Polycrystalline MgAl2O4 Spinel, transparent from two hundred nanometers to six microns, offers a unique combination of optical and physical properties. A superior dome and window material with respect to rain and particle erosion, solar radiation, high temperatures and humidity, it is resistant to attack by strong acids, alkali solutions, sea water and jet fuels. Residual microporosity from the powder process used for fabricating Spinel which previously limited the use of Spinel to thin wall thicknesses and small sizes, has been significantly reduced by advanced hot press and hot isostatic press (HIP) technology. It is now possible to manufacture high quality shallow domes up to seven inches in diameter with a two tenths inch thick wall thickness. Eight inch diameter flat windows have been produced for an advanced missile system. Proof of process near hemispherical 8 inch dome blanks have been fabricated. Recent measurements of refractive index, homogeneity, scatter and surface roughness are available for design purposes. Improvement in the optical quality and in size/shape capability along with several successful prototype tests demonstrate that Spinel is ready for inclusion in appropriate production systems.

Lanthana-strengthened yttria (LSY) is a window and dome material for 3 - 5 micrometers IR applications. Scattering, an important optical property of LSY, was measured using a scatterometer method and integrating sphere technique. The scatterometer results were correlated with the integrating sphere results. The mechanisms of forward scattering in LSY are discussed.

A vacuum emissometer utilizing a CO2 laser for high temperature sample heating has been designed and built for use with a Fourier Transform spectrometer. A two-color pyrometer technique is used to calculate sample temperatures. Oxides such as sapphire, spinel, yttria, ALON and fused silica are experimentally characterized from 600 to 2000 K and from 500 to 5000 cm-1. A glowing yttria sample has also been characterized over the spectral range of 8500 to 13500 cm-1. Good agreement with a quantum mechanical multiphoton model for the complex index of refraction, also developed at APL, is obtained.

Transparent dome elements for future higher speed missile systems place stringent requirements on the mechanical properties of the dome material. Currently no material meets all the requirements. Among the five candidate oxide materials, sapphire appears to be the best choice based upon material properties, thermal shock resistance and status of material production. A disadvantage of sapphire is that it must be produced in single crystal form because of its anisotropic structure. A new approach to produce near net-shaped sapphire domes from the melt is discussed. Multiple near-net-shaped sapphire domes of various sizes and curvature have been produced.

Close attention to crystal growth parameters and characterization of the crystal's thermal environment during growth has led to improvement in the crystal structure of EFG grown dome blanks. These near net shape 80 mm sapphire blanks have been fabricated to produce high quality finished domes. New measurements of the coefficient of thermal expansion (CTE), thermal conductivity, optical scatter, rain erosion and the thermal coefficient of refractive index (dn/dT) as a function of wavelength have been performed and the data are presented.

A polishing process for fabricating high-quality optical sapphire windows is presented. This process was successfully used to produce very high aspect ratio, as high as 1500:1, sapphire windows for use in optical imaging systems. During the development of the process as unusual print-through phenomenon was discussed. This replication of surface features on the blocking bodies, such as bumps or grooves, onto the polished window surface produced distortion in the final wavefront and degraded window performance. A method of bonding smaller, thick sapphire panes into a larger optical window is also presented. A glass fritting process utilizing two different temperature frits is discussed. The optical errors encountered during fritting which contribute to the optical error budget and methods for minimizing their effect on the wavefront of the final window are also reviewed. An 8' diameter X 0.435' optical sapphire window was produced using this fritting technology.

Optical polishing effect of purity 99.96% cerium oxide extracted by sulfuric acid solution from raw monazites is discussed. Researches show polishing efficiency of the cerium oxide calcined at 1100 +/- 25 degree(s)C (and then rapidly cooled) can increase by 30 - 40% as compared with that of the un-calcined, and that it is especially suitable for polishing hard optical glass and crystals, and window and dome materials; to a certain extent, those which produce flocculent additives round the cerium oxide grains can raise polishing efficiency, decrease or eliminate 'drags' and 'orange peel' of polishing surface and improve polishing surface quality result, and each additive has its optimum amount to a certain optical material. In addition, content of other rare-earth oxides or non-rare-earth oxides affecting polishing surface quality has been studied.

Infrared (IR) windows and domes for use with electro-optic (EO) systems operating in the 8- to 12-micrometer region are currently fabricated from the following limited number of materials: germanium (Ge), zinc sulfide (ZnS), and a zinc selenide (ZnSe) sandwich. A multiyear development effort at Texas Instruments (TI) has been produced another such material: gallium arsenide (GaAs). A novel crystal growth process has been developed that allows for economical growth of either high-resistivity (10⁷ ohm-cm) or conductive [> 1 (ohm-cm)^-1] GaAs crystals of various thicknesses in sizes up to 12 by 12 inches. This unique growth process permits production of GaAs, which is highly uniform or has a tailored electrical conductivity. With high IR transparency from approximately 1 to 14 micrometers, the high resistivity and conductive GaAs remain operationally useful to 400 degree(s) and 200 degree(s)C, respectively. The low resistivities allow for effective electromagnetic interference (EMI) shielding of 60 dB or more in the gigahertz region in highly transmissive IR windows made of this conductive GaAs. Besides fabrication of traditional precision optical components like lenses and prisms, TI has developed precision fabrication techniques for domes, windows, and segmented windows made of GaAs. These same techniques have also been designed to lower the mean size and distribution of the traditional optical component fabrication flaws, thereby increasing the strength and Weibull modulus of GaAs to 19.1 Kpsi and 4.7, respectively. These high strengths and the corresponding survival probabilities are comparable to those of chemical vapor deposition grown ZnS. TI has also developed techniques to further toughen GaAs, specifically for window and dome applications, in terms of increased strength through increasing the critical stress intensity factor (K_IC) by changing the GaAs microstructure. Toughening in terms of impact resistance to rain during aerodynamic flight has also been developed by means of a protective coating. Antireflective coatings for surface-impedance matching have also been developed. This paper describes the properties of these various GaAs technologies.

Complex electro-optical (E-O) systems may be integrally mounted within the airframe or housed in separate pods on aircraft. Sensor fields of view and integrated laser target designator/rangefinder beams commonly sweep over large solid angles. This is accomplished by gimbaled mounts that are normally line-of-sight stabilized. If the E-O system is gimbaled as a whole, simple flat windows suffice. Internally gimbaled systems require domes or multi- segmented (faceted) windows. In any event, windows form the environmental barrier to the outside world for the E-O system and often must accommodate spectra other than just infrared (FLIR) wavebands. Optical windows can easily be the single most expensive optical component of a system. This paper considers various window designs and discusses their unique properties and possible pitfalls. Besides optical properties--durability, thermal, electromagnetic shielding, and other environmental aspects are treated.

LWIR-transmitting materials are of interest for medical, sensing, and communication applications where optical, mechanical and chemical stability during use are essential. Currently available materials are limited to crystalline chalcogenides and halides, chalcogenide glasses, and chalcohalide glasses whose long wavelength transmissivity is obtained at the expense of thermal and quite often chemical stability. This paper reviews preliminary results from an investigation of the glass forming tendencies of a group of compounds that have been chemically designed for optimal transparency in the 8 - 12 micrometers region and that also possess good mechanical and chemical stability.

New infrared glasses based on chalcogen halides specially tellurium halide are described. These TeX glasses exhibit low optical loss in the 8 - 12 micrometers region and a strong dependence of their thermal properties on the chemical composition. The variation of the refractive index versus temperature T, wavelength (lambda) and composition leads to information on optical dispersion. These ductile, plastic glasses can be molded with an excellent duplication of optical surface and drawn into IR optical fibers with attenuation as low as 0.2 dB/m in the 8 micrometers region.

Silicon is used extensively as a transmissive optic in the mid infrared (IR) region of the spectrum. It has not been used in the far IR primarily due to an absorption band at about nine microns. This absorption is the result of both intrinsic lattice absorption and an impurity band absorption due to oxygen (O₂). The absorption can be minimized by reducing or eliminating the impurity band component. In this work we report results obtained from transmission scans for low oxygen 'O₂ Free' silicon. We compare these results with those obtained for typical Czochralski (CZ) silicon. Absorption coefficients are calculated from the transmission data. Tabular data and graphical representations of the data are shown in the eight to twelve micron region of the spectrum.

Results are presented from a program of tests carried out on bulk type IIa (natural) diamond. The objectives were to assess diamond as a material for optical windows exposed to severe environmental conditions, and to determine benchmarks for the performance of synthetic, film-grown diamond. Measurements have been carried out of transmittance and reflectance from the UV to the far-IR, IR transmittance up to 700 degree(s)C, and absorptance at specific CO₂ laser wavelengths (determined by laser calorimetry). Water jet impact and simulated sand erosion resistance have been evaluated. The laser-induced damage threshold at 10.6 micrometers has been measured.

An oxy-acetylene flame can produce diamond films at significantly higher deposition rates than those associated with either microwave plasma or hot filament assisted chemical vapor deposition. We have established the growth conditions necessary to achieve good quality diamond on silicon substrates. The addition of hydrogen to the gas mixture has been shown to give good quality material at enhanced growth rates. The growth rate has been increased further by using a growth-etch cycling process. This is achieved by periodically pulsing extra oxygen into the gas stream to change from depositing to etching conditions. Under etching conditions the non-diamond carbon in the film is rapidly removed leaving the diamond behind. This allows the use of high rate growth conditions that would otherwise produce poor quality material. The morphology and Raman spectra of films produced by these techniques are presented. The scale-up of the deposition system to cover areas as large as 15 X 20 mm is reported. This is accomplished by rastering a burner consisting of a line of small flames.

Diamond grown with chemical vapor deposition (CVD) processes is currently being considered for use as a long wave infrared (LWIR) dome material for advanced missiles. In order to assess diamond's suitability for this application, a complete understanding of the optical properties of CVD diamond is needed. This includes a determination of the relative amount of bulk and surface scattering, and a measurement of the absorption in thick CVD diamond films. In this paper, we present scattering data for visible (0.633 micrometers ) and infrared (10.6 micrometers ) wavelengths from optically smooth thick diamond films. Scattering data from the aluminized front and back surfaces of the films is also reported. These measurements, together with first order scattering theory, provide a means for determining the component of scattered radiation which is due to bulk scattering. The bulk absorption is also estimated from a detailed energy balance using reflectance, transmittance and scattering measurements.

While the optical and thermal properties of CVD diamond have been previously addressed, its mechanical properties have received little attention. A preliminary study of strength and fracture toughness in free-standing DC arc-discharge CVD diamond is reported in this paper. The materials studied is characterized by near-randomly oriented grains ranging in size from 3 to 30 microns. Disks ranging from 40 to 800 microns in thickness and from 7 to 16 mm in diameter were tested for flexure strength using a ball-on-ring apparatus. Both polished and unpolished disks were studied resulting in strength values averaging over 100,000 psi. Fracture surface analysis showed that the fracture occurred transgranularly as is generally observed for other brittle polycrystalline materials. The fracture toughness value of 6.5 +/- 1.2 MPa-m^1/2 determined by the analysis is enhanced over the previously measured single crystal value of 3.4 MPa-m^1/2 reflecting the increased area of the rough fracture surface. The indentation toughness was shown to have a similar value providing additional evidence of this enhancement.

Optical, mechanical and erosion protective characteristics of boron and gallium phosphide have been evaluated as single films and within anti-reflection multilayers. These coatings are shown to combine broad-band infra-red transmission with environmental durability, specifically in relation to abrasion resistance and elevated temperature performance up to 500 degree(s)C. Rain erosion protection of all common IR optical materials is demonstrated from single water jet impact and whirling arm tests. Protective characteristics in relation to solid particle impact are described. Productionizing of phosphide coating processes is well advanced in relation to control, scaling and handling of hazardous feedstocks.

Thick germanium carbine films (GeC) are successfully grown on various Zinc Sulfide and Germanium substrates at temperatures up to 350 degree(s)C by two methods: Plasma Enhanced Chemical Vapor Deposition (PECVD) in gas mixtures of methane and germane and by Reactive Radio-Frequency Sputtering (RRFS) starting from a germanium target in a sputtering medium of methane and argon. The optical and mechanical properties of the GeC coatings depend on the composition determined by the deposition parameters. The refractive index at 633 nm varies from 4.9 to 4.3 for a carbon content ranging from 3 to 25% and the correlated refractive index in the 8 to 12 micrometers range is found to be between 3.96 and 3.1. For these coatings, the absorption coefficient is ranging from 270 to 40 cm^-1. All films are amorphous in nature with domains ranging from 13 to 20 angstroms. The hydrogen content varies from 2 to 25% coming from C:H, Ge:H and C:Ge:H bonding. The XPS analysis shows the Ge:C precipitation kinetic for high deposition temperature or annealed films. The rain erosion resistance of GeC films and GeC with a protective diamond like-carbon (DLC) coating on top is measured for 1.2 mm water drop with an impact velocity ranging from 210 to 265 m/s on the Saab-Scania whirling-arm rig (Linkoping, Sweden).

Texas Instruments (TI) has an ongoing development effort for protective coatings to enhance the environmental durability, particularly against water and particle impact, of infrared (IR) (8 to 12 micrometers ) transmissive windows and domes on airborne platforms. This program has produced a very effective, rain-erosion-resistant coating consisting of polycrystalline or epitaxial gallium phosphide (GaP). The GaP coating has been grown on germanium (Ge), gallium arsenide (GaAs), and zinc sulfide (ZnS) window/dome materials using a metal-organic chemical vapor deposition (MOCVD) process. The coatings have low IR absorption coefficients of 2 cm^-1 at 10.6-micrometers wavelength, as measured by laser calorimetry. At a thickness of 20 micrometers , these GaP coatings degrade the transmission of the window/dome materials by only 1 percent. These high-transmission coatings have been shown to be very effective in protecting the window/dome materials from rain impact damage, as evidenced in testing by single-waterdrop impact, multiple-impact jet apparatus and whirling arm rain erosion. The details of the properties of these GaP IR protective coatings are presented and discussed.

The amount and type of damage a sample receives during waterdrop impact experiments depends not only on the size and impact velocity of the waterdrop, but also on the microstructure of the underlying substrate, and its impact history. The geometry of the ring fractures resulting from single impacts is strongly affected by the morphology (i.e. grain size and orientation) of the substrate material. Furthermore, repeated impacts on or near previously impacted sites will create damage which depends not only on the morphology of the substrate material, but also on nature of the previous damage. Impact resistance refers to a previously unimpacted samples ability to withstand damage from individual waterdrop impacts. Durability refers to a samples ability to withstand extended exposures to high speed rain fields. Rain protective coatings can be applied to substrates to significantly enhance their survivability. Coating have been shown to increase a substrate's Damage Threshold Velocity (DTV) and to significantly reduce the cumulative damage that samples receive during prolonged exposures to high speed rain fields.

Quantitative measurements of the influence of high-speed rain impact damage on uncoated water clear ZnS as a function of velocity and impact exposure were made using Nomarski microscopy, total integrated scattering, integrated spectral transmission and equibiaxial flexural testing. The rotating arm tests assessed the response of the material to nominal 2 mm diameter drops at normal incidence. The measure of drop exposure selected for these studies was the density of drop impacts expected to strike the specimen for the test conditions (rain rate, velocity, run time, and spatial distribution of drops). The expected exposure is compared to measurements on PMMA calibration specimens. The influence of high durability coatings on the water clear ZnS was assessed using the same characterization methods. The limitations imposed by statistical variability illustrate the difficulty of comparisons made without adequate control of the parameters.

Conventional ZnS, clear ZnS, ZnSe, and ZnS/ZnSe sandwich materials along with 8 to 12 micrometers anti-reflection (AR) coatings have been used as windows for forward looking infrared (FLIR) thermal imaging electro-optical sensors (such as those incorporated on PAVE TACK, F-18, and LANTIRN pods). Conventional ZnS also has been used as dome material for IR Maverick missiles and other missile applications. All of these systems have separate windows/systems for target designation, rangefinding, and low light level television (LLLTV) applications. New generation system require that a single window provide multispectral capabilities to perform various functions. A graded index AR coating developed at Hughes Danbury Optical Systems (HDOS) provides the multispectral capabilities and is highly durable for subsonic aircraft and missile applications. The spectral performance, durability, rain- erosion, and some sand and dust data of such a coating are presented in this paper. The data is also presented for this coating in conjunction with grids for EMI attenuation. The transmission of the coating as a function angle of incidence is also presented.

An optical surface can be microscopically textured in a pattern with physical features whose dimensions are below that of the operational wavelength to produce a physical gradient in the effective optical index of refraction. The performance of such 'moth-eye' surface features, typically cones or pyramids, can be predicted based on dielectric mixture models by use of the optical properties of the base material and air. The performances of LWIR antireflective moth- eye surfaces formed in silicon, germanium and diamond are consistent with theoretical predictions.

In comparison with presently available LWIR (8 - 12 micrometers ) window materials, diamond has unsurpassed optical and thermomechanical properties. However, the manufacture of bulk diamond optical components is still in its infancy and many years effort are required to develop the technologies necessary to the fabrication of large windows and domes. In the short term the coating of current LWIR materials (particularly ZnS) with a protective layer of polycrystalline diamond would result in a significant improvement in performance. At GMMTL a high temperature microwave plasma assisted chemical vapor deposition (MPACVD) system has been developed which produces excellent quality diamond films on silicon. It is not so easy to apply this technique to the coating of ZnS since atomic hydrogen in the microwave plasma attacks and rapidly etches the ZnS surface and, in addition, there is a large thermal expansion mismatch between ZnS and diamond that causes decohesion of the film. To solve this problem an advanced physical vapor deposition (PVD) process has been used to deposit interlayers of various LWIR transparent materials onto ZnS to give protection from the plasma and also allow some thermal stress relief. The interlayers have been found to be extremely robust with respect to current nucleation treatments and have allowed continuous diamond films of thickness in excess of 1 micron to be formed onto ZnS. The significance of nucleation treatment and diamond morphology are discussed in the context of gaining the optimal performance both in terms of layer adhesion and IR properties.

The development of diamond coatings has opened up new possibilities in the field of dome technology. The Cavendish Laboratory has long been involved in liquid impact studies of candidate window materials using a jet technique. This technique has been incorporated into a computer controlled automated Multiple Impact Jet Apparatus which has now been used to characterize the rain erosion properties of a variety of window systems including diamond coated samples. The results presented here compare the threshold velocities of damage for the new materials with those for current windows.

A very important consideration in determining the survival of infrared windows exposed to hydrometeor environments is the condition of the hydrometeors at the time of impact and the extent of the damage they may produce. The condition of the hydrometeors at the impact point is determined by the altitude and velocity of the space vehicle and the location of the infrared window. Computational procedures have been developed to evaluate the shape, orientation, and velocity of waterdrops (ice or sand particles) at the time of impact for the range of initial waterdrop diameters in a given rainfield. Specific examples are provided for the flowfields around supersonic vehicles with a front-mounted hemispherical dome and with a side-mounted window. The results from these analyses are used to establish the waterdrop impact test conditions which are unique to these specific flight conditions and vehicle configurations. It is then necessary to identify the waterdrop impact testing capabilities which can best satisfy the resulting requirements for damage assessment. This approach is in contrast to the typical approach which evaluates the survival of the infrared-transmitting window relative to the existing waterdrop impact testing capabilities which may not be at all representative of the vehicle flight conditions.

The effects of supersonic waterdrop impacts on infrared-transmitting materials are relatively unknown. This is due to the general lack of testing capabilities for these impact conditions, however current flight requirements are placing an increased emphasis on this velocity regime. The GRC Multiparticle Impact Facility was used to obtain multiple nylon bead simulations of waterdrop impacts on zinc sulfide specimens. Nylon bead impact damage on zinc sulfide is shown to be quite similar to that due to waterdrop collisions for several of the impact conditions. The validity of the nylon bead/waterdrop impact correlation is being more fully developed as the range of comparable impact conditions is increased. The nylon bead impact data base for zinc sulfide includes three impact angles (90 degree(s), 45 degree(s), and 30 degree(s)) and three bead diameters (1.6 mm, 2.0 mm, and 3.2 mm) for a range of impact velocities.

Physically-based optical property models of solids are a convenient means of representing the complex index of refraction as a function of frequency and temperature. This modeling approach is especially convenient considering the wide spread use of personal computers and the uncomplicated mathematical form of the models. Models provide a convenient method of cataloging measurements and interpolated between measurements. Several useful models covering absorption and scattering phenomena are presented. Together, these models allow prediction of optical properties over the spectral range from microwaves to the electronic band gap. Temperature dependence of the optical properties cover a more restricted range, but some models predict optical properties from liquid helium to melting temperatures. We have developed an optical properties code incorporating the following models: the classical (one- phonon) oscillator model, our multi-phonon model, the Urbach tail and weak absorption tail models, free-carrier model, and an empirical scatter model. These models require measured parameters which are given for common materials. Comparisons of model calculations of the refractive index, the absorption coefficient, and scattering coefficient to experimental data are presented.

EO/IR windows are a significant challenge for the weapon system sensor designer who must design for high EO performance, low radar cross section (RCS), supersonic flight, durability, producibility and affordable initial and life cycle costs. This is particularly true in the 8 to 12 micron IR band at which window materials and coating choices are limited by system design requirements. The requirements also drive the optimization of numerous mechanical, optical, materials, and electrical parameters. This paper addresses the EO/IR window as a system design challenge. The interrelationship of the optical, mechanical, and system design processes are examined. This paper presents a summary of the test results, trade studies and analyses that were performed for multi-segment, flight-worthy optical windows with superior optical performance at subsonic and supersonic aircraft velocities and reduced radar cross section. The impact of the window assembly on EO system modulation transfer function (MTF) and sensitivity will be discussed. The use of conductive coatings for shielding/signature control will be discussed.

It is the purpose of this paper to demonstrate that the concept of a 'universal' figure of merit for thermal shock has no merit since the ability of an IR dome to survive transient thermal stresses depends not only on intrinsic material properties but also on the thermal environment as characterized by the Biot number (Bi). For this reason, the thickness of the dome plays an essential role because it may have an impact on the heat-flow regime (thermally thick or thermally thin) and, therefore, on peak thermal stresses. Furthermore, in a thermally thin regime (Bi < 1), the resistance to thermal shock will be enhanced by making the dome as thin as possible, that is, as determined by structural requirements, which are not reflected in the derivation of the Hasselman parameters. The procedure outlined in this paper provides a direct measure of the thermal shock resistance (TSR) in the sense that it yields the 'ultimate' thermal shock temperature, i.e., the allowable recovery temperature rise above the wall temperature at the onset of the shock.

An experimental investigation was undertaken to examine the aero-optic performance of multi- aperture windows for use in hypersonic endo-atmospherical vehicles. A series of imaging tests was conducted through a two-dimensional flat plate model of a multi-aperture window that was incorporated into the Teledyne Brown Dual Nozzle Aero-Optic Simulator (DNAOS). This simulator brought two high-velocity gas streams together in an enclosed test region to form an approximate Mach 2 mixing/shear layer, creating the turbulent properties found in hypersonic flight. The same series of tests was conducted looking through a monolithic flat plate window. The images recorded through both window schemes were analyzed to determine image distortion and results were compared to demonstrate the various optical phenomenon associated with multi-aperture windows.