DURABLE VISIBLE/LASER/MEDIUM WAVE INFRARED COMPOSITE WINDOW

Abstract: A durable window (16) which is scaleable in size is provided for transmitting 0.4 to 0.9 micrometer visible and near infrared wavelengths, 1.06 and 1.54 micrometer laser and 3 to 5 micrometer medium wave infrared wavelengths. One embodiment of the present invention comprises a 5 to 20 micrometer thick hard sapphire coating (34) on a multispectral zinc sulfide substrate (32). This window provides a higher transmission and therefore higher acquisition range for infrared imaging systems, while still providing the same exterior durability as bulk sapphire. Since there is less absorption, there is also less emission from the window of the present invention in comparison with a bulk sapphire window. Accordingly, background noise is reduced. Sapphire-coated multispectral zinc sulfide windows are also considerably less expensive than the expensive bulk sapphire substrate alternative. Furthermore, multispectral zinc sulfide substrates are available in large sizes up to 30 inches (0.762 m) in diameter. The application of a durable sapphire coating allows a straightforward scalability which can be used to produce the large size windows (i.e., 10 to 20 inches in dimension) necessary for multisegmented "greenhouse"-type window assemblies integrated into high-speed aircraft airframes.

Description: DURABLE VISLBLE/LASERyMEDIUM WAVE INFRARED COMPOSITE WINDOW BACKGROUND OF THE INVENTION 1. Field ofthe Invention The present invention relates generally to durable windows required in multis¬ pectral imaging systems for high-speed aircraft. More particularly, the present inven¬ tion is directed to durable windows transmitting the visible and near infrared band- passes ranging from 0.4 to 0.9 micrometers in wavelength, the 1.06 and 1.54 microme¬ ter wavelength laser bandpasses and the medium wave infrared (MWIR) bandpass ranging from 3 to 5 micrometers in wavelength. 2. Description of Related Art Integration of infrared imaging systems in fixed wing aircraft and other high¬ speed aircraft requires a highly durable infrared window which can survive rain ero¬ sion. This highly durable infrared window must also be able to provide high transmis¬ sion of the 1.06 and 1.54 micrometer wavelength laser bandpasses and the medium wave infrared 3 to 5 micrometer wavelength bandpass. In some systems it may also be desirable to provide high transmission in the visible and near infrared bandpasses rang¬ ing from 0.4 to 0.9 micrometers in wavelength. Furthermore, large size windows with 10 to 20 inch dimensions are necessary for multisegmented "greenhouse"-type window assemblies integrated into high-speed airframes. There is currently no available window resistant to rain erosion which transmits the 1.06 and 1.54 micrometer wavelength laser and medium wave infrared bandpasses and that is available in sizes larger than 7 inch x 7 inch. Available window technologies for transmitting the aforementioned laser and medium wave infrared bandpasses typically comprise sapphire or multispectral zinc sul¬ fide. Bulk sapphire meets rain erosion requirements for high-speed aircraft and has good transmission in the visible and near infrared ranging from 0.4 to 0.9 micrometers in wavelength and the 1.06 and 1.54 micrometer laser wavelengths. However, sapphire has poor transmission at the long end of the 3 to 5 micrometer wavelength bandpass due to fundamental lattice vibration absoφtion. Furthermore, absoφtion in this band¬ pass is manifested as emission from the warm window, increasing system background noise. Sapphire is also not available in sizes larger than about 7 inches x 7 inches. Multispectral zinc sulfide, on the other hand, has good transmission at the long end of the 3 to 5 micrometer wavelength bandpass, as well as in the 1.06 and 1.54 mi¬ crometer laser wavelengths and is readily available in large sizes with over 20 inch di¬ mensions. However, this material also has extremely poor resistance to rain erosion and does not meet rain erosion requirements for high-speed aircraft. Thus, there remains a need for a window which transmits the 1.06 and 1.54 mi¬ crometer laser and MWIR bandpasses and, in some cases, the visible and near infrared ranging from 0.4 to 0.9 micrometers in wavelength, which is resistant to rain erosion, and is available in sizes larger than 7 inches x 7 inches. SUMMARY OF THE INVENOON In accordance with the invention, a window for transmitting infrared radiation is provided. The infrared-transmitting window comprises a sapphire layer formed on a sub- strate comprising multispectral zinc sulfide. As an example, the thickness of the sapphire layer is in the range of about 5 to 20 micrometers. The window ofthe present invention is durable and can transmit the visible and near infrared ranging from 0.4 to 0.9 micrometers in wavelength, the 1.06 and 1.54 micrometer laser wavelengths and the 3 to 5 micrometer medium wave infrared wave- lengths. The window of the present invention provides a higher transmission and therefore higher acquisition range for infrared imaging systems, while still providing the same exterior durability as bulk sapphire. Since there is less absoφtion, there is also less emission from the window of the present invention in comparison with a bulk sapphire window. Accordingly, background noise is reduced. The window of the pres¬ ent is also lower in cost than the expensive bulk sapphire counteφart. The present in¬ vention provides a readily scaleable window necessary for the large size windows (i.e., 10 to 20 inches) required for multisegmented "greenhouse"-type window assemblies integrated into high-speed aircraft airframes. There is no currently available window or window combination for this application. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view, depicting an airframe of a high speed aircraft equipped with a multispectral infrared imaging system; FIG. 2, on coordinates of transmission and wavelength, is a plot ofthe transmission of bare multispectral zinc sulfide prior and subsequent to the 470 mph (miles per hour), 20 minute whirling arm rain erosion test; FIG. 3, on coordinates of transmission and wavelength, is a plot ofthe transmission of various thicknesses of sapphire; and FIG. 4 is a cross-sectional view ofthe composite sapphire-coated multispectral zinc sulfide window ofthe present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, wherein like reference numerals designate like ele¬ ments throughout, an airframe 10 of a high speed aircraft equipped with a multispectral infrared imaging system is depicted. The multispectral infrared imaging system is housed in a sensor ball 12 shown in FIG. 1. The sensor ball 12 resides within a mul¬ tisegmented "greenhouse"-type window assembly 14 comprising an array of flat windows 16 oriented around the sensor ball for the protection thereof The "greenhouse"-type win¬ dow assembly 14 is so named due to its resemblance and similarity in construction to a greenhouse which also comprises an array of flat windows. As mentioned above, a multispectral infrared imaging system on a high-speed aircraft requires windows (or a window) 16 that transmits selected visible and infrared wavelengths and that is also resistant to rain erosion In the current state of the aπ, the window 16 for transmitting the visible and near infrared ranging from 0 4 to 0 9 micrometers, and the 1.06 and 1.54 micrometer wavelength laser and 3 to 5 micrometer medium wave bandpasses comprises either multispectrai zinc sulfide or sapphire Multispectral zinc sulfide, as mentioned above, however, has extremely poor resistance to rain erosion characterized by an increase in transmission loss with rain exposure The transmission curves for multispectral zinc sulfide are shown in FIG. 2 with respect to rain erosion. Resistance to rain erosion is tested using a standard rain erosion test for fixed wing aircraft, which comprises exposing the sample to a 470 mph (miles per hour), 90° angle of incidence, 20 minute whirling arm in rain. Curve 18 and Curve 20 show the transmission of multispectral zinc sulfide in the 400 to 2400 nanometer wavelength range prior and subsequent to the standard rain erosion test, re¬ spectively. Curve 22 and Curve 24 show the transmission of multispectral zinc sulfide in the 2 to 14 micrometer range prior and subsequent to the standard rain erosion test, respectively. Substantial degradation in optical performance of the multispectral zinc sulfide is seen to occur with rain erosion. Bulk sapphire passes the standard rain erosion test with no degradation. As mentioned above, however, sapphire has poor transmission at the long end of the 3 to 5 micrometer wavelength bandpass. Transmission curves for various thicknesses of sapphire are shown in FIG. 3. Curve 26, Curve 28, and Curve 30 show the transmis¬ sion for 0.040 inch thick sapphire, 0.125 inch thick sapphire, and 0.247 inch thick sap- phire, respectively. For conventional window thicknesses of 0.5 inch, the medium wave infrared loss would be even higher than that shown for Curve 30, which depicts transmission for 0.247 inch thick sapphire. Also, as mentioned above, the absoφtion in this bandpass is manifested as emission from the window 16, which is warmed by the absorbed energy. This emission increases system background noise. In accordance with the present invention, a window 16 which is durable and multispectral, i.e., transmitting 0.4 to 0.9, 1.06, 1.54, and 3 to 5 micrometer wave¬ lengths, is provided The infrared-transmitting window 16 of the invention is shown in FIG. 4. The window 16 of the present invention comprises a multispectral zinc sulfide substrate 32 coated with a sapphire layer 34 about 5 to 20 micrometers thick. The sap¬ phire layer 34 is formed on the multispectral zinc sulfide substrate 32 via conventional techniques which are well-known and thus form no part of this present invention. These conventional techniques include techniques for depositing sapphire such as sputtering, ion-assisted deposition, etc. In another embodiment of the present invention, at least one anti-reflection coating is deposited on the multispectral zinc sulfide substrate 32 prior to the forma¬ tion of the sapphire layer 34. The anti-reflection coating reduces the reflection loss at the multispectral zinc sulfide/sapphire interface. The multispectral zinc sulfide substrate 32 provides a substrate with essentially no absoφtion in the visible and near infrared 0.4 to 0.9 micrometer wavelength range and the 1.06 and 1.54 micrometer wavelength laser and medium wave infrared, i.e., 3 to 5 micrometers wavelength, bandpasses. The multispectral zinc sulfide substrate 32 is also available in large sizes, currently as large as 30 inches. The sapphire layer 34 provides a durable layer for the exterior of the window 16 which enables the window to survive high-speed rain erosion. The sapphire layer 34 also only absorbs a slight amount of MWIR, i.e., 3 to 5 micrometer wavelength, en¬ ergy, especially compared to a bulk 0.5 inch thick piece of sapphire; see FIG. 3 The absoφtion for 0.5 inch thick sapphire can be extrapolated from FIG. 3. This low ab¬ soφtion is also manifested as low emission, thus reducing background noise. Since multispectral zinc sulfide has no absoφtion in the 3 to 5 micrometer bandpass, the only absoφtion would come from the thin sapphire layer 34 comprising a 5 to 20 microme¬ ter thick sapphire. As determined from FIG. 3, this absoφtion is insignificant co - pared with that for a bulk substrate. Additionally, the multispectral zinc sulfide windows are available in sizes up to about 30 inch in diameter and deposition ofthe sapphire layer 34 is readily scaleable to these larger sizes. The application of a durable sapphire coating allows a straightfor¬ ward scalability which can produce the large size, i.e., 10 to 20 inch dimension, win- dows 16 necessary for multisegmented "greenhouse"-type window assemblies 14 inte¬ grated into high-speed aircraft airframes. Thus, a survivable window can be provided for transmitting the 1.06 and 1.54 micrometer wavelength laser and 3 to 5 micrometer medium wave infrared bandpasses for applications such as the large multisegmented windows integrated into high-speed aircraft airframes as well as providing as an op¬ tion, transmission in the visible and near infrared range from 0 4 to 0 9 micrometers in wavelength There is no currently available window 16 for this application The window 16 ofthe present invention provides the following advantages ( 1 ) It is durable and will survive ram erosion on high-speed aircraft (2) It transmits higher amounts of infrared energy at the medium wave infrared bandpasses than currently available sapphire windows (3) The increased transmission through the window 16 is manifested as m- creased recognition range, i e , higher acquisition range for infrared imaging systems (4) Since there is less absoφtion with the window 16 of the present invention, there is also less emission from the hot window than for a bulk sapphire window, thus reducing background noise (5) The window 16 of the present invention does not possess the birefringence effects common to bulk sapphire windows (6) The multispectral zinc sulfide/sapphire coating combination of the present invention is less expensive than a bulk sapphire window (These advantages listed above apply to the window of the present invention whether the window is smaller or larger than 7 inches x 7 inches ) (7) Additionally, the present invention provides a window 16 which is readily scaleable and thus necessary for the large size, l e , 10 to 20 inch dimension, windows required for multisegmented "greenhouse" -type window assemblies 14 integrated into high-speed aircraft airframes Further, the upper limit of window size is only dependent on (a) the size of multispectral zinc sulfide substrates available and (b) the deposition technology employed to deposit sapphire Thus, there has been disclosed an infrared transmitting window which is resistant to rain erosion for use at 1 06 and 1 54 micrometer laser and medium wave infrared wave¬ lengths It will be readily apparent to those skilled in this aπ that vaπous changes and modifications of an obvious nature may be made, and all such changes and modifications are considered to fall within the scope ofthe invention, as defined by the appended claims

Claims: CLAIMS What Is Claimed Is: 1. A window for transmitting visible and infrared radiation, comprising a sapphire layer formed over a multispectral zinc sulfide substrate. 2. The window of Claim 1 wherein said visible and infrared radiation includes wavelengths selected from the group consisting of 0.4 to 0.9 micrometers, 1.06 microme¬ ter, 1.54 micrometer, and 3 to 5 micrometers. 3. The window of Claim 1 wherein said sapphire layer has a thickness of 5 to 20 micrometers. 4. The window of Claim 1 wherein said multispectral zinc sulfide substrate is larger than 7 inches wide and 7 inches long. 5. The window of Claim 4 wherein said window is integrated into a greenhouse- type window assembly. 6. The window of Claim 1 wherein at least one anti-reflection coating is formed on said multispectral zinc sulfide substrate, said sapphire layer being formed on said anti- reflection coating. 7. An airframe including at least one window for transmitting visible and infrared radiation, comprising a sapphire layer formed over a multispectral zinc sulfide substrate. 8. The airframe of Claim 7 wherein said visible and infrared radiation includes wavelengths selected from the group consisting of 0.4 to 0.9 micrometers, 1.06 microme¬ ter, 1.54 micrometer, and 3 to 5 micrometers. 9. The airframe of Claim 7 wherein said sapphire layer has a thickness of 5 to 20 micrometers. 10. The airframe of Claim 7 wherein said window for transmitting visible and infra- red radiation is larger than 7 inches wide and 7 inches long. 11. The airframe of Claim 10 wherein said window for transmitting visible and in¬ frared radiation is integrated into a greenhouse-type window assembly. 12. The airframe of Claim 7 wherein said window for transmitting visible and infra¬ red radiation comprises at least one anti-reflection coating formed on said mukispectral zinc sulfide substrate, said sapphire layer being formed on said anti-reflection coating. 13. A method of making a durable window for transmitting visible and infrared ra- diation, comprising (a) providing a multispectral zinc sulfide substrate; and (b) forming a sapphire layer over said multispectral zinc sulfide substrate. 14. The method of Claim 13 wherein said visible and infrared radiation includes wavelengths selected from the group consisting of 0.4 to 0.9 micrometers, 1.06 microme¬ ter, 1.54 micrometer, and 3 to 5 micrometers. 15. The method of Claim 13 wherein said sapphire layer has a thickness of 5 to 20 micrometers. 16. The method of Claim 13 wherein at least one anti-reflection coating is formed on said multispectral zinc sulfide substrate prior to forming said sapphire layer over said multispectral zinc sulfide substrate. 17. The method of Claim 13 wherein said sapphire layer is formed by deposition techniques selected from the group consisting of sputtering and ion-assisted deposition.

Inventors: BAKER GEORGE F, BORDEN MICHAEL R, BAKER, GEORGE, F., BORDEN, MICHAEL, R.
Applicants: HUGHES AIRCRAFT CO, HE HOLDINGS, INC. DOING BUSINESS AS HUGHES ELECTRO
Application year: 1996
Main classification: G02B1
IPC codes: B32B7, B64C1, B64D47, C23C14, G02B1