Proceedings Volume 6545

Window and Dome Technologies and Materials X

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Proceedings Volume 6545

Window and Dome Technologies and Materials X

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Volume Details

Date Published: 29 April 2007
Contents: 9 Sessions, 31 Papers, 0 Presentations
Conference: Defense and Security Symposium 2007
Volume Number: 6545

Table of Contents

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Table of Contents

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  • Front Matter: Volume 6545
  • Long-wavelength Infrared Transparent Materials
  • Glasses
  • Micro- and Nano-crystalline Optical Materials and Structures
  • Modeling, Characterization, and Fabrication I
  • Modeling, Characterization, and Fabrication II
  • Optical Coatings and Surface Structures
  • Visible to Mid-wavelength Infrared Transparent Materials and Applications
  • Poster Session
Front Matter: Volume 6545
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Front Matter: Volume 6545
This PDF file contains the front matter associated with SPIE Proceedings Volume 6545, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
Long-wavelength Infrared Transparent Materials
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Development of hot-pressed and chemical-vapor-deposited zinc sulfide and zinc selenide in the United States for optical windows
By the mid 1950s, there was a need for infrared-transmitting materials with improved optical and mechanical characteristics for military and commercial instruments. The newly invented "heat-seeking" missile also required a more durable infrared-transmitting dome. Some properties of ZnS were known from studies of natural minerals. More properties of pure ZnS and ZnSe were measured with single crystals grown in Air Force and industrial laboratories in the 1950s. In 1956, a team led by William Parsons at the Eastman Kodak Hawk-Eye Works in Rochester, New York began to apply the technique of hot pressing to make infrared-transmitting ceramics from powders. This work led to commercial production of six materials, including ZnS (IRTRAN® 2) and ZnSe (IRTRAN® 4) in the 1960s. Because the hot pressed materials could not be made in very large sizes and suffered from undesirable optical losses, the Air Force began to look for alternative manufacturing methods around 1970. Almost immediately, highly successful materials were produced by chemical vapor deposition under Air Force sponsorship by a team led by James Pappis at the Raytheon Research Division in Waltham, Massachusetts. Chemical-vapor-deposited materials replaced hot pressed materials in most applications within a few years. From a stream of Air Force contracts in the 1970s and early 1980s, Raytheon produced two different grades of ZnS for windows and domes, one grade of ZnSe for high-energy CO2 laser windows, and a composite ZnS/ZnSe window for aircraft sensor pods. In 1980, a competitor called CVD, Inc., was formed by Robert Donadio, who came from the Raytheon Research Division. CVD began with a license from Raytheon, but soon sued Raytheon, arguing that the license violated the Sherman Antitrust Act. Raytheon countersued for breach of employment contracts and misappropriation of trade secrets. In 1984, a jury ruled in favor of CVD, which went on to build a lucrative business in ZnSe and ZnS. CVD was eventually purchased, first by Morton, and later by Rohm & Haas. II-VI, Inc. was formed in 1971 by Carl J. Johnson and James E. Hawkey to produce CdTe optics for industrial CO2 lasers. When Raytheon introduced ZnSe into the market in 1974, it was obvious that ZnSe was superior to CdTe, so II-VI purchased ZnSe from Raytheon to produce optical components. The supply of ZnSe was never stable enough for II-VI, which therefore began its own effort to deposit ZnSe in 1975. In 1980, II-VI became an investor in and customer of CVD, Inc., buying a substantial portion of the ZnSe that could be supplied by both Raytheon and CVD. Still pressed to meet customer demand, II-VI built its first ZnSe production furnace in the period 1983-1986. A second furnace came on line in 1988 and two more were operational by 1990. Finally attaining excess capacity, II-VI became a supplier of ZnS as well as ZnSe. In 1990, Raytheon exited the ZnS and ZnSe business, leaving it mainly to CVD and II-VI.
International development of chemical vapor deposited zinc sulfide
The materials community realized that zinc sulfide (ZnS) was an important optical material for infrared windows over forty years ago. Chemical vapor deposition (CVD) quickly became the method of choice for producing large ZnS windows and domes. In addition to the development initiated in the United States, several international efforts for understanding the processing and properties of CVD ZnS are notable. This paper summarizes the published history of non-U.S. CVD ZnS development including the significant efforts in the United Kingdom, the former Soviet Union, Israel, China, and Japan.
Optical properties of epitaxial single-crystal chemical-vapor-deposited diamond
Giorgio Turri, Ying Chen, Michael Bass, et al.
Epitaxial single-crystal chemical-vapor-deposited diamond was obtained from Element Six Ltd. (Ascot, UK) and from Apollo Diamond (Boston, MA). Both companies provided 5 x 5 mm squares with thicknesses ranging from 0.5 to 1.5 mm. In addition, Element Six provided 10-mm-diameter disks with a thickness of 1.0 mm. The absorptance of all specimens at 1064 nm was measured by laser calorimetry, with good agreement between independent measurements at the University of Central Florida and at QinetiQ (Malvern, UK). Depolarization at 1064 nm and ultraviolet absorption properties are also reported.
Spectral characterization of diffractively structured GaAs using the ARISTMS
Spectral results including reflectance, transmittance, rTIS, and tTIS are presented for diffractively structured GaAs using the Automated Rasterable Integrated Spectrometric and Total Integrated Scatter Measurement System (ARISTMS). The data is for the bandwidth of 10&mgr;m to 12&mgr;m over a range of incidence angles between 0° to 75°. A description of the diffractively structured GaAs and the operation of the ARISTMS are given.
Glasses
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Fluorinated silicate glass for conventional and holographic optical elements
This presentation is a survey of results of a long-term research at the laboratory of photoinduced processes at CREOL/UCF. A highly homogeneous and transparent sodium-zinc-aluminum-silicate glass doped with fluorine and bromine was developed. Glass is transparent from 220 to 2700 nm. It is a crown-type optical glass having refractive index at 587.5 nm nd=1.4959 and Abbe number νd=59.2. This glass shows low dependence of refractive index on temperature dn/dt<10-7 1/deg. Absorption coefficient in the near IR region is about 10-4 cm-1. Glass can withstand multi-kilowatt laser beams. Nonlinear refractive index is the same as for fused silica. Laser damage threshold for 8 ns is about 40 /cm2. This glass becomes a photosensitive one by doping with silver and cerium. It demonstrates refractive index decrement after exposure to UV radiation followed by thermal development and therefore is used for phase volume hologram recording. Spatial modulation of refractive index resulted from precipitation of nano-crystalline phase of sodium fluoride. The main mechanism of refractive index decrement is a photoelastic effect resulted from strong tensions generated in both crystalline and vitreous phases because of difference in their coefficients of thermal expansion. Volume Bragg gratings recorded in this glass, show extremely narrow spectral and angular selectivity and have low losses combined with high tolerance to laser radiation. These gratings possess a unique ability to produce laser beam transformations directly in angular space. This feature paves a way to creation of high power lasers with stable narrow emission spectra and diffraction limited divergence.
Development of a laser glass for the National Ignition Facility
Joseph S. Hayden, John H. Campbell, Stephen A. Payne
We review the development of a new glass formulation and manufacturing technology for a neodymium-doped phosphate based laser glass used in the LLNL National Ignition Facility (NIF) and the French Laser MegaJoule (LMJ). The glass development process built on both accumulated experience and the utilization of glass science principles, and the resultant new glass offers superior laser properties in combination with improvements in physical properties to enhance manufacturing yield. Essentially in parallel, a continuous melting production line was also conceived, designed and operated to meet both the schedule and cost targets of the NIF. Prior to 1997, phosphate laser glasses were manufactured by a discontinuous pot-melting process with limited production rate and associated high costs. The continuous melting process met several technical challenges, including producing glass with low residual water content and absence of inclusions which become damage sites when used in the NIF laser system.
Micro- and Nano-crystalline Optical Materials and Structures
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Spark plasma sintering and forming of transparent polycrystalline Al2O3 windows and domes
Dongtao Jiang, Dustin M. Hulbert, Umberto Anselmi-Tamburini, et al.
Spark plasma sintering(SPS) method was used to produce transparent alumina and it proved to be a cost-effective method due to short processing cycle. It was found that the optical transmittance of alumina is greatly influenced by SPS sintering parameters. A maximum transmittance of 85% has been achieved by sintering at 1300°C for 5min. Annealing in air at 1250°C for 24h significanly increased mid-infrared transmission. Utilizing SPS, transparent polycrystalline alumina domes have been successfully produced by combining sintering and forming into one step in minutes instead of hours needed when using conventional methods. This is a near-net-shape forming method such that only a minor amount of machining or polishing is needed. The present forming method provides an unprecedented opportunity to make optically transparent domes at much lower costs.
Nano-composite optical ceramics for infrared windows and domes
Currently available IR transparent materials typically exhibit a trade-off between optical performance and mechanical strength. For instance, sapphire domes are very strong, but lack full transparency throughout the 3-5 micron mid-wave IR band. Yttria is fully transparent from 3-5 microns, but lacks sufficient strength, hardness, and thermal shock resistance for the most demanding aero-thermal applications. Missile system designers must limit system performance in order to accommodate the shortcomings of available window and dome materials. Recent work in the area of nanocomposite ceramics may produce new materials that exhibit both excellent optical transparency and high strength, opening the door to improved missile performance. The requirements for optical nanocomposite ceramics will be presented and recent work in producing such materials will be discussed.
Polycrystalline yttrium aluminum garnet (YAG) for IR transparent missile domes and windows
Polycrystalline Yttrium Aluminum Garnet (YAG) is being considered as an attractive material candidate for IR transparent missile domes and reconnaissance windows, due to its superior optical clarity and mechanical properties compared to the incumbent material choices. YAG possesses a very uniform index of refraction with minimal variation. Its fracture strength, hardness, and toughness also rank high among various other optically transparent materials and can be optimized further through grain size minimization. Polycrystalline YAG has been in development for several years at Raytheon for laser gain and IR transparency applications. Recent advances in optical loss characterization and optimization, scale-up efforts, and the fabrication of non-planar geometries such as hemispherical domes will be presented. In addition, the YAG material trade study conducted to date on thermal, optical, mechanical properties are discussed.
Modeling, Characterization, and Fabrication I
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Optical properties of Nd-doped and undoped polycrystalline YAG
High optical quality polycrystalline yttrium aluminum garnet (YAG) is now available. The optical properties of pure polycrystalline YAG and 1% Nd doped polycrystalline YAG are reported from the midwave infrared to the visible. The absorption and scatter properties are represented in terms of standard models.
Erosion studies of infrared dome materials
Roger M. Sullivan, Andrew Phelps, James A. Kirsch, et al.
The testing reported in this paper operationalized the material requirement: An infrared transparent dome material must be at least as good as magnesium fluoride in rain tests and substantially better than magnesium fluoride in sand tests. Sand erosion test conclusions, based on changes in midwave infrared transmission, are that CleartranTM with the protective coating system tested is not substantially more resistant to large grain sand erosion damage than magnesium fluoride. ALONTM and spinel are substantially more resistant to large grain sand erosion damage than magnesium fluoride. There is no significant transmission difference due to small grain sand erosion observed between any of the tested coupons. Qualitative analysis of coupon damage after exposure to an artificial rain field on a whirling arm showed that ALONTM and spinel are at least as rain erosion resistant as magnesium fluoride, but the coated CleartranTM coupons delaminated rapidly under the same rain test conditions. Testing coupons exposed sequentially to the milder sand condition followed by the whirling arm rain erosion test demonstrated that magnesium fluoride rain resistance is diminished in the combined test, but that ALONTM and spinel retain their robust resistance. Coated CleartranTM delaminated under the combined conditions as well. It is noteworthy that the results reported for the midwave infrared range also apply to the near infrared region above 1 micron.
BRDF and BSDF models for diffuse surface and bulk scatter from transparent windows
A novel approach based on the generalized Van Cittert-Zernike theorem is used to characterize the scatter properties of window materials and coated surfaces. The scattered light is categorized based on the level of coherence of the scattered light. A closed form model is applied to a wide range of illumination frequencies and material types. Diffuse scattered light is represented in a straightforward manner. Comparisons between measurements and model fits will be presented.
Application of nondestructive optical techniques in the detection of surface and subsurface defects in sapphire
Ikerionwu A. Akwani, Douglas L. Hibbard, Keith T Jacoby
Advancements in optical manufacturing and testing technologies for sapphire material are required to support the increasing use of large aperture sapphire panels as windscreens for various electro-optical system applications. It is well known that the grinding and polishing operations employed to create optical surfaces leads to the introduction of surface stress and sub-surface damage which can affect critical opto-mechanical performance characteristics such as strength and durability. Traditional methods for measuring these defects are destructive and, therefore, unsuitable as in-process, high volume inspection tools. A number of non-destructive optical techniques were investigated at Exotic Electro-Optics under funding by the Office of Naval Research and the Air Force Research Laboratory including Raman spectroscopy, laser polarimetry and the Twyman effect to characterize process-induced defects in sapphire panels. Preliminary experimental results using these techniques have shown that surface stress and sub-surface damage may be non-destructively measured. Raman spectroscopy has shown promise in quantifying surface stress, laser polarimetry is of questionable utility and the Twyman effect may be used qualitatively to monitor relative stress and sub-surface damage. This information will ultimately provide a better understanding of the overall manufacturing process leading to optimized process time and cost.
Characterization of AFB sapphire single crystal composites for infrared window application
H.-C. Lee, H. E. Meissner
Next generation weapons platforms may require 30" x 30" sapphire windows. Since these sizes exceed what can be manufactured directly, a concept is proposed and experimental data are furnished in this report on the viability of increasing the window dimensions by Adhesive-Free-Bonding (AFB®) of smaller starting components by their edges. The bonding scheme has been evaluated for single crystal sapphire but is expected to also work equally well for other IR window materials. The bonding mechanism is explained with Van der Waals theory of attractive forces and confirmed experimentally by applying the bending plate theory. The gap at the interface between two components is deduced from the measured roughness of the polished surfaces that are brought into optical contact and subsequently heat-treated, and is estimated to be about 2 Å rms. Stress relief at AFB® interfaces has been established. Experimental data of flexural strength determined by four-point bending at room temperature is reported. The data indicates that AFB® composite specimens and equivalently prepared blank samples fracture at statistically same loads under standardized testing conditions. Failure of composites has not been observed at the interface and only at random flaws that are a result of sample preparation.
Simulation and experimental results of sub-aperture transmitted wavefront measurements of a window using a time-delayed source
It is often desirable to measure an optical component whose aperture exceeds the capacity of the measurement device. However, stitching of sub-aperture measurement data into a single measurement of an optical component is a challenging problem since mechanical motions of the test component relative to the reference surface of an interferometer can not be made with interferometric accuracy. Even more challenging than the need to compensate for rigid body motion between the sub-aperture measurements is the need to account for imperfections in the reference surface itself. In this paper we show, both in simulation and experimentally, how the use of a time-delayed source (TDS) simplifies the stitching of transmitted wavefront measurements from domes and windows. This is accomplished by making it possible to obtain phase-shifted interferometric measurements using only the light reflected by two surfaces from a dome or window without the use of a reference surface.
Modeling, Characterization, and Fabrication II
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Transmitted wavefront metrology of hemispheric domes using scanning low-coherence dual-interferometry (SLCDI)
Modern missile domes are up to 7 inches in diameter, subtending an angular aperture of 180 degrees. Quantifying the transmitted wavefront of these domes is critical for quality control, but such optics are diffcult or impossible to measure using conventional interferometric techniques. To address this issue, we have developed a non-contact measurement process that uses a technology similar to optical coherence tomography (OCT) to map the optical thickness of the dome over its full aperture. The technique has been termed Scanning Low Coherence Dual Interferometry (SLCDI), and has the unique ability to measure the optical thickness of component layers within multilayer domes to an accuracy of 0.1 micron. In this paper we demonstrate the capability of SLCDI by measuring the optical thickness of a seven inch diameter BK7 dome at a sampling resolution of 0.2 mm. SLCDI yields results comparable to those from a Zygo interferometer, and the two methods agree to within 0.2 micron. From this we conclude that SLCDI is an effective tool for measuring the optical quality of hemispheric domes.
Time-delayed source and interferometric measurement of domes and windows
Measurement of the transmitted wavefront of domes and windows is a long-standing problem. One may use a large return sphere and measure the interference cavity without the dome present and again with the dome present. The difference between the two measurements is a double-pass measurement of the transmitted wavefront of the dome. Even so, the long coherence length of the source results in many extraneous fringe patterns. Windows may be tested by using a collimated source and return flat. A time-delayed source (TDS) having a short-coherence length is used to obtain a single interference pattern due only to interference of light reflected by the two surfaces of a dome or window. Standard phase shifting algorithms may be used with the TDS to measure the optical thickness of a dome or window without errors due to multiple reflections. Since most of the interferometer is common-path, environmental sensitivity is reduced and alignment is straightforward compared to typical interferometers. Finally, since there is no reference surface, stitching of sub-aperture measurements is simplified.
Laser-assisted pre-finishing of optical ceramic materials
Jay C. Rozzi, Odile H. Clavier, Michael D. Barton
At Creare, we are developing a laser-assisted, pre-finishing system that enables the single-point diamond turning of super-hard ceramics into hemispheres, ogives, and other shapes that are ready for final optical finishing. Currently, super-hard ceramic materials cannot be affordably processed due to the low material removal rates and the high amount of sub-surface damage associated with current processes. Our innovation uses a low-power, far-infrared laser to heat, but not ablate, a thin layer of material prior to its removal. By heating the ceramic material, plastic-like deformation at the cutting edge is fostered by high-temperature dislocation motion. In doing so, the cutting forces are reduced which enables attendant reductions in tool wear, surface and sub-surface damage, and processing time. Our paper will summarize the development of our innovation, describe the process, discuss the machine tool, and review the latest results.
Developments in the finishing of domes and conformal optics
The final finish and characterization of windows and domes presents a number of difficult challenges. Furthermore, there is a desire to incorporate conformal shapes into next generation imaging and surveillance systems to provide significant advantages in overall component performance. Unfortunately, their constantly changing curvature and steep slopes make fabrication of such shapes incompatible with most conventional polishing and metrology solutions. Two novel types of polishing technology, Magnetorheological Finishing (MRF®) and Magnetorheological Jet (MR JetTM), along with metrology provided by the Sub-aperture Stitching Interferometer (SSI®) have several unique attributes that give them advantages in enhancing fabrication of hemispherical domes and even conformal shapes. The advantages that MRF brings to the precision finishing of a wide range of shapes such as flats, spheres (including hemispheres), cylinders, aspheres and even freeform optics, has been well documented. The recently developed MR Jet process provides additional benefits, particularly in the finishing the inside of steep concave domes and other irregular shapes. Combining these technologies with metrology techniques, such as the SSI, provides a solution for finishing current and future windows and domes. Recent exciting developments in the finishing of such shapes with these technologies will be presented. These include new advances such as the ability to use the SSI to characterize a range of shapes such as domes and aspheres, as well as progress in using MRF and MR Jet for finishing conventional and conformal windows and domes.
Contact mechanics models and algorithms for dome polishing with UltraForm Finishing (UFF)
UltraForm Finishing (UFF) is a new deterministic subaperture computer numerically controlled (CNC) polisher. Because UFF uses compliant tools with large contact patches, the depth of removal is prescribed by adjusting the tool crossfeed velocity. The equations for the depth of removal as the tool traverses an axisymmetric part are derived. The form correction problem consists in solving these equations by adjusting the tool crossfeed velocity to achieve a desired removal profile. The solution must satisfy constraints on the tool velocity and acceleration. Solutions for flats, spheres and aspheres are achieved by treating the problem as a constrained optimization after writing the depth of removal equations in matrix form. The solutions were validated experimentally. The removal function is evaluated by making a removal spot for one set of process parameters. Its variations, as a function of the process parameters, are predicted by using Hertz contact theory and the Preston equation. To prevent tool-part collisions and to analyze part and spot measurements, algorithms were developed for the tool path and evaluation of metrology inputs.
Improving surface figure and microroughness of IR materials and diamond turned surfaces with magnetorheological finishing (MRF)
Optics manufactured for infrared (IR) applications are commonly produced using single point diamond turning (SPDT). SPDT can efficiently produce spherical and aspheric surfaces with microroughness and figure error that is often acceptable for use in this region of the spectrum. The tool marks left by the diamond turning process cause high surface microroughness that can degrade performance when used in the visible region of the spectrum. For multispectral and high precision IR applications, surface figure may also need to be improved beyond the capabilities of the SPDT process. Magnetorheological finishing (MRF®) is a deterministic, subaperture polishing technology that has proven to be very successful at simultaneously improving both surface microroughness and surface figure on spherical, aspheric, and most recently, freeform surfaces. MRF has been used on many diamond turned IR materials to significantly reduce surface microroughness from tens of nanometers to below 1 nm. MRF has also been used to successfully correct figure error on several IR materials that are not diamond turnable. This paper will show that the combination of SPDT and MRF technologies enable the manufacture of high precision surfaces on a variety of materials including calcium fluoride, silicon, and nickel-plated aluminum. Results will be presented for microroughness reduction and surface figure improvement, as well as for smoothing of diamond turning marks on an off-axis part. Figure correction results using MRF will also be presented for several other IR materials including sapphire, germanium, AMTIR, zinc sulfide, and polycrystalline alumina (PCA).
Optical Coatings and Surface Structures
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High durability antireflection coatings for silicon and multispectral ZnS
Shay Joseph, Orna Marcovitch, Ygal Yadin, et al.
In the current complex battle field, military platforms are required to operate on land, at sea and in the air in all weather conditions both day and night. In order to achieve such capabilities, advanced electro-optical systems are being constantly developed and improved. These systems such as missile seeker heads, reconnaissance and target acquisition pods and tracking, monitoring and alert systems have external optical components (window or dome) which must remain operational even at extreme environmental conditions. Depending on the intended use of the system, there are a few choices of window and dome materials. Amongst the more common materials one can point out sapphire, ZnS, germanium and silicon. Other materials such as spinel, ALON and yittria may also be considered. Most infrared materials have high indices of refraction and therefore they reflect a large part of radiation. To minimize the reflection and increase the transmission, antireflection (AR) coatings are the most common choice. Since these systems operate at different environments and weather conditions, the coatings must be made durable to withstand these extreme conditions. In cases where the window or dome is made of relatively soft materials such as multispectral ZnS, the coating may also serve as protection for the window or dome. In this work, several antireflection coatings have been designed and manufactured for silicon and multispectral ZnS. The coating materials were chosen to be either oxides or fluorides which are known to have high durability. Ellipsometry measurements were used to characterize the optical constants of the thin films. The effects of the deposition conditions on the optical constants of the deposited thin films and durability of the coatings will be discussed. The coatings were tested according to MIL-STD-810E and were also subjected to rain erosion tests at the University of Dayton Research Institute (UDRI) whirling arm apparatus in which one of the coatings showed no rain drop impact damage at all.
iDLC: hardcoat for chalcogenide glass and other IR materials
K. Rogers, John Ward, Y. Guimond
A novel IR transmissive hard coating that offers protection to harsh environmental conditions on GASIR® and other IR materials for thermal imaging and sensing applications. iDLC has been developed to maximise both spectral and environmental performance for GASIR®. This coating can be applied to the outside surface of molded optics and windows and offers high spectral efficiencies from 1.4μm to 15μm. The ability to deposit a multi-layer structure allows broad band high efficiency anti-reflection coatings to be produced. Compared to conventional DLC, this coating offers significantly less absorption, lower reflection and thus allows higher transmission over a wide spectral band. Tests have shown that the coating offers exceptional resistance to abrasion, salt spray and humidity. The process used to manufacture iDLC has been configured for production volumes and offers a process for a wide range of applications on IR electro-optic materials.
Effects of mesh voids on insertion loss of metallic mesh coatings
Jennifer Halman, Keith Ramsey, Vashti Sawtelle
Metallic mesh thin-film coatings have been used for many years to provide electromagnetic interference (EMI) shielding on infrared windows and domes. During the fabrication of these conductive, micron-sized mesh patterns, mesh voids or holes in the mesh pattern occasionally occur. Voids in the mesh degrade the EMI shielding or insertion loss of the mesh coating. In the past, we have shown that a small number of 1-mm voids do not degrade the insertion loss significantly for 20-dB insertion-loss mesh coatings. In this paper, we present a theory that provides an approximation for the number and size of mesh voids that can be tolerated without degrading the EMI shielding properties of a mesh coating. We also measured the insertion loss of several typical metallic-mesh coatings with and without voids and compared the results with our simple insertion loss model. Our analysis shows that tens of very small voids may have only minimal impact on the EMI shielding properties of a metallic mesh coating. Even a single 3-mm diameter void may not degrade the shielding properties significantly.
Update on the development of high performance anti-reflecting surface relief micro-structures
Microstructures built into the surfaces of an optic or window, have been shown to suppress the reflection of broad-band light to unprecedented levels. These antireflective (AR) microstructures form an integral part of an optic component, yielding an AR property that is as environmentally robust, mechanically durable, and as radiation-hardened as the bulk material. In addition, AR microstructures built into inexpensive glass windows, are shown below to exhibit a threshold for damage from high energy lasers of nearly 60 J/cm2, a factor of 2 to 4 increase over published data for conventional thin-film dielectric material AR coatings. Three types of AR surface relief microstructures are being developed for a wide variety of applications utilizing light within the visible to very long wave infrared spectrum. For applications requiring broad-band operation, Motheye AR textures consisting of a regular periodic array of cone or hole like structures, are preferred. Narrow-band applications such as laser communications, can utilize the very high performance afforded by sub-wavelength structure, or SWS AR textures that consist of a periodic array of simple binary, or step profile structures. Lastly, Random AR textures offer very broad-band performance with a simple manufacturing process, a combination that proves useful for cost sensitive applications such as solar cells, and for complex devices such as silicon and HgCdTe sensor arrays. An update on the development of AR microstructures is discussed for many specific applications. Data from SEM analysis, reflection and transmission measurements, environmental durability testing, and laser damage testing, is shown for AR microstructures fabricated in silicon, fused silica, borofloat glass, ZnGeP, AMTIR, As2Se3, As2S3, and GaAs.
Visible to Mid-wavelength Infrared Transparent Materials and Applications
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An improved soft-chemistry approach to the preparation of spinel powders
Spinel powders for the production of transparent polycrystalline ceramic windows have been produced using a number of traditional ceramic and sol-gel methods. We have demonstrated that magnesium aluminate spinel powders produced from the reaction of organo-magnesium compounds with surface modified boehmite precursors can be used to produce high quality transparent spinel parts. In previous work, the spinel powders were prepared by the reaction of surface-modified boehmite nanoparticles with magnesium acetylacetonate. While the magnesium acetylacetonate can produce small quantities of high quality spinel powders, it use for large scale production of spinel powders is problematic. Through a thermodynamic analysis we have identified a new high-purity, low-cost, low-toxicity organomagnesium compound that reacts the with surface modified boehmite nanoparticles to produce a spinel precursor. The magnesium doped precursor readily transforms into pure phase spinel at temperature between 900°C and 1200°C.
Polycrystalline transparent spinel domes for multimode seeker applications
A. A. DiGiovanni, A. LaRoche, L. Schubel, et al.
Magnesium aluminate spinel is a durable, broadband, electro-optical material that can be readily manufactured into transparent domes for multimode seeker applications. Actual spinel domes have suffered from manufacturing difficulties and light-scattering inclusions. The program described herein has solved many of the difficulties to achieve better optical properties and better process yields.
Critical parameters for grinding large sapphire window panels
Joseph R. Bashe, Gene Dempsey, Ikerionwu A. Akwani, et al.
Advances in optical manufacturing and testing technologies for sapphire material are required to support the increasing use of large-aperture sapphire panels as windscreens for various electro-optical system applications. Single surface grinding is a crucial process step in both the figuring and finishing of optical components. Improper grinding can make subsequent polishing operations more difficult and time consuming. Poor grinding can also lead to the introduction of surface stress and sub-surface damage which can affect critical opto-mechanical performance characteristics such as strength and durability. Initial efforts have been completed at Exotic Electro-Optics under the funding of the Office of Naval Research and the Air Force Research Laboratory to investigate a number of process enhancements in the grinding of a-plane sapphire panels. The information gained from this study will ultimately provide a better understanding of the overall manufacturing process leading to optimized process time and cost. EEO has completed two sets of twelve-run Plackett-Burman designs of experiment (DOE) to study the effects of fundamental grinding parameters on sapphire panel surfaces. The relative importance of specific process parameters on window characteristics including surface roughness, stress, sub-surface damage are reported.
Improvements in large window and optics production
Fabrication of large optics has been a topic of discussion for decades. As early as the late 1980s, computer-controlled equipment has been used to semi-deterministically correct the figure error of large optics over a number of process iterations. Magnetorheological Finishing, MRF®, was developed and commercialized in the late 1990's to predictably and reliably allow the user to achieve deterministic results on a variety of optical glasses, ceramics and other common optical materials. Large and small optics such as primary mirrors, conformal optics and off-axis components are efficiently fabricated using this approach. More recently, specific processes, MR Fluids and equipment have been developed and implemented to enhance results when finishing large aperture sapphire windows. MRF, by virtue of its unique removal process, overcomes many of the drawbacks of a conventional polishing process. For example, lightweighted optics often exhibit a quilted pattern coincident with their pocket cell structure following conventional pad-based polishing. MRF does not induce mid-frequency errors and is capable of removing existing quilt patterns. Further, odd aperture shapes and part geometries which can represent significant challenges to conventional polish processing are simply and easily corrected with MRF tools. Similarly, aspheric optics which can often present multiple obstacles-particularly when lightweighted and off-axis−typically have a departure from best-fit sphere that is not well matched with to static pad-based polishing tools resulting in pad misfit and associated variations in removal. The conformal subaperture polishing tool inherent to the QED process works as well on typical circular apertures as it does on irregular shapes such as rectangles, petals and trapezoids for example and matches the surface perfectly at all points. Flats, spheres, aspheres and off-axis sections are easily corrected. The schedule uncertainties driven by edge roll and edge control are virtually eliminated with the MRF process. This paper presents some recent results of the deterministic finishing typified by the QED product line and more specifically of its large-aperture machines, presently capable of finishing optics up to one meter in size. Examples of large sapphire windows and meter-class aspheric glass optics will be reviewed. Associated metrology concerns will also be discussed.
Poster Session
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Reduced angle-shift infrared bandpass filter coatings
B. M. Lairson, J. Mosier, K. Gibbons, et al.
Infrared optics are often expected to perform over a wide range of angles of incidence, over a selected bandpass. For the best performance, it is often desired that the shift of spectral features, and the splitting between s and p polarizations, be minimized. These issues can be mitigated to a large extent by design, particularly over a narrow range of angles. However, the availability of high index materials throughout a stack design can greatly improve the performance at a given coating thickness, or greatly reduce the overall thickness required to achieve a design. Here we discuss several designs that have been achieved via hydrogenated silicon in multilayers, which demonstrate improved performance at oblique angles of incidence.