Data on laser-induced damage threshold (LIDT) in optical materials show the damage energy fluence dependence on the laser wavelength. Also, these LIDT measurements depend on the type of materials and its physical properties. The fundamental mechanism of materials damage can be due to ablation, melting, vaporization and/or thermal stresses. In general, optical materials require a very high-energy fluence for its damage. Therefore, the LIDT in optical materials can be considered as a limiting case for the materials damage due to the propagation of the ELectro- magnetic radiation. In this paper, equations for materials damage by a laser radiation are derived based on the melting and vaporization processes. The measured and calculated data are found to agree well.

Automated damage testing of KDP using LLNLs Zeus automated damage test system has allowed the statistics of KDP bulk damage to be investigated. Samples are now characterized by the cumulative damage probability curve, or S-curve, that is generated from hundreds of individual test sites per samples. A HeNe laser/PMT scatter diagnostic is used to determine the onset of damage at each test site. The nature of KDP bulk damage is such that each scatter signal may possess many different indicator of a damage event. Because of this, the determination of the initial onset for each scatter trace is not a straightforward affair and has required considerable manual analysis. The amount of testing required by crystal development for the National Ignition Facility (NIF) has made it impractical to continue analysis by hand. Because of this, we have developed and implemented algorithms for analyzing the scatter traces by computer. We discuss the signal cleaning algorithms and damage determination criteria that have lead to the successful implementation of a LabView based analysis code. For the typical R/1 damage data set, the program can find the correct damage onset in more than 80 percent of the cases, with the remaining 20 percent being left to operator determination. The potential time savings for data analysis is on the order of approximately 100 X over manual analysis and is expected to result in the savings of at least 400 man-hours over the next 3 years of NIF quality assurance testing.

Over the past tow years extensive experiments has been carried out to determine the nature of bulk damage in KDP. Automated damage testing with small beams has made it possible to rapidly investigate damage statistics and its connection to growth parameter variation. Over this time we have built up an encyclopedia of many damage curves but only relatively few samples have been tested with large beams. The scarcity of data makes it difficult to estimate how future crystal will perform on the NIF, and the campaign nature of large beam testing is not suitable for efficient testing of many samples with rapid turn-around. It is therefore desirable to have analytical tools in place that could make reliable predictions of large-beam performance based on small-beam damage probability measurements. To that end, we discuss the application of exponential and power law damage evolution within the framework of Poisson statistics in this memo. We describe the result of fitting these models to various damage probability curves on KDP including the heavily investigated KDP214 samples.

Laser induced materials modifications in the bulk and on the surface of KDP and DKDP are studied using fluorescence image and spectroscopy. Photoluminescence is observed at damaged regions following above threshold exposure with an emission peak centered at 550-nm. In addition, surfaces exposed to > 100 high power, 355-nm laser pulse reveal a reduced surface finishing quality as evidenced by an associated emission under UV photoexcitation. The emission spectra from the laser-induced damage sites and the laser degraded surfaces are similar suggesting the generation of similar defect species.

Severe scattering losses from KDP crystals have been correlated with the exposure of porous sol AR coated crystal to ambient humidity. The scattering is attributed to formation of etch pits which develop under the coating on the KDP surface along crystallographic axes. This high angle scattering can in turn produce laser damage of downstream optics either through modulation of the beam or by optic contamination from ablation of adjacent metal structures. We have developed a simple tool to characterize the evolution of scatter from sol-coated KDP surface.s We have measured the rate of etch pit formation as a function of relative humidity and surface treatment using both microscopy and scattering. We will discuss various surface treatments which can be utilized to retrad or eliminate the environmental degradation of KDP crystals.

Following initiation at absorbing surface flaws, UV laser- induced damage to polished fused-silica surfaces continues to grow upon subsequent illumination. In this study photoluminescence spectroscopy was used to detect the formation of a modified, absorbing layers of silica that could be responsible for the continued growth of the damage site. For damage sites created with pulsed 355 nm illumination, three characteristic photoluminescence peaks are detected within the damage sites when excited with a 351 nm CW beam. Two of the peaks are likely due to the well- known E' and NBOHC defects associated with oxygen vacancies and broken Si-O bonds, respectively. The third, and dominant, peak at 560 nm has not been clearly identified, but is likely associated with a change in stoichiometry of the silica. The relative intensities of the peaks are non- uniform across individual damage sties. The photoluminescence data is being combined with insights from various optical and optical and electron microscopies to develop an understanding of laser-induced damage sties. The objective is to develop strategies to slow or stop the growth of the damage sites.

This paper introduces an error analysis of the international standard procedure for the measurement of optical absorption, ISO 11551. In this paper expression are developed for the uncertainty in a given absorptance measurement. The expressions for the uncertainty are developed using a standard propagation of error technique for both the pulse and gradient method. The paper includes uncertainty calculations based on typical experimental parameter values and their associated uncertainties for both methods. It is shown that measurements made by the pulse method have smaller uncertainty than gradient method measurements.

IR thermal imagin has been sued to study absorption in coated optical surface.s THis technique has demonstrated the ability to rapidly determine coating quality on thermally insulating substrates such as fused silica. The application of this technique to coatings deposited on thermal conductive substrates such as sapphire, silicon or copper is discussed. Data and the result of modeling are compared to show the limitations and potential of this technique for measurement and study of different classes of optical components.

An extension of the Keldysh formulation for electron tunnel ionization has resulted in a description of laser-induced dielectric breakdown with the following features: threshold optical field is predictive based on material properties such as ionization potential, number density, and refractive index. Time to damage is predicted to occur at or slightly beyond the peak laser intensity. Threshold breakdown fields depend on gas pressure as p^-1/3. Threshold field varies with pulse duration as t^-1/4. The perceived anomalously high strong-field ionization rate of O₂ as compared to Xe is explained. Acceptable agreement with measured threshold is seen for a range of materials with 0.000036 < n - 1 < 1.4, and threshold field varies approximately as (Lambda) ^0.77 for 0.248 micrometers < 1 < 2.94 micrometers . Specific pulse shapes can be simulated and their effects predicted.

Transient surface morphology changes in dielectric materials induced by laser irradiation were investigated with time- resolved interferometry. Deformation images were acquired at various delay times after exposure to single pulses on fresh sample regions. Above the ablation threshold, we observe prompt ejection of material and the formation of a single unipolar compressional surface acoustic wave propagating away from the ablation crater. For calcite, no deformation - either transient or permanent - is discernable at laser fluences below the threshold for material ejection. Below- threshold behavior was investigated using a phosphate glass sample with substantial near IR absorption. KG3 exhibits the formation of a small bulge roughly the size of the laser spot that reaches its maximum amplitude by approximately 5 ns. At lower laser fluences, diffusion of thermal energy away from that region causes a much weaker and boarder bulge to appear on a slower time scale. At higher laser fluences, a pair of strong, unipolar rarefaction surface acoustic waves is launched, separating from the central region at roughly 17 and 22 ns. Details of the transient interferometry system will also be given.

The ion beam assistance during the film growth is one of the most useful method to obtain dense film along with improved optical and structural properties. Afnia material is widely used in optical coating operating in the UV region of the spectrum and its optical properties depend on the production method and the physical parameters of the species involved in the deposition process. In this work afnia thin films were evaporated by an e-gun and assisted during the growth process. The deposition parameters, ion beam energy, density of ions impinging on the growing film and the number of arrival atoms from the crucible, have been related to the optical and structural properties of the film itself. The absorption coefficient and the refractive index were measured by spectrophotometric technique while the microstructure has been studied by means of x-ray diffraction. A strictly correlation between the grain size, the optical properties and the laser damage threshold measurements at 248 nm was found for the samples deposited at different deposition parameters.

Exfoliation of optical thin films at high humidity in the air is serious problem. Deterioration of three kind of optical thin films deposited on three types of substrate with different water-resistance at high humidity condition was studied. Thin films deposited on fused silica glass with highest water-resistance showed a best result for the exfoliation problem. Silica film seamed to be best for the surface observation at high humidity, but the laser -induced damage threshold was considerably lowered.

Multiple-pulse laser induced bulk damage of various types of silica glasses at wavelengths of 532 and 355 nm with laser pulse width of approximately 6 ns repetition rate of 10 Hz were measured. At 532 n, no apparent difference was observed in multiple pulse laser-induced bulk damage threshold (MLIDT) except for As fused quartz (FQ) produced by electrical melting of natural quartz powder whose MLIDT at a constant number of pluses is less than those of the other silicas. MLIDT at 355 nm were different among samples. The MLIDT of FQ disperse much more than that of synthetic fused silica. This could be derived from the distribution of the metallic impurity content in FQ.

Nonlinear-optical crystals are attractive materials for the high-power frequency converter with high damage threshold, phase matching characteristics, wide transparency range, and large effective nonlinear coefficients. Especially as to the power laser applications, its laser-induced damage threshold determines the limit of performance in the optical system. The threshold depends not only on the intrinsic material parameters but also on the laser beam parameters is use. We have investigated the bulk damage threshold of several crystals at a single-shot operation for frequency converter depending on the laser irradiation direction and its polarization. For KDP and CLBO crystals, the damage threshold in the direction of c-axis is about tow times higher than that in the a- or b-axis at 1.064 micrometers of wavelength. This result is consistent with the molecular bonding structure in different directions of the crystal. The relation between the bulk damage pattern and the crystal structure is also discussed.

High-power all solid-state UV lasers are highly demanding for many applications because of their compactness and ease of operation. An effective technique for UV generation is cascaded sum-frequency generation pumped by the output of near-IR solid-state lasers. The performance of these laser systems is limited by the laser power handling capability of the nonlinear optical crystals that employed for frequency up-conversion.

The gray-tracking of KTiOPO₄ nonlinear crystals, which is used for second harmonic generation of solid state lasers, have been investigated. The susceptibilities of gray-tracking were evaluated by measuring the reduction of transmittance of the crystal with the second harmonics pulse of Nd:YAG laser, and were compared with the optical absorption coefficients and contaminants in each crystal. As result, the susceptibility indicated a dependence on the initial absorption at 532-nm wavelength. Moreover, it was prevented with increasing hydroxyl concentration in the crystals. We propose a new approach toward the improvement of gray-tracking for KTP crystals annealing process under control if the humidity.

The photoacoustic beam deflection technique at (lambda) equals 248 nm was employed for the measurement of the laser damage threshold of single films of magnesium fluoride deposited on superpolished calcium fluoride and fused silica substrates. Different samples were investigated. All films have similar thickness but are deposited by various techniques in different laboratories. The samples are fully characterized, both from the optical and the structural point of view, in the framework of a European Project on 'UV-coatings'. The results of these measurements are reported, along with the data on laser damage threshold, to find out which are the most significant parameters for laser applications.

During the last few years enormous technical progress has been achieved in the development of ultrashort pulse lasers. The new generation of these laser systems are near the threshold to innovative applications in industrial environments for precision materials processing. As a consequence, an increasing demand for optical components with high laser damage resistance and extended lifetime can be noticed. In order to investigate the damage threshold and lifetime, the existing multiple-pulse laser-induced damage measurement facility at the Laser Zentrum Hannover has been adapted for the ultrashort pulse regime. During our measurements, the most important substrate materials and selected model layer systems were investigated, and the influence of optimized coating processes was studied. The reslut indicate a distinct reduction of the damage threshold with the exposed pulse number.

HR layer stacks with increasing number of HL pairs of fluoride material deposited on different substrates have been successfully investigated with respect to the laser radiation damage threshold at 248 nm and 193 nm. In this paper the investigation has been extended on resonant as well as non-resonant oxide coatings deposited by ion beam sputtering (IBS). Compared to conventional evaporation and plasma ion assisted evaporation technique, IBS coatings exhibit a higher packing density, thus preventing water to enter the film which woudl degrade the quality of the coatings. In contrast, the thermal stress is increased in IBS layer stacks. The measurements were carried out by use of the pulsed thermal lens technique enabling us to measure the advent of otpical breakdown induced by laser fluences in the subdamage range.

A mode-mismatched surface thermal lens technique with pulsed top-hat beam excitation and near field detection scheme is used to measure in situ the thermoelastic response of UV dielectric coatings under excimer laser irradiation with fluences from below to above damage threshold (LIDT). At fluences below LIDT, the thermal lens amplitude is caused by the surface displacement of the optical coating samples, due to the coating absorption and the thermal expansion of the substrate. Measurements are made on highly reflective (HR) dielectric coating on quartz and copper substrates. For copper substrate HfO₂/SiO₂ HR mirror, the damage of the coating is caused by thermal stress due to the strong thermal expansion of the copper substrate. For quartz substrate sample, on the other hand, the thermal stress caused by the thermal expansion of the substrate is relatively weak and the coating is damaged by the defect- absorption induced melting and evaporation of the coating. The copper substrate sample therefore shows lower LIDT than the quartz substrate sample. For HfO4-2)/SiO₂ HR coating on copper sample, we also observed a decrease of thermal diffusion rate with increasing the fluence form below to above LIDT, which indicates that delamination occurs at interface of the coating/substrate and/or between different coating layers.

Light scattering is an optical loss mechanism, which reduces the efficiency of imagin systems and beam guiding arrangements in many applications of high power excimer lasers. Therefore, the measurement of total scattering by optical components plays an important role for the development and optimization of thin film components and high power optics for the DUV/VUV-spectral range. In this work, a set-up for the measurement of total scattering (TS) is described and the limitations of the different functional parts are considered. The imaging properties of the Coblentz hemisphere, which is employed for collection of the scattered radiation from the specimen, are investigated in respect to the precision of the scatter measurements in the DUV/VUV spectral range. Also, the technical demands for the beam preparation system, the signal detection and the calibration of the set up are compared to specifications of the current Draft International Standard ISO/DIS 13696 for the measurement of total scattering. The study is concluded by typical TS measurements at 193nm and preliminary result for scatter measurements at 157nm.

Optimization of deposition technologies improves the laser resistivity of fluoride multilayer coatings at wavelengths as short as 193 nm.

HfO₂ is one of the most important high index thin film materials for the manufacture of interference coatings in the DUV spectral region down to 248 nm. High quality coatings and multilayer interference systems in conjunction with SiO₂ as low index material can be deposited by various PVD technologies including reactive e-beam evaporation (RE), ion assisted deposition (IAD) and plasma ion assisted deposition (PIAD). Thin HfO₂ films with optical thickness up to 3(lambda) /4 were deposited by RE, IAD and PIAD onto fused silica. The optical and structural properties of these films were investigated. The optical properties are related to the film structure and film density. The interaction of UV radiation with photon energies close to the band gap of HfO₂ with different films was studied. LIDT at 248 nm were determined in the 1- on-1 and the 1000-on-1 test mode in dependence on the deposition technology and the film thickness. LIDT values of all investigated films decrease with increasing thickness due to the higher absorption and defect density. Additionally, data on the radiation resistance of interference coatings containing HfO₂ will be presented.

New absorption measurements for aluminum oxide optical coatings at 193nm are presented. Apart from the strong linear absorption at this wavelength the data indicate a nonlinear absorption within the thin dielectric layer. By varying the laser thickness, the intrinsic contribution of the layer material to the overall absorption was separated from the contribution of the substrate and the interface. In addition, the conditioning behavior of the coatings was examined. A strong long term conditioning in the linear absorption was found for Al₂O₃ containing systems. Comparing the absorption and conditioning behavior of the single layers and a high-reflective system, we can show that the absorption properties of the HR-system are determined by its Al₂O₃ layers.

DUV optical components are examined with respect to their optical losses at 193nm. Scattering and calorimetric absorption measurements were performed. The used calorimetric measurement setup allows absolute absorptance measurement with ppm resolution. By varying the energy density on the sample, linear as well as nonlinear absorptance can be determined. The total-scattering measurement setup allows the determination of both forward and backward scattering independently. We examined bare fused silica substrates, single layer coatings and thin film stacks. The obtained data give a detailed overview over the strength and the origins of the losses due to the different mechanisms for these optical components.

A mode-mismatched surface thermal lens technique with pulsed top-hat beam excitation and near field detection scheme is developed to measure in situ the thermoelastic response of UV dielectric coatings and bulk materials under excimer laser irradiation. The thermal lens technique is demonstrated to be not only convenient for an accurate determination of the laser-induced damage threshold (LIDT), but also sensitive to measure the thermoelastic response of dielectric coatings irradiated with fluences much below the LIDT, and hence, to carry out time resolved predamage investigation. The minimum detectable surface displacement of approximately 0.002nm is achieved with a simple experimental configuration. Nonlinear absorption of UV dielectric materials and coatings are demonstrated. The surface thermal lens technique is also a convenient technique to distinguish different damage mechanisms: thermal stress induced damage or melting induced damage, depending on the thermo-elastic properties of the substrate. Hence, this technique allows to indicate qualitatively the relative contribution of linear and nonlinear absorption as possible causes for laser damage. Moreover, the nonlinear effect in laser conditioning of a LaF₃/MgF₂ highly reflective dielectric coating has been observed experimentally.

The key technologies for modern production processes with enhanced spatial resolution, require high performance DUV- excimer laser optics with enhanced optical properties. Major challenges imposed onto the requested new generation of optical elements are concentrated on lowest absorption and scattering as well as stability against highest pulse number throughput. These targets are the driving force within the German Joint Research Project 'OPUS II', which is dedicated to the development of high quality optical components for the DUV spectral range. As a major contribution to these investigations, sets of reflecting stacks with four different numbers of layer pairs of LaF₃/MgF₂ were produced by 6 partners of the consortium and characterized in respect to their optical performance and structural properties. The characterization includes spectrophotometric measurements from the VUV up tot eh mid RI range. calorimetric absorption measurements at 193 nm, and a comparative study in total scatter behavior at 193 nm, which was performed by three laboratories within the project. Also, besides the intrinsic stress and the surface topography of the layers, the non-linear absorption behavior of selected samples have been determined. The results are presented and discussed with respect to possible applications.

By direct observation of individual damage sites on the surface of a component, the local fluence of every damage site was obtained. This more accurate measurement was used to evaluate the real defect density at each fluence. These results were compared to some others, calculated from damage fluences distributions, obtained from R/1 test. Results presented here concern fused silica with a regular polish. The experimental set up used for this work was equipped with a 3.7ns Nd:YAG pulsed laser at 1 (omega) . The laser beam diameter was 1.1mm and the angle of incidence was close to 0 degrees.

A simple model is proposed, featuring cooperation of many defects to the damage process. For identical, randomly and independently distributed defects, the response of the optical components to a spatially gaussian laser pulse is calculated. The absorption response of the component is a probability distribution, which depends on the values of mean defect density and beam equivalent area. For a damage threshold expressed as a value of the absorbed energy, the statistical distribution of damage in obtained theoretically, as a function of fluence. The damage fluence distribution resembles the lognormal function found in many recent experimental measurements. Damage statistics were also calculated for a diverse spatial dimensions of the beam. This cooperative model of a damage is compared to the description of defect-related damage developed in particular by Mike Feit et al to correlate small beam damage test to large area irradiation results. In the latter work ,defects react one by one, contrary to the present view.

A laser damage measurement campaign was realized on PHEBUS high power laser on two different high reflecting HfO₂/SiO₂ mirrors centered at the wavelength of 1.053 micrometers at 45 degrees incidence. The two tested mirrors were deposited using e-beam technology: a large 620 X 440 mm² LIL mirror was made with an oxide HfO₂ target, and a 100 mm-diameter mirror with a metal Hf target. The test were performed with a 40 mm wide beam. Damages were detected by light scattering on a separate facility. Macroscopic and microscopic images of the damages were taken. A statistical analysis of these data is proposed to compare the mirrors. It is also interesting to compare large beam damage data to small beam laboratory statistics.

Simulations of laser matter interaction at intermediate energies is a key issue in predicting and quantifying laser damage in the LMJ or the NIF facilities. We have done simulations of laser interaction with several metals from the solid state to the plasma by solving the Helmholtz equation in our hydrodynamic code DELPOR. We are comparing our result to time dependent reflectivity measurements on Aluminum and Iron and discuss the influence of the transitions of phase of the materials in laser mater interaction.

We describe the experimental set-up used to determine the optical densities and the laser damage threshold of laser goggles. Optical densities have been measured at 1064 nm using 3ns laser pulses on an upgraded automatic test bench, which also allows for the determination of laser damage threshold of otpical components. We have performed experiments at constant fluence by varying different parameters such as the angle of incidence, the light polarization, the laser repetition rate, and the spot size. Multiple shot data are obtained in real time, which enables us to detect optical density variations during the laser exposure. From our study, we can conclude that a few goggles show the well known effect of saturable absorbers. This means that, above a given thresholds energy, the goggles become transparent. Finally, we will present a simple model which allows to understand the behavior of the laser damage threshold of polycarbonate goggles under these various conditions.

Very high damage threshold Hafnia Silica mirror coating have been developed successfully thanks to a clear understanding of the main limitations of the available mirrors. Coating with a 3ns pulse width threshold above 100J/cm² are reported. These coatings proved to be much more resistant than very high quality bare silica substrate tested the same way. The limitation of these coatings is still extrinsic. The potential of this technology for large size high damage mirrors for fusion laser application is discussed.

Previous studies of for sol-gel coated multilayer UV mirrors have shown a significant decrease in laser induced damage threshold (LIDT) between the single component layer coatings and ten pair multilayer coatings. Further investigation has shown that the LIDT decreased rapidly after the first two pairs however no further decrease was seen with the subsequent deposition of more layers. In this paper, the effect on the LIDT of starting and finishing with high index material was assessed as a function of the number of layers. The effects on LIDT and environmental stability, i.e. the reaction to changes in temperature and pressure, of using a silica or Teflon half wave overcoat were also investigated. Different coating treatments, e.g. baking each layer applied to the substrate, were also investigated with the aim of improving both the LIDT and the number of layers which could be deposited, and hence the reflectivity of the mirrors, without the occurrence of crazing. Investigations into the damage morphologies were made and differences between the samples compared.

The resistance of Chemical Vapor Deposited (CVD) diamond samples to high power, continuous wave CO₂ lasers has been investigated. It was found that the major possibility of failure was through thermal hoop stresses at the disc edge and these formed as a result of a thermal runaway effect in the samples. CVD diamond's uniquely high thermal conductivity means that the thermal runaway can be prevented in almost all practical applications by effective cooling of the disc edge. If this is done, the laser damage threshold becomes so high that it is difficult to accurately quantify using standard laser systems. Other parameters which determine CVD diamond's performance in IR window applications, such as: modulation transfer function, samples flatness and absorption, have also been studied.

The improvement of optical components for high power laser applications is still topical. Indeed the different signal cant progress made these last years, had allowed to improve the damage resistance of optical components by in particular, the identification of micronic precursors centers. A new challenge today is the identification of precursor centers of damage with size in the range of few nanometers. This kind of defects seems to play an important role in the laser damage process. In any case the challenge is to find an efficient tool able to detect these defects which are invisible with usual techniques as optical microscope or standard scattering. The technique of Laser Modulated Scattering (LMS) has been performed to reach this challenge. This new tool presented last year in the Boulder symposium, is based on a very high sensitivity detection of photothermal response of the defect. The LMS has been performed via two different setup arrangements. The first one uses tow beams as in the configuration of a standard Photothermal microscope, and the second one uses only one beam. In this article we first briefly remind the principle of the LMS technique with one and two beams. Then we will show by different results, the advantages of using an optical fiber to collect the scatted light instead of a block beam system used before. One of the main advantages of the setup using a fiber, is that it is easily possible to realize an angular study of scattering which allows a best understanding of the physical origin of the defect-induced scattered signal. The last part of this work consists of a series of stimulation of angular scattering LMS curve, in order to quantify the sensitivity and the powerfulness of this technique.

The purpose of the paper is to transfer the experience of production of laser-induced damage images. At the beginning the specific system for production of the images is shown and characteristics of all elements of the system are described. General steps of creation of laser-induced damage images are discussed in detail and they are illustrated by numerous examples.

Creation of laser-induced damage imags inside transparent materials is one of the technical and art applications of laser induced damage phenomena. The purpose of the paper is to present the laser induced damage image technology and to discus problems of the optimization of the system producing the images. At the beginning a brief historical overview and patent situation are presented. An exemplary block-diagram of laser etching system for simultaneous production of several images inside several articles is shown and some peculiarities due to specific formation of laser induced micro-damages are discussed. Several methods for generating 3D images and portraits allowing reproduction of them within an optically transparent materials with the same resolution like computer processed images, without sharp to point structure and without significant fluctuations of gray shades are disclosed. Technique of image reproduction without undesirable regular arrangement of etch points is presented.

Optical windows consisting of layers of different materials created at elevated temperatures usually exhibit substantial residual stresses. These stresses are cause by intrinsic strains, in addition to thermal strains that originate from the bonding of the layers, which generates internal forces and moments that must be balanced to achieve mechanical equilibrium. For isotopically elastic structures, this leads to planar contraction or elongation accompanied by spherical deformation, thus resulting in a partial relaxation of the stresses. There is a vast amount of literature relating to this topic. Analytical solutions have been proposed for thin films on a thick substrate, but a closed-form solution for multiple layers of arbitrary thickness has only been available since 1987 and has not yet been fully exploited. It is the purpose of this contribution to take advantage of Townsend's model for deriving 'user-friendly' formulas that properly describe the strains, the stresses, and the curvature of chemically vapor-deposited multilayered optical window blanks; interfacial shear stresses do not alter the curvature or the normal stress distribution off the edges and are beyond the scope of this paper.

Using the LOCALF method, we have examined electric field distributions for high and low dielectric susceptibility cases through systematic variations of defect concentration and orientation. The addition of nonlinear susceptibility terms to the conventional linear contributions in the dielectric formalism incrementally perturbs both the electric field intensities and predicted dielectric constants, having a greater influence on low dielectric materials. These effects are modulate by the relative ratios of the linear and nonlinear susceptibility terms for a material due to polarization-derived changes. Nonlinear effects can have a strong influence on the magnitude of these changes; potentially obscuring predicted changes in dielectric response due to microstructural detail.

Most quarterwave-stack laser reflectors and IR blocking filters have the capability of reflecting nearly 100 percent of the light over an extended wavelength range at all angles of incidence. These so-called 'perfect mirrors' have been around for many years, but the omnidirectional property has been recognized only recently. This paper reviews the properties of omnidirectional reflectors and points out an interesting feature related to the optimum design that may not have been anticipated from the point of view of an optical-coating designer. It is also shown that the wavelength region of omnidirectional performance of a single quarterwave-stack reflector can be extended by using standard thin-film design methods. For example, four adjacent quarterwave-stack reflectors can provide close to 100 percent reflectance at all angle over the complete visible spectrum. Important features of the designs that limit their usefulness as omnidirectional band-pass or edge filters are presented.

Recent developments in the laser spallation technique to measure the tensile strength of thin film interfaces will be presented. In this technique, a laser-generated stress wave in the substrate pries off the coating deposited on its free surface. The interface strength is quantified by recording the free surface velocity of the coating using an interferometer. Examples from metal/ceramic, ceramic/oxide and polymer/semiconductor interface systems will be presented to demonstrate the ability of the technique to pick up strength changes resulting of interfacial adhesion. The sensitivity of the technique to pick up strength changes resulting from different processes and surface variables will be demonstrated. Potential application of the technique to measure the strength and reliability of interfaces in geometrically heterogeneous multilayer ICs and electronic substrate structures will also be discussed.

We have used a 3-color imagin technique to obtain time- resolved series of images during nanosecond laser damage in bulk DKDP crystals. In contrast to single-pump, single-probe time-resolved imaging techniques, we are able to correlate behavior during single damage events. This enables us to observe a range of morphological dynamics that is otherwise difficult to study, including: the propagation of elastic sound waves and the liquid/solid melt front from the damage nucleation site and the dynamics of crack formation and propagation.

The need for microstructural characterization extends beyond defect detection and control of fabrication processes. Accurate description of the dielectric properties is required to predict the macroscopic properties of heterogeneous or composite materials and to implement materials-by-design concepts. Traditionally, light scattering has been the tool of choice because of its noninvasive character. However, many forms of condensed matter scatter light very strongly and complications due to multiple scattering seem to prevent the direct use of such investigation methods. Recent advances in the theory of multiple light scattering show that retrieval of structural information is possible in spite of an overall randomness of the scattered field. We developed the phenomenological understanding and the methodologies necessary to correlate the morphological details of the scattering centers with the measurable macroscopic properties.

For more than 35 years, a wide range of biomedical research has been conducted in order to understand the biophysical factors which influence laser induced retinal injury. Although the optical effects which influence retinal imaging and the initial physical events which lead to the absorption and dissipation of the laser energy are well understood, the stages of biological damage which take pace after the deposition of energy are not so well understood. The greatest body of research was initial centered on the interaction of laser energy with ocular tissues. Much of the research was to derive occupational health and safety standards that provide maximum permissible exposure limits. These limits are based both upon the theoretical understanding and the large body of experimental data. Current laser safety research has recently focused almost exclusively on deriving retinal injury thresholds for sub- nanosecond exposures. Setting limits in this temporal region has been difficult, since there have been conflicting data sets and there are limited data to extrapolate to other spectral regions. Because of the transparency of some ocular tissues, the ocular injury studies offer an opportunity to study all interaction mechanism with greater ease of viewing the effects directly with greater clarity than working with other biological tissues.

Mechanisms of photic injury to the eye can be categorized as photochemical, photothermal or photodistruptive. Exposure wavelength, exposure duration, ocular tissue characteristics and response criteria are key factors in the delineation of the ocular injury mechanisms. Depending on the exposure condition, one or all of the laser-tissue interaction mechanisms can be involved. Although photic injury to the eye was initially assumed to involve thermal mechanisms, more recent research has demonstrated that ocular effects can be produced by light exposure without a significant retinal temperature rise. Photochemical mechanisms are also implicated in UV photic injury to the cornea and lens. Exposure of the retina to short visible wavelengths for prolonged durations results in photochemical retinal damage with negligible localized retinal temperature elevation. For exposure conditions where photochemical mechanisms are dominate, the reciprocity of irradiance and exposure duration is apparent. The latency until observation of a photochemical lesion is often 24-48 hours whereas a thermal lesion is observed immediately or within a few hours after the exposure. Action spectra for photochemical injury to the eye are discussed in the context of ocular injury thresholds and current permissible exposure limits.

The action spectrum for light-induced damage to the retina results from wavelength dependent transmission of the pre-retinal ocular media, wavelength dependent absorption in retinal chromophores, and chromatic aberration of the eye optics. While various light/tissue interaction mechanisms have been implicated, thermal mechanisms dominate in the red and near-infrared for all exposure durations and in the visible for exposures shorter than a few seconds. A number of investigators have measured the transmission of the eye and the spectra of retinal absorbers, and thermal models based on these data predict the broad features of the action spectrum. Dose/response studies with lasers conducted over the past 40 years have mainly validated the thermal models. However, thresholds for laser induced chorioretinal injury using q-switched pulses show a variability ofthreshold with small changes in wavelength

Absorption of pulsed laser radiation by individual melanosomes produces located heating and microactivation bubble formation around the particles, with transient bubble lifetimes of a few hundred nanoseconds. Intracellular cavitation in the retinal pigment epithelium (RPE) leads to prompt cell death. Threshold laser fluence for cavitation bubble formation and RPE cell damage was measured as the exposure spot size was varied from 20 to 200 micrometers using a RPE tissue explant model. The threshold energy for cell killing decreased with decreasing spot diameter but the fluence was nearly constant for all spotsizes. How this compares with data from in vivo animal studies is discussed.

Laser energy absorbed in cells can affect the cellular materials through a variety of physical mechanisms. For pulses longer than a microsecond in duration, damage at threshold levels is due to thermal effects. However, for supra-threshold pulses, or for threshold levels at shorter pulse durations, other physical mechanisms may be the source of damage. The most likely mechanisms are the generation of large pressures, and the creation of bubbles. This is especially true for heavily pigmented cells, such as the retinal pigment epithelium, where large energy densities can be attained in the strongly absorbing pigments. We describe how the generation of large pressures depends on laser pulse duration and energy, as well as the properties of the absorbing pigment. Extremely large negative pressures can be generated in the core or the absorber, possibly resulting in the explosion of the absorbing pigment within the cell. We also discuss the conditions for which damaging bubbles are likely to be formed.

The Air Force has led a research effort to investigate the thresholds and mechanisms for retinal damage from ultrashort laser pulses. The results suggest that nonlinear optical phenomena mitigate the eventual damage threshold of the retina, while the fundamental mechanisms for damage remain unchanged from 100 fs to 10 microsecond(s) . The result of this research is a recommendation for the establishment of maximum permissible exposure limits in the visible and near IR that reflect the results of the nonlinear interaction. We review the progress made in determine trends in retinal damage from laser pulses from one nanosecond to one hundred femtoseconds for visible and near-IR wavelengths including variations in spot size and number of pulses. We discuss the most likely damage mechanisms, including nonlinear optical interactions pertinent in this pulse width regime and discuss relevance to laser safety.

Although most laser injuries involve a beam which is tightly focused on the retina, some laser or laser-derived sources are inherently non-focusable and produce a large image. These sources include nonlinear optical limiters, which might be used to protect the eye against injury. Nonlinear refraction, scattering, and other processes in these devices degrade the focusability of the beam. The resultant retinal image is complex in shape, contain both focusable and non- focusable components. This paper will review the influence of these processes in different types of limiter and will examine the issues which affect retinal damage threshold for these and other large-image sources. Recent experimental data will be discussed in the light of physical models.

The correspondence between theory and experiment is frequently an issue of debate in the area of laser damage. This paper sets out to assess the ability of analytical techniques to predict the out-of-band laser damage thresholds of thin films in a spectral regime where the films are strongly absorbing. The study was cantered on a group of silicon oxynitride dielectric mirrors fabricated for use at 532 nm using samples deposited by microwave plasma deposition, magnetron sputtering and ion-assisted deposition on sapphire substrate. Laser damage measurements were carried out using a pulsed laser at 10.6 micrometers , where the silicon oxynitride is highly absorbing. The laser- material interaction was modeled using a 1D, coupled radiation, thermal diffusion code which calculated the degree of temperature rise as a function of fluence using known optical constants for the materials. Melt thresholds were predicted to occur at fluences of 1-1.3 J/cm², closely matching the experimental data. Damage sites at higher fluences had a characteristic ripple morphology with approximately 3.5 micrometers spacing. A comparison is made with behavior found at 532 and 1064 nm.

The dependence of retinal damage threshold on laser spot size was examined for two pulsewidth regimes; nanosecond- duration Q-switched pluses from a doubled Nd:YAG laser and microsecond-duration pulses from a flashlamp-pumped dye laser. Threshold determination were conducted for nominal retinal image sizes ranging form 1.5 mrad to 100 mrad of visual field, corresponding to image diameters of approximately 22 micrometers to 1.4 mm on the primate retina. Together, this set of retinal damage threshold reveals the functional dependence of threshold on spot size. The threshold dose was found to vary with the area of the image for larger image sizes. The experimental results were compared to the predictions of the Thompson-Gerstman granular model of laser-induced retinal damage. The experimental and theoretical trends of threshold variation with retinal spot size were essentially the same, with both data sets showing threshold dose proportional to image area for spot sizes >= 150 micrometers . The absolute values predicted by the model, however, were significantly higher than experimental values, possibly because of uncertainty in various biological input parameters, such as the melanosome absorption coefficient and the number of melanosomes per RPE cell.

In our application, dichroics in a high average power, near- IR, laser system have short operating lifetimes. These dichroics were used as the resonator fold mirrors and permitted the transmission of the pumping argon (Ar) ion laser light. Representative samples of two different dichroic optics were taken off-line and the transmission performance monitored in various scenarios. Irradiating these topics under resonator vacuum conditions, resulted in a degradation of transmission with time. Irradiating these optics in a rarefield oxygen atmosphere, the transmission remained steady over a period of days. The transmission loss observed in the optic tested in vacuum was somewhat reversible if the optic was subsequently irradiated in a rarefield oxygen atmosphere of 10 T of air also prevented the transmission degradation. In addition, test were performed to demonstrate that the optic damage was not caused by the ultra UV component in the Ar ion laser. Mechanisms that may account for this behavior are proposed.

Jefferson Lab's IR Demo FEL Facility includes an associated 600 m² user facility containing six separate laboratory areas. In the summer of 1999 we began delivery of beam int two of these labs as part of our commissioning of the FEL optical transport and laser safety systems. The high average power capability in the mid-IR, along with an ultrafast high PRF temporal structure makes this laser a unique source for both applied and basic research. While commissioning, we conducted several test, primarily of laser-materials interactions that take advantage of the unique characteristics of this FEL. An overview of the FEL facility and its current performance, along with a synopsis of current and future experiments, will be presented.

Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput superior performance of the optical components with lowest absorption and scattering losses are necessary. In the framework of the German Joint Research Project OPUS II efforts are made to investigate the optical properties, the radiation resistance and long term stability of single layers and layer system of interest in the DUV. The evaluation of optical coatings and coating system on different substrate materials was carried out by scattering experiments, atomic force microscopy, IR spectroscopy, calorimetric absorption measurements, and determination of laser induced damage threshold. Additionally, from the spectralphotometric measurements the optical behavior of the films was examined.