An interferometric technique is described that uses a Fabry-Perot type configuration to measure the coefficient of thermal expansion (CTE) and the thermo-optic coefficient (dn/dT) of transparent materials concurrently. A number of measurements of the CFE and dn/dT are made for both oxide and fluoride based glasses. These measurements show that the technique is accurate, reproducible, and is very effective in determining the temperature dependence of both the thermo-optic and thermal expansion coefficients.

Linearly polarized light remains linearly polarized after reflection from a transparent material at oblique incidence. The reflected polarization angle is determined from the extinction position of the analyzer. If the incident polarization angle is 45 deg, the reflected polarization angle gives the ratio of the reflected p-wave to s-wave. This value can be used to determine the index of refraction from Fresnel equations. With our instrument, the uncertainty in the deduced refractive index is +/- 0.0004. This method is fast, convenient, and versatile enough to provide accurate results on small laboratory samples. In addition to measuring the refractive index, the method is sufficiently accurate to characterize the homogeneity of transparent materials.

Absorption and pulsed laser induced damage of antireflective coatings on ZnSe and high reflecting metal mirrors were studied. As deposition materials for the AR-coatings germanium zinc sulfide zinc selenide cadmium selenide and different fluorides were selected. For protecting and enhancing of metal mirrors zinc sulfide and zinc selenide were used in conjunction with ThF4 as low index material. The films were deposited by thermal evaporation. Absorption measurements were performed with the aid of a laser calorimeter and a cw C2aser Single-shot and multiple-shot (prf 3 Hz) damage thresholds were measured by a computer controlled test facility using 100 ns pulses from a TEA C02-laser. The gaussian beam was focussed down to a diameter of 1. 4 mm on the sample surface. A scatter measurement system and an on-line video microscope were applied for damage detection. Damage morphology was investigated by Nomarski- and scanning electron microscopy. Key words: C02-laser laser induced damage single-shot multiple-shot laser calorimetry photothennal deflection spectroscopy ar-coating metal mirror video image processing. 1.

In this paper wavelength dependent damage phenomena in SiO2 overcoats on TiO2/SiO2 and ZrO2/SiO2 HR coatings are systematically investigated with the help of various techniques, including a dynamical study of the damage process by time-resolved photothermal deflection technique, microscopic analysis of the absorption sites by CW-modulated photothermal microscopy, and observation of the coating microstructures and damage morphology by TEM and Nomarski optical microscopy. From the data for 355 nm and 1.06 micron radiation and a theoretical calculation of the temperature distribution inside the coating, we conclude that damage occurs first either in the HR coating or in the overcoat depending on the wavelength of the incident radiation.

Pulsed phototherinal deflection technique (pulsed-PDT) is employed to make insitu investigations of laserinduced damage in optical coatings. Preliminary experiniental results show that the technique is capable of providing a lot of new information about not only the damage thresholds but also the damage processes of which of particular interest are the transient damage 1ehavior and the cumulative effect. key words: laser damage optical coating in-situ investigation photothermal deflection pulse duration and wavelength effect thermal property. 1 .

Even unusual care in polishing leads to optics with somewhat damaged working faces. The question is to what degree? the physical nature of this damage depeids on the ductility or plasticity of the constniction material. Glassy materials (glass brittle metals and semiconductors) fracture while ductile materials work harden. Beryffium suffers from both modes. To physically model damage we oversimplify and picture a surface layer with two features a depth and an intensity of the IvjIk property changes. A statistical alternative to intensity is the visible flaw density. We use a variety of analytical mathods but mainly x-ray diffraction metallography and instrumented microindentation and frequently use them in combination. Our problem is to develop a general hasis and means of characterizing subsurface damage. To illustrate the diversity difficulty and our progress with the problem we will report experiences with a variety of examples. The talk will consider damage in ductile regimeN ground classically polished fused silica BK-7 pyrex optics esuperu polished BK-i precision ground silicon nitride and aluminaceramics and precision CBN turned beryllium.

Inorganic polymers are characterized by both in-plane it''and out-of-plane it bonding interactions in contrast to organic materials where iteffects dominate the optical properties. Involvement of the it'' bonding system results in higher electronic transition energies with the first transition being a it'' ic process. Second hyperpolarizabiities are affected by both it bonding systems and are reduced by higher transition energies and lower transition probabilities. In this paper we present electronic structure calculations of the static hyperpolarizabilities of a series of model compounds for inorganic polymers to illustrate the relationship between their electronic structure and nonlinear optical properties. 1. 0

Laser-induced morphological damage initiated by single 10 ns, 1.06 micron Nd:YAG laser pulses was observed on three types of silicon PIN photodiodes. When the samples were irradiated at fluences above the melting threshold, the damage morphologies were observed to vary according to the type of coating and the fluence. Nomarski microscopy at 200X was used to analyze the laser-induced topography at the damage sites. The topography was characterized generally by ripples, ridges and boiling marks in order of increasing fluence. However, all test fluence levels remained below the threshold for electrical degradation and no increase in leakage current was detected. The detector responsivity at the damage sites was mapped by means of a probe laser. The sample was translated within its plane in order to obtain a series of raster scans across each damage site. A comparison of the topographical data and responsitivity data suggests that local variations in responsivity were correlated with laser-induced surface roughness and with the removal of the thin-films.

A simple theoretical expression is developed which allows correlation between laserinduced breakdown threshold in solids liquids and gases. It is shown that the breakdown threshold for single-layer thin films surfaces of dielectrics the bulk of solid dielectrics linear liquids and gases all follow a linear fit to the expression N 2/3/(n2 -1) where N is the atomic number density and n is the refractive index. The threshold for sol-gels is compared with conventional thin film thresholds and is shown to agree reasonably well with the relative threshold predicted by the simple expression. The gas breakdown threshold vs pressure is compared with the predicted dependence. Finally maximum values for solid liquid and gas thresholds are plotted against the expression and are seen to fall on a single straight line across three decades of physical constants. 1.

Different film deposition techniques have been found to generate distinct grain morphologies and defect distributions, which can result in a wide distribution of local field intensities. We have developed a lattice element model of an inhomogeneous medium, which we use to selfconsistently determine the local internal electric field and polarization. We use this method to show the sensitivity of the polarization to defect shape. The results of this method predict the local field to have a large range of values within the film and have been used to identify regions where the local field is a maximum.

This paper describes the development and implementation of MIS-36477 a Missile Specification for laser damage certification of designator optical components. This MIS is the first official government specification issued for laser damage testing of optics for military laser systems. We describe the history behind the development of laser damage testing for military systems and the gradual emergence of standards for this type of test. The necessity of a standard for laser damage testing was clearly indicated by the inability to correlate results from different test facilities1''2. The development ofde facto standards within the industry is discussed along with the rationale behind the generation of this particular specification. The MIS is described in detail and it is clear that considerably more than just a fluence level and laser parameters need to be defined to have a thorough and widely applicable specification.

At excimer wavelengths the photon energy either exceeds or is comparable to bandgap energies of most materials. This severely limits the materials suitable for optical coatings and is also responsible for the rapid decrease in observed damage threshold fluences as laser wavelength drops. Most materials traditionally used in coating fabrication are unsuitable at the shorter excimer wavelengths. For longer wavelength coatings materials are chosen for properties such as index contrast and ease of deposition. We are currently reevaluating the suitability of selected materials for damage resistant optical coatings at the KrF excimer wavelength. The low and medium index fluorides are promising as is aluminium oxide. We report the results of damage experiments on both single layer films and dielectric reflectors fabricated from promising combinations of these materials. 1.

A microwave plasma assisted chemical vapor deposition (MPACVD) process has been used to deposit modulated refractive index profile dielectric films to meet specific reflectance/transmittance coating characteristics. The MPACVD process has been proven to allow the deposition of dielectric thin films of excellent optical quality which have lower scatter and less strain than most conventional coatings. An important feature of the process is that both the deposition rate and the composition of the film can be adjusted under computer control during the deposition process. This degree of control allows the deposition of both graded and event more complicated refractive index profiles. The technique therefore allows the successful realization of highly sophisticated coating designs (e.g., Fourier). The process has been used to fabricate a range of dielectric designs, including narrow band high reflectivity coatings with minimal sidebands, using silicon oxynitride on fused silica substrates. This paper discusses aspects of the MPACVD process, including deposition rate, control, uniformity and deposition quality, the Fourier design program and the characteristics of a range of coating trials. The measurements made have included optical, laser and limited environmental testing.

In a collaborative effort with some french industrial and university laboratories and also in contact with our colleagues from the Lawrence Livermore National Laboratory (LLNL), we have developed at Limeil-Valenton National Laboratory (LVNL) various laser-performant and inexpensive dielectric mirrors made of a resin-replicated substrate and a specific colloidal sol-gel coating. To date, we have obtained 8" to 12" diameter molded optical surfaces with a flatness as good as A./10 peak to valley at 633-nm wavelength with a roughness ranging 10-15 A RMS. We have been able to acheive coatingreflectivity exceeding 99 % using room temperature spin-coating deposition of alternate silica and alumina quarterwave layers, and conserving the original substrate flatness. Such sol-gel coated replicated mirrors averaged 13.5 i/cm2 in a single shot irradiation mode and 15.1 i/cm2 with a laser-annealing procedure at 1064-nm and with 3-ns pulses. The present cost of such a product is about 4-6 times cheaper than a similar component conventionally e-beam coated.

Motivated by interest in optical firing systems for initiating explosives laser-induced damage thresholds have been investigated in step-index multimode fibers having pure fused silica cores. A compact multimode Nd/YAG laser operated at a pulsewidth of 16 ns was used for the experiments. The focusing geometry for introducing the beam into the fiber was chosen to avoid damage along the core/cladding interface as observed in previous studies. Five lots of twenty fibers each were tested with polishing steps varied between successive lots to produce improved fmishes on the fiber end surfaces. Each fiber was subjected to a sequence of progressively increasing energy densities up to a value more than 80 J/cm2. Initial damage was monitored by observing scattered HeNe laser light from the fiber faces using magnified video cameras. The majority of the fibers damaged initially at the rear fiber face once a " laser conditioning" process at the front fiber face was completed. In this process a visible plasma was generated at the front face for one or more laser shots. Rather than produce progressive damage at the front surface this process apparently improved the surface finish in nearly all cases resulting in improved resistance to damage. Other modes of damage along the fiber length were observed either at locations of handling stresses or at the location of highest static tensile stress corresponding to the fiber's minimum bend radius.

Graded index coatings of silicon oxynitride have been deposited using ion assisted deposition (IAD). During the IAD process the coated surface is bombarded with low energy reactive ions such that the chemical properties of the coating can be changed in a controlled fashion. We have shown that the chemical composition of SiN(x)O(y) can be varied continuously from silicon nitride to silicon oxide. As a result, the index of refraction of the coatings can be varied between 2.1 and 1.45 by varying the gas mixture in the coating system. The process has been used to deposit graded index antireflection coatings and rugate filters. Several diagnostic techniques were employed to examine the optical, mechanical, and chemical properties of the coatings.

Ion beam figuring has been demonstrated to be a deterministic efficient flexible technique for removing material from optical surfaces. Recent interest in using this process to produce high quality optical components has driven the need to fully characterize the resulting surfaces. We have performed a polishing parameter matrix investigation to optimize fused silica (Corning 7957) surfaces for subsequent ion milling. Samples were characterized for surface scatter surface absorption surface roughness subsurface damage and laser damage as a function of mill depth. Small defects (pits) were evident on surfaces after milling a few microns with pit density dependent to some degree upon the surface preparation technique. The defects were often in lines apparently following a surface or subsurface scratch in the materiaL Surface scatter decreased significantly (up to lOX) and laser damage threshold increased in some cases by 400. Laser damage was not correlated with defects in the material. Key words: ion beam milling laser damage scatter fused silica absorption. 1.

Thermal stress induced fracture limits the power output of high average power glass laser slabs1. The fracture of laser glass is initiated by the onset of unstable propagation of subsurface microcracks in which local tensile stress exceeds the critical level. The production of subsurface flaws is inevitable for a standard high precision fabrication process. Applying a compressive stress surface layer can be used to counterbalance the thermal tensile stress imposed upon the surface during operation. We have developed an ionexchange process submerging the phosphate glass into a mixed alkali nitrate salt melt. An ionic size differential induces a compressive ayer of tens of microns. The process avoids water corrosion and preserves the finished optical surface quality during ion-exchange. Furthermore we have optimized the process taking into account network relaxation as well as ion-exchanged concentration profile. To characterize the surface compressive stress we have employed a biaxial flexure bending test. We have found that a finished HAP-4 laser glass with a tensile breaking strength of 200MPa can be strengthened to twice its original strength namely 400MPa. The process has been scaled up to production in which large laser glass slabs or most other phosphate glass can be treated routinely. We have strengthened polished HAP-4 slabs( 42cm X 14cm X 1cm) to up to twice their original thermal shock resistance without deteriorating their optical surface quality. 1.

Single crystal Raman spectra of the anatase phase of titania have been acquired as a simultaneous function of temperature and pressure in a heated diamond anvil cell. Measured data are used to construct frequency contours in P space for particular allowed vibrational modes. The magnitude of stress in thin films deposited on various substrates and its temperature evolution have been evaluated from correlations between the P frequency maps and measured temperature-dependent mode frequencies exhibited by the films. A simple anharmonic oscillator model has been used to relate the temperature-dependence of the square of the mode frequency to the first order aiiharmonic contribution to the potential energy from which the thermal expansion coefficient (a) of polycrystalline films may be estimated. The stress dependence of a for bulk titania has been determined from this data and is compared with the predicted response of several stressed titania films. 1.

The laser-induced damage thresholds of diamond films deposited on silicon substrates were investigated. Experiments were performed with both CW and pulsed lasers operating at a wavelength of 10. 6 jim. Analytical studies were conducted to determine the thermal and thermomechanical states as well as the electric field strengths of the films and the substrates at the damage thresholds in an effort to identify the damage mechanism. The evidence suggests that the diamond films are damaged by stress-induced cracking which occurs when the underlying silicon approaches melt. 1 .

Free electron heating in SiO2 in the presence of high intensity laser excitation at 1 micron wavelength was investigated. The formalism is based on a Monte Carlo integration of the Boltzmann transport equation which has successfully explained dc transport data. The simulations were based on experimentally determined electron-hole pair excitation rates and energy dependent electron-photon scattering rates. Average power loss of conducting electrons to the lattice via phonon excitations increases rapidly with incident laser power. This allows for strong free carrier induced lattice heating and melting well below the onset of carrier multiplication by impact ionization. Chance momentum reversal in phase with the oscillating electric field results in a long high energy tail in the electron energy distribution. For high laser intensities this high energy tail will lead to strong carrier multiplication. High temperatures may suppress carrier multiplication. Free carrier heating, single shot laser breakdown, and avalanche induced prebreakdown lattice heating are discussed. A new mechanism for single shot prebreakdown laser heating which combines impact ionization and free electron heating is proposed.

In combination with known thermomechanical fatigue data for Mo we have applied the Transient Photothermal Deflection (TPD) technique to develop a model f the N-on-i damage ofMo mirrors to predict their multipulse lifetimes. In laser-damage experiments to verify the model mechanically polished Mo mirrors were irradiated with 10 ns Nd:YAG laser pulses at 1064 nm at a 10 Hz rep rate. In the TPD experiments the approximately 600 jtm diameter Nd:YAG laser spot was probed off axis by a smaller HeNe laser beam whose deflection was detected by a fast bicell photodetector and amplifier. Digitized photodetector waveforms indicated that the surface angular deflection could be converted into surfae disp1zement. In addition thermal modelling of the vertical heat distribution enabled the peak surface-deflection signal to be converted into peak surfwe temperawre. The thermomechanical model was verified by both the experimental and model results. Conventional mechanical fatigue data for Mo were used to derive a predictive equation for the laser-accumulation lifetime of Mo mirrors. Experiments were performed with one to 1O'' pulses per site yielding laser-damage thresholds and accumulation curves. The accumulation behavior predicted from measurennts of mechanical fatigue was in excellent agreement with the measured behavior. It is possible that data on high-cycle ( mechanical fatigue can be used to predict the performance of optical surfaces at equally large values of N.

During multishot ablation with 248 nm excimer laser pulses for each single laser shot the shock wave emerging from the ablated material was monitored by the acoustic mirage effect. The shockwave parameters turned out to depend sensitively on the nature of the ablated material. In particular during ablation of a polymeric film/Si02/Si multilayer system distinct changes in the deflection signal were found when the ablation was driven through the interface between layers. Inspection by optical microscopy and depth profil ing was used as cross check. 1.

Photoacoustic spectrososcopy is used to characterize the surface absorption of polished fused silica substrates and thin films deposited on fused silica substrates. The extreme sensitivity of this technique allows measurement of surface absorptions of a few tenths of a part per million. Characterization of samples with surfaces finished using a variety of methods is reported. Key words: Photoacoustic ion beam absorption thin films. 1.

Fluoride coatings produced by thermal evaporation suffer from high scatter losses ageing and cracking due to high tensile stress. These problems impose severe limitations to the production of low loss multilayer coatings for the VUV range. A key position for improved performance is the microstructure of the layers. The aim of our investigations is to improve the microstructure of A1F3- and LaF3-'' films by ionbeamsputtering. Scatter measurements of single layers revealed lower values for lBS than for boat evaporation. Unfortunately sputtered fluoride films nave high absorption losses caused by decomposition of the coating material. By sputtering in reactive atmospheres and annealing we were able to reduce the absorption losses significantly. Antireflective as well as high reflective coatings were produced. Reflection and transmission values were obtained with a VUV-spectrophotometer. Damage tests at the 193 mu ArF laser wavelength were performed at the Laser-Laboratorium Gttingen. Key words: ion-beamsputtering fluoride films UVcoatings VUV-coatings color-center laser damage A]. F3 MgF2 LaF3. 1.

The refractometry program at the National Institute of Standards and Technology (NIST) was abandoned ten years ago because of a lack of support. A serious attempt is being made to reestablish this program and to upgrade the existing refractometers and the measurement capabilities to meet the present needs of the optical community. The purpose of this paper is to present the current status of the refractive-index-measurement capabilities at NIST the immediate plans for the future and the results of some recent measurements. Refractive index data will be given from 0. 5461 to 1. 083 pm for three samples of zinc selenide from different manufacturers. 1 .

The nature of subsurface damage incurred during polishing of fused silica varies greatly with the polishing process. Although subsurface damage is known to lower laser damage thresholds for fused silica surfaces1''2 the effect on thresholds of coated dielectric surfaces has not been previously documented. This study describes features of subsurface damage due to polishing by ten commercial fabricators and compares laser damage thresholds of a high reflection coating on these surfaces.

We document the concept and the design method for dome lenses that have a compressive pressure applied to the periphery. Dome (meniscus) lenses can eliminate both birefringence and tensile stresses if used in evacuated spatial filters where a pressure difference is present across the lenses. Calculations show that spatial filter birefringence is an insignificant 3 nm within the portion of a 740-mm Nova spatial filter dome lens illuminated by the laser beam. Membrane stresses are everywhere compressive, varying from -1.2 to -2.4 MPa, and are nearly uniform over the illuminated portion of the lens. Eliminating sources of birefringence on the Nova laser could increase the energy on target with Type 11/Type II frequency conversion from '-50% to the '70% level attainable with Type I/Type II frequency conversion. Eliminating tensile stresses may reduce the damage and catastrophic failure that has occurred in some Nova spatial filter input lenses. The major disadvantage of using dome lenses on Nova is the cost of new lenses which could range up to $50,000 per lens for the material and $20,000 per lens for finishing.

We present recent measurements of excited-atom and ion emission from KCl surfaces illuminated by vacuum-ultraviolet synchrotron radiation (h-nu = 8-28 eV) and ultraviolet laser light (h-nu = 4 eV). At low intensities characteristic of the synchrotron experiments, excited atoms are desorbed by simple valence-band excitation process involving the metallization of the KCl surface. At the higher intensities typical of laser desorption and ablation, we observe a strong decrease in K emission as a function of the number of laser shots, but an essentially constant yield of Cl. K(+) and Cl(-) emission at high intensities show similar behavior. The energetics of these desorption phenomena can be treated in a bond-orbital model which shows that creation of a single valence hole is sufficient to excite an ion to an anti-bonding state.

Theoretical investigations of stresses associated with thin-film coatings usually focus on evaluating the normal stresses in the film layers and the thick substrate. They do not address interfacial stresses which are responsible for such effects as film delamination and substrate bowing. Thermally-induced shearing stresses in CVD-diamond coated windows made of optically transmitting materials are known to be of concern because of the lattice-expansion mismatch and the enormous elastic modulus of diamond. The purpose of this paper is to reformulate Suhir''s analytical method for assessing interfacial stresses in multilayered structures [J. App!. Mech. 55. 14? (1988)] and to apply relevant formulas to the problem of mitigating the impact of thermoelastic stresses in diamondcoated ZnS windows. Specifically it is demonstrated that the critical shear and the central deflection are both governed by the first-interface longitudinal force per unit length F0 z(hj1) where h and ai refer to the film thickness and the normal stress in each layer which suggests that the presence of a " buffer" in tension may result in major improvements. Issues relating to the biaxial elastic modulus of diamond are discussed in Appendix where it is shown that state-of-the-art diamond films behave as expected based on single-crystal elastic constants. 1 .

Laser-induced surface breakdown in fused silica has been studied as a function of pulse width in the nanosecond regime. The third harmonic of a Nd:YAG laser was used to produce 7.5 ns duration pulses (FWHM) at 355 nm. A novel system using optical delay lines was used to extend these pulses to variable widths between 7.5 and 400 ns. At each pulse width, the beam was focused onto the surface of a commercially available fused silica flat and the breakdown fluence was determined. The breakdown fluence threshold was found to scale as the pulse width to the 0.8 power, significantly higher than the 0.5 power reported elsewhere for similar cases. Experiments were also performed on 200 jim core fused silica optical fibers and the results Obtained were consistent with a 0.8 power scaling law. This strong scaling law led to a dramatic increase in the amount of 355 nm light that could be transmitted through 200 p.m core fibers - from 1-2 mJ at 7.5 ns up to over 30 mJ at 400 ns. An experiment was also performed to probe the recovery time of fused silica (the time separation between pulses such that their effects are independent). This time was determined to be less than 25 ns.

Low energy electron diffraction (LEED) Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS) were used to characterize Al2O(1 120) surfaces before and after exposure to Nd:YAG 1064 nm 1. 17 eV photons. The clean and well-annealed (T5 surface exhibited a (12x4) LEED pattern. Irradiation of the surface with laser fluences of approximately 3. 5J/cm2 30 of the visible damage threshold degraded the surface to the point that the (12x4) diffraction beams were no longer detectable. Characterization of the laser-irradiated surface by AES failed to show any departure from the (12x4) surface unlike electron beam damage of alumina where the desorption of oxygen leaves metallic aluminum. However subsequent annealing to 1700 K was sufficient to restore the original pattern. ELS of the (12x4) surface showed the presence of surface electronic states in the bulk band gap with energy losses between 3 and 5eV. Time-of-flight (TOF) spectra of laser-desorbed species obtained with a quadrupole mass spectrometer revealed that at fluences less than those leading to catastrophic surface damage the desorbing flux was predominantly aluminum (atoms and ions). The energy of the desorbing aluminum was approximately 8 eV the energy required to promote an electron from the valence band maximum to an exciton localized on the aluminum atom. At fluences which produce visible surface damage oxygen molecules are observed in addition to aluminum in the desorbing flux. 1. 0

Laser induced damage to InAs the material or interest br use as a nonlinear medium or the dinamic holography phase conjugation etc. at 1O. 6pni was studied. Damage thresholds or InAs surraces both bare and antireflectin coated have been measured Lor single pulses or 2OILs duration and beam spot diameter O. 5cm. The dependence or thresholds on the surrace preparation procedure was round. The perrormed investiga tions oI damaged sites with help or optical and scanning electron mic roscopes and Xray microprobe analiser made evident that the surrace heating leaded to the damage. The role o surrace absorption and rerlectivity are exposed. 1 .

Surface and bulk laser damage probability distribution functions are derived for a Gaussian beam and a power-law defect ensemble. Closed-form solutions are derived and plotted for six and three values of the defect ensemble parameter p respectively in the surface and bulk damage distribution functions. The derived degenerate (p 1) and non-degenerate (p 0) bulk damage equations are least-squares fitted to measured laser damage statistics in PMMA. The results show that the power law defect ensemble is a reasonable description of the defects in the damage-tested PMMA and that the ensemble is more degenerate in character than uniform. I.

Laserinduced damage of optical materials due to the presence of radiation at two different frequencies has received little attention in the literature even though this is not an uncommon occurrence in the laboratory. Recent studies on the inultiphoton ionization of single atoms have shown that the ionization threshold depends strongly on the frequencies of the incident fields and their polarizations. In this paper we explore the extent to which these results can be applied to laserinduced damage. The results for a single atom are presented for the case where one field has a comparatively high frequency and low intensity. This field is responsible for the ionization. The other field has a higher intensity and much lower frequency such that it does not ionize by itself. However it substantially modifies the ionization threshold due to the first field. Both fields are circularly polarized. The behavior of the ionization rate near the thresholds is found to depend strongly on whether the fields have the same or opposite circular polarizations. We also speculate on how these results will influence laser induced damage thresholds.

In this paper we report the results of comparative measurements of laser-induced damage (LID) in optical material from the USA and the USSR. Bulk LID thresholds were measured in very pure alkali halide crystals at 1064 nm and 532 nm and surface LID thresholds in selected nonlinear optical crystals were measured at 10.6 microns. The results show that the LID thresholds of the USSR and the USA samples are approximately the same except for KC1 where the USA samples have a factor of 2 higher LID threshold electric field then the Soviet sample.

Two photon absorption (TPA) coefficients in HgCdTe have been calculated for CO2 laser at 40 77 150 and 300 K using Basov formula both in the parabolic and nonparabolic band structure approximations. Temperature and composition dependent energy gaps have been included in these calculations. Reasonable agreement is obtamed with available experimental data. 1.

Research on Inertial Confinement Fusion (ICF) has progressed rapidly in the past several years. As a consequence LLNL is developing plans to upgrade the current 120 kJ solid state (Nd3-phosphate glass) Nova laser to a 1 . 5 to 2 megajoule system with the goal of achieving fusion ignition. The design of the planned Nova Upgrade is briefly discussed. Because of recent improvements in the damage resistance of optical materials it is now technically and economically feasible to build a megajoule-class solid state laser. Specifically the damage threshold of Nd3- doped phosphate laser glass muliilayer dielectric coatings and non-linear optical crystals (e. g. KDP) have been dramatically improved. These materials now meet the fluence requirements for a 1. 5-2 MJ Nd3-glass laser operating at 1054 and 351 nm and at a pulse length of 3 ns. The recent improvements in damage thresholds are reviewed threshold data at both 1064 and 355 nm and the measured pulse length scaling are presented. 1.

A new commercially available perfluorinated organic polymer has been used to prepare a combined quarterwave AR and protective coating for KDP and other optical materials. Coatings are applied from solution at room temperature by spin or dip they are fully dense and have a refractive index of 1. 29. The laser damage threshold at 1064 nm and 355 nm is the highest that we have ever measured for an AR coating material. 1.

Multilayer Hf02/Si02 high reflectors (HR) and polarizers show a permanent increase in their 1(4-nm damage thresholds followinglaserconditioningatsubthreshold fluences. Threshold increasesof2-3x are typical. In an effort tobetter understand the condifioning effect we have made laser conditioning and electronic property measurements on single layers of these two materials. The laser damage threshold of 1-. un thick e-beam deposited Si02 was increased by laser conditioning for wavelengths ranging from 355 to 1064 nm. The damage threshold of 2 srnglelayers was not influenced by sub4hreshold illumination. As-deposited thin films of a-Si02 are known to contain paramagnetic electronic defects. We have used electron paramagnetic resonance (EPR) to study the concentrations and types of defects present in single layer and multilayer films of Hf02 5102. E'' and oxygen hole centers with concentrations on tI order of 107/cm3 have been measured in the Si02 layers. A previously unreported defect has been observed for Hf02. The concentrationofdefects was studied bothbeforeand afterlaserconditioningand damagewith 1064-nm photons. These electronic structure measurements are discussed in relation to an electronic defect model for laser conditioning of dielectric multilayers. 1.

We have conducted an extensive series of laser damage measurements on highly reflective (HR) dielectric coatings which have yielded 1064-nm thresholds as high as 40 J/cm for 8- to 10-ns pulses at pulse-repetition frequencies (PRF) of 10 Hz. Moreover by laser conditioning these coatings with subthreshold pulses the thresholds of some coatings were raised to levels exceeding 70 J/cm. These are the highest threshold dielectric HR coatings that we have tested in this regime. The coatings were originally developed to produce HR-overcoated metal mirrors for free-electronlaser (EEL) app1icationsat high PRF. Our tests included coatings deposited on both dielectric substrates and molybdenum (Mo) substrates. In each category we also examined coatings with a pre-coat of Mo between the substrate and the HR stack. The improved dielectric HR stacks effectively shielded the Mo from the laser irradiation so that the thresholds of virtually all Mo samples exceeded levels of the best dielectric-enhanced and dielectric-HR-coated metal mirrors we have tested to date. In addition to the low PRF measurements we also conducted 1064-nm damage tests at 6-kHz PRF using 65-ns pulses from the Kilroy damage test facility. The coatings survived thermal loading of fluences ranging from 2 to 10 J/cm2 with respective small spot sizes on the order of 1 . 2 to 0. 3 mm (1 /e? diameter). 1.

The propagation of intense 3rd harmonic light (0351 μm) through large optical components of the Nova laser results in fracture damage of the center of the component. This damage is caused by an intense acoustical wave brought to focus in the center by reflecting off the circular edge of the optic. The source of this wave is light generated by transverse stimulated Brillouin scattering (SBS). By taking into account the transient gain characteristics of the SBS, we can correctly predict the pulse energy that would cause damage for any time variation in intensity in the pump beam, and predict the relative intensity of the Brillouin light. Our model is based on the transient behavior of a first order linear system.

It has been recently shown that the optical damage thresholds of multilayer coatings can be increased by up to a factor of two by conditioning the coating at laser fluences below the optical damage threshold. We have investigated the laser conditioning mechanism of Hf02-Si02 coatings by observing the laser-induced emission of neutral constituents of the coatings during laser conditioning. We observe the ejection of Hf, Si and oxygen (0 or 02, or both) neutrals starting at fluences of about 10 J/cm2, considerably below our measured unconditioned damage threshold of about 16-18 J/cm2. We observed a threshold pulse fluence for laser conditioning, which is also about 10 J/cm2. This identical threshold for neutral emission and laser conditioning suggests that the neutral emission is caused by the laser conditioning process. The emitted neutrals have extremely high kinetic energies, on the order of several tens of eV. On the basis of this information, we propose that the laser conditioning process is caused by microscopic damage that involves cracking of the coating layers, resulting in the ejection of the high-energy particles via a "fractoemission" process. This microscopic damage apparently raises the damage threshold by either reducing the absorption of coating defects or by relieving the thermal stresses that cause cracking, particle emission, and possible plasma formation by laser ionization of the neutrals.

We recently performed an extensive series of measurements of the bound electronic nonlinear refractive index n2 of a variety of solids at several wavelengths. We found that as the photon energy approached Eg (the band-gap energy) that n2 changed from positive to negative. This observed wavelength dispersion of n2 can be well explained for wavelengths well below the fundamental absorption edge using a Kramers-Kronig transformation on the two-photon absorption coefficient fi which we had previously studied. While this theory fit the data well for O. IEghW 8Eg (Eg iS the band-gap energy) there was a significant deviation toward larger negative values of n2 near the fundamental absorption edge. We speculated that the AC Stark effect could account for this deviation.

This paper reports the effects of poflshing material on the laser damage threshold of optical coatings. Two identical sets of BK7 glass winckws were polished to a laser quality finish. One set was polishi with cerium dioxide ( Ce02) and the second set was polished with a soft zirconia (Zr02). Samples of each type were coated with electron beam evaporated dielectric coatings. After coating the samples were then tested to determine their relative cma threshold. The results for each polish type were then compared. Implications for the use of zirconia as an alternative polishing material for high power laser optics is presented. 1.

xUv (10 nm -100 nm) free-electron lasers (EELs) are potentially important light sources for advanced lithography and materials applications. The average power of an XUV EEL oscillator may be urnited by thermal loading of the resonator mirrors. We analyze the requirements for the thennal performance of the mirrors of a metal multifacet-mirror ring resonator for use at 12 nm. We use analytical methods and numerical approaches which include simulations with the 3-D EEL code FELEX. Thermal distortion of mirror surfaces leads to optical wavefront aberrations which reduce the focusability of the light beam in the gain medium (wiggler/electron beam) and limit the laser performance.

High-power glass laser systems for laser fusion use many BK-i glasses and the laser-induced damage of this glass is a serious problem. Since the BK-i glass is generally melted in platinum (Pt) crucibles the laser damage due to metallic Pt particles occurs inside the glass. The Pt particles cause fractures when irradiated at a fluence higher than 3 J/cm with 1064 nm 15 ns laser pulse. The size of the fracture increases with irradiation fluence and the damaged glass has to be replaced. In special cases BK-i glass is melted in ceramic crucibles but there is a problem of cristobalite production inside the BK-i glass. The laser damage threshold of cristobalite is about 4. 5-5. 5 JIcm with 1064 nni 15 ns laser pulse. We have found average damaged particles in BK-i glasses melted in Pt and ceramic crucibles to be 200-400 and 10 particles per liter respectively. At present laser damage in BK-i glass is being improved by changing a composition ratio of melting materials.

Atomic force microscopy (AFM) is a powerful tool for studying the surface morphology of dielectric thin film coatings. Results of AFM scanning of antireflection and high reflection coatings are presented. These results demonstrate the effectivenss of AFM in providing high resolution monitoring of processing defects, laser damage, and laser conditioning and aging in e-beam evaporation deposited films.

Lasers are dynamic sources of energy which are ever changing. The laser is prepared for the application through a series of optics varying in quality and complexity. Fluctuations in the laser and the imperfections in the optical train will affect to some degree of significance the performance of the beam at the work site. Neasuring these variables and their effects on the laser performance has been very inconsistent. Many measurement techniques are based on the perception of the person making the measurement. Inconsistencies in perception affect the results and performance quality of the system. A method for beam diagnostics has been developed that removes the human perception and offers consistency in measurements of laser beams. The Laser Beam Analyzer (LBA-10) manufactured by Spiricon is a portable self contained beam diagnostic instrument that provides numerical and visual analysis of laser characteristics. Data is taken using CCD and vidicon cameras each chosen for the wavelength of interest. The signal is then digitized for analytical analysis. The system offers algorithms for numerical comparison of beam quality and performance. This userfriendly system provides real time measurements calculations on the fly and the ability to manipulate and compare data. Accurate analysis of uniformity distribution shape and many other characteristics can be monitored consistently and with a high degree of confidence. Analysis of the laser source is simplistic. Information pertaining to the laser beam is accurate comprehensive consistent for both CW and pulsed laser sources. Variables within the system can be understood and monitored for significant changes enabling the system to operate at optimal parameters.

The effect of thermal treatment and nuclear irradiation on the far infrared spectrum of polycrystal CsI was studied using pure and cerium doped crystals. Samples were exposed to neutrons and gamma radiation from a neutron activation tube. Thermal treatment was conducted in a 5.5 kilowatt furnace. Ionizing radiation caused significant infrared transmission losses at doses less than or equal to 1 MRad and the threshold wavelength of transmittance shifted to longer wavelengths. Transmission losses in cerium doped crystals were less than in pure crystals. Heat treatment also caused a significant reduction in transmittance. These results indicate that high temperature annealing will not decrease radiation damage.

Multilayer dielectric optical coatings produced by high temperature plasma-assisted chemical vapor deposition (PCVD) have been previously shown to have very high surface and bulk damage thresholds (above 40 J/cm). Because these experimental coatings are deposited on tubular substrates conventional wavelength scanning cannot accurately measure the coating peak reflectance and bandwidth. Measurement of the variation of transmittance with incidence angle at fixed wavelength permits analysis of the coating spectral response. The results indicate that the PCVD coatings behave as nearly " ideal" rugate ifiters. Their optical performance agrees weliwith that predicted fora rugateby Southwell''s coupled-wave theory and by the characteristic-matrix model. These 1000-layer-pair filters have maximum reflectances exceeding 99. 9 peak reflectance wavelengths within 0. 5 of the design wavelength and FWHM bandwidths narrower than 10 mm Minor perturbations to the ideal rugatesinusoidal profile do not appredably affect the coating optical perfonnance. Comparison with calculations suggest that the only significant deviation of th ePCVD structure from that of an ideal rugate is a small (0. 7) drift in the index period. Excellent optical performance and high damage resistance makes PCVD rugate coatings potentially useful for several high power laser applications.

we investigated the laser damage thresholds and optical performances of fluorides multi-layered reflectors, consisting of middle index fluoride and Na3AlF, for high power KrF laser applications. It was found the damage thresholds of those reflectors are significantly improved by laser annealing. Those reflectors with annealing resisted against the fluence of 18-27 J/cm2 (this fluence varies on combinations of various fluorides) for 248 n, 15 nsec, 10Hz pulses.

With increasing laser power the transient behavior of the optical components becomes more and more important giving rise to investigations in this field e. g. [1 Especially the time dependence of the optical properties which influence the laser beam parameters is of interest In this paper the heating of optical components caused by the absorption of a small part of the laser power as well as the resulting deformation of the phase front of tl laser beam have been studied with finite element analysis. For this the spatial temperature profile the mechanical deformation and the stress of the optical component have been calculated as a fuixtion of laser power laser mode proffle and irradiation time. Using the temperature dependence of the index of refraction the spatially resolved optical path difference compared to the unirradiated component has been calculated with a ray tracing algorithm. The results for various optical components (mirmrs windows coated and uncoated) are given and compared to experimental investigations.

This paper presents results of a successful four year effort funded by the Air Force to develop a measurement system for the study of optical-thin-film thermal transport properties. Data are presented on several coated samples. Characteristic features of the data are interpreted in terms of behavior expected on the basis of solutions of the heat diffusion equation. Comparisons are made between observed coating data and model predictions based on handbook data on coating materials in bulk format These comparisons show close agreement for nasurable properties in the special cases studied. The thta are found to be free of unexpected strncture. 1.

Fundamental aspects of multishot laser damage are considered: thermal instability in an optical medium due to pulseto--pulse accumulation o absorbing derects mechanisms o laser induced detect generation. Eeotive methods or suppressing a deeot rormation to increase laser damage resistance are discussed and illustrated by experimental data or various type optical materials. Key words: multishot and singleshot laser damage instability inclusion accumulation eect 5 thermochemical mechanochemicalprocesses thermoelastic stresses leser damage resistance of crystals and polymers laser conditioning. 1 .

For optical materials mirrors and antirerleotion coatings the laser damage resistance resul ts for XeCl Kr ArP eximer laser wavelengths are presented.