Aluminum Oxynitride (ALON® Optical Ceramic) combines broadband transparency with excellent mechanical properties. ALON’s cubic structure means that it is transparent in its polycrystalline form, allowing it to be manufactured by conventional powder processing techniques. Surmet has established a robust manufacturing process, beginning with synthesis of ALON® powder, continuing through forming/heat treatment of blanks, and ending with optical fabrication of ALON® windows. Surmet has made significant progress in our production capability in recent years. Additional scale up of Surmet’s manufacturing capability, for larger sizes and higher quantities, is currently underway. ALON® transparent armor represents the state of the art in protection against armor piercing threats, offering a factor of two in weight and thickness savings over conventional glass laminates. Tiled and monolithic windows have been successfully produced and tested against a range of threats. Large ALON® window are also of interest to a range of visible to Mid-Wave Infra-Red (MWIR) sensor applications. These applications often have stressing imaging requirements which in turn require that these large windows have optical characteristics including excellent homogeneity of index of refraction and very low stress birefringence. Surmet is currently scaling up its production facility to be able to make and deliver ALON® monolithic windows as large as ~19x36-in. Additionally, Surmet has plans to scale up to windows ~3ftx3ft in size in the coming years. Recent results with scale up and characterization of the resulting blanks will be presented.

Transparent ceramics are finding applications in demanding optical applications were traditional mineral salts and amorphous materials are limited and single crystals are not practical. Polycrystalline ceramics offer a unique combination of mechanical, electrical and optical properties that allow window and dome applications and possibilities that were previously not possible. Transparent ceramics are being developed for use in a number of applications with each material possessing a distinctive set of properties that address a particular application. The current status of CeraNova’s fine grain transparent ceramic programs for dome and window applications will be presented with emphasis on their exceptional material properties for specific applications.

Excellent transmission in the Mid IR wavelength range coupled with good mechanical properties make the use of spinel ceramics very attractive for high performance windows and domes. MER has concentrated its current development efforts in the scale up to windows as large as 25”x32” and domes/hyperdomes as large as 10” in diameter. The MER spinel technology allows producing complex 3-D shape parts like hyper-hemispherical domes and other aspheric lenses. The thickness can reach 1" and above. MER has shown the feasibility of producing several windows 25”x32” and 18”x22” per run. Optimization of transmittance and strength, minimization of the stress birefringence, keeping the variation of the index of refraction to a minimum, has been the main objective. MER also pursued edge bonding technology, where large, thick, panes were edge bonded into a final large window. High optical and IR transparency in the 0.3 – 5.5 μm wavelength range is obtained. Optical metrology measurement of a polished 18”x22”x0.86” window indicated tolerable levels of high frequency variation in the index of refraction homogeneity as well as acceptable low values of residual stress birefringence. MER is also scaling up production to several 7" diameter hemispherical domes blanks per run. After rendering, polishing, and coating, defect free domes in conformance with optical, mechanical, and dimensional specifications have been produced. Details of MER’s technology achievements to produce low cost, high strength, transparent magnesium aluminum spinel windows and domes are described. Optical and mechanical properties were measured and are provided.

While transparent Spinel ceramic’s mechanical and optical characteristics are ideal for many Ultraviolet (UV), visible, Short-Wave Infrared (SWIR), Mid-Wave Infrared (MWIR), and multispectral sensor window applications, commercial adoption of the material has been hampered because the material has historically been available in relatively small sizes (one square foot per window or less), low volumes, unreliable supply, and with unreliable quality. Recent efforts, most notably by Technology Assessment and Transfer (TA and T), have scaled-up manufacturing processes and demonstrated the capability to produce larger windows on the order of two square feet, but with limited output not suitable for production type programs. ArmorLine Corporation licensed the hot-pressed Spinel manufacturing know-how of TA and T in 2009 with the goal of building the world’s first dedicated full-scale Spinel production facility, enabling the supply of a reliable and sufficient volume of large Transparent Armor and Optical Grade Spinel plates. With over $20 million of private investment by J.F. Lehman and Company, ArmorLine has installed and commissioned the largest vacuum hot press in the world, the largest high-temperature/high-pressure hot isostatic press in the world, and supporting manufacturing processes within 75,000 square feet of manufacturing space. ArmorLine’s equipment is capable of producing window blanks as large as 50" x 30" and the facility is capable of producing substantial volumes of material with its Lean configuration and 24/7 operation. Initial production capability was achieved in 2012. ArmorLine will discuss the challenges that were encountered during scale-up of the manufacturing processes, ArmorLine Optical Grade Spinel optical performance, and provide an overview of the facility and its capabilities.

Low-thermal-expansion tungstate materials have the potential to be used as thermalshock- resistant midwave (3-5 μm) infrared windows. Material properties that favor thermal shock resistance are high strength, high thermal conductivity, low elastic modulus, and low thermal expansion. Sapphire, for example, owes its high thermal shock resistance to high strength and high thermal conductivity. In principle, it is possible to obtain even higher thermal shock resistance if a window material with near-zero thermal expansion can be made. This paper assesses recent work on Zr(WO₄)₂ and Al_0.5Sc_1.5(WO₄)₃. It is concluded that multi-phonon absorption in the midwave spectral region limits the optical capabilities of tungstate materials. These materials have more absorption—and therefore, more emission—than aluminum oxynitride in the 4-5 μm wavelength region.

The mechanical properties of several transparent ceramics were investigated to determine if their use might lighten next generation spacecraft windows. The measured fracture toughness and slow crack growth parameters were used as inputs to functions describing the required mass for a desired window life. Transparent magnesium aluminate (spinel, MgAlO₄) and AlON exhibit superior slow crack resistance relative to fused silica, which is the historical material of choice. For spinel, slow crack growth, strength and fracture toughness are significantly influenced by the grain size, and alumina rich phases and porosity at the grain boundaries lead to intergranular fracture in coarse grain spinel. The results imply that transparent ceramics can lighten window panes from a slow crack growth perspective.

The requirements for modern aircraft are driving the need for conformal windows for future sensor systems. However, limitations on optical systems and the physical properties of optically transparent materials currently limit the geometry of existing windows and window assemblies to faceted assemblies of flat windows held in weight bearing frames. Novel material systems will have to be developed which combine different materials (e.g. ductile metals with transparent ceramics) into structures that combine transparency with structural integrity. Surmet’s demonstrated ability to produce novel transparent ceramic/metal structures will allow us to produce such structures in the types of conformal shapes required for future aircraft applications. Furthermore, the ability to incorporate transparencies into such structures also holds out the promise of creating multi-functional windows which provide a broad range of capabilities that might include RF antennas and de-icing in addition to transparency. Recent results in this area will be presented.

As sensor technology and applications have advanced over the years, the size of sensor windows has grown substantially to satisfy current and future demands. Rubicon Technology, with their strong history in scaling sapphire crystal growth and large scale production processes, has successfully produced large sapphire blanks using a highly modified horizontal directional solidification process. Several prototypes have been synthesized up to 1.75 inches thick, 14 inches wide and 20 inches long. Crystal properties and optical characteristics such as transmission and refractive index homogeneity will be presented on several polished bubble-free windows with excellent results. This research sets the standard for high quality monolithic sapphire sheets large enough for use as seamless integrated optical windows in both military and civilian applications.

The aim of this work was to prepare transparent ceramics with large size and complex-shapes by a new water-soluble gelling agent poly(isobutylene-alt-maleic anhydride). Alumina was used as an example of the application of the new gelling system. A stable suspension with 38vol% was prepared by ball milling. Trapped bubbles were removed before casting to obtain homogenous green bodies. The microstructure and particle distribution of alumina raw material were tested. The thermal behavior of the alumina green body was investigated, which exhibited low weight loss when compared with other gelling processes. The influence of solid loading and gelling agent addition were studied on the basis of rheological behavior of the suspension. The microstructures of alumina powders, green bodies before and after de-bindering process, were compared to understand the gelling condition between alumina particles and gelling agent.

To advance an area of technology requires an understanding of the current state of the art. For the past thirty years the standard material for durable window applications has been single crystal sapphire. Sapphire has proven to be a difficult material to replace although many attempts have been made. A variety of polycrystalline materials have been developed and characterized. Also, engineered structures such aerodynamic outer shapes with corrective optics to replace hemispherical shapes have been studied. Recently polycrystalline materials with nanometer-sized grains have been developed. A review of midwave optical window material development is presented with an emphasis on optical properties.

Nanocerox produces oxide nanopowders via flame spray pyrolysis that have proven effective in the processing of a host of high quality optical ceramic materials. In order to produce LWIR windows to compete with ZnS, however, oxide materials are not suitable. Nanocerox has therefore developed aqueous synthesis techniques for the production of zinc sulfide nanopowders. The proprietary processing technique allows control of primary particle size, high purity, low levels of agglomeration, and cost effective synthesis. Crystallinity, particle size, and purity of the powders will be presented. Characterization of parts fabricated from these powders via sinter/HIP processing will also be discussed, including optical performance and microstructural characterization.

A large body of literature was reviewed with the aim of identifying binary and ternary systems for producing long-wave infrared transmitting glass-ceramics for window applications. Known optical and physical property data was summarized for many ternary sulfides as well as their constituent binary sulfides. Some phosphide and arsenide chalcopyrite structures were reviewed as well. Where available, data on the transmission range, energy gap, refractive index, and hardness were tabulated. Several glass-forming systems were identified containing Ga₂S₃, GeS₂, or As₂S₃.

The design of a modern optical system often raises new challenges for manufacturers of high end optical components. One such challenge, which has become more and more common, is the requirement for highly durable hemispherical domes to allow for wide field of view. There are many difficulties to overcome before the final product can be made. In this paper we present some of the major difficulties of developing such domes made from ZnS grown by chemical vapor deposition (CVD). First, the CVD process which introduces the challenge of removing the grown raw dome from the graphite mold without causing cracking and breakage is discussed. Then, the challenges introduced by the electron beam (EB)-gun evaporation method, most commonly used for evaporating the anti-reflection coating, are presented. Amongst these challenges, the mounting of the dome in-side the coating chamber, the coating uniformity over the dome's curvature and the coating's environmental durability are the most difficult problems to overcome. The paper presents how computerized modeling along with experimental procedures can be combined to minimize the difficulties in the production processes and improve the overall product quality and yield.

Fused silica, YAG crystals, and spinel ceramics substrates have been successfully patterned through reactive ion etching (RIE). Reflection losses as low as 0.1% have been demonstrated for fused silica at 1.06 microns. Laser damage thresholds have been measured for substrates with ARSS and compared with uncoated and/or thin-film anti-reflection (AR) coated substrates. Thresholds as high as 100 J/cm² have been demonstrated in fused silica with ARSS at 1.06 microns, with ARSS substrates showing improved thresholds when compared with uncoated substrates.

Infrared and visible windows have been fabricated with surface structures that function as broadband and large angle-of-incidence antireflective (AR) coatings. The AR coating is composed of nanoscale pillars formed by reactive ion etching of either a sacrificial buffer layer or the underlying optical substrate using a random array of nickel dots as an etch mask. The Ni dots are formed using a rapid thermal annealing process. The size and spacing of the Ni dots are found to be highly dependent on the initial Ni film thickness and the annealing parameters, as well as the nature of the surface interaction between the Ni and the buffer layer. The diameter and spacing of the resulting pillars can be parametrically tuned between <100 nm to <1 μm to yield broad antireflection properties from visible through the long-wave infrared. Cleartran (ZnS) and ZnSe optical substrates treated (both sides) with the AR coating process have exhibited a greater than 90% transmittance over the short-wave infrared and mid-infrared wavelengths (1-5 μm), and at incident angles up to 70°. The process for fabricating the surface structures is suitable for low-cost application of broadband, wide-angle, AR coatings on curved optical surfaces.

The deposition of nano-crystalline ZnS/diamond composite protective coatings on silicon, sapphire, and ZnS substrates, as a preliminary step to coating infrared transparent ZnS substrates from powder mixtures by the aerosol deposition method is presented. Advantages of the aerosol deposition method include the ability to form dense, nanocrystalline lms up to hundreds of microns thick at room temperature and at a high deposition rate on a variety of substrates. Deposition is achieved by creating a pressure gradient that accelerates micrometer- scale particles in an aerosol to high velocity. Upon impact with the target substrate the particles fracture and embed. Continued deposition forms the thick compacted lm. Deposition from an aerosolized mixture of ZnS and diamond powders onto all targets results in linear trend from apparent sputter erosion of the substrate at 100% diamond to formation of a lm with increasing fractions of ZnS. The crossover from abrasion to lm formation on sapphire occurs above about 50% ZnS and a mixture of 90% ZnS and 10% diamond forms a well-adhered lm of about 0.7 μm thickness at a rate of 0.14 μm/min. Resulting lms are characterized by scanning electron microscopy, pro lometry, infrared transmission spectroscopy, and x-ray photoemission spectroscopy. These initial lms mark progress toward the future goal of coating ZnS substrates for abrasion resistance.

Temperature non-uniformity in a heated window can result in a significant distortion in the transmitted wavefront. Aberrations are introduced by actual physical distortion of the window due to differential thermal expansion and by localized optical path variations due to the change in index with temperature (dn/dT) of the substrate material. Typically, the second factor is the more pronounced. This effect represents a significant limitation in the performance of windows with non-symmetric geometries made from materials that exhibit combinations of high dn/dT and low thermal conductivity.

LightWorks Optical Systems (LWOS) has recently developed a software tool capable of quantitatively modeling the thermal distribution of a heated window in operation. This capability allows the design team to optimize the heater layer sheet resistance (whether the layer is a metallic grid or a transparent conducting oxide thin film) and the configuration of the bus-bar (electrode) connections prior to any hardware fabrication. Consideration of both of these factors is critical to achieving a uniform thermal distribution at the specified temperature across a given window.

This presentation will describe the recent efforts of LWOS to establish the capability for quantitatively modeling the temperature homogeneity across a heated window based on window material and dimensions, heater layer characteristics and bus-bar configuration. Data will be presented that demonstrates the validity of these models via comparison to actual heated windows observed under heated conditions.

Magnesium aluminate spinel is a durable electro-optical material with high visible through mid-IR transmission. Technology Assessment and Transfer (TA and T) reports on their efforts to integrate electromagnetic interference protection into spinel domes through the use of metallic grids. TA and T is developing an approach to embed noble metal EMI grids in spinel domes by encapsulating the grid with a germanate glass that has matching refractive index and coefficient of thermal expansion to spinel. Achievements to date and an outlook on this method as a viable manufacturing route are presented.

Electromagnetic sensing is a promising technology for precisely locating conductive grid structures that are buried in optical ceramic domes. Burying grid structures directly in the ceramic makes gridded dome construction easier, but a practical sensing technology is required to locate the grid relative to the dome surfaces. This paper presents a novel approach being developed for locating mesh grids that are physically thin, on the order of a mil, curved, and 75% to 90% open space. Non-contact location sensing takes place over a distance of 1/2 inch. A non-contact approach was required because the presence of the ceramic material precludes touching the grid with a measurement tool. Furthermore, the ceramic which may be opaque or transparent is invisible to the sensing technology which is advantageous for calibration. The paper first details the physical principles being exploited. Next, sensor impedance response is discussed for thin, open mesh, grids versus thick, solid, metal conductors. Finally, the technology approach is incorporated into a practical field tool for use in inspecting gridded domes.

A comprehensive suite of NDE/NDI methods has been developed to rapidly inspect transparent ceramic domes for wide range of critical defects in order to identify and mitigate dome manufacturing issues and improve yield through the reduction or elimination thereof. A matrix of hot-pressed (HP), hot isostatically pressed and pressureless sintered (PS) spinel samples with a range of defect types and a variety of surface conditions were inspected using six different, yet complimentary, nondestructive evaluation (NDE) methods. In summary, the combination of three such methods demonstrated the capability to detect all the critical defects with statistical significance for shop floor inspection of spinel domes. The equipment is relatively inexpensive and easy to use, and offers significant payoffs of improved yields and reduced finishing costs by detecting defects prior to the expensive grinding and polishing steps.

Hard ceramic optical materials such as sapphire, ALON, Spinel, or PCA can present a significant challenge in manufacturing precision optical components due to their tough mechanical properties. These are also the same mechanical properties that make them desirable materials when used in harsh environments. Tool wear and tool loading conditions during the grinding process for these materials can be especially problematic. Because of this, frequent dressing and reshaping of grinding wheels is often required. OptiPro systems is developing an ultrasonic grinding process called OptiSonic to minimize the forces during grinding and make the grinding process more efficient. The ultrasonic vibration of the grinding wheel allows for a grinding process that has the capacity for longer tool life and reduced tool wear for a more deterministic process. This presentation will discuss the OptiSonic process and present current results.

There is a need for precisely figured large sapphire windows with dimensions of up to 20 inches with thicknesses of 0.25 inches that will operate in the 1- to 5-micron wavelength range. In an effort to reduce manufacturing cost during grinding and polishing, OptiPro Systems is developing technologies that provide an optimized deterministic approach to making them. This development work is focusing on two main areas of research. The first is optimizing existing technologies, like deterministic microgrinding and UltraForm Finishing (UFF), for shaping operations and precision controlled sub-aperture polishing. The second area of research consists of a new large aperture deterministic polishing process currently being developed at OptiPro called UltraSmooth Finishing (USF). The USF process utilizes deterministic control with a large aperture polishing tool. This presentation will discuss the challenges associated with manufacturing large sapphire windows and present results on the work that is being performed to minimize manufacturing costs associated with them.

Future optical systems are moving away from traditional spherical optics. The anticipated benefits are numerous for freeform optics as they provide better aerodynamic characteristics for aircraft, lighter weight for space missions, and smaller size for medical procedures.

Currently the design and utilization of conformal and freeform shapes are costly due to the difficulties introduced with fabrication and metrology of these parts. Techniques for creating these complex optical surfaces are still in development for traditional optical materials. OptiPro has a unique opportunity create manufacturing solutions through computer controlled multi-axis optical generating, polishing, and metrology machines. OptiPro Systems is continuing to develop advanced optical manufacturing technologies. OptiPro has made toric and freeform arch shapes. OptiPro’s existing manufacturing platforms include its eSX grinding, UltraForm Finishing, and UltraSurf non-contact surface scanning system, which will be used for grinding, polishing, and measuring conformal and freeform shapes.

Freeform surfaces are initially generated using deterministic micro-grinding with diamond bonded tools. Tool paths with up to five axes of simultaneous motion are required to generate and polish the optical figure of conformal surfaces. Sub-aperture corrective polishing will need to vary the amount of time the tool contacts at each location in order to remove the proper amount of material. These locations and dwell times are derived from a surface figure error map provided by OptiPro’s UltraSurf. Research and development of the freeform manufacturing process will be presented.

Conformal windows and domes improve aerodynamic quality of missiles and aircraft but introduce significant optical aberrations. These aberrations can be compensated, provided both window and corrective optics are fabricated to high tolerances. Highly accurate measurement of conformal optics is required for success of the fabrication process. This paper describes the development of the Interferometric Tomography – a new tool for metrology of conformal aspheric optics, including optics with very high aberrations. The metrology system is designed to measure wavefront aberrations as well as the optical figure of both surfaces.

Freeform optical shapes or optical surfaces that are designed with non-symmetric features are gaining popularity with lens designers and optical system integrators. This enabling technology allows for conformal sensor windows and domes that provide enhanced aerodynamic properties as well as environmental and ballistic protection. In order to provide ballistic and environmental protection, these conformal windows and domes are typically fabricated from hard ceramic materials. Hard ceramic conformal windows and domes provide two challenges to the optical fabricator. The material hardness, polycrystalline nature and non-traditional shape demand creative optical fabrication techniques to produce these types of optics cost-effectively. This paper will overview a complete freeform optical fabrication process that includes ultrasonic generation of hard ceramic surfaces, high speed VIBE polishing, sub-aperture figure correction of polycrystalline materials and final testing of freeform surfaces. This paper will highlight the progress made to each of the processes as well as the challenges associated with each of them.

The continuous drive towards higher performance intelligence, surveillance and reconnaissance (ISR) and high-energy laser (HEL) systems has translated into new requirements for high-performance windows. In these applications a wide range of materials needs to be considered, ranging from amorphous glass (such as fused silica), polycrystalline materials (such as CleartranTM) or hard ceramics (such as AlON, spinel and sapphire). A wide range of sizes (up to and including meter class optics) and geometries are also considered for these applications (high aspect ratio plano surfaces remain prevalent, of course, but “free-form” shapes are also being envisioned and implemented routinely, including conformal windows). As is always the case, increasingly tighter specifications, driven by lower wavelength IR systems as well as visible and/or multispectral systems, require continually more sophisticated metrology techniques to verify and validate. Development of manufacturing processes needed to yield pristine optical surfaces capable of operating at higher laser fluences and/or for highly brittle ceramics capable of withstanding a wide range of temperature, operating pressure and stress are also considered. New high-durability thin film coatings capable of withstanding increasingly harsher environments have been developed for these applications. In a defense environment where cost pressures continue to require less expensive manufacturing processes, several advances are discussed. This paper will present a wide range of examples dealing with these materials, geometries, specifications, metrology and thin film coating developments.

Patterning of gold-black infrared absorbing films by stencil lithography and hardening by polymer infusion is reported. Gold black nano-structured films are deposited through a thin metal shadow mask in a thermal evaporator in ~400 mTorr pressure of inert gas, followed by ethyl cyanoacrylate fuming through the same mask to produce rugged IR absorptive patterns of ~100 micron scale dimensions. Infrared absorptivity is determined by transmission and reflectivity measurements using a Fourier spectrometer and infrared microscope. Results indicate that the optimized hardening process reduces the usual degradation of the absorptivity with age. This work has potential application to infrared array bolometers.