This PDF file contains the front matter associated with SPIE Proceedings Volume 10179, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

ZnS-based materials have a long history of use as x-ray luminescent materials. ZnS was one of the first discovered scintillators and is reported to have one of the highest scintillator efficiencies. The use of ZnS for high energy luminescence has been thus far limited to thin powder screens, such as ZnS:Ag which is used for detecting alpha radiation, due to opacity to its scintillation light, primarily due to scattering. ZnS in bulk form (chemical vapor deposited, powder processed, and single crystal) has high transmission and low scattering compared to powder screens. In this paper, the performance of single crystalline ZnS is evaluated for low energy x-ray (<10 keV) imaging. For these applications, a scintillator needs to be thick enough to absorb the incoming x-rays and to provide sufficient gain, but thin enough to allow for a good spatial resolution. The scintillators also need to have a good radiation hardness, a fast decay time, and low levels of afterglow. We present a trade study which compares the calculated scintillation gain and absolute efficiency for low energy x-rays (<10 keV) comparing thin (<100 μm) ZnS to CsI:Tl, Bi₄Ge₃O₁₂ (BGO), and Y₃Al₅O₁₂:Ce (YAG:Ce). The study also gives insight into the spatial resolution of these scintillators. Further, photoluminescence (PL) and PL excitation (PLE) of several undoped ZnS single crystals is compared to their Radioluminescence (RL) spectra. It was found that the ZnS emission wavelength varies on the excitation source energy.

The characteristics of broadband transmission, environmental durability, and laser damage resistance are critical for silica glass exit aperture windows for their use in kW-level, high energy laser systems. The use of conventional antireflective (AR) coatings on windows for high energy lasers operating in the near infrared is impacted by laser induced damage that occurs under high power irradiation as well as the potential for delamination in operational environments. Novel methods for fabricating antireflective surface structures (ARSS) directly on optics have resulted not only in reduced reflection loss, but also in other advantages to AR coatings as well. The ARSS approach involves sub-wavelength surface structures fabricated directly into the actual surface of the window, eliminating the need for a coating of extraneous materials. We will report on results for ARSS fabricated on silica glass windows. Recently we have reported broadband, low reflectance (< 0.02% at 1 µm) for silica glass windows with random ARSS, fabricated using reactive ion etching. These windows have shown remarkably high laser damage thresholds of 100 J/cm2 at 1.06 µm, which is 5x the threshold measured for a conventional AR coating. We will also present results for MILSPEC durability tests on silica windows, both with and without ARSS, for rain and sand erosion as well as salt fog testing, conducted at a government facility. We will also report on scale up of ARSS on silica windows of large sizes (33 cm), making them practical for system implementation.

Infrared (IR) transmissive moth eye-like substrates, including randomly patterned fused silica and various periodically patterned germanium substrates, were surface modified using a simple process. Goniometric analysis showed that the surface modification altered the surface wettability of each substrate, rendering them superhydrophobic. Following the surface modification, it was determined that the desirable IR transmission and antireflective properties of each substrate type were maintained. Furthermore, the hydrophobicity, IR transmission and antireflective capabilities of the substrates were shown to be significantly enhanced in comparison to native, non-patterned fused silica and germanium substrates that underwent the same processes. The results of this work provide an opportunity for the development of enhanced utility for infrared transmissive optics in wet or humid conditions.

This tutorial paper explains the maximum likelihood method for Weibull analysis of mechanical strength of ceramic materials and how strength scales with area under stress. Weibull parameters for a variety of infrared window materials are presented. Calculation of the Weibull static probability of survival is illustrated.

Samples of fine-grain, transparent polycrystalline alumina (CeraNova Corp) and multispectral zinc sulfide (Cleartran) were tested to determine mechanical strength and slow crack growth parameters. Mechanical strength measurements of coupons were fit to a Weibull equation that describes the material strength and its distribution. Slow crack growth parameters were calculated using the procedure set forth by Weiderhorn.¹ This paper describes the derivation of Weibull and slow crack growth parameters from strength measurements over a range of stress rates and how these parameters are used to predict window lifetime under stress. Proof testing is employed to ensure that a window begins its life with a known, minimum strength.

The understanding and characterization of low level absorption in window materials is important for applications involving high energy lasers and hot windows in front of detectors. Low concentration impurities are important as well as disorders and defects. Such mechanisms can produce weak absorption that can manifest itself as background continuum absorption between the band gap and the multiphonon absorption edges. This so called weak absorption tail has been characterized in amorphous semiconductors and glasses, but not as completely in crystalline or polycrystalline materials that are typical durable window materials.

Low-level absorption in the visible and near infrared has been reported for single crystal o-ray sapphire and a limited set on other crystalline based material. A survey of reported measurements in regions of high transparency for such materials is presented.

A common window material for applications that require the simultaneous use of visible and infrared wavelengths is zinc sulfide, which offers a high transmittance between 400 nm and 12 μm. Depending on the manufacturing process, zinc sulfide can, however, exhibit large scattering losses (>10%) which degrade the imaging quality in particular in the visible spectral range. In this contribution, the different sources for light scattering such as volume imperfections, surface roughness, and subsurface damage are analyzed individually for hot isostatic pressed chemical vapor deposited zinc sulfide and correlated to the structural properties resulting from the fabrication process.

A microwave cylindrical cavity combined with a laser has been investigated to characterize the temperature dependence of widow materials in the Air Force Research Laboratory (AFRL). This paper discusses the requirements of high temperature RF material characterizations for transparent ceramic materials, such as ALON, that can potentially be used for multispectral windows. The RF cylindrical resonator was designed and the numerical model was studied to characterize the dielectric constant of materials. The dielectric constant can be extracted from the resonant frequency shift based on the cavity perturbation method (CPM), which is sensitive to the sample size and shape. Laser heating was applied to the material under test (MUT), which could easily be heated above 1000°C by the laser irradiation, in order to conduct CPM at high temperature. The temperature distribution in a material was also analyzed to investigate the impact of the thermal properties and the sample shape.

The refractive index of polycrystalline α-alumina prisms with an average grain size of 0.6 μm is reported for the wavelength range 0.9 to 5.0 and the temperature range 293 to 498K. Results agree within 0.0002 with the refractive index predicted for randomly oriented grains of single-crystal aluminum oxide. This paper provides tutorial background on the behavior of birefringent materials and explains how the refractive index of polycrystalline alumina can be predicted from the ordinary and extraordinary refractive indices of sapphire. The refractive index of polycrystalline alumina is described by

𝑛𝑛² − 1 = (A+B [𝑇𝑇²−𝑇𝑇²₀]) +Dλ² /λ2−(λ₁+C [𝑇𝑇²−𝑇𝑇²₀])² + λ²−λ₂²

where wavelength λ is expressed in μm, T_o = 295.15 K, A = 2.07156, B = 6.273× 10^-8, λ₁ = 0.091293, C = –1.9516 × 10^-8, D = 5.62675, and λ₂ = 18.5533. The slope dn/dT varies with λ and T, but has the approximate value 1.4 × 10^-5 K^-1 in the range 296–498 K.

Transparent ceramics are finding increasing use in optical applications with demanding operating conditions. Polycrystalline ceramics provide a unique combination of mechanical, dielectric and optical properties for sensor window applications that were previously not possible. The mechanical strength of CeraNova’s transparent alumina and spinel was measured by an equibiaxial strength test method. The results of the tests and their analysis, included those at elevated temperatures for transparent alumina, will be presented.

Transparent magnesium aluminate spinel (MgAl2O4) ceramic has excellent transmission from the UV to mid-wave IR. It is rugged with strength that is 5x that of glass. Spinel also has better IR transmission compared to sapphire and ALON. Because of its superior mechanical and optical properties, it is considered as a sensor window for numerous military platforms. At the Naval Research Laboratory (NRL), we have focused on process developments to facilitate wider acceptance of spinel for various applications. These developments include purification of spinel to reduce the absorption and scattering losses, as well as new processes to make conformal spinel windows and also to reduce manufacturing and finishing costs. In this presentation, we will provide an update on all the spinel activities at NRL

The requirements for modern aircraft based reconnaissance systems 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.

ALON consists primarily of aluminum and oxygen, similar to that of alumina, with a small amount of nitrogen added to help stabilize the cubic gamma-AlON phase. ALON’s chemical similarity to alumina, translates into a robust manufacturing process. This ease of processing has allowed Surmet to produce ALON windows and domes in a wide variety of geometries and sizes.

Spinel (MgAl₂O₄) contains equal molar amounts of MgO and Al₂O₃, and is a cubic material, that transmits further into the Infrared than ALON. Spinel is produced via powder processing techniques similar to those used to produce ALON. Surmet is now applying the lessons learned with ALON to produce conformal spinel windows and domes as well.

Aluminum Oxynitride (ALON^® Transparent Ceramic) and Magnesia Aluminate Spinel (Spinel) combine broadband transparency with excellent mechanical properties. Their cubic structure means that they are transparent in their polycrystalline form, allowing them to be manufactured by conventional powder processing techniques.

Surmet has scaled up its ALON^® production capability to produce and deliver windows as large as 4.4 sq ft. We have also produced our first 6 sq ft window. We are in the process of producing 7 sq ft ALON^® window blanks for armor applications; and scale up to even larger, high optical quality blanks for Recce window applications is underway.

Surmet also produces spinel for customers that require superior transmission at the longer wavelengths in the mid wave infra-red (MWIR). Spinel windows have been limited to smaller sizes than have been achieved with ALON. To date the largest spinel window produced is 11x18-in, and windows 14x20-in size are currently in process. Surmet is now scaling up its spinel processing capability to produce high quality window blanks as large as 19x27-in for sensor applications.

In the transparent ceramics processing, the green body elaboration step is probably the most critical one. Among the known techniques, wet shaping processes are particularly interesting because they enable the particles to find an optimum position on their own. Nevertheless, the presence of water molecules leads to drying issues. During the water removal, its concentration gradient induces cracks limiting the sample size: laboratory samples are generally less damaged because of their small size but upscaling the samples for industrial applications lead to an increasing cracking probability. Thanks to the drying step optimization, large size spinel samples were obtained.

The authors developed production process of polycrystalline Zinc Sulfide (ZnS) materials which have been widely applied to windows and domes for infrared sensor systems. Commercially available ZnS powders of ca. 5 um particle sizes were used as a starting material and Spark Plasma Sintering method (SPS) was applied to the powders for firing process. It was found that the densification of the sintered materials was inhibited by outgassing from ZnS powders during the sintering process (ca. 400 Celsius). Thermal desorption spectroscopy analyses revealed the components of outgassing, such as hydrogen sulfide, sulfur oxide and organic molecules. Based on these analyses, the optimum conditions on heating rate and starting temperature of uniaxial pressurization were investigated to remove the outgassing. The polycrystalline ZnS materials fired under the optimized SPS conditions have such characteristics as better transmittance than 65 % and good uniformity in both 3 - 5 um and 8 - 12 um wavelength regions. These results show the importance of removing outgassing from starting materials.

Freeform and conformal optics have the potential to dramatically improve optical systems by enabling systems with fewer optical components, reduced aberrations, and improved aerodynamic performance. These optical components differ from standard components in their surface shape, typically a non-symmetric equation based definition, and material properties. Traditional grinding and polishing tools are unable to handle these freeform shapes. Additionally, standard metrology tools cannot measure these surfaces. Desired substrates are typically hard ceramics, including poly-crystalline alumina or aluminum oxynitride. Notwithstanding the challenges that the hardness provides to manufacturing, these crystalline materials can be highly susceptible to grain decoration creating unacceptable scatter in optical systems. In this presentation, we will show progress towards addressing the unique challenges of manufacturing conformal windows and domes. Particular attention is given to our robotic polishing platform. This platform is based on an industrial robot adapted to accept a wide range of tooling and parts. The robot’s flexibility has provided us an opportunity to address the unique challenges of conformal windows. Slurries and polishing active layers can easily be changed to adapt to varying materials and address grain decoration. We have the flexibility to change tool size and shape to address the varying sizes and shapes of conformal optics. In addition, the robotic platform can be a base for a deflectometry-based metrology tool to measure surface form error. This system, whose precision is independent of the robot’s positioning accuracy, will allow us to measure optics in-situ saving time and reducing part risk. In conclusion, we will show examples of the conformal windows manufactured using our developed processes.

158 kg sapphire single crystals were grown by horizontal directional solidification method. Bubble-free panels with usable rectangular dimensions of 457 mm x 914 mm x 38 mm (18 x 36 x 1.5 inches) can be cut from the crystals. These are the largest-area sapphire panels ever produced by any technique. Bubble-free sections of the crystal 300 mm x 457 mm and up to 60 mm thick (12 x 18 x 2.36 inches) can also be produced. Growths were performed with graphite heating elements under a controlled atmosphere with automated power control. Results of transmission measurements performed on 5 mm thick samples are presented and the transmission is similar to sapphire grown by other techniques.

Sapphire windows are routinely being used in demanding aerospace applications due to their high strength and desirable optical and material properties. Sapphire is particularly useful in addressing the increasing need for systems that provide a wider range of capabilities in a single package. In general, refractive index homogeneity of the component materials can have a significant impact on overall optical system performance. This leads to the need for a deeper understanding of the shape and magnitude of index inhomogeneity in large sapphire windows to ensure predictable, high quality operation. Thin, sapphire slices from a sapphire crystal boule grown via the Heat Exchanger Method (HEM) have been previously evaluated for refractive index homogeneity over a 25.4cm (10.0”) aperture. The resultant transmitted wavefront error (TWE) from those measurements has now been used to model typical optical systems to quantify the effects on system-level performance attributed to representative amounts of index inhomogeneity in the sapphire window. The results of this modeling effort are presented in the following paper.

Metrology of freeform shapes has traditionally been difficult, especially at the sub-micron level. Sub-aperture polishing techniques and diamond turning allow optical designers to incorporate freeform surfaces into their systems. Contact measuring systems typically lack the accuracy or resolution required for optical qualification and can potentially damage the surfaces. 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 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. Incorporating datums from mechanical prints into the non-contact measuring method is especially important for freeform surfaces. UltraSurf has the ability to measure a wide range of surface roughness and has the degrees of freeform needed to scan datums and surfaces. The metrology method of UltraSurf and the non-contact probes will be presented. Form, figure, and thickness data will highlight the capabilities of UltraSurf to measure freeform surfaces.

Conformal optics require special manufacturing techniques to produce them to optical tolerances. In many cases the materials used are very hard optical ceramics that present additional manufacturing challenges due to their hardness and grain structure. OptiPro has developed grinding technologies such as OptiSonic grinding, as well as sub-aperture polishing technologies like UltraForm Finishing (UFF) to manufacture these challenging components. We have also developed a custom computer aided manufacturing (CAM) software package, ProSurf, to generate the complex tool paths for both grinding and polishing processes. One of the main advantages of ProSurf over traditional CAM software packages is that it uses metrology feedback for deterministic corrections. The metrology input can be obtained from OptiPro’s 5-axis UltraSurf metrology system, which is capable of measuring these complex shapes to sub-micron accuracies. Through the development of these technologies much work has been performed in creating, measuring and analyzing the alignment fiducials or datum’s used to qualify the location of the optical surfaces. Understanding the sensitivity of the optical surface to any datum misalignment is critical to knowing not only where the part is in space, but how good the optical surfaces are to each other. Working with the optical designer to properly tolerance surfaces to these datums is crucial. This paper will present the technologies developed by OptiPro to manufacture conformal windows as well as information related to the optical surfaces sensitivity to datums and how accurately those datums can be measured.

Through a collaborative effort between the Virginia Commonwealth University and Raytheon, a peridynamic model for sand impact damage has been developed1-3. Model development has focused on simulating impacts of sand particles on ZnS traveling at velocities consistent with aircraft take-off and landing speeds. The model reproduces common features of impact damage including pit and radial cracks, and, under some conditions, lateral cracks. This study focuses on a preliminary validation exercise in which simulation results from the peridynamic model are compared to a limited experimental data set generated by NASA’s recently developed micro-particle gun (MPG). The MPG facility measures the dimensions and incoming and rebound velocities of the impact particles. It also links each particle to a specific impact site and its associated damage. In this validation exercise parameters of the peridynamic model are adjusted to fit the experimentally observed pit diameter, average length of radial cracks and rebound velocities for 4 impacts of 300 μm glass beads on ZnS. Results indicate that a reasonable fit of these impact characteristics can be obtained by suitable adjustment of the peridynamic input parameters, demonstrating that the MPG can be used effectively as a validation tool for impact modeling and that the peridynamic sand impact model described herein possesses not only a qualitative but also a quantitative ability to simulate sand impact events.