27th Annual Boulder Damage Symposium: Laser-Induced Damage in Optical Materials: 1995

Characterization of absorption and degradation on optical components for high-power excimer lasers

Klaus R. Mann, Eric Eva, Bernd Granitza

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At Laser-Laboratorium Gottingen, the performance of UV optical components for high power excimer lasers is characterized, aiming to employ testing procedures that meet industrial conditions, i.e. very high pulse numbers and repetition rates. Measurements include determination of single and multiple pulse damage thresholds, absorption loss and degradation of optical properties under long-term irradiation. Absorption of excimer laser pulses is investigated by a calorimetric technique which provides greatly enhanced sensitivity compared to transmissive measurements. Thus, it allows determining both single and two photon absorption coefficients at intensities of standard excimer lasers. Results of absorption measurements at 248 nm are presented for bare substrates (CaF₂, BaF₂, z-cut quartz and fused silica). UV calorimetry is also employed to investigate laser induced aging phenomena, e.g. color center formation in fused silica. A separation of transient and cumulative effects as a function of intensity is achieved, giving insight into various loss mechanisms.

Segmented adaptive optic mirrors for laser power beaming and other space applications

Harold E. Bennett

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In order to effectively beam laser power into space to power satellites or to remove space debris in mid or high earth orbit very large mirrors (perhaps 12 m in diameter or more) and an adaptive optic systems to penetrate the atmosphere are required. Mirrors with adaptive optic segment sizes less than the equivalent. Fried coefficient for atmospheric turbulence (typically 3 - 5 cm at zenith in the visible region of the spectrum) are optimum for atmospheric penetration. These new mirrors may have hundreds of thousands of segments. The behavior of such mirrors under high powered laser irradiation is not clear, although for a large mirror both average and peak irradiation levels will be very low. Attention must be paid to penetration of laser energy into the gaps between segments and to the cumulative effect of edge diffraction. These problems do not appear to be severe and this new class of optics appears to offer us new possibilities for use in space. It may change the way in which we look at telescopes for space applications.

Review of the state-of-the-art measurements for and the phenomenon of anomalously low thermal conductivities of dielectric thin films

Joseph A. Carpenter Jr.

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Over the past decade, researchers in the laser optics, semiconductor electronics, and solid- state physics fields have shown that thin films of dielectrics a few micrometers or less in thickness can have measured thermal conductivities values significantly lower than if in normal bulk form. The values can be lower by several orders of magnitude and often decrease with decreasing film thickness. These phenomena have been observed in thin films of various dielectrics deposited or grown by various methods on various substrates. Preliminary explanations for these observations have centered around atomic- or molecular-sized defects or distortions in conjunction with high thermal resistances at or near the interface between the thin film and the substrate. This work has also fostered the initial development and assessment of apparently viable techniques for making measurements of thermal conductivities of thin films in the direction normal to the interface. The state-of-the-art of this work is reviewed.

Review of structural influences on the laser damage thresholds of oxide coatings

Erich J. Hacker, Hans Lauth, Peter Weissbrodt

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The laser damage thresholds of optical coatings lie, as a rule, markedly below those of the respective bulk materials. This is due to diverse specific realstructure properties with regard to composition, crystallography, microstructure and the physico-chemical structure of the interfaces. These properties depend in a highly complex and sensitive way on the substrate treatment, coating techniques and deposition conditions. With evaporated and sputtered oxide coatings as example, some correlations between structural thin film properties (e.g. crystallography, microstructure, anisotropy, chemical composition, defects) and the ultraviolet (248 nm) or near infrared (1064 nm) laser damage thresholds are discussed with concern to a further increase of the damage resistance. It is evident from data that an approach to the problem requires complex investigations of the technology-structure-properties relationships.

Ultrafast laser semiconductor interactions

L. A. Vern Schlie

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Studies of the ultrafast (< 100 fs) interactions of infrared, sub-100 fs laser pulses with IR, photosensitive semiconductor materials InGaAs, InSb, and HgCdTe are reported. Both the carrier dynamics and the associated Terahertz radiation from these materials are discussed. The most recent developments of femtosecond (< 100 fs) Optical Parametric Oscillators has extended the wavelength range from the visible to 5.2 micrometers . The photogenerated semiconductor free carrier dynamics are determined in the 77 to 300 degree(s)K temperature range using the Transmission Correlation Peak (TCP) method. The electron-phonon scattering times are typically 200 - 600 fs. Depending upon the material composition and substrate on which the IR crystalline materials are deposited, the nonlinear TCP absorption gives recombination rates as fast as 10's of picoseconds. For the HgCdTe, there exists a 400 fs electron-phonon scattering process along with a much longer 3600 fs loss process. Studies of the interactions of these ultrashort laser pulses with semiconductors produce Terahertz (THz) radiative pulses. With undoped InSb, there is a substantial change in the spectral content of this THz radiation between 80 - 260 degree(s)K while the spectrum of Te-doped InSb remains nearly unchanged, an effect attributed to its mobility being dominated by impurity scattering. At 80 degree(s)K, the terahertz radiation from undoped InSb is dependent on wavelength, with both a higher frequency spectrum and much larger amplitudes generated at longer wavelengths. No such effect is observed at 260 degree(s)K. Finally, new results on the dependence on the emitted THz radiation on the InSb crystal's orientation is presented.

Packaging-induced failure of semiconductor lasers and optical telecommunications components

Julia A. Sharps

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Telecommunications equipment for field deployment generally have specified lifetimes of <100,000 hr. To achieve this high reliability, it is common practice to package sensitive components in hermetic, inert gas environments. The intent is to protect components from particulate and organic contamination, oxidation, and moisture. However, for high power density 980 nm diode lasers used in optical amplifiers, we found that hermetic, inert gas packaging induced a failure mode not observed in similar, unpackaged lasers. We refer to this failure mode as packaging-induced failure, or PIF. PIF is caused by nanomole amounts of organic contamination which interact with high intensity 980 nm light to form solid deposits over the emitting regions of the lasers. These deposits absorb 980 nm light, causing heating of the laser, narrowing of the band gap, and eventual thermal runaway. We have found PIF is averted by packaging with free °2 and/or a getter material the sequesters organics.

Manufacturing experience in reducing environmentally induced failure of laser diodes

Richard A. Jollay

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Packaged life testing of laser diodes revealed a dramatic degradation of product lifetime. Failure analysis revealed the presence of carbon based compounds deposited on the emission regime causing catastrophic failure. This phenomenon was limited to packaged product. Testing was begun to identify the source, understand the mechanism, and initiate corrective action. Analysis revealed that the epoxy used in assembly was the dominate source of carbon. Contaminated epoxy had been introduced into production by a material batch change at the vendor. It was also learned that deposition only occurred in the dry, oxygen free atmosphere of a hermetically sealed package. It was necessary to develop incoming test procedures and process monitoring to ensure that material of acceptable quality was used. This paper will discuss test methods and results obtained in controlling this failure mechanism.

Comprehensive characterization of micro-arcing related particles

Yuri S. Uritsky, J. T. Pan, Terry Francis, et al.

Contamination control in semiconductor manufacturing and particle deposition on wafer surfaces

Benjamin Y.H. Liu

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Particulate conthmin4tion is an important problem in semiconductor device inanufactuiing. Very large scale integrated circuit (VLSi) devices such as the current generation of 16 megabit DRAM (dynamic random access memory) have typical device feature sizes on the order ofO.5 pm. Particles as small as 0.05 tm can cause a killer defect to form and the loss of product yield. Contaminant particles can deposit on the wafer surface from the gas or liquid in which they are suspended by the process of diffusion, sedimentation, and electrostatic attraction. In the case of deposition of gas-borne particles (aerosols), additional effects due of thermophoresis or diflutsiophoresis can occur. This paper discusses the fundamental issues in contamination control in semiconductor manufacturing and the mechanisms of particle depositions on wafer surfaces taking into account the different deposition mechanisms. Results of the theoretical calculation sill be compared th the available experimental data.

Photochemically induced surface contamination: mechanisms and effects

Graham S. Arnold, Kenneth T. Luey

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Spacecraft function in a hostile environment of sunlight, charged particles, micrometeroids and debris, and their own self-induced contamination environment. Contamination can occur during fabrication and ground processing, or by very long term outgassing and transport processes on orbit. One of the most deleterious effects of contaminant films is that they increase the solar absorptance of optics, such as thermal control mirrors and solar cell cover slips. This paper will discuss the role of vacuum ultraviolet induced photochemistry in the deposition of contaminant films during the multi-year life of a spacecraft. Laboratory and spaceflight measurements leading to a one-photon "Langmuir" type model for the deposition mechanism will be presented. Measurements of visible and ultraviolet optical properties of photodeposited films will be described. The implications of these process for terrestrial optical systems, including a case history of similar effects in an ultraviolet laser system studied in this laboratory will be discussed.

Clean-cavity contamination in gas lasers

Christine E. Geosling

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A mechanism has been identified which can affect the long-term power output of a high powered gas laser. This mechanism, which is brought about by a process of mobile ion migration in the glass cavity material, is driven by the electric fields set up by the operation of the laser itself. Characteristics of this mechanism and one successful design to overcome its deleterious effects will be discussed. Another mechanism of concern in gas lasers is UV damage to optics. Experiments were carried out in which the UV sensitivity characteristics of new coating materials and designs were determined in a plasma exposure chamber. A method to correlate the UV exposure in the chamber with that under actual laser operating conditions was devised and will be described. Use of this chamber as a testing device eliminates the cost and time required to build a laser for each new material/design iteration. It can also be used as a process control tool for random UV sensitivity checks of production components.

Cleanliness and damage measurements of optics in atmospheric-sensing high-energy lasers

Gale A. Harvey, Thomas H. Chyba, Marc C. Cimolino

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Langley Research Center has several atmospheric remote sensing programs which utilize high energy pulsed lasers. These lasers typically have many damaged optics after several million shots. Damage is defined herein as color changes and/or optical flaws seen in microscopic inspection, and does not necessarily relate to measured performance degradation of the optic. Inspections and measurements of some of these optics indicate that energy thresholds for several million shots damage is about an order of magnitude lower than that for single shot damage. Damage initiation is often at micron size areas at the coating interface, which grows and sometimes develops as erosion of the top of the coating. There is a wide range in polish and coating quality of new optics, even on different faces of the same optic. Military Standard 1246C can be used to provide overall particulate, and molecular film, or nonvolatile residue cleanliness scales. Microscopic inspections and photography at 10X to 500X with brightfield (perpendicular) and darkfield (oblique) illumination are useful in assigning cleanliness levels of new and in-service optics. Microextraction (effecting concentration of molecular films to small areas) provides for enhanced optical detection and surface film chemical analysis by electron-microscope energy-dispersive-spectroscopy. Similar measurement techniques can be used to characterize and document optical damage initiation and optical damage growth. Surface contamination interferes with and complicated measurements of polish and coating quality, and of optical damage. Our work indicates ultrasonic cleaning, and packaging of optics in Teflon sleeves or cups is advantageous over conventional cleaning and packaging for characterization of new optics.

Contamination damage in pulsed 1-um lasers

Floyd E. Hovis, Bart A. Shepherd, Christopher T. Radcliffe, et al.

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Although thin film coating technology has evolved to the point that damage thresholds of several hundred MW/cm² can be routinely achieved, sealed laser systems which must be operated for extended times or at elevated temperatures frequently experience failure due to optical damage. This damage, which is frequently due to the build up of gas phase contaminants in the sealed optical compartment, occurs in spite of the fact that the lasers were designed such that the intracavity intensities are only a few tens of MW/cm². Since much of our work involves designing Q-switched Nd:YAG lasers that operates over extreme environmental conditions, eliminating contamination damage at 1 micrometers is of particular interest to us. In this paper we will describe our current understanding of contamination induced damage at 1 micrometers and give an overview of the processes that can be used to eliminate such damage in fielded systems.

Comparison between 355 and 1064-nm damage of high-grade dielectric mirror coatings

Axel Bodemann, Norbert Kaiser, Mark R. Kozlowski, et al.

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Advanced reactive e-beam evaporation process was used to deposit HfO₂/SiO₂ HR coatings for 355 nm high power laser applications. Atomic force microscopy studies and Nomarski microscopy have shown that the defect density of these coatings is extremely low exhibiting nearly no nodular defects known for an increased susceptibility to laser damage in the IR spectral region. Standard damage testing (conditioned and unconditioned) was conducted at LLNL at 355 nm (3 ns) for normal (0 degree(s)) and nonnormal-incident designs (45 degree(s)). Damage thresholds between 5 J/cm² and 8 J/cm² were obtained. No significant conditioning effect could be demonstrated. The same evaporation technique was used to manufacture normal incident HfO₂/SiO₂ HR coatings for 1064 nm wavelength from 2 different types of evaporant grade HfO₂ as well as from a Hf metal source. Damage test results, as well as defect concentrations and conditioning effect, were compared to the 355 nm samples. Moreover, care was taken on the detection of the origin of damage at fluences near the damage thresholds.

Thin film laser damage mechanisms at the YAG third harmonic

Jean DiJon, Jean Hue, Aysegul Disgecmez, et al.

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The morphology of the first damage occurring on either single layer materials or multilayer HR coatings are studied using Scanning Electron Microscopy. For single layers obtained by electron beam evaporation or ion beam sputtering, the damages are always centered on a small size center of about 0.1 micrometers . This center triggers the degradation which presents a well defined shape and size. Two kinds of damage mechanisms occur, either melting of the layer or mechanical breaking of the layer. The same behavior is observed on HR coatings with damage centers located at the interfaces between the high and low index materials of the stack. From these experiments an impurity dominated model is proposed which enables the main features of UV damages to be understood.

Electrodynamic instability as a reason for bulk and surface optical damage of transparent media and thin films

Vitali E. Gruzdev, Mikhail N. Libenson

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Electromagnetic field instability in nonabsorbing dielectric nonhomogeneity is considered to be one of possible mechanisms of field localization and abrupt amplitude increasing in transparent dielectric materials. Instability threshold estimates and some parametric dependences of the threshold amplitude are obtained making use of model of field instability in dielectric sphere. It is discussed using of the model for analysis of optical breakdown processes in transparent media.

R-on-1 automatic mapping: a new tool for laser damage testing

Jean Hue, Pierre Garrec, Jean DiJon, et al.

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Laser damage threshold measurement is statistical in nature. For a commercial qualification or for a user, the threshold determined by the weakest point is a satisfactory characterization. When a new coating is designed, threshold mapping is very useful. It enables the technology to be improved and followed more accurately. Different statistical parameters such as the minimum, maximum, average, and standard deviation of the damage threshold as well as spatial parameters such as the threshold uniformity of the coating can be determined. Therefore, in order to achieve a mapping, all the tested sites should give data. This is the major interest of the R-on-1 test in spite of the fact that the laser damage threshold obtained by this method may be different from the 1-on-1 test (smaller or greater). Moreover, on the damage laser test facility, the beam size is smaller (diameters of a few hundred micrometers) than the characteristic sizes of the components in use (diameters of several centimeters up to one meter). Hence, a laser damage threshold mapping appears very interesting, especially for applications linked to large optical components like the Megajoule project or the National Ignition Facility. On the test bench used, damage detection with a Nomarski microscope and scattered light measurement are almost equivalent. Therefore, it becomes possible to automatically detect on line the first defects induced by YAG irradiation. Scattered light mappings and laser damage threshold mappings can therefore be achieved using a X-Y automatic stage (where the test sample is located). The major difficulties due to the automatic capabilities are shown. These characterizations are illustrated at 355 nm. The numerous experiments performed show different kinds of scattering curves, which are discussed in relation with the damage mechanisms.

ArF excimer laser-induced absorption in soot-remelted silicas

Nobu Kuzuu

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Characteristics of ArF excimer laser induced absorption in various types of fused silicas synthesized by VAD soot remelting methods were investigated. Fused silica containing 30 and 70 ppm of OH and no Cl, shows absorption bands at 215 nm ascribed to E' center (identical with Si(DOT)) and absorption band at 260 nm ascribed oxygen related center. A sample containing about 800 ppm of Cl and no OH shows only 215-nm band. A OH and Cl free silica, have preexisting absorption band at 245 nm ascribed to oxygen deficient center (ODC; identical with Si(DOT)(DOT)(DOT)Si identical with) and weak band at 215 nm; the 245-nm band change into the 215-nm band by irradiating with the ArF laser. A model to describe these phenomena are proposed: the glass networks considerably deformed from ideal network and terminal groups, such as Si-OH and Si-Cl, reduce some amount of highly strained bonds. In the sample containing only Cl, Si-Cl could be major precursor of the E' center and only weak 215-nm band is observed. In silica containing 30 - 70 ppm OH and no Cl, bond breakage is major process to produce ArF laser induced defects. Samples without such terminal groups, some of the strained bonds cannot retain in the glass network, and defect structures such as ODC and E' centers will be formed even in as-synthesized material.

Peculiarities of photoexcitation and heating of surface in nano-optics

Mikhail N. Libenson, George A. Martsinovsky

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Peculiar physical phenomena at a surface under superlocal light action of optical near-field sources are studied theoretically. Stationary spatial distributions of nonequilibrium carriers concentration and temperature are found. The results point to the essential delocalization of near-field action on surface. The consideration is done for semiconductors and valid for metals as a particular case of the problem parameters.

Laser damage processes in cleaved and polished CaF2 at 248 nm

Michael Reichling, S. Gogoll, E. Stenzel, et al.

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Single-shot irradiation of single crystal CaF₂ with 248 nm/14 ns laser light results in various degrees of degradation and damage depending on the applied laser fluence. Phenomena range from subtle, non-topographic surface modification only detectable by secondary electrons of scanning electron microscopy (SE-SEM) over cracking along crystallographic directions to the ablation of crystalline material. Significant differences are found for cleaved and polished surfaces. Findings from SEM investigations, in-situ photoacoustic mirage measurements and visual inspection of irradiated samples from a comprehensive picture of the stages of laser-induced damage.

Laser-induced surface thermal lensing for thin film characterizations

Zhouling Wu, Pao-Kuang Kuo, Y. S. Lu, et al.

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Laser-induced surface deformation is a widely used technique for studying thin film coatings. In this technique the deformation is typically detected by using the well-known optical beam deflection technique. In this paper we report a different technique in studying the deformation by using optical diffraction effect. In this detection scheme the laser-induced thermal bump behaves as a curved reflection mirror which can either focus or defocus a second probe laser beam through optical diffraction, depending on the specific geometry used. This surface thermal lensing effect is demonstrated to be a sensitive and easy-to-handle method for thin film characterization. The potentials of this technique and its advantages over the conventional method for thin film absorption measurement and defect characterization are discussed.

Accuracy and precision of laser damage measurements made via binary search techniques

Jonathan W. Arenberg

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This paper presents an estimation of the accuracy and precision for several variants of the binary search technique for the determination of laser damage threshold. First the variants of the binary search technique are introduced. Next a normalized version of the defect ensemble model is introduced. A Monte Carlo calculation is made using the normalized probability distribution and the four BST variants. The results are analyzed to yield the median and upper limit on the measured threshold for various number of spots tested. It is shown that there is a rapid increase in the accuracy and precision of the test with increasing numbers of spots included. Also shown are performance differences among the test BST protocols under examination. The paper concludes with rules of thumb for making accurate measurements via the BST.

Uncertainty in damage-frequency threshold measurements

Jonathan W. Arenberg

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A Monte Carlo model was developed to investigate the distribution of laser damage threshold (LDT) results obtained via the Damage Frequency Method (DFM). The Monte Carlo model was used to determine the correlation in LDT accuracy and precisIon and the parameters of the optic, test laser and test parameters. It is seen the DFM underestimates the true LDT. The degree of underestimatIon is shown to be related to the slope of the probabIlity versus test fluence (P,4) Line. The degree of underestimation is shown further to be positively correlated to the slope of the line (P,4), with the lower slope cases producing the lowest estimates. Rules of thumb are given to determine the degree of underestimation to allow for comparison between test results.

Design of a certification test for laser damage resistance

Jonathan W. Arenberg

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This paper reports on the design and procedure of test protocol for use as a certification test for laser damage resistance. A formulation for the estimated survival probability as a function of incident fluence is derived.

248-nm laser interaction studies on MgF2/LaF3 optical coatings by mass spectroscopy and x-ray photoelectron spectroscopy

Axel Bodemann, Norbert Kaiser, L. Raupach, et al.

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A UHV surface analysis system combined with an optical setup was used in the present work to study the conditioning mechanism on MgF_2?/LaF₃ HR coatings at 248 nm excimer laser wavelength. During laser irradiation of the sample the laser-induced emission of contaminants and coating material from the sample surface was recorded by a quadrupole mass spectrometer. To analyze changes in composition and chemical bonds of the coating surface XPS-measurements were performed before, during and after irradiation in dependence on sample design, number of pulses and oxygen atmosphere.

Theoretical model for laser energy deposition in intrinsic optical materials and thermomechanical effects

Florian Bonneau, Bernard Cazalis

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On the basis of a microscopic theory for the interaction of intense laser irradiation with free electrons over the range from visible to near infrared, we calculate space-time energy deposition in SiO₂ for several wavelengths and laser pulsewidths. Most of the parameters used in the simulations are extracted from a Monte Carlo integration of the Boltzmann transport equation. We investigate the relative roles of multiphoton ionization, avalanche multiplication and free carrier absorption as intrinsic processes of carrier generation and pulse absorption for different laser irradiation conditions. The intensity profile distortion in the time domain of the transmitted pulse due to the rapid build-up of the plasma density is discussed. In the field of ultrashort laser pulses, damages exhibit a morphology dramatically different from that observed with long pulses: thin layers of material are removed by ablation. Therefore, we interfaced the space-time energy deposition with a 1D Lagrangian hydrodynamic code and estimated thermomechanical effects: stress evolution, spalling.

Survivability demonstration and characterization of a multibillion-shot Q-switched Nd:YAG laser

Joseph Louis Dallas, M. A. Stephen, Robert S. Afzal

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As part of NASA's suite of Earth Observing Satellites, Goddard Space Flight Center is developing a laser altimeter for measuring the polar ice sheet mass balance. The Geoscience Laser Altimeter System (GLAS) requires a diode pumped, Q-switched, Nd:YAG laser transmitter producing 150 mJ, 4 ns pulses at a 40 Hz repetition rate in a single transverse mode. The mission lifetime goal is five years (6.3 billion shots). The projected performance of the GLAS laser can be limited by a number of failure sources including optical damage to the components and degradation of the pump laser diodes. To the authors' knowledge, no data exists describing the multi-billion shot accumulative exposure effect Q-switched, 4 ns pulses have on intracavity optical components. To obtain multi-billion shot results in a reasonable time, an accelerated repetition rate (500 Hz) version of the GLAS oscillator was built with modifications for thermal management. An Accelerated GLAS Exposure System (AGES) was developed to autonomously monitor the laser's vital signs. Upon analysis of the stored data, the system dynamics were decoupled to identify the sources of degradation. Over 7 billion shots were accumulated during AGES' non-stop 5.5 month operation.

Laser-induced absorption at 355 nm in silica studied by calorimetry and photothermal deflection

Jean DiJon, E. Van Oost, Catherine Pelle, et al.

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Laser calorimetry is used to measure low losses of optical samples on the third harmonic of a YAG laser. During the experiments, degradation of bare SiO2 substrate was observed. This degradation consists of an absorption increasing with the laser pulse number. The observed evolution depends on the kind of silica tested and particularly on the OH content of the material. The observe evolution required both changes in the electron content of the existing traps and the creation of new traps or color centers. Using an Argon laser at 351 nm, relaxation of YAG induced absorption was observed. This point enables an evolution mechanism to be proposed related to charge modification of the existing traps and to a multiphoton absorption process during the YAG irradiation.

Influence of the number of double layers on the damage threshold of Al2O3/SiO2 and LaF3/MgF2 mirrors at 248 nm

Eric Eva, Klaus R. Mann, Uwe B. Schallenberg, et al.

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KrF-laser induced damage thresholds (LIDT) of HR stacks were investigated as a function of the number of quarterwave layers. The findings were interpreted in terms of calorimetric absorption measurements and scatter defect density counts as well as an analysis of the standing wave electric field. Higher numbers of high-purity Al₂O₃/SiO₂ layers resulted in enhanced LIDT by shielding the substrate surface increasingly well. Contrary to this, LIDT decreased with increasing numbers of e-beam evaporated LaF₃/MgF₂ layers. This was accompanied by elevated absorptance, defect density and conditionability at higher stack numbers.

Sol gel optical thin films for an advanced megajoule-class Nd:glass-laser ICF driver

Herve G. Floch, Philippe F. Belleville, Philippe M. Pegon, et al.

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It is well established by manufacturers and users that optical coatings are generally prepared by the well known Physical Vapor Deposition (PVD) technology. In the authors' opinion sol-gel technology is an effective and competitive alternative. The aim of this paper is to emphasize on the sol-gel thin film work carried out at CEA Limeil-Valenton and concerning the technology for high power lasers. We will briefly discuss the chemistry of the sol-gel process, the production of optical coatings and the related deposition techniques. Finally, the paper describes performance of sol-gel optical coatings we have developed to fulfill the requirements of a future 1.8 MJ I500TW (351 nm) pulsed Nd:glass laser so-called << LMJ << (Laser MegaJoules). This powerful laser is to be used for our national Inertial Confinement Fusion (ICF) program, to demonstrate at the laboratory scale, ignition of deuterium-tritium fusion fuel. Moreover, the aim of this article is, hopefully, to provide a convincing argument that coatings and particularly optical coatings, are some of the useful products available from sol-gel technology, and that exciting developments in other areas than high power laser technology are almost certain to emerge within the coming decade. Keywords : sol-gel, oxides, colloidal suspensions, optical coatings, laser damage

Ultrasensitive detection of photothermal signal of thin films by using an optical amplification system

S. T. Gu, Zhouling Wu, Peter Diehr, et al.

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A simple optical amplification system is introduced to improve the sensitivity of both the photothermal deflection detection scheme and the surface thermal lensing technique. Model calculations and experimental data show that the photothermal signals can be easily amplified by more than an order of magnitude and that the technique can be readily applied to weak absorption measurements for optical thin film coatings.

CMO YAG laser damage test facility

Jean Hue, Jean DiJon, Philippe Lyan

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The CMO YAG laser damage test facility, which is equipped with a 30 Hz laser, is presented in this paper. The main points are described: (1) The characteristics of the laser beam and the in situ damage detection technique (a scattered light measurement system) are perfectly suited to work up to the frequency of the laser. They are monitored in real time, and work at three wavelengths: 1064 nm, 532 nm, 355 nm. (2) The laser beam characteristics are preserved during the laser damage tests even for only one shot or frequencies lower than 30 Hz due to a fast shutter. (3) With this same shutter, it is possible to automatically stop the laser on the pulse which induces the first damages. These automatic capabilities enable the samples to be tested quickly. (4) A Nomarski microscope supplied with a 16-bit CCD camera enables the test sites to be photographed before and after the laser interaction. Image processing enables us to extract the first damages. (5) Six pulse widths are available (between 3 ns and 13 ns). Therefore, with all these characterization tools, many kinds of laser tests may be considered. These different features are illustrated by experimental results (1-on-1 test or R-on-1 test).

Environmental stability of CO2 laser optics

Michael Kennedy, Wilfried Plass, Detlev Ristau, et al.

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Transmissive and reflective optics for high power CO₂ lasers were exposed to define relative humidities and temperatures. Degradation effects were investigated by means of absorptance, reflectance, laser-induced damage threshold measurements and microscopic inspection. The LIDT measurements were performed in the short and long-pulse regime involving different test routines as 1 on 1, S on 1 and R on 1. For distinct coatings, microscopic investigation reveal a size-increase of the nodular defects. This increase results in a reduction of the defect induced LIDT, while the absorptance and reflectance remain unchanged. Alkali-halide components show an intensity-dependent laser-induced conditioning effect. This effect is analyzed with respect to different environmental parameters.

Photothermal study of optical components at 10.6 um: finite element calculations and experiments

Rene Krupka, Adolf Giesen

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Up to now theoretical calculations of the photothermal response of optical components have only been performed using analytical approximations of the adequate thermo-mechanical equations. In contrast, this paper presents finite-element (FE) calculations for the description of the photothermal behavior of optical components at 10.6 micrometers . The FE-analysis allows the calculation of the surface temperature- and deformation-profile for bare substrates and two additional coating layers with respect to their thermo-optical properties. Also the behavior of the interfaces (substrate-coating, coating layer1 - coating layer2) can be calculated. These FE- model calculations of the photothermal response were performed with respect to the thermal properties and absorption characteristics of the coating and substrate respectively. The results are compared to experimental results obtained using the photothermal deflection technique (PDT). Additionally, the FE-model used allows the calculation of the time resolved response of the optical component to pulsed irradiation. The calculations of the temporal behavior are studied for a pulsed PDT situation and are compared to measurements. The results can also be used to determine the heat conductivity of the coatings of optical components. Some measurements and comparisons with the calculations are shown and discussed in detail.

OH content dependence of ArF excimer-laser-induced absorption in type III fused silica

Nobu Kuzuu

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OH content dependence of the intensity of ArF-excimer laser induced absorption in fused silica synthesized under reducing condition was investigated. The absorption spectra of each silica has a peak of 220 nm, and the intensity of which increases with decreasing OH content. Above OH content about 1150 ppm, ArF laser induced absorption was not observed. Annealing in He ambient, suppressed the creation of the 220 nm band, and no absorption was observed in the sample containing greater than 800 ppm of OH. The annealing in the H₂ ambient was not so effective as in He ambient. These results indicate that the fused silica synthesized under reducing conditions and containing OH more than about 1150 ppm was found to be desirable material for excimer laser optics. The fused silica containing more than 800 ppm of OH annealed in the He ambient can also be used as excimer laser optics.

UV damage threshold of molecular-beam-deposited fluoride coatings

Sven Laux, Klaus R. Mann, Ute Kaiser, et al.

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Optical coatings for the ultraviolet spectral region consisting of fluorides have been made by molecular beam deposition in ultrahigh vacuum. The laser induced damage threshold at (lambda) equals 248 nm of the NdF₃ single layers has been improved by means of reactive deposition. It was found that the threshold of fluoride antireflection coatings for 248 nm is significantly lower than that of single layers and does not show significant dependence on reactive gas conditions. A surface smoothing has been obtained due to an interrupted growth of very thin sublayers crystallizing in different crystal lattices. The damage threshold of interrupted grown antireflection coatings is somewhat higher than that of a conventional one.

Thermoelastic wave model of the photothermal and photoacoustic signal

Peter Meja, Bernhard Steiger, Pier Paolo Delsanto

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By means of the thermo-elastic wave equation the dynamical propagation of mechanical stress and temperature can be described and applied to model the photothermal and photoacoustic signal. Analytical solutions exist only in particular cases. Using massively parallel computers it is possible to simulate the photothermal and photoacoustic signal in a most sufficient way. In this paper the method of local interaction simulation approach is presented and selected examples of its application are given. The advantages of this method, which is particularly suitable for parallel processing, consist in reduced computation time and simple description of the photoacoustic signal in optical materials. The present contribution introduces our model, the formalism and some results in the 1D case for homogeneous nonattenuative materials. The photoacoustic wave can be understood as a wave with locally limited displacement. This displacement corresponds to a temperature variation. Both variables are usually measured in photoacoustics and photothermal measurements. Therefore the temperature and displacement dependence on optical, elastic and thermal constants is analyzed.

Absorption and laser damage threshold studies of ion-assisted pulsed-laser-deposited oxide films

Guenter Reisse, Steffen Weissmantel, Bernd Keiper, et al.

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The laterally resolved absorption and the laser damage thresholds at 1.06 micrometers wavelength of yttria and hafnia films prepared by pulsed laser deposition with oxygen ion bombardment of the growing films were investigated. Depending on the laser and ion beam parameters films with low average absorption can be prepared by that method. Consequently, high predominantly intrinsic absorption induced laser damage thresholds D₁ comparable with those of continuously evaporated films can be prepared. The more defect induced laser damage thresholds D₀, however, are largely determined by a certain number of micron- sized particulates embedded inside the films. Their number can be reduced by optimizing the laser pulse power density and the laser beam cross-section on the target, while their influence on laser damage thresholds can be reduced by increasing the ratio of oxygen ion bombardment to growth rate.

Round-robin test on optical absorption at 10.6 um

Detlev Ristau, Hansjoerg S. Niederwald, D. Erdelyi, et al.

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The approved Draft International Standard 11551 on test methods for absorptance of optical laser components recently passed the international voting procedure. The utility of this standard practice document is the subject of the present round robin test. In order to cover a broad range of CO₂-laser optical components, different types of metal mirrors and transmissive ZnSe-optics were included in the master sample set. After an initial inspection, this set passed through a series of optical laboratories in the United States, the United Kingdom, and Germany. The absorption of the samples was measured by calorimetric methods according to ISO DIS 11551, and the measurement results were compiled by the coordinating institute. The evaluation of the experiment was not started until all tests were completed. The results of the round robin test are discussed, compared and evaluated with respect to ageing mechanisms in optical coatings for CO₂-laser systems. Although a great variety of different test facilities was employed by the round robin partners, a good agrement of the absorption values was observed for the wavelength of 10.6 micrometers . This demonstrates the versatility of the approved Draft International Standard 11551 for the calorimetric measurement of optical absorption in CO₂-laser components.

Can scaling be extended to gigahertz repetition rates?

Jerry Ray Bettis, Glenn T. Bennett

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A scaling relation for laser-induced damage is developed which allows scaling from single- pulse thresholds for a given set of conditions to repetition-rate thresholds at various spot sizes and pulse durations. Damage test results on AgGaS₂, for single, 26.5 psec pulses are scaled to 350 psec pulses at 95 MHz repetition rate. Damage test results for 300 psec pulses at 75 MHz and 150 psec pulses at 95 MHz are compared with these scaled thresholds.

Theoretical model of multiple-shot bulk damage in silicate glasses under conditions of multiphoton generation of color centers

O. N. Bosyi, Oleg M. Efimov, Leonid B. Glebov, et al.

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A model of multiple shot bulk damage under conditions of multiphoton excitation of glass matrix has been considered. It has been based on the nonsteady thermal self-focusing due to accumulation of color centers and increasing of absorption coefficient of medium at the wavelength of incident radiation. These representations have allowed to explain not only the shape of accumulation curves but the influence of spot size and color centers generation efficiency on the thresholds of single and multiple shot damage, and the influence of variable- valence impurity ions on the process' characteristics too.

High-efficiency dielectric multilayer gratings optimized for manufacturability and laser damage threshold

Jerald A. Britten, Michael D. Perry, Bruce W. Shore, et al.

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Ultrashort pulse, high-intensity lasers offer new opportunities for the study of light-matter interaction and for inertial confinement fusion. A 100 Terawatt laser operating at 400 fs and 1.053 micrometers is operational at LLNL, and a 1000 Terawatt (Petawatt) laser will come online in early 1996. These lasers use large-aperture (40 cm and 94 cm diameter, respectively) diffraction gratings to compress the amplified laser pulse. At present, holographically produced, gold overcoated photoresist gratings are used: these gratings represent the fuse in the laser chain. Higher laser damage thresholds and higher diffraction efficiencies are theoretically possible with multilayer dielectric gratings (MDG's). A number of design parameters regarding both the multilayer stack and the etched grating structure can be optimized to maximize the laser damage threshold and also improve the processing latitude for the interference lithography and reactive ion etching steps used during manufacture of these gratings. This paper presents model predictions for the behavior of hafnia/silica MDG's both during processing and in operation, and presents experimental data on the diffraction efficiency and short-pulse laser damage threshold for optimized witness gratings.

Midinfrared power delivery through chalcogenide glass-clad optical fibers

Lynda E. Busse, John A. Moon, Jasbinder Singh Sanghera, et al.

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Chalcogenide glass cladded optical fibers of As-S-Se core/As-S cladding compositions are being developed at the Naval Research Laboratory. Results of testing these fibers for high power transmission in the 2 - 6 micrometers wavelength region using various laser sources are presented. These sources include a CW CO laser (5.4 micrometers ) and a pulsed OPO (2 - 4 micrometers ). In addition, telluride glass fibers (transmission from 3 - 12 micrometers ) have been tested with CW CO₂ laser input. Power handling capability has been assessed during laser irradiation by both visual observation of fiber endfaces using a microscope/camera apparatus and by measuring any reduction in power transmission.

Experimental data on the fiber fuse

Donald D. Davis Jr., Stephen C. Mettler, David J. DiGiovanni

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A phenomenon which results in the catastrophic destruction of an optical fiber core has been observed at laser powers on the order of 3 X 10⁶ watts/cm² in the core. This phenomenon is characterized by the propagation of a bright visible light from the point of initiation toward the laser source. The term `fiber fuse' has been used because of the similarity in appearance to a burning fuse. The fiber fuse has been shown to occur when the end of the fiber is contacted, and it has also initiated spontaneously from splices and in-core fiber gratings. This paper will report experimental data gathered on the nature of the fiber fuse, and will discuss their relevance to the mechanisms which have been proposed to date.

Laser damage threshold of gelatin and copper phthalocyanine-doped gelatin optical limiter

Mark C. Brant, Daniel G. McLean, Richard L. Sutherland, et al.

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We demonstrate optical limiting in a unique guest-host system which uses neither the typical liquid or solid host. Instead, we dope a gelatin gel host with a water soluble Copper (II) phthalocyaninetetrasulfonic acid, tetrasodium salt (CuPcTs). We report on the gelatin's viscoelasticity, laser damage threshold, and self healing of this damage. The viscoelastic gelatin has mechanical properties quite different than a liquid or solid. Our laser measurements demonstrate that the single shot damage threshold of the undoped gelatin host increases with decreasing gelatin concentration. The gelatin also has a much higher laser damage threshold than a stiff acrylic. Unlike brittle solids, the soft gelatin self heals from laser induced damage. Optical limiting test also show the utility of a gelatin host doped with CuPcTs. The CuPcTs/gelatin matrix is not damaged at incident laser energies 5 times the single shot damage threshold of the gelatin host. However, at this high laser energy the CuPcTs is photo bleached at the beam waist. We report photo bleached sites by annealing the CuPcTs/gelatin matrix.

Influence of impurity ions on nonlinear coloration of alkali silicate glasses

Oleg M. Efimov, Yurii A. Matveev, Andrei M. Mekryukov

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A study was made of the role of iron impurity ions in the formation and decay of color centers in alkali-silicate glasses under conditions of multiphoton absorption of laser radiation. An investigation was carried out on sodium-silicate glasses which are typical representatives of crown glasses. In the experiments the glasses of the Na₂O:CaO:SiO₂ equals 22:3:75% compositions synthesized from materials of the `specially pure' grade were used. The impurity was introduced in the form of the compound Fe₂O₃ in amounts of 0.1% by weight into some of the samples. Oxidative conditions during synthesis ensured that the main part of the iron ions in the glasses was in the form of Fe³⁺. Since the band gap of alkali- silicate glasses is approximately 6 eV, the third and fourth harmonics of a neodymium laser were used for nonlinear coloration of these glasses. It was shown that an increase in the concentration of Fe₂O₃ enhanced the residual absorption by stable hole color centers and reduced considerably the lifetime of unstable electron color centers. Repeated irradiation of the glasses containing a small amount of the impurity increased the residual absorption, whereas the induced absorption (measured immediately after the action of a radiation pulse) did not change from one pulse to the next and was governed solely by the radiation irradiance. An increase in the Fe₂O₃ concentration destroyed this effect.

Inclusion and overlayer effects on the electric fields in dielectric films

Kim F. Ferris, Gregory J. Exarhos, Steven M. Risser

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The surface and bulk microstructures inherent to the fabrication process of dielectric films, affects both the distribution of the local electric field intensities and its associated dielectric properties. In this paper, we have used a finite element electric field model to examine the effects of a low dielectric surface layer, alignment of a low dielectric component within the dielectric film, and the interaction of these two influences on the local electric fields and dielectric constants. Columnar microstructure and alignment of dielectric components perpendicular to the surface norm are shown to enhance electric intensities and dielectric constants. Predicted dielectric constants are compared against conventional effective medium approximation results.

Laser conditioning of optical coatings: a statistical study of defects by atomic force microscopy

Anne Fornier, C. Cordillot, D. Bernardino, et al.

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Laser conditioning is a process which consists in illuminating a coating by a laser below damage threshold in order to increase this threshold value. In a previous study we had reported that laser conditioning actually results in minimization or elimination of surface defects present after coating elaboration. The present paper reports a statistical study whose goal is to seek a correlation between defects size and shape, and defect evolution during laser conditioning. In order to achieve this, hundreds of defects are individually analyzed before and after laser conditioning using Atomic Force Microscopy. The observed samples are e-beam deposited highly reflecting multilayers Zr02/Si02 and Hf02/Si02 mirrors, and also single layers of Si02 and Zr02.

Laser damage thresholds in fused silica structured light gratings

Alan M. Frank, Calvin H. Gillespie

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Computer-generated, ion-milled, holographic structures and gratings are being used for multiple splitting of high-power pulsed laser beams. The structures were manufactured by Teledyne Brown Engineering in Huntsville Alabama and both a linear grating and a 2D structure were tested. Damage thresholds were measured in single-shot exposure using a 300 mj, 1 micrometers wavelength laser with a nominal 15 ns gaussian pulse. Energy densities on the gratings were adjusted with a focusing lens and the energy distribution was mapped with an imaging profiler. The gratings were examined dynamically for sparking and breakdown. They were examined after exposure using phase contrast microscopy. Initial results indicate the grating surface is always damaged before the smooth surface independent of the beam direction. The initial indication is that the average energy density threshold for single-shot damage is in the excess of 12 j/cm².

Liquid disinfection using power impulse laser

S. Gribin, Viktor Assaoul, Elena Markova, et al.

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The presented method is based on the bactericidal effect of micro-blast induced by various sources (laser breakdown, electrohydraulic effect...). Using elaborated conception of physical phenomena providing liquid disinfection it is possible to determine optimal conditions of water treatment. The problem of optimization is solved using methods of mathematical modeling and special experiments.

Mechanism of impulse-light self-focusing on shock lenses in transparent media

Serguei Gribine, Boris Spesivtsev

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Mathematical and physical models have been suggested of the phenomenon, explaining the lowering of the damage threshold of a transparent dielectric by self-focusing of light energy on the refraction index dynamic inhomogeneity, arising in the compression wave which appears in transparent media around absorbing inhomogeneities in the process of their absorbing a part of light impulses. On the basis of this model the dependence have been obtained connecting parameters of light impulse, physical and chemical properties of the medium and absorbing inhomogeneity characteristics with the level of local concentration of light energy.

Theory of laser-induced damage to optical coatings: dependence of damage threshold on physical parameters of coating and substrate materials

M. F. Koldunov, Alexander A. Manenkov, I. L. Pokotilo

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A problem of a laser induced damage to dielectric coatings due to thermal explosion of the absorbing inclusions is investigated. It is shown that major parameters governing damage characteristics of the coating are band gaps of coating and substrate materials. In particular, damage threshold dependences on a coating thickness and a position of the inclusion in the coating or on the coating-substrate interface are connected with an effect of this fundamental parameter. Experimental data are in qualitative agreement with theoretical predictions.

Pulse-width and pulse-shape dependencies of laser-induced damage threshold to transparent optical materials

M. F. Koldunov, Alexander A. Manenkov, I. L. Pokotilo

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Theory of pulsewidth dependence of laser induced damage threshold (LIDT) in transparent solids is presented. The damage is supposed to be initiated by thermal explosion of absorbing inclusions. The investigation of thermal explosion is based on an analysis of the heat transfer equation and a new approach to solving this equation is developed allowing to study kinetics of thermal explosion without any modeling presentation of an absorption mechanism. It is shown that the key parameter determining a dependence of LIDT upon a laser pulsewidth, (tau) _p, is the heat transfer time, (tau) , from an inclusion to a surrounding medium. At (tau) _p >> (tau) a damage threshold is characterized by a laser radiation intensity, whereas at (tau) _p << (tau) --by an energy density. The pulsewidth dependence of the LIDT has been investigated for rectangular and gaussian shapes of laser pulses and it has been established that the dependencies considerably differ in these two cases in a range of (tau) _p approximately (tau) . An effect of damage statistics, connected with a random spatial distribution of inclusions in a material, is also investigated. For the case of one-type inclusions (single-(tau) inclusions) it is shown: the statistics does not change a functional form of the pulsewidth dependence of the LIDT and correct only the LIDT values by a spot-size factor. Theoretical results are compared with experimental data published by different research groups for the laser damage in a nanosecond-picosecond region.

CVD-grown diamond: a new material for high-power CO2 lasers

Mathieu Massart, Piet Union, G. A. Scarsbrook, et al.

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In CO₂ laser engineering, combining high output power with low distortion of the laser beam is an ongoing challenge, leading to a search for optics with low absorption and high thermal conductivity. As CVD diamond has recently become available in larger sizes and with better surface quality, this material can now be assessed for use in high power CO₂ laser optics. This paper presents the systematic study of diamond as a substrate material for optics at 10.6 microns. CO₂-laser calorimetry has been used for the measurement of absorption of laser power in uncoated and antireflection coated diamond optics. The bulk absorption coefficient of natural and CVD diamond is more than a magnitude higher than that of ZnSe, however, a laser window needs to be antireflection coated, and this (together with the ability to use thinner windows of diamond because of its greater strength) reduces the increase in overall absorption for the window to about a factor of three (or approximately 7%). In high power applications this is more than compensated for by the substantially higher thermal conductivity of diamond. Laser induced damage threshold measurements have been made on antireflection coated diamond optics. These measurements have been performed using a TEA CO₂-laser with a peak pulse width of 150 ns at 10.6 microns, and the results are reported here.

Laser-induced damage to glass surfaces

Shao-Xian Meng, Xiaoqin Wang, Fuyi Guan, et al.

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Investigation of damage to glass surfaces by high power laser beams is presented in this paper. Surface damage thresholds for K8 glass induced by 200 ps laser pulse at near normal incidence are measured. The reason for the difference between the entrance and exit damage thresholds is briefly explained.

AFM-mapped nanoscale absorber-driven laser damage in UV high-reflector multilayers

Semyon Papernov, Ansgar W. Schmid, J. Anzelotti, et al.

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Submicrometer lateral-size craters that develop independently of the presence of micron-scale growth nodules and whose number density follows the intensity profile of the full laser beam are the dominant laser-damage features in 351-nm high-reflector production coatings on the University of Rochester's 60-beam OMEGA laser. Coupled with the observation that the smallest measured damage craters allow for initiating absorber sizes not larger than 10 nm, the experimental evidence points toward randomly distributed nano-cluster absorbers as the sources for energy transfer from the optical field to the porous film medium.

Nd:YAG laser-induced damage on ultrathin silicon samples

Wolfgang Riede, Jerome B. Franck

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The single shot laser induced damage onset and morphology of ultrathin silicon wafers is investigated using a Q-switched, single longitudinal and transverse mode Nd:YAG laser operating at 1.06 micrometers . The wafers had a thickness of 2.5 - 33 micrometers with identical front and back surface polish and <100> orientation. Comparisons are made for simultaneous front and back surface damage at the lowest level detectable surface modification. The morphology of laser damage on front and back surface due to the application of multiple shots on one site (N/1) at higher fluence values was also monitored. A scatter probe system consisting of a laser beam analyzer and a CCD array was used. It allowed the in-situ observation of the development of damage on the samples.

Analysis of high-resolution scatter images from laser damage experiments performed on KDP

Michael J. Runkel, Bruce W. Woods, Ming Yan, et al.

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Interest in producing high damage threshold KH₂PO₄ (KDP) and (D_xH_1-x)₂PO₄ (KD^*P, DKDP) for optical switching and frequency conversion applications is being driven by the system requirements for the National Ignition Facility (NIF) at Lawrence Livermore National Lab (LLNL). Historically, the path to achieving higher damage thresholds has been to improve the purity of crystal growth solutions. Application of advanced filtration technology has increased the damage threshold, but gives little insight into the actual mechanisms of laser damage. We have developed a laser scatter diagnostic to better study bulk defects and laser damage mechanisms in KDP and KD^*P crystals. This diagnostic consists of a cavity doubled, kilohertz class, Nd:YLF laser (527 nm) and high dynamic range CCD camera which allows imaging of bulk scatter signals. With it, we have performed damage tests at 355 nm on four different `vintages' of KDP crystals, concentrating on crystals produced via fast growth methods. We compare the diagnostic's resolution to LLNL's standard damage detection method of 100X darkfield microscopy and discuss its impact on damage threshold determination. We have observed the disappearance of scatter sites upon exposure to subthreshold irradiation. In contrast, we have seen scatterers appear where none previously excited. This includes isolated, large (high signal) sites as well as multiple small scatter sites which appear at fluences above 7 J/cm² (fine tracking). However, we have not observed a strong correlation of preexisting scatter sites and laser damage sites. We speculate on the connection between the laser-induced disappearance of scatter sites and the observed increase in damage threshold with laser conditioning.

Optical limiting and excited-state absorption in fullerene derivatives

Laura B. Smilowitz, Duncan W. McBranch, Victor I. Klimov, et al.

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We present measurements of intensity-dependent transmission (optical limiting), Z-scan, and time resolved excited-state absorption, for several fullerenes and fullerene derivatives in solution, and for Co thin films and porous glass composites. Specific derivatives investigated include the isomers 5,6 pheny1-C-butyñc acid cholesteryl ester (5,6 PCBCR) and 6,6 PCBCR. We report optical limiting results for several wavelengths in the visible and near-infrared. Excited-state absorption is measured from 500-1000 nm with <1 picosecond resolution. The spectral and temporal information about the excited state can be used in combination with ground state measurements of absorption cross section to predict optical limiting behavior due to reverse saturable absorption. These predictions are confirmed by intensity dependent transmission measurements made at different wavelengths. We find that fullerenes functionalized for optimized solubility display enhanced optical limiting properties.

Determination of the cohesive force between layers in multilayer materials using the laser adhesimeter

Boris Spesivtsev

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The method of determination of the adhesion in multi layer materials using impulse lasers is elaborated. The laser adhesimeter allows to measure the adhesion in laboratory conditions and in a process of getting the material without interrupting a technological process.

Laser damage studies of tantala and zirconia sol gel coatings

D. M. Spriggs, Paul A. Sermon, Mariana S. W. Vong, et al.

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Tantala and zirconia sol gel coatings, with simple preparation methods and high single layer laser damage thresholds at 1064 nm and 355 nm have been investigated. The sols are ethanol based and the coatings have high refractive indices and abrasion resistance. In situ microscopy and interferometric image suggest a thermal mechanism for damage at 1064 nm and inter- particle bonding, high strength and low porosity are linked to the high damage thresholds observed. The absence of exothermic reactions in the coatings at high temperatures and the closeness of absorption band edges to the laser wavelength are also considered to be factors influencing damage thresholds.

Comparison of nodular defect seed geometries from different deposition techniques

Christopher J. Stolz, Robert J. Tench, Mark R. Kozlowski, et al.

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A focused ion-beam milling instrument, commonly utilized in the semiconductor industry for failure analysis and IC repair, is capable of cross-sectioning nodular defects. Utilizing the instrument's scanning on beam, high-resolution imaging of the seeds that initiate nodular defect growth is possible. In an attempt to understand the origins of these seeds, HfO₂/SiO₂ and Ta₂O₅/SiO₂ coatings were prepared by a variety of coating vendors and different deposition processes including e-beam, magnetron sputtering, and ion beam sputtering. By studying the shape, depth, and composition of the seed, inferences of its origin can be drawn. The boundaries between the nodule and thin film provide insight into the mechanical stability of the nodule. Significant differences in the seed composition, geometry of nodular growth and mechanical stability of the defects for sputtered versus e-beam coatings are reported. Differences in seed shape were also observed from different coating vendors using e-beam deposition of HfO₂/SiO₂ coatings.

Influence of microstructure on laser damage threshold of IBS coatings

Christopher J. Stolz, Francois Y. Genin, Mark R. Kozlowski, et al.

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Multilayer coatings deposited by ion-beam sputtering with amorphous layers were found to have lower damage thresholds at 1064 nm than similar coatings with crystalline layers. Interestingly, at higher fluences the damage was less severe for the amorphous coatings. The magnitude of the difference in damage thresholds between the two different microstructures was strongly influenced by the size of the tested areas. To better understand the microstructure effect, single layers of HfO₂ with different microstructures were studied using transmission electron microscopy, ellipsometry, and a photothermal deflection technique. Since the laser damage initiated at defects, the influence of thermal diffusivity on thermal gradients in nodular defects is also presented.

Ultrashort-pulse optical damage

Brent C. Stuart, Michael D. Feit, Steve M. Herman, et al.

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Advances in high peak power short-pulse laser systems are currently limited by laser-induced damage to optical components by the intense short pulses. We have investigated the damage thresholds and mechanisms of pure dielectrics, and gold and multilayer-dielectric mirrors and diffraction gratings, with pulses ranging from 1 ns down to 0.1 ps. Theoretical modeling of the damage process is in quantitative agreement with measurements for both metals and dielectrics. In the dielectrics, we find a change in pulse width scaling of the threshold fluence near 20 ps, below which the excited electrons generated by multiphoton and avalanche ionization have insufficient time to couple their energy to the lattice during the pulse. For the shortest pulses the damage process becomes dominated by multiphoton ionization, leading to a very strong dependence on the electric field strength (extremely localized ablation) and a relative insensitivity to sample defects.

Multilayer optical coatings based on half-wave-layer pairs

Marshall Thomsen, Zhouling Wu

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Highly reflective coatings and polarizing beam splitters are generally designed with alternating layers of high and low index of refraction materials, with the thickness of each layer corresponding to an optical path length of a quarter of a wavelength. We present numerical results suggesting that the most important constraint is that the optical path length of a pair of layers should sum to one half of a wavelength, but within that constraint there is a fair amount of latitude to vary the proportions of the two layers without significantly affecting the coating's optical properties. Specifically, we suggest that in a hafnia/silica highly reflective coating, it may be possible to reduce the hafnia component significantly (by 20%) without compromising the reflectivity. Such a reduction could prove beneficial for damage thresholds since hafnia seems to be the primary source of seeds for nodule defects and hafnia's lower thermal conductivity increases the likelihood of thermal damage.

Effect of silica overlayers on laser damage of HfO2-SiO2 56 degrees incidence high reflectors

Christopher C. Walton, Francois Y. Genin, Robert Chow, et al.

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A series of hafnia/silica, oblique incidence (56 degree(s)), 1064 nm high reflectors (HRs) were prepared and coated with silica overlayers of varying optical thickness from (lambda) /2 to 4(lambda) in order to determine the effect of an overlayer on the laser-damage resistance of the HRs. The stress and laser damage thresholds for S and P polarization of the HRs were measured, and the damage sites for P polarization examined by Atomic Force Microscopy. All the multilayers were found to be in compression, with an intrinsic stress increasing with overlayer thickness. The presence of an overlayer and its thickness did not affect the damage threshold significantly. However, the presence of an overlayer greatly influenced the size and morphology of the damage. First, the overlayer prevented catastrophic `burns' of the hafnia top layer. Second, as the overlayer thickness increased, two distinct damage morphologies were found: jagged pits and round craters. The diameter of these pits and craters then increased somewhat with thicker overlayers. The depths of the pits and craters also increased with overlayer thickness, and the depths showed failure occurring at the interfaces below the hafnia layers. The side-wall angles of the craters were shallower with thicker overlayers, but there was no angle dependence for the pits. The craters showed fracture-like features and a small hillock or pit on their bottom surfaces. No correlation of damage morphology to conditioning or fluence was found.

UV-laser investigation of dielectric thin films

K. Ettrich, Holger Blaschke, Eberhard Welsch, et al.

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Utilizing thermal Mirage technique, UV laser damage resistivity studies on LaF₃/MgF₂, Al₂O₃/SiO₂, and HfO₂/SiO₂ multilayer stacks have been performed at (lambda) equals 248 nm, (tau) equals 20 ns. Investigating these stacks by changing the number of (HL) pairs and the substrate material, optical and thermal coating properties were shown to be responsible for UV single-shot laser damage. Similarly, the damage threshold of selected samples is to be influenced by the deposition technique. Furthermore, multishot damage measurements on LaF₃/MgF₂ high-reflecting multilayer coatings reveal the accumulation of laser energy in the predamage range.

Overview of photothermal characterization of optical thin film coatings

Zhouling Wu, Marshall Thomsen, Pao-Kuang Kuo, et al.

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Photothermal techniques are widely used in thin film characterizations and are particularly useful in studying laser-induced damage in optical coatings. The specific applications include measuring weak absorption, characterizing thermal conductivity, detecting local defects, as well as monitoring laser-interaction dynamics and determining laser damage thresholds. In this paper we take an overview of the principle of photothermal techniques, the various detection methods, and the progress made during laser decade in applying these techniques to optical thin films. Further potential and limitations of the techniques will also be discussed, with emphasis on in-situ studies of laser-interaction with thin films and local defects.

Impurity contamination in fast-grown KDP

Ming Yan, James J. De Yoreo, Natalia P. Zaitseva, et al.

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Potassium dihydrogen phosphate (KDP) has traditionally been used as a nonlinear optical material for frequency conversion to produce second and third harmonic radiation. A high laser induced damage threshold for KDP crystals is required for high power laser applications, such as laser fusion. High quality KDP crystals for such applications can be produced by a recently developed rapid crystal growth method. We report the results of an impurity contamination study in raidlygrown KDP crystals. Using absorption spectroscopy, we identified the impurity contamination in the different growth sectors of the crystals. We show that the level of contamination depends on the growth rate achieved during the rapid growth. The impurities observed by absorption spectroscopy are identified as the origin of lattice distortion and optical birefringence in the KDP crystals. The study of impurity incorporation during crystal growth is important for understanding the damage mechanism of KDP.

Stress and defect damage of CO2 laser optics: time of damage analysis

Wilfried Plass, Adolf Giesen

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Laser damage experiments in the long-pulse regime show for coated CO₂-laser optics two distinct damage mechanisms. These damage mechanisms can be distinguished by their time of damage behavior. The time of damage is measured with a previously presented setup, which was refined to improve the accuracy of the measurements. The two mechanisms are interpreted as induced by either defects or stresses in the coating. For the defect induced case, damage occurs at or before the peak fluence of the laser pulse, while for the stress induced case, the damage is observed laser. For both mechanisms, analytical transient heat flow calculations are discussed. While for the defect induced damage, a good thermal contact of the small defects to the host explains the observed behavior, for the stress induced damage a 1D heat-flow model of a film-substrate system is used taking into account for actual temporal profile of the laser pulse. The experimental data of coated metal mirrors as well as ZnSe- and Germanium optics are very well described by this model even if bulk material parameters are used for the film. Some samples show both damage mechanisms. In this case the defect induced LIDT is lower than the stress induced one. The separation of these mechanisms is useful for an effective improvement of optical coatings.

In-situ optical coatings on subsurface damage-removed substrate

Kunio Yoshida, T. Hirao, Tomosumi Kamimura, et al.

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The development of high power lasers requires highly damage-resistant optical coatings. Present multilayer dielectric coatings, do not have sufficient laser-induced damage thresholds (LIDTs) to pulsed lasers, particularly in the short wavelength region. LIDT strongly depends on the absorption coefficient of optical coatings and the impurities on the optical substrate. The absorption coefficient of optical coatings can be minimized by optimizing the deposition conditions. However, the impurities on the optical substrate cannot be completely removed by standard optical cleaning techniques. In this paper, the significant improvement of LIDT of optical coatings on subsurface-damage removed fused silica glass due to ion beam etching is presented.

CW/CO2 laser damage in optical components: thermal modeling and surface characterization

Rune Holm, Keith E. Puttick, Detlev Ristau, et al.

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This paper reports progress towards characterizing the laser induced damage threshold (LIDT) for zinc selenide and copper components in CW/C02 laser optical systems. The work has been undertaken by industrial partners and research centres supported by the CRAFT initiative of the European Commission. Specimen surfaces have been characterized by a wide range of experimental methods, including absorption calorimetry. Thermal modelling on the basis of these observations indicates that the temperature rise in irradiated components is dominated by surface absorption, and that the maximum temperature rise is proportional to the ratio of the total power P to focal spot size d rather than to the areal power density. A CW/C02 laser damage facility has been constructed by one of the industrial partners. Preliminary. experiments have determined P/d values at the LIDT an order of magnitude less than those predicted by the measured level of surface absorption.

Optically tuning a dichroic multilayer stack for a high-fluence laser application

Robert Chow, Gary E. Loomis, Camille M. Bibeau, et al.

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We report on the design and successful fabrication of a dichroic multilayer stack using a procedure that allowed shifting from high reflectance to high transmittance within 89 nm and surviving high laser fluences. A design approach based on quarter-wave thick layers allowed the multilayer stack to be optically tuned in the last layers of the stack. In our case, this necessitated removing the samples from the coating chamber for a transmittance scan prior to depositing the last layers. This procedure is not commonly practiced due to thermal stress- induced failures in an oxide multilayer. However, D. J. Smith and co-workers reported that reactive e-beam evaporated hafnia from a Hf source produced laser-resistant coatings that had less coating stress compared to coatings evaporated from a HfO₂ source. Tuned dichroic coatings were made that had high transmittance at 941 nm and high reflectance at 1030 nm. The coating was exposed for 5 minutes to a 100 kW/cm² 1064 nm (180-ns pulsewidth, 10.7 kHz) laser beam and survived without microscopic damage. The same coating survived a 140 kW/cm² of laser intensity without catastrophic damage before optical tuning was performed.

Comparison of ion exchange and cw CO2 laser treatment of Nd-doped phosphate laser glass

Gong Hui, Li Chengfu

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In recent years, the effect of laser pre-irradiation and ion exchange on glasses surface were widely carried out to stabilize their damage thresholds. But comparison of ion exchange and CW CO₂ laser treatment is never studied, this paper is devoted to the investigation of this question. Nd-doped phosphate laser glasses were heated with CW CO₂ laser radiation and were strengthened by ion exchange. Laser damage thresholds of the surface were measured with 1064 nm 10 ns pulses focused to small spots irradiation. Both ion exchange treatment and CW CO₂ laser treatment result in residual compress stress occurred at surface, peak-to- volley and microcracks decreased in surface appearance, and damage thresholds of surfaces increased by a factor of over 2. Polariscope, reflected optical microscope and atomic force microscope are used for stress, damage morphologies and surface topography analysis on glass surface. It is shown that laser condition mechanism is consistent with ion exchange treatment mechanism.

Structure, impurity composition, and laser damage threshold of the subsurface layers in KDP and KD*P single crystals

Vitaly I. Salo, L. V. Atroschenko, Serge V. Garnov, et al.

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The effect of the structure and degree of faultness of the subsurface layer which appears during the abrasive mechanical treatment of KDP and KD^*P single crystals on the value of their surface laser damage threshold was studied in this paper. It was shown that flattening of the relief polycrystalline layer, decrease of the impurity (Ca, Al, Mg, c, etc.) concentration introduced by the abrasive and supplementary materials, used for the mechanical treatment of single crystals, as well as lowering the concentration of dislocations lead to a significant rise of the surface resistance of the studied crystals to the laser radiation damage. However, at a most fine polishing the value of the surface laser damage resistance was not higher than 40% with respect to the corresponding value of the bulk material. Considered were typical patterns of the damage center on the surface of crystals exposed to the laser radiation in connection with the structure and depth of the damaged layer. It was found that the degradation of a strongly damaged surface under the effect of laser radiation is accompanied by the emergence of the burned or melted material areas with no crystallographically oriented cracks formed. The typical pattern the surface of the laser damage center with a clearly pronounced crystallographic orientation of cracks approaches in its view the bulk damage as the degree of faultness of the subsurface layer decreases.

Effect of ionizing radiation on the properties of KDP single crystals

Marina I. Kolybayeva, Viacheslav M. Puzikov, Igor M. Pritula, et al.

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The radiation induced chances in the optical absorption spectra or KDP single crystals and laser damage threshold value or the irradiated. at room temperature samples having essentially d.irrerent impurity composition (DP I and KDP II) have been considered in this paper. In the samples or the rirst type (JP I) the impurity content was 5.1O rnass% and. in those or the second. type (KDP II) their concentration was in the range or i 1O — I 1O rnass%. In the irradiated. crystals or both types there observed was a shirt or the short wavelength absorption ed into the visible part o the spectiim with a simultaneous emergence or the band. havirg the maximum at X = 0.26 .un. As it rollows rrom the obtained data the characters or the dependence of the absorption coericient value on the dose is derined. by the type of irradiation. It should. be. noted. that the absolute value or the absorption coerriclent or the mentioned. band is not adequate ror the crystals with d.irrerent content or impurities. Apart rrom the said. band. in the II — type crystals one may observe a siiiricant increase or the background absorption over the whole optical spectrum which is proportional to the radiation dose and visually revealed in the brown-red colouring or the irradiated samples.

Correlation between laser and mechanical strength of KDP-type crystals

L. V. Atroschenko, Marina I. Kolybayeva, Vitaly I. Salo, et al.

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Several mec1nisms o laser damage o waLer soluble KDP type crystals have been suggested by now, each of them corresponding to one or another aspect o a nonresonance interaction o radiation with material. However, no su.Uicient attention has been rocuseci on the correlation Q the laser damage threshold o crystals with other stn.icture — sensitive properties .The authors were rlrst to 1nc1 a correlation between laser and mechanical strength o DP and. KD*P crystals arid the way or their increase. The statistic processing of the results of measuring laser damage tbresholcl (W) and. fregile strength (a) for several tens of samples showed. a direct dependence between these two most important characteristics. For the KDP crystals doped with lead the rise of a I .5 times Is accompanied by the increase of W 3.7 times as compared with the undopeci ones. The crystals doped with chromium demonstrated the decrease of W almost by an order at a simu.ltaneou.s lowering of a I .7 times. The isothermal annealing of crystals near the temperature of the phase transition from the tetragorial modification to the rnonoclinic one results in both W and. a rise. An explanation of the mechanical strengthening of the material and. its correlation with laser damage threshold Is given in this paper.

Beam profiling at 30 Hz for absolute fluence measurement

Gary T. Forrest

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The original SensorPhysics beam profiler introduced in 1990 has been upgraded to utilize standard computers and cameras to capture 640 X 480 images at 30 Hz. These images are analyzed to calculate the peak fluence (J/cm2), beam diameter, total energy, per cent saturation, peak location, centroid location and beam area for a series of pulse. The maximum, minimum and average values are calculated as are the values for the averaged image. Incomplete data is automatically excluded. Data is displayed in a variable sized pane under Windows^TM 3.11. The LaserTest utilizes a combined VGA-framegrabber- memory card to acquire sequential pulses at full resolution at 30 Hz. A SensorPhysics high peak power pyroelectric detector is being integrated into the Windows^TM based platform to allow simultaneous measurement of laser energy on every pulse. A similar approach has been implemented on a self contained notebook computer platform.

Laser material test results from Russian high-power carbon monoxide laser

Ray O. Johnson, Boris M. Dymshits, G. V. Ivanov, et al.

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Researchers from the United States and Russia conducted laser-material interaction tests at the LOK Company, St. Petersburg, Russia. These tests were conducted using a one-of-a-kind, continuous wave, supersonic, e-beam-sustained carbon monoxide (CO) laser. The purpose of these tests were to characterize the laser while performing collaborative research between scientists from Russia and the United States. Additionally, the testing verified previously- reported laser characteristics. All planned laser-material interaction tests were successfully conducted. Several material samples were irradiated by the CO laser to allow calculation of the laser energy and power levels. Statistical errors were reduced by testing materials with different characteristics at varying laser energy and power levels. Laser-material interaction tests were also conducted at varying distances from the laser output window to assess beam quality and divergence.

Ion-assisted deposition of optical coatings

Zhengxiu Fan, Rui-Ying Fan, Xuefei Tang

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Ion assisted deposition technique and effects of this technique on properties of optical coatings are investigated in this paper. We have produced thin-film samples by bombarding with different ions such as oxygen, argon and mixture of the two during deposition, of which the refractive index, the extinctive coefficient and the laser induced damage threshold are analyzed in detail.

Improving the laser resistance of Al mirror and Si window by depositing dielectric protection layers

Quingchun Zhao, Zhengxiu Fan

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From two aspects: speeding up the heat conduction and reducing the absorption of the incidence laser energy, we studied the detailed means to improve the laser resistance of aluminum mirror and silicon window by depositing dielectric films and discussed its' feasibility. In the laser induced damage test by 1.06 micrometers IR laser and copper vapor laser, the damage thresholds of samples prepared by depositing dielectric protection layers have all been improved. It, in turn, verifies the feasibility of our protecting technology.

Effect of interface absorption on temperature field of optical coatings

Quingchun Zhao, Zhengxiu Fan

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On the basis of full consideration to interface absorption in optical coatings, we work out a model of simulative interface absorption layer. We make calculation then analysis to the temperature field of kinds of multilayers. The real-time result by photothermal displacement technology for laser induced damage to samples supports our model.

Measurement of nonlinear index by a relay-imaged top-hat z-scan technique

Tsutomu Shimada, Norman A. Kurnit, Mansoor Sheik-Bahae

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Measurement of the nonlinear index of a number of materials of interest for the National Ignition Facility have been performed at 1064 nm and 355 nm by a modified version of the `top-hat' Z-scan technique and the results compared with the more standard gaussian-beam Z- scan technique. The top-hat technique has the advantages of higher sensitivity and smaller uncertainties introduced by beam-quality considerations. We have made what we feel to be an additional improvement by placing the defining aperture for the top hat at the front focal plane of the lens that focuses the beam into the sample and then reimaging the input aperture with a second lens onto a ccd camera. Reimaging eliminates diffraction fringes and provides a stationary image even for a wedged sample; recording the entire image permits minimization of spurious effects such as varying interference fringes.

Peculiarities of the dynamics of the relief induced by two-beam laser interference on silicon surface near the melting threshold

Mikhail N. Libenson, Vladimir S. Makin, Yu. I. Pestov, et al.

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To study the material mechanisms of laser-induced silicon microrelief formation the periodic space-modulated heating up to melting point was used. On the basis of measurements of dynamics of surface thermal radiation and of the relief height the conclusion about material mechanisms of relief formation had been made.

27th Annual Boulder Damage Symposium: Laser-Induced Damage in Optical Materials: 1995

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