bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiple-Shot Laser Damage Thresholds of Ultraviolet Reflectors at 248 and 308 Nanometers
Authors: Foltyn SR , Newnam BE
Published: March 2009

Multiple-shot damage thresholds of dielectric reflectors have been measured at 248 and 308 nm. Standard irradiation conditions were a 10-ns pulsewidth, 0.6-mm spot diameter and 35-Hz pulse repetition frequency. The reflectors, from various sources, were composed of oxide and fluoride films.

Although damage was generally initiated at visible film defects, there was no correlation between damage susceptibility and the appearance of these defects. At levels near threshold, damage was most often observed as an increase in white-light scatter of a site with no growth upon continued irradiation; at higher levels, the damage site grew with successive shots.

Test sites were subjected to at least 103 shots and some sites received as many as 2.5 × 104 shots; however, with only one exception damage was found to occur within the first few shots or not at all.

Reflectors at 248 nm typically had damage thresholds in the 1.0–1.8 J/cm2 range with two samples exhibiting unexpectedly high thresholds of 2.8 and 3.0 J/cm2. In some cases, a subthreshold pre-irradiation treatment resulted in a 20–25% enhancement in damage resistance.
A Novel Technique for Investigating Impurity Initiated Short Pulse Laser Damage in Thin Films
Authors: Wiggins TA , Walker TW , Guenther AH
Published: March 2009

A preliminary study has been conducted to investigate the effects of spot size on the laser induced damage threshold in thin films. Damage was produced by the interference fringes of two beams from a frequency-doubled Nd-YAG laser with a nearly Gaussian spatial profile. Fringes of spacing from 5 to 125μm were used on ZnS, ZrO2 and ThF4/ZnS films. The damage threshold and morphology were observed and are reported.

Microscopic examination indicated that the appearance within a damaged region, which is as narrow as 1μm, is not different from that produced by a single beam whose half-intensity diameter is a hundred times larger. Interesting results concerning the relative importance of scratches and other defects were revealed by Nomarski microscopic observation of the damage morphology.

It has been reported that laser damage to thin films is initiated primarily by small-size dielectric impurities and the dependence of the damage threshold on pulse length, film thickness, wavelength, and material investigated. In the present study it was observed that the damage threshold increased with decreasing film thickness in agreement with previous observations. However, there was no significant increase or decrease in threshold for the two-beam case over that for a single beam when the increased energy density due to interference effects is considered. In concert with previous observations it is concluded from the damage morphology and the absence of a spot size dependence that the source of damage initiation is impurities smaller than 1μm for these 5 ns pulse lengths.
The Use of Ion-Beam Deposited Diamond-Like Carbon for Improved Optical Elements for High Powered Lasers
Authors: Aisenberg S , Stein M
Published: March 2009

The use of thin films of ion-beam deposited diamond-like carbon (DLC) appears to provide a number of properties that could result in improved optical elements (windows, mirrors) for high powered lasers. Many of these properties have already been described.

Of particular importance for high powered lasers is the ultra-smooth nature of the ion-beam deposited DLC plus its transparency, chemical intertness, and barrier properties. Several mechanisms predict and explain why DLC films are observed to be smoother than the substrate, and can result in improved transmission and reflection coefficients. The problem of high power surface breadkown associated with microscopic irregularities in the substrate can be reduced as a result of the ion-beam deposition energy and the smoothing effects of a film of ion-beam diamond-like carbon. The relationship of DLC to high power breakdown at surface defects, as well as plasma breakdown will be discussed to show how the ion-beam diamond-like carbon coating can improve performance.
Characterization of Small Absorptions in Optical Coatings
Authors: Anderson WJ , Hansen WN
Published: March 2009

Probably the main causes of laser damage in thin films used as optical coatings are impurities and non-stoichiometry, especially at interfaces. This paper considers the problems of identifying absorbing species, locating them in the optical structure and measuring the amount of absorbing species present. The ability to have large electromagnetic fields of the desired distribution in a multilayer system is used to optimize multiple reflection spectroscopy for the characterization of thin layers. The absorption coefficient caused by impurities in several Th F4 films are calculated as a function of angle of incidence and polarization. The position of these impurities is predicted by comparing the absorption coefficients measured with several techniques. The method is general and can be applied to give accurate results on systems of multiple phases and varying substrate. While the method is valid for general stratified media, its accuracy depends upon the information content of the particular reflection spectrum.
Stress-Intensity Factors for Radial Cracks in a Partially Autofrettaged Thick-Wall Cylinder
Authors: Pu SL , Hussain MA
Published: March 2009

Using a finite-element method and thermal simulation, stress-intensity factors are determined for a uniform array of equal depth radial cracks emanating from the internal boundary of a pressurized, autofrettaged thick-wall cylinder. The computation of the same by methods of load relief and weight function also are examined. The combination of finite elements and weight functions is found very effective and is used in this paper for multiple-radical cracks in a partially autofrettaged tube. Extensive numerical results are presented for a cylinder having an external diameter twice that of the internal diameter. It shows that the autofrettaged tube with two diametrically opposed cracks remains, in general, the weakest configuration. For more than two cracks, the higher the number of cracks is, the smaller the stress-intensity factor will be.
Geometry Variations During Fatigue Growth of Surface Flaws
Authors: Jolles M , Tortoriello V
Published: March 2009

Recent investigations have indicated that fatigue growth of surface flaws cannot be adequately predicted solely by stress-intensity factor (SIF) analysis. This is due to variations in the stress field triaxiality along the flaw border. A fatigue growth analysis of surface cracks is performed accounting for the variation in stress field triaxiality using the concepts of crack closure. Changes in the geometric parameters describing the flaw size and shape are studied for remote tensile load. For an isotropic material where the Paris equation adequately describes the fatigue crack growth rates, it is found that all subcritical flaws, regardless of initial geometry, extend such that they eventually will have identical size and shape, and that the process is independent of stress range and material properties. Experiments were performed to assess the accuracy of the analysis. Accounting for the constraint variation through the use of the concepts of crack closure yields excellent results.
Influence of Nonuniform Thermal Stresses on Fatigue Crack Growth of Part-Through Cracks in Reactor Piping
Authors: Dedhia DD , Harris DO , Lim EY
Published: March 2009

The growth of part-through part-circumferential semielliptical surface cracks in nuclear primary piping of finite wall thickness is computed. Crack extension is considered to be controlled by root mean square (RMS)-averaged stress-intensity factors associated with each axis of the semielliptical crack, which allows the consideration of varying aspect ratio with crack depth. Crack extension due to nonuniform through-wall stresses is computed by the use of a recently developed influence function for circumferential cracks in pipes. The crack growth due to uniform and nonuniform stresses is presented by the use of crack trajectories, which pictorially display the change in crack geometry with stress history. These trajectories are presented for crack growth due to heat-up and cool-down and reactor trip transients of a nuclear plant. These crack growth trajectories for uniform stress are presented in a manner that makes it possible to compute crack growth for any stress history.
Influence of Specimen Size and Shape on the Singularity-Dominated Zone
Authors: Chona R , Irwin GR , Sanford RJ
Published: March 2009

Improved computational methods have been used to determine, from photoelastic fracture patterns, those stress field parameters (in addition to the stress-intensity factor) that are associated with different fracture test specimen geometries. The variations with crack tip position of these nonsingular terms in modified-compact-tension and rectangular-double-cantilever-beam specimens have been studied. The results have been utilized to formulate criteria that can be used to quantify the concept of the singularity-dominated zone around a crack tip in specimens of finite dimensions.
Total Fatigue Life Calculations in Notched SAE 0030 Cast Steel Under Variable Loading Spectra
Authors: Glinka G , Stephens RI
Published: March 2009

A method of fatigue life calculation based upon low-cycle fatigue and fracture mechanics concepts is presented. Two arbitrary crack initiation criteria were analyzed regarding experimental results with SAE 0030 cast steel and theoretical solutions proposed by other authors. Four different load histories based upon the Society of Automotive Engineers (SAE) transmission history using a keyhole compact specimen with two different notch diameters were analyzed. The results showed that the criterion Δa = 0.25 mm (0.01 in.) for crack initiation is practical. However, the exact value of crack length at initiation is not of the greatest importance in total fatigue life calculations. Similarly, fatigue crack growth retardation was not as important as that found with other spectra discussed in the literature. It was found that proper modeling of fatigue crack growth near notches can improve fatigue life calculations. With the present state of knowledge it was possible to calculate the total life within a factor of two. The calculation methods developed primarily for wrought metals were quite acceptable for this cast steel.
A Superposition Model for Corrosion-Fatigue Crack Propagation in Aluminum Alloys
Authors: Kim YH , Manning SD
Published: March 2009

The mechanism for fatigue crack growth in aluminum alloys under a chemically aggressive environment is discussed, based on the current understanding of hydrogen embrittlement phenomena. This mechanism is discussed quantitatively in terms of the three-term superposition model proposed by Wei et al. A diffusion-controlled model, characterizing the cycle-dependent interaction of fatigue loading and environmental attack, is developed, based on the assumption that crack growth enhancement results from microvoid nucleation due to hydrogen accumulation at inhomogencities ahead of a crack tip. This model is evaluated with limited data on Aluminum 7075-T6. The model developed in this paper accounts for the significant parameters affecting corrosion-fatigue crack growth enhancement. Integration of this model into the superposition scheme is discussed, including the application to predicting crack growth behavior in a corrosive environment for spectrum loading.
Correlation of Smooth and Notched Body Stress Corrosion Crack Initiation
Authors: Brose WR
Published: March 2009

A method is presented for predicting the stress corrosion crack initiation (environment-induced crack initiation) resistance of notched components from the results of smooth (unnotched) specimen tests. The predictive procedure is based on the maximum principal surface strain failure criterion and uses Neuber's rule to estimate notch root strains. It is capable of handling elastic-plastic notch stress-strain behavior. The method is shown to be accurate in predicting the results of tests on two notched geometries, representing very different stress gradients, and a number of notch radii corresponding to a wide range of stress concentration factors. A material-environment system was employed in which hydrogen-induced cracking occurs, specifically a 1194 MPa (173 ksi) yield strength NiCrMoV steel exposed to 345 kPa (50 psig) hydrogen sulfide gas. Some additional data pertaining to the definition of crack initiation and the effect of constraint were generated and discussed in terms of their effect on the accuracy of the prediction procedure. The results of the work indicate that the stress corrosion crack initiation resistance of structures and machine parts which contain stress raisers can be predicted by laboratory tests with specimens of much simpler geometries.
An Elastic-Plastic Fracture Mechanics Prediction of Stress-Corrosion Cracking in a Girth-Welded Pipe
Authors: Abou-Sayed IS , Ahmad J , Brust FW , Kanninen MF
Published: March 2009

Despite the fact that weld-induced residual stresses arise as a result of thermoplastic material deformation, current analyses of crack growth in welds assume that linear elastic fracture mechanics (LEFM) conditions are valid. In particular, the stress-intensity factor K, the LEFM-based measure of the crack driving force, is calculated using the linear superposition techniques that strictly hold for linear clastic material behavior. Another common idealization is that only the normal component of the residual stress field that acts on the crack plane is significant.

To assess the importance of these assumptions, a heuristic elastic-plastic fracture mechanics analysis procedure is developed in this paper and applied to determine the rate of stress-corrosion crack growth in an axially-loaded 102-mm (4-in.)-diameter girth-welded Type 304 stainless steel pipe. It is found that the more rigorous elastic-plastic analysis predicts higher crack growth rates than do the currently used LEFM-based calculations. It is concluded that more rigorous prediction methods may be needed for safety assessments of welded structures than those now in use.
Static and Dynamic J-R Curve Testing of A533B Steel Using the Key Curve Analysis Technique
Authors: Joyce JA
Published: March 2009

Elastic-plastic J-R curves were obtained for compact specimens of A533B-02 plate tested at average load point velocities of 2.5 × 10−4 m/min and 0.25 m/s directly from the load displacement record using a key curve analysis technique. The slow velocity results compare well with J-R curves obtained on the identical specimens using an unloading compliance technique. The key curve method is shown to accurately predict the extent of crack extension. The J-R curves obtained from the high speed loading are shown to be elevated with respect to the slow test results in both the initial JIc and the tearing modulus T by from 50 to 100 percent.
Experimental Verification of Tearing Instability Phenomena for Structural Materials
Authors: Vassilaros MG , Joyce JA , Gudas JP
Published: March 2009

The objective of this investigation was to extend the range of tearing instability validation experiments utilizing the compact specimen to include high-toughness alloys. J-integral (JI) tests of ASTM A106, ASTM A516 GR70, HY-80, and HY-130 steels were performed in a variably compliant screw-driven test machine. Results were analyzed with respect to the JI-R curves of the materials, and various models of Tapplied for the compact specimen. Tearing instability theory was validated for these high-toughness materials. For the cases of highly curved JI-R curves, it was shown that the actual value of Tmaterial at the point of instability should be employed rather than the average Tmaterial value. The Tapplied analysis of Paris and co-workers applied to the compact specimen appears to be nonconservative in predicting the point of instability, whereas the Tapplied analysis of Ernst and co-workers appears to be accurate, but requires precision beyond that displayed in this program. The generalized Paris analysis applied to the compact specimen and evaluated at maximum load was most consistent in predicting instability.
Transparent Polymers as a New Class of Optical Materials for Lasers
Authors: Dyumaev KM , Manenkov AA , Maslyukov AP , Matyushin GA , Nechitailo VS , Prokhorov AM
Published: March 2009

Possibilities of using transparent polymer materials for production of different optical elements (active elements with lasing dyes, bleachable O-switching filters, etc.) for high-power lasers are discussed. Laser damage resistance of various polymers is particularly investigated. The dependences of laser-induced damage threshold on the initial monomer purity, chemical composition and temperature of polymers, frequency and duration of laser pulses, on the focal spot, as well as statistics and morphology of damages, the accumulation effect in damage in a multiple irradiation regime, light nonlinear scattering and subthreshold UV-glow have been investigated to elucidate the mechanisms of laser-induced damage in polymers.

Absorbing defects of different kinds have been established to play an initiating role in the damage process. Purification of the initial monomers was found to result in high values of laser damage thresholds of polymers comparable with those for crystals and glasses, although considerable differences are revealed in the polymer damage characteristics compared to other optical materials.

Analysis of these peculiarities is presented and it is concluded that the molecular characteristics of plasticating agents are decisive in the mechanism of microdamage appearance initiated by absorbing defects, and that visco-elastic properties of polymers have a considerable effect on the growth of damages from micro- to macrodimensions.

A theoretical model of laser damage, as well as practical, effective ways of increasing laser damage resistance of polymer materials are discussed.
Optical Damage, Nonlinear Transmission, and Doubling Efficiency in LiIO3
Authors: Van Stryland EW , Williams WE , Soileau MJ , Smirl AL
Published: March 2009

Laser-induced damage thresholds of single crystal LiIO3 have been studied using pico-second pulses at 1.06 μm and 0.53 μm. These thresholds depend on wavelength, crystal orientation, and on the number of times the sample has been irradiated. In addition, the doubling efficiency at high irradiance levels was observed to be a decreasing function of irradiance beyond a critical value. We present evidence to show that this results from the onset of optical parametric down conversion. In separate nonlinear transmission studies, reversible nonlinear transmission of 1.06 μm light was measured, and in self-diffraction experiments, both reversible and irreversible optically-induced complex index of refraction changes at 0.53 μm were observed.
Studies of Laser-Produced Damage to Transparent Optical Material in the UV Region and in Crossed UV-IR Beams
Authors: Gorshkov BG , Epifanov AS , Manenkov AA , Panov AA
Published: March 2009

The values of the damage thresholds of some alkali halide crystals, sapphire and fused quartz at λ = 0.266 μm are presented.

The role of electron avalanche in the damage of real wide-gap dielectrics in the UV region is discussed.

Some peculiarities of the interaction of intense UV radiation with dielectrics (nonlinear absorption, photoionization of impurities, self-focusing and others) are considered.

Experiments on laser damage induced by two synchronous, crossed laser beams with different frequencies (λ1 = 1.06 μm and λ2 = 0.266 μm) are described. This method makes it possible to establish the influence of “seeding” electrons on the development of laser-produced damage, as well as to determine if the damage mechanisms are similar at different frequencies.

In order to record directly and estimate concentrations of the free carriers excited by UV laser radiation, d.c. photoconductivity experiments have been carried out.
Laser Damage Measurements at 492 nm Using a Flashlamp-Pumped Dye Laser
Authors: Marrs CD , Faith WN , Dancy JH , Porteus JO
Published: March 2009

A triaxial flashlamp-pumped dye laser has been developed into a characterizable source for laser damage studies. The temperature of the dye is maintained constant to ±0.2°C via three heat exchangers and flow regulation of the dye, coolant, and refrigerant. Using LD 490 laser dye and maintaining a temperature of 1.1 ± 0.2°C cooler than the temperature of the coolant, the laser produces 0.18 J, 0.5 μsec pulses at 492 nm. The pulse-to-pulse energy stability is ±3%. The spatial profile of the focused beam was measured in orthogonal directions in the plane of damage of the focus lens. The orthogonal profiles were flat-topped Gaussians with 1/e2 widths of 270 μm. Laser damage measurements have been performed at 492 nm on high reflectance dielectric mirrors, and no correlation was found between the reflectance or total integrated scatter and the axial fluence at which damage was observed. The damage morphology observed on the dielectric mirrors was dominated by failure of the coatings at defects (selective damage sites). In addition, multithreshold laser damage measurements were performed on polished bulk Mo and on the following diamond-turned metals: bulk Al alloy, bulk Cu, electrodeposited Ag, and electrodeposited Au. Comparisons are made between calculated and experimentally measured slip and melt thresholds.
Accumulation and Laser Damage in Optical Glasses
Authors: Maldutis EK , Balickas SK , Kraujalis RK
Published: March 2009

Bulk damage of the optical glasses by single and multiple periodic laser pulses of nanosecond duration has been investigated. In the case of multiple irradiation, damage is shown to occur with the energies considerably lower than the damage threshold by a single pulse.

The dependence of the accumulating damage threshold on the number and frequency of the irradiation pulses, the wavelength of laser radiation and the conditions of its focussing into the volume of the sample has been observed.

The bulk damage of glass K-8 induced by a single pulse of radiation with 1.06 μm and 0.53 μm wavelength is shown to be determined by different mechanisms. By employing absorptive-spectral analysis, the degradation of the medium under laser irradiation with intensity ten times lower than damage threshold was observed. The mechanisms of the accumulation and bulk damage are also discussed.
A Technique for Increasing the Optical Strength of Single-Crystal NaCl and KCl Through Temperature Cycling
Authors: Franck JB , Soileau MJ
Published: March 2009

This paper relates a technique for increasing the optical strength of NaCl and KCl single-crystal samples. The 1.06-μm pulsed laser damage thresholds were increased by factors as large as 4.6 for some bulk NaCl single crystal, namely the laser grade NaCl purchased from Harshaw Chemical Company. The bulk laser damage breakdown threshold (LDBT) of the crystal was measured prior to and after heat treatment using a Nd:YAG laser operating at 1.06 μm in the TEMoo spatial mode with a pulse width of 9 nsec, full width at half maximum. After the LDBT of the untreated sample was measured, it was mounted in a quartz tube and placed in an oven for the heat treatment. The quartz tube was continuously flushed with dry nitrogen gas throughout the heat treatment cycle. The sample was slowly heated to a predetermined annealing temperature: for NaCl, up to 800°C (approximately 1°C below its melting temperature). The sample was maintained at this temperature for a short time and was then removed from the oven to allow rapid cooling. Samples taken to near-melt temperature required repolishing because of surface sublimation, which occurs at elevated temperatures. After repolishing, the bulk LDBT was remeasured and found to be up to 4.6 times greater than the value measured for the untreated crystal. For samples annealed at lower temperatures, bulk and surface LDBT's were studied; for cleaved and polished surfaces, changes in the damage morphology were found.
Thermo-Optic Coefficient (∂n/∂T) of 1.3 μm Laser Window Materials
Authors: Harris RJ , Gangl ME
Published: March 2009

The thermo-optic coefficient (∂n/∂T) of several 1.3 μm laser window materials has been measured in the visible and near infrared spectral regions. These materials include CaF2, Al2O3, MgO, MgF2, YAG, YLF, water clear ZnS, Kigre Q-98 glass, Corning CORTRAN glasses, and LiF. The measurements were made in the temperature range of 25 to 100 degrees C using a laser interferometric technique. The lasers used were HeNe lasers operating at 0.6328 μm, 1.15 μm, and 3.39 μm and a Nd:YAG laser operating at 1.3 μm.
Development of Laser Mirrors of Very High Reflectivity Using the Cavity-Attenuated Phase-Shift (CAPS) Method
Authors: Herbelin JM , McKay JA
Published: March 2009

It has been possible to obtain mirrors of very high reflectivity by following the simple four-step procedure described herein. The key to success is the ability to measure the scattering and other losses of the substrates and dielectric coatings to ensure that the specifications are being met. These measurements are especially critical in the important cleaning process. The cavity-attenuated phase-shift (CAPS) method is ideally suited for performing these important measurements, permitting us to obtain mirrors with reflectivities of R = 0.99975 ± 0.00005.
Damage Thresholds to Metal Mirrors by Short-Pulse CO2 Laser Radiation
Authors: Figueira JF , Thomas SJ , Harrison RF
Published: March 2009

Measurements of damage to metal surfaces induced by intense nanosecond pulses of ir radiation are reported. Single-shot damage thresholds of Cu, stainless steel, molybdenum and aluminum surfaces have been measured for various angles of incidence and the predicted increase in damage thresholds for grazing incidence optical components have been experimentally verified for the first time at 10-μm.

Multiple-shot damage tests have also been performed and practical lifetime curves for Cu mirrors have been established. Scanning electron micrographs of the mirror show the damage mechanism to be the development of a fine scale (1 μm) microstructure on the copper surface.
Directed Energy Production of Novel Metallic Surfaces
Authors: Draper CW , Bernasek SL
Published: March 2009

1. Introduction

2. Overview

3. Surface alloying with lasers

4. Surface analysis

5. Novel metallic surface alloys

6. Discussion

7. Reference
Dimensional Stability of Zerodur and ULE Mirrors Undergoing Thermal Cycling
Authors: Shaffer JJ , Bennett JM , Bennett HE
Published: March 2009

Zerodur and ULE quartz have been shown to have exceptional dimensional stability over extended time periods and to be insensitive to small temperature variations when they are maintained near room temperature. However, it is not clear what their dimensional stability will be after cycling through a more extended range of temperatures such as might occur during bakeout and film deposition when applying dielectric multilayer high-reflectance coatings. Since mirrors are required to maintain tolerances as severe as λ/100 visible equivalent in some laser applications, the safe limits for temperature cycling are of considerable interest, and a preliminary experiment was performed to investigate this question. During the experiment, a convenient method for interferometrically verifying the thermal expansion coefficient of these low-expansion materials was developed and will be described. In one series of experiments, a 6-inch-diameter Zerodur optical flat of the highest quality having a better than λ/40 average peak-to-valley optical figure was oven heated to a peak temperature and then allowed to cool in air. No permanent change was observed up to a temperature of 200°C, but at 250°C the mirror took a permanent set. After the 300°C cycle, this set increased to λ/5. Under similar conditions, the ULE sample was unaffected; it was then water quenched after heating, a much more severe test. Even at oven temperatures of 350°C, the ULE sample developed no permanent set. These experiments suggest that caution should be employed when heating Zerodur mirrors above 150 to 200°C. There is no comparable restriction on ULE mirrors.
Thermo-Elastic Action of the Powerful High Pepetition Rate Laser Radiations on the Solid State Surface
Authors: Apollonov VV , Chyotkin SA , Khomich VY , Prokhorov AM
Published: March 2009

The phenomena of the thermoelastic behavior of materials under powerful, high-repetition-rate laser radiation are discussed. An analysis is based on the Duamel integrals for the thermo-stress characteristics which correspond to CW and high-repetition-rate laser irradiations.

Relationships for the upper-level laser intensities which are connected with admissible elastic surface distortion, plastic flow of the material, fatigue distributions, and melting of the surface layer are calculated.
The Promising Use of Some Heat Carriers in High Intensity Laser Optics
Authors: Apollonov VV , Bystrov PI , Chyotkin SA , Goncharov VG , Khomich VY , Prokhorov AM
Published: March 2009

Heat and thermodeformation characteristics of laser mirrors based on powder and felt porous structures cooled by dielectric and liquid metal heat carriers are theoretically investigated. In the case of the heat carriers considered, it is possible to widely vary the regimes of intense heat mass transfer in porous heat exchangers of laser mirrors, whereas use of liquid metal cooling with porous structure fabricated from low coefficient thermal expansion material opens new possibilities in development of especially accurate reflectors with very high optical damage thresholds.
Laser Damage Thresholds of Thin Film Optical Coatings at 248 nm
Authors: Rainer F , Milam D , Lowdermilk WH
Published: March 2009

We have measured the laser-induced damage thresholds for 248 nm wavelength light of over 100 optical coatings from commercial vendors and research institutions. All samples were irradiated once per damage site with temporally multi-lobed, 20-ns pulses generated by a KrF laser. The survey included high, partial, and dichroic reflectors, anti-reflective coatings, and single layer films. The samples were supplied by ten vendors. The majority of samples tested were high reflectors and antireflective coatings. The highest damage thresholds were 8.5 to 9.4 J/cm2 respectively. Although these represent extremes of what has been tested so far, several vendors have produced coatings of both types with thresholds which consistently exceed 6 J/cm2. Repeated irradiations of some sites were made on a few samples. These yielded no degradation in threshold, but in fact some improvement in damage resistance. These same samples also exhibited no change in threshold after being retested seven months later.
Degradation of Dielectric Films by XeF Excimer Intermediates
Authors: Loudiana M , Schmid A , Dickinson JT
Published: March 2009

Many excimer laser designs require optical component surfaces to be in contact with both optical radiation and reactive components of the gain medium. The resulting environmental degradation of coatings on mirrors or windows is analyzed by isolating intermediates from the XeF gain medium and measuring quantitatively corrosive interaction between these species and the dielectric surfaces. Particular attention will be given to SiO2 films whose reaction with XeF2 and NF2 with and without additional stimulation by ions will be surveyed. Temperature effects of the film substrate as well as the importance of synergistic effects for the stability of the SiO2 coatings will be discussed.
Pulsed D2 - F2 Chain-Laser Damage To Coated Window and Mirror Components
Authors: Amimoto ST , Whittier JS , Whittaker A , Chase A , Hofland R , Bass M
Published: March 2009

Large-spot laser damage thresholds have been measured for bowl-feed-polished CaF2 and sapphire windows (bare and antireflection-coated) and for highly-polished copper mirrors (bare and carbyne-coated) at DF chain-laser wavelengths (3.58–4.78μm). The chain reaction between F2 and D2 was initiated by a magnetically-confined electron beam, producing DF-laser outputs of 10–20 J in pulses of 0.6 – 0.9 μsec (FWHM) duration. Energy extracted from a transmission-coupled unstable resonator was focussed using a CaF2 lens. A soft-aperture technique was employed to suppress effects of Fresnel diffraction so that uniform (top-hat) intensity profiles were obtained along the focussing beam. With this laser system, commercially-available antireflection-coated CaF2 and Al2O3 samples were measured to have damage thresholds in the range 21–27 J/cm2. Significantly larger damage thresholds were found for uncoated, polished samples of Al2O3, but damage resistance of uncoated polished CaF2 was measured to be equal to that of the best antireflection-coated CaF2 samples. A highly polished copper mirror was found to have the highest damage threshold of all the materials tested (58 J/cm2). Carbyne films of diamond-like hardness, a type of carbon coating, were applied to polished copper mirrors and bowl-feed-polished CaF2 surfaces. Such carbyne coatings as were prepared in this work contained numerous carbon-bearing particles that were easily damaged (∼10 J/cm2). However, regions of the irradiated carbyne film that were free of carbon particles withstood high laser fluences (25 J/cm2), suggesting that improvements in carbyne film preparation would yield attractive protective coatings of high damage resistance at DF wavelengths.
Influence of Cleaning Solvents, Sunlight, Humidity, and HF Gas on Pulsed Damage and Optical Characteristics of 3.8-μm Multilayer Coatings
Authors: Porteus JO , Archibald PC , Bethke JW , Dancy JH , Faith WN , Franck JB , Temple PA
Published: March 2009

Samples representing state-of-the-art 3.8-μm laser optical components were sequentially exposed to potentially harmful ingredients of an airborne-operating environment and tested for degradation after each exposure. Two enhanced-reflection-coated mirror designs, Mo(substrate)/Ag/ZnS/(ThF4/ZnS)4 and Si(substrate)/Ag/Si/(Al2O3/Si)4, were studied, along with the antireflection-coated window designs, CaF2 (substrate)/PbF4/ThF4/PbF4 and ZnSe(substrate)/ZnSe/ThF4/ZnSe. Samples were exposed to controlled moderate doses of (1) methanol or acetone applied as cleaning solvents, (2) simulated atmospheric solar radiation at 6-km altitude, (3) 100% humidity ambient, and (4) an ambient containing 10% HF gas. Testing consisted of the following measurements at 3.8 μm: multithreshold pulsed damage profiles, absorptance, scattering, and absolute reflectance/transmission. While cleaning solvents and sunlight generally produce a reduction of thresholds for uniform damage, the influence of humidity and HF is more subtle, resulting in a significant increase in some cases. The windows respond differently than the mirrors, a result that is evidently related to an observed difference in the dominant damage characteristics. Optical characteristics do not correlate well with damage thresholds, suggesting that modification of thermal or other material properties plays a more important role.
Index, Thickness and Birefringence of Thin Films by Guided Waves
Authors: Feldman A , Farabaugh EN
Published: March 2009

A guided wave technique has been used to measure the refractive index and thickness of a thin film. The film, which contained approximately 90 mol % MgO and 10 mol % SiO2 was produced by coevaporation of the two constituents onto a fused silica substrate. Measurements were performed at three visible wavelengths obtained from an argon-ion laser. From the positions of the mode coupling angles, we have calculated the refractive index and thickness of the film. The film was found to have a large birefringence which is attributed to internal stresses. A signature for the birefringence can be observed in the scattered m-line spectrum of the film, in which the order of the TEo and TMo modes is interchanged as compared to an isotropic film. The optical determination of the film thickness is 1.098 ± 0.003 μm as compared to 1.11 μm measured by a mechanical method. The measured values of refractive index agree well with values calculated on the basis of the Drude model.
Limits to the Validity of Evaluating a Quarter-Wave, High-Reflectance Multilayer Through Analysis of Its Secondary Structure
Authors: Bennett HE , Burge DK
Published: March 2009

Information on the perfection of high-reflectance multilayer films can be obtained from a study of the secondary structure surrounding the principal reflectance maximum. The wavelengths at which the adjacent minima occur are a good indication of the correctness of the optical film thicknesses achieved in a quarter-wave multilayer stack. Calculations indicate that the depths of these minima are much more sensitive to absorption in the film layers than is the reflectance at the maximum. If the thicknesses are correct and if volume absorption in the film materials is the main source of decreased reflectance, excellent correlation is found between the measured peak reflectance and that calculated from the depth of the more easily measured secondary reflectance minima. Errors in film thickness or significant interface absorption arising from impurity segregation, surface plasmon excitation, or other sources complicate this simple picture, however, so that the peak reflectance should also be measured directly.
A Review of 1064-nm Damage Tests of Electron-Beam Deposited Ta2O5/SiO2 Antireflection Coatings
Authors: Milam D , Rainer F , Lowdermilk WH , Swain J , Carniglia CK , Tuttle Hart T
Published: March 2009

Damage tests of Ta2O5/SiO2 antireflection films deposited under a variety of conditions showed that thresholds of films deposited at 175 C were greater than thresholds of films deposited at either 250 C or 325 C. Deposition at high rate and low oxygen pressure produced highly absorptive films with low thresholds. Thresholds did not correlate with film reflectivity or net stress in the films, and correlated with film absorption only when the film absorption was greater than 104 ppm. Baking the films for four hours at 400 C reduced film absorption, altered net film stress, and produced an increase in the average damage threshold.
Optical Properties of Hydrogenated Amorphous Carbon (a-C:H) - a Hard Coating for IR-Optical Elements
Authors: Bubenzer A , Dischler B , Nyaiesh A
Published: March 2009

Hydrogenated amorphous carbon films a-C:H were deposited on glass and germanium substrates. The films are transparent in the IR and are extremely hard (Mohs's hardness of about 8). The a-C:H coatings were prepared according to the method of Holland in an RF excited discharge sustained by various hydrocarbon gases.

Thickness, density, refractive index (at 0.3 μm and 2 – 10 μm) and relative hydrogen content were determined. Variations in IR-refractive index and relative hydrogen content could be correlated with deposition conditions. With a refractive index of approximately 2 a-C:H is an ideal AR-coating for germanium (n = 4). Laser calorimetric measurements of optical absorption at 10.6 μm give a loss as low as 4% for a 1.3 μm thick coating on germanium (λ/4 for n = 2 at 10.6 μm). Preliminary damage tests with a CO2 laser (600 W, cw) were performed.
The Deposition of Diamondlike Carbon Thin Films on CaF2
Authors: Moravec TJ
Published: March 2009

As part of an AFML program, deposition conditions were studied to produce diamondlike carbon thin films on CaF2. Halide materials are difficult to deposit diamondlike carbon coatings onto because of the moderate or large coefficient of thermal expansion of the halides which causes thermal induced separation of the coating. In addition, the adhesion of the carbon films to CaF2 is poor. Several pretreatments were attempted to improve adhesion without much success. We describe a simple thermal technique that resulted in carbon films adhering most of the time to CaF2 mechanical test bars and optical discs. The problem of adherence, however, has not been solved for this film-substrate system.
A Predictive Tool for Evaluating the Effect of Multiple Defects on the Performance of Cooled Laser Mirrors
Authors: Palmer JR , Bennett HE
Published: March 2009

A technique is suggested for evaluating the distance two defects on the surface of a fluid-cooled laser mirror must be separated before they can be considered independently. The basic ingredient of the correlation is taken from a technique suggested by Max Jakob for steady-state conduction with heat generation in a circumferential fin. Using this technique with concomitant Bessel functions of zero order and first kind, a distribution of temperatures in a circumferential fin at various radii may be evaluated for cylindrical heat generators acting in concert.
Recent Progress in the Studies of Laser-Induced Intrinsic Damage of Transparent Solids: Deterrent Lack Effect of Seed Electrons in Avalanche Ionization Process
Authors: Epifanov AS , Garnov SV , Gomelauri GV , Manenkov AA , Prokhorov AM
Published: March 2009

Peculiarities of laser-induced damage in transparent dielectrics caused by electron avalanche in the case of deterrent lack of seed electrons are discussed. Statistical models of the avalanche process initiated by multiphoton ionization of host atoms or impurities are described. Based on the breakdown probability expressions derived, the damage threshold dependence upon the temperature and the focal spot size are discussed. The theoretical results are found to be in good qualitative agreement with the experimental data for the laser-induced damage in alkali-halide crystals.
Effects of Higher Order Nonlinearities on Second Order Frequency Mixing
Authors: Harter DJ , Brown DC
Published: March 2009

The theory which has been developed to study the effects of higher order nonlinearities in second-order frequency mixing can be applied to the case of up-conversion of high powered lasers. We have considered the effect of the intensity dependence of the refractive index upon phase-matching for second-order frequency mixing. We have also considered the effects of second-order generation on higher-order nonlinearities which have to be considered in the design of other components of high power lasers. This includes the effect of second-order frequency generation on small-scale self-focusing and on the whole beam phase profile.
Simple Theory of Microwave Absorption in Alkali Halides
Authors: Sparks M , King DF , Mills DL
Published: March 2009

The intrinsic microwave absorption coefficient β of alkali halides is well explained by lifetime-broadened two-phonon difference processes, in contrast to energy-conserving two- and three-phonon processes as previously thought. A simple closed-form expression for β with no adjustable parameters gives excellent agreement with the magnitude, frequency dependence, and temperature dependence of β in NaCℓ, KBr, and KI. As ω increases from ω<ωc/3 to ω=ωc+γ293, the frequency and temperature dependence of β at 293 K (or 1000 K) change from the low-frequency limit β∼Tℓω2 to β∼Thω2+, where 2.0 ≤ ℓ ≤ 2.6 (or 2.2 ≤ ℓ ≤ 4.0) and 1.1 ≤ h ≤ 1.2 (or 0.57 ≤ h ≤ 1.6) for the three materials. Here ωC = 20 cm−1 for NaCl is the frequency difference between the transverse acoustical and transverse-optical phonon modes at the Brillouin zone edge and γ is the combined phonon inverse lifetime at 293 K. The temperature dependence of the phonon frequencies and other parameters must be included in order to explain the temperature dependence of the experimental results. The present lifetime-broadened results agree with previous energy-conservation results (apart from smoothing) and with experimental results in the region ω>ωc+γ. Previous energy-conserving results disagree with the T and ω dependence and magnitude of the experimental results in the region ω<ωc.
Increased Breakdown Thresholds in Air by Admixing an Electronegative Gas
Authors: Volkin HC
Published: March 2009

The intrinsic breakdown by large area laser beams of gases containing the attaching species SF6 mixed with clean dry Air has been analyzed in detail. The results show that SF6 is equally effective in inhibiting avalanche breakdown of these gases for near infrared pulses longer than 100 nanoseconds as for DC fields. The breakdown intensities were obtained for various pulse durations at 10.6 μm by numerically solving the Boltzmann equation for the energy distribution n(ε) of electrons in the generated plasma. Published electron collision cross sections considered the best available for our application were used. The calculated breakdown intensity for very long pulses of constant intensity in air at 1 atm and 300K is I* = 2.7 × 109 w/cm2, which agrees within experimental uncertainty with the measured value. At any given intensity, the power absorbed by the plasma and the power losses in the various electron collision processes (elastic, inelastic excitation of vibrational and electronic levels, ionization, and attachment) are evaluated in our calculations. Discrete photon effects in power absorption and photoelectric processes are expected to be small for wavelengths larger than 1 μm, so the frequency scaling (I*/ω2) of classical field theory should hold for our results over the near infrared region.
Influence of Compressibility on the Elastic-Plastic Field of a Growing Crack
Authors: Gao Y-c
Published: March 2009

The plastic singular field at the tip of a crack growing steadily in a perfectly plastic medium is solved in this paper by use of the basic equations and contiguity conditions given in a previous paper [1].2 The fifth-order nonlinear differential equation has been integrated to a second-order equation. The five contiguity conditions are given in very simple form. By use of the general solution of the asymptotic equation and the contiguity conditions, the various possible cases of tip-field structures are discussed and the unique reasonable solution is obtained. For the angles of each sector, the numerical solution is given. In this paper, it is found that the strains also possess the singularity of ℓn(A/r) in the sector ahead the crack tip, even if for the case of ν = ½. Finally, a fracture criterion based on the analysis of singularity of strains is given.
An Elastoplastic Finite-Element Investigation of Crack Initiation Under Mixed-Mode Static and Dynamic Loading
Authors: Ahmad J , Barnes CR , Kanninen MF
Published: March 2009

The fracture mechanics techniques needed for materials that fracture in a highly ductile manner must give explicit attention to the extensive plastic deformation surrounding the crack tip. Large-scale crack-tip plasticity has two effects. First, because of crack-tip blunting, crack growth initiation tends to occur under conditions for which conventional linear elastic fracture mechanics analyses are invalid. Second, significant amounts of stable crack growth can occur prior to fracture instability. To cope with these essentially inelastic processes, elastic-plastic fracture mechanics techniques are now being widely pursued [1].2 The research reported in this paper builds on these developments to provide the basis for fracture mechanics treatments under the more general conditions involved in dynamic mixed-mode loading.

Current work in elastic-plastic fracture mechanics is largely confined to quasi-static loading conditions and to crack growth in the opening mode (Mode I). Yet, in impact-loaded structures, crack initiation could occur under dynamic conditions with a combination of both the opening mode and the sliding mode (Mode II). Thus, it is necessary to obtain a quantitative understanding of crack growth initiation, propagation, and arrest under initial combined-mode dynamic loading conditions. The research described here is aimed at developing this understanding through a program of integrated experimental and finite-element analysis work.