After the chemical compositions of sapphire and ruby were unraveled in the middle of the 19th century, chemists set out to grow artificial crystals of these valuable gemstones. In 1885 a dealer in Geneva began to sell ruby that is now believed to have been created by flame fusion. Gemnologists rapidly concluded that the stones were artificial, but the Geneva ruby stimulated A. V. L. Verneuil in Paris to develop a flame fusion process to produce higher quality ruby and sapphire. By 1900 there was brisk demand for ruby manufactured by Verneuil's method, even though Verneuil did not publicly announce his work until 1902 and did not publish details until 1904. The Verneuil process was used with little alteration for the next 50 years. From 1932-1953, S. K. Popov in the Soviet Union established a capability for manufacturing high quality sapphire by the Verneuil process. In the U.S., under government contract, Linde Air Products Co. implemented the Verneuil process for ruby and sapphire when European sources were cut off during World War II. These materials were essential to the war effort for jewel bearings in precision instruments. In the 1960s and 1970s, the Czochralski process was implemented by Linde and its successor, Union Carbide, to make higher crystal quality material for ruby lasers. Stimulated by a government contract for structural fibers in 1966, H. LaBelle invented edge-defined film-fed growth (EFG). The Saphikon company, which is currently owned by Saint-Gobain, evolved from this effort. Independently and simultaneously, Stepanov developed edge-defined film-fed growth in the Soviet Union. In 1967 F. Schmid and D. Viechnicki at the Army Materials Research Lab grew sapphire by the heat exchanger method (HEM). Schmid went on to establish Crystal Systems, Inc. around this technology. Rotem Industries, founded in Israel in 1969, perfected the growth of sapphire hemispheres and near-net-shape domes by gradient solidification. In the U.S., growth of near-net-shape sapphire domes was demonstrated by both the EFG and HEM methods in the 1980s under government contract, but neither method entered commercial production. Today, domes in the U.S. are made by "scooping" sapphire boules with diamond-impregnated cutting tools. Commercial markets for sapphire, especially in the semiconductor industry, are healthy and growing at the dawn of the 21st century.
Refractive index matching glass coatings have been applied to mechanically-ground sapphire blanks using a modified glazing technique. The as-fired coatings are optically clear and well adhered, producing coated sapphire windows with up to 88 percent in-line transmittance and excellent optical imaging characteristics. Coated sapphire windows up to 150 x 230 mm in size have been produced, with additional scale-up to at least 300 x 350 mm planned for the near future. Glass-coated sapphire (GCS) can be rapidly polished in a small fraction of the time required for sapphire itself, thereby substantially reducing the cost of transparent armor. Glass-coated sapphire windows are also being evaluated for precision airborne reconnaissance and FLIR systems, to determine the limits, if any, to transmitted wavefront quality. The feasibility of applying index matching glass coatings to sapphire dome shapes has also been demonstrated. Index matching glass has also been used as a bonding material to fabricate actively cooled sapphire windows with internal channels for hypersonic missiles.
The goal of this effort was to design, develop, and demonstrate diffractive anti-reflection structures (DARS) on gallium arsenide (GaAs). Structures were designed and fabricated in GaAs intended to reduce the reflectance to infrared radiation from 1-10 microns wavelength. Design trade studies were performed to determine the optimum overall depth and period of the structure. The wafers were coated with UV sensitive photoresist and exposed in our interferometric stepper and our reduction stepper. Patterned areas were approximately 1cm x 1cm square. The wafers were then developed and measured to determine that the appropriate size and shape had been achieved prior to etching the pattern into the substrate. The wafer was etched in a plasma reactor to transfer the developed pattern into the GaAs. The depth and period of the surface was characterized using an atomic force microscope and a Scanning Electron Microscope. Reflectance spectra were measured for several angles of incidence.
The U.S. Army Aviation and Missile Command, Aviation and Missile Research, Engineering, and Development Center (AMRDEC) is currently developing the Compact Kinetic Energy Missile (CKEM) which achieves hypersonic velocities at sea level. The system incorporates guidance to the target and requires active guidance technology. CKEM's kinetic energy warhead requires an accurate guidance sub-system in order to achieve high probability of kills at long range. Due to the severity of the aerothermal environments, minimized reaction time for small time to target conditions, and the communication degrading effects of the missile's energetic boost motor, a state of the art guidance technique is being developed by the AMRDEC Missile Guidance Directorate called Side-Scatter Laser Beam Rider. This technology incorporates a 1.06 micron laser to receive an off-axis laser guidance link to communicate guidance information from the launch site to the missile. This concept requires the use of optical windows on board the missile for the missile-borne laser energy signal receivers. The current concept utilizes four rectangular windows at 90° increments around the missile. The peak velocity during flight can reach approximately 6300 ft/sec inducing severe aerothermal heating and highly transient thermal gradients. The Propulsion and Structures Directorate was tasked to design and experimentally validate the laser window. Additionally, flight tests were conducted to demonstrate the laser guidance technology. This paper will present the laser window design development process as well as aerothermal testing to induce flight like environments and assess worst case thermostructural conditions.
Edge Defined film Fed Growth (EFG) SaphikonTM sapphire crystals have been grown and successfully processed into windows measuring 225 x 325 mm with a thickness of 5.6 mm. More than 40 windows have been completed and assembled into customer hardware and delivered. The polished and coated windows have exhibited average transmission >93% from 1 to 5 mm and wavefront measurements of <0.1 waves rms (@ 0.633 μm) over a 125 mm aperture. Optical measurement data are presented and aspects of the crystal growth and polishing processes are discussed.
The world's largest sapphire boules up to 340-mm diameter are produced by the Heat Exchanger Method (HEM). In order to meet all applications, the highest purity crackle is used so the product has impurity levels very near the detectability limit of Glow Discharge Mass Spectroscopy (GDMS). The charge size of production 340-mm diameter sapphire boules was increased from 55-kg to 70-kg, and larger 380-mm diameter, 84-kg boules were produced. These boules were used to produce 315-mm diameter, 132-mm high sapphire cylinders to meet customer requirements. Efforts have been taken to produce a nearly flat top surface of HEM-grown boules with minimal undulations along the sidewalls to allow fabrication of larger sapphire pieces for production boules.
Large area sapphire windows have been fabricated by edge-bonding multiple panes. A 4-pane edge-bonded 320 x 410 x 7 mm sapphire window with excellent optical characteristics has been successfully finished. Two different bonding methods were used to build up the 4-pane window blank. Pairs of commercially available EFG sapphire panes were first bonded using a 1500°C bonding process. The bonded pairs were then joined using a 1100°C process. Bond strengths for the two methods are approximately 130 MPa (20 kpsi). Optical finishing was completed using standard methods for sapphire with no significant increase in finishing time caused by the bonds. There are no deleterious optical effects or visible optical distortion due to the bond lines. The edge bonding technology can now produce 600 x 600 mm flat window blanks. Conformal windows have also been produced using the edge bonding method. Very high bond strengths of 250 MPa (37 kpsi) have been attained on smaller samples using an optimized solid ceramic fillet.
Half of a set of sapphire disks was exposed to fast neutrons (0.8-10 MeV) at a fluence of 1022 neutrons/m2. Each 25-mm-diameter x 1-mm-thick disk was then exposed to a 10.6 μm CO2 laser (121 W/cm2) while the central 12.7-mm-diameter region of the disk was shielded from the laser. c-Plane disks that had not been exposed to neutrons survived 76% longer than a-plane disks that had not been exposed to neutrons. Neutron irradiation had no significant effect on time to failure of c-plane sapphire. However, neutron irradiation increased the survival time of a-plane sapphire by 30%-a result that was significant at the 99.9% confidence level. c-Plane disks were expected to fail in tension at the center of the disk. The calculated tensile stress at the mean failure time was ~700 MPa and the center temperature was ~400°C. By contrast, a-plane disks failed near the boundary between the shielded central region and the exposed outer annulus. The radial stress at this location is tensile and the hoop stress is compressive. Failure origins were at surface scratches. Rhombohedral twinning was observed in many a-plane disks, but there was no fractographic evidence that r-plane twinning caused failure. The mechanism by which neutron irradiation increases the time to failure of a-plane disks is unknown.
There are presently three materials (sapphire, ALON and spinel) which exhibit a desirable combination of material properties such as hardness, strength, and transmission in MWIR that are considered for various window/dome applications. Of the three, sapphire exists in a number of service applications. It is, however, the most expensive of the three and depending on application, can have significant drawbacks owing to its birefringent nature. ALON, by comparison is less expensive, benefits from greater development efforts, is an easily shaped polycrystalline ceramic, optically does not possess the birefringent nature of sapphire, but requires very high formation temperatures for the starting powders and equally long processing times for fabricated parts. The remaining material, transparent spinel, offers improved optical performance over the spectrum from UV to MWIR, comparable mechanical properties, and can be fabricated at much lower temperatures and shorter times than the other materials making it less expensive to produce. Data will be described which compares the transparency and mechanical properties and discusses the relevant processing efforts for spinel products.
For the past 15 years, the SPIE Window and Dome Technologies and Materials Conferences have been a community focal point for dissemination of information on the state of the art in materials, properties, and design, evaluation, and test methods for high-performance windows. This paper reviews developments over that time, the current state of window development, and future expectations. Examples are used to illustrate both progress and the need for continued development.
The Center for Optics Manufacturing is developing computer-aided manufacturing technology to produce affordable high quality spherical and non-spherical optical surfaces. Advances in MRF magnetorheological finishing), a computer-controlled deterministic finishing technology, now demonstrate a capability to polish plano, spherical, aspherical, and cylindrical optics, with round or non-round apertures, to better than 0.05 wave p-v (peak-to-valley), 5.0 Å rms surface microroughness, and no subsurface damage. MRF is a paradigm shift in optics manufacturing that redefines the
capabilities of the industry and will ultimately enable the affordable manufacture of any freeform optical shape.
Proc. SPIE 5078, Recent advances in the application of computer-controlled optical finishing to produce very high-quality transmissive optical elements and windows, 0000 (26 September 2003); https://doi.org/10.1117/12.487799
Large aperture (20-inch diameter) sapphire optical windows have been identified as a key element of new and/or upgraded airborne electro-optical systems. These windows typically require a transmitted wave front error of much less than 0.1 waves rms @ 0.63 microns over 7 inch diameter sub-apertures.
Large aperture (14-inch diameter by 4-inch thick) sapphire substrates have also been identified as a key optical element of the Laser Interferometer Gravitational Wave Observatory (LIGO). This project is under joint development by the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology under cooperative agreement with the National Science foundation (NSF). These substrates are required to have a transmitted wave front error of 20 nm (0.032 waves) rms @ 0.63 microns over 6-inch sub-apertures with a desired error of 10 nm (0.016 waves) rms.
Owing to the spatial variations in the optical index of refraction potentially anticipated within 20-inch diameter sapphire, thin (0.25 - 0.5-inch) window substrates, as well as within the 14-inch diameter by 4-inch thick substrates for the LIGO application, our experience tells us that the required transmitted wave front errors can not be achieved with standard optical finishing techniques as they can not readily compensate for errors introduced by inherent material characteristics.
Computer controlled optical finishing has been identified as a key technology likely required to enable achievement of the required transmitted wave front errors. Goodrich has developed this technology and has previously applied it to finish high quality sapphire optical windows with a range of aperture sizes from 4-inch to 13-inch to achieve transmitted wavefront errors comparable to these new requirements.
This paper addresses successful recent developments and accomplishments in the application of this optical finishing technology to sequentially larger aperture and thicker sapphire windows to achieve the challenging transmitted wave front error requirements defined above.
The anode characteristic differences of n- and p- type germanium in aqueous solutions has been investigated in the early 1950's. n-type electrodes were always pitted after being made anode above the breakdown potential, whereas p-type electrodes exhibited an electro-polished surface finish.
Diamond Like Carbon (DLC) coated germanium has been used extensively as the material of choice in harsh sub-surface and surface seawater environments. It has been widely demonstrated that DLC coated germanium provides limited protection and that electrolytic etching of the germanium still occurs.
This paper investigates the corrosion resistance properties of Boron Phosphide on germanium in a naval environment by presenting measured, comparative data from tests and field trials.
Missile domes made from Sapphire or other ceramic materials must be attached to metallic structures with different Coefficients of Thermal Expansion (CTE's) without blocking aperture or causing optical deformation due to mounting. High speed, high temperature environments are demanding requiring a strong, robust mount while maintaining optical location and figure. Brazing using compliant, CTE matched materials and active braze alloys have been highly successful in solving these problems for missiles. Descriptions of several different design approaches will be discussed and the advantages of each presented.
Chemical Vapour Deposited (CVD) diamond can now be fabricated in the form of large planar windows (up to 120mm in diameter and 2mm thick) and hemispherical domes (up to 70mm in diameter) suitable for operation as ultra-robust, aerospace infrared (IR) apertures. This paper describes the optical properties of such components. Many of the optical properties of large area CVD diamond windows are governed by its polycrystalline structure, in this paper its fracture
strength properties are also related to its structure. It is shown that 3-point bend techniques are an appropriate method for testing the strength of CVD diamond and that its strength is dictated by internal bulk flaws that are similar in size to the grains of the diamond. Its unsurpassed rain and sand erosion properties are briefly discussed and its shown that its solid particle erosion properties are related to its grain structure and that in liquid impact its properties are also significantly affected by its polycrystalline nature.
ZnS and ZnSe are two of the most extensively used longwave infrared optical window materials. Standard grade ZnS exhibits excellent transmittance properties from the 7-12 micrometer region. Post-deposition hot-isostatic pressing converts the standard grade ZnS to multispectral ZnS. Multispectral ZnS is transparent from the ultraviolet to the longwave region. ZnSe is optically superior to any grade of ZnS, but significantly weaker. In this paper, the experimental characterization of the multiphoton absorption edge in standard and multispectral grade ZnS and ZnSe as a function of temperature and frequency is presented. A broadband FTIR transmissometer is used to acquire data at temperatures ranging from 10 to 800 K for both materials. The frequency range is from 600 to 5000 cm-1 for ZnS and 400 to 5000 cm-1 for ZnSe. Using this experimental data set a multiphonon absorption model is developed that represents the experimental data over all temperatures and frequencies.
Diamond and Silicon Carbide (SiC) are two of the most durable infrared transmitting window materials available today. Diamond is transparent from 0.25 - 3 μm, exhibits weak absorption in the mid-infrared, and is again transparent from 8 μm well into the microwave range. Silicon Carbide has a single infrared transmission window from a 0.4 - 6 μm. In this paper, experimental characterization of the multiphonon absorption in CVD diamond and various grades of SiC as a function of temperature and frequency is presented. A broadband FTIR transmissometer is used. The temperature range is from 10 to 800 K and the frequency range is from 500 to 5000 cm-1. Using this experimental data set up updated multiphonon absorption model is developed that represents the experimental data over all temperatures and frequencies.
The experimental characterization of multiphonon absorption in polycrystalline GaP and GaAs as a function of temperature and frequency is presented. Becaues GaP and GaAs have moderate bandgaps, free carrier absorption is examined at high temperature as well. The longwave transparency and excellent thermal and mechanical properties of GaP make it a candidate for future high-stress environment applications. In this paper, a broadband FTIR transmissometer is used with a frequency range from 500 to 5000 cm-1 for GaP and 400 to 5000 cm-1 for GaAs. Spectral measurements were performed from 10 to 800 K for GaP and 10 to 295 K for GaAs. In addition, high temperature laser transmittance measurements using HeNe lasers (632.8 nm and 3.39 μm) and a CO2 (10.6 μm) laser were conducted up to 1100 K. Using this experimental data set, an updated multiphonon and free carrier absorption model is developed that represents the experimental data over all temperatures and frequencies.
There is a continuing need for durable and protective coatings for long wavelength infrared (LWIR) windows and domes as a result of the environmental and mechanical vulnerability of most LWIS transparent materials. Diamond coatings would be ideal except for the fact that relatively high deposition temperatures are required to deposit films having low optical absorption. Diamond-like carbon films deposited at low temperatures are typically too absorbing or highly stressed. Certain transition metal oxide films can be used successfully for many applications, are very durable and can be deposited by traditional thin film deposition methods. In this study, Y2O3, ZrO2 and HfO2 films are deposited and characterized, in particulara their absorption coefficients as a function of wavelength are derived at wavelengths in the LWIR. Durable oxide coatings are deposited over full-size LWIR windows.
Commerically available yttrium oxide nanopowders were evaluated as starting materials for preparation of transparent materials. The objective is an yttria optical ceramic exhibiting approximately one micrometer grain size to provide increased strength and thermal shock resistance. Three vendors were selected to provide nanoscale powders for testing and evaluation. They were compared to a conventional (5 μm) powder previously used to prepare optical quality ceramic yttria. While all of the selected nanopowders had impurity levels that were too high to allow processing to full transparency, two of the samples were processed to full density and moderate transparency was produced in one. In preparation for processing via Hot Isostatic Press (HIP) samples were sintered to a closed pore state at temperatures as low as 1400 °C, and with soak times as short as 12 minutes at 1550 °C. The use of ultrasonic attenuation as a technique for measuring particle size distributions in slurries was explored and found to be an invaluable tool when colloidally processing nanopowders. Finally, the areas most important for continued improvements were identified.
Polycrystalline transparent infrared windows with good optical and mechanical properties are needed. Starting from nanopowders, and sintering to ultrafine grained dense materials, offers the possibility of tuning the final grain size in order to simultaneously optimize the optical and mechanical properties. We have developed a chemical synthesis process to produce nanoparticles of single phase oxides, such as MgO and Y2O3. The synthesis process has been scaled in-house to produce kilogram quantities per batch. The primary particle size of powders is in the 15-35nm range, and the aggregate size is in the 150-200 nm range. In addition to using conventional sintering techniques such as hot pressing, these nanopowders are being sintered to full density and a high degree of transparency using a novel microwave sintering process, which has the added advantages of uniformly and rapidly heating a green compact. In preliminary studies, fully dense MgO, with LiF as sintering aid, was synthesized with a final grain size in the 1-3 micron range. Effects of processing parameters, such as hot pressing temperature, pressure, and LiF content, on microstructure and transparency were studied.
New grades of sintered (polycrystalline) corundum ceramics have been shown to exhibit a ballistic shielding power close to SiC/B4C composites when manufactured with a grain size of about 500 nm. It is demonstrated here that these Al2O3 ceramics become transparent when their residual porosity is decreased to less than 0.05 %. Specifically, in the IR range between about 2 and 6 μm their transmissivity equals that of sapphire approaching the upper theoretical limit for wavelengths of 2.5-4.5 μm. This opens the way to new possible applications such as IR domes. These optically and mechanically homogeneous ceramics can be manufactured with a wall thickness up to 15 mm by a wet casting approach. The technology enables the manufacture of complex hollow spheres which after sintering are transparent in visible light without polishing.
BaO-Ga2O3-GeO2 (BGG) glasses have the desired properties for various window applications in the 0.5-5 μm wavelength region. These glasses are low cost alternatives to the currently used window materials. Fabrication of a high optical quality 18" diameter BGG glass window has been demonstrated with a transmitted wave front error of λ/10 at 632 nm. BGG substrates have also been successfully tested for environmental weatherability (MIL-F-48616) and rain erosion durability up to 300 mph. Preliminary EMI grids have been successfully applied on BGG glasses demonstrating
attenuation of 20dB in X and Ku bands. Although the mechanical properties of BGG glasses are acceptable for various window applications, it is demonstrated here that the properties can be further improved significantly by the glassceramization process. The ceramization process does not add any significant cost to the final window material. The crystallite size in the present glass-ceramic limits its transmission to the 2-5 μm region.
With the advent of the uncooled detectors, the fraction of infrared (IR) imaging system cost due to lens elements has risen to the point where work was needed in the area of cost. Since these IR imaging systems often have tight packaging requirements which drive the optical elements to have complex surfaces, typical IR optical elements are costly to manufacture. The drive of our current optical material research is to lower the cost of the materials as well as the element fabrication for IR imaging systems. A low cost, moldable amorphous material, Amtir-4, has been developed and characterized. Ray Hilton Sr., Amorphous Materials Inc., Richard A. LeBlanc, Amy Graham and Others at Lockheed Martin Missiles and Fire Control Orlando (LMMFC-O) and James Johnson, General Electric Global Research Center (GE-GRC), along with others have been doing research for the past three years characterizing and designing IR imaging systems with this material. These IR imaging systems have been conventionally fabricated via diamond turning and techniques required to mold infrared optical elements have been developed with this new material, greatly reducing manufacturing costs. This paper will outline efforts thus far in incorporating this new material into prototype IR imaging systems.