Proceedings Volume 2966

Laser-Induced Damage in Optical Materials: 1996

Harold E. Bennett, Arthur H. Guenther, Mark R. Kozlowski, et al.
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Proceedings Volume 2966

Laser-Induced Damage in Optical Materials: 1996

Harold E. Bennett, Arthur H. Guenther, Mark R. Kozlowski, et al.
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 13 May 1997
Contents: 7 Sessions, 80 Papers, 0 Presentations
Conference: Laser-Induced Damage in Optical Materials: 1996 1996
Volume Number: 2966

Table of Contents

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Table of Contents

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  • Materials and Measurements
  • Surfaces and Mirrors
  • Thin Films
  • Fundamental Mechanisms
  • Mini-Symposium: Laser-Induced Damage in Optical Fibers
  • Thin Films
  • Fundamental Mechanisms
  • By Title Only
  • Materials and Measurements
  • Thin Films
  • Fundamental Mechanisms
  • Thin Films
  • Fundamental Mechanisms
  • Materials and Measurements
  • Fundamental Mechanisms
  • Materials and Measurements
  • Thin Films
  • Surfaces and Mirrors
  • Thin Films
  • Surfaces and Mirrors
  • Fundamental Mechanisms
  • Materials and Measurements
  • Surfaces and Mirrors
  • Fundamental Mechanisms
  • Related Optical Fiber Contributions
  • Fundamental Mechanisms
  • Materials and Measurements
  • Thin Films
  • By Title Only
  • Thin Films
  • Fundamental Mechanisms
  • Materials and Measurements
  • Related Optical Fiber Contributions
  • By Title Only
  • Thin Films
  • Fundamental Mechanisms
  • Thin Films
  • Materials and Measurements
  • Thin Films
  • Materials and Measurements
  • Thin Films
  • Materials and Measurements
  • Thin Films
  • Materials and Measurements
  • Fundamental Mechanisms
  • Surfaces and Mirrors
  • Fundamental Mechanisms
  • Thin Films
  • Surfaces and Mirrors
  • Thin Films
  • Fundamental Mechanisms
  • Thin Films
  • Fundamental Mechanisms
  • Related Optical Fiber Contributions
  • Surfaces and Mirrors
  • Thin Films
Materials and Measurements
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Development of high-power solid state laser for inertial fusion energy driver
Kunio Yoshida, Masanobu Yamanaka, Masahiro Nakatsuka, et al.
The design study of the laser fusion power plant KOYO has been conducted as a joint program of universities, national laboratories, and industries in Japan and also with international collaborations. In the design of KOYO, the gain scaling of direct drive implosion with 0.35 micrometers wavelength laser light is used. A driver of diode pumped solid state laser generates 4 MJ/pulse with 12 Hz and the output pulses are switched to deliver the laser energy success five to four chambers, which operate with 3 Hz. The chamber wall is protected with thick liquid metal which flows down in a SiC woven tube. Following to the conceptual design study, the critical key issues which may affect the technical and economical and economical feasibility of the commercial power plant KOYO have been examined. Research and development of some key technologies have been performed. As the results of the studies on KOYO, it is concluded that thee technical and economical feasibility of laser fusion reactor is well in our scope to reach.
Surfaces and Mirrors
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Laser-induced damage and fracture in fused silica vacuum windows
Jack H. Campbell, Patricia A. Hurst, Dwight D. Heggins, et al.
Laser induced damage, that initiates catastrophic fracture, has been observed in large, fused silica lenses that also serve as vacuum barriers in high-fluence positions on the Nova and Beamlet lasers. In nearly all cases damage occurs on the vacuum side of the lens. The damage can lead to catastrophic crack growth if the flaw size exceeds the critical flaw size for SiO2. If the elastic stored energy in the lens in high enough, the lens will fracture into many pieces resulting in an implosion. The consequences of such an implosion can be severe, particularly for large vacuum systems. Three parameters control the degree of fracture in the vacuum barrier window: (1) the elastic stored energy, (2) the ratio of the window thickness to flaw depth and (3) secondary crack propagation. Fracture experiments have ben carried our on 15-cm diameter fused silica windows that contain surface flaws caused by laser damage. The results of these experiments, combined with data from window failures on Beamlet and Nova have been sued to develop design criteria for a 'fail-safe' lens. Specifically the window must be made thick enough such that the peak tensile stress is less than 500 psi and the corresponding ratio of the thickness to critical flaw size is less than 6. Under these conditions a properly mounted window, upon failure, will break into only tow pieces and will not implode. One caveat to these design criteria is that the air leak through the window before secondary crack growth occurs. Finite element stress calculations of a window before and immediately following fracture into two pieces show that the elastic stored energy is redistributed if the fragments 'lock' in place and thereby bridge the opening. In such cases, the peak stresses at the flaw site can increase leading to further crack growth.
Thin Films
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Progress in optical coatings for the midinfrared
Keith L. Lewis, Andrew M. Pitt, M. Corbett, et al.
There has been a growing interest in new laser systems for use at mid-IR wavelengths, driven by requirements for a number of diverse applications. Developments in mid-IR coating technology are following three major thrusts. In the first, the potential of ion-assisted process is being exploited and the technology is already becoming well established in the optical coating industry. In the second, the benefits of using gas-phase precursors are being explored, driven by the potential of avoiding the generation of second phase inclusion in the coatings with associated reduction in laser damage threshold.Research is also addressing the use of nanocomposite materials with an engineered microstructure to produce coatings with dielectric properties of choice. This paper presents a review of progress in some of these areas with special emphasis on the comparison of the properties of materials at DF laser wavelengths. Evidence is presented which suggests that laser damage thresholds at 3.8 micrometers are determined by residual hydroxyl-related absorption in the films. Ion- assisted processes do not necessarily provide the best way forward for the highest damage thresholds.
Fundamental Mechanisms
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Ultrafast laser-induced structural changes in semiconductors
Eric Mazur, Eli N. Glezer, Li Huang, et al.
We present experimentally determined values of the dielectric function of GaAs following femtosecond laser excitation. The data at photon energies of 2.2 and 4.4 eV show that the response of the dielectric function tot he excitation is dominate by changes in the electronic band structure and not by the optical susceptibility of the excited free carriers. The behavior of the dielectric function indicates a drop in the average bonding-antibonding splitting of GaAs following the excitation, which leads to a collapse of the band gap. The changes in the electronic band structure results from a combination of electronic screening, many-body effects, and structural deformation of the lattice caused by the destabilization of the covalent bonds. Broadband measurement of the dielectric function over the range 1.5-3.5 eV reveals ultrafast laser-induced heating at low fluence, disordering at intermediate fluence, and an ultrafast semiconductor-to-metal transition at high fluence.
Mini-Symposium: Laser-Induced Damage in Optical Fibers
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Overview of power delivery and laser damage in fibers
Fiber optic delivery systems are an important component of high power laser systems used for applications ranging from laser surgery to industrial cutting and welding. Conventional silica fiber optics may be used to deliver Nd:YAG laser powers in excess of 2,000 W whereas lasers operating beyond 2 micrometers require non-silica fiber optics. Specifically, IR transmissive fiber optics for Er:YAG or CO2 laser beam delivery require the use of special fiber optics such as sapphire, fluoride or chalcogenide glass, or hollow waveguides. The optical and mechanical properties of these unique guides and their power handling capability are reviewed along with those of silica fibers operating in the UV and near IR wavelength region of the spectrum.
IR laser power transmission through silver halide crystals and polycrystalline fibers
Lev Nagli, D. Burstein, Shaul Shalem, et al.
Laser-induced breakdown (LIB) thresholds in AgClxBr1-x crystals and fibers were studied under CO2 laser pulses and CW excitation. The value of LIB threshold Pc of the bulk crystals is about 7.2 X 108 W/cm2 for AgCl and 4 X 109 W/cm2 for AgBr under 60 ns TEA laser excitation. The LIB threshold in fibers is much smaller; about 2 X 108 W/cm2. The absorption of the crystals at 10.6 micrometers changes from 8 X 10-5 cm-1 for AgCl to 2 X 10-5 cm-1 for AgBr; for fibers with the same composition absorption is much greater. The dependence of the LIB and IR absorption on composition, and mechanical and temperature treatments suggests that the LIB in silver halide crystals and fibers is due to the avalanche electrons in a high electric field. The initial free electrons for this process are supplied by ionization of the cation vacancy - charged dislocation complexes. The relatively lower optical stability of the fibers is due to the increased concentration of defects formed in the hot extrusion of the crystal. A simple thermal annealing method for reduction of the IR absorption is proposed.
Mid-IR high-power transmission through chalcogenide fibers: current results and future challenges
Lynda E. Busse, John A. Moon, Jasbinder Singh Sanghera, et al.
We present current results and discuss future potential for laser power transmission through chalcogenide fibers in the 2-11 micrometers wavelength region. Results include both high average power and high peak power laser transmission. Multiple pulse laser induced damage thresholds are presented for polished glass surfaces, bulk glass and fiber endfaces. Important issues for optimum power delivery are discussed, including antireflection coatings on fiber endfaces and a method for smoothing the fiber output spatial distribution.
Possibilities and limitations of optical fibers for the transmission of excimer laser radiation
Karl-Friedrich Klein, Georg Hillrichs, Peter Karlitschek, et al.
For fiber-delivery systems with UV-lasers the candidates are mainly optical fibers with an undoped high-OH silica core and a F-doped silica cladding. However, there re there important limits to UV-applications: surface damage, two- photon-absorption and defect-generation during operation. In the last two years, UV-improved fibers with significantly reduced defects have been developed. This improvement is most pronounced at 248 and 193 nm, because at these critical wavelengths the induced losses are strongly influences by the main broadband UV-defects with absorption maxima at 165 nm and 215 nm. We will summarize the results including the influence of the main parameters. In addition, the transmission capacity for the 308 nm wavelength is of interest due to medical and industrial applications. At this wavelength the influence of the nonlinearities is much lower; however, the induced losses in standard fibers are still an important factor. To show the advantages of the UV- improved fibers, the transmission characteristics at 308 nm wavelength will be described in more detail, for the first time.
Investigations on fibers for high-average-power Nd:YAG lasers for industrial applications
Norbert Reng, Thomas Beck, Martin Ostermeyer
For industrial applications of high-average power Nd:YAG- lasers the laser power is usually transmitted through all- silica optical fibers. The transmission properties of different types of step index and graded-index fibers are investigated, using a multimode high-power Nd:YAG rod laser with 2000 W output power in CW and Q-switch mode. The fibers are step index and graded-index fibers with 400 and 600 micrometers core diameters, different cladding to core ratios and different types of coating materials. The dependence of the output beam parameters, waist diameter and divergence, and the resulting power transmission are given. The upper limits for the maximum beam parameters and maximum laser powers which can be coupled into fibers per limits for the maximum beam parameters and maximum laser powers which can be coupled into fibers without loss, as well as the dependence of the output beam profiles upon the intensity distribution at the fiber input are briefly discussed. The end faces of polished and cleaved fibers are compared. Different set-ups for coupling several laser beams into one fiber, in order to increase the maximum laser power to be transmitted, are discussed. By these means more than 6kW laser power could be transmitted.
Heat-flow modeling and visualization of catastrophic self-propagating damage in single-mode optical fibers at low powers
Raman Kashyap, Andre H. Sayles, G. F. Cornwell
Damage to optical fibers by 'self-propelled self-focusing' occurs at relatively low optical powers. This phenomenon is modeled using a finite-element technique. The description is based on an induced fiber-absorption change with temperature which has been verified experimentally. A lumped absorption of a few percent used in the model gives good agreement with measurements of the velocity of damage-propagation.
Comparative evaluation of fiber fuse models
Donald D. Davis Jr., Stephen C. Mettler, David J. DiGiovanni
A phenomenon which results in the catastrophic destruction of the guiding properties of an optical fiber has been observed at laser power densities 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 start when the end of the fiber is contacted. It has also been initiated spontaneously from mechanical splices. This paper reports experimental data gathered on the fiber fuse, and discusses their relationship to proposed physical mechanisms.
Thin Films
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Stress and environmental shift characteristics of HfO2/SiO2 multilayer coatings
J. F. Anzellotti, Douglas J. Smith, Robert J. Sczupak, et al.
HfO2/SiO2 polarizer coatings for 1054 nm have been produced that have low stress at explicit environmental conditions without the employment of backside stress- compensation films. In this process hafnia is condensed from a metallic melt and silica from an oxide source, both via electron-beam evaporation. Specifically, this process has been adopted for multilayer designs with stringent requirements on spectral control and wavefront distortion. Efforts to meet these requirements have prompted various investigations of coating stress and spectral behavior, especially under changing environmental conditions. Results have shown that coating stress and optical thickness vary significantly with humidity. THese quantities have been measured under both ambient air and dry nitrogen atmospheres. The effects of coating parameters on stress and environmental stability have been examined for an experimental hafnia/silica polarizer coating. The aforementioned parameters are hafnia deposition rate, oxygen pressure during hafnia deposition, and oxygen pressure during silica deposition. Results indicate a strong correlation of coating stress to oxygen pressure during the silica evaporation. Data on the aging of stress in hafnia/silica coatings will also be presented. The HfO2/SiO2 process has ben utilized in high-laser-damage- threshold coatings for the OMEGA laser system and for National Ignition Facility development coatings at the Laboratory for Laser Energetics.
Fundamental Mechanisms
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Comparison of the damage frequency method and the binary search technique
This paper presents a direct comparison of the two major laser damage threshold measurement techniques, namely the binary search technique and the damage frequency method. The estimates of the laser damage threshold produced by each technique are compared using three different probability functions. A simple damage defect ensemble model for the damage probability is used as a control. The other probability distributions represent non-ideal cases and have tails in the region near threshold. It is shown that in the latter cases that both test procedures have difficulty in accurately and precisely reporting the threshold. The paper concludes that neither technique as presently formulated is sufficiently accurate or precise enough for applications requiring high confidence in the threshold measurement.
Spatial fluence profile for certification test of laser damage resistance: call for comments
This paper presents a technique for certifying the laser power handling capability of a laser optic. This paper is being presented so that community comments can be collected and incorporated before the issuance of a draft international standard based in the technique defined in the paper in 1997. The paper derives the curve for the probability of the optic surviving a series of exposures to laser irradiation as a function of the laser fluence profile, the number of damage initiation sites on the optic and the fraction of the test optic irradiated at or above a specific level. Of general interest to the community is the discussion of the proper fluence profile for the certification test.
By Title Only
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Investigation of the influence of particle surface emission on the laser damage of optical materials
Valerie A. Astapenko, V. F. Pershin, Oleg E. Sidoryuk, et al.
The analysis of the under-threshold phenomena in the surface layer of the lithium niobate under intense laser irradiation was carried out. The accumulation effect leading to the laser damage was considered in the framework of the influence of the particle emission from the investigated sample surface on the electronic processes in the surface layer. The model of the oxygen depletion of the surface due to the intense nanosecond laser pulses is suggested. The influence of the absorbed surface layer on this process is considered. This model is in agreement with the experimental data of the laser damage threshold dependence on the oxidizing/reduction properties of the environment.
Materials and Measurements
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Guest-host optical limiters with high-laser-damage threshold
Mark C. Brant, Michael E. De Rosa, Hao Jiang, et al.
Conventional guest-host optical limiting materials utilize either a liquid solvent or solid as the matrix for nonlinear absorbing chromophore dopants. Concentration gradients of the chromophore in the matrix can improve optical limiting performance. However, low viscosity liquid solutions can not retain a concentration gradient while polymer solid matrices damage at low laser fluences. We report on a novel approach of using an elastic polymer and viscoelastic gels for guest- host optimal limiting matrices. We achieve high bulk laser damage thresholds in the hosts and maintain a concentration gradient of the chromophore. By softening the epoxy we significantly enhance its bulk laser damage threshold. We characterize this effect by measuring the damage threshold as a function of viscoelastic properties. In addition, optical limiting was demonstrated in all the hosts doped with nonlinear phthalocyanine chromophores.
Thin Films
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High reflector absorptance measurements by the surface thermal lensing technique
Robert Chow, John R. Taylor, Zhouling Wu, et al.
Surface thermal lensing is an alternate configuration of a photothermal deflection system that was used to measure low levels of optical absorption. The thermal lensing configuration facilitated the alignment of the pump and prove laser beams by using a larger diameter probe beam. This technique was applied to high performance optical coatings, specifically high reflectors at 511 nm, zero degrees angle of incidence. The absorptance of these coatings was previously measured using a high power copper vapor laser system. A high power copper laser beam is focused onto an approximately 2 mm diameter spot. A thermal camera senses the temperature rise with respect to the rest of the coating. The temperature change, power density and beam diameter were used with an empirical formula that yields optical absorption. The surface thermal lensing technique was able to resolve absorption levels lower than that achieved with the copper laser method.
Fundamental Mechanisms
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Arrest of self-focusing collapse with quadratic nonlinearities
Alfredo DeRossi, Gaetano Assanto, Stefano Trillo, et al.
Since the early work of Kelley in 1965, self-focusing in transparent dielectric materials was recognized as one of the mechanisms of optical induced damage in laser materials. Recently the experimental demonstration of stable solitary waves in Quadratic materials has shown that the catastrophic collapse of laser beams can be arrested in materials with a pure second order response. New ways to solve the self- focusing damage mechanism in laser materials can result from such an experimental discovery. Indeed choosing host laser materials with a quadratic nonlinearity can considerably delay the appearance of the laser beam collapse. We present theoretical considerations and numerical calculations on materials design concepts which can in practice resolve the self-focusing collapse in the presence of a quadratic nonlinearity.
Thin Films
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Nano absorbing centers: a key point in the laser damage of thin films
Jean DiJon, T. Poiroux, C. Desrumaux
In order to investigate a possible origin of absorbing centers responsible for laser damage of thin films in the UV, the laser-induced damage threshold of metal implanted bare SiO2 substrates is studied. Post-implantation thermal annealing of the samples is associated with a large change in the laser damage morphology, a decrease of the absorption at 355 nm as well as a decrease in the measured threshold values compared with non-annealed samples. All these phenomena are understood considering that nanoclusters of metals are produced during annealing. Such clustering is very likely to occur due to the thermodynamical properties of the metal dielectric system. The same behavior observed on thin films enables the proposition to be made that the same kind of clusters exist on thin films before any post- deposition process.
Fundamental Mechanisms
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Role of self-focusing in laser-induced breakdown of water caused by nano- and picosecond pulses
Oleg M. Efimov, Andrei M. Mekryukov, Vladimir S. Popikov, et al.
An experimental investigation of laser-induced breakdown of water and subthreshold phenomena using pulsed laser radiation in the nanosecond to picosecond region has been performed. It has been shown that self-focusing and suspended particles have a strong influence on laser-induced breakdown of water. A significant decrease in transmittance for an input irradiance 1 to 2 orders of magnitude less than the laser-induced breakdown threshold of water as observed, as well as a strong spot size dependence of this threshold. Besides the investigated processes result in a breakdown threshold of water for small spot sizes that is nearly 10 times larger than that of glass due to a sharp increase in scattering from inhomogeneities formed as a result of self- focusing. For large spot sizes the breakdown threshold of water is considerably less than for the same glasses due to breakdown initiated by suspended particles.
Materials and Measurements
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Photoinduced processes in silicate glasses exposed to IR femtosecond pulses
Oleg M. Efimov, Kai M. Gabel, Serge V. Garnov, et al.
The optical properties variation in silicate glasses after exposure to high-power femtosecond laser radiation at 0.85 micrometers have been studied. The laser spectral line broadening leading to the supercontinuum generation in visible and UV spectral regions was observed in all studied glasses. Color center generation and intrinsic luminescence were found in boro- and alkali-silicate glasses. It is believed that these processes result from linear and/or two-photon absorption of the short-wavelength part of this supercontinuum which causes glass matrix ionization. No color center absorption in the visible region was observed in fused silica at irradiances up to the laser damage threshold. It was concluded that there is no significant ionization of fused silica under exposure to IR femtosecond laser pulses with irradiance below laser induced damage threshold.
Pulse-length dependence of absorptance and degradation rate of fused silica at 248 nm
Eric Eva, Klaus R. Mann, Stephan Thomas
Optical components for industrial excimer laser applications such as semiconductor lithography steppers have to withstand more than 1010 laser pulses without suffering from significant cumulative absorption increase due to color center formation. Lithography lasers are now approaching pulse durations of 10 ns and repetition rates of 1000 Hz, a domain unexplored in previous degradation studies. With the aid of an optical pulse-extension unit and a modified laser resonator, we measured absorptance and absorption changes in Suprasil 312 UV-grade fused silica calorimetrically at pulse lengths of 7-73 ns. At 100 mJ/cm2, no degradation was detectable even after 500,000 pulses, but there was slight evidence of an absorption decrease due to laser cleaning. At 400 mJ/cm2, both absorptance and degradation rate increased towards shorter pulses and hence higher intensities, but even over the full pulse-length range, the changes remained moderate. Furthermore, degradation at 300 Hz was lower than at 150 Hz, indicating that a simultaneous bleaching effect might be involved which favors future high- repetition rate applications.
High-resolution calorimetric absorption measurements on optical components for excimer lasers
Eric Eva, Klaus R. Mann
In the past, we presented several works showing that laser calorimetry can be applied in the UV-spectral range with greatly enhanced sensitivity compared to transmission measurements. We have now finished developing an improved laser calorimeter which is two orders of magnitude more temperature sensitive than our previous setup. This added resolution can be employed to measure absorptance as a function of irradiation parameters such as energy density or pulse repetition rate over an even broader range. In particular, the irradiation energy dose per measurement can be kept low enough to allow cumulative laser-induced absorption increase to be monitored with very high resolution. We used the new setup to cast some light on the scaling laws of absorptance in fused silica as a function of the ample thickness. It was possible to distinguish between surface and bulk processes.
Fundamental Mechanisms
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Modeling of laser damage initiated by surface contamination
Michael D. Feit, Alexander M. Rubenchik, Douglas R. Faux, et al.
We are engaged in a comprehensive effort to understand and model the initiation and growth of laser damage initiated by surface contaminants. This includes, for example, the initial absorption by the contaminant, heating and plasma generation, pressure and thermal loading of the transparent substrate, and subsequent shockwave propagation, 'splashing' of molten material and possible spallation, optical propagation and scattering, and treatment of material fracture. The integration use of large radiation hydrodynamics codes, optical propagation codes and material strength codes enables a comprehensive view of the damage process. On the entrance optical surface, small particles can ablate nearly completely. In this case, only relatively weak shockwaves are launched into the substrate, but some particulate materials may be left on the surface to act as a diffraction mask and cause further absorption. DIffraction by wavelength scale scattering centers can lead to significant intensity modulation. Larger particles will not be completely vaporized. The shockwave generated in this case is larger and can lead to spallation of contaminant material which then may be deposited in the substrate. A gaseous atmosphere can lead to radiation trapping with concomitant increases in temperature and pressure near the surface. In addition, supersonic ionization waves in air may be generated which greatly extend the plasma plume spatially and temporally. Contaminants on the exit optical surface behave differently. They tend to heat and pop off completely in which case significant damage may not occur. Since plasma formed at the interface of the optic and absorbing particle is confined, much stronger pressures are generated in this case. Imaging of contaminants resulting in 'writing' a diffraction pattern on the exit surface due to contamination on the entrance surface has been observed experimentally and predicted theoretically. Such imprinted damage regions can seed damage from subsequent pulses.
Laser intensity modulation by nonabsorbing defects
Nonabsorbing defects can lead to laser damage. Defects such as voids, microcracks and localized stressed concentrations, even if they differ from the surrounding medium only be refractive index, can serve as positive or negative lenses of the incident laser light. The resulting interference pattern between refracted and diffracted light can result in intensity increases on the order of a factor of 2 some distance away from a typical negative microlens, and even larger for a positive microlens. Thus, the initial damage site can be physically removed from the defect which initiates damage. The parameter that determines the strength of such lensing is (Ka)2 (Delta) (epsilon) where the wave number K is 2(pi) /(lambda) , 2a is the linear size of the defect and (Delta) (epsilon) is the difference in dielectric coefficient between matrix and scatterer. Thus, even a small change in refractive index results in a significant effect for a defect large compared to a wavelength. Geometry is also important. 3D as well as linear and planar microlens can all have strong effects. The present paper evaluates the intensification due to spherical voids and high refractive index inclusions.
Microstructural and dielectric susceptibility effects on predictions of dielectric properties
Kim F. Ferris, Gregory J. Exarhos, Steven M. Risser
In modeling the dielectric properties of inhomogeneous materials, the treatment of the electric field interaction s differentiate the usual modeling formalism and their accuracy. In this paper, we show that the performance of effective medium methods is dependent upon a number of variables - defect concentration, alignment, and the dielectric constant of the material itself. Using our previously developed finite element model of an inhomogeneous dielectric, we have developed models for a number of dielectric films of varying dielectric constant and microstructures. Alignment to of defects parallel to the applied field and the larger defect aspect ratios increase the overall dielectric constant. The extent of these effects is dependent on the dielectric constant of the bulk component.
Materials and Measurements
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Pulsed laser damage on ZnS optics
Christophe Hubert-Habart, Enrique Fontana, Vincent Baudinaud
AEROSPATIALE Corporate Research Center conducts since 1989 experimental works on the interaction of lasers on optical materials. Those works are part of an approach to understand physical mechanisms leading to the observed damages. Several means of diagnostic have been developed to study the pulse duration and wavelength dependence of those processes. The damages observed in the nanosecond and the millisecond regime can be explained by a change in the mechanism. This observation is based on morphological examinations of the damage and on measured pulse duration and wavelength dependence. Furthermore, a numerical analysis of the temperature in the samples has been computed to strengthen this observation.
Thin Films
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Characterization of optical coatings: damage threshold/local absorption correlation
Anne Fornier, C. Cordillot, D. Bernardino, et al.
Simultaneous absorption and scattering mappings are used to study the influence of local defects on the laser damage threshold of thin films. The same area is mapped for absorption an d scatter before and after irradiation at the threshold fluence. The study is performed for an uncoated fused silica substrate and two single-layer films deposited on fused silica substrates at a wavelength of (lambda) equals 1.06 micrometers . Initial results seem to indicate that the irradiation can create and enhance absorption and scattering defects.
Characterization of HR coatings for the megajoule laser transport mirrors
Anne Fornier, C. Cordillot, D. Bernardino, et al.
One of the concerns with the megajoule laser design is the laser-induced damage threshold of the transport mirrors. Earlier studies have shown that the main constraint on the laser damage threshold comes from nodules at the mirror surface. It is therefore important to restrict the number of such nodules. SFIM-ODS, in close collaboration with CEL-V, has initiated a special study to characterize these nodules as precisely as possible. The objective of the study is twofold: (1) to determine the origin of the nodules and subsequently to adapt the mirror fabrication process in order to limit their formation, (2) to analyze their shapes and dimensions in order to ascertain which nodules are critical for laser-induced damage. To understand the origin of the nodules and their effect on the laser damage threshold, the mirrors are characterized using various methods: (1) absorption and scatter mapping: does the presence of nodules result in specific absorption patterns, (2) surface analysis by atomic force microscopy: to characterize nodule shape and dimensions, (3) focused ion beam cutting of nodules: to locate the seed initiating the nodule and to characterize the seed shape and composition, (4) laser damage threshold measurements: to determine the laser damage threshold of them mirror and study the behavior of nodules under laser irradiation depending on their dimensions and shape.
Surfaces and Mirrors
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High-temperature measurements of reflectivity and heat capacity of metals and dielectrics at 1064 nm
Serge V. Garnov, Vitali I. Konov, Olga G. Tsarkova, et al.
The information on optical and thermal properties of materials at high temperatures is the base for any theoretical analysis of laser matter interaction processes. To evaluate the absolute value of energy deposited in the interaction area and to estimate the laser damage threshold one need possessing the knowledge on reflection/absorption coefficients and heat composition, irradiation and environmental conditions, surface and bulk temperature etc. the task of its direct numerical calculation becomes very difficult and the obtained theoretical estimations are unreliable. Up to now the only way of get the reliable values of the parameters mentioned is the experimental procedure. Among the other experimental techniques developed for measuring reflection/absorption coefficients at high temperatures, the based on an integrating sphere method is the most suitable, reliable and accurate one. The installation for high sped, high temperature measurements of optical and thermal properties of metals and dielectric materials is described. The system includes a specially designed photometric sphere, a chopped CW:Nd:YAG laser with output power up to 250W, which heats samples and simultaneously serves as a probing beam, and a high speed optical pyrometer. The set-up is fully computerized. The real temperature of targets is determined by a simultaneous measuring of brightness temperature and reflectively at about the same wavelengths. Different metals, ceramics laser active media - Al2O3:Ti3+ single crystals, and diamond CVD films have been investigated in air, vacuum and argon atmosphere at heating rates of 103-104 K/sec. The obtained data on high temperature optical and thermal properties of materials can be used at surface laser damage processes modeling.
Laser-induced damage of fused silica at 355 and 1064 nm initiated at aluminum contamination particles on the surface
Francois Y. Genin, K. Michlitsch, James Furr, et al.
Contamination particles of controlled size and shape were deposited onto 1.14 cm thick fused silica windows by sputtering Al through a mask. The particles were 1 micrometers thick circular dots, 10 to 250 micrometers in diameter. Al shavings were also deposited on the windows to investigate the effects of particle-substrate adhesion. The silica windows were then illuminated repetitively using a 3-ns, 355 nm and an 8.6-ns, 1064 nm laser. The tests were conducted at near normal incidence with particles on the input and output surfaces of the window. During the first shot, a plasma ignited at the metal particle and damage initiated on the fused silica surface. The morphological features of the damage initiated at the metal dots were very reproducible but different for input and output surface contamination. FOr input surface contamination, minor damage occurred where the particle was located; such damage ceased to grow with the removal of contaminant material. More serious damage was initiated on the output surface and grew to catastrophic proportions after few shots. Output surface contaminants were usually ejected on the initial shot, leaving a wave pattern on the surface. No further damage occurred with subsequent shots unless a shot cracked the surface; such behavior was mostly observed at 355 nm and occasionally for large shavings at 1064 nm. The size of the damaged area scaled with the size of the particle. The onset of catastrophic damage on the output surface occurred only when particles exceeded a critical size. The damage behavior of the sputtered dots was found to be qualitatively similar to that of the shavings. The artificial contamination technique accelerated the study by allowing better control of the test conditions.
Thin Films
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Effect of electric field distribution on the morphologies of laser-induced damage in hafnia-silica multilayer polarizers
Francois Y. Genin, Christopher J. Stolz, T. A. Reitter, et al.
Hafnia-silica multilayer polarizers were deposited by e-beam evaporation onto BK7 glass substrates. The polarizers were designed to operate at a wavelength of 1064 nm at Brewster's angle. The polarizers were tested with a 3-ns laser pulse at 45 degrees, 56 degrees, and 65 degrees incidence angle in order to vary the electric field distribution in the multilayer, study their effects on the damage morphology, and investigate the possible advantages of off-use angle laser conditioning. The morphology of the laser-induced damage was characterized by optical and scanning electron microscopy. Four distinct damage morphologies were observed. These damage morphologies were found to depend strongly on the angle of incidence of the laser beam. In particular, massive delamination observed at 45 degrees and 56 degrees incidence, did not occur at 65 degrees. Instead, large and deep pits were found at 65 degrees. The electric field distribution, the temperature rise and the change is stress in the multilayer were calculated to attempt to better understand the relationship between damage morphology, electric field peak locations, and maximum thermal stress gradients. The calculations showed a two-fold increase in stress change in the hafnia top layers depending on the incidence angle. The stress gradient in the first hafnia- silica interface was found to be highest for 45 degrees, 56 degrees, and 65 degrees, respectively. Finally, the maximum stress was deeper in the multilayer at 65 degrees. Although the limitations of such simple thermal mechanical model are obvious, the results can explain that outer layer delamination is more likely at 45 degrees and 56 degrees than 65 degrees and that damage sites are expected to be deeper at 65 degrees.
Growth of laser-induced damage during repetitive illumination of HfO2/SiO2 multilayer mirror and polarizer coatings
As designers want to increase the peak fluence of high power laser, it becomes necessary to tolerate some damage on mirrors and polarizers. To quantify how the different types of damage morphologies initiate and grow during repetitive illumination, hafnia-silica multilayer mirror and polarizer coatings were laser damage tested. The coatings were prepared by e-beam evaporation and irradiated with a 3-ns- pulse at 1064 nm. The morphology of laser-induced damage was recorded after each shot to determine the damage threshold. The results of the tests were summarized on damage stability maps plotting the average damage size as a function of the number of shots for fluences ranging from 10 to 40 J/cm2. The maps indicate that the commonly observed damage morphologies have distinct growth behaviors and influence the value of the functional damage threshold differently. While pits are stable up to fluences as high as 40 J/cm2, flat bottom pits can grow during repetitive illumination above a critical fluence of about 35 J/cm2. Scalds are formed in the first shot and never grow at fluences below 40 J/cm2. Finally, delaminates are highly unstable and have the potential for damaging the coating catastrophically above 15 J/cm2. The results show that the delaminate damage morphology should be prevented. This knowledge has allowed coatings developments efforts to focus on eliminating the origin of such damage morphology.
Surfaces and Mirrors
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Near-field optical microscopy: application to investigation of surface laser-induced damage of transparent optical materials
Vitali E. Gruzdev, Mikhail N. Libenson, George A. Martsinovsky, et al.
Surface laser-induced damage of optical materials presents a major fundamental problem of light-matter interaction. The nature of surface optical damage is usually related to the presence of pre-existing and laser-induced inhomogeneities in optical properties of the material or inhomogeneities of the laser beam. SEM, STM, AFM and other techniques used to investigate the optical surface inhomogeneities do not provide direct measurements of the surface optical properties with high spatial resolution. Near-field optical microscopy (NFOM) has principal advantages compared to the mentioned techniques being applied to the investigation of defects characteristics and kinetics of evolution of surface laser-induced damage of optical materials: (1) NFOM provides optical data measurements at the wavelength of the damaging radiation with about 30 nm resolution; NFOM combined with AFM gives a topographical image of surface simultaneously with the optical data; (2) NFOM provides spectroscopy information; (3) NFOM technique provides measuring space field distribution induced by small surface and near-surface defects.
Fundamental Mechanisms
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New model for high-power electromagnetic field instability in transparent media
A model of high-power field instability is developed to describe local abrupt increasing of electromagnetic field intensity in transparent dielectric. Small local enhancement of the field amplitude is initiated by low-absorbing spherical inclusion which size is less than radiation wavelength. Exceeding threshold of optical bistability results in abrupt increasing of field amplitude in the defect that also leads to local increasing of field amplitude in the host material in the vicinity of the inclusion. Bearing in mind nonlinear dependence of refractive index of the host material on light intensity we develop a model to describe 'spreading' of initial defect up to size appropriate for the first resonant field mode to be formed. Increasing of refraction index due to nonlinear light-matter interaction and existence of high-Q eigenmodes of dielectric sphere can both cause positive feedback's and result in field instability in the medium. Estimates are obtained of the threshold value of incident-field amplitude. This model is applied to analysis of some experimental data on optical breakdown of transparent dielectrics and thin films, both impurity-induced and intrinsic. The model can also be used to consider bulk optical breakdown by tightly focused light beams.
Materials and Measurements
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Damage thresholds in laser-irradiated optical materials
Franck Guignard, Michel L. Autric, Vincent Baudinaud
An experimental study on the damage induced by laser irradiation on different materials, borosilicate glass, fused silicate, molded and stretched polymethylmethacrylate (PMMA), has been performed. The irradiation source is a 1KL pulsed cold cathode electron gun preionized TEA CO2 laser. Damage mechanisms are controlled by the in-depth absorption of the 10.6 micrometers radiation according to the Beer-Lambert law. PMMA is damaged following a boiling process. Stretched PMMA is fractured first, releasing stresses, then boiled like molded PMMA at a higher energy. BK7 crazed after the irradiation due to thermomechanical stresses, silicate melt and vaporized. Optical damages have been characterized by measuring the contrast transfer function through the irradiated samples.
Surfaces and Mirrors
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High-strength optical glasses for high-fluence laser applications
Suresh T. Gulati
Fused silica spatial filter lenses for high-fluence lasers like Nova and Beamlet can experience optical damage on the vacuum side if the tensile stress due to pressure differential exceeds the threshold level. The damage is initiated by slow crack growth and culminates into catastrophic crack growth when the stress intensity approaches the critical value. If the elastic energy released by lens flexure, which is proportional to square of the stress, is high the lens can implode into several fragments causing further damage to associated equipment. This paper presents an alternate approach for designing and manufacturing high strength optical glasses which will not experience tensile stresses in excess of threshold level. Consequently, lenses made from these glasses will not be subjected to slow crack growth and their laser-damage threshold will be significantly higher. Furthermore, the lens thickness can be reduced by 50 to 70 percent thereby improving its optical performance over a wide range of wavelengths. The trade-offs between strength, lens thickness and stored elastic energy are discussed.
Fundamental Mechanisms
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Toward predicting the laser damage threshold of large-area optics
Jean Hue, Francois Y. Genin, Stephen M. Maricle, et al.
As the size of optics has increased, so has the difficulty in effectively measuring and defining their laser damage threshold. This is exemplified in the case of optical coatings being developed for the National Ignition Facility in the USA and the laser megajoules in France. Measuring the threshold on small witness samples rather than on the full aperture optic presents obvious advantages. In this article, the threshold of large-area components is being addressed in two general ways that both use experimental mapping data. First, a model based on the fruitful concept of the R-on-1 threshold distribution is shown to predict the threshold of a large optic with a high degree of confidence. At the same time, it is determined that the average R-on-1 threshold provides a reliable and accurate value to evaluate the coating improvements. To acquire the essential data, an automated damage test bench has been developed by the laboratory of 'Couches minces pour l'Optique' at the CEA. Secondly, the damage threshold has to be defined according to the final use of the component. To address this issue, the Lawrence Livermore National Laboratory has defined a functional damage threshold to set limits on the maximum damage size. An empirical power law dependence of average damage size on peak fluence was found. This relation can be used to predict the damage behavior of large-aperture optics exhibiting the same damage morphology.
Related Optical Fiber Contributions
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Suppression of solarization effects in optical fibers for 266-nm laser radiation
The creation of color centers in the Si-O network of a fused silica fiber by UV laser photons leads to solarization and hence to decreasing optical UV-transmission of the fiber. So fibers are only of little use for UV laser applications. We now found a way to improve the fiber properties by reduction of solarization. The transmission behavior of these modified fibers for 266 nm pulsed laser radiation has been studied. Spectroscopic experiments give some insight into the creation processes and the repair dynamics of the color centers. The influence of the OH-content in the fiber was also investigated. Based on this work, it is possible to realize fiber optic beam delivery systems for frequency quadrupled Nd:YAG lasers and for other high power UV laser sources. These results may also be useful for the improvement of bulk fused silica for UV applications.
Fundamental Mechanisms
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Two-photon spectroscopy of free-carrier generation in wide-band gap crystals
Sergei M. Klimentov, Serge V. Garnov, Alexander S. Epifanov, et al.
Multiphoton absorption is able to produce primary free electron concentration, launching avalanche ionization mechanism in the transparent solids. We applied transient photoconductivity technique to investigate free carrier generation, caused by two-photon absorption in the alkali halide undoped crystals. Using radiation of tunable optical parametric oscillator-amplifier, the excitation spectra and temperature dependencies for conduction band electron concentration were measured. Two variations of experiment included optical excitation by 'equal' quanta and by combination of tunable radiation with a fixed wavelength laser source. As it was shown, an efficient free electron producing was observed not only for interband optical transitions, but also for two-photon exciton absorption at room temperature. An analysis of the data obtained and comparison to conventional photoconductivity spectra indicated thermal ionization of the exciton states in the temperature range above 300 K. The corresponding exciton activation energy in KI was estimated and found to be consistent with the location of energy levels, allowed for two-photon exciton transitions. The results obtained qualify transient photoconductivity and as an effective tool for investigation of multiphoton excitation and following electron transitions in the wide-band gap crystals.
Wavelength dependence of laser damage in coatings
M. F. Koldunov, Alexander A. Manenkov, I. L. Pokotilo
A model of laser-induced damage in optical coatings due to photoionization thermal explosion of absorbing inclusions proposed and analyzed in our previous work reasonably explained dependencies of the laser induced damage threshold (LIDT) on a material band-gap energy and a coating thickness. Continuing the analysis of the model in this paper we present results on a laser radiation wavelength dependence of LIDT. It is shown hat his dependence is determined mainly by two factors: wavelength dependencies of inclusion absorption cross section and absorption coefficient of conduction band electrons. A computer analysis done taking into account both factors. Effects of coating thickness and damage statistics are also investigated. Theoretical results are compared with experimental data.
Laser-induced damage criterion
M. F. Koldunov, Alexander A. Manenkov, I. L. Pokotilo
An adequate choice of a damage criterion has a key significance for development of a laser induced damage (LID) theory in transparent dielectrics. In this connection a comparative analysis of damage criteria applicable to different damage mechanisms is done in this paper. A consistent formulation of a thermoelastic damage criterion is given and one of the most important LID characteristics - the pulsewidth dependence of the LID threshold is obtained. An applicability range of the damage criterion and the pulsewidth dependence are discussed. Theoretical results for the pulsewidth dependence are compared with available experimental data and a good agreement is found in a very wide pulsewidth range from nanoseconds to femtoseconds.
Two-dimensional thermoelastic wave propagation
Peter Meja, Bernhard Steiger
The photothermal and photoacoustical effects can be described by means of thermo-elastic wave equations. However, analytical solutions describing those processes in optical components consisting of several optical coatings are not known. We have applied the local interaction simulation approach in order to simulate the generation and propagation of thermo-elastic waves in homogeneous and layered 2D media. A sharp interface model was applied at each layer interface. A few selected examples of thermo- elastic wave propagation in 2D are presented.
Materials and Measurements
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Adaptation of the ISO/DIS 11551 to absorptance measurement of low thermal conductors
Peter Meja, Ulrike Broulik, Ute Pfeifer, et al.
The ISO/DIS 11551 is valid only for materials with high thermal conductivity. The application of this standard to the calorimetric measurements of sample with low thermal conductivity like glass results in large deviations between the measured and the actual values of absorptance. These deviations depend on the applied methods of analysis and on several parameters. In this paper the influence of irradiation time and detection place is presented. Those influences are described theoretically and compared with the experimentally results, where a good agreement were achieved. It will be shown that by using a modified standard the absorptance measurement of samples with low thermal conductivity is possible.
Thin Films
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Comparative studies on titanium and tantalum oxide thin film structures of laser mirrors deposited by an ion-assisted gun
Shigetaro Ogura, Qi Tang
For obtaining radiation less damageable laser mirrors, a preliminary optimization of film fabrication suitable for the analysis of laser damage mechanism has been done as the first step. Here, the optimization requires not only the stable fabrication process but also the ideal film structure i.e., the amorphous and smooth film structure simultaneously, eliminating latently unwanted secondary effects such as light scattering during laser damage test. For this purpose, we adopted the ion assisted deposition method and modified the deposition conditions for titanium and tantalum oxide films, both of which compose typical high index layers, and where SiO2 layers are also chosen as low index layers because of their amorphous and smooth nature, in alternative multilayer laser mirrors. Surface and cross sectional film structures and film crystallinity are compared and characterized, using a high resolution SEM and a x-ray diffractometer, respectively. The fabrication process and the film structures obtained even by IAD method for tantalum pentaoxide lack stability similar to those films by a conventional EB method. The fabrication process for titanium oxide films are stable even by a conventional EB method, but it produces the columnar film morphology and rough film surface and layer boundary simultaneously. Therefore, further optimization of titanium films, has been done by IAD method to improve the film morphology, introducing new fabrication parameters such as the determination of the best starting materials among various titanium oxides, adopting the new mixed gases of Ar and oxygen, suitable input power for ion gun, and arrival ratio of the ionized vapor ions. Then the structure of titanium oxides, adopting the new mixed gases of Ar and oxygen, suitable input power for ion gun, and arrival ratio of the ionized vapor ions. Then the structure of titanium oxide films deposition conditions of the ratios of oxygen and argon gases from 1:1 to 1:2 and ion power of 500 to 750 V during the titanium oxide deposition at the substrate temperature of 100 to 150 degrees C. Otherwise, the conventional EB and IAD methods with)2 gas only produce the columnar film structures and rougher boundaries than those by IAD with the mixed Ar + O2 gases. Related crystallinity and optical properties, such as refractive index, absorption, and index inhomogeneity, are also reported.
Heat transfer from localized absorbing defects to the host coating material in HfO2/SiO2 multilayer systems
Semyon Papernov, Ansgar W. Schmid
Finite-element analysis was applied in calculating the heat- transfer effects from localized absorbing defects to the host coating material in multilayer HfO2/SiO2 system, irradiated by 1-ns pulses. Thermoconductivity was considered the major heat-transfer mechanism. A comparison with high-resolution laser-damage morphology studies carried out by means of atomic-force microscopy allowed estimating peak temperatures reached by the absorbing defects. These peak-temperature values as well as the lateral length scales of the empirically found laser-damage features point toward the importance of radiative energy transfer from the heated defect as the driving mechanism of laser-damage initiation.
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Annealing of KDP crystals in vacuum and under pressure
Igor M. Pritula, Marina I. Kolybayeva, Vitaly I. Salo, et al.
The effect of the high temperature annealing on the absorption spectra and laser damage threshold of KDP crystals was studied in the present paper. The experiments on isothermal annealing were performed under pressure in the atmosphere with specific properties. The composition of the atmosphere was selected to be chose to that of the desorbing gas component determined during annealing in vacuum. The mentioned conditions allowed to conduct annealing in the temperature range of 230-280 degrees C without a degradation of the sample. The variations in the absorption spectra showed that the effect of the annealing is most strongly revealed in the short-wave region of the spectrum. The real decrease of the absorption coefficient in the samples after annealing demonstrate that at temperatures approximately 230-280 degrees C. The processes ensuring the improvement of the structure quality are stimulated in the volume of the crystals. These conclusions are also confirmed by the data on laser damage threshold of KDP crystals.
Thin Films
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Light scatter from high reflectors deposited by reactive dc magnetron sputtering
David W. Reicher, J. Sobczak, James Paul Black, et al.
Scatter from optical thin film coatings is a significant problem for high power laser optics. Theoretical aspects of the problem have ben well explored for scattering due to surface roughness. Over the past twenty years the surface roughness of optics has been significantly reduced. Improvements in optical surface fabrication and film deposition techniques have progressed to the point that even for complex coatings, surfaces of less than 1nm rms roughness are routinely achievable. As the surface roughness of optics decreases, bulk scatter, rather than topographic scatter, may be the major scatter source in these smooth surfaces. Atomic force microscopy can profile the surface of a coating with atomic resolution. By comparing the power spectral density (PSD) derived from the surface profile with the PSD derived from angle resolved scatter measurements, some conclusions can be reached on this question. Data from analysis of dual wavelength high reflectors deposited by reactive DC magnetron sputtered Nb2O3/SiO2 and ZrO2/SiO2 structures will be presented, allowing analysis of results for these film materials.
Fundamental Mechanisms
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Status of optics on the OMEGA laser after 18 months of operation
The 60-beam OMEGA laser has sustained approximately 1000 target shots without significant damage to the optics. Approximately 3000 optics on the OMEGA laser system were closely monitored during their installation, and inspections continue throughout the operation of the system. A review of the condition of these optics at each stage of the laser and a summary of the peak incident fluences are presented. The most severe damage on OMEGA is seen on the input, fused- silica, spatial filter lenses. Since these optics are under vacuum, inspection of damaged lenses occurs on a more frequent cycle to track the growth of the defect and to maintain the system's safety. An optic is replaced well before massive failure is expected to occur. Other optics on the system that exhibit different types of damage are BK7 spatial filter lenses, focus conversion crystals, primary pickoff lenses, calorimeters, and liquid-crystal optics. Laser glass and development optics such as distributed phase plates are not covered in this review.
Materials and Measurements
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Effect of low-angle quasi-particles on the laser damage threshold of KDP single crystals
Vitaly I. Salo, V. F. Tkachenko, M. A. Rom
The thin structure of KDP crystals was studied using multicrystal x-ray diffraction analysis of high resolution and two-crystal topography for reflection. KDP crystals grown from aqueous solutions are characterized by the impurity striated structure caused by the fluctuation of the growth processes, the latter results in the nonuniform penetration and distribution of impurities and formation of low angle turns during a layer-by-layer growth. Low angle quasi-boundaries separating the strips of the normal growth are perhaps of a nondislocational origin. A qualitative correlation between the concentration of low angle quasi- boundaries in the crystal and the value of bulk laser damage threshold has been found.
Related Optical Fiber Contributions
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Optimized fiber delivery system for Q-switched Nd:YAG lasers
Interest in the transmission of high intensities through optical fibers is being motivated by an increasing number of applications. Using different laser types and fiber materials, various studies are encountering transmission limitations due to laser-induced damage processes. For a number of years we have been investigating these limiting processes during the transmission of Q-switched, multimode, Nd:YAG laser pulses through step-index, multimode, fused- silica fiber. We have found that fiber transmission is often limited by a plasma-forming breakdown occurring at the fiber entrance face. This breakdown can result in subtle surface modifications that leave the entrance face more resistant to further breakdown or damage events. Catastrophic fiber damage can also occur as a result of a variety of mechanisms, with damage appearing at fiber entrance and exit faces, within the initial 'entry' segment of the fiber path, and at other internal sites due to fiber fixturing and routing effects. System attributes that will affect breakdown and damage thresholds include laser characteristics, the design and alignment of laser-to-fiber injection optics, and fiber end-face preparation. In the present work we have combined insights gained in past studies in order to establish what thresholds can be achieved if all system attributes can be optimized to some degree. Our multimode laser utilized past modifications that produced a relatively smooth, quasi-Gaussian profile. The laser-to-fiber injection system achieved a relatively low value for the ratio of peak-to-average fluences at the fiber entrance face, incorporated a mode scrambler to generate a broad mode power distribution within the initial segment of the fiber path, and had improved fixturing to insure that the fiber axis was collinear with the incident laser beam. Test fibers were from a particular production lot for which initial-strength characteristics were established and a high-stress proof test was performed. Fiber end faces were prepared by a careful mechanical polishing schedule followed by surface conditioning using a CO2 laser. In combination, these factors resulted in higher thresholds for breakdown and damage than we had achieved previously in studies that utilized a simple lens injection system. Probability distribution functions were fitted to the threshold data, providing a means for estimating the probability for transmission failure at lower laser energies.
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Application of the erosion-plume-products-induced gas discharge for the registration of optical material surface laser damage
The possibility of the application of the erosion plume products induced gas discharge for the registration of the laser damage of the optical materials is considered. The method is shown to be convenient for the application in the automatic sample testing installations.
Thin Films
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Investigation of the degradation of antireflection coatings of nonlinear crystal under the action of high-average-power laser radiation
Oleg E. Sidoryuk, O. D. Volpyan, A. A. Zilov
The analysis of the local heating role in the processes of the degradation of the antireflection coatings of the nonlinear crystals when they have been used in the CW and quasi-CW YAG laser was carried out. The broad sample range of the oxide, nitride and fluoride films deposited on the LiNbO3, BaNaNbO5, LiO3, KTP and BBO crystals was considered. Investigation of these samples was carried out under the intense heating of their surfaces by the action of the CO2 laser pulses. The analysis of the obtained results with account for the chemical reaction thermodynamics, thermotension and other factors is presented.
High laser-induced damage threshold polarizer-coatings for 1054 nm
Douglas J. Smith, J. F. Anzellotti, Semyon Papernov, et al.
Polarizer coatings developed for the OMEGA laser are performing well without sustaining any significant damage. Similar polarizers developed for the National Ignition Facility have exceptionally high damage thresholds when tested with a 1-ns pulse at 1054 nm. Polarizers for OMEGA were originally developed using Ta2O5/SiO2 multilayers. All final polarizers before the frequency conversion cell were made using this method. A new coating was developed for a polarizing beamsplitter with more stringent optical and laser-damage requirements. The new coating used a HfO2/SIO2 system with the hafnia formed by reactive evaporation from a hafnium metal melt. The new process provided better film control, lower defect counts, better stress control, and higher damage thresholds. Beamsplitter coatings made from both processes were installed in the OMEGA laser. After 1.5 years of operation the Ta2O5/SiO2 beamsplitters are developing signs of damage on OMEGA while the HfO2/SiO2 coatings show no damage. The HfO2/SiO2 process was also used to develop polarizer coatings for the NIF. Damage- threshold results from 1-on-1 testing will be presented for both types of polarizers. Experimental results show that the coating damage threshold is not strongly dependent on deposition parameters, allowing use of these parameters to control film stress. The damage thresholds are higher for s- polarized incident light, and different damage morphologies for the two polarizations have been observed. A base layer of scandium oxide that allows the coating to be safely stripped does not affect the polarizer damage threshold.
Mechanism of thin layer damage due to collision of laser-induced near-surface shock waves
Boris Spesivtsev, Serguei Gribine, Viktor Assaoul, et al.
The problem of laser induced damage for thin layer on surface of condensed medium is considered. Specific mechanism of the damage at the place between light absorbing inhomogeneities is discussed in terms of irradiated collision of shock waves induced by micro explosions of the irradiated inhomogeneities. Collision of shock waves stimulates air ionization and absorption for laser irradiation. The process could lead to microexplosion far from initial microexplosions. Models for the phenomena includes: (1) estimation of shock wave parameters form single microexplosion; (2) thermodynamics parameters of air inside region of two shock wave collision; (3) estimation of layer damage.
Fundamental Mechanisms
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Deposit of shock wave generation mechanism into size effect
Boris Spesivtsev, Serguei Gribine, A. Efremov
Level of light damage threshold for bulk and layer glass is considered versus diameter of laser beam. Mechanism of absorbed energy dissipation through shock and pressure waves is used for interpretation of the phenomena. Estimations on base of simple model are in agreement with experiments.
Thin Films
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Laser damage studies of 351-nm HR silica-zirconia sol-gel coatings
D. M. Spriggs, James E. Andrew, Nicholas J. Bazin
Zirconia and silica sol gels have been spin and dip coated on 50mm diameter fused silica substrates to produce 351nm HR multilayer stacks and very high laser damage thresholds. Multilayer coating design software has been used to estimate the refractive indices of individual layers. A spectrophotometer has been used to evaluate the absorption coefficient of the zirconia layers in the UV region. The coatings were typically found to craze after about 6 to 9 layer pairs had ben deposited. Laser damage thresholds up to 8J/cm2 have been measured using a frequency tripled Nd:YAG laser at 351nm and 0.7ns pulse duration using constant and ramped fluence sequences. The coatings and laser damage morphologies have been evaluated using optical, interferometric and electron microscope techniques.
Effect of SiO2 overcoat thickness on laser damage morphology of HfO2/SiO2 Brewster's angle polarizers at 1064 nm
Christopher J. Stolz, Francois Y. Genin, T. A. Reitter, et al.
HfO2/SiO2 Brewster's angle polarizers are being developed at Lawrence Livermore National Laboratory for the National Ignition Facility. Damage threshold studies using a 3-ns pulse length 1064-nm laser have revealed a number of different damage morphologies such as nodular ejection pits, plasma scalds, and overcoat delaminations. Of these laser damage morphologies, delaminations have the most negative impact on fusion laser performance. By increasing the thickness of the SiO2 overcoat, the delamination morphology is eliminated without significantly modifying the spectral characteristics of the coating. A model of the thermal mechanical response of the overcoats is presented for various SIO2 overcoat thicknesses. The overcoat thickness influences the electric-field profile resulting in different thermal gradients between the outer SiO2 and HfO2 layers. This modeling effort attempts to understand the relationship between the thermal stress distribution in the overcoat and the occurrence of delamination.
Materials and Measurements
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KrF laser-induced absorption in synthetic fused silica
Stephan Thomas, Bodo Kuehn
In order to estimate the long-term behavior of synthetic fused silica under 248 nm irradiation, KrF laser beam transmission has been measured on 40 cm long samples. Due to this long optical path length, the measuring sensitivity for induced absorption is high enough to apply relatively low pulse energy densities in the range from 100 mJ/cm2 down to 15 mJ/cm2. The maximum number of pulses was 750 million with a repetition rate of 1000 Hz. The damage behaviors of the two synthetic fused silica types Suprasil 2 and Suprasil 312 differ markedly. Suprasil 2 exhibits a rapid damage process at the beginning of irradiation followed by a slower linear absorption increase afterwards and the 'strong absorption transition'. Induced absorption in Suprasil 312 increases s-shape like during irradiation reaching a plateau level for higher number of pulses. We describe both damage behaviors with phenomenological model equations. These are used to extrapolate to parameter ranges, that can not be investigated easily.
Thin Films
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Integrated optic modulator and splitter damage at 1053 nm
Russell B. Wilcox, Donald F. Browning
We are designing and developing a single mode fiber laser and modulation system for use in an inertial confinement fusion research laser, the National Ignition Facility (NIF). Our fiber and integrated optic oscillator/modulator system generates optical pulses of around 30 nanoseconds duration, at one kilohertz, with up to 500 nanojoules of energy. This is enough to potentially damage some of the single mode fiber and waveguide components. To test these components, we have built a test system using a diode-pumped Nd:YLF laser, producing 10 microjoules in 120 nanoseconds at 500 hertz. This system has been used to test commercial lithium niobate integrated optic modulators, silica-on-silicon waveguide splitters, lens-coupled dichroic mirror splitters, and other fiber optic components. We present results of damage tests and efforts to improve performance.
Materials and Measurements
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Investigations of laser damage in KDP using light-scattering techniques
Bruce W. Woods, Michael J. Runkel, Ming Yan, et al.
Interest in producing high damage threshold KH2PO4 (KDP) and (DxH1-x)2PO4 (DKDP) for frequency conversion and optical switching applications is driven by the requirements of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory. At present only the best crystals meet the NIF system requirements at the third harmonic and only after a laser conditioning process. Neither the mechanism for damage in bulk KDP nor the mechanism for conditioning ins understood. As part of ta development effort to increase the damage thresholds of KDP and DKDP, we have been developing techniques to pinpoint the locations where damage will initiate in the bulk material. After we find these locations we will use other measurement techniques to determine how these locations differ from the other surrounding materials and why they cause damage. This will allow crystal growers to focus their efforts to improve damage thresholds. Historically, damage thresholds have increased it is believed as a consequence of increased purity of the growth solution and through the use of constant filtration during the growth process. As a result we believe that damage is caused by defects in the crystals and have conducted a series of experiments using light scatter to locate these defects and to determine when and where damage occurs. In this paper we present results which show a low correlation between light scatter from bulk defects in KDP and the initiation sites for damage. We have also studied the effects of thermal conditioning on light scatter, strain induced birefringence and damage threshold. We have seen evidence that regions of high strain also exhibit lower damage threshold than the surrounding lower strain material. When thermally conditioned, these crystals show a decrease in some of the strong linear scattering features and a decrease in the strain birefringence while the damage threshold in these regions increased to that of the surrounding bulk material.
Thin Films
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Temperature field design of optical thin film coatings
Zhengxiu Fan, Qian Zhao, Zhouling Wu
Thermodynamics of thin film coatings under laser irradiation is a fundamental process for laser-induced studies as well as many other relevant applications, including optical data storage and laser processing/laser conditioning. Temperature distribution within a thin film coating depends on not only the coating materials but also its design. When the design of the thin film coating changes, a variation of the temperature field will emerge. Based on this understanding the idea of optimizing the temperature field is introduced in the design of optical thin films for high power laser applications. By analyzing photothermal process, the temperature distribution within a thin film multilayer, together with required optical performance, is used as the major criterion for the design. The results of optimized design for a high reflective coating and a photothermal functional coating are shown as examples to demonstrate the temperature field design idea.
Materials and Measurements
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Investigation of impurities and laser-induced damage in the growth sectors of rapidly grown KDP crystals
Ming Yan, Richard A. Torres, Michael J. Runkel, et al.
We report the experimental results of impurity contamination and laser-induced damage investigations on rapidly grown potassium dihydrogen phosphate (KDP) crystals. Using absorption spectroscopy and chemical analysis, we determined the impurity distribution in the different growing sectors of KDP single crystals. The level of impurity was dependent on the starting materials and growth rate. We also studied the influence of impurities on the laser-induced damage in fast grown KDP. The laser damage threshold in the impurity- rich prismatic sector is same as in the high purity pyramidal sector within the experimental error. Meanwhile, the laser damage threshold (LDT) at the boundary of the prismatic and pyramidal sectors is less than half of that in the bulk. Furthermore, we found that the thermal annealing of the crystal eliminated the weakness of this sector boundary and increased its LDT to the same level as in the bulk of the crystal. Our result suggests that laser damage occurred in the vicinity of a high, localized strain field.
Thin Films
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Effects of thin film production parameters on laser-induced damage in optical coatings
Qian Zhao, Zhengxiu Fan, Hong Qiu, et al.
Laser induced damage in optical coatings is a complicated problem, which is attributed to a lot of factors, including environment parameters, laser parameters and thin film production parameters. Among the three factors, production parameters interest us most. We produce a series of samples with different roughness levels of the substrates, thin film material and vacuum environment within the chamber, etc. Using 1.06 micrometers Nd:YAG laser as our test laser, we make 1- on-1 laser damage test to these samples. Some interesting phenomena are found. We calculate the laser induced local heating within the thin film multilayers, and make analysis to the experimental phenomena. Conclusions are given.
Materials and Measurements
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Pyroelectricity and its role in optical damage of potassium titanyl phosphate crystals
D. Wayne Cooke, Bryan L. Bennett, Ross E. Muenchausen, et al.
The origin of optical damage in potassium titanyl phosphate (KTP) crystals has been vigorously investigated since its introduction as a nonlinear optical material in 1976. It is well known that this material exhibits a laser damage threshold that limits its use in many high average-power applications, especially frequency doubling of Nd-doped lasers. Both photochromic and electrochromic damage can be induced in KTP. Until recently, it was thought that these two types of damage were distinctly different, possibly involving different mechanisms; however, new data show that electrochromic-like damage can be induced in KTP by laser irradiation only, implying the existence of an internal electric field.We have recently observed bursts of light when heating KTP crystals at 0.1-1.0 K/s in the temperature range 8-675 K. The scintillations correspond to molecular nitrogen emission occurring during the electrical breakdown of air near the crystal surface, and imply the existence of pyroelectric fields in KTP exceeding 30 kV/cm. These fields wee induced by 10.6 micrometers laser irradiation. The observation of pyroelectric effects, heretofore not considered in KTP damage models, provides an important new insight into the possible cause of the recently observed 'electrochromic- like' photochromic damage in KTP.
Fundamental Mechanisms
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Ultrafast laser-induced microexplosions in transparent materials
Eli N. Glezer, Li Huang, Richard J. Finlay, et al.
Submicron-diameter structures can be produced inside many transparent materials by tightly focused 100-fs laser pulses. The ultrafast energy deposition creates very high temperature and pressure inside the region, initiating a 'microexplosion'. Material is ejected from the center and forced into the surrounding volume, forming a void surrounded by densified material. Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.
Surfaces and Mirrors
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Distribution of impurities in the surface layers of KDP and KD*P single crystals after their mechanical treatment
L. V. Atroschenko, Abraham B. Blank, E. S. Zolotovitskaya, et al.
The detailed investigation of the composition and the impurity concentration in surface layers of KDP and KD.P single crystals was carried out. The relation of surface laser damage threshold to bulk one in these crystals was not more than 40 percent. The content of impurities - Al, Mg, Ca, Fe, Si - was determined by x-ray photoelectron and atomic emission spectroscopy and carbon content by coulonometry. The depth profiling curves of the samples subjected to different kinds of mechanical treatments were obtained. It was found our that the penetration depth of impurities depends on the kind of treatment and abrasives. The deviation of the component composition in surface layers from the stoichiometric ratio in the bulk was also established. It was shown that the impurities included in the crystal lattice of KDP and KD.P are responsible for the active absorption of the laser radiation and for the decrease of the radiation threshold power when the crystal is not yet damaged.
Fundamental Mechanisms
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Modeling of filamentation damage induced in silica by 351-nm laser pulses
David Milam, John T. Hunt, Kenneth R. Manes, et al.
A major risk factor that must be considered in design of the National Ignition Facility is the possibility for catastrophic self-focusing of the 351-nm beam in the silica optical components that are in the final section of the laser. Proposed designs for the laser are analyzed by the beam-propagation code PROP92. A 351-nm self-focusing experiment, induction of tracking damage, was done to provide data for validation of this code. The measured self- focusing lengths were correctly predicted by the code.
Thin Films
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cw laser compaction of aqueous-solution-deposited metal oxide films
Zirconium dioxide films were spin cast onto silica or silicon substrates from an aqueous solution comprised of the precursor metal nitrate and an organic complexant such as glycine. The hydrated films so derived consist of an amorphous organic phase in which the metal cations and nitrate anions are homogeneously dispersed. Heating to temperatures above 200 degrees C leads to film dehydration followed by an auto-catalyzed oxidation reaction whereby the bound nitrate oxidizes the organic matrix leaving behind an intact stoichiometric and crystalline metal oxide film. Films are characterized using AFM, XRD, and optical methods. Transformation processes in these films have been studied in detail by means of spectroscopic ellipsometry and laser induced fluorescence from films doped with a suitable rare earth probe ion such as Sm+3. In the latter case, the measured fluorescence emission spectra are used to identify the hydrated, dehydrated, amorphous and crystalline metal oxide phases which evolve during processing. These transformations also have been induced upon visible CW laser irradiation at fluences in excess of 1 MW/cm2. Under these conditions, the film dehydrates and compacts within the footprint of the incident laser beam rendering this region of the film water insoluble. Post irradiation washing of the film with water removes all vestiges of the film outside of the beam footprint suggesting a possible use of this technique for lithography applications. Films subjected to laser irradiation and post irradiation heating have been characterized with respect to thickness, phase composition, crystallite size and optical constants.
Optical properties of thin films deposited by reactive-Rf-magnetron sputtering
Kunio Yoshida, Tomosumi Kamimura, Kanyoshi Ochi, et al.
Optical coatings deposited by conventional evaporation processes have a number of shortcomings including weak adhesion which depends upon the subsurface condition of substrate, and the change of spectral characteristics and film stress in the relative humidity. Thin films coated by reactive-rf-magnetron sputtering improve these problems. In this paper, optical properties of thin films deposited by reactive-rf-magnetron sputtering are presented.
Surfaces and Mirrors
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Computer-enhanced photon tunneling microscopy for surface characterization of optical materials
James D. Kleinmeyer, J. Derek Demaree
Surface characterization of optical materials is a time consuming and complex process. The complexity of these materials provides a challenging task for determining the effects of such factors as polishing, surface defects, subsurface fracture and variations in optical surface coatings. Our laboratory has incorporated several types of non-intrusive techniques for surface analysis and sample characterization. The newest and one of the most promising techniques is Computer Enhanced Photon Tunneling Microscopy. This paper describes our current photon tunneling microscopy (PTM) system and its use in the characterization of the surface topography of ion-implanted optical sapphire. Examination of implanted surfaces by PTM can be used to determine the amount of material removed by ion sputtering as well as the roughening of the surface by the implantation.
Thin Films
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Damage-resistant thin film plate polarizer
A thin-film polarizer with a high polarization ratio for single laser wavelengths can be realized if a Fabry-Perot filter design is used.THe spectral width of the polarization region of the filter polarizer is smaller than that of the usual long-wave pass filter, but there are not the practical difficulties of depositing multilayer systems with a large number of layers having non-quarter-wave thicknesses. Experimental results of a HfO2/SiO2 Fabry-Perot filter polarizer at 355 nm are presented. The laser-induced damage thresholds of the polarizer are enhanced by minimizing E-field intensities and minimizing absorption.
248-nm laser interaction studies on LaF3/MgF2 optical coatings by cross sectional transmission electron microscopy
Zsolt Czigany, Miklos Adamik, Norbert Kaiser
LaF3MgF2 high-reflectance optical multilayer coatings were deposited by e-beam evaporation and investigated by cross-sectional transmission electron microscopy concentrating on the evolution of the film structure during growth and due to laser irradiation. The irradiated samples show enhanced texture compared to the as deposited films, indicating that recrystallization took place due to the laser irradiation. The waviness of the interfaces between the single quarter-wave sublayers in the multilayer system increase with increasing distance from the substrate.This effect may result in increased scatter losses, especially in the ultraviolet spectral region.
Influence of substrate cleaning on LIDT of 355 nm HR coatings
Jean DiJon, Pierre Garrec, Norbert Kaiser, et al.
An advanced high-purity reactive e-beam evaporation process was used to deposit Al2O3/SiO2 HR coatings for 355 nm high-power laser applications. Both 1:1 and R:1, 6 ns pulse width, laser-induced damage threshold (LIDT) tests for normal and non-normal incident design were performed to study the influence of quartz substrate cleaning. Damage test results indicate very high LIDT values on clean substrates. Some tested points have R:1 LIDT above 20 J/cm2. Post-cleaning of coated substrates degrades LIDT.
Fundamental Mechanisms
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Lifetime survivability of contaminated target-chamber optics
Frank Rainer, Andrew T. Anderson, Alan K. Burnham, et al.
Target chambers used for inertial confinement fusion expose laser optics to a very hostile environment not only from high-fluence laser irradiation but also x-ray irradiation and particulate debris from targets and chamber wall materials. Expendable debris shields provide the first line of defense to more costly optics upstream in the laser beam path to contaminants generated within the target chamber. However, the replacement of a large number of debris shields is also an expensive proposition so that extending their usable lifetime within the chamber is of crucial importance. We have conducted tests to show that optics can both be cleaned and damaged by laser irradiation at 355 nm after being contaminated with potential chamber-wall materials such as B4C and Al2O3. Such optics can survive from one to hundreds of laser shots depending on the degree of contamination and laser fluence levels. Similarly, we have studied the survivability of optics that have been exposed to direct contamination from representative target materials irradiated in the target chamber. We have also studied the effects on optics that were not directly exposed to targets yet received secondary exposure form the above directly-exposed samples.
Thin Films
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Large-area sol-gel highly reflective coatings processed by the dipping technique
Philippe F. Belleville, Philippe M. Pegon
The Centre d'Etudes de Limeil-Valenton is currently involved in a project which consists of the construction of a 2 MJ/500TW pulsed Nd:glass laser devoted to inertial confinement fusion research. With 240 laser beams, the proposed megajoule-class laser conceptual design necessitates 44-cm X 44-cm X 6-cm cavity-end mirrors representing more than 50-m2 of coated area. These dielectric mirrors are made of quarterwave stacks of SiO2 and ZrO2-PVP and are prepared from colloidal suspensions using the sol-gel route. After a sustained search effort, we have prepared (SiO2/ZrO2- PVP)10 mirrored coatings with up to 99 percent reflection at 1053-nm and for different incidence use. Adequate laser-conditioned damage thresholds ranging 14-15 J/cm2 at 1053-nm wavelength and with 3-ns pulse duration were achieved. Large-area mirrors with good coating uniformity and weak edge-effect were produced by dip-coating at room temperature and atmospheric pressure.
Fundamental Mechanisms
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Multiple-pulse damage thresholds of optical components for excimer lasers
Klaus R. Mann, Bernd Granitza, Eric Eva
For characterization of the long-term stability of UV optical components a set-up for determination of multiple- pulse damage thresholds has been developed. Using a high power excimer laser, 'S-on-1' damage thresholds were measured at 248nm for bare UV substrates as well as high- and anti-reflective dielectric coatings, indicating characteristic dependencies on the number of applied laser pulses and pulse repetition rates. It could be demonstrated that the multiple pulse threshold of dielectric coatings strongly depends on the thermal conductivity of the substrate. In addition, stability test were performed at 308nm on Al2O3/SiO2HR coatings applying 108 pulses of up to 400mJ/cm2. Although the samples did withstand this long-term irradiation without damage, a slight degradation of the optical properties and a laser- induced contamination layer on the irradiated area were observed.
Related Optical Fiber Contributions
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In-situ characterization of visible and near-infrared Raman throughput from fiber optics in a high intensity gamma radiation environment
Robert J. Donohoe, Joseph A. Skelton
The use of fiber-optic-based Raman spectroscopy as a diagnostic in waste tanks and streams has been suggested both as a safety measure and a process chemistry control. The chief advantages of Raman for these applications are its ability to provide highly resolved, distinctive spectroscopic signatures of neutral and ionic molecular species in complex matrices with fairly rapid data acquisition (often less than one minute) from remote distances, easily in excess of 100 meters. Disadvantages include the rather insensitive nature of the Raman measurement, the possibility of optical interferences, especially from stray light or photoluminescence, and the problematic application of Raman for quantitative determination of analytes. Other considerations include the small sampling volume, costs of instrumentation and facilities requirements.
Surfaces and Mirrors
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Surface damage resistance of calcite crystals to nanosecond-pulse 694-nm laser radiation
Surface damage thresholds of polished calcite crystals were measured with a pulsed ruby laser at the wavelength of 694.3 nm. Three grades of calcite used for laser polarizers were tested: Grade A, Schlieren-Free, and Scatter-Free, as characterized by the manufacturer, Karl Lambrecht Corporation of Chicago. The test samples were cut and polished with the surfaces parallel to the optic axis. Sample dimensions were 1 cm X 1 cm and 2 to 3 mm thick. The ruby laser oscillator was passively Q-switched to produce 12-ns pulses in a single-transverse and longitudinal mode. Samples were irradiated at near-normal incidence to the 1-cm2 surfaces with a laser 1/e2 spot diameter of approximately 0.12 mm. Tests were conducted with a single shot per site, and photoelectric detection of a laser- induced surface spark was used as a sensitive method for detecting the onset of damage. Damage threshold of 30, 60, and 50 mJ/cm2, were determined for the three grades, respectively. No significant threshold differences were observed between test with the laser polarization parallel and perpendicular to the optic axis. Laser-induced damage, visible by scattering of a collinear He-Ne laser beam, was most frequently observed at the rear surfaces, although it sometimes occurred at the front surface alone or in the interior. On the basis of standing wave electric-field analysis, the predicted energy density at the rear surface was more than twice that at the front surface, and laser damage at the rear surface was expected to occur at a correspondingly lower fluence. Early damage at randomly distributed surface imperfections appeared to preclude such a consistent correlation.
Thin Films
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In-situ measuring and monitoring of the laser conditioning of optical coatings
Qian Zhao, Zhengxiu Fan, ZhiJiang Wang
By now laser conditioning has been proved to be a useful method to improve the optical coatings and raise the laser induced damage threshold of the thin film coatings. However, whatever happens during the laser conditioning process and which ever factor influences the laser conditioning effect are still not very clear. We produce a series samples with different roughness levels of the substrates, multilayer design, thin film material and the vacuum environment of the chamber. We make laser conditioning test to these samples by 1.06 micrometers laser. Photothermal deflection technology is employed to observe the conditioning process in-situ. A series of experimental data are obtained. We give our explanation. The mechanism of laser conditioning is also discussed.