Sapphire optical materials have limited index of refraction homogeneity. This homogeneity can limit the degree of transmitted wavefront error achievable with current, conventional optical finishing practices. Current practices can not typically compensate well for the localized inhomogeneities in the sapphire substrates resulting in limited transmitted wavefront values. Emerging transmitted wavefront requirements exceed those achievable with current practices. Hughes Danbury Optical Systems recently completed a successful demonstration program in which computer controlled polishing was applied to the fabrication of very low transmitted wavefront error sapphire window. This technique involves measuring the windows in transmission and then polishing them in localized areas to remove the wavefront errors arising from the material index inhomogeneity. The net effect of each localized correction is a high fidelity transmitted wavefront over each subaperture. In the demonstration completed, we stated with windows fabricated to the limit of current, conventional practices. Applying computer controlled polishing, the transmitted wavefront quality was rapidly improved by a factor of up to five over the starting value. These results not only satisfied emerging requirements, but the process also resulted in satisfying parallel requirements of extreme surface smoothness and scatter as defined by the bi- directional transmittance distribution function. This paper addresses the process developed, its results, benefits and applications.
The new military requirements that call for the use of eyesafe lasers for range finding capabilities on aircraft, ships, tanks, and other vehicles has put an extra burden on the transmission of conductive coatings. Originally, the window coatings have low resistivity, less than 7.0 ohm/square, for EMI control were required to transmit up to 1.06 micron. With the new eyesafe lasers for range finding, the requirements of near infrared transmission have been pushed toward longer wavelengths. The windows are now required to have good transmission at 1.543 microns (eyesafe laser wavelength). The traditional indium-tin oxide coating has poor transmission varying from 20% to 50% due to free carrier absorption. A new conductive coating developed at HDOS has good transmission in photopic and xenon tracker regions and it has good transmission at 1.543 micron. This coating also has good environmental durability. The spectral performance, environmental durability, rain erosion resistance, acid resistance, and sand abrasion resistance of this coating are also significant.
Thorium fluoride ThF4 has been used over the last several decades as a low index coating material for many optical applications such as in anti-reflection (AR) coatings, high reflective coating, beam splitters, coating dichroics and optical filters. ThF4 has excellent optical and mechanical properties but unfortunately is slightly radioactive. New guidelines from many government agencies require that ThF4 shall not be used as coating material for safety reasons. Also new stringent requirements on the disposal of radioactive material in many states within the USA have made it economically difficult to use. This paper therefore deals with AR coatings which do not have radioactive materials in them. Three types of coatings based on applications have been discussed in this paper. These are namely (1) AR coatings for long wavelength infrared region (LWIR), (2) AR coatings for 1.06 micrometer and long wavelength infrared regions (LWIR), and (3) AR coatings for multiple wavelength regions namely VIS/NIR/MWIR (medium wavelength infrared region) and VIS/NIR/MWIR and LWIR. The spectral performance, mechanical and environmental durability, and rain-erosion test data is presented in this paper.
Antireflection coated ZnS and Ge substrates erode under severe operational environmental conditions. High velocity water drop impact and high velocity sand particle impact are primarily military concerns that originated with the advent of faster aircraft. High speed flight through rain and sand storms seriously erodes forward facing components such as infrared transmitting windows and/or domes. This erosion of windows and/or domes causes reduction in transmission, resulting in the reduction of detection and recognition sensitivity of the electro-optical sensor. A single film of one quarterwave thick hard-carbon coating has been used on germanium to increase optical transmission (reducing Fresnel's reflection losses on Ge surface) as well as to reduce rain and sand impact damage to some extent, at a lower speed. At high speed, the damage becomes more severe, resulting in unacceptable large transmission losses. Recently, new hard carbon coatings have been developed for Ge which have substantially increased the damage threshold of the coated substrates. The rain erosion test was performed at Wright-Patterson AFB facility in Dayton, Ohio, and the sand tests were performed at PDA Engineering in Santa Ana, California. In addition, a multilayer AR coating utilizing hard carbon film as one of the low index films has also been developed at Hughes for ZnS substrates. The optical properties, rain erosion, sand erosion, and sand abrasion test results of these coatings are also presented in this paper.
Antireflection coated ZnS and Ge substrates erode under severe operational environmental conditions. High velocity water drop impact and high velocity sand particle impact are primarily military concerns that originated with the advent of faster aircraft. High speed flight through rain and sand storms seriously erodes forward facing components such as infrared transmitting windows and/or domes. This erosion of windows and/or domes causes reduction in transmission, resulting in the reduction of detection and recognition sensitivity of the electro-optical sensor. A single film of one quarterwave thick hard-carbon coating has been used on germanium to increase optical transmission (reducing Fresnel's reflection losses on Ge surface) as well as to reduce rain and sand impact damage to some extent, at a lower speed. At high speed, the damage becomes more severe, resulting in unacceptable large transmission losses. Recently, new hard carbon coatings have been developed for Ge which have substantially increased the damage threshold of the coated substrates. The rain erosion test was performed at Wright-Patterson AFB facility in Dayton, Ohio, and the sand tests were performed at PDA Engineering in Santa Ana, California. In addition, a multilayer AR coating utilizing hard carbon film as one of the low index films has also been developed at Hughes for ZnS substrates. The optical properties, rain erosion, sand erosion, and sand abrasion test result of these coatings are also presented in this paper.
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.
Infrared coatings are required on both surfaces of infrared domes to increase transmission and enhance system
performance. These domes are integral parts of either a Missile System or an electro-optical sensor attached to military
vehicles. The convex surface ofthe dome is exposed to the harsh environment ofthe outside world, and therefore requires
a coating which not only increases transmission but is able to withstand humidity, fluids, exhaust, sand and rain
environments. This paper deals with the techniques that were developed to adapt a rain erosion resistant coating for ZnS
Optical Flats Windows to curved surfaces. The fabrication process testing and results are described in this paper.
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