This PDF file contains the front matter associated with SPIE Proceedings Volume 10985, including the Title Page, Copyright Information, Table of Contents, Author and Conference Committee lists.

Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. However, potential material candidates such as spinel and sapphire have scattering and absorption levels near the 1 μm wavelength region that are undesirable for future HEL applications. Indeed, absorption measurements in these materials indicate that residual loss in this region can be associated with a weak absorption tail. This tail has been experimentally characterized for amorphous semiconductor materials, long-wave infrared glasses, and certain optical fiber materials, but no comprehensive characterization has been done on crystalline window materials. Measurements of both weak scattering and absorption are essential for understanding loss mechanisms in these low loss materials. In this work, scattering measurements on spinel and sapphire samples obtained from different vendors are reported for wavelengths of 532 nm, 633 nm, 1064 nm, and 1550 nm. These measurements were obtained using a bidirectional scatterance probability distribution function instrument. The surface and bulk absorptions of these materials at wavelengths of 355 nm, 532 nm, and 1064 nm were also measured using photothermal common path interferometry. Additionally, a photoacoustic technique was used to obtain the absorption measurements at 1064 nm. In addition, ultraviolet-visible spectroscopy measurements were made to detect any impurity bands. These measurements contribute to a better understanding of the low-loss phenomenon near the 1 μm wavelength in both spinel and sapphire.

Single crystal YAG doped with a rare earth element has been commercially available for six decades. Polycrystalline YAG of high quality, suitable for use as laser gain media or optical windows has been available since 2004, and only from a single vendor. II-VI Incorporated seeks to be the premier supplier of high quality polycrystalline Yttrium Aluminum Garnet for laser gain media and infrared windows here in the United States. We present recent optical property characterization of polycrystalline (ceramic) YAG synthesized by II-VI. Optical transmission (1064 nm) data, FTIR and UV-vis-NIR spectroscopy, and photothermal common-path interferometer (PCI) absorption data are presented. Mechanisms for absorption and scatter produced by residual pores and impurities are discussed. Ceramic YAG properties are compared with single crystal (boule grown) YAG.

The data reported in journals on angle resolved scatter measurements for highly transparent window materials is scarce. An experimental facility with enough sensitivity to measure such low level scatter is described. A dynamic range of at least eight orders of magnitude is required to measure the peak of the specular component down to the random diffuse component far removed from the specular direction. Single crystal CaF₂ and polycrystalline CVD ZnSe are measured at wavelengths from the visible to the midwave infrared. The use of thick and thin samples allows the distinction between surface scatterance and bulk scatter.

There is an increasing industry need for high quality materials which can be used in high energy laser (HEL) applications. Currently, II-VI OS is growing large sapphire substrates for use in the defense industry on aerospace applications that operate in the visible and mid-wave infrared regions. II-VI OS is capable of producing a-plane and cplane sapphire substrates of sufficient size for envisioned HEL applications, and in the following paper will present optical and material data on properties of interest. Specifically, data will be presented on BTDF and BRDF at 1.06μm in both s- and p-polarizations at angles of incidence (AOI) ranging from 0 to 60 degrees. These measurements are performed on a-plane and c-plane windows grown, fabricated and polished at II-VI OS Additionally, other critical properties evaluated include absorption at 1.06μm, as well as thermal expansion coefficient and Knoop hardness of cplane sapphire.

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 is being developed as a sensor window for numerous military platforms. At the U.S. 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 for use as an exit aperture on High Energy Laser (HEL) systems and various cost-effective densification methods to reduce manufacturing costs. In this paper, we will provide an update on some of the ongoing spinel activities at NRL.

In this study, effects of coating material and thickness on damage characteristics of an electro-optical material impacted by small particles have been investigated. Six distinct sample types were prepared: (1) multispectral zinc sulfide (MS-ZnS), (2) MS-ZnS with rain erosion protection (REP), (3) MS-ZnS with thin durable antireflection coating (DAR), (4) MS-ZnS with thick DAR, (5) MS-ZnS with REP and thin DAR, and (6) MS-ZnS with REP and thick DAR. The samples were subjected to impact by glass spheres of diameter ranging from 150 micrometers to 700 micrometers at velocities ranging from 40 m/s to 200 m/s. The impact events were captured using two high-speed cameras at a rate of 125,000 frames per second. Taking advantage of the transparency of the samples, one camera captured the impact events from a vantage point normal to the impacted plane, on the other side of the impacted face. The other camera captured the events from a side view. Using the synchronized footage from both cameras, impact sites were paired with the impacting spheres and their velocities were measured. Nearly two hundred impacts were analyzed. The analysis included optical and scanning electron microscopy. Through the characterization effort, impact pit diameter, lateral crack diameter, and radial cracks lengths were measured. When coatings were present, delamination and peeling of the coatings were quantified. The trends in damage morphologies as functions of impacting sphere diameter and inbound velocities were explored. Thresholds for lateral and radial cracking phenomena were identified. The measurements were compared against theoretical values from indentation theory.

Satellite based laser communication terminals (LCT’s) rely on the optomechanical stability and imaging performance of their beam expander and receiver telescope lens assemblies under extreme environmental conditions with respect to temperature and solar irradiation. Direct or partial sun exposure of the entrance aperture of any telescope optics will result besides a possible stray light background in a gradual transient heat load and thus thermal deformation of the optical elements. This can result into temporary degradation of the optical performance, making a datalink impossible within this time. We present a design and functional verification of a high reflectivity (HR) sun blocking filter (SBF), that is able to protect LCT telescopes against solar or earth albedo based electromagnetic radiation over a very broad spectral range from 450 nm up to > 5 μm while simultaneously providing a high transmissivity (HT) of > 95% for the LCT laser radiation at 1064.55 nm. The SBF is based on a 3.25” diameter sapphire substrate in high performance optical quality. Both sides of the filter have a multilayer coating with distributed functionalities accounting for broadband HR, low loss anti reflection (AR) coating and HT at the laser wavelength of 1064.55 nm. The HT coating design at 1064.55 nm is based on an induced transmittance of a thin ( < 25 nm) Ag layer, that is stacked within an interference coating, tuning the HT to the desired laser wavelength of the used LCT. Design verification is presented with respect to spectral transmission/reflection, transmitted wave front error and birefringence under all relevant operational conditions.

Advanced window coatings being developed for drone applications need to be multi-functional. A typical window design includes a hydrophobic outer coating, a solar filter rejecting unwanted spectrum, an EMI layer and/or heater to prevent icing and anti-reflection films. Hydrophobic and hydrophilic thin film materials are available from various vendors. We present our experience in using these materials and their comparative performance with respect to film adhesion and durability. The window can be a common aperture for several different sensors. Adding an EMI shield using a transmitting conducting oxide (TCO) is challenged when visible and NIR transmission is needed. The trade-off between required conductivity and NIR transmission is presented.

High power continuous-wave (CW) and high pulse energy laser optical systems can suffer damage from untreated optics due to undesirable Fresnel reflections. With high energy pulse lasers, traditional anti-reflection (AR) thin-film coated optics are susceptible to localized field enhancement regions, due to multiple boundaries, and experience laser induced damage on both entry and exit interfaces. Sub-wavelength anti-reflective structured surfaces (ARSS) have been shown to have a higher laser-induced damage threshold than traditional AR coatings. Previously published work detailed nanosecond-pulsed laser-induced damage on planar fused silica (FS) substrates with random ARSS (rARSS) treatment (λ=1064 nm). This study details and compares laser damage and fatigue testing of rARSS treated FS via continuous-wave, 2 kW irradiation (λ=1075 nm). Gaussian laser output was focused to ~ 60-μm-diameter (1/e²), to increase incident intensity, yielding a maximum power density of ~ 70.7 MW/cm². Laser power and duration were controlled while monitoring the window’s optical transmission and entry/exit surface temperatures. CW durations at maximum power up to 5 min were used. Peak temperatures were recorded for untreated FS, as well as double side treated rARSS samples.

A peridynamic model for sand impact damage has been developed. 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 application of the model to systems with coatings. Two different coating materials were included in the modeling effort. The required material parameters of critical stretch, which governs the material failure and fracture, and yield strength, were extracted from peridynamic simulations of experimental results of indentation tests. The model was then applied to impact conditions for which experimental measurements of damage characteristics were available. Peridynamic approach enabled investigation of the mechanical properties of interfaces between the coatings and the substrate. This study demonstrates current computational approach and its quantitative ability to simulate sand impact events.

Aluminum Oxynitride (ALON® Transparent 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’s has been producing and delivering ALON® windows as large as 4.4 sq. Ft. for both Armor and Sensor window applications. We have also recently scaled up our ALON® process to produce windows as large as 8 sq. Ft. for sensor window applications. Initial prototype windows of this size have been produced. These windows exhibit the same excellent levels of transmission, haze and clarity as seen in smaller ALON® windows that we have produced.

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.

Freeform optical components are gaining popularity with designers due to their ability to improve optical and aerodynamic performance for many applications. The challenges involved with the manufacturing and metrology of these shapes, which have little or no symmetry, has been discussed at previous talks and conferences. This paper will focus on the challenges that Optimax faced as we scaled up our freeform polishing process from parts with approximately 150 mm diameters, to polishing components with diameters over 600 mm. The large format platform, designed, built, and programmed at Optimax, utilizes a pick-and-place style, 6-axis robotic arm for the polishing motion. In order to scale up the platform from our existing robotic polishers, a larger robotic arm was used. The associated challenges include: timing considerations for both the polishing and metrology, obtaining sufficient material removal for reliable measurements, and difficulties modelling robot joint positions for collision prevention. These issues have been investigated and mitigated through proprietary techniques and novel solutions, some of which will be explored in this paper. One such technique currently under development at Optimax is deflectometry; which is a noninterferometric method involving fringe reflection and ray tracing to calculate the mid-spatial frequency (MSF) error on a part surface. Deflectometry is able to measure MSF error two orders of magnitude faster than the current method, and has been implemented in-situ, mitigating another challenges involved with larger freeform optics: the logistics of moving them around a shop floor safely.

OptiPro Systems, LLC has been recognized as a U.S. leader in computer-controlled equipment for manufacturing high precision optical components found in medical, military, aerospace, night vision and many other industrial and commercial applications. OptiPro’s success in winning several Small Business Innovative Research (SBIR) grants through the Navy, NASA and the Army have led to the commercialization of new machines, software and processes for grinding, polishing and measuring complex optics such as windows and domes. OptiSonic machines incorporate the latest in ultrasonic machining technology by adding 20-40 kHz of adaptive oscillation to the grinding tool, which reduces force on the tool and part during processing. OptiSonic machines have proven to increase processing speed and precision when grinding various hard optical materials that are ideal for hypersonic applications. UltraForm Finishing (UFF) machines are capable of deterministically finishing windows, domes, and other complex optical elements through a patented sub-aperture polishing technique. This method involves a moving belt of polishing material wrapped around a precision compressive wheel. UltraSurf machines deliver ultra-precision non-contact metrology though the utilization of high-quality machine components and the latest non-contact probe technologies. The measuring probe is scanned over the optical surface while maintaining perpendicularity and a constant focal offset. Finally, our PROSurf freeform computer-aided manufacturing (CAM) software is used to develop advanced toolpaths for each of these platforms. We will present an overview of each of these technologies and discuss how they are advancing the precision optics manufacturing world.

A 1989 SPIE proceedings paper described the first version of an optical properties computer code developed at the Applied Physics Laboratory. The emphasis was on durable infrared window materials. In the thirty years since the beginning of this code development a lot of progress has been made concerning the physics based models used to represent the complex index of refraction and the number of materials characterized. A key feature is providing the frequency and temperature dependence of optical properties. Room temperature data is available for a wide range of materials, but temperature dependent data is much harder to find. The status of the code development along with example outputs will be reported.

Erosion of external optics in sensing systems leads to degradation in performance. Sand impact is the most common form of window and dome erosion. Erosion from sand impact can be separated into two categories: erosion generated while on the wing of a platform in flight (known in this paper as “Platform Induced erosion”) and erosion generated on the ground during high wind events like sand storms (known in this paper as “natural environment erosion”). When qualifying materials for external optics, both categories of sand impact erosion must be evaluated. Platform Induced erosion testing is primarily performed at facilities such as the US Air Force's Particle Erosion Test Facility using a testing setup commonly known as the “sand rig”. Natural environment erosion testing, commonly referred to as “blowing sand”, is performed at various facilities around the United States with varying test setups. When interpreting data from a test setup, it is desirable to understand the variability of data collected over multiple runs and from multiple days. Understanding the sources of variability is also important. This paper presents information collected by Raytheon on fused silica test samples at the Particle Erosion Test Facility’s “sand rig” to assess the variability of data obtained from this facility. Possible sources of variability will be discussed and recommendations for performing sand erosion testing will be presented.