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Future trends in optical coatings for high-power laser applications

Description: Inertial Confinement Fusion (ICF) research has historically been a driver in the development of high performance, high damage threshold optical coatings. This is particularly the case now as the ICF community develops plans for a proposed 1.8 mega-joule solid state (Nd{sup +3}-phosphate glass) laser system. The new system, the National Ignition Facility, is possible in part due to advances in optical coatings technology including the laser-conditioning of multilayer dielectrics and broadened applications for room-temperature deposited coatings. Sol-gel AR coatings are the standard for large, high-power laser optics and sol-gel HR coatings are being developed. For mirror and polarizer coatings, e-beam-deposited dielectric continue to provide the highest damage threshold coatings, but their laser damage thresholds and optical performance are limited by {mu}m-scale defects and poor control over layer thickness, respectively. More energetic deposition techniques such as IAD and IBS, now popular in the commercial market, offer both advantages and disadvantages in this high-damage-threshold coatings market.
Date: July 1, 1994
Creator: Kozlowski, M. R. & Thomas, I. M.
Partner: UNT Libraries Government Documents Department

The role of defects in laser damage of multilayer coatings

Description: Laser induced damage to optical coatings is generally a localized phenomenon associated with coating defects. The most common of the defect types are the well-known nodule defect. This paper reviews the use of experiments and modeling to understand the formation of these defects and their interaction with laser light. Of particular interest are efforts to identify which defects are most susceptible to laser damage. Also discussed are possible methods for stabilizing these defects (laser conditioning) or preventing their initiation (source stabilization, spatter particle trapping).
Date: December 21, 1993
Creator: Kozlowski, M. R. & Chow, R.
Partner: UNT Libraries Government Documents Department

2-1/2-D electromagnetic modeling of nodular defects in high-power multilayer optical coatings

Description: Advances in the design and production of high damage threshold optical coatings for use in mirrors and polarizers have been driven by the design requirements of high-power laser systems such as the proposed 1.8-MJ National Ignition Facility (NIF) and the prototype 12- kJ Beamlet laser system. The present design of the NIF will include 192 polarizers and more than 1100 mirrors. Currently, the material system of choice for high-power multilayer optical coatings with high damage threshold applications near 1.06 {mu}m are e-beam deposited HfO{sub 2}/Si0{sub 2} coatings. However, the optical performance and laser damage thresholds of these coatings are limited by micron-scale defects and insufficient control over layer thickness. In this report, we will discuss the results of our 2-1/2-D finite-element time- domain (FDTD) EM modeling effort for rotationally-symmetric nodular defects in multilayer dielectric HR coatings. We have added a new diagnostic to the 2-1/2-D FDTD EM code, AMOS, that enables us to calculate the peak steady-state electric fields throughout a 2-D planar region containing a 2-D r-z cross-section of the axisymmetric nodular defect and surrounding multilayer dielectric stack. We have also generated a series of design curves to identify the range of loss tangents for Si0{sub 2} and HfO{sub 2} consistent with the experimentally determined power loss of the HR coatings. In addition, we have developed several methods to provide coupling between the EM results and the thermal-mechanical simulation effort.
Date: July 1996
Creator: Molau, N. E.; Brand, H. R.; Kozlowski, M. R. & Shang, C. C.
Partner: UNT Libraries Government Documents Department

A comparison of nodular defect seed geometeries from different deposition techniques

Description: A focused ion-beam milling instrument commonly utilized in the semiconductor industry for failure analysis and IC repair, is capable of cross-sectioning nodular defects. Utilizing the instrument`s scanning on beam, high-resolution imaging of the seeds that initiate nodular defect growth is possible. In an attempt to understand the origins of these seeds, HfO{sub 2}/SiO{sub 2} and Ta{sub 2}O{sub 5}/SiO{sub 2} coatings were prepared by a variety of coating vendors and different deposition processes including e-beam, magnetron sputtering, and ion beam sputtering. By studying the shape, depth, and composition of the seed, inferences of its origin can be drawn. The boundaries between the nodule and thin film provide insight into the mechanical stability of the nodule. Significant differences in the seed composition, geometry of nodular growth and mechanical stability of the defects for sputtered versus e-beam coatings are reported. Differences in seed shape were also observed from different coating vendors using e-beam deposition of HfO{sub 2}/SiO{sub 2} coatings.
Date: December 29, 1995
Creator: Stolz, C.J.; Tench, R.J.; Kozlowski, M.R. & Fornier, A.
Partner: UNT Libraries Government Documents Department

Laser-induced damage of fused silica at 355 and 1065 nm initiated at aluminum contamination particles on the surface

Description: 1-{mu}m thick circular dots, 10-250 {mu}m dia, were deposited onto 1.14 cm thick fused silica windows by sputtering Al through a mask. Al shavings were also deposited on the windows to investigate 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 input and output surfaces of the windows. During the first shot, a plasma ignited at the metal particle and damage initiated on the fused silica surface. The morphology of the damage at the metal dots were 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 (pits and cracks) was initiated on the output surface (especially at 355 nm) 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 (usually the first 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 (except when catastrophic damage occurred). Onset of catastrophic damage on output surface occurred only when particles exceeded a critical size. 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.
Date: January 1, 1997
Creator: Genin, F.Y.; Michlitsch, K.; Furr, J.; Kozlowski, M.R. & Krulevitch, P.
Partner: UNT Libraries Government Documents Department

Towards predicting the laser damage threshold of large-area optics

Description: As the size of optics increases, such as in the optical coatings being developed for the National Ignition Facility in US and the Laser MegaJoules in France, the difficulty also increases in measuring and defining their laser damage threshold. Measuring the threshold on small witness samples ({le}cm) rather than full aperture optic (=m) is advantageous, and in this article, the threshold of large-area components is addressed in two ways. First, a model based on the R-on-l threshold distribution is shown to predict the threshold of a large optic with a high degree of confidence. The average R-on-l threshold provides a reliable, accurate value to evaluate coatings. An automated damage test bench has been developed at CEA. Secondly, the damage threshold has to be defined according to final use of the component. LLNL has defined a functional damage threshold to set limits on maximum damage size. An empirical power law dependence of average damage size on peak fluence was found; this can be used to predict the damage behavior of large-aperture optics exhibiting the same damage morphology.
Date: October 1, 1996
Creator: Hue, J.; Genin, F.Y.; Maricle, S.M. & Kozlowski, M.R.
Partner: UNT Libraries Government Documents Department

Contamination effects on optical damage

Description: The development and construction of high fluence lasers for inertial confinement fusion such as the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) or the Laser Megajoule in France continues to generate strong interest in the behavior of optical components under intense laser irradiation. the design of such lasers has created significant technological challenges in the area of laser glass, KDP crystal growth, surface finishing, and fused silica damage in the ultraviolet (UV). The damage of fused silica lenses at 355 nm is of particular concern since the optical design requires peak fluences that are very close to the damage threshold of the most resistant fused silica lenses available today.
Date: March 2, 1998
Creator: Genin, F.Y.; Kozlowski, M.R. & Feit, M.D.
Partner: UNT Libraries Government Documents Department

Characterization of defect geometries in multilayer optical coatings

Description: Laser-induced damage in optical coatings is generally associated with micrometer-scale defects. A simple geometric model for nodule-shaped defects is commonly used to describe defects in optical coatings. No systematic study has been done, however, to prove the applicability of that model to standard optical coating deposition. Some defects are known not to have a classic nodule geometry. The present study uses atomic force microscopy (AFM) and scanning electron microscopy to characterize the topography of coating defects in a HfO{sub 2}/SiO{sub 2} multilayer mirror system. Focused ion-beam cross-sectioning is then used to study the underlying defect structure. This work develops a model for defect shape such that the overall geometry of a coating defect, particularly seed size and depth, can be inferred from non-destructive evaluation measurements such as AFM. The relative mechanical stabilities of nodular defects can be deduced based on the nodule`s geometry. Auger analysis showed that the seed material that causes nodular defects in HfO{sub 2}/SiO{sub 2} multilayers is a hafnia oxide. Such characterization capabilities are needed for understanding the enhanced susceptibility of particular defects to laser damage and for developing improved techniques for depositing low-defect density coatings.
Date: December 1, 1993
Creator: Tench, R. J.; Chow, R. & Kozlowski, M. R.
Partner: UNT Libraries Government Documents Department

Large-area conditioning of optics for high-power laser systems

Description: In order to reach the high fluence goals of the Beamlet laser at Lawrence Livermore National Laboratory (LLNL), the polarizers in the system must be laser conditioned to increase their damage thresholds. Research has shown that by using a six-step raster-conditioning program, the damage thresholds of the HfO{sub 2}/SiO{sub 2} multilayer polarizer coatings can be increased to meet the Beamlet 3-ns design goal of 8 J/cm{sub 2}. Because of the large size of the polarizers (73 cm {times} 37 cm {times} 9 cm), a large-area conditioning facility was constructed capable of rastering a one-meter optic, weighing as much as 400 pounds, at any specified use angle. A large translational stage moves the optic in a raster pattern through a stationary, 10-Hz rep-rated, 1064-nm beam with 10-ns pulses. A scatter measurement diagnostic allows on-the-fly evaluation of laser-induced damage and logs the coordinates of the damage. Laser energy is measured pulse-to-pulse in order to ensure stability during a scan. A small amount of minor coating damage does occur during the process, but the damage does not grow upon further irradiation. This damage causes only a small increase in total scatter compared to that due to the preexisting defects, and would not influence the system performance.
Date: December 21, 1993
Creator: Sheehan, L. M.; Kozlowski, M. R.; Rainer, F. & Staggs, M. C.
Partner: UNT Libraries Government Documents Department

Influence of defect shape on laser-induced damage in multilayer coatings

Description: Atomic force microscopy (AFM) was used as a diagnostic for laser damage testing of lim-scale nodule defects in e-beam deposited HfO2/SiO{sub 2} multilayer mirror coatings. Earlier experiments in our laboratory showed that there was a significant increase in damage susceptibility for defects with heights greater than 0.6 {mu}m. In the present study, e-beam HfO{sub 2}/SiO{sub 2} HR coatings from two additional vendors were also studied. Atomic force microscopy characterization of the defects showed that the defect shape became less ``classic`` as the defect dimensions, especially height, increased. The characteristic shapes (spatter, multi-lobe, etc.) of the larger defects also varied between vendors. Using the AFM and optical microscopy, laser damage experiments (1064-nm, 3-ns pulses) were performed on various defects found in the coatings of the two new vendors. The data showed once again that the damage susceptibility increased as the height of the defect increased. There was also some difference between vendors in the damage susceptibility of the defects of a given size. Spatter defects, found predominantly in the coatings of one vendor showed the highest susceptibility to damage.
Date: June 1, 1994
Creator: Kozlowski, M. R.; Tench, R. J.; Chow, R. & Sheehan, L.
Partner: UNT Libraries Government Documents Department

Investigation of the microstructure of coatings for high power lasers by non-optical techniques

Description: The microstructure of optical coatings strongly influence their resistance to high fluence laser, scatter properties, as well as, their mechanical and environmental stability. The relative merits of non-optical techniques, such as: scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and focused ion beam, are discussed as they apply to optical multilayer coatings. The combination of these techniques provides a unique method to analyze defects in coatings. The long term objective of this work is to: understand the initiation and growth mechanisms of defects in optical coatings, investigate failure mechanisms of laser coatings, and suggest methods for reducing the number of defects during the deposition process. To date, our defect analysis using non-optical techniques has focused on hafnia/silica multilayers for high power lasers. In summary, the information we have compiled about the defect seeds: indicates that seed size has an influence upon the mechanical stability of the whole defect, indicates that seed shape and chemical composition reveal potential seed sources in the coating system, and demonstrates that defects can be initiated either as a single event or continuously during the deposition process. Also, it is shown that different vendors have characteristic defects and seeds.
Date: June 1, 1994
Creator: Tench, R. J.; Kozlowski, M. R. & Chow, R.
Partner: UNT Libraries Government Documents Department

Defect geometries and laser-induced damage in multilayer coatings

Description: A correlation between laser-induced damage and the height of a coating defect was found from an examination of HfO{sub 2}/SiO{sub 2} mirrors made by three different coating vendors. The nodular defects in these reactive e-beam deposited mirrors were studied using the combination of SEM, optical microscopy, FIB and AFM techniques. Each vendor had small defects in common, but characteristically different large defects. Also the majority of seeds that caused the defects were made of hafnia, not silica. The apparent mechanical stability of the defects within the coating plays a major role in the laser resistance (1064 nm and l0ns) of a high damage threshold coating.
Date: July 1, 1994
Creator: Tench, R. J.; Kozlowski, M. R. & Chow, R.
Partner: UNT Libraries Government Documents Department

Current 3 {omega} large optic test procedures and data analysis for the quality assurance of National Ignition Facility

Description: A reliable metric is required to describe the damage resistance of large aperture 3{omega} transmissive optics for the National Ignition Facility (NIF) laser. The trend from single site testing to the more statistically valid Gaussian scanning test requires a well modeled experimental procedure, accurate monitoring of the test parameters, and careful interpretation of the resulting volumes of data. The methods described here provide reliable quality assurance data, as well as intrinsic damage concentration information used to predict the performance expected under use conditions. This paper describes the equipment, test procedure, and data analysis used to evaluate large aperture 3{omega} optics for the NIF laser.
Date: December 22, 1998
Creator: Schwartz, S; Feit, M D; Kozlowski, M R & Mouser, R P
Partner: UNT Libraries Government Documents Department

Depth profiling of polishing-induced contamination on fused silica surfaces

Description: Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for UV optics. Here secondary ion mass spectroscopy (SIMS) was used to measure depth profiles of finished process contamination on fused silica surfaces. Contaminants detected include the major polishing compound components (Ce or Zr from CeO2 or ZrO2), Al presently largely because of the use of Al2O3 in the final cleaning process (Fe, Cu,Cr) incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-100 ppm range, except for Al with exceeds 1000 ppm. A strong correlation has been shown between the presence of a gray haze damage morphology and the use of CeO2 polishing compound. No strong correlation was found however between high levels of Ce, or any other contaminant and the low damage threshold was observed. In fact one of the strongest indications of a correlation is between increased damage thresholds and increased Zr contamination. This suggests that the correlation between redeposition layer and laser damage threshold is not simple an absorbing contaminant issue.
Date: December 20, 1997
Creator: Kozlowski, M.R.; Carr, J.; Hutcheon, I,; Torres, R.; Sheehan, L. Camp, D. & Yan, M.
Partner: UNT Libraries Government Documents Department

Effect of Silica overlayers on laser damage of HfO{sub 2}-SiO{sub 2}56{sup degree} incidence high reflectors

Description: A series of hafnia/silica, oblique incidence (56{degrees}), 1064 nm high reflectors (HRs) were prepared and coated with silica overlayers of varying optical thickness from {lambda}/2 to 4{lambda} in order to determine the effect of an overlayer on the laser-damage resistance of the HRs. The stress and laser damage thresholds for S and P polarization of the HRs were measured, and the damage sites for P polarization examined by Atomic Force Microscopy (AFM). All the multilayers were found to be in compression, with an intrinsic stress increasing with overlayer thickness. The presence of an overlayer and its thickness did not affect the damage threshold significantly. However, the presence of an overlayer greatly influenced the size and morphology of the damage. First, the overlayer prevented catastrophic ``burns`` of the hafnia top layer. Second, as the overlayer thickness increased, two distinct damage morphologies were found: agged pits and round craters. The diameter of these pits and craters then increased somewhat with thicker overlayers. The depths of the pits and craters also increased with overlayer thickness, and the depths showed failure occurring at the interfaces below the hafnia layers. The side-wall angles of the craters were shallower with thicker overlayers, but there was no angle dependence for the pits. The craters showed fracture-like features and a small hillock or pit on their bottom surfaces. No correlation of damage morphology to conditioning or fluence was found.
Date: January 5, 1995
Creator: Walton, C.C.; Genin, F.Y.; Chow, R.; Kozlowski, M.R.; Loomis, G.E. & Pierce, E.
Partner: UNT Libraries Government Documents Department

Laser damage performance of fused silica optical componets measured on the beamlet laser at 35nm

Description: A statistics-based model is being developed to predict the laser-damage-limited lifetime of UV optical components on the NIF laser. In order to provide data for the model, laser damage experiments were performed on the Beamlet laser system at LLNL (aperture: 34 cm x 34 cm). Three prototype NIF focus lenses were exposed to 351 nm pulses (1.5 ns or 3 ns) during four experimental campaigns, each consisting of 23 to 38 pulses at NIF relevant fluences. Each lens was sol-gel AR coated and all laser exposures were performed in a vacuum environment. Through inspections of the lens before, during and after the campaigns, pulse-to-pulse damage growth rates were measured for damage initiating both on the surfaces and at bulk inclusions. Radial growth rates measured for rear surface damage was typically 10x higher than that measured in the bulk or at the front surface. No significant correlation of growth rate to precursor type was indicated. For 5 J/cm², 3 ns pulses the typical radial growth rate was nominally 20 µm/pulse. Average growth rates measured on three lenses made by two manufacturers were in good agreement. While the growth rate clearly increased with fluence, the data obtained was insufficient to quantify the dependence. The growth rates reported here were 20x-50x higher than values predicted from off-line studies of bare surfaces in air.
Date: December 22, 1998
Creator: Kozlowski, M R; Maricle, S; Mouser, R; Parham, T; Schwartz, S; Wegner, P et al.
Partner: UNT Libraries Government Documents Department

Effects of wet etch processing on laser-induced damage of fused silica surfaces

Description: Laser-induced damage of transparent fused silica optical components by 355 nm illumination occurs primarily at surface defects produced during the grinding and polishing processes. These defects can either be surface defects or sub-surface damage.Wet etch processing in a buffered hydrogen fluoride (HF) solution has been examined as a tool for characterizing such defects. A study was conducted to understand the effects of etch depth on the damage threshold of fused silica substrates. The study used a 355 nm, 7.5 ns, 10 Hz Nd:YAG laser to damage test fused silica optics through various wet etch processing steps. Inspection of the surface quality was performed with Nomarski microscopy and Total Internal Reflection Microscopy. The damage test data and inspection results were correlated with polishing process specifics. The results show that a wet etch exposes subsurface damage while maintaining or improving the laser damage performance. The benefits of a wet etch must be evaluated for each polishing process.
Date: December 22, 1998
Creator: Battersby, C.L.; Kozlowski, M.R. & Sheehan, L.M.
Partner: UNT Libraries Government Documents Department

Effect of Vacuum on the Occurrence of UV-Induced Surface Photoluminescence, Transmission Loss, and Catastrophic Surface Damage

Description: Vacuum degrades the transmittance and catastrophic damage performance of fused-silica surfaces, both bare and silica-sol anti-reflective coated. These effects may be important in certain space application of photonics devices. When exposed to hundreds of 355-rim, 10-ns laser pulses with fluences in the 2-15 J/cm{sup 2} range, transmittance loss is due to both increased reflectance and absorption at the surface. Spectroscopic measurements show that the absorbed light induces broadband fluorescence from the visible to infrared and that the peak photoluminescence wavelength depends cumulative fluence. The effect appears to be consistent with the formation of surface SiO{sub x} (x<2) with progressively lower x as cumulative fluence increases. Conversely, low fluence CW UV irradiation of fluorescent sites in air reduces the fluorescence signal, which suggests a photochemical oxidation reaction back to Si0{sub 2}. The occurrence of catastrophic damage (craters that grow on each subsequent pulse) also increases in a vacuum relative to air for both coated and uncoated samples. In both cases, the 50% damage probability for 100 one-mm sites decreases from about 45 to 35 J/cm{sup 2} for superpolished fused silica at pressures in the 10{sup -6} Torr range. The damage probability distribution in 10 Torr of air is close to that at one atmosphere of air. The damage morphology of the crater formed in vacuum differs substantially from that in air and has a more melted appearance and does not show cracking and flaking. These differences are possibly due to more coupling of the plasma shock wave into the surface with air present but slower heat dissipation in a vacuum. While it is attractive to propose that formation of sub-stoichiometric silica on the surface in a vacuum environment enhances the probability of catastrophic damage, initial experiments have not yet been able to establish a mechanistic link between the two phenomena.
Date: July 20, 2000
Creator: Burnham, A K; Runkel, M; Demos, S G; Kozlowski, M R & Wegner, P J
Partner: UNT Libraries Government Documents Department

Mechanisms to explain damage growth in optical materials

Description: Damage growth in optical materials used in large aperture laser systems is an issue of great importance when determining component lifetime and therefore cost of operation. Understanding the mechanisms and photophysical processes associated with damage growth are important in order to devise mitigation techniques. In this work we examined plasma-modified material and cracks for their correlation to damage growth on fused silica and DKDP samples. We employ an in-situ damage testing optical microscope that allows the acquisition of light scattering and fluorescence images of the area of interest prior to, and following exposure to a high fluence, 355-nm, 3-ns laser pulse. In addition, high-resolution images of the damage event are recorded using the associated plasma emission. Experimental results indicate that both aforementioned features can initiate plasma formation at fluences as low as 2 J/cm{sup 2}. The intensity of the recorded plasma emission remains low for fluences up to approximately 5 J/cm{sup 2} but rapidly increases thereafter. Based on the experimental results, we propose as possible mechanisms leading to damage growth the initiation of avalanche ionization by defects at the damage modified material and presence of field intensification due to cracks.
Date: January 10, 2001
Creator: Demos, S G; Kozlowski, M R; Staggs, M; Chase, L L; Burnham, A & Radousky, H B
Partner: UNT Libraries Government Documents Department

Mechanisms to Explain Damage Growth in Optical Materials

Description: Damage growth in optical materials used in large aperture laser systems is an issue of great importance when determining component lifetime and therefore cost of operation. Understanding the mechanisms and photophysical processes associated with damage growth are important in order to devise mitigation techniques. In this work we examined plasma-modified material and cracks for their correlation to damage growth on fused silica and DKDP samples. We employ an in-situ damage testing optical microscope that allows the acquisition of light scattering and fluorescence images of the area of interest prior to, and following exposure to a high fluence, 355-nm, 3-ns laser pulse. In addition, high-resolution images of the damage event are recorded using the associated plasma emission. Experimental results indicate that both aforementioned features can initiate plasma formation at fluences as low as 2 J/cm{sup 2}. The intensity of the recorded plasma emission remains low for fluences up to approximately 5 J/cm{sup 2} but rapidly increases thereafter. Based on the experimental results, we propose as possible mechanisms leading to damage growth the initiation of avalanche ionization by defects at the damage modified material and presence of field intensification due to cracks.
Date: December 18, 2000
Creator: Demos, S G; Kozlowski, M R; Staggs, M; Chase, L L; Burnham, A & Radousky, H B
Partner: UNT Libraries Government Documents Department

Growth of laser-induced damage during repetitive illumination of HfO2-SiO2 multilayer mirror and polarizer coatings

Description: As peak fluence of high power lasers is increased, it becomes necessary to tolerate damage on mirrors, polarizers. To study how 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 damage morphology was recorded after each shot to determine the types of damage that cause massive unstable failure and lower the optic`s functional damage threshold. Results were summarized on damage stability maps plotting the average damage size vs number of shots for fluences ranging from 10 to 40 J/cm{sup 2}. The maps indicate that the commonly observed damage morphologies (pits, flat bottom pits, scalds, outer layer delamination) 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/cm{sup 2}, flat bottom pits can grow during repetitive illumination above a critical fluence of about 35 J/cm{sup 2}. Scalds are formed in the first shot and never grow at fluences below 40 J/cm{sup 2}. Finally, delaminates are highly unstable and have the potential for damaging the coating catastrophically above 15 J/cm{sup 2}. Results show that delaminate damage should be prevented; this knowledge has allowed coatings development efforts to focus on eliminating the origin of such damage morphology.
Date: January 1, 1997
Creator: Genin, F.Y.; Stolz, C.J. & Kozlowski, M.R.
Partner: UNT Libraries Government Documents Department

Laser conditioning methods fo hafnia silica multiplayer mirrors

Description: Large aperture multilayer hafnia silica high reflector coatings at 1064 nm, deposited by reactive electron-beam deposition, were prepared to examine different laser conditioning methods for manufacturing high fluence optics in the National Ignition Facility. Laser conditioning is a process where the damage threshold of the coating is increased or the damage that is created is minimized so that it does not grow upon further irradiation. Two laser conditioning methods were examined for coatings deposited from only oxide starting materials. Off-line laser conditioning consists of raster scanning a mirror past a 1 mm diameter Gaussian beam over the entire clear aperture; a process that takes approximately 24 hours per scan. On-line laser conditioning consisted of a large aperture 300 mm x 300 mm beam from the Beamlet laser that irradiated the entire full clear aperture of a series of mirrors; a process that was limited by a 2-4 hour shot rate. In both cases a six-step process was used with the mirror first irradiated at a low fluence, then successively higher fluences increased in equal increments up to the peak laser operating fluence. Mirrors that were only partially laser conditioned damaged catastrophically while fully conditioned mirrors survived fluences exceeding the safe operating Beamlet fluence. An alternative off-line laser conditioning method was examined for coatings deposited from hafnia or metallic hafnium sources. Single-step laser conditioning consists of off-line raster scanning an optic at the peak operating fluence, thus decreasing the laser conditioning cost by reducing the number of scans and required laser conditioning stations to process all the mirrors for the National Ignition Facility. Between pulses the optic is stepped approximately one fourth of the l/e* Gaussian beam diameter so each area of the coating is irradiated by different segments of the beam starting at a low fluence at the outer edge ...
Date: January 6, 1998
Creator: Stolz, C.J.; Sheehan, L.M.; Maricle, S.M. Schwartz, S.; Kozlowski, M.R.; Jennings, R.T. & Hue, J.
Partner: UNT Libraries Government Documents Department

Study of the effects of polishing, etching, cleaving, and water leaching on the UV laser damage of fused silica

Description: A damage morphology study was performed with a 355 nm Nd:YAG laser on synthetic UV-grade fused silica to determine the effects of post- polish chemical etching on laser-induced damage, compare damage morphologies of cleaved and polished surfaces, and understand the effects of the hydrolyzed surface layer and waste-crack interactions. The samples were polished , then chemically etched in buffered HF solution to remove 45,90,135, and 180 nm of surface material. Another set of samples was cleaved and soaked in boiling distilled water for 1 second and 1 hour. All the samples were irradiated at damaging fluencies and characterized by Normarski optical microscopy and scanning electron microscopy. Damage was initiated as micro-pits on both input and output surfaces of the polished fused silica sample. At higher fluencies, the micro-pits generated cracks on the surface. Laser damage of the polished surface showed significant trace contamination levels within a 50 nm surface layer. Micro-pit formation also appeared after irradiating cleaved fused silica surfaces at damaging fluences. Linear damage tracks corresponding cleaving tracks were often observed on cleaved surfaces. Soaking cleaved samples in water produced wide laser damage tracks.
Date: December 23, 1997
Creator: Yoshiyama, J.; Genin, F.Y.; Salleo, A.; Thomas, I.; Kozlowski, M.R.; Sheehan, L.M. et al.
Partner: UNT Libraries Government Documents Department

Influence of microstructure on laser damage threshold of IBS coatings

Description: Ion-beam sputtering (IBS) coatings were developed for the laser gyro industry to meet significantly different requirements than those of fusion lasers. Laser gyro mirrors are small (< 25 mm) and require low losses (< 30 ppm typical) and high stability with long exposures to low power laser energy. In contrast, fusion laser optics are large (< 1 meter), have significantly reduced loss requirements (< 5,000 ppm) and high damage thresholds (> 26 J/cm{sup 2} at 1,064 nm with 3-ns pulses). As part of the National Ignition Facility (NIF) coating development effort, IBS coatings are being studied to explore the possible benefits of this technology to NIF optics. As an initial step to achieving the NIF size and damage threshold requirements, the coating process is being scaled to uniformly coat a 20 x 40 cm{sup 2} area with reduced spectral, reflected wavefront, and laser damage threshold requirements. Here, multilayer coatings deposited by ion-beam sputtering with amorphous layers were found to have lower damage thresholds at 1,064 nm than similar coatings with crystalline layers. Interestingly, at higher fluences the damage was less severe for the amorphous coatings. The magnitude of the difference in damage thresholds between the two different microstructures was strongly influenced by the size of the tested area. To better understand the microstructure effects, single layers of HfO{sub 2} with different microstructures were studied using transmission electron microscopy, ellipsometry, and a photothermal deflection technique. Since the laser damage initiated at defects, the influence of thermal diffusivity on thermal gradients in nodular defects is also presented.
Date: January 5, 1996
Creator: Stolz, C.J.; Genin, F.Y.; Kozlowski, M.R.; Long, D.; Lalazari, R.; Wu, Z.L. et al.
Partner: UNT Libraries Government Documents Department