Silver gallium selenide (AgGaSei) crystal for efficient second-harmonic-generation of C02 laser lines has been demonstrated using a pulsed laser. However, the use of this crystal in continuous wave (CW) lasers is limited due to its low laser damage threshold. In this paper, laser damage threshold measurements obtained using a 9 µm C02 laser will be discussed. The data obtained for the frequency doubling of 9 µm to 4.5 µm will be presented.

Optical Parametric Oscillator (OPO) based laser sources have potential applications not only in the environmental Lidars, semiconductor spectroscopy, but also in communications, medical and military applications. Their development is suffering from the lack of optical components that can withstand high power damage and be broad (wavelength wise) enough to fulifill the tuning range they are capable of. Unfortunately, such optics are very difficult to produce. This paper adresses questions on how to improve all dielectric coatings to meet OPOs requirements and displays some designs and coatings, where the damage threshold levels presently need improvement. Some damage threshold results are presented.

Single layer coatings of hafnia are included with results obtained with titania, tantala, zirconia and silica. These coatings were deposited by a reactive low voltage ion plating (RLVIP) and conventional reactive e-beam process. Layer thicknesses were optically a A14at 532 nm. The RMS surface roughness changes between the uncoated and coated were on the sub-angstrom level. The spectral transmittance curves demonstrated the material thickness homogeneity aspect of the RLVIP process. Laser damage thresholds were made from high peak power (1.06 .tm and 0.532 jim, 10 ns) and high average power (511 nm and 578 nm) laser damage test facilities. Given a set of coatings from one process, the damage thresholds tended to increase as the refractive index decreased. The e-beam silica coating had the highest damage threshold, exceeding 98 J/cm2 (1.06 jtm, 10 ns, 10 Hz). In addition, the surface absorption and refractive indices of the coatings were measured, and the X-ray diffraction patterns of the hafnia coatings made.

Amorphous Ti02 sol-gel films are irreversibly transformed to a crystalline anatase phase when heated to temperatures in excess of 575 K or subjected to intense pulsed or CW laser irradiation. The laser-induced transformation is initiated as a result of impurity absorption and subsequent heating, and results in densification and relative changes in compressive stress of the film. Isothermally annealed films exhibit a decrease in compressive stress as crystallization proceeds while an increase in compressive stress followed by a decrease in stress is observed when crystallization is laser-induced. Raman spectroscopy has been used to characterize the crystallization ingrowth kinetics and is used in this work as a real time probe of both film temperature and localized stress which can be evaluated from shifts in lattice phonon frequencies measured in real time during laser irradiation. The laser not only induces the phase transformation but excites inelastic Raman scattering from which film stress and temperature can be estimated. A second approach for the determination of these parameters requires incorporation of a thin ruby film between the titania and silica substrate. Here, the wavelength shift of the laser-induced ruby fluorescence can be used to quantify interfacial stress; the fluorescence lifetime measurements are used to determine temperature. The advantages and limitations of these techniques for evaluating transient stress and temperature evolution in thin titania films subjected to CW laser irradiation will be discussed.

The Phebus Neodymium glass Laser system located at Centre d'Etudes de Limeil Valenton (CEL-V) is the most powerful in Europe (20 Id, 1 ns at = 1053nm and up to 8 U, 1 ns at = 351 nm). A major concern in the design of this system was the damage thresholds of the 1 meter diameter turning mirrors and of the large polarizers (0.3 m2). MATRA DEFENSE S .A. , in a joint effort with CEL-V, using the well known reactive e-beam process, has investigated and produced a new generation of efficient ZrO/SiO and HfO/SiO high reflective and polarizing coatings, leading to very high laser-conditioned damage thresholds. In addition to good flatness figures (t/4 to ?i /10 at He-Ne wavelength), excellent roughness values (5-10 A RMS) and excellent spectral characteristics, laser-conditioned damage thresholds were in the range of 25-30 J/cm2 and 12- 15 J/cm2 respectively for mirrors and polarizers at 1 .064 m wavelength for a 3 ns pulse. Such performances are respectively 2 and 6 times higher than those of same components in the Ti02/5i02 conventional design. Laser-conditioning effect, enhancing the damage threshold by at least a factor of 2, was observed as a durable improvement

Ktecb bas modified the 2-D thermomecbanical response code PRONTO-TFT to include a model of the interaction of laser energy with optical coatings and substrates. PRONTO-TFT is the 2-D successor to the 1-D PUFF-TFT response code for energy deposition in thin films. Both codes were developed by Ktech under sponsorship of the Air Force Phillips Laboratory. In this paper, we present the method used in PRONTO-TFT to calculate the laser energy deposition within an optical system and bow this computation is incorporated into the thermal and mechanical material models. Examples of high power laser interactions with optical systems are used to demonstrate the utility of the code.

This research investigates the damage morphology and electrical performance of variously fabricated PIN photodiodes under single-pulse laser irradiation. Specially-ordered and industrial-standard photodiodes were irradiated with various fluences from a 10 ns Nd:YAG laser at 1064 nm. Morphological damage was defined as any residual surface damage observed at 200X magnification under a Normarski optical microscope. Electrical damage entailed any significant change in reverse bias current between the before and after laser-irradiated I-V curves. The electrical transient photocurrent was monitored and recorded for anomalous electrical response. It is presumed that the different fabrication processes will affect the electrical response of the photodiode. The photodiode sample sets differed from one another by the doping mechanism (ion implantation or planar diffusion), the crystallinity of the substrate prior to ion implantation (crystalline or amorphous), and the anneal or regrowth processing (furnace or rapid thermal anneal). All diodes were p⁺-n photodiodes produced by implanting boron into a high-resistivity, n-doped silicon substrate. The junction depth of all the diodes was between 0.5 and 1.2 micrometers .

Previous observation of micron-sized particle removal from optical surfaces by pulsed laser irradiation were explained on the basis of a simple analytical model. The model was based on the idea that thermal expansion during the pulse generated forces and accelerations large enough to detach absorbing particles. Several simplifying assumptions were made in developing the model. In this work more exact and detailed treatment of the laser pulse and thermal transport effects is presented. The previous approximate model is shown to be semiquantitatively accurate.

Thin films of titanium dioxide have been deposited using ion-assisted deposition with oxygen ions produced using: (1) a Heitmann ion source (HIS) for ions with energy less than 100 eV and (2) a broad beam Kaufman ion source (KIS) for ions in the energy range 100 to 500 eV and current densities up to 100 (mu) A/cm². It has been observed that the refractive index of the films increases up to 300 eV and the extinction coefficient only nominally up to 300 eV. The maximum refractive index obtained was 2.49 at an energy of 300 eV and 50 (mu) A/cm² current density. The refractive index of the films deposited using the HIS was lower than those deposited using the KIS. The refractive index of the HIS films increased with increasing substrate temperature with very little change in the extinction coefficient.

Supporting laser/materials interactive testing for the past 15 years, the Laser-Hardened Materials Evaluation Laboratory (LHMEL) has efficiently performed low-cost high-volume testing of materials samples placed in various environmental simulation conditions. The capabilities of the upgraded LHMEL facility carbon dioxide laser and related test support systems are described. The LHMEL facility is part of Wright Laboratories Materials Directorate and is located at Wright-Patterson Air Force Base, Ohio. Two lasers producing a 10.6 micron wavelength, continuous wave output and having a flat-top spatial intensity distribution are currently available for testing. The test parameters achievable with the 15 kW LHMEL I and the 100 kW LHMEL II devices are discussed. In addition, various test environmental simulation capabilities are described. Vacuum environments in the 1 X 10^-6 torr range are routinely achieved for samples ranging in size from 1 cm to 120 cm. Atmospheric velocities approaching Mach 1 for samples up to 7 cm are provided. Mechanical loads up to 55,000 lbs of force can be incorporated into ambient or high velocity testing schemes. Finally, the capability of the facility data acquisition system is described.

Results including 351-nm laser damage threshold data are presented from a study of a perfluorinated copolymer being considered for use in high peak-power lasers. The objective of this work is the hermetic sealing of hygroscopic, optical, nonlinear-conversion crystals by this material with a simultaneous decrease in Fresnel losses at the crystal surfaces. Dramatic effects on short-pulse (0.8 ns) damage thresholds by solvents and/or polymer functional moieties will be shown.

In this paper we report on an empirical study of the absorption and laser damage threshold behavior of uncoated KCI and KBr windows at 10.6 micrometers . Absorption measurements were performed with the aid of CO₂ laser calorimeter. Single-shot damage thresholds at different spot sizes were measured by a computer-controlled test facility using 100 ns pulses from a TEA-CO₂ laser. A video microscopy system was installed for on-line damage detection. Onset-damage thresholds (0% LIDT) are compared with the 50% damage thresholds. Damage morphology was investigated by Nomarski microscopy.

The attempt to eliminate subsurface damage in polished materials is a major objective in optical and semiconductor fabrication. The level of subsurface damage in optical components is proportional to the surface scatter and related to the laser damage threshold of the optic. The float polishing process has been shown to produce surfaces with low subsurface damage on ferrite materials. We have ground samples of rough cut Corning 7940 fused silica using synthetic polycrystalline diamond. These samples were then float polished on a precision machine manufactured by Toyoda Machine Works Limited. Our surfaces were characterized using differential phase interference microscopy, total internal reflection microscopy, and scatterometry. We will describe the fabrication process and report the results of the surface and subsurface characterization.

A previous investigation of laser-induced damage mechanisms and corresponding thresholds in step-index, multimode fibers was motivated by an interest in optical systems for firing explosives. In the initial study, the output from a compact, multimode Nd/YAG laser was coupled into fiber cores of pure fused silica. End-face polishing steps were varied between successive fiber lots to produce improved finishes, and each fiber was subjected to a sequence of progressively increasing energy densities up to a value of more than 80 J/cm². Essentially all of the tested fibers experienced a 'laser conditioning' process at the front fiber face, in which a visible plasma was generated for one or more laser shots. Rather than produce progressive damage at the front surface, however, this process would eventually cease and leave the surface with improved damage resistance. Once past this conditioning process, the majority of fibers damaged at the rear end face. Other modes of damage were observed either at locations of fixturing stresses or at a location of high static tensile stress resulting from bends introduced to the fiber. Although the previous results were encouraging in terms of achieving useful damage thresholds, a number of areas for further study were indicated. In the present study, a similar experimental procedure was used to address these areas. The relative permanence of front-surface laser conditioning was examined by re-testing fibers that had experienced this process at least a year previously. End-face mechanical polishing was again examined by testing fibers prepared using a refined polishing schedule. Attempts to use a single fixture to hold an entire lot of fibers throughout end-face polishing and damage testing met with mixed results, with fiber positions subjected to fixturing stresses likely sites for initial damage. In an effort to prepare fiber faces with the improved damage resistance observed with front faces following 'laser conditioning,' two schedules for CO₂-laser polishing of end faces were developed and evaluated. Finally, to improve resistance to damage at sites with significant static stresses, fiber samples which passed a much higher tensile proof test during manufacturing were tested. The current experiments were conducted with a new laser having a shorter pulsewidth and a significantly different mode structure. The beam was injected into the fiber using a geometry that had been successful in the previous study in minimizing a damage mechanism which can occur at the core/cladding interface with the first few hundred fiber diameters. However, the different mode structure of the new laser apparently resulted in this mechanism dominating the current results.

Previous studies have investigated the possible effects of thin film thermal conductivity on laser damage thresholds and the phenomenon of anomalously low thermal conductivity values for materials in thin film form. Here we discuss the primary microstructural mechanisms for the high thermal resistance found in thin films. By studying a number of thin film systems, we found that many factors contribute to the reduction in conductivity, and that primary mechanisms for thermal resistance depend on film material and processing conditions. Rare earth-transition metal films were studied to illustrate the effect of columnar microstructure, ZrO₂ to show the effects of varying crystal structure, YBa₂Cu₃O_7-(delta ) to show the effects of crystal structure anisotropy and hillock growth, and AIN to show the effects of impurities, dimensional effects, and nanocrystalline microstructure.

Results are presented for preliminary investigations into the way optical elements that are exposed to corrosive environments exhibit an increase in transmissive scatter the longer that such exposure exists. Near angle (i.e. < 1 degree(s)) scatter data is presented for fused silica substrates exposed to fluorine atmospheres of differing concentrations. The fluorine reacts with any water vapor present to form hydrofluoric acid which etches materials such as silica. The scatter probe wavelength used was 633 nm and scattering angles from 4 to 15 mR studied. Results are presented in bidirectional transmission distribution function (BTDF) format as well as photographs of the resulting diffraction patterns arising from passing a HeNe laser beam through the bulk of the sample which seem to show a periodic roughness profile. It is suggested that surface cleaning prior to exposure provides seeding sites, or micro scratches, for preferential attack by hydrofluoric acid.

A well controlled damage testing facility is vital for establishing confidence in production coatings for high power laser operation and for the understanding of damage phenomena. An upgraded damage test facility at LLNL now allows better control in measuring damage thresholds and absorption of thin film coatings as well as examination of sites where damage first occurs. A description of the system configuration using lasers from the Laser Isotope Separation (LIS) at the Laser Demonstration Facility (LDF) as well as the methods used to acquire damage thresholds will be given.

An effort is being made to understand the limits of survivability and damage for optical components exposed to a visible laser operating continuously at a high repetition rate over 4 kHz. Results of this work are reported and related to the materials and manufacturing conditions for the coatings and substrates as well as defects seen at the surface under laser illumination. These results were obtained for a variety of optical coatings and conditions using lasers from the Laser Demonstration Facility, part of the Atomic Vapor Laser Isotope Separation (AVLIS) Program at LLNL. Better understanding of the reliability of optical components in this environment could lead to improvements in design and manufacture that would result in reduced size for the laser optical system and correspondingly lower costs for the facilities that can use this technology.

A Q-switched Nd:YAG laser had been used to damage perspex. Damage was initiated on the entry face and within the bulk of the material. Acoustic transients or shock waves were produced centered on the laser focus and high spatial and temporal resolution Mach Zehnder interferometry was used to measure the acoustic pressure field associated with the interaction. The well defined laser pulse had an energy of 4.6 +/- 0.2 mJ with a corresponding energy fluence of 260 J cm^-2. This is approximately an order of magnitude larger than the bulk damage threshold at 1.06 mm. Abel inversion and integration of the interferograms gave the total energy dissipated in the polymer in the form of acoustic transients, which was approximately 4 (mu) J. This result suggests that most of the laser pulse is either used to produce the damage void or scattered without absorption from the damage site.

Absorption characteristics of optical coatings have been observed using infrared photothermal radiometry as a noncontact diagnostic technique. The local laser-induced temperature rise was measured as a function of position on the coatings. The data was used to calculate the absorption of the coatings and to identify point sites with atypical absorption. The coating samples were tantala/silica multilayers designed to be high reflectors at 1.32 micrometers . The tests were done with a 1.32 micrometers , continuous-wave Nd:YAG laser and a HgCdTe infrared camera.

Total internal reflection microscopy (TIRM) is an inspection method that yields an image of the defects in a surface and/or a dielectric thin film. By installing a TIRM system in a vacuum-deposition chamber, the formation of thin-film defects (visible as point-scatter sites) is directly observable. By using such a system, we have examined the effects of deposition conditions on thin-film defect generation. In this preliminary investigation, the rate of defect accumulation in ZrO₂ thin films produced by electron-beam (e-beam) evaporation displays a dependence on substrate cleanliness, with cleaner surfaces yielding lesser defect-formation rates. In addition, the presence of film crystallites can be observed by in-situ TIRM, and shows a dependence on the oxygen partial pressure during deposition.

The determination of absorption at the wavelengths 1.06 and 10.6 micrometers was carried out by means of the photothermal displacement spectroscopy method. The analysis of the signal generation demonstrates the necessity of comprehensive correction calculations to obtain the absolute values. The investigation of the influence of postdeposition baking on the absorption revealed the generation of absorption centers in consequence of the heat treatment, e.g., at Ta₂O₅ films. The deposition of BaF₂ films at the substrate temperature 150 degree(s)C for 10.6 micrometers wavelength led to a smoother absorption behavior and simultaneously to a reduction of scattering compared to deposition at room temperature. Damage threshold measurements at the wavelength 248 nm also proved the negative influence of postdeposition baking on the coating performance. Further experiments in the ultraviolet wavelength range tested the association of laser damage thresholds of the high index material and the multilayer system.

In this contribution we demonstrate that surface conditions and microstructures of thin films strongly modify the thermal transport in these materials. Our technique is based on the pulsed photothermal deformation technique, refined by utilizing transient thermal gratings. Such gratings introduce a preferred direction in the surface plane for heat conduction. By using a nanosecond pump laser and analyzing the data with an appropriate model, the technique is capable of distinguishing between vertical and lateral channels for heat diffusion. Data are presented for thermally evaporated gold films of different thicknesses (20 - 2500 nm) on BK7 substrates, a commercial Au plate with random surface structure, and a Ni foil with a well oriented texture. The measured thermal diffusivity of the Au plate is about 25% lower than the literature value, due to a random surface roughness. For the Ni foil we find a strongly anisotropic diffusivity, either 0.19 +/- 0.03 or 0.05 +/- 0.002 cm²/s, depending on whether the direction of the grooves is parallel or perpendicular to the grating vector. For the gold films we observe the typical thickness dependent behavior, which most likely originates from disorder in the films that leads to an increased lateral thermal resistance.

Both cw and pulsed photothermal deflection techniques have been applied to make in-situ investigations of laser conditioning of optical coatings. Two different kinds of lasers have been used for the conditioning, namely 488 nm-cw-Ar⁺ and 1.06 micrometers -10 ns- Nd:YAG. The samples investigated include thermally evaporated TiO₂, ZrO₂, HfO₂, Al₂O₃, SiO₂, MgF₂, and ZnS single layers as well as TiO₂/SiO₂, ZrO₂/SiO₂, and HfO₂/SiO₂ HR coatings. While the conditioning effects on the damage thresholds vary from sample to sample and depend on the specific irradiation parameters, a good correlation is observed between the in-situ measured change in absorption and the improvement of the damage resistance. Also presented in this paper are the detection of the change in defect density and thermal conductivity of the irradiated area. The latter, however, is small for most of the samples.

The thermal transport behavior of CVD diamond films with different thicknesses (1 - 100 micrometers ) has been investigated by high frequency modulated photothermal reflectance technique. By analyzing the frequency dependent phase behavior of the laser-induced thermal waves, we have obtained the thermal conductivity of the samples as well as the thermal resistance at the film-substrate interfaces. With the tabled values of the density and the specific heat, we then calculate the thermal diffusivities and compare them with previously reported data on similar samples. The results demonstrate the technique a powerful tool for investigating the demanding diamond coatings.

This paper reports the recent progress in our research on defect characterization of optical thin films. The technique itself, based on photothermal microscopy through optical beam deflection, has been reported earlier by several groups including ourselves. The work here focuses on the differentiation between optical and thermal defects by photothermal imaging at different frequencies. We first present a brief introduction of the principle and the experimental method, and then the data of samples with artificially introduced surface and subsurface structures. The results demonstrate that the technique is capable of depth-profiling for absorption defects as well as of differentiating between optical and thermal imperfections. Finally, as typical examples of the applications of the technique, the results of various practical optical coatings are presented, including TiO_{2$, ZrO(subscript 2}, SiO₂, and ZnS single layers as well as TiO₂/SiO₂ AR and HR coatings. The effects of undercoats to AR and overcoats to HR are also investigated and compared with the damage experiments.

In this paper we report our recent measurements of thermal conductivities of optical coatings by two photothermal methods, namely photothermal reflectance and photothermal deformation technique. We will first give a brief introduction of the principles and the apparatus for the experiments, and then present the measured thermal conductivities of various coatings including oxide layers (SiO₂, ZrO₂, Ta₂O₅) on BK₇ glass, a MgF₂ film on MgF₂ substrate and metallic films (Au). The results are compared between the two methods as well as with those previously reported by other techniques and those of the related bulk materials.

Single layers and high reflecting QWOT stacks for 1064 nm of Al₂O₃, HfO₂, SiO₂, Ta₂O₅, TiO₂, and ZrO₂ were deposited on polished BK7 and fused silica substrates. All materials were evaporated with a conventional electron beam evaporator. The growing film surface was bombarded with ions from a microwave excited ECR ion source. Refractive indices were determined by spectrophotometric analysis of single layers. Laser calorimetry was used to measure absorption of light. Investigations on laser induced damage were performed by damage frequency measurements. Damage frequencies were based on more than 500 single tests per sample, ('one on one' method). From this data a certain value of damage threshold in accordance with ISO/DIS 11254 could be obtained. All results were compared with the corresponding values of conventional produced coatings.

Diamond exhibits unusually favorable properties in terms of mechanical strength, thermal conductivity, and optical transmission, which make this material highly attractive for infrared (IR) applications that involve severe heat loads. Until recently, diamond has been available only in the form of relatively small crystals, but this situation is evolving rapidly as a result of major advances in the art of growing diamond by chemical vapor deposition (CVD) techniques. Success in producing large, free-standing deposits having properties that match those of natural diamond has stimulated enormous interest and has given rise to much speculation about CVD diamond as an 'ideal' optical material for a wide range of engineering uses in the IR; the objective of this paper is to present a critical assessment of some of the issues that arise in connection with using diamond as a window material for high-power lasers. In a high-average-power environment, defect-free single-crystal diamond, preferably (111)-orientated to avoid anisotropic elastic responses, may indeed provide an outstanding window-material candidate for operation in the near IR. The power-handling capability, however, will be limited by thermal lensing, which can only be eliminated if absorption-free anti-reflection coatings become available; another limitation concerns the edge heat-transfer coefficient, which must be substantially above the state of the art to avoid nullifying the advantage derived from the superior thermal conductivity of diamond at low temperatures. Pressure-induced and thermally induced stresses are of no consequence, but diamond windows are not particularly promising for very-high-peak-power operation considering the non-linear index of refraction, which may cause highly damaging self-focusing effects.

Conventional evaporation as well as IAD and IBS processes were used to develop low-loss dielectric mirrors and antireflection coatings for the wavelength range 130 to 250 nm. First, single layers of SiO₂, Al₂O₃, HfO₂, AlF₃, MgF₂, LaF₃, NdF₃, and GdF₃ were deposited. The refractive indices, extinction coefficients and packing densities of the layers were determined by spectrophotometric methods in order to investigate their dependence on deposition parameters. With optimized parameters, HR and AR coatings were deposited. Their transmission and reflection coefficients were measured and compared to values calculated from single layer data and interface roughness data. Integrated scatter measurements were performed on single layers as well as AR and HR coatings for 248 nm. Results of damage threshold measurements at 193 and 248 nm are presented.

The modification of an optical coating surface with laser exposure at fluences below the damage threshold resulted in localized minimization of asperities. Topological changes with time were mapped using an atomic force microscope. It is suggested that charge accumulation on the dielectric coating drives the surface morphology to higher uniformity.

We have developed a super-lightweight mirror usable for laser fusion for the first time. The porous fused silica with density of 0.2 g/cm³ was sandwiched between two fused silica plates. The porous silica and the fused silica plates were contacted under high temperature of about 1500 - 1600 degree(s)C. The weight and optical flatness of the mirror (3500 mm^(phi ) X 36 mm^t) were about 1.7 kg and (lambda) /8, respectively. A larger diameter mirror of 530 mm^(phi ) is being fabricated.

Optical polishing method of large diameter KDP crystals was developed. The surface finishing of KDP crystals was conducted using specially prepared lapping plate and sluny of diamond powder in oil. The optical flatness and the surface roughness of optically polished KDP crystal (2O9mm'x16mmt) were about X/6 and 7- 10 Arms, respectively. The surface damage threshold of about 10 J/cm2 at 1064nm with a pulsewidth of ins was obtained. Key words : laser damage, KDP crystal, optical flatness, surface roughness

Improvements in magnetron sputtering technology have made it possible to deposit compound thin films at total pressures as low as approximately 1 X 10^-4 torr. Deposition at these lower pressures increases the mean free path of molecules within the vacuum chamber, thereby allowing for greater adatom energies on the substrate surface. By increasing adatom energy, low-pressure dc-magnetron sputtering can lead to the deposition of dense metal-oxide films that are resistant to the adsorption of atmospheric water (H₂O). We report results showing improved environmental stability in single-layer films of silica (SiO₂) and alumina (Al₂O₃) deposited using a 8-in-diameter dc magnetron source. Metal targets were used, with argon (Ar) as the sputtering gas and oxygen (O₂) as the reactive gas. The silicon target was doped with 5-percent Al to improve its electrical conductivity. The effects of target voltage, Ar partial pressure, and O₂ partial pressure were studied. In addition, several source geometries were tested in order to optimize the coating thickness uniformity. Film moisture content was characterized by spectral transmittance near the H₂O optical absorption band at 2.8 micrometers . Deposition at total pressures < 3 X 10^-4 torr resulted in SiO₂ coatings with minimal H₂O content, while all Al₂O₃ coatings exhibited no H₂O content.

Diffusion bonding of single crystal Nd:YAG with another Nd:YAG or undoped YAG involves precision polishing, assembly by optical contacting and heat treatment. Preliminary data on bonding kinetics indicate an approximate (time)^1/5 dependence which describes bulk sintering. Optical homogeneity across the interface did not exhibit a discontinuity for composites of Nd:YAG/Nd:YAG while a step at the Nd:YAG/undoped YAG interface is assumed to be due to a difference in refractive index. Parallel to the interface, stress birefringence values of up to about 5 nm/cm have been observed for Nd:YAG/Nd:YAG and about 20 nm/cm for Nd:YAG/undoped YAG. Corresponding values perpendicular to the interface are about 2 to 3 nm/cm for both types of composites. Reflectivity of about 3% and scattering of about 5% has been measured at 89 degrees incident angle while values below 0.03 and 0.05% respectively have been estimated for normal incidence.

Optical damage mechanisms for submicron thick, electron beam deposited HfO₂ and SiO₂ films on BK-7 substrates have been investigated by monitoring the emission of neutral constituents during excitation with time-delayed pairs of 70 ps laser pulses at a wavelength of 1064 nm. In silica, and probably also HfO₂, linear absorption is the mechanism for energy deposition into the films by the laser beams. Sporadic ablation observed for the HfO₂ films may be related to optical conditioning of multilayer HfO₂-SiO₂ high-reflector coatings.

A laser-based two-pulse correlation technique is used to investigate the mechanisms of photon absorption and the decay of surface excitation on Al₂O₃(1120). The mechanism for emission of positive or neutral particles depends on the wavelength and/or pulsewidth. In our previous study for a pulsewidth of 80 picoseconds at 1064 nm the lifetime of the surface excitation leading to detectable emission and further to optical surface damage was measured to be approximately 200 picoseconds. However, for 800 femtosecond pulses at 616 nm the measured lifetime is a few nanoseconds. In both cases a low-order absorption process is indicated.

In the past two years we have made a sixfold expansion of our laser-damage database. Our primary emphasis has been with the fundamental 1064-nm irradiation generated by Nd:YAG. Because of the increasing need for high-threshold optics designed to operate in the UV, we include data covering the harmonics at 532, 355, and 266 nm. This is further supplemented with results of excimer-laser damage testing at 351 and 248 nm. The presented summaries cover over eight years of complete data plus selected results spanning over a fourteen-year history of damage testing at LLNL using thirteen different laser systems. Besides the range of wavelengths, our parameter space covers pulse durations from < 1 ns to 84 ns, repetition rates from single shots to 6000 Hz, and irradiation modes from single shots to a variety of multiple-shot laser-conditioning techniques.

Atomic force microscopy was used to determine in-situ the nm-scale morphological changes that occur on dielectric optical coatings as a result of laser illumination. Of particular interest is a process called laser conditioning in which the damage threshold of the films is increased by a factor of 2 to 3 when the film is first illuminated at fluences below the damage threshold. The optical coating studied was a highly refractive dielectrical multilayer mirror (HR) consisting of many alternating quarter-wave layers of HfO₂ and SiO₂ at 1.06 micrometers . The top layer was a (lambda) /2 SiO₂ overcoat. Laser beam specifications were: 1.06-micrometers wavelength, 8- to 10-ns pulsewidth. Laser beam spot sizes ranging from 85 micrometers to 1.4 mm in diameter. The maximum scan range of the AFM was 80 micrometers . A survey of the as-deposited surface shows mostly hillocks approximately 200 nm in diameter and 10 nm in height. The predominant surface irregularity is micrometers -scale domes associated with the well known nodule defects. Laser illumination causes nodule defects to be easily ejected from the coating surface. Further damage may propagate from the resulting craters. These nodule defects therefore determine the damage threshold of the film. Using the AFM we have shown that for illumination at fluences below the nodule ejection threshold we observe a decrease in the surface roughness of the nodule defects and hillock structure of the as-deposited film. The subtle changes in these surface features may be an indication that the film is being mechanically stabilized, thus providing the observed conditioning effect.

The rate of surface fogging on KDP crystals has been associated with several parameters in the diamond turning and subsequent cleaning process. Fogging can also occur during use because of environmental degradation. We have determined that reaction of the KDP surfaces with additives in the diamond turning oil has been a major factor in fogging and have found a replacement oil that eliminates this effect. We have also developed an antireflective silicone- sol coating that also reduces surface environmental deterioration during use by a factor of four.

We measure the nonlinear refractive coefficient n₂ in BaF₂, LiF, MgF₂, Al₂O₃ and diamond using the Z-scan technique at (lambda) equals 1.06, 0.532, and 0.355 micrometers . Two-photon absorption in diamond at 0.355 micrometers is also measured. The results are in good agreement with a two-parabolic band theory developed for semiconductors.

Fluoride HR coatings consisting of middle and low refractive index fluorides often show a higher damage threshold than oxide coatings in the UV range. However, the light scattering level of fluoride HR coatings tend to be higher than oxide ones. In this study, the scattering and surface roughness of fluoride HR coatings for KrF laser were investigated, and also compared with those of oxide HR coatings using Al₂O₃ as high index materials.

Investigations of the pulse length dependence of laser induced damage thresholds in the ultraviolet range suffer from the limited possibilities to increase the excimer laser pulse duration. The presented paper describes the application of an external pulse extension unit for damage threshold measurements with an effective pulse length from 25 to 200 ns at 248 nm. High accuracy results were obtained due to the fact that the same laser spot size and spatial profile as well as the same damage detection scheme could be used for all pulse durations. Especially the results for bare and single layer coated substrates strongly support a t^0.5- law of the damage threshold, which is the indication for a thermal breakdown mechanism. First results on double pulse experiments with different separation and amplitude ratios of the pulses reveal interesting details of the interaction process between laser radiation and optical surfaces.

NASA has plans to monitor the health of Planet Earth using remote sensing techniques from polar orbiting satellites. By using active remote sensors, such as lasers, global, height- resolved, data can be obtained on essential earth variables. Performance specifications and first order designs for current and proposed remote sensing instruments have been developed. Optical damage impacts the performance of these sensors in the attainment of efficiency and reliability.

Experiments in three different RF-based (13.56 MHz) plasma reactors located in the U.S. and two different microwave-based (2.45 GHz) deposition systems located in Europe were conducted and the laser damage threshold results at 1064 nm as well as some uniformity and reflectivity data are given. The RF-based plasma reactors fail to produce multilayers with visible optical interference effects. Microwave-based plasma systems were successful in preparing rugate-type dielectric reflectors with index modulation ((Delta) n) up to 0.034 and reflectivities up to 95%. The laser damage threshold was strongly dependent on reactor geometry. A vertical tube furnace arrangement gave the highest damage thresholds (> 40 J cm^-2) when cylindrical or rectangular cross-section tubes were used. Planar substrate geometries were a factor of 3 to 10 lower. RF-based coating systems were also low (5 to 12 J cm^-2) regardless of the deposition chemistry used or the reactor geometry tried.