This PDF file contains the front matter associated with SPIE Proceedings Volume 9453, including the Title Page, Copyright information, Table of Contents, Invited Panel Discussion, and Conference Committee listing.

Spinel ceramic exhibits excellent optical and mechanical properties, but its widespread use in high volume applications has been limited primarily due to the high cost associated with hot pressing and finishing. While, we have previously demonstrated techniques to reduce finishing costs, in this paper we report on the use of microwave sintering to make spinel ceramic at significantly lower cost than traditional hot pressing. We also identify preferred grain growth as well as an intra-granular fracture mode.

Ideal exit aperture windows for high-energy laser (HEL) should possess low absorption and scattering losses and be environmentally rugged and strong in order to protect the laser gain medium without compromising the light propagating through the window. Spinel is an ideal candidate for this application due to its high mechanical strength, high thermal conductivity, and excellent optical transmission between 0.2~5 μm. However, spinel ceramics fabricated with commercial powders often show inhomogeneity and suffer from absorption and scattering caused by various types of intrinsic and extrinsic impurities present in the powders. Here, we report on a convenient and economical powder purification method to significantly lower the absorption loss of transparent spinel ceramics using commercial powders. Acid washing was successfully used to reduce absorption loss in spinel ceramic fabricated using commercial powder from >20,000 ppm/cm down to 75 ppm/cm.

An automated controlled heat extraction system (CHES)® has been developed to produce 300 kg, 500 mm diameter sapphire boules. The versatile process allows production of boules in a-, r- or c- orientations. The technological advancements come with major cost reductions making sapphire for consumer electronics, LEDs and large optical windows a reality. In addition, these features allow large scale production with a fast ramp rate.

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 controls every aspect of the manufacturing process, beginning with synthesis of ALON® powder, continuing through forming/heat treatment of blanks, ending with optical fabrication of ALON® windows. Surmet has made significant progress in its production capability in recent years. Additional scale up of Surmet’s manufacturing capability, for complex geometries, larger sizes and higher quantities, is underway. The requirements for modern aircraft are driving the need for conformal windows for future sensor systems. However, limitations on optical systems and the ability to produce windows in complex geometries currently limit the geometry of existing windows and window assemblies to faceted assemblies of flat windows. Surmet’s ability to produce large curved ALON® blanks is an important step in the development of conformal windows for future aircraft applications.

Yttrium Aluminum Garnet (YAG) is an important laser host material. Ideal host materials have low loss at the laser transition frequency. This becomes more important as the gain length increases or a low gain transition is of interest. Unfortunately, single crystal YAG suffers from relatively high scatter caused by strain induced index of refraction variations generated by the growth method. For this reason polycrystalline YAG has been developed with virtually no strain. Furthermore, this material can be doped with concentrations that vary spatially. This can provide a tremendous advantage in matching the gain volume to the mode volume in a laser. However, because of the grain boundaries and porosity, polycrystalline materials have scatter loss. Angle resolved, in-plane scatter measurements of polycrystalline YAG and Nd:YAG are reported from 405 to 1064 nm. This covers the range of interest for laser operation but also with enough bandwidth to derive a physical understanding of the scatter mechanisms. A model is also applied to provide physical insight and interpolation and meaningful extrapolation of the experimental results.

Semi-Active Laser (SAL) guidance systems were developed starting in the mid-1960's and today form an important class of precision guided weapons. The laser wavelengths generally fall in the short wave infrared region of the spectrum. Relative to passive, image based, infrared seekers the optical demands placed on the domes or windows of SAL seekers is very modest, allowing the use of low cost, easily manufactured materials, such as polycarbonate. This paper will examine the transition of SAL window and dome science and technology from the laboratory to battlefield, with special emphasis on the story of polycarbonate domes.

Mechanical strength measurements of transparent ceramic window material coupons are customarily fit to a Weibull equation that describes the strength and its distribution. Predictions of window lifetime under stress are commonly based on slow crack growth parameters obtained by measuring the mechanical strength of coupons over a range of constant stress rates. This tutorial paper describes how to derive Weibull and slow crack growth parameters from strength measurements and how to use those parameters to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.

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. Premature tool wear or tool loading during the grinding process is a common result of these tough mechanical properties. Another challenge is the requirement to create geometries that conform to the platforms they reside in, but still achieve optical window tolerances for wavefront. These shapes can be complex and require new technologies to control sub aperture finishing techniques in a deterministic fashion. In this paper we will present three technologies developed at OptiPro Systems to address the challenges associated with these materials and complex geometries. The technologies presented will show how Ultrasonic grinding can reduce grinding load by up to 50%, UltraForm Finishing (UFF) and UltraSmooth Finishing (USF) technologies can accurately figure and finish these shapes, and how all of them can be controlled deterministically, with utilizing metrology feedback, by a new Computer Aided Manufacturing (CAM) software package developed by OptiPro called ProSurf.

Advancements in optical manufacturing technology allow optical designers to implement freeform and conformal shapes in their systems. Metrology of the shapes has traditionally been difficult, especially at the sub-micron level. Contact measuring systems typically lack the accuracy required for optical qualification and can damage the surface. Interferometric systems are unable to handle high spherical departures and may require complicated lateral calibration to generate feedback for deterministic grinding and polishing. OptiPro has developed UltraSurf, a noncontact coordinate measuring machine to determine the form, figure, and thickness of freeform and conformal optics. We integrated several non-contact sensors that acquire surface information through different optical principles. Each probe has strength and weaknesses relative to an optic’s material properties, surface finish, and figure error. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. Measurements of freeform and conformal shapes will be presented. The scanning method of UltraSurf and the non-contact probes will also be shown. The form, figure, and thickness data will highlight the capabilities of UltraSurf to measure freeform surfaces. Comparisons between accuracy and measureable surface departure will be made with current metrology systems such as coordinate measuring machines, interferometers, and profilometers. Additionally, methods for defining a freeform or conformal surface for metrology analysis and manufacturing will be discussed.

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 which challenge 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, finishing 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 specifically focusing on the use of fiducials in the manufacturing and measurement process and the adaptation of stitching interferometry to the measurement of a freeform conformal window.

We discuss the development and applications of a new approach to Diamond-Like Carbon (DLC) coating that provides the durability of traditional DLC coatings, with the addition of significantly more transmission at visible wavelengths and greater transmission in the IR. We developed a deposition system design that incorporates multiple coating technologies, allowing for multiple material design approaches. This has enabled the manufacture of DLC coatings with improved extended spectral properties, suitable for applications in which the coating must withstand airborne particulate impacts, corrosive fluids, environmental extremes, and abrasive physical handling, while offering better than typical transmission in the visible or infrared wavelength regions, or both.

Metallic mesh coatings are used on visible and infrared windows and domes widely to provide shielding from EMI (Electromagnetic Interference). In this paper, different EMI mesh geometries are compared with each other regarding various performance parameters. But to decide the best fitting EMI mesh geometry to particular optic system is a little bit complicated issue. Therefore, we try to find a simple optimization methodology to decide best EMI mesh geometry design that fits our particular high performance ISR (Intelligence, Surveillance and Reconnaissance) systems.

Sapphire’s hardness, strength, and UV-IR transmittance make it an excellent candidate for IR window and transparent armor applications. At Saint-Gobain Crystals, Edge-defined Film-fed Growth (EFG) sapphire crystals are currently being manufactured for IR window and transparent armor applications in sizes up to 305x510x11 mm. However, the demand for even larger sapphire panels continues to increase. In order to aid in the development of larger pieces, a nondestructive measurement has been developed to map planar stress in Clear Large Area Sapphire Sheet (CLASS). The measurement works by utilizing optical excitation of trace amounts of Cr³⁺ impurities. The resulting luminescence produces a sharp emission doublet whose exact wavelength is dependent on spacing between Cr³⁺ and O^2- ions in sapphire, and therefore the strain in the sample. By recording several data points over an array, it is possible to construct a stress map of large sapphire sheets and gain valuable information on the growth conditions of the sapphire ribbon.

Wide band gap Oxide based diluted magnetic semiconductors (ODMS) exhibit unique magnetic, magneto-optical and magneto-electrical effects and can be exploited as spintronic devices. Theoretical studies of transition metal (TM) doped zinc oxide which belongs to these class of materials has been attracting significant research interest in the recent years. In this paper, the electronic band structures, and band gap energies of ZnO doped with transition metal have been analyzed by ab initio calculations based on the density functional theory using quantum espresso PWscf code. For the band gap calculations, we have used both local density approximation (LDA) and generalized gradient approximation (GGA). The magnetic and optical properties of the materials have been studied using the above method. For all the theoretical calculations, the model structures of transition metal-doped ZnO were constructed by using the 16 atom supercell with one Zn atom replaced by a transition metal atom. The results are useful in understanding the band gap variations with doping and other related properties in oxide based diluted magnetic semiconductors such as ZnO.

The ability to deploy advanced sensor and seeker systems in harsh environments is often restricted by the mechanical durability of the external electromagnetic window or dome. Mission environments may range from long flights at high speeds through rain, ice, or sand to exposure at slower speeds to debris on runways or from helicopter downwash. While significant progress has been made to characterize, understand, and model rain damage, less is known about modeling damage in windows and domes caused by impacts from solid particles such as stones, pebbles, and sand.

This paper highlights recent progress made to simulate particle impact damage in zinc sulfide (ZnS) using peridynamics (PD). Early versions of the PD model of sand impact damage simulated the sand particle as a rigid disk. Results from these early models indicated that the extent of damage in relation to the size of the impacting particle was significantly larger than the actual damage observed by experimentation. In order to identify possible explanations for this discrepancy, the shape, impact orientation and mechanical properties of the impacting particle were modified to more closely resemble actual sand particle impacts, that is, the particle was made friable (deformable and breakable). The impacting geometries considered include sphere, flat face of a cylinder, cube-face, cube-edge, and cube-corner. Results confirm that modification of the impacting particle’s mechanical properties, shape and impact orientation lead to better agreement between experimental observations and simulation results.

The mechanical durability of the external electromagnetic window or dome of a sensor often limits the environments in which the sensor or seeker system can be deployed. More durable window and dome materials will allow platforms to fly longer and faster and sustain lower maintenance and replacement costs. Unfortunately, no good models exist for predicting the performance of window and dome materials under harsh erosion environments, especially when the aperture substrates are protected by advanced coating systems.

Recently, Peridynamic (PD) models of sand impact damage have been shown to produce the same phenomenological damage as is observed experimentally in zinc sulfide (ZnS). This paper discusses improvements in the PD impact simulation model which now allow it to simulate coated substrates and non-parallel impact events (where the flat impactor face is no longer parallel to the substrate but tilted by some small impact angle.) Two different substrates are considered, one with the properties of ZnS and another which is twice as strong and stiff as ZnS. Finally, the variation in damage as a function of impact angle is discussed. These modeling results demonstrate the versatility of the peridynamic model of sand impact damage and its potential for identifying trade space and providing design guidance during the development of more durable apertures.

Modern optical systems are often required to function in severe environmental conditions for prolonged periods without suffering from performance degradation. Essential parts of such electo-optical systems are windows, domes and other optical elements. These elements are almost always coated with an efficient, multispectral and highly durable antireflection coating. The complexity further increases when these coatings are applied on curved surfaces, such as hemispherical domes that are usually used in order to allow a wider field of regard. The durability of the coated optical component is dependent upon many different parameters such as deposition method and process parameters and the adhesion between layers and substrate. However, one very important parameter, which can have a significant impact on the durability and optical performance, is the stress state of the applied anti-reflection layers. This subject is mostly left untreated mainly due to the difficulty of characterization and modeling techniques and lack of thin film mechanical constants which sometimes significantly differ from the bulk constants of the same material. The stress state of the optical part is mainly determined by the mechanical properties of the coating materials and substrate, geometrical shape of the part and the thickness of the layers. In this work, both analytical and experimental approaches were used for characterization of stress distribution in thin optical films. Various single and multiple thin films were deposited using the electron beam evaporation technique onto Si 6'' wafers. The film thickness was measured using an ellipsometer and the bi-axial module of the different films was studied by measuring the change in radius of curvature of the Si substrate and applying Stoney's equation. The results were compared to the analytical Timoshenko solution of stress state in a single and multiple film structure. Knowing the stress state of a multi-layer coating would allow the engineer to design a part with the desired stress state which alongside the optical design, is expected to bring about a significant improvement in the durability and optical performance.

HfO₂ and SiO₂ multilayers were used to enhance spectral reflectance, laser damage resistance, and environmental durability of aluminum alloy-based reflective optics. Multiband spectral reflectance was realized, ranging from ultraviolet to middle-wave infrared. Laser-induced damage threshold tests were performed at 1064 nm. Post laser damage morphology analysis was conducted, indicating defect-initiated and absorption-initiated damage mechanisms. The defect-initiated damage was associated with seeds residing at the substrate-coating interface. The absorption-initiated damage was separated from the defect-initiated damage by using an aluminum overcoat on the Al-alloy surfaces. Laser-induced damage thresholds associated with the two damage processes were predicted.

Angle resolved light scattering was measured at 1064 nm on an HfO₂/SiO₂ multilayer mirror deposited via modified plasma ion-assisted deposition. A total backscatter of approximately 4.0×10^-4 was calculated by integrating the angle resolved scattering curves over the backscattering hemisphere. In this way, also the an integrated near angle backscatter of 1.2×10^-4 was derived for angles from 0° to 2° and an integrated near angle scatter of 6×10^-5 was determined in the most critical range from 0.5° to 2°. 12% of the total backscatter originated from the near angle region between 2° and 4°. 36%, 62%, 76% and 86% of the total backscatter were created from 2° to 10°, 20°, 30° and 40°, respectively. Good agreement was obtained for the total backscatter values calculated from the ARS measurements and the top-surface roughness derived by AFM measurements.

The brittle nature of electromagnetic (EM) window and dome materials limits electrical and magnetic performance due to impact of sand particles, hailstones and raindrops. The damage and fracture patterns due to such impacts are well documented with distinct association to the impact type. However, the underlying mechanisms that lead to those patterns are not well understood. Adding to the complexity, multiple layers of coatings with varying thicknesses are applied to the external surfaces of these structures, which affects the extent and nature of the impact damage. A physics -based analysis method that captures correct damage and fracture patterns due to particle impact is well warranted.

In this paper, Peridynamic (PD) Theory is demonstrated as a simulation methodology for fracture analysis of EM windows and domes under particle impact. This theory involves reformulation of classical continuum mechanics in integral form (no spatial derivatives), alleviating the stress singularity problem common to previous fracture analysis approaches. The PD theory enables accurate description of failure events via natural generation and accumulation of defects, cracks, and damage; it can capture complex, 3-D and multiple non-coplanar crack initiation and propagation. The fracture behavior of materials is influenced by an important material parameter, critical stretch, which is specific to PD theory. This study offers a combined experimental-computational method to extract the critical stretch parameter for glass and ceramic materials based on simulations of indentation tests. The critical stretch parameter extracted from indentation simulations is subsequently used for simulations involving sand impact. The predicted damage field is in very good agreement with the experimentally observed fracture patterns.