Optical quality polycrystalline spinel (MgAl₂O₄) has been sought as a visible- and infrared-transmitting material since the 1960s because of its potential for transparent armor and durable sensor windows. Its physical properties were known from synthetic crystals available since ~1950 from Linde Air Products. In the late 1960s, methods to process powder into transparent, polycrystalline spinel were investigated at North Carolina State University, General Electric Co., AVCO, and Westinghouse, mainly with Government support. The leading figure in the development of polycrystalline spinel was Don Roy, who began work on spinel at Coors Ceramics around 1970, initially for transparent armor. In the late 1970s, both Coors Ceramics and Raytheon Research Division were funded to make spinel for the infrared dome of the Advanced Short-Range Air-to-Air Missile, an application that disappeared by 1980. In the late 1980s, there was another burst of activity when spinel was a candidate for the Stinger Missile. By 1990, Raytheon had dropped spinel and the material was spun off by Coors Ceramics to Alpha Optical Systems, whose technical effort was led by Don Roy. With low commercial sales potential for spinel, Alpha was dissolved in 1993. RCS Technologies took over a Government contract seeking 200-mm spinel domes for the Harrier aircraft, but this effort ended in 1996 and RCS was dissolved. In 1998, the Army enlisted TA&T to make spinel for transparent armor. Other potential applications appeared and TA&T received numerous Government development contracts. Demand for the still-unavailable spinel drew Surmet to begin development in 2002. In early 2005, spinel is under active development at TA&T and Surmet.

The aluminum oxynitride phase known as ALON is prepared as a ceramic with promising physical and optical properties for infrared transparent domes and windows. It was first reported in the late nineteen fifties. James McCauley of the Army Materials and Mechanics Research Center (AMMRC) began work in the nineteen seventies that led to a firm foundation for understanding the phase diagram of AlN-Al₂O₃, which in turn led to the first transparent ceramic ALON samples. Technology transfer from the AMMRC to Raytheon Corporation resulted in further development of the material, which was qualified for missile dome applications. In the nineteen nineties a renaissance in interest in ALON occurred when ALON composites were shown to have superior qualities as transparent armor. The first decade of the twenty first century has seen Surmet Corporation acquire ALON optical ceramic technology from Raytheon Company and transition the research and development technology to production.

The dome or window on a sensor suite seems, at first glance, to be a relatively low tech item. In reality, it can be one of the most costly items in the system. The choice of materials is highly dependent on the sensor, the anticipated operating conditions, and other requirements such as electromagnetic interference or radar cross section issues. The situation is further complicated when multiple sensor bands are used. Some dome materials are suitable for visible or near infrared applications, some for midwave infrared applications, and others for long wave infrared applications. Materials are also available which can be used for dual band sensors such as visible and midwave sensors. The Army is currently developing a tri-mode seeker containing semi-active laser, midwave infrared, and millimeter sensors all using a common aperture. This added complexity is nowhere more apparent than in the missile dome. Optically transparent infrared optical materials tend to have relatively high dielectric constants. Millimeter wave radomes typically have low dielectric constants. Electromagnetic shielding for an optical seeker frequently consists of some type of grid that serves as a wideband filter. Electromagnetic shielding for millimeter wave missiles may use complicated frequency selective surfaces that block all but the frequency of interest. Unfortunately, those frequency selective surfaces tend to be predominantly metal and are opaque in the optical regime. This paper will discuss the unique requirements that are placed on a tri-mode seeker as well as efforts to meet those requirements.

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. The new powder production method allows fine control over the starting particle size, size distribution, purity and stoichiometry. The new process involves formation of a boehmite sol-gel from the hydrolysis of aluminum alkoxides followed by surface modification of the boehmite nanoparticles using carboxylic acids. The resulting surface modified boehmite nanoparticles can then be metal exchanged at room temperature with magnesium acetylacetonate to make a precursor powder that is readily transformed into pure phase spinel.

Densification and microstructural evolution of transparent magnesium aluminate spinel (MgAl₂O₄) were examined through systematic experiments that focus on the role of impurities in the starting powders. Up to 1 wt. % LiF was added to very fine spinel powder synthesized by a metal-exchange method. The material was densified by pressing at elevated temperatures. Very high transmissivity was achieved under appropriate conditions. A layered spinel/LiF/spinel specimen was also fabricated with the intent of better understanding the role of LiF on microstructure development. Its presence can lead to accelerated grain growth, or growth suppression, depending on a complex interplay of chemistry, concentration and processing schedule.

Transparent magnesium aluminate spinel is an attractive material for use in a wide range of optical applications including windows, domes, armor, and lenses, which require excellent transmission from the visible through to the mid IR. Theoretical transmission is very uniform and approaches 87% between 0.3 to 5 microns. Transmission characteristics rival that of ALON and sapphire in the mid-wave IR, making it especially attractive for the everincreasing performance requirements of current and next-generation IR imaging systems. Future designs in missile technology will require materials that can meet stringent performance demands in both optical and RF wavelengths. Loss characteristics for spinel are being investigated to meet those demands. Technology Assessment and Transfer Inc. (TA&T), have established a 9000 ft² production facility for optical quality spinel based on the traditional hot-pressing followed by hot isostatic pressing (HIPing) route. Additionally, TA&T is developing pressureless sintering - a highly scalable, near net shape processing method based on traditional ceramic processing technology - to fabricate optical components. These two main processing approaches allow the widest variety of applications to be addressed using a range of optical components and configurations. The polycrystalline nature of spinel facilitates near net shape processing, which provides the potential to fabricate physically larger optical parts or larger quantities of parts at significantly lower costs compared to single crystal materials such as sapphire. Current research is focused at optimizing the processing parameters for both synthesis routes to maximize strength and transparency while minimizing the cost of fabrication.

New military requirements have reinvigorated the need for transparent magnesium aluminate (MgAl₂O₄) spinel. Surmet has developed a process that yields high quality transparent spinel at production scale. Several issues related to the extreme requirements of processing ultrafine spinel powders are described. Transmission data is presented for a significant dataset of parts made by this process. More recently, the process has been expanded to include a capability for producing domes for the Joint Common Missile program. Domes at nominal 6” and 7” diameter have been successfully fabricated. Despite early challenges related to the forming portion of the process, a repeatable capability for these domes has been demonstrated. Several challenges remain in spinel processing in order to support additional military requirements. In particular, the strength of the material needs further improvement. Also, improvements in optical quality with regard to inclusions are needed.

Aluminum Oxynitride (ALON^TM Optical Ceramic) is a transparent ceramic material which combines transparency from the UV to the MWIR with excellent mechanical properties. ALON’s optical and mechanical properties are isotropic by virtue of its cubic crystalline structure. Consequently, ALON is transparent in its polycrystalline form and can be made by conventional powder processing techniques. This combination of properties and manufacturability make ALON suitable for a range of applications from IR windows, domes and lenses to transparent armor. The technology for producing transparent ALON was developed at Raytheon and has been transferred to Surmet Corporation where it is currently in production. Surmet is currently selling ALON into a number of military (e.g., windows and domes) and commercial (e.g., supermarket scanner windows) applications. The capability to manufacture large ALON windows for both sensor window and armor applications is in place. ALON windows up to 20x30 inches have been fabricated. In addition, the capability to shape and polish these large and curved windows is being developed and demonstrated at Surmet. Complex shapes, both hyper-hemispherical and conformal, are also under development and will be described.

The optical property characterization of Spinel and AlON samples, as provided by SURMET, is presented. Several experiments are performed to characterize the optical properties of the materials. A broadband FTIR transmissometer acquired data at temperatures ranging from 298 to 800 K covering a frequency range from 1887 cm^-1 to 4000 cm^-1. These measurements provided information on the broadband spectral properties of the samples and the temperature dependent shift of the multiphonon band edge. Laser transmission measurements were performed at 632.8 nm and 3.39 mm to provide very accurate transmission values at the two fixed wavelengths. Finally, BSDF measurements on uneroded/eroded sample pairs were performed at 632.8 nm and 3.39 mm. These measurements indicated that the erosion process introduced to these samples would have minimal affect on the imaging performance of the windows in the mid-infrared.

ALON^TM Optical Ceramic is a durable window material for UV, Visible and Mid IR window and dome applications. The mechanical, thermal, and optical properties of ALON products produced commercially by Surmet Corporation have been measured and this new data will be presented. Comparisons to previously measured data will be made. Optical quality, low scatter ALON having high strength that is nearly double previously reported has been made. Average strength values of 700 MPa at 21°C and 631 MPa at 500°C have been measured for ALON specimens prepared by precision surface finishing techniques. Polished optical domes tested have survived severe thermal shock tests. These strength levels are comparable to those for single crystal sapphire. Strength, thermal conductivity, thermal expansion, refractive index, emissivity and absorption coefficient will be presented. The possible mechanisms for the increased strength will be discussed.

Many defense systems have a critical need for high-precision, complex optics. However, fabrication of high quality, advanced optics is often seriously hampered by the lack of accurate and affordable metrology. QED's Subaperture Stitching Interferometer (SSI^®) provides a breakthrough technology, enabling the automatic capture of precise metrology data for large and/or strongly curved (concave and convex) parts. QED’s SSI complements next-generation finishing technologies, such as Magnetorheological Finishing (MRF^®), by extending the effective aperture, accuracy and dynamic range of a phase-shifting interferometer. This workstation performs automated sub-aperture stitching measurements of spheres, flats, and mild aspheres. It combines a six-axis precision stage system, a commercial Fizeau interferometer, and specially developed software that automates measurement design, data acquisition, and the reconstruction of the full-aperture figure error map. Aside from the correction of sub-aperture placement errors (such as tilts, optical power, and registration effects), our software also accounts for reference-wave error, distortion and other aberrations in the interferometer’s imaging optics. The SSI can automatically measure the full aperture of high numerical aperture surfaces (such as domes) to interferometric accuracy. The SSI extends the usability of a phase measuring interferometer and allows users with minimal training to produce full-aperture measurements of otherwise untestable parts. Work continues to extend this technology to measure aspheric shapes without the use of dedicated null optics. This SSI technology will be described, sample measurement results shown, and various manufacturing applications discussed.

An optical train is designed and built to take a Ø1 inch collimated output sample beam from a 10.6 μm wavelength Wyko IR3 interferometer, and by use of a fast aperture-ratio lens, allow the diverging rays to pass through a steeply curved optical test dome, encounter a concave mirror and return to the interferometer for wavefront analysis. The advantage over off-the-shelf hardware is an ability to capture at one instant a large area or even the entire clear aperture, of a dome. The key to the design is the fast, f/0.65, highly-aspheric, diamond-turned, ZnSe lens, and the equally fast, very thick, deeply concave mirror. Other components allow for placement and rotation of the optical dome under test. Operation at 10.6 μm allows loose fabrication tolerances for the surfaces in the visible wavelengths, yet the system is of reference quality in the infrared. The subsystem is modular so that it may be easily removed to gain access to the standard output port of the interferometer for other purposes.

Boron phosphide films prepared by PECVD have been characterised as a function of phosphine flow rate during deposition. The films were characterised by x-ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM), Nano-indentation and Scanning electron microscopy. The effect of phosphine flow rate during deposition on the dispersive, polar and acid-base components of the surface energy of the films was investigated. The components of the surface energy were determined by the Owens-Wendt (OW) and the Van-Oss-Chaudhry-Good (VOCG) methods. Both the Lifshitz-Van der Waaals dispersive interaction and the electron donor/electron acceptor acid-base components were found to depend on the phosphine flow rate during film preparation. Our results indicate the potential of Boron Phosphide films for tribological and engineering applications beyond their current application as protective coatings for soft infra-red transmitting substrates.

Exotic Electro-Optics (EEO) recently completed a series of MIL-STD-810F, Method 510.4 sand erosion tests at multiple commercial testing sites. During this testing process, it became apparent that no two environmental test vendors are alike, even if MIL-STD-810F is specified in all cases. Three different test laboratories performing the same Method 510.4 sand test on identically fabricated samples yielded three different results. Ultimately, it is the responsibility of the Engineer to confirm that the test vendor’s equipment, processes, and procedures produce a test environment that is applicable and a result that is accurate based upon the customer specified test requirement and the MIL-STD-810 methodology. Some critical factors that determine the utility of a test are particle concentration, air velocity, particle size and composition, and the ability to maintain these parameters over test duration of up to 90 minutes. EEO has identified a number of parametric details critical to maximizing the stability and accuracy of MIL-STD-810F, Method 510.4, Procedure II sand testing. These strategies will be presented.

EFG sapphire sheet measuring 305 x 510mm and 225 x 660mm have been produced in quantity. The average optical transmission of 6.15 mm thick uncoated polished panels is 84.0% ± 0.5 at 700 nm. This value assures good transmission throughout the 500 to 5000 nm spectral range. Effective absorption coefficients for this spectral range and thickness are calculated and presented. An a verage index inhomogeneity of 6 ppm ± 2 has been measured and is the requirement for panels polished to 0.1λ at this thickness (@633 nm).

There is an increasing demand for large sapphire windows for a number of defense related programs. Some of these emerging requirements call for windows that are on the order of half a meter in size with tight surface figure and transmitted wavefront requirements. Magnetorheological Finishing (MRF^®) is a deterministic polishing process capable of rapidly converging to the required surface figure. MRF finishing of sapphire has been demonstrated with surface accuracies better than 0.07 μm peak-to-valley (0.010 μm RMS) and surface microroughness less than 1.0 nm RMS on circular and square apertures. As a sub-aperture polishing technique, MRF provides a mechanism for effectively addressing and correcting a variety of optical surface features. This is of particular interest when correcting the transmitted wavefront on windows. The process allows for correction of the optical wavefront when it is aberrated due to inhomogeneity in the material in addition to the errors in the surface. Another benefit is that MRF has been shown to remove subsurface damage left from prior fabrication steps and can improve surface roughness of pre-polished sapphire. We report on a predictable, lower-cost process for fabricating large-scale sapphire windows.

Edge Defined Film-fed Growth (EFG^TM) Saphikon^® sapphire crystals have been grown as large, thick sheet. The sheet is then precision-polished and coated into an infrared or laser transmission compatible window. The sapphire windows are subsequently assembled into a multi-panel configuration for advanced targeting, navigation, or reconnaissance applications. As future aerospace programs will require windows with larger apertures, material characteristics and uniformity such as refractive index homogeneity will increase in importance. Optical measurements, x-ray topography data and rocking curve analysis are presented The crystalline properties as they relate to refractive index inhomogeneity and wave front distortion are discussed.

Signifigant enhancements of the flexural strength of a- and c- plane sapphire by means of "super polishing" was first reported be McHargue and Snyder [Proc. SPIE 2013, 135 (1993)]. The improvement was attributed to the removal of residual mechanical polishing damage. More recently, a comprehensive series of eperiements was carried out by Crystal Systems for the specific purpose of assessing the effects of various polishing procedures on the high-temperature strength of c-plane sapphire. Subsequent testing at room temperature confirmed that chemomechanical polishing improves both the effective strength and the strength distriution. In this contribution we take advantage of the methodology previously used by Klein et al. [Opt. Eng. 41, 3151 (2002)] to perform a correct Weibull statistical analysis of biaxial flexure-strength data genenrated in the course of Crystal Systems' investigations. We demonstrate that chemomechanical polishing procedures can improve the high-temperature characteristic strength of c-plane sapphire by 150% and the room-temperature Weibull modulus by 100%.

Exotic Electro-Optics (EEO) has completed a study of how the edge finish of an A-plane sapphire sample affects its flexural strength when tested using the 4-point bend test method. Flexural bar samples were fabricated out of a sapphire panel that was polished to production quality using EEO's standard production methods. All samples were configured to meet the requirements for a C-size sample as defined by ASTM C-1161. The only difference between the three sample groups was the edge finish applied to the sample - conventionally ground, fine ground or a commercial polish edge finish. The edge finish on each sample was quantitatively characterized prior to strength testing. All samples were visually inspected prior to testing to identify any potential fracture initiation points. The samples were then tested using an Instron Universal tester per ASTM C-1161 in the UDRI Ceramics and Glasses Laboratory. After testing, a visual inspection was performed to identify the fracture initiation surface and location. Observations confirmed that all sample data was valid (all fractures initiated inside the two inner load dowels), no fractures were initiated on the edges, and no fractures initiated at any of the suspect sites noted in the pre-test visual inspection. The data was post processed using standard statistical and Weibull analysis methodologies. The results showed no significant difference when comparing the flexural strength of the three edge finish groups. The data suggest that the surface quality of the planar surfaces and the bevels is more critical than the finish of the full edge.

Polycrystalline alumina (PCA) has great potential for providing performance comparable to or better than single-crystal sapphire, yet offers the opportunity for low-cost powder based manufacturing. CeraNova has demonstrated transparent PCA, by processing the material to simultaneously achieve 100% density and sub-micron grain size. CeraNova PCA displays low scatter in the infrared, with high transmittance (>85%) in the 3.0-4.0μm region, comparable to sapphire. In addition, the sub-micron grain size leads to high hardness, high strength and high thermal shock resistance. This fine-grain PCA is a viable sapphire replacement for dome applications, including those that require aerodynamic shapes readily possible by powder processing. Such shapes present not only processing challenges, but also surface finishing issues. Results from a current program to address these issues in creating an ogive dome of PCA are discussed.

Windows for infrared sensors on missiles and aircraft must be of high optical quality and durable enough to protect delicate sensors from harsh operating conditions. If the grain size is smaller than the wavelength of the IR signal, then optical materials with excellent mechanical properties and IR transmission will be likely. This paper describes work being done for IR window applications on Yttrium oxide, which represents one potential candidate material for infrared windows because of its high infrared transparency, and a nanocomposite of Alumina:Zirconia having a grain size under 50 nm. Nanoparticles of yttria precursors and alumina-zirconia are synthesized using a sonochemical technique. These are passivated and dispersed as a colloid. Various colloidal processing approaches are utilized to make solid preforms, which are further processed to remove liquid, organics and to convert precursors to crystalline materials. Concentrated colloids can be used to directly fabricate a solid preform using more than one consolidation technique. The preform becomes the starting point for further processing and densification to form an IR transparent material. Pressure-assisted sintering is being done using sinter forging done on solid preforms that were partially sintered. Near 100% of the theoretical density was attained at modest temperatures with this method. Samples containing zirconia need to be further oxygenated between 1200°C-1275°C for 12 to 24 hours. While this work is still on-going, one sample showed 50% IR transmission. IR transmission was also found to increase with longer oxygenation times.

Polycrystalline infrared transparent materials with good optical and mechanical properties are needed for the fabrication of infrared windows and domes. We have developed a synthesis process to produce MgO nanopowders. The average aggregate size of powder is in the range of 150 - 160 nm, with a narrow aggregate size distribution. Additionally, a protocol for compaction of nanocrystalline powders to achieve high green density (>50% of theoretical density) and microstructural uniformity has been described, as these factors are critical to sinter compacts to theoretical density with minimum number of flaws. Some of the samples were hot pressed and subsequently hot isostatically pressed (HIPed) for complete densification. Processing parameters are discussed to achieve uniformly dense sample with high degree of translucency. Some of the fully densified samples had a grain size as low as 100 - 300 nm.

We have synthesized nanophase yttria via gas-phase condensation with CO₂-laser heating. Transmission Electron Microscopy (TEM) shows particles of about 3 - 8 nm in diameter. X-ray diffraction (XRD) indicates a diffracting domain size L440 of 2 - 5 nm, and a lattice constant a0440 ranging from 10.60 Å to 10.70 Å. Vacuum sintering, two-stage sintering, and combined vacuum/two-stage sintering are used to consolidate the Y₂O₃ nanoparticles into ceramics.

New materials with improved mechanical properties and high optical transmission in the full 3-5 micron MWIR region wavelength are required. Commercially available polycrystalline transparent Yttria, with >100 micron average grain size, does not perform satisfactorily in demanding applications because of its modest strength. One way to improve strength is to develop an ultra-fine grained material with acceptable optical transmission properties. To realize fine grains it is necessary to use other phases to inhibit grain growth during fabrication. A promising processing method comprises: (a) synthesis of an extended metastable solid solution by plasma melting and quenching, and (b) consolidation of the metastable ceramic powder to form dense submicron-grained (<1 micron) composites. Two ceramic composites containing 20 and 50 vol% of second phase are evaluated in this study. Optical transmission, hardness, and indentation fracture toughness are measured and correlated with structure.

Nanophase yttria co-doped with erbium and ytterbium (Er,Yb:Y₂O₃) was successfully synthesized using gas-phase condensation. As-prepared nanoparticles were about 2 nm in diameter, while annealed nanoparticles were about 20 nm in diameter. Two-stage sintering of the nanoparticles led to ceramics with grain sizes of less than 160 nm in diameter, while vacuum sintering led to ceramics with grain sizes of about 5 μm. The annealed nanopowder, as well as the two-stage and vacuum sintered ceramics showed fluorescence from erbium and ytterbium ions.

With the ever increasing demands for optical quality YAG, ceramic laser gain materials present attractive advantages in design, fabrication, and cost without any penalty in performance compared to their single crystal counterparts. Due to intrinsic differences in the production method, ceramic gain media can be fabricated faster, larger, more affordably, and at higher and more uniform doping. This investigation into polycrystalline YAG included optical, mechanical, and microstructural characterization -- focusing primarily on Konoshima Chemical Company ceramic YAG, the current global state of the art.

The development and use of arsenic trisulfide glass in infrared systems began in 1950. The glass was produced commercially in ton quantities by a number of companies in the US and Europe and used in NIR and MWIR systems mostly for commercial use. In the 60's, it became apparent that new optical materials capable of longer wavelength transmission would be needed for thermal imaging systems beginning to be considered. The government sponsored exploratory development programs aimed at providing selenium and tellurium containing glasses. By mid 1970's, two selenium based glass compositions had been developed and were in production in the U.S. Many tons of these two glasses have been produced and used in the 1980's, 1990's and present day for thermal imaging systems both military and commercial. The fact that only two glass compositions have been used so extensively for such a long period is due in part to the size of the effort required to identify, characterize and produce a new glass to a state where it will be considered by the optical designer for use in new systems. But new compositions may be needed to provide a glass better suited for a different application. Glasses developed by Amorphous Materials for different applications will be described.

The U.S. Naval Research Laboratory (NRL) has developed two unique materials with excellent properties for various military and commercial applications in the UV-Vis-IR wavelength range. These materials are: an amorphous Barium Gallo-Germanate (BGG) glass and a polycrystalline Magnesium Aluminate Spinel. The BGG glass is made using traditional glass melting techniques, and was developed as a low cost alternative to the currently used window materials. Large prototype windows have been fabricated for a Navy reconnaissance program. BGG windows have been successfully tested for environmental ruggedness (MIL-F-48616) and rain erosion durability up to 300 mph. BGG glass is currently under development and evaluation for High Energy Laser (HEL) applications. A new process has been developed to sinter spinel to clear transparency with very high yields. This process has been used to make various sizes and shapes (flats and domes) and is readily scalable to industrial sizes to produce large windows & domes for various applications. NRL has also developed modified BGG glasses, which are compatible with Spinel and ALON substrates for bonding.

Glass materials based on rare earth oxides and aluminum oxide can provide a combination of infrared transparency, strength, hardness, and environmental stability in a formable material. This article describes a new family of rare earth oxide-aluminum oxide glass materials that can be made by casting from melts formed in platinum crucibles. The glasses transmit light in the wavelength range from 0.3 to 5 μm in sections of ~0.3 cm, they have a Vickers hardness of 800-1000, and exhibit excellent environmental stability typical of refractory oxide materials. The composition of the glass can be adjusted to achieve refractive indices in the range 1.7-1.8 and Abbe numbers of 30-60. The materials are promising candidates for passive optical elements or as a host for optically active ions such as Yb or Nd that provide laser action or absorb at laser line wavelengths.

The process of selecting suitable materials for high-energy laser windows involves considerations realting to (a) the flexural strength, (b) the thermal stresses, and (c) the optical distortion. Optical distortion ocnsiderations strongly favor low-absorbtion materials ythat exhibit a negitive thermo-optic coefficient (dn/dT) in conjunction with minimal stress-birefringence (q&dverline; -q⊥ ≃0). For this reason, calcium floride has been the primary candidate for many years, but the efforts to strengthen this material have not been successful. Recently, a new glass compostion-oxyfloride glass (OFG)-has been promoted as an ideal solution in the sense that it will allow fabricating large "athermal" windows for operation at the chemical oxygen-iodine laser wavelength. It is, therefore, of interest to properly assess the merits of OFG in comparison to CaF₂, which we do here on the basis of available (Dec '04) property data for fusion-cast CaF₂ and OFG. Oxyfloride glass was found to be deficient in regard to thermal diffusivity, which may lead to excessive coating-induced compressive stresses, and stress- birefringence, which rules out creating a distortion-free window. It is suggested that future efforts should be directed at strengthening CaF₂ in view of this material's exceptionally low absorbtion and almost no stress-birefringence

Single Point Diamond Turning (SPDT) has been a cost effective technique to achieve the required figure and roughness specification on a wide range of infrared (IR) optics. SPDT is one of the few technologies that can efficiently generate aspherical surfaces, and as recent developments such as fast-tool servos mature, “free-form” surfaces are becoming feasible as well. Optical end-user requirements for a wide range of industries are continuing to tighten, driven, for example by multi-spectral systems that require good performance at shorter wavelengths in addition to IR. In many cases, specified shape tolerances can exceed SPDT capabilities. Additionally, SPDT typically leaves “turning marks” (affecting micro-roughness) that can be detrimental to performance. In some cases, surface integrity (e.g. sub-surface damage) can also be of concern. Magneto-Rheological Finishing (MRF^®) has the proven ability to simultaneously improve roughness, figure, and surface integrity in a fast and cost effective manner. MRF is a deterministic, sub-aperture polishing technology, and is typically employed as the last manufacturing step. MRF can deterministically remove from tens of nanometers to microns worth of material, while efficiently “converging” to the specified requirements. Conversely, SPDT has proven to be very effective in removing the hundreds of microns (if not mm) sometimes required to “pre-shape” an aspheric surface before its final polish. After a brief introduction of MRF, this paper will discuss how SPDT and MRF processes can complement one another very effectively. Examples of MRF results on a wide range of IR materials will be presented.

A new compliant sub-aperture optical finishing technique is being investigated for the removal of mid-spatial frequency artifacts and smoothing of hard polycrystalline infrared ceramics for aspheric applications and conformal shaped optics. The UltraForm concept was developed by OptiPro Systems, Ontario, NY, and is a joint process development effort with the Center for Optics manufacturing (COM). The UltraForm tool is a pressurized, elastomeric bladder in the shape of a toroid. Finishing pads are attached to the periphery, allowing the use of a wide variety of pad materials and abrasive selections. Experimentation has been conducted using both slurry mixes and fixed abrasive pads. The toroidal tool is rotated while the compliant tool is compressed into contact with the surface. Currently this process has specific interest for the finishing of conformal ALON Domes. Also to be discussed will be new versions of the UltraForm Tools which are currently be developed and tested.

In order to enhance missile performance, future window and dome designs will incorporate shapes with improved aerodynamic performance compared with the more traditional flats and spheres. Due to their constantly changing curvature and steep slopes, these shapes are incompatible with most conventional polishing and metrology solutions. Two types of a novel polishing technology, Magnetorheological Finishing (MRF^®) and Magnetorheological (MR) Jet, could enable cost-effective manufacturing of free-form optical surfaces. MRF, a deterministic sub-aperture magnetically assisted polishing method, has been developed to overcome many of the fundamental limitations of traditional finishing. MRF has demonstrated the ability to produce complex optical surfaces with accuracies better than 30 nm peak-to-valley (PV) and surface micro-roughness less than 1 nm rms on a wide variety of optical glasses, single crystals, and glass-ceramics. The polishing tool in MRF perfectly conforms to the optical surface making it well suited for finishing this class of optics. A newly developed magnetically assisted finishing method MR Jet^TM, addresses the challenge of finishing the inside of steep concave domes and other irregular shapes. An applied magnetic field coupled with the properties of the MR fluid allow for stable removal rate with stand-off distances of tens of centimeters. Surface figure and roughness values similar to traditional MRF have been demonstrated. Combining these technologies with metrology techniques, such as Sub-aperture Stitching Interferometer (SSI^®) and Asphere Stitching Interferometer (ASI^®), enable higher precision finishing of the windows and domes today, as well as the finishing of future conformal designs.

Night vision and related thermal imaging systems play a critical role in the protection of our nation's security. These systems record images using video cameras designed for operation in the infrared (IR) region of the light spectrum. As with any imaging system, increased functionality and new information is gained when discrete portions of the observed light spectrum are analyzed separately using optical filters. Highly discriminating filters are needed to increase the sensitivity of atmospheric chemical sensors, to enable multi-spectral imaging and secure laser communications links, and to protect imaging systems from damage due to attack by high power laser weapons. Today, the performance of IR light filters is inadequate for many applications. Filters capable of efficient rejection of multiple discrete wavelength bands, combined with high transmission for wavelengths outside the rejection bands, do not exist. A new type of narrow-band optical filter capable of protecting critical imaging systems from attack from laser weapons operating at multiple wavelengths, is being developed. Based on rugged surface-structure wave-guide resonant holograms, the new filters will be capable of rejecting better than 99% of IR light within each notch, while maintaining the same level of transmission outside each notch covering a broad range of the IR spectrum. The theory, design and fabrication of surface structure, laser-line rejection and transmission filters built upon infrared transmitting materials, will be described. Optical performance data for prototype structures will be presented.

A process to diffractively structure GaAs for enhanced optical performance is described. The benefits of diffractively structuring an EOIR window material include improved FOR/FOV, consistent broadband performance, the ability to design and implement hyper-spectral characteristics directly into the substrate without incorporating a complex anti-reflective coating. Progress to date will be discussed including design evolution, process implementation, and optical characterization using the Automated Rasterable Integrated Spectrometer and TIS Measurement System (ARISTMS). Results will be presented on 100mm diameter samples.

Rugged infrared transmitting materials have a high refractive index, which leads to large reflection losses. Multi-layer thin-film coatings designed for anti-reflection (AR), exhibit good performance, but have limited bandwidths, narrow acceptance angles, polarization effects, high costs, and short lifetimes in harsh environments. Many aerospace and military applications requiring high optical transmission, durability, survivability, and radiation resistance, are inadequately addressed by thin-film coating technology. Surface relief microstructures have been shown to be an effective alternative to thin-film AR coatings in many infrared and visible-band applications. These microstructures, etched directly into the window surface and commonly referred to as “Motheye” textures, impart an optical function that minimizes surface reflections without compromising the inherent durability of the window material. Reflection losses are reduced to a minimum for broad-band light incident over a wide angular range. For narrow-band applications such as laser communications, a simpler type of AR surface structure called a sub-wavelength, or "SWS" surface, is used. In general, both the Motheye and SWS surface textures will exhibit the same characteristics as the bulk material with respect to durability, thermal issues, and radiation resistance. The problems associated with thin-film coating adhesion and stress, are thus eliminated by design. Optical performance data for AR structures fabricated in fused silica, sapphire, Clear ZnS, ZnSe, cadmium zinc telluride (CZT), silicon, and germanium, will be presented.

The corrosion susceptibility of Boron phosphide films prepared by PECVD, was studied in saturated saline solution as a function of phosphine flow rate during deposition. The chemical composition of the Boron phosphide films was determined by x-ray photoelectron spectroscopy (XPS) analysis. The investigation involved open circuit potential measurements (OCP) over several hours, potentiodynamic polarisation and electrochemical impedance Spectroscopy (EIS) measurements. The corrosion rate of the Boron Phosphide films was found to vary with changes in the phosphine flow rate during deposition. The results of our investigation also showed that Boron phosphide coated stainless steel plates had superior corrosion resistance, when compared to bare uncoated stainless steel plates. This opens up the potential for the application of Boron Phosphide films as a protective coating to improve the corrosion resistance of metals and alloys for various engineering applications.

Multi-spectral (WC)-ZnS has high transparency in a wide spectral range, from 0.4μm to 12μm. However, the relatively low hardness of WC-ZnS components limits their use especially for air-borne military systems where conditions such as particle or rain erosion may occur. In this work, a protective coating for WC-ZnS components was developed. The protective coating reduces damage caused by rain drop impacts without affecting the transparency of the uncoated component. An anti-reflective (AR) coating was evaporated over the protective coating to improve the transmission of the component. The microstructure of the protective coating was found to be of columnar grains, tens of nanometers in diameter. Rain erosion tests of the coated WC-ZnS samples conducted at the University of Dayton Research Institute (UDRI) whirling arm apparatus showed that they fully comply with the commonly accepted OCLI specification #6040012 for rain erosion resistant coating for forward looking infra-red (FLIR) grade ZnS. The damage threshold velocity (DTV), measured at the Cambridge University Multiple Impact Jet Apparatus (MIJA), increased from 120m/sec for an AR coated WC-ZnS sample to 178m/sec for a WC-ZnS sample coated with a 17μm protective coating and an AR coating. In this paper we show that the improved rain erosion protection was achieved without causing any change in the transparency of a WC-ZnS substrate. We also thoroughly discuss how the protective coating thickness and WC-ZnS sample thickness affect the damage caused by rain erosion.

Durable coatings of silicon-carbon-oxy-nitride (a.k.a. SiCON) are being developed to protect high-speed missile windows from the environmental loads during flight. Originally developed at Rockwell Scientific Corporation (RSC) these coatings exhibited substantial promise, but were difficult to deposit. Under a DoD DARPA SBIR Phase I program, Surmet Corporation, working closely with RSC, is depositing these coatings using an innovative vacuum vapor deposition process. High rate of coating deposition and the ease of manipulating the process variables, make Surmet’s process suitable for the deposition of substantially thick films (up to 30 μm) with precisely controlled chemistry. Initial work has shown encouraging results, and the refinement of the coating and coating process is still underway. Coupons of SiN and SiCON coatings with varying thickness on a variety of substrates such as Si-wafer, ZnS and ALON were fabricated and used for the study. This paper will present and discuss the results of SiN and SiCON coatings deposition and characterization (physical, mechanical and optical properties) as a basis for evaluating their suitability for high speed missile windows application.