This PDF file contains the front matter associated with SPIE Proceedings Volume 7302, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

A. Verneuil developed flame fusion to grow sapphire and ruby on a commercial scale around 1890. Flame fusion was further perfected by Popov in the Soviet Union in the 1930s and by Linde Air Products Co. in the U.S. during World War II. Union Carbide Corp., the successor to Linde, developed Czochralski crystal growth for sapphire laser materials in the 1960s. Stepanov in the Soviet Union published his sapphire growth method in 1959. Edge-Defined Film-Fed Growth (EFG), which is similar to the Stepanov method, was developed by H. Labelle in the U. S. in the 1960s and 1970s. The Heat Exchanger Method (HEM), invented by F. Schmid and D. Viechnicki in 1967 was commercialized in the 1970s. Gradient solidification was invented in Israel in the 1970s by J. Makovsky. The Horizontal Directional Solidification Method (HDSM) proposed by Kh. S. Bagdasorov in the Soviet Union in the 1960s was further developed at the Institute for Single Crystals in Ukraine. Kyropoulos growth of sapphire, known as GOI crystal growth in the Soviet Union, was developed by M. Musatov at the State Optical Institute in St. Petersburg in the 1970s and 1980s. At the Institute for Single Crystals in Ukraine, E. Dobrovinskaya characterized Verneuil, Czochralsky, Bagdasarov, and GOI sapphire. In 1995, she emigrated to the United States and joined S&R Rubicon, founded near Chicago by R. Mogilevsky initially to import sapphire and ruby. Mogilevsky began producing sapphire by the Kyropoulos method in 1999. In 2000 the company name was changed to Rubicon Technology. Today, Dobrovinskaya is Chief Scientist and Rubicon produces high quality Kyropoulos sapphire substrates for solid-state lighting. In 1995, H. Branover of Ben Gurion University and a sole investor founded Gavish, which is Hebrew for "crystal." They invited another veteran of the Ukrainian Institute for Single Crystals, V. Pishchik, to become Chief Scientist. Under Pishchik's technical leadership and J. Sragowicz's business leadership, Gavish now makes finished products for the semiconductor and medical industries from HDSM, Stepanov, and Kyropoulos sapphire.

Demand for larger aperture sapphire IR windows is increasing. To withstand the higher dynamic and pressure forces exerted on them these larger windows require thicker material. Edge Defined Film-fed Growth (EFG)^TM Sapphire crystals have traditionally been grown with a thickness of ≤ 11 mm, then finished and polished to a nominal thickness of 5.5 mm. We present optical characteristics data here for Class^225(R) EFG^TM sapphire sheet that is being grown up to 22 mm thick and finished at 16.8 mm.

Sapphire is a widely used material for optical, electronic and semiconductor applications due to its excellent optical properties and very high durability. Optical and mechanical properties of sapphire depend on many factors such as the starting materials that are used to grow crystals, methods to grow sapphire crystals, etc. Demand for highest purity and quality of sapphire crystals increased ten fold for the last several years due to new applications for this material. In this work we studied the effect of starting materials and crystal growth methods on the optical and mechanical properties of sapphire, especially concentrating on the effect of hydrogen on the properties of sapphire. It was found that the infrared (IR) absorption which is traditionally used to measure the hydrogen content in sapphire crystals cannot be reliably used and the data obtained by this method provides a much lower hydrogen concentration than actual. We have shown for the first time that Nuclear Magnetic Resonance techniques can be successfully used to determine hydrogen concentration in sapphire crystals. We have shown that hydrogen concentration in sapphire can reach thousands of ppm if these crystals are grown from Verneuil starting material or aluminum oxide powder. Alternatively, the hydrogen concentration is very low if sapphire crystals are grown from High Purity Densified Alumina (HPDA®) as a starting material. HPDA® is produced by EMT, Inc through their proprietary patented technology. It was found that optical and mechanical properties of sapphire crystals grown using EMT HPDA® starting material are much better than those sapphire crystals grown using a starting material of Verneuil crystals or aluminum oxide powder.

CeraNova's transparent polycrystalline alumina (CeraLumin^TM)a has sub-micron grain size (300-500nm) and high transmittance in the mid-wave infrared (>85% in the 3-5µm MWIR region). The fine, uniform grain size imparts high hardness, high strength, and high thermal shock resistance. Polycrystalline alumina is a viable alternative to sapphire for domes, particularly for aerodynamic shapes which are readily fabricated by powder processing. Both hemispheric and ogive domes (sub-scale and full-size) have been successfully molded and densified to transparency. Hemispheric domes have been optically finished. Current efforts include a focus on scale-up, fabrication, and metrology of aerodynamic domes. This paper presents recent analyses of microstructure, optical properties, and mechanical properties.

The demand for large ALON® windows has continued to increase since the material transitioned to Surmet Corporation for commercialization. Two applications which represent opposite ends of the requirements spectrum in terms of required optical performance and cost sensitivity are Reconnaissance windows and transparent armor. Consequently, the approaches to producing large area windows for both applications are quite different. While Recce applications require windows of the highest possible optical quality and stringent refractive index homogeneity across the large aperture sizes of Recce sensors, the optical requirements for transparent armor windows are substantially looser. Furthermore, optical performance is paramount for Recce applications while transparent armor applications are more strongly driven by cost considerations. Surmet has developed processes for producing large (i.e., up to ~17x30-in) ALON® window blanks of extremely high optical quality and refractive index homogeneity, for Recce applications. This material has been optically fabricated into finished windows and characterized for transmitted wavefront and homogeneity. Recent results will be presented. Large area transparent armor windows have been produced using a tiling approach. Since transparent armor laminates consist of multiple layers (i.e., ALON/Glass/Polycarbonate) Smaller ALON® tiles can be face bonded onto the underlying glass and polycarbonate layers to produce very large windows. Excellent ballistic results have been obtained using a tiled configuration. Recent results will be presented.

Future advanced communications systems will utilize lasers which operate at 1.55 μm backed up by an RF links. To the extent that such systems utilize a common aperture, dual IR/RF windows and domes will be required. The durability of such windows, with respect to rain and sand erosion damage, is an important consideration as damaged surfaces will lead to significant optical degradation for operation at 1.55 μm. This requirement drives the materials choices toward more durable materials such as ALON® Optical ceramic, spinel and sapphire. Single layer windows, with appropriately selected thicknesses of these materials can be used for narrow RF wavebands, but are not adequate for applications requiring broadband RF transparency. To this end, multilayer windows, with durable outer layers of ALON have been developed, and built. Recent results will be presented.

In the framework of many EL-OP projects there was increasing interest in large optical windows for protecting infrared sensing systems. There are many solutions for dual visible and midwave infrared windows, but for windows larger than 300 mm in diameter only clear Zinc Sulfide is available. When large area windows are required Zinc Sulfide becomes heavy, costly, and with significant optical scattering. Sapphire and ALON^TM (Aluminum -oxynitride) were chosen as candidates for the transparent ceramic due to their high strength, availability and low optical scattering. As mentioned above these materials are not available in the form of large windows thus a joining technique was necessitated. Three main joining processes were investigated: Adhesive Bonding, Optical Glass Bonding -OGB and diffusion bonding, which were expected to result in good optical transmittance and moderate to good mechanical performance.

Polycrystalline spinel serves as an alternative to materials such as sapphire and magnesium fluoride that are currently being used in electromagnetic window applications such as missile domes, where high strength, high hardness and high transmittance in the visible and infrared spectra are required. The cubic crystal lattice of spinel imparts an isotropy to the bulk optical property, which eliminates optical distortion due to birefringence that occurs in sapphire and other non-cubic materials. The current study is to find a reliable manufacturing process to produce large magnesium aluminate spinel domes from powder consolidation efficiently. A binder-less dry ball milling process was used to deflocculate the spinel powder to increase its fluidity in an effort to ease the shape-forming. Dry ball milling time trials were conducted at several intervals to determine the appropriate level of time required to break up both the hard and soft agglomerates associated with the virgin spinel powder. The common problems encountered in dry powder shape-forming are crack growth and delamination of the green body during cold isostatic pressing (CIPing). The cracking and the delamination are due to the buildup of stress gradients on the green body that are created by the frictional force between the powder and the die wall or mold wall. To understand the stresses during the CIPing process, a finite element analysis of stresses on the green body was conducted. The simulation was used to evaluate the effect of die tooling and process characteristics on the development of stress gradients in the green body dome. Additionally, the effect of friction between the die wall and powder was examined by the simulation. It was found that by mitigating the frictional forces, cracking and delamination on the green body could be eliminated. A stepped-pressure CIPing technique was developed to reduce stress gradient build-up during CIPing. Also, oleic acid lubricant was applied to the die wall to reduce the wall friction between the powder and the die itself. As a result of these two above-mentioned methods, it was demonstrated that it is possible to consolidate a binder-free powder into large defect-free domes.

We report on the bond strength at the interfaces of Adhesive-Free-Bonded (AFB®) single crystal sapphire composite bars as deduced from the fracture toughness. Fracture toughness (K_IC) characterizes the resistance of a bulk brittle material to fractural failure caused by unstable crack propagation. Correlation between the well-defined initial flaw size and the apparent failure strength gives the fracture toughness that is a function of the average bond strength at the fractured interfaces. To ensure that the fracture interface coincides with the AFB® interface, we make a pre-notch at the AFB® interface to be the initial surface flaw using a 532 nm wavelength marking laser. The measured failure strength yields the fracture toughness of the AFB® composites. We have found in this study that the average fracture toughness of AFB composite samples is 2.51 MPa*√m and that of non-composite control samples is 2.39 MPa*√m. The correlating fracture energy is 13.1 J/m² and 11.9 J/m², respectively. The apparently greater fracture toughness of composite samples compared to that of non-composite samples conforms with the hypothesis that attributes the origin of the bonding forces at the AFB interfaces to the Lodon-Van der Waals interaction between two solid bodies.

A significant challenge in the fielding of transparent MgAl₂O₄ (spinel) ceramic parts for a variety of military applications is the limited availability and fairly high cost of starting powder with consistent quality and performance. In addition, available powders often require additional processing (particularly the addition of a sintering aid such as LiF) prior to ceramic forming and sintering. Although the current sources of commercial spinel powder are limited, separate Mg and Al oxides or hydroxides are among the most widely produced ceramic powders on the market. If stoichiometric combinations of such powders could be substituted with modest effort into existing procedures for transparent spinel manufacture, significant gains could be made in cost, availability, and consistency of the resulting ceramic bodies. To this end we have studied the suitability of various commercial sources of MgO, Mg(OH)₂, γ-Al₂O₃, and AlOOH for transparent MgAl₂O₄ production. Our methods have been kept simple to facilitate comparisons between trials and to maintain a focus on eventual manufacturing feasibility. Stoichiometric mixtures of Mg and Al powders are thoroughly mixed in an aqueous slurry. The solids are collected, dried, calcined, milled with LiF (as a sintering aid), and sieved. The powders are sintered into dense ceramics with standard hot pressing and hot isostatic pressing procedures. Resulting ceramic transmission is measured and correlated with the purity, surface area, and phase composition of the prepared powders.

Exotic Electro-Optics recently conducted a basic evaluation of state-of-the-art spinel material from the standpoint of optical fabrication. The goal of this study was to characterize the behavior of several spinel samples as they passed through a complete optical fabrication sequence. Overall, the material was found to be compatible with conventional fabrication processes. Methodologies used in the manufacture of heritage optical components were employed successfully, without significant modification, in the fabrication of the spinel windows. A standard anti-reflective coating was used to coat polished spinel samples. Good coating-to-substrate adhesion was observed and the coated optic exhibited the expected spectral performance. In this paper, Exotic Electro-Optics reports on the results of this work.

Magnesium aluminate spinel is a durable, broadband, electro-optical material that can be readily manufactured into transparent domes for multimode seeker applications. Technology Assessment & Transfer reports on the results of its research and development effort to resolve manufacturing issues with regard to transparent spinel domes. The specific areas of study have been cost, quality, and ability to scale up to full production. Alternative manufacturing approaches were evaluated and compared.

This paper describes MER's recent advances on the development of high strength, transparent magnesium aluminum spinel technology for large IR windows and domes. The novel spinel material exhibits high optical and IR transparency in the 0.2 - 5.5 μm wavelength, is very resistant to abrasion, with density higher than 99.9% of theoretical, with very fine and uniform grain size, and flexural strength of 300 MPa. Spinel domes technology has been scaled up to produce hemispherical 180° aperture domes in sizes up to 7" in diameter using freeze casting technology to produce the green dome preforms. MER is also pursuing the production of large size spinel windows by either producing monolithic large single windows or by edge bonding several smaller size windows. Both approaches present challenges. Production of monolithic large size windows is limited by equipment size, availability, and investment capital while the edge bonding approach requires perfect transparency and strength at the bonded edge. MER together with Precision Photonics Corp. are developing high strength, edge bonded, transparent magnesium aluminum spinel windows for next generation aircraft and other defense armor applications which require windows as large as 30"x30"x0.5" at an affordable cost. MER has further improved strength of the spinel by accurate control of the average grain size and grain size scatter while remarkable transmission is obtained by elimination of the intergrain/intragrain porosity, and by eliminating all possible contamination. The spinel bonding technology under development consists of chemically activated direct bonding (CADB®), an epoxy-free solution-assisted optical-contacting process developed by Precision Photonics Corporation (PPC).

High optical quality spinel is being developed as exit aperture (windows and domes) for various electro-optic (EO) and infrared (IR) systems operating in harsh environmental conditions. These applications require windows with low absorption loss, low scattering loss, good index homogeneity, low transmitted wavefront error and high strength. Windows are also required in very large sizes and thicknesses for specialty applications. We have demonstrated fabrication of high optical quality spinel windows with high strength and environmental ruggedness. We achieved record low absorption loss of 6 ppm/cm at 1.06 μm and transmitted wavefront error of better than λ/10 at 633 nm. We are also developing technology for making very large and very thick spinel windows.

Spinel (MgAl₂O₄) is a good candidate material for transparent armor and IR window applications. Traditionally, transparent spinel has suffered from poor strength and difficult polishing owing to its large, bimodal grain structure. Starting from a spinel nanopowder, spinel ceramics with a grain size of less than 2 microns have been made with better than 80% in-line transmittance at 632 nm wavelength for 3/8" thick samples. A ring-on-ring test has been used to measure biaxial flexural strength on samples machined to 0.8 mm thickness. The average strength was found to exceed 480 MPa.

(100) oriented polycrystalline diamond films are of benefit to applications such as optical windows and coatings due to its smoother as-grown surface and potentially better optical performance than randomly oriented diamond films. (100) Highly Oriented Diamond (HOD) films have been successfully grown on polished silicon wafer using the plasmaenhanced chemical vapor deposition technique (PECVD). The seeding procedure was based on Bias Enhanced Nucleation (BEN), which provided a high-density and uniform diamond nucleation on the entire two inch diameter silicon wafer. During the diamond deposition step subsequent to the BEN process, nitrogen gas was added in the standard methane/hydrogen processing gas mixture. The addition of small amount of nitrogen has three effects: 1) It increases the growth rate almost 3 times. 2) It stabilizes and enhances the (100) orientation growth. 3) It makes the HOD growth possible at high pressures (over 100 Torr) and high temperatures (over 1300 K). The diamond film has been characterized by confocal Raman and SEM, and an optimal temperature window for HOD growth has been identified. The growth rate of the (100) HOD growth rate reached > 16 μm/hour.

In this paper we report an on going research to correlate between optical coating survivability and military (MIL) standards. For this purpose 8 different types of coatings were deposited on 1" substrates of sapphire, multi-spectral ZnS (MS-ZnS), germanium, silicon and BK7. All coatings underwent MIL standard evaluation as defined by customer specifications and have passed successfully. Two other sets were left to age for 12 months at two different locations, one near central Tel-Aviv and one by the shoreline of the Mediterranean Sea. A third set was aged for 2000 hours at a special environmental chamber simulating conditions of temperature, humidity and ultra-violet (UV) radiation simultaneously. Measurements of optical transmission before and after aging from all 3 sets reveal, in some cases, major transmission loss indicating severe coating damage. The different aging methods and their relation to the MIL standards are discussed in detail. The most pronounced conclusion is that MIL standards alone are not sufficient for predicting the lifetime of an external coated optical element and are only useful in certifying the coating process and comparison between coatings.

Data is presented for the erosion resistance and pulsed laser damage threshold of anti-reflecting (AR) microstructures built in the surface of the infrared light transmitting window materials sapphire, ALON, and diamond. It was found that the erosion resistance of AR microstructures (ARMs) in sapphire is comparable to the resistance of sapphire with no AR treatment. Such environmental durability, combined with the enhanced light transmission of windows incorporating ARMs, provides system designers with an effective solution for applications requiring high transmission over long mission times operating in abrasive environments. In addition, the optical power handling capacity of sapphire and ALON windows was investigated through pulsed laser damage threshold measurements with a laser source operating in the near infrared at a wavelength of 1573nm. As with prior results reported for ARMs in fused silica and borosilicate glass, the measured damage threshold of 19 J/cm² for ARMs treated sapphire windows is comparable to the damage level measured for untreated sapphire windows, and this level is at least two times higher than that found with the most durable thin-film AR coatings designed for fused silica. The damage thresholds measured for untreated and ARMs treated ALON windows was also comparable, but at a level more than four times less than the sapphire windows. Lastly, the long-wave infrared light transmission of high performance ARMs fabricated in clear diamond windows is presented. The Air Force Research Laboratoy's Laser Hardened Materials Evaluation Laboratory at WPAFB tested the damage threshold of the ARMs treated diamond windows along with untreated diamond windows using their pulsed CO2 laser setup operating at 9.56μm. Although the results of the tests using two different laser settings were quite variable and inconsistent due to the nature of the diamond material, the damage thresholds measured were in the 50 to 100 J/cm² range, a level much higher than can be achieved with thin-film AR coatings.

The optical performance of windows and domes are subject to degradation from rain and sand erosion damage in harsh flight environments. While durable window and dome materials, such as ALON®, spinel and sapphire are more or less impervious to rain and sand erosion damage in the captive carry environments, the coatings use to provide antireflection (AR) function are not. Rain and/or sand erosion damage of the outer AR coating leads to degradation of the windows optical performance, even when the underlying window itself is not damaged. Surmet has been working on design and development of physical gradient index (Moth- Eye) structures based AR surfaces etched directly into the surface of the ALON substrate. By eliminating the need for less durable coating materials, these structures offer high optical performance without compromising durability. The difficulty of this approach is that the same durability that makes ALON impervious to erosion damage makes it very difficult to etch. Processes have been developed at Surmet which facilitate the etching of fine deep features into ALON surfaces required for broadband AR function. Recent results will be presented.

The story of germanium's development as an infrared dome and window material exemplifies the manner in which the complementary efforts of industry, academia and government can work together to solve scientific and technical challenges. Germanium is one of a few materials which have been used for infrared transparent windows in both the long-wave 8-12 micron region, as well as the mid-wave 3-5 micron region. Due to its legacy as one of the first commercial semiconductors, germanium became available in high purity as a bulk commodity. This paper will examine the history of how germanium came to be used as an infrared window and what enabling technologies have supported its use.

Samples of CVD ZnS from the United States, Germany, Israel, and China were evaluated using x-ray diffraction, transmission and Raman spectroscopy, and biaxial flexure testing. Visible and near-infrared scattering, 6 μm absorption, and ultraviolet cut-on edge varied substantially in tested materials. Transmission cut-on (ultraviolet edge) blue-shifts with annealing and correlates with visible color but not the 6 μm absorption. Raman scattering for CVD ZnS, presented here for the first time, was similar for all ZnS tested. Crystallographic hexagonality and texture was determined and correlated qualitatively with optical scattering. All CVD ZnS tested with biaxial flexure exhibit similar fracture strength values and Weibull moduli. This survey suggests that despite over 30 years of production as an infrared window, the optical properties of CVD ZnS and their variability still defy easy explanation.

A series of experiments was performed to ascertain the effects of various metals on the structure and properties of hot isostatic pressed (HIPped) chemical vapor deposited (CVD) ZnS. Samples were HIPped without metal and with Fe, Co, Ni, Cu, Pd, Ag, Pt, and Au foils. It was found that metals promote recrystallization of CVD ZnS to a greater or lesser extent. A processing temperature of 750 °C for 16 hours was chosen to assess the effect of the metals, since HIPping without metal under these conditions does not recrystallize ZnS. Fe and Co have little or no effect on recrystallization, Ni and Au have moderate effect, and Pt, Pd, Ag, and Cu have the greatest effect. Ag and Cu, however, seem to have problems with indiffusion of the metal. Recrystallization is correlated with improvement in transmission characteristic of multispectral ZnS. "Interrupted HIP" experiments were conducted at 900 °C for 1 hour and at 750 °C for 2 hours to assess the relative effects of temperature and metal on the recrystallization. At 900 °C recrystallization proceeded in the bulk even without metal, but was accelerated by certain metals. At 750 °C, recrystallization took place only on the surface in contact with certain metals and not in the bulk. The role of contact of the metal to the ZnS surface was further explored by comparing Pt HIP experiments with foil fully in contact, foil with air gap, and sputtered metal Pt. Some possible mechanisms for the role of the metal in promoting recrystallization of ZnS are suggested.

A conventional polishing study was conducted with infrared material zinc sulfide with the goal of producing defect-free polished surfaces in predictable amounts of time. Utilizing the measured electro-kinetic properties of the zinc sulfide and polishing abrasives, polishing slurries were selectively altered and the resulting removal rates and surface roughness values were measured. This paper will serve as a baseline for developing an empirical model for optimizing both surface roughness and removal rate for two different types of abrasives with ZnS.

New generations of infrared transmitting optical domes are currently being developed to improve the drag, range, speed, and payload capabilities of missiles. Traditionally, these domes have been hemispheres, which can be well characterized with conventional optical interferometers. These interferometers, however, are not generally well-suited to the new shapes, such as tangent ogives, because the transmitted and reflected wavefronts can differ by many wavelengths from the planar or spherical wavefronts that are normally used as a reference. In this paper, we present an innovative technique to characterize unconventional optical components such as aspheric domes, mirrors, and freeform optics. The measurements are based on an innovative instrument that combines an instantaneous digital phase-shifting infrared interferometer with a dynamic spatial light modulator that extends the range of the interferometer. The goal of the measurement is to determine the wavefront error, within a small fraction of a wavelength, caused by the deviation of the optical component from a perfect geometrical shape of any type (i.e. not spherical). Experimental results are presented from several infrared components.

OptiDomes are six-inch diameter concentric F/0.53 spherical fused silica domes manufactured by Optimax Systems Inc. The purpose of these domes is to act as a standard for metrology testing of various testing methods for measuring the surface quality, mechanical attributes and/or the transmitted wave front error of hemispherical/spherical domes. Each of the OptiDomes was fabricated to pre-specified quality levels including domes that have induced errors. This paper gives additional manufacturing information, detailed handling instructions, procurement information as well as initial measurement results.

A time-delayed source interferometer manipulates the output of a short-coherence length source so that light reflected from the two surfaces of a nominally constant-thickness optical component interfere. The interference pattern is a measure of optical thickness variation and can be phase-shifted. The approach is well suited to optical components that are nominally constant thickness over some portion of the surface. Interferometers suited to the measurement of windows, hemispherical domes and tangent ogives have been built. Data acquisition, calibration tooling and processing methods are described for the stitching of phase data.

OptiPro Systems is developing a non-contact measurement system using state of the art motion control while minimizing the axes of motion during the measurement. The goal is to precisely scan concave and convex surfaces of aspheric, deep parabolic, and ogive shapes without the limitations associated with other measurement methods. The metrology systems will use different computer controlled slicing techniques to create a topographical surface map of the surface form with a high accuracy non-contact probe. To achieve this precise scan the measurement system will incorporate sub-micrometer precision air bearings for the linear and rotary axes motion to minimize the effect of non-repeatable mechanical errors. Calibration of the measurement system will use high precision reference spheres. Finite element modeling and estimate has been used to predict and possibly compensate for mechanical flexures. OptiPro has built a "breadboard" measurement system using a Professional Instruments air bearing and a STIL white light measurement pen. The results from the measurement of a near full hemispheric dome measurement will be presented as well as a comparison to the same dome measured using a stitching interferometer. The final system will incorporates complete computer controlled axes requiring as little operator training and set up as possible. The prototype system will utilize a non-contact pen for measurement. Current developments include the utilization of the STIL white light pen and the OptiGauge optical probe which utilizes invisible 1310 nm infrared light. The current system design and performance will be presented.

Despite increasing demand for freeform optical elements, at present there are no commercial systems to measure such components. In previously published research we demonstrated that a scanning low-coherence dual-wavelength interferometer can accurately measure the transmitted wavefront of hemispheric dome optics by mapping the optical thickness of the dome as a function of probe probe position. To address the issue of more generalized freeform surfaces, we have developed a new probe for the interferometer. This probe incorporates a reference surface and simultaneously projects four beams. This allows the instrument to measure both the position and orientation of the surface with respect to the probe as the probe is scanned over the object. Both the interior and exterior surfaces can be measured simultaneously. Furthermore, by overlapping the measurement regions, the redundant data can be used to minimize some forms of measurement error.

OptiPro Systems has developed a robust 5-axes computer controlled platform, for implementation of the sub-aperture UltraForm Finishing (UFF) process specifically focused on finishing AlON, spinel and transparent polycrystalline alumina (PCA) steep concave, convex and ogive shaped infrared domes and aspheres. Traditional manufacturing of optical components typically involves a three-stage process: grinding, lapping and polishing. The lapping and polishing stages are focused at reducing the surface roughness while preserving the integrity of the form acquired during grinding. Polishing of non spherical and irregular shapes is nearly impossible using traditional full aperture techniques. However, finishing these non-spherical and irregular shapes is possible using UltraForm Finishing. A brief description of the evolution of the UltraForm hardware and processes will be presented, with the current hardware developments. A review of the results with regard to form/figure and roughness improvements on glass, AlON and transparent PCA will be presented using a variety of grinding and finishing abrasives. Differences in the abrasive materials, some bound, and others loose in a slurry have a large impact on the process cycle time and resultant surface roughness.

The final finish and characterization of windows and domes presents a number of challenges in achieving desired precision with acceptable cost and schedule. This becomes more difficult with advanced materials and as window and dome shapes and requirements become more complex, including acute angle corners, transmitted wavefront specifications, aspheric geometries and trending toward conformal surfaces. Magnetorheological Finishing (MRF®) and Magnetorheological Jet (MR Jet®), along with metrology provided by Sub-aperture Stitching Interferometry (SSI®) have several unique attributes that provide them advantages in enhancing fabrication of current and next generation windows and domes. 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. Recent advancements include the ability to finish freeform shapes up to 2-meters in size as well as progress in finishing challenging IR materials. Due to its shear-based removal mechanism in contrast to the pressure-based process of other techniques, edges are not typically rolled, in particular on parts with acute angle corners. MR Jet provides additional benefits, particularly in the finishing of the inside of steep concave domes and other irregular shapes. The ability of MR Jet to correct the figure of conformal domes deterministically and to high precision has been demonstrated. Combining these technologies with metrology techniques, such as SSI provides a solution for finishing current and future windows and domes in a reliable, deterministic and cost-effective way. 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 with increasing size and complexity of design will be presented.

A model has been created based on scattering that describes the λ^-2 dependence of the extinction in bulk samples of CVD ZnS. The model is a version of surface scattering from internal surfaces of layers with different refractive index. The form of the model was inspired by observation of a lamellar nanostructure in CVD ZnS composed of alternating layers of thickness on the order of 10 to 100 nm. The scattering model produces a family of solutions which depends on the difference in refractive index (Δn), the layer thickness, and the roughness. Reasonable ratios of roughness to layer thickness require Δn for CVD ZnS with higher values than can be explained solely by the Δn between sphalerite and wurtzite phases of ZnS. Other evidence suggests a substantial oxygen component in CVD ZnS that could result in the lower refractive index Zn(O,S) necessary for the model. Differences in transmission for CVD ZnS, elemental ZnS, and multispectral ZnS can be explained simply by a different magnitude of Δn between the layers. Absolute transmission is modeled satisfactorily from the band edge to 10 μm using this approach. Extracted Δn's from transmission measurements of various samples correlate well with measured hexagonality from x-ray diffraction.

Metallic mesh thin film coatings have been used for many years to provide electromagnetic interference (EMI) shielding on infrared windows and domes. The level of EMI shielding effectiveness (SE) of metallic mesh coatings when used in a high frequency application is understood and characterized. Conversely, the level of SE of these metallic mesh coatings when used in a low frequency application has been called into question. In a recent study, we applied an appropriately designed metallic mesh coating to a sapphire window, mounted that window in a fixture, and tested the SE of the window assembly over a frequency range that envelopes the various military platforms covered in MIL-STD-461 (10 kHz to 18 GHz) for a radiated emissions test. The test plan was devised in such a way as to independently assess the individual contributions of the aperture, the mounting, and the metallic mesh coating to the total shielding. The results of our testing will be described in this paper. Additionally, the test results will be compared to the predicted SE for both the aperture and the metallic mesh coated window in order to validate the predictive model. Finally, an assessment of the appropriateness of the use of metallic mesh coatings for EMI shielding in a low and/or broad range frequency application will be made.

Metallic mesh coatings are used on visible and infrared windows and domes to provide shielding from electromagnetic interference (EMI) and as heaters to de-fog or de-ice windows or domes. The periodic metallic mesh structures that provide the EMI shielding and/or resistive electrical paths for the heating elements create a diffraction pattern when optical or infrared beams are incident on the coated windows. Over the years several different mesh geometries have been used to try to reduce the effects of diffraction. We have fabricated several different mesh patterns on small coupons of BK-7 and measured the transmitted power and the diffraction patterns of each one using a CW 1064 nm laser. In this paper we will present some predictions and measurements of the diffraction patterns of several different mesh patterns.

Grain boundaries scatter light in polycrystalline materials consisting of birefringent crystals. Apetz and van Bruggen developed a model for grain-boundary scattering based on Rayleigh-Gans-Debye light-scattering theory and demonstrated its application to polycrystalline alumina. This paper reports the measurements of in-line transmittance in polycrystalline magnesium fluoride with different grain sizes and compares the results with the grain-boundary scattering model. Good agreement was obtained between the model predictions and the measured data for grain sizes varying between 0.2 and 2.3 μm for light in the wavelength range, 0.633-5.5 μm.

The development of high-energy lasers has focused attention on the need to assess the mechanical strength of optical components made of fused silica (Si0₂). The strength of this material is known to be highly dependent on the stressed area and the surface finish, but has not yet been properly characterized in the published literature. Recently, Detrio and collaborators at the University of Dayton Research Institute (UDRI) performed extensive ring-on-ring flexural strength measurements on fused Si0₂ specimens ranging in size from 1 to 9 inches in diameter and of widely differing surface quality. In this contribution, we report on a Weibull statistical analysis of the UDRI data-an analysis based on the procedure outlined in Proc. SPIE 4375, 241 (2001). We demonstrate that: (a) a two-parameter Weibull model, including the area-scaling principle, applies; (b) the shape parameter (m is asymptotically equal to 10) is essentially independent of the stressed area as well as the surface finish; (c) the characteristic strength (1-cm2 uniformly stressed area) obeys a linear law, σ_C(in MPa) is asymptotically equal to 160 - 2.83x PBS®(in ppm/sr), where PBS® measures the surface/subsurface "damage." In this light, we evaluate the cumulative failure probability of optically polished and superpolished fused Si0₂ windows as a function of the biaxial tensile stress, for uniformly stressed areas ranging from 0.3 to 100 cm².

L2 Tech, Inc. is in development of an innovative infrared-transparent glass ceramic material with low-thermal expansion (<0.5 ppm/°C) and high thermal-shock resistance to be used as windows and domes for high speed flight. The material is an inorganic, non-porous glass ceramic, characterized by crystalline phases of evenly distributed nano-crystals in a residual glass phase. The major crystalline phase is zirconium tungstate (ZrW₂O₈) which has Negative Thermal Expansion (NTE). The glass phase is the infrared-transparent germanate glass which has positive thermal expansion (PTE). Then glass ceramic material has a balanced thermal expansion of near zero. The crystal structure is cubic and the thermal expansion of the glass ceramic is isotropic or equal in all directions.

Multiple percentages of neodymium doped polycrystalline yttrium aluminum garnet (YAG) from 0 to 10% are now commercially available. This paper summarizes a detailed characterization of this material from the mid infrared to ultraviolet. Characterization includes material transmittance and scatter (BSDF) measurements for multiple doping levels of polycrystalline samples. Material Characterization is presented in the form of standard models.

Ceramic laser gain materials have been in development since the 1960's but it was not until the resurgence in research and development in the 1990's that they showcased equivalent laser performance to their single crystal counterparts. Ceramics offer numerous distinctive advantages over single crystal and are considered to be the key enabler in power scaling of solid state lasers. Ceramics can be made larger, at lower cost, and with additional degrees of engineering design freedom than single crystals, such as higher doping concentration, more uniform or tailored distribution of dopants, and feasibility to be fabricated into monolithic composite structures without bonding. At Raytheon, powder processing methodology has matured to meet the optical requirement, scale-up challenge, and laser performance characteristics in Yb, Nd, and Er doped ceramic YAG materials. This communication presents the latest results obtained by Raytheon on the US fabricated ceramic laser materials.

Reliability and protection of various sensors, optical windows and displays that are used in aircraft, satellites and spacecraft for communications, navigation, display and management of multiple systems are becoming more challenging due to their long-term performance requirements. This paper describes a new development in the area of Parylenes and its role in advancing the effectiveness of various types of sensors, optical windows, displays (including OLEDs and FOLEDs) and electronics. In addition, it highlights various characteristics of the new material that addresses current challenges of the optical materials under harsh and corrosive environments.

A new Time Delayed Source (TDS) has been designed to work with a transmitted wavefront interferometer. The TDS splits a short-coherence wavelength source into two halves, introduces a time delay between the two halves and then recombines the two halves. The recombined wavefronts are fed into a transmitted wavefront interferometer to measure the optical path difference (OPD) between the two surfaces of a part. The time delay is adjusted to match the separation between the two surfaces of the part allowing phase shifting techniques to be used to measure the OPD. The measurement approach can be adapted to measure nominally constant thickness windows, spherical domes and tangent ogives through the design of suitable wavefront matching optics. Producing the wavefront to measure a tangent ogive is accomplished by a series of sub-apertures, each measuring a section of a tangent ogive. The tangent ogive is placed on a rotary stage, and as it rotates, sub-aperture measurements are taken and later stitched together to generate a complete measurement of the part. Alignment of each sub-aperture instrument is essential to obtain an accurate measurement. The setup and alignment of a tangent ogive transmitted wavefront interferometer is also described.