This PDF file contains the front matter associated with SPIE Proceedings Volume 6403, including the Title Page, Copyright information, Table of Contents, Foreward, International Program Committee listing, Symposium Welcome, and the Summary of Meeting.

Femtosecond laser ablation is an important process in micromachining and nanomachining of microelectronic, optoelectronic, biophotonic and MEMS components. It is also important in the damage of optical components and materials. A thorough understanding of all aspects of femtosecond matter interaction processes in the near-threshold regime is required if one wants to have complete control of these processes. Two aspects of the interaction process for metals and semiconductors are examined in detail in the present paper, namely the effect of a more complete model for the temperature dependent electron thermal conductivity in metals and the avalanche ionization process in semiconductors. These are included in two temperature and molecular dynamics modeling calculations respectively. The proper inclusion of these processes allows the model calculations to better reproduce published experimental measurements for copper and silicon.

We report on the refractive index grating formation by filamentary propagation of femtosecond pulses in fused silica. The relevant exposure and work cycles are considered both experimentally and through numerical study, involving a model of light filaments supported by conical wave, capable to capture permanent glass refraction index changes.

Laser damage studies are made on a phase mirror used for laser beam shaping in high power laser applications. The phase mirror is composed of a glass substrate with defined patterns to encode a phase, on top of which a multilayer mirror is deposited. We describe in this paper the LIDT obtained (at 1064nm, 6ns) and the laser damage test procedure, adapted to the geometry, that has been used. A morphologic analysis of the damage sites is made with Nomarski and Atomic Force Microscopy, to obtain information on the damage initiation and its localization on the structured component. The results are completed with simulations of the electric field within the multilayer by using a wave propagation computer code. We obtain localization and values of the light intensification occurring in the structure, that we correlate to experimental measurements.

High performance optical coatings are an enabling technology for many applications - navigation systems, telecom, fusion, advanced measurement systems of many types as well as directed energy weapons. The results of recent testing of superior optical coatings conducted at high flux levels have been presented. Failure of these coatings was rare. However, induced damage was not expected from simple thermal models relating flux loading to induced temperatures. Clearly, other mechanisms must play a role in the occurrence of laser damage. Contamination is an obvious mechanism-both particulate and molecular. Less obvious are structural defects and the role of induced stresses. These mechanisms are examined through simplified models and finite element analysis. The results of the models are compared to experiment, for induced temperatures and observed stress levels. The role of each mechanism is described and limiting performance is determined.

We use an infrared thermal imaging system in combination with a fluorescence microscope to map the dynamics of the local surface temperature and fluorescence intensity under cw, UV excitation of laser-modified fused silica within a damage site. Based on a thermal diffusion model, we estimate the energy deposited via linear absorption mechanisms and derive the linear absorption coefficient of the modified material. The results indicate that the damage growth mechanism is not entirely based on linear absorption. Specifically, the absorption cross-section derived above would prove insuffcient to cause a significant increase in the temperature of the modified material under nanosecond, pulsed excitation (via linear absorption at ICF laser fluences). In addition, irreversible changes in the absorption cross-section following extended cw, UV laser exposure were observed.

A thermal model is considered to better understand Laser-Induced Damage and conditioning mechanism in KH₂PO₄ (KDP) and D_2xKH_2(1-x)PO₄(DKDP) crystals. We mainly focus on two points, the probed volume of the laser beam and the optimization of the conditioning process. Our predictions are in agreement with recent experimental data.

The selection of a suitable laser-window material involves considerations relating to "thermal lensing," that is, the process of beam distrotion caused by thermally induced phase-aberrations, in addition to issues relating to the stress field generated by beam-induced temperature gradients. The purpose of this paper is to obtain improved figures of merit (FoM) for ranking high-energy laser-window materials in regard to thermal lensing and thermal stresses. We address this task in the following manner: (a) We provide proper analytical expressions for describing how beam-induced optical distortions and beam-induced hoop stresses control the allowable beam fluence; (b) We re-evaluate the role of axial stresses, which may lead to failure through compressive yielding or thermal shock, and derive appropriate FoMs based on allowable irradiances; (c) We illustrate the procedure through FoM evaluations of six window-material candidates for operation at the chemical oxygen-iodine laser wavelength (1.315 µm). This methodology confirms that low-absorption, impurity-free fused silica is the window material of choice for contemplated high-energy laser systems operating in the near-infrared.

The presence of a nearby free surface means the morphology of surface damage sites is inevitably different from that of bulk damage sites. In both, the material is subject to compressive stress waves from the initial release of laser energy. However, reflection at the free surface leads to a tensile stress wave. Because material strength is much less against tension than compression, surface sites will be more extensive than bulk sites, all else being equal. We analyze the extent of damage as a function of the amount and position of energy released and compare to experimental results.

We present recent results of molecular dynamics simulations to illustrate the processes and mechanisms in damages to silica glass, including densification, cavitation, fragmentation and agglomeration via photon, electron, ion and neutron radiations and stresses. Radiation of glass creates point defects (vacancies and interstitials), and subsequent structure relaxation induces densification. Nanovoid below a certain size and rapid-quenching of silica liquid can also densify a glass. Hot spots due to photon-absorbing impurities in glass may cause local densification and cavitation as well. Densification can also be induced by compressional stress, and spall, by tensile stress. The densified glasses, regardless of the exact processes, share similar structural and vibrational properties, for example, the five-fold coordinated Si atoms. Densification is essentially a kinetic frustration during structure relaxation driven by excessive free energy, e.g., due to defects or stresses. The point-defect mechanism is dominant for densification without compression and complemented by thermal spike mechanism in thermal processes. Defects, thermal effects and stresses may interplay in a general damage process in silica glass.

There is general agreement that localized absorbing defects are a major factor affecting thin-film performance, and laserinduced damage in films designed for UV, nanosecond-scale, pulsed-laser applications is driven by nanoscale absorbers. Low number densities and size (few nanometer), however, prevent any characterization of these defects and, consequently, deterministic film improvement. This situation also hampers further development of localized defectdriven damage theory, since initial conditions for modeling remain uncertain. Recently, a new approach for studying laser interaction with thin-film nanoscale defects was implemented in which well-characterized, isolated artificial absorbing defects (gold nanoparticles) were introduced inside the thin film. This work is a review in which we discuss main findings from experiments with gold nanoparticles, such as delocalization of absorption during the laser pulse, importance of the defect boundary conditions (contact with the matrix), and competition of pure thermal and stressdriven mechanisms of damage-crater formation. These experimental results will be compared with theoretical results of damage-crater formation in such model thin films using both phenomenological modeling and detailed calculations of the kinetics of the damage process. An outlook on future thin-film-damage studies using model systems with artificial defects is also presented.

During the life of a high-power laser chain, optical components may be damaged due to local high fluence levels in the inhomogeneous beam. The origin of the laser damage can be impurities, surface defects or flaws and cracks resulting from polishing, or it may be produced by self-focusing in the component. The aim of this study is to better understand the correlation between a surface crack on a silica optical component and laser damage. To accomplish this, calibrated indentations were made on silica samples. Observations of the sites were made with an optical microscope, and three different morphologies were recognized. Then the zones containing the indentations were irradiated (single shot mode) with a Nd Yag laser at 355 nm for various fluences. Subsequent observations of the sites were made with an optical microscope, with the aim of correlating site morphology and laser-induced damage. Some sites were believed to have undergone laser conditioning. They were further irradiated (raster scan mode) at high fluence, and some evidence for a laser conditioning effect was obtained.

Correctly determining the lifetime of optical components is a major issue in the operation of high power laser facilities such as the Laser Megajoule developed by the Commissariat a l'Energie Atomique (CEA). Laser damage that occurs at the surface is a main cause of optical aging, and may lead to dramatic degradation of the focal spot. To estimate the effect of such defects, we measured and calculated the distortion of the focal spot induced by "model defects." These "model defects" are circular silica dots randomly distributed on a silica substrate. The experiments were conducted in the ANTALIA facility at the Centre d'Etude Scientifique et Technique d'Aquitaine (CESTA). We performed numerical calculations of beam propagation with the Miro software, developed by the CEA. We obtain a remarkable correlation between measurements and simulations in the central part of the focal spot for large defects. However, experimental noise and measurement dynamics become serious problems when we confine our attention to smallerdefects (<500 micron) or to the diffuse light around the central part of the focal spot. We present some modifications of the ANTALIA experimental setup designed to overcome these problems.

Contamination by metallic particles has been known to reduce the laser damage threshold on high power laser optics. To simulate the presence of metallic particle on the Ligne d'Integration Laser optics, silica substrates were arti.cially polluted by square aluminum dots of 5 × 5 micron² and 50 × 50 micron², respectively. The metallic dot sites were irradiated by a Nd:YAG laser at 1064 nm with different fluences. The sites were analyzed by Nomarski microscopy, optic profilometry and photothermal microscopy. For both sizes of metallic dots, vaporization of metal can be observed. We study in this paper the dot size influence on the surface cleaning process and the effect of the pre-irradiation mode (1 shoot or several shots).

The current research focused upon ascertaining the extent of induced laser damage that occurs due to the outgassing species from adhesives that will contaminate the optics. The adhesives that are being studied are actual flight materials that are being used in mounting optics in existing 1064nm LIDAR laser systems. Three different adhesives were tested in our vacuum system. Each sample was loaded onto an effusive source and a PID controller controls the set point temperature of the adhesive. The optics that were tested were anti-reflective coated BK7 windows. An oil free vacuum pump system was used to pump the system down to approximately 10^-5 Torr. The vacuum pressure of the system was measured by use of a thermocouple gauge and a Bayard Alpert ion gauge. The test windows were irradiated with a 20 Hz Nd:YAG laser at 1064 nm with a nominal fluence of 1 J-cm^-2 for at least 1 million shots. All sample windows are analyzed by use of bright field and dark field light microscope. Under the test conditions that were performed, varying extent of damage with different morphologies were observed; making it difficult to specify a single damage mechanism that would adequately explain the vast differences observed.

The lifetime of optical components submitted to high laser fluences is degraded under organic contaminant environment. The molecular background of the Ligne d'Integration Laser (LIL), prototype of the future Laser Megajoule, might reduce the laser damage threshold of exposed fused silica surfaces. This paper reports the interaction effects between pure model contaminant deposits and a pulsed 1064 nm laser radiation on the coming out of mirror damage. The experimental setup allowed us to condense nanolayers of model contaminants on optics, the deposit impacts were then investigated by Laser Induced Damage Threshold (LIDT) tests in Rasterscan mode. In order to highlight physical processes emphasizing the emergence of optics damage, we characterized the irradiated deposit using interferometric microscopy analysis and spectrophotometric analysis. The challenge was to determine physical and phenomenological processes occurring during the irradiation of a pure contaminant deposit with a 1064 nm pulsed laser and to study the impact of this model contaminant on the LIDT of dielectric SiO₂/HfO₂ mirrors.

Under vacuum conditions, the accumulation of low fluence laser pulses generally leads to an organic contamination of the surface irradiated. This phenomenon reduces the optical component lifetime. Experimental conditions such as laser characteristics, environment composition and structure of the coating strongly influence the contamination mechanisms. Silica being the most employed material for optical coatings, this study aims at describing the laser-induced contamination influence of silica coatings deposition techniques. E-Beam evaporated and Ion Beam Sputtered silica thin films have been exposed to several billions 600 mJ/cm² - 532 nm laser pulses under vacuum. This paper presents the observations made on laser-induced contamination and discusses the physical mechanisms involved.

We examine the effect of pulse duration on both density and morphology of laser-induced damage in KDP and SiO₂. In both materials the density of damage sites scales with pulse duration to the ~ 0.4 power for 351-nm pulses between 1 and 10 ns. In SiO₂ three types of damage sites are observed. The sizes of the largest of these sites as well as the size of KDP damage sites scale approximately linearly with pulse duration. Similarities of damage in very different materials points to properties of laser-induced damage which are material independent and give insight to the underlying physics of laser-induced damage.

Growth of laser initiated damage is a potential lifetime limiter of laser optics. While laser initiated damage occurs most often on the exit surface of optical components, some damage sites can occur on the input surface. We have investigated the growth of laser initiated damage in fused silica when the damage occurs on the input surface of the optic. We have measured both the threshold for growth as well as the lateral growth rate at 351 nm. The lateral growth of damage on the input surface is best described as having a linear dependence on shot number. The rate of growth has a linear dependence on fluence, with an extrapolated threshold of approximately 6 J/cm². This behavior will be contrasted to growth of damage when located on the exit surface. The behavior will be compared to growth of input surface damage when the irradiation wavelength is 1053 nm or 527 nm.

Surface damage caused by high fluence, 351 nm light to fused silica optics can adversely affect the performance of fusion class laser systems like that of the National Ignition Facility (NIF). It is typically initiated as a small pit and grows in both diameter and depth during normal operation with cracks that extend into the bulk. Mitigation of this growth has been previously reported using a 10.6 micron CO₂ laser. Here, we report growth mitigation with the 4.6 micron light from a frequency-doubled, 9.2 micron CO₂ laser. The motivation for using 4.6 microns is >25 times longer absorption length in fused silica at room temperature compared to that at 10.6 micron. Mitigation of subsurface cracks at 10.6 micron required ablation of material to the depth of the cracks. In contrast, it was possible to mitigate the subsurface cracks using 4.6 micron light without significant ablation of material. Damage sites as large as 500 microns in diameter with cracks extending to 200 microns in depth were successfully mitigated with 4.6 microns.

Over the past two years we have developed MRF tools and procedures to manufacture large-aperture (430 X 430 mm) continuous phase plates (CPPs) that are capable of operating in the infrared portion (1053 nm) of high-power laser systems. This is accomplished by polishing prescribed patterns of continuously varying topographical features onto finished plano optics using MRF imprinting techniques. We have been successful in making, testing, and using large-aperture CPPs whose topography possesses spatial periods as low as 4 mm and surface peak-to-valleys as high as 8.6 microns. Combining this application of MRF technology with advanced MRF finishing techniques that focus on ultraviolet laser damage resistance makes it potentially feasible to manufacture large-aperture CPPs that can operate in the ultraviolet (351 nm) without sustaining laser-induced damage. In this paper, we will discuss the CPP manufacturing process and the results of 351-nm/3-nsec equivalent laser performance experiments conducted on large-aperture CPPs manufactured using advanced MRF protocols.

Multilayer dielectric (MLD) diffraction gratings are an essential component for the OMEGA EP short-pulse, highenergy laser system. The MLD gratings must have both high-optical-diffraction efficiency and high laser-damage threshold to be suitable for use within the OMEGA EP Laser System. Considerable effort has been directed toward optimizing the process parameters required to fabricate gratings that can withstand the 2.6-kJ output energy delivered by each beam. In this paper, we discuss a number of conventional semiconductor chemical cleaning processes that have been investigated for grating cleaning, and present evidence of their effectiveness in the critical cleaning of MLD gratings fabricated at LLE. Diffraction efficiency and damage-threshold data were correlated with both scanning electron microscopy (SEM) and time-of-flight secondary ion-mass spectrometry (ToF-SIMS) to determine the best combination of cleaning process and chemistry. We found that using these cleaning processes we were able to exceed both the LLE diffraction efficiency (specification >97%) and laser-damage specifications (specification >2.7 J/cm²).

The performance of sacrificial and plasma mirrors has been investigated on the HELEN laser chirped pulse amplification [CPA] beam line. Sacrificial mirrors are initially highly reflective surfaces that degrade during the course of a pulsed laser experiment. They are being considered for protecting the off axis parabolic surfaces used to focus CPA lasers from plasma physics target generated debris and shrapnel. Plasma mirrors are initially low reflectivity surfaces that transmit low intensity beams but produce a reflecting plasma surface during the course of the laser pulse. They are being investigated to prevent prepulse effects in plasma physics experiments and increase the contrast ratio of the incident laser beam.The sacrificial mirrors were operated at 45 degrees angle of incidence and an average input beam diameter of ~14 mm with intensities in the range 8 TW/cm² to 44 TW/cm². Dielectric protected silver and gold coatings as well as dielectric multi layers were studied as the mirror surfaces for directing all of the short pulse [500fs] laser beams onto tantalum foil targets of 10 microns thickness. Proton emissions from the foils monitored by radiochromic film were used to evaluate the beam irradiance achieved from the mirror surfaces. Glass witness plates were used to evaluate debris and shrapnel emissions from the mirror surfaces, the diagnostics and the target foils. The plasma mirrors were operated in a similar configuration but with beam diameters of ~8mm and irradiances of 57 TW/cm² to 235 TW/cm². Uncoated and sol gel anti-reflection coated fused silica were used as the high intensity mirror surfaces. The influence of surface coating on laser damage morphology will be described as well as post shot inspection of debris distributions.

A process to stabilize laser-initiated surface damage on KDP/DKDP optics by micromachine contouring using a single-crystal diamond ball nose end mill is shown to mitigate damage growth for subsequent laser shots. Our tests show that machined circular contours on output surfaces of uncoated doubler (KDP) and tripler (DKDP) crystals are stable for laser exposures at 351nm, ~8ns pulses at ~12J/cm² fluences. Other tests also confirmed that the machined contours on the output surface of an uncoated tripler are stable for combined 1053nm and 351nm, ~8ns pulses at ~12J/cm² total fluences (~6J/cm² each wavelength) for greater than 100 shots. Laser damage tests have also been conducted on an array of machined contours on the output surface of an AR coated tripler at 351nm, ~1ns pulses up to ~8J/cm² fluences. The machined shapes have been as large as 1.5 mm in diameter and 0.25 mm deep, and as small as 250 microns in diameter and 25 microns deep. Both Gaussian and conical shaped contours have been successfully tested. Computer modeling and measurement of laser beam propagation through the actual contoured shapes have been conducted to confirm downstream intensification is manageable.

The space environment presents some unique problems for optics. Components must be designed to survive variations in temperature, exposure to ultraviolet radiation, particle radiation, atomic oxygen and contamination from the immediate environment. To determine the importance of these phenomena, a series of passive exposure experiments have been conducted which included, among others, the Long Duration Exposure Facility (LDEF, 1984-1990), the Passive Optical Sample Assembly (POSA, 1996-1997) and most recently, the Materials on the International Space Station Experiment (MISSE, 2001-2005). The MISSE program benefited greatly from past experience so that at the conclusion of this 4 year mission, samples which remained intact were in remarkable condition. This study will review data from different aspects of this experiment with emphasis on optical properties and performance.

Laser optics being used in space laser systems are usually exposed to high vacuum conditions under the absence of air or oxygen. In the past, several space-based laser missions have suffered from anomalous performance loss or even failure after short operation times. To mitigate the risks involved with long-term operational conditions, a laser damage test bench has been developed and is operated at the German Aerospace Center (DLR) to test laser optics in the IR, VIS, and in the UV spectral range. The testing is performed under application oriented conditions, i.e. under high-vacuum using dry pump systems. The main goal of the test campaign is to identify the critical components in terms of their laser damage threshold for very high pulse numbers applied per site. Characteristic damage curves according to ISO 11254 are evaluated for each component under investigation for up to 10 000 shots per site. The characteristic damage curves are used for the estimation of the performance at very high pulse numbers.

It is well known that optical dielectric coatings show a change in performance when altering the environmental condition from air to vacuum. Evacuating or venting a set-up will shift the spectral characteristic and also the damage behavior of the specimen. With respect to the spectral shift it has been observed that dense dielectric coatings manufactured by Ion Assisted Deposition and Ion Beam Sputtering do not show this modification. This work was performed to investigate AR coatings of different deposition processes to determine whether the LIDT of dense layers can also be kept stable in vacuum. It was found that the damage threshold of these dense coatings is also stable in an evacuated environment.

To evaluate the impact of particulate contamination in laser induced damage of optical material, an experimental program is established. The first step consists in the Ligne d'Integration Laser (LIL) particle contamination sampling. Carbonated cellophane tapes, antireflection coated and uncoated silica samples were inserted in the LIL laser chain, in six different zones to collect particles. The second step is the pollution characterization. Polluted cellophane tapes are analysed by Scanning Electron Microscopy and Energy Dispersive Spectrometry. The density and the nature of particles collected in the Amplification Section are found to be homogenous throughout this section. The pollution collected in the Frequency Conversion and Focusing system is more complex. One of its features is a larger proportion of silica particles. The last step consists in the silica samples irradiation. Antireflection coated and uncoated silica samples are examined by optical microscopy, then irradiated at 1064 nm or 355 nm and examined again. No damage growing under several irradiations is observed. We show a cleaning effect efficient for particles larger than 20 microns.

Ophthalmic antireflection coatings are not normally considered to be in the same category as other traditional optical coatings with respect to environmental damage. However, as a group, eyeglass lens wearers tend to subject their optical-coated eyewear to a broader and more aggressive range of environmental aggressions than at first imagined. This paper presents the environmental aggressions and, in some detail, the resultant coating defects observed in coated ophthalmic optics. Further, development of test methods for defect replication, to enable product improvements will be discussed. Real-life environments combine thermal, chemical, and mechanical "aggressions" which spectacle lenses are subjected to. These aggressions generate optical coating defects and failure modes involving abrasion, corrosion, and loss of adhesion. In addition, market forces driven by retail customer perceptions lead to product liabilities not normally considered to be of any consequence in traditional optical coating applications.

This paper introduces a theory for material erosion in proximity to a laser driven EUV source, with a xenon target. The mechanism hypothesized is x-ray induced damage. A semi empirical photo ablation model is developed using the laser induced damage threshold at 1.06 microns to set the critical energy density for material removal. The model also includes absorption of the plasma generated xrays and is shown to agree well with experiment. With the theory validated, the paper concludes with a calculation of a safe operating distance and how this distance could be calculated for other optic materials and plasma targets.

The cavity optics within high power free-electron lasers based on energy-recovering accelerators are subjected to extreme conditions associated with illumination from a broad spectrum of radiation, often at high irradiances. This is especially true for the output coupler, where absorption of radiation by both the mirror substrate and coating places significant design restrictions to properly manage heat load and prevent mirror distortion. Besides the fundamental lasing wavelength, the mirrors are irradiated with light at harmonics of the fundamental, THz radiation generated by the bending magnets downstream of the wiggler, and x-rays produced when the electron beam strikes accelerator diagnostic components (e.g., wire scanners and view screens) or from inadvertent beam loss. The optics must reside within high vacuum at ~ 10^-8 Torr and this requirement introduces its own set of complications. This talk discusses the performance of numerous high reflector and output coupler optics assemblies and provides a detailed list of lessons learned gleaned from years of experience operating the Upgrade IR FEL, a 10 kW-class, sub-ps laser with output wavelength from 1 to 6 microns.

Thin film superlattice materials can exhibit physical, optical and mechanical properties very different and superior to those of single layer counterparts. In the past fifteen years, hard coating, optical and electrical device technologies have advanced beyond the use of single layer coatings with the development of nanoscale compositionally modulated coatings, or superlattices and nanocomposites. A typical superlattice consists of hundreds to thousands of nm-scale layers with alternating compositions and/or crystalline phases. It is possible to engineer the electrical and mechanical properties by choice of layer thicknesses and compositions. Typical layer thicknesses are between 2 and 100 nm. We report of three types of superlattice coatings: (1) AlN/Si3N₄ optical superlattice for abrasion protection of ZnS IR windows, (2) Al/Cu structural superlattices and (3) advanced thermoelectric superlattices. All superlattice coatings were deposited by DC and RF reactive magnetron sputtering. The AlN/Si3N₄ superlattice had layer thicknesses of 2 nm and exhibited a nanohardness of 35 GPa. The Al/Cu superlattice had layer thicknesses of 1.5 nm and a hardness near 6.5 GPa and is being developed for lightweight optics for space applications. The thermoelectric superlattice demonstrated a figure of merit (ZT) ~ 1.5 and is being developed for power generation from waste heat sources.

The development of advanced and reliable techniques for the production of optical coating systems with a continuous variation of the refractive index opens the way towards a new generation of optical components in laser technology and modern optics. The present paper is dedicated to an Ion Beam Sputtering (IBS) concept for the production of coatings with gradual index layers and Rugate filters. On the basis of a spectrophotometric online-control system, Rugate filter coatings were produced with high precision and reliability. In addition to the optical performance, especially the laser damage properties of the coating systems were investigated with respect to defined mixtures of two coating materials and the influence of gradual index layer designs. A dramatic increase of the laser induced damage threshold was observed for the produced Rugate coatings. The experimental results are discussed considering the special properties of gradual coating systems.

Glancing angle deposition (GLAD) is a novel way to produce nanostructural thin films with engineered porosity, and it is possible to make new optical components in laser systems. In this paper, ZrO₂, SiO₂ and TiO₂ thin films were grown by electron beam evaporation with GLAD technique. Different microstructures were observed. The optical properties, such as transmittance and refractive index were characterized. As application of the GLAD thin films, several optical components were designed and fabricated, such as graded-index rugate filter, broadband antireflection coating and phase retardater for visible and near infrared laser systems. Finally, laser-induced damage threshold were measured and discussed.

Hafnium oxide (HfO₂) is undoubtedly one of the most desirable high-index optical coatings for high power laser applications. One of the key goals in the fabrication of oxide films with high Laser Induced Damage Threshold (LIDT) is to minimize the number of film imperfections, in particular stoichiometric defects. For HfO₂ films deposited by ion beam (reactive) sputtering (IBS) of a hafnium metal target, stoichiometry is controlled by the injection of molecular oxygen, either close to the substrate or mixed with the sputtering gas or some other combination. Good stoichiometry is important to reduce the density of unoxidized particles buried in the coatings, which affect the LIDT. This work evaluates the potential advantages of using pre-activation of oxygen in the IBS of HfO₂, with special emphasis on its impact on LIDT and film stress. For the experiments, oxygen was activated by an independent plasma source and then introduced into a commercial IBS chamber. The optical properties of the films were characterized using spectrophotometry and ellipsometry. Their structural quality and composition were determined from x-ray diffraction and x-ray photoelectron emission spectroscopy. The stress was determined from interferometer measurements. For optimized conditions, 2.5 J/cm² LIDT was measured on HfO₂ films at λ=800 nm with 1 ps and 25 mJ pulses from a chirped amplification Ti:Sapphire laser. In the range of oxygen variations under consideration the effects on LIDT are shown to be minimal.

High power laser systems are one of the most rapidly growing areas in the development of laser technology. This also leads towards higher requirements for environmental stability of optical components and their resistance to laser radiation. There are some reports showing that porous dielectric coatings are more resistant to intense laser radiation, however they have smaller environmental stability than denser coatings, which are more sensitive to laser radiation. The influence of important technological parameters (deposition rate, substrate temperature, energy of ions) on optical and microstructural properties of high reflection dielectric coatings based on Nb₂O₅/SiO₂, and Ta₂O₅/SiO₂ in VIS spectral region is presented. Furthermore the LIDT measurements using repetitive nanosecond laser pulses of Nb₂O₅/SiO₂ and Ta₂O₅/SiO₂ high reflecting optical coatings based on ISO 11254-2 standard are presented.

Laser damage characteristics of infrared substrates and coatings at 2-micron wavelength have been rarely studied until yet, even if the need of optical components with high laser-induced damage threshold in the mid-infrared is important. Use of an infrared nanosecond laser, tunable in the range 2 to 5 microns, allowed us to develop an automatic test facility for the determination of accurate LIDT curves for different test procedures. We choose to particularly study polycrystalline zinc selenide (ZnSe) material used as substrates for infrared dielectric coatings. Irradiation of ZnSe substrates with parallel laser beam shows that surfaces always break clearly before bulk material, this shows that surfaces must be carefully prepared. We particularly exhibit the influence of polishing processes on substrates LIDT. Then the influence of different cleaning methods before coatings deposition is studied. Practical implications for the fabrication of highly laser resistant multilayer coatings at 2-microns are finally discussed.

The absorption of ArF laser pulses in calcium fluoride, fused silica as well as in highly (HR) and partially (PR) reflecting fluoridic coatings is directly measured using the laser induced deflection technique (LID). For the calcium fluoride sample it is proved that the LID technique allows to separate surface and bulk absorption by measuring only one sample with the size 20 x 20 x 10 mm³. At a laser pulse fluence Φ = 36 mJ/cm² and a repetition rate f = 1 kHz the bulk absorption coefficient and the surface absorption are determined to 0.0029 cm^-1 and 0.00043 (two surfaces), respectively. For the HR and PR coatings the ArF laser absorption is 0.0004 for Φ= 22 mJ/cm² (f = 1 kHz) and 0.0066 for Φ= 40 mJ/cm² (f = 1 kHz), respectively. For the example of the PR coating the influence of high coating scattering on the LID measurements is discussed and an appropriate measuring procedure is derived and applied to avoid the scattering influence. In addition to the established LID setup requiring rectangular substrate dimensions a modified setup is introduced enabling the measurement of cylindrical optical elements. The principle of the new LID setup is explained and first measurements at fused silica are presented.

Standard DUV mirror systems with conventional quarterwave design were deposited from oxide materials by ion beam sputtering deposition (IBS) and from fluoride materials by conventional thermal evaporation for the wavelength 193 nm. In addition, a protected fluoride mirror system was manufactured consisting of a conventional fluoride stack with a dense SiO₂ protection layer. In a comparative study, these mirror systems were characterised in respect to their optical properties and absorption in the VUV spectral range. Subsequently, the value of the laser-induced damage threshold (LIDT) of the mirrors was determined in an S-on-1 procedure. All DUV measurements were conducted under the conditions of nitrogen purging. It was observed that all mirror system exhibit a similar optical performance and loss levels at 193 nm. However, it was found for the LIDT value, that for IBS oxide system the damage mechanism is defect induced at a comparable low level, whereas the LIDT value of evaporated fluoride mirror is absorption induced, with 1-on-1 values of up to 6 J/cm². The protected fluoride mirror exhibits value in the intermediate range.

Microscopic imaging methods are valuable tools to analyze damage morphologies of laser optics for ns and fs applications. In the fs-regime, the morphology of TiO₂/SiO₂ coatings with modified field strength distributions were investigated, whereby a characteristic morphology was caused by the special designed vertical field strength profile, depending on the local power density. In the ns-regime, the morphology of the damage sites has shown significant differences between the quarter wave stacks and the gradual index systems without abrupt interfaces in the functional layers. Typically, these Rugate high reflectors did not show catastrophic damage. Rather the damage becomes apparent by the creation of colour centres.

Investigations in fs-laser damage mechanisms within the recent years indicate that damage mechanisms in the fs-range are based on electronic interaction schemes in the material. Usually, a direct correlation of the power handling capability to the band gap structure of the material and the field strength distribution in the optical system is observed. The present work is focused on the optimization of high refractive index coating materials by mixing with silica. The different compositions of mixed materials are manufactured with an IBS coating process using a zone target. This technique allows for a continuous variation of the material composition. In addition, new coating designs were developed to adapt the contents of silica within the layers to the high field strengths. By combining these techniques a significant increase of the laser damage threshold could be accomplished.

A general method of designing multilayer dielectric (MLD) gratings for ultrashort pulse compressor is presented, which is based on the integration of Fourier spectrum decomposition and rigorous modal method. Numerical calculations show that the shape and energy of the -1^st order reflected pulse is greatly dependant on the reflection bandwidth of the MLD grating, which can be greatly improved by etching gratings into a MLD coating with broad reflection bandwidth. In order to improve the damage resistant ability, the average intensity in the MLD grating is used as another criterion to optimize the grating structure. The fabricated MLD gratings provide diffraction efficiencies higher than 95% at Litrrow angle of 51.2 degree for 1053nm short pulse. Damage tests show that 3.5J/cm² LIDT can be obtained under the irradiation of 12- ns pulse light.

In recent years, there has been a growing interest in further development of sol-gel method which can produce ceramics and glasses using chemical precursors at relative low-temperatures. The applications for sol-gel derived products are numerous. Department of General and Inorganic Chemistry with Laser Research Center of Vilnius University and Institute of Physics continues an ongoing research effort on the synthesis, deposition and characterization of porous solgel. Our target is highly optically resistant anti-reflective (AR) coatings for general optics and nonlinear optical crystals. In order to produce AR coatings a silica (SiO₂) sol-gel has been dip coated on the set of fused silica substrates. The optical properties and structure of AR-coatings deposited from hydrolysed tetraethylorthosilicate (TEOS) sol were characterized in detail in this study. The influence of different parameters on the formation of colloidal silica antireflective coatings by dip-coating technique has been investigated. All samples were characterized performing, transmission electron microscopy, UV-visible spectroscopy, atomic force microscopy, ellipsometric, total scattering and laser-induced damage threshold measurements. Herewith we present our recent results on synthesis of sol-gel solvents, coating fabrication and characterization of their optical properties.

The development of high power lasers and optical micro-components requires optical characterization techniques for studying behavior of optical materials under illumination, laser damage phenomena and ageing. More usual optical characterization tools are based on measurements of absorption, scattering and luminescence; they are non destructive evaluation techniques. It is important to combine several tools which allow getting complementary information. Optical tools can be used in damage initiation studies or to characterize properties of damaged areas. Because defects involved in laser damage initiation are sub-micrometer sized, both high spatial resolution and high sensitivity are required to detect defects as small as possible. Furthermore optical tools have to be implemented in damage set-up and at the same wavelength for a detailed analysis of damage mechanisms. We present an overview of recent developments in the field of optical characterization in connection with laser damage. Especially, a high resolution photothermal deflection microscopy has been coupled with a damage set-up to detect nano-absorbing precursors of damage and to study their behavior under irradiation. Thus model defects such as gold inclusions of various sizes have been followed through irradiation and results are compared with numerical simulations. Optical characterization allows to get determining information if several techniques are associated with numerical simulations.

Surface incandescence properties of proton implanted fused silica have been researched with a focused CO₂ laser. We have discovered that in the initial stage of incandescence a thermoluminescent peak appears. We call it blackbody thermoluminescence. In our silica samples, with a 100 micron spatial resolution, the blackbody thermoluminescence mapping reveals surface and sub surfaces defects made by the polishing process. We show how laser damage and laser conditioning are the same two facets of this blackbody thermoluminescence occurrence.

Waveguiding was used to measure the extinction coefficient of a thin film while it was being deposited in a vacuum chamber. Experimental results are presented and compared to calculations and measurements by other techniques.

The laser damage threshold and absorption efficiency of a variety of carbon based thermal coatings for laser power and energy measurements have been investigated. Carbon based paint, carbon fibers, as well as single wall carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs), were applied to a water cooled copper substrate. The heating of the water was measured to determine power absorbed by the sample during laser exposure. Before and after exposure to 10.6 µm laser radiation, optical and electron microscopy as well as Raman spectroscopy were employed to evaluate the coating topology and composition. These early measurement results demonstrate that a MWCNT coating has a damage threshold of approximately 1686 W/cm², which is four times as large as that measured for SWCNTs and fifteen times greater than that of carbon based paint.

An infrared camera system has been used to measure absorption in optical coatings and substrates. Laser light is directed at the test sample and milliwatts of power are absorbed. The camera images the surface of the sample and provides a direct measurement of the 8-12 micron radiation emitted. By considering the effective emissivity of the sample and the ambient temperature, the surface temperature of the sample is obtained. Through the use of an equivalent "reference" sample which is not heated by the laser, background variations may be effectively eliminated. The application of standard calorimetric methods to infrared imaging as well as the availability of improved sensors such as the microbolometer array has led to our ability to resolve temperature excursions as low as 0.01°C with a S/N of 20 for typical samples. The IR imaging method has been used to evaluate many optical coatings and window materials for the Airborne Laser program. Because the method is noncontact, it has been used to directly measure absorption on large optical surfaces. In some instances, defects have been observed and mapped using this method. Variations in absorption which might be predicted from the coating design have been measured directly. The IR imaging technique thus offers great flexibility and sensitivity comparable to precision calorimetry.

Comprehensive calorimetric absorption measurements were performed for CaF₂ crystals at irradiation wavelengths 193 nm and 157 nm. By using samples with different thickness a separation of surface and bulk absorptance could be achieved and thus, single- and two-photon absorption coefficients could be determined. For the surface absorptance, a dependence of the polishing grade of the sample was observed at 193 nm. The presented results support earlier proposed models of the absorption mechanisms in wide band-gap materials. For an assessment of the optical quality of DUV optics, a high-sensitivity wavefront analyzer system based on the Hartmann-Shack principle is employed. The device accomplishes precise online monitoring of wavefront deformations of a collimated test beam transmitted through the laser-irradiated site of a sample. Due to the achieved sub-nm resolution, it can be used as an alternative to interferometric measurements for 'at wavelength' testing of optics, e.g. for on-line registration of thermal lensing effects or compaction in fused silica.

We investigated the formation of UV laser induced deposits on uncoated and coated fused silica optics under vacuum conditions in presence of outgassing materials. As contamination samples epoxy, silicone and polyurethane containing materials were used. To realize low partial pressures of the contaminants in the gas phase they were slightly heated (40°C). The formation of the depositions was monitored in situ and online by detecting the fluorescence emission of the deposits, excited by the UV laser beam. The influence of different optical coatings on the deposit formation was studied. By analysing the surface profiles of the deposits, growth rates were estimated. Time-of-flight secondary ion mass spectroscopy was used for chemical characterization of the deposits.

Applications of excimer lasers in the deep ultraviolet (DUV) for optical lithography, medicine and material processing are steadily growing together with drastically increasing requirements for low-loss optics. This leads to crucial requirements for at-wavelength characterization tools. For a thorough investigation of optical losses, all mechanisms contributing to the total loss have to be taken into account, comprising scattering at surfaces, thin film interfaces, and in bulk materials. Because of the strong wavelength-dependence of scattering (~1/λ²,1/λ⁴), this in particular holds for DUV optical components designed for high-end applications at 193 nm. Therefore, a system for the measurement of angle resolved and total scattering at 193 nm and 157 nm was developed at the IOF in Jena. The system enables at-wavelength scattering measurement and analysis of DUV optical components. Examples of investigations are discussed such as scatter analysis of all-fluoride thin film coatings on differently polished substrates and bulk scatter properties of synthetic fused silica.

Previous work [1] has shown the optimum pulse length range for laser-conditioning tripler-cut DKDP with 355 nm (3ω) light lies between 200 ps and 900 ps for damage initiated at 3 ns. A 3ω, 500 ps (500 ps) table-top laser system has been built at Lawrence Livermore National Laboratory (LLNL) [2] to take advantage of this optimal conditioning pulse length range. This study evaluates parameters important for practically utilizing this laser as a raster-scan conditioning laser and for determining the effectiveness of various conditioning protocols. Damage density vs. test fluence (ρ(Φ) was measured for unconditioned and 500-ps laser-conditioned (conditioned) DKDP with 3ω, 3 ns (3 ns) test pulses. We find a 2.5X improvement in fluence in the 3 ns ρ(Φ) after conditioning with 500 ps pulses to 5 J/cm². We further determine that the rate of improvement in ρ(Φ)decreases at the higher conditioning fluences (i.e. 3.5 - 5 J/cm²). Single-shot damage threshold experiments at 500 ps were used to determine the starting fluence for our 500 ps conditioning ramps. We find 0%, 70%, and 100% single-shot damage probability fluences of 4, 4.5, and 5 J/cm², respectively at 500 ps. Bulk damage size distributions created at 3 ns are presented for unconditioned and conditioned DKDP. The range of diameters of bulk damage sites (pinpoints) in unconditioned DKDP is found to be 4.6 ± 4.4 µm in agreement with previous results. Also, we observe no apparent difference in the bulk damage size distributions between unconditioned and conditioned DKDP for testing at 3 ns.

In this paper, we present different procedures of laser conditioning realized on KDP doubler crystals. First, components are treated either with an excimer laser (SOCRATE facility, 351 nm, 12 ns) or a Nd: YAG laser (MISTRAL facility, 355 nm, 7 ns). Then damage tests are performed at 2ω (532 nm - 5 ns BLANCO facility) and 3ω (355 nm - 2.5ns LUTIN facility) in order to estimate the conditioning gain for these two wavelengths. For the best procedures, results show that it is possible to increase laser damage threshold at 532 nm so that it becomes compatible with the nominal specifications of the LMJ. Moreover, tests realized at 355 nm highlight also an encouraging improvement for the laser conditioning of tripler crystals.

In this paper we examine how optical techniques can be used for impurities and defects detection in KH₂PO₄ (KDP) components. This is important in so far as some of these defects are responsible for a weaker than expected laser-induced threshold in these materials. Photothermal deflection, polariscopy, fluorescence and photoexcitation are investigated with the aim of localizing and identifying the laser-induced damage precursors. Impurities concentration is measured directly by ICP-AES and Fe is accordingly checked to be at the origin of a higher absorption in the prismatic sectors of rapidly grown KDP crystals. We also exhibit a fluorescence signal in the near-ultraviolet range by pumping at 248 nm; in rapidly grown crystals, in the same way as iron, the incorporation rate of the fluorescent centers is shown to depend on the growth sector.

This study is concerned with the identification of the defects responsible for laser damage observed on KDP/DKDP frequency triplers used in high power lasers. We reported at BDS 2005 a non destructive high energy X-ray topographic setup able to characterize lattice imperfections in optics. Results obtained using this technique on KDP and DKDP crystals are reported and discussed. The influence of each type of defect, observed or likely to exist in optics, is discussed in light of damage mechanisms recently published. Finally, an experimental setup presumably able to reveal those defects is proposed.

For large aperture solid state lasers, the laser resistance of the optical component remains an important limitation for the performances and the maintenance costs. Since decades, laser induced damage has been intensively studied in order to understand and control the origin of the phenomenon. LID measurements are commonly performed with table top lasers whose characteristics change from one to another and, sometimes, the scaling laws do not permit to explain the experimental differences. For example, we have previously demonstrated that, in KH₂PO₄ (KDP) crystals, the laser beam size can influence strongly the determination of the damage probability. Here, we present a systematic study realized on KDP crystal to quantify the influence of the beam size on the LIDT (Laser Induced Damage Threshold) measurement at 355 nm. The use of an unique Gaussian beam ranged from micronic to sub-millimetric sizes permits to highlight different types of laser-damage precursor. LIDT measurements realized with beams of small (lower than 100 microns at 1/e²)or large (upper than 400 microns at 1/e²)dimensions give information about the behavior of material regarding precursor defects.

We investigate the laser-induced damage resistance at 355 nm in DKDP crystals grown with varying growth parameters, including temperature, speed of growth and impurity concentration. In order to perform this work, a DKDP crystal was grown over 34 days by the rapid-growth technique with varied growth conditions. By using the same crystal, we are able to isolate growth-related parameters affecting LID from raw material or other variations that are encountered when testing in different crystals. The objective is to find correlations of damage performance to growth conditions and reveal the key parameters for achieving DKDP material in which the number of damage initiating defects is minimized.

Laser induced damage in potassium dihydogen phosphate (KDP) has previously been shown to depend significantly on pulse duration for 351-nm Gaussian pulses. In this work we studied the properties of damage initiated by 1053-nm temporally Gaussian pulses with 10ns and 3ns FWHM durations. Our results indicate that the number of damage sites induced by 1053-nm light scales with pulse duration (τ) as τ₁/τ₂)^0.17 in contrast to the previously reported results for 351-nm light as (τ₁/τ₂)^0.35. This indicates that damage site formation is significantly less probable at longer wavelengths for a given fluence.

We report on laser-induced damage threshold (LIDT) and UV-laser excited defect formation measurements in large aperture KDP crystals developed as doublers and triplers for mega-Joule laser. Measurements of LIDT were performed according to the ISO 11254-2 standard for repetitive pulses with duration ~ 4 ns and repetition rate of 10 Hz. The results for different laser wavelengths (1064, 532 and 355 nm) and polarizations are presented. The largest LIDT was observed for 532 nm pulses and the 1064 nm wavelength had a strong dependence on laser polarization. The LIDT values at 532 nm and 355 nm also depended on the crystal cutting angle, which is different for doublers and triplers. A comparison of LIDT with earlier reported crystal absorptance at different wavelengths is also performed. The UV-laser induced defect formation was investigated by the means of pump-probe technique. The excitation was performed with a single pulse of ns Nd:YAG laser (355 or 266 nm wavelength) and probing with another Nd:YVO₄ laser system (532 nm) operating at 1kHz. This gave us a temporal resolution of 1ms. The transient absorption of defect states relaxed non-exponentially and fully disappeared in ~10 s. A comparison is made between crystal grown by distinct growth methods and between different laser polarizations. An influence of laser conditioning on UV induced defect state formation is also revealed.

Lasers for space applications require miniaturized high power components that can be operated at low voltages. RbTiOPO₄ (RTP) is a highly efficient electro-optical material, which is used in particular for the realization of low voltage and high repetition rate Pockels cells. RTP can be operated in two crystal orientations (x-cut and y-cut). In both cases, the incoming linear polarization is oriented at 45^o to the z-direction. In this study, laser damage is investigated in RTP crystals. More precisely, we focus on the correlation between the laser damage characteristics and the used crystal orientation. The laser damage tests were carried out at 1064 nm with a standard 6 ns Q-switched Nd:YAG laser and the polarization was oriented as for Pockels cell operation at 45^o to the z-axis of the crystals. This work reveals that the Laser Induced Damage Threshold (LIDT) is two times higher for x-cut than for y-cut RTP crystals. Reflection and transmission measurements show that this LIDT anisotropy can not be explained by an evident loss mechanism like Stimulated Raman Scattering (SRS).

The Mercury laser uses ytterbium-doped strontium fluorapatite (Yb:S-FAP) crystals as the gain medium with a nominal clear aperture of 4 x 6 cm. Recent damage test data have indicated the existence of bulk precursors in Yb:S-FAP that initiate damage starting at approximately 10 J/cm² at 9 ns under 1064 nm irradiation. In this paper, we report on preliminary results on bulk damage studies on Yb:S-FAP crystals.

We report standardized absorption and scattering losses measurements of the nonlinear crystals LiInSe₂ and LiInS₂ in IR range by high average power 1064 nm radiation and tunable radiation of optical parametric oscillator (OPO) based on a periodically poled lithium niobate (PPLN) pumped by a diode-pumped, Q-switched TEM₀₀ mode Nd:YVO₄ laser operated at 1064 nm.

A number of fused silica samples were evaluated for their resistance to densification by deep ultraviolet (UV) radiation at 193nm wavelength. Density changes for all the samples equal the product of a material dependent constant and the absorbed two-photon dose to a sub-linear power of about 2/3. This dose dependence is consistent with earlier compaction studies using UV, electron and gamma radiation. We propose a fictive temperature model to describe fused silica structure; and the observed stretched power dependence of compaction on deposited energy for ionization damage can be explained by a simple network relaxation process. Experimental observations of isothermal-annealing behavior of UV-induced compaction in fused silica agree very well with our theoretical prediction; e.g. strong correlation between thermal recovery of compaction and the compaction rates for different fused silica samples; preheat-treatment can manipulate the compaction damage rates.

The objective of this work is to understand catastrophic optical damage in nanosecond pulsed fiber amplifiers. We used a pulsed, single longitudinal mode, TEM₀₀ laser at 1.064 &mgr;m, with 7.5-nsec pulse duration, focused to a 7.45-&mgr;m-radius spot inside a fused silica window, to measure the single shot optical breakdown threshold irradiances of 4.7E11 and 6.4E11 W/cm² respectively for pure fused silica, and for a 1% Yb³⁺ doped fused silica preform of Liekki's Yb1200 fiber. These irradiances have been corrected for self focusing which reduced the area of the focal spot by 10% relative to its low field value. Pulse to pulse variations in the damage irradiance in pure silica was >2%. The damage induction time appears to be much less than 1 ns. We found the damage morphology was reproducible from pulse to pulse. To facilitate our morphology study we developed a technique for locating the position of the focal waist based on the third harmonic signal generated at the air-fused silica interface. This gives a precise location of the focal position (± 10 &mgr;m) which is important in interpreting the damage structure. The surface third harmonic method was also used to determine the diameter of the focal waist. Earlier reports have claimed the damage irradiance depends strongly on the size of the focal waist. We varied the waist size to look for evidence of this effect, but to date we have found none. We have also studied the temporal structure of the broadband light emitted upon optical breakdown. We find it consists of two pulses, a short one of 16 ns duration, and a long one of several hundred ns. The brightness, spectra, and time profiles of the white light provide clues to the nature of the material modification.

An adhesive method that creates properties of heatproof, waterproof, and transparent to ultraviolet ray of 200 nm and under in the wavelength without adhesive strain was developed by putting one silica glass to another with the silicone oil that had been photo-oxidized by Xe2 excimer lamp. The measurement by the ZYGO interferometer showed that there was neither adhesive strain nor bubbles, and the bonding strength of 18MPa was achieved. To compare the heat resistance of the photo-oxidized silicone oil with that of general-purpose adhesives such as silicone rubber, water glass, and epoxy resin, the shearing tensile strength test was conducted after exposing at high temperatures from 25 to 500 °C. As a result, the silicone rubber adhesive exfoliated at 110 °C, and the epoxy resin adhesive, at 150 °C; however, the photo-oxidized silicone oil had the bonding strength of 6.5MPa at 500 °C.

Laser damage at 3ω, 351 nm, of fused silica optical components is a major concern for LMJ maintenance. Indeed, even a low density of damage sites is unacceptable due to the exponential growth of surface damage with a series of laser shots. A technique is now used to prevent the growth of initiated damage sites : this mitigation technique consists in a local melting and evaporation of silica by CO₂ laser irradiation on the damage site. Even if the growth is stopped in most cases, we showed previously that some of the mitigated sites re-initiate on their peripheral area, where most of redeposited debris are located. To further increase the efficiency of mitigation technique, the treatment was improved by varying the spatial profile of the CO₂ laser beam. We present here the new set-up and the results obtained in terms of laser damage resistance: about 98% of the mitigated sites sustained 200 shots of a 10 J/cm² 3ω YAG laser without damage.

High power and high energy laser sources are used in a large variety of industrial and scientific applications for material processing. The most common are welding, soldering, cutting, drilling, laser thermal annealing, micro-machining, ablation and micro-lithography. For optimised processes the most important laser sources today are: CO₂-lasers, Nd- YAG lasers, high-power diode lasers, excimer lasers or fiber lasers. Beside the right choice of the suitable laser source the right choice of high performance optics for generating the appropriate beam profile is of high importance for the applications. In many cases homogenous top-hat square or rectangular light fields as well as light lines are indispensable or add strong advantages to the application. This takes into account that gaussian shaped laser foci are not the ideal solution. Refractive micro-lenses and micro-lens arrays based on damage resistant materials are an efficient, compact and flexible solution to achieve adequate intensity distributions on the work piece. LIMO has a unique production technology based on computer-aided design that enables the manufacture of high-precision microlens arrays with free programmable surfaces. Thus, specific beam profiles with superior uniformity and efficiency can be generated. Compact beam shaper modules with prealigned optics have been developed. These modules simply have to be placed into the collimated input beam and the required intensity profile is generated at the target without any complicated alignment.

Polymethyl methacrylate (PMMA) is a versatile polymeric material that is well suited for fabrication of many commercial optical components: lenses, fibers, windows, phase waveplates and others. Our focus is achromatic zero-order waveplates made of anisotropic PMMA which can be used to modify the state of polarization of electromagnetic radiation. Such waveplates have a broad range of application in devices where polarized radiation is used. For example, when tunable lasers are used or when spectropolarimetric measurements are performed, one needs an achromatic waveplate providing a specific retardation in a wide wavelength range. Herewith anisotropic properties of PMMA subjected to one-axis stretching are analyzed and the technology for manufacturing such achromatic and super-achromatic, one-axis-stretched PMMA waveplates is described. This technology excludes any mechanical processing of waveplate component surfaces. Technical characteristics of achromatic and super-achromatic waveplates manufactured of PMMA including results of laser-induced damage threshold (LIDT) measurements are discussed below.

The automated laser damage testing system at REO has been in operation for over a year, providing quantitative and detailed information on laser damage of ion beam sputtered (IBS) thin films in a production setting. Results have accumulated in a database, which can be queried in complex ways. We present statistical analysis on event curves (number vs. fluence) for various defect size groups. We examine the differences in event curves for high-threshold and lower-threshold IBS optics. We also present results of experiments on laser conditioning of IBS thin films.

An automated laser damage test system has been developed by the National Ignition Facility small optics metrology group. The Small Optics Laser Damage (SOLD) system measures the fluence at which laser damage occurs in optical coatings and substrates following the requirements of MEL01-013-OD. Irradiation of the sample is by a 1064nm, 8ns pulse with a 1mm 1/e² diameter. The test protocol requires raster scanning of a 1cm² area at increasing fluence levels. Real-time high-resolution imaging of the surface during raster scanning enables automated detection and sizing of defects to 10 microns. Improved imaging resolves actual size of damage events while the automated damage detection removes the subjectivity of the human operator in thresholding damage events. In addition, a map is created enabling additional functions such as excluding damage sites on future scans and to returning to the damage site for growth testing.

High power laser systems require nearly contamination free optics to maintain desired transport efficiency and to minimize optic damage. The required cleanliness is generally achieved through practices that preclude or remove foreign particle contamination. However, laser optic systems may also be contaminated by vapor-borne contaminants from material outgassing, by particles ablated from surfaces exposed to amplifier or laser light, or by contact with items used in the production and cleaning of optics and components. To minimize such contamination on the optics of the National Ignition Facility (NIF), a rigorous screening test program was introduced. This test program replicates conditions in the beam path as well as conditions during production and cleaning. The former is represented by sol-gel exposure tests and by subjecting materials to amplifier flashlamp light and 1ω laser light. The latter is represented by organic solvent extraction tests and surface contact tests for items that could contact optic surfaces. This paper will discuss the methodology for, and administration of, these tests and present results for selected materials.

Information-based materials discovery offers a structured method to evolve materials signatures based upon their physical properties, and to direct searches using performance-based criteria. In this current paper, we focus on the crystal structure aspects of an optical material and construct an information-based model to determine the proclivity of a particular AB composition to exhibit multiple crystal system behavior. Exploratory data methods used both supervised (support-vector machines) and unsupervised (disorder-reduction and principal-component) classification methods for structural signature development; revealing complementary valid signatures. Examination of the relative contributions of the materials chemistry descriptors within these signatures indicates a strong role for Mendeleev number chemistry which must be balanced against the cationic/anionic radius ratio and electronegativity differences of constituents within the unit cell.