam, magnetron sputtering) were investigated. Ex-situ characterization of deposits and damage morphology was performed by differential interference contrast, fluorescence, and atomic force microscopy. The tests were run with pulsed UV light at 355 nm. Partial pressure of contamination material in the range of 10^-4 mbar induced a drastic reduction of laser damage threshold compared to values obtained without contamination.

Surface modification of fused silica windows caused by the laser ablation of surface-bound microparticles is investigated. Using optical and electron microscopies between laser pulses, we detail the ablation, fragmentation and dispersal of 2-150 μm diameter particles of various materials. Following complete ablation and ejection of all debris material, surface pitting was found to be highly dependent on material type and particle size. Subsequent light propagation modeling based on pit morphology indicates up to ~4x intensification. Understanding this class of non-local, debris-generated damage is argued to be important for effective design of high-power optical windows and debris-mitigation strategies.

Crater formation that accompanies laser-induced damage is the result of material ejection following the rapid, localized heating to temperatures on the order of 1 eV. The objective of this work is to compare the material ejection behavior in fused silica and KDP crystals as captured using time-resolved shadowgraphy. These two materials are of fundamental importance in ICF class laser systems but they also represent materials with significantly different physical properties. We hypothesize that these different properties can affect the material ejection process.

The National Ignition Facility has recently achieved the milestone of delivering over 1.8 MJ and 500 TW of 351 nm laser energy and power on target, which required average fluences up to 9 J/cm² (3 ns equivalent) in the final optics system. Commercial fused silica laser-grade UV optics typically have a maximum operating threshold of 5 J/cm². We have developed an optics recycling process which enables NIF to operate above the laser damage initiation and growth thresholds. We previously reported a method to mitigate laser damage with laser ablation of the damage site to leave benign cone shaped pits. We have since developed a production facility with four mitigation systems capable of performing the mitigation protocols on full-sized (430 mm) optics in volume production. We have successfully repaired over 700 NIF optics (unique serial numbers), some of which have been recycled as many as 11 times. We describe the mitigation systems, the optics recycle loop process, and optics recycle production data.

In the past, the degradation of 405 nm fiber-coupled diode laser systems was investigated in detail with focus on the input end. The coupling and transmission loss of the laser light was associated to the growth of a periodic structure on the input surface. To reduce this damage, a short launch-fiber with a good surface quality was used on the input end surface. Thereby the power transmission was stabilized for at least one month. However, damage structures appeared on the output surface of the single-mode fiber. To investigate this effect, damaged samples were taken after different periods of time and examined with a scanning electron microscope (SEM). Bulges with a submicron periodic structure were found in the core region, too. Additionally, measurements of spectral loss were performed, showing the formation of color centers in the deep UV along the length of the fiber.

We solve the time-independent (o.d.e.) propagation equations for the c.w. operation of an array of externally coupled fiber amplifiers with internal reflection, in which Kerr and resonant nonlinearities play a part. A transcendental equation for each amplifier is obtained, collectively yielding multiple distinct array solutions, which are characterized in terms of their mutual phase coherence. We find that the two types of nonlinearity (Kerr and resonant) affect the solutions in distinct parameter regimes. The relation of Strehl ratio to output power at fixed wavelength and feedback level reveals that phase-locking may occur due to nonlinearity as opposed to mode selection, in accordance with recent experiments (H-S Chiang, J.R. Leger, J. Nilsson and J. Sahu, Optics Letters (2013)). The individual lasing instability (“rogue” lasing) anticipated by A.E. Siegman in 2004 we observed only at low feedback levels in a small number of cases.

Damage mechanisms in thin films are reviewed from femtosecond pulse to CW laser illumination. Special emphasis is given to the role of native and laser induced defects, recent successes and the need for better diagnostic tools.

A model explaining a number of experimental observations on nanosecond laser induced damage in KTP is presented. According to this model the nanosecond laser damage mechanism in KTP is composed of two successive steps. The generation of transient laser damage precursors by multi-photo absorption and the subsequent heating of conduction band electrons provided by the damage precursors. The strong synergy of 1064 nm and 532 nm wavelengths during simultaneous irradiation is quantitatively explained as well as the polarization-dependent anisotropy of the damage threshold. We further give some details on a multi-pulse irradiation experiment carried out with verified smooth temporal pulses that confirmed the existence of statistical pseudo-fatigue at 1064 nm.

We describe the evolution of laser damage spots on bulk nickel generated by multipulse femtosecond laser irradiation with a 100 μm x 100 μm square flat-top beam profile as a function of the laser fluence and the number of pulses incident on the target. This large-area irradiation simulates conditions associated with the interaction of femtosecond laser pulses on a remote target. The larger area laser damage sites are characterized either by a series of self-organized surface structures with micro- and nanoscale features or a deep circular pit rather than a crater that mirrors the beam profile. Furthermore, the ablation rate of the deepest feature sharply increases above a laser fluence of 2 J/cm²; this increase is associated with the creation of a deep circular ablation pit generated during ablation with the first few pulses on the sample that continuously grows upon multipulse irradiation due to the focusing of incident laser energy into the pit by the sloped pit surfaces.

An imaging of strongly excited thin film dielectric coating is done by the means of femtosecond time-resolved off-axis digital holography (TRDH). Ta₂O₅ single layer coating have been investigated at different time moments in transmission mode. The evolving damage process was recorded in series of microscopic amplitude and phase contrast images. Different processes were found to occur and namely: Kerr effect, free-electron generation, ultrafast lattice heating and shock wave generation. The trends in electronic contribution are qualitatively reproduced by the theoretical model while the other effects require additional studies.

Interpretation of spatial and time resolved images of rear surface ns laser damage in dielectrics requires understanding of the dynamic interaction of the incoming laser beam with the confined expanding plasma in the material. The detailed kinetics of the plasma, involving both expansion and retraction, depends on details of reflection and absorption in the hot material. The growth of the hot region is treated using a model previously developed to understand laser peening. The pressure is found to scale as the square root of laser intensity and drops off slowly after energy deposition is complete. For the conditions of our experimental observations in fused silica, our model predicts a pressure of about 9 GPa and a surface expansion velocity of about 1.5 km/sec, in good agreement with experimental observation.

The ELI-beamlines project will use high power ultrafast lasers with high average powers up to 1kW and peak powers up to 10PW. The project presents a major challenge in terms of damage threshold of ultrafast mirror coatings and gratings. In order to assess the LIDT of ultrafast coatings in the expected environments a test station was constructed in the PALS facility. The setup can use beams from a 25TW Ti:Sapphire laser system with a 10Hz repetition rate. Testing is performed mainly with a secondary 1 kHz 40fs while contamination levels are investigated.

We propose a systematic approach that may apply to many complex interactive networks, such as biological or electronic neural assemblies, which was partly inspired by mathematical features of phased laser arrays. Using an appropriate quasi-logarithmic transformation, a Fox-Li integral equation of linearly coupled phased laser arrays is mapped to a semi-equivalent coupled oscillator description, of which the interaction term is decomposed into orthogonal projections. Based on traditional ideas of symmetry, orthogonality, completeness, and the physical concept of criticality, techniques are proposed for the description of the dynamics and organization of massively nonlinearly interconnected networks, which may serve as memories, or perform computational operations in biological neuron assemblies, or models of evolution, pathology, ecological and social networks, individual and collective behavior, etc.

In the determination of the laser-induced damage threshold of optics, the correct estimation of damage probabilities is essential. In this publication, a simple procedure based on physical considerations is proposed to optimize the calculation of the damage probabilities by using a cumulative algorithm. The predicted status of test sites at higher and lower fluences than actually tested provides the basis for a new data reduction. It is shown that the proposed algorithm increases the statistically relevant amount of data per fluence interval dQ_i by using virtual test sites. Thus, the uncertainty σ_i in the calculation of the damage probabilities is reduced significantly and the subsequent linear regression of the damage probabilities will have a reduced least squared error. Non-linear regression of the damage probability according to defect-induced damage models as published numerous times in recent years are also performed to utilize a better confidence level which will be shown exemplarily.

During the 1990s the International Standard for absorptance testing of optical coatings was developed. Based on the method of laser calorimetry and after years of theoretical and empirical work, ISO 115511 was revised and published in its current version in 2003. Laser calorimetry is based on the measurement and evaluation of the temperature increase caused by absorption in the sample exposed to laser radiation. In dependence on the thermal diffusivity of the sample, a temperature distribution develops in the irradiated sample. Therefore, finding a correlation of temperature increase and absorption becomes a complex task. This challenge was met by keeping the sample geometry to a standard size and simulating the thermal distribution for a number of optical materials. By this, LZH developed a calorimetric test set-up that offers fully calibrated absorptance values for sample diameters of 25 mm (or 1 inch) with a total error of below 13 % and a relative measurement error of below 5%. However, this technique is limited to the mentioned sample geometry. This work presents a new approach to adjust the measurement configuration to numerous sample sizes of standard circular laser components. Finite element analysis and experimental verification is presented for exemplary values of the samples diameters. Based on a new sample mount concept, this procedure allows utilizing all flexibility in test wavelength and angle of incidence, combined with the sensitivity level sufficient for current optical materials.

In this study influence of temporal effects are investigated within a context of laser-induced damage threshold (LIDT) measurements. 1-on-1 LIDT testing has been performed with laser operating in single- and multilongitudinal mode regimes. Four fused silica samples were chosen for investigation. Qualitative differences in the damage morphology and damage probability curve have been observed. Analysis of these phenomena was performed by employing Monte Carlo simulations representing the statistical interaction between laser irradiation and randomly distributed damage precursors. The results and findings of this study are reported and discussed in detail.

We report on the continuation of a comparative study of different fused silica materials for ArF laser applications. After selecting potentially suited fused silica materials from their laser induced absorption and compaction obtained by a short time testing procedure, accelerated life time tests have been undertaken by sample irradiating at liquid nitrogen temperature and subsequent direct absorption measurements using the laser induced deflection (LID) technique. The obtained degradation acceleration strongly differs between fused silica materials showing high and low OH contents, respectively. As a result, a difference in the absorption degradation mechanism between high and low OH containing fused silica is proposed. Consequently two different scenarios for an acceleration of the absorption degradation are derived.

This paper presents a first look at the application of maximum likelihood estimation methods to S on 1 testing by comparing results with an analysis that is typical of our previous reports and consistent with ISO 21254. In traditional, ISO tests, the data collected from an S on 1 test is processed to give a set of fluences representing the no-damage or safe operating fluence (SOF) as a function of the number of shots. The (SOF,N) ordered pairs are then fitted to a model and the model is used to extrapolate the SOF to large values of N. In the present report, the entire data set from an ISO S on 1 test is processed via maximum likelihood methods to estimate the probability curve as a function of fluence, P(Φ). The probability of survival to N shots is calculated, under the assumption that P is independent of N, to give the final results. The maximum likelihood method shows promise for application to S on 1 testing.

The determination of surface damage densities of thick optical components is tricky due to the occurrence of non-linear effects (Brillouin and Kerr) that affect the beam propagation through the optics. It is then compulsory to record the beam parameters, mainly the temporal profile, in order to predict and calculate fluence and/or intensity on the rear surface taking into account the non-linear beam propagation. Experiments have been realised with the use of large beams and several phase modulations were activated, leading to numerous peak intensities due to the occurrence of temporal amplitude modulations. Results are first compared in the case of thin optics in order to separate the intrinsic absorptions by the defects which are the weak points of the optics to the effect of the non-linear propagation. The correspondence between the length of the filaments and the beam parameters has been realised in order to highlight the relevant beam parameters that have to be considered for the damage test of thick optics. The whole of measurements and modeling permit us to measure more accurately the rear surface damage of thick optics due to intrinsic defects.

An 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 range were performed for both the characterization of optical materials and complete F-Theta lenses, utilizing a 500 W Yb fiber laser (λ = 1070 nm) to induce thermal load. Different combinations of bulk materials and AR coatings were examined to minimize absorption and to evaluate potential approaches for thermal compensation. Additionally, bulk and surface / coating absorption coefficients were determined by means of curve-fitting procedures. Furthermore, F-Theta lenses were tested to gain understanding of the thermal behavior of the entire optical system.

A capability to suppress laser-induced ionization of dielectric crystals in controlled and predictable way can potentially result in substantial improvement of laser damage threshold of optical materials. The traditional models that employ the Keldysh formula do not predict any suppression of the ionization because of the oversimplified description of electronic energy bands underlying the Keldysh formula. To fix this gap, we performed numerical simulations of time evolution of conduction-band electron density for a realistic cosine model of electronic bands characteristic of wide-band-gap cubic crystals. The simulations include contributions from the photo-ionization (evaluated by the Keldysh formula and by the formula for the cosine band of volume-centered cubic crystals) and from the avalanche ionization (evaluated by the Drude model). Maximum conduction-band electron density is evaluated from a single rate equation as a function of peak intensity of femtosecond laser pulses for alkali halide crystals. Results obtained for high-intensity femtosecond laser pulses demonstrate that the ionization can be suppressed by proper choice of laser parameters. In case of the Keldysh formula, the peak electron density exhibits saturation followed by gradual increase. For the cosine band, the electron density increases with irradiance within the low-intensity multiphoton regime and switches to decrease with intensity approaching threshold of the strong singularity of the ionization rate characteristic of the cosine band. Those trends are explained with specific modifications of band structure by electric field of laser pulses.

The temperature dependence of the laser-induced damage threshold on optical coatings was studied in detail for laser pulses from 123 K to 473 K at different temperature using Nd:YAG laser (wavelength 1064 nm and pulse width 4 ns) and Ti:Sapphire laser (wavelength 800 nm and pulse width 100 fs, 2 ps, and 200 ps). The six kinds of optical monolayer coatings were prepared by electron beam evaporation and the coating materials were SiO₂, Al₂O₃, HfO₂, ZrO₂, Ta₂O₅, and MgF₂. For pulses longer than a few picoseconds, the laser-induced damage threshold of single-layer coatings increased with decreasing temperature. This temperature dependence was reversed for pulses shorter than a few picoseconds. We describe the physics models to explain the observed scaling. The electron avalanche is essential to explain the differences in the temperature dependence. In other words, the balance between linear process such as electron avalanche etc. and nonlinear process such as multiphoton ionization etc. will be able to decide the tendency of the temperature dependence. The proposed model also gives one of possibility for an extremely high LIDT optics.

We present, to our knowledge, the first adaptation of the Particle-In-Cell (PIC) simulation method for use in the study of femtosecond pulse laser damage, including the first implementation of the Morse potential for PIC codes. We compare the PIC method to a wide variety of currently used modeling schemes, ranging from purely ab-initio molecular dynamics simulations to semi-empirical models with many fitting parameters, and show how PIC simulations can provide a complementary approach by filling the gap in theoretical methodology between the two cases. We detail the necessity and implementation of an inter-atomic pair-potential in PIC studies of laser damage. Lastly, we use our model to treat the full laser damage process of a copper target, and show that our results compare well to simple scaling laws for crater size.

We develop and characterize high index of refraction thin films by e-beam evaporation of Ti3O5, Ta2O5 and Nb2O5 in reactive, ion-assisted deposition processes. We then deposit broad bandwidth high reflection (HR) coatings based on quarter-wave stacks of these high index layers alternating with SiO2 low index layers. The HR band is centered at 1054 nm and designed for 45o angle of incidence. We compare the laser induced damage thresholds of these coatings in order to explore tradeoffs between their laser damage properties and HR bandwidths.

A photothermal apparatus with submicron resolution (~0.5 μm) using a focused 532 nm pump beam is demonstrated. A coaxial probe (633 nm HeNe) detects changes in reflectivity. In addition to detecting modulation of the probes total reflected power caused by perturbation of the Fabry-Perot resonance, a pinhole in confocal geometry detects changes to the probe beam caused by thermal lensing. Based on signal-to-noise measurements made with a film of known absorption, a sensitivity of ~10 ppm is predicted with 200 mW of pump power. The instrument is applied to mapping of local absorption spikes in a Sc₂O₃ film and characterization of laser-induced ripple pattern in a HfO₂ film.

We developed high-resistant anti-reflection (AR) coating by using Al₂O₃/SiO₂ multilayer for Yb:YAG thin disk amplifier. The AR coating was designed both for 940 nm of pump laser at an incident angle of 30 degrees and for 1030 nm of seed laser at 5 degrees. The Al₂O₃/SiO₂ multilayer was deposited by using the electron beam evaporation technique on a fused silica substrate and then the laser induced damage threshold was evaluated. The sample was irradiated by 1030 nm laser with 520 ps duration delivered from the Yb:YAG thin-disk regenerative amplifier. The measured damage threshold of the Al₂O₃/SiO₂ AR coating was 75 J/cm².

Laser-induced-damage-threshold (LIDT) of polarizing Brewster-angle beam splitters based on two different layer system designs was measured using a laser apparatus working at 1060 nm wavelength with 10 ns pulse length and 1-on-1 test mode. Two sets of samples with different design of layer system using TiO₂/SiO₂ coating materials were examined. Both BK7 and fused silica substrate materials were used for manufacturing of samples. The measured damage thresholds in S- and P-polarization were compared with computed values of the internal electric field inside of the layer system and with computed values of absorption as a measure of integral interaction of laser beam throughout the layer system.

In the past years the usage of mixed oxides coatings lead to an important improvement of laser damage threshold and quality of optical elements. In this study influence of post treatment procedure - ex-situ annealing - is examined in terms of quality, optical constants and laser induced damage threshold (LIDT) of mixed HfO₂ and SiO₂ coatings. Monolayer thin films containing different fractions of HfO₂ are deposited with ion beam sputtering technology (IBS.) All samples are post annealed at different temperatures and optimal regimes are defined. Refractive index and absorption coefficient dispersion is evaluated from transmission spectra measurements. Surface roughness of all samples is characterized before and after deposition and annealing, using atomic force microscopy (AFM). Microstructural changes are identified from changes in surface topography. Further, optical resistance was characterized by 5.7 ns duration pulses for 355 nm wavelength laser radiation, performing 1-on-1 sample exposure tests with high resolution micro-focusing approach for monolayer samples and S-on-1 tests for multilayer reflectors. Morphology of damaged sites was analyzed through optical microscopy. Finally, conclusions about annealing effect for mixed HfO₂ and SiO₂ monolayer and multilayer coatings are made.

We have investigated the properties and laser damage behavior of Ta₂O₅/SiO₂ quarter wave stacks designed for λ=1 μm operation by substituting the Ta₂O₅ layer by either Y₂O₃ or HfO₂ and the SiO₂ by Al₂O₃ in the top 3 pairs of the multilayer stack. The high reflectors were deposited by dual ion beam sputtering. Laser damage at 1 μm using 350 ps showed enhanced performance when the Ta₂O₅/SiO₂ stack had HfO₂ or Y₂O₃ in its top few pairs.

The effects of long-term exposure to high intensity 532 nm radiation on various dielectric-coated optics are studied. To investigate potential photodarkening effects on optical surfaces, an accelerated life test platform was constructed where optics were exposed to 532 nm radiation from a short-pulse, high repetition rate fiber amplifier at total doses up to 1 trillion shots. The first run of trillion-shot tests were conducted on e-beam deposited and ion beam sputtering (IBS) coated high reflecting mirrors with onsurface intensities ranging from 1.0-1.4 GW/cm². It was found that the e-beam coated mirrors failed catastrophically at less than 150 billion shots, while the IBS coated mirror was able to complete the trillionshot test with no measurable loss of reflectivity. Profiling the IBS mirror surface with a high-resolution white light interferometer post-irradiation revealed a ~10 nm high photocontamination deposit at the irradiation site that closely matched the intensity profile of the laser spot. Trillion-shot surface exposure tests were also conducted at multiple surface sites of an LBO frequency doubling crystal at ~1.5 GW/cm² at multiple surface sites. The transmitted power and on-surface beam size were monitored throughout the tests, and periodic measurements of the beam quality and waist location of the transmitted light were also made using an M² meter. No changes in transmitted power or M² were observed in any of the tests, but 3D surface profiling revealed laser-induced contamination deposits at each site tested.

The 266 nm AR coatings consisting of Sc₂O₃/SiO₂ and HfO₂/SiO₂ were deposited on fused silica and CaF₂ substrates. The laser damage resistance (LDR) was measured to determine the laser fluence that a coating can withstand without damaging when exposed to a large number of pulses. The LDRs of Sc₂O₃/SiO₂ AR coatings were higher than those of HfO₂/SiO₂ AR coatings. The absorption, subsurface damage and the surface roughness of the substrates were measured and were correlated with the LDR of AR coatings. The LDR generally decreased as the subsurface damage size increased. The substrate with the largest subsurface damage showed the relatively high absorption of ~220 ppm at 266 nm compared to other substrates with the smaller subsurface damage. The LDR of Sc₂O₃/SiO₂ AR coating on it was the lowest (9.9 J/cm²). No correlation between the surface roughness and the LDR of AR coatings was found. Improving the polishing process was concluded to be an important factor in increasing the LDR of the AR coating. The laser damage morphology on AR coating was also studied.

We present a method to repair damaged optics using laser-based chemical vapor deposition (L-CVD). A CO₂ laser is used to heat damaged silica regions and polymerize a gas precursor to form SiO₂. Measured deposition rates and morphologies agree well with finite element modeling of a two-phase reaction. Along with optimizing deposition rates and morphology, we also show that the deposited silica is structurally identical to high-grade silica substrate and possesses high UV laser damage thresholds. Successful application of such a method could reduce processing costs, extend optic lifetime, and lead to more damage resistant laser optics used in high power applications.

In this work we report an experimental investigation of subsurface damage (SSD) in conventionally polished fused silica (FS) substrates which are widely used in laser applications and directly influence performances of optical elements. Two procedures were developed: 1 - acid etching and 2 - superpolishing. Additionally, surface roughness and total integrated scattering (TIS) measurements were performed to find correlation between the main surface properties and laser induced damage threshold (LIDT) as circumstantial evidence of elimination of SSD. Different durations of acid etching have been used to study LIDT of FS substrates. These experiments revealed that the optimal etching time is ~1 min. for a given acid concentration. Laser induced damage threshold of etched and SiO₂ layer coated FS samples increased ~3 times, while of the ones that were not coated - 4 times. It has been revealed that for nonetched surface a single nano- to micro-scale absorbing defect ensemble most likely associated with polishing particles within Beilby layer was dominant, while damage morphology in ~1 min etched FS sample had no point defects observed. More than 5 times lower roughness value (RMS) was obtained by superpolishing procedure using colloidal silica abrasive particles. LIDT of such superpolished fussed silica substrates was also strongly increased and compared with conventional CeO₂ abrasive polishing.

The MegaJoule laser being constructed at the CEA near Bordeaux (France) is designed to focus more than 1 MJ of energy at 351 nm, on a millimetre scale target in the centre of an experiment chamber. The final optic assembly of this system operating at a wavelength of 351 nm is made up of large fused silica optics, working in transmission, that are used to convey and focus the laser beam. Under high fluences (i.e. more than 5 J/cm² for 3 ns pulses), the limited lifetime of final optical assembly is a major concern for fusion scale laser facilities. Previous works have shown that surface finishing processes applied to manufacture these optical components can leave subsurface cracks (SSD), pollution or similar defects that act as initiators of the laser damage. In this work, we used epi-fluorescent light scanning microscopy (ELSM) and Stimulated Emission Depletion (STED) in confocal mode with fluorescent dye tagging to get a better knowledge of size and depth of these subsurface cracks. Magnetorheological fluid finishing technique (MRF) was also used as a tool to remove these cracks and thus assess depths measured by confocal microscopy. Subsurface cracks with a width of about 120 nm are observed up to ten micrometers below the surface.

Fused silica optics in the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors are extremely sensitive to optical scattering and absorption losses induced by both particulate and hydrocarbon contamination. At full power, the optical surfaces are illuminated with up to 200 kW/cm². Additionally, the round-trip test mass cavity loss budget is limited to 70 ppm total from all sources. Even low-level contaminants can result in laser damage to optics during the operation the interferometers, and/or the unacceptable reduction of overall detector sensitivity. These risks are mitigated by a two-pronged approach: quantifying contamination sources and the extent of contamination, then reducing sources and cleaning optics in-situ. As a result of these ongoing efforts, we now have a better understanding of what the contamination levels and sources are, and have made significant improvements to methods of controlling contamination, thus protecting the optics from losses and laser damage in the Advanced LIGO Interferometers.