Abstract:
The present invention provides a protective device for sensitive infrared sensors as Forward Looking Infrared imagers (FLIRs). A prior device using materials with higher order susceptibilities to electric polarization, which provides protection against extremely intense radiation from high-power lasers is combined with a low energy optical power limiters such as the chalcogenide glass and the vanadium dioxide which respond reversibly to infrared radiation.

Description:
The invention described herein may be manufactured, used, and licensed by the U.S. Government for governmental purposes without the payment of any royalties thereon. 
   BACKGROUND OF INVENTION 
   1. Field 
   This invention pertains to optical radiation limiters used as protective devices for imaging devices such as the FLIR (Forward Looking Infrared Imager). 
   2. Prior Art 
   Far infrared imagers must be highly sensitive in order to operate with the small temperature differentials that define many targets. Because of their sensitivity and the material used for such detector elements, e.g. mercury cadmium telluride, they are highly vulnerable to damage from low energy threat laser radiation. In an earlier patent application entitled, “Frustrated Total Internal Reflection Optical Power Limiter (FTIR)”, Ser. No. 648,140, a high energy protective device, using, e.g. nematic liquid crystals, is described, which limits the input radiation, if it exceeds a certain level, thus protecting the detector elements. 
   The detector material is damaged at an input fluence of between 2 and 7 J/cm 2  a level easily achieved by commercially available CO 2  lasers. The FTIR device has not yet been fabricated that can fully protect the detector materials in a system configuration. There are chalcogenide and vanadium oxide materials, however, that can be triggered at lower range fluence levels such that they can protect the detector material. When used alone, however, protective devices made from these materials suffer from too low a dynamic range. 
   An object of this invention is to extend the protection range of a device in the earlier mentioned application above, in order to close the vulnerability window, by adding a different class of light sensitive materials, similar to those mentioned above, to lower the switching threshold of the FTIR while maintaining the upper limit of the dynamic range. 
   SUMMARY OF THE INVENTION 
   The invention involves the combination of nonlinear optical limiter elements, FTIR, with a more light sensitive optical power limiter such as a chalcogenide glass device or a vanadium oxide device. At very low input levels, the device transmits the radiation with little attenuation. At somewhat higher levels, the more light sensitive device is activated, attenuating the input radiation by absorption in a chalcogenide device or by reflection in a vanadium oxide device thus protecting the detector elements at such levels. At very high levels, the FTIR is activated and the input radiation is completely reflected, thus protecting the chalcogenide or vanadium oxide switch and the detector elements at those levels. By combining the two devices in series, the resultant device has a low switching threshold with a high dynamic range, the properties of each device complimenting one another. 

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a detailed view of a first embodiment of applicant&#39;s novel protective device. This device utilizes a high power limiter such as one disclosed in an earlier application entitled Frustrated Total Internal Reflection Power Limiter, Ser. No. 648,140 filed 5 Sep. 1984. As disclosed in that application, the limiter must be located at an intermediate focal point, which can be created by adding beam folding mirrors to a first generation FLIR, but which is an integral feature of second generation FLIRS. A similar device is disclosed in Ser. No. 268,461 “Optical Power Limiter Utilizing Nonlinear Refraction”, filed 1 Nov. 1989 by Gary L. Wood, et al. The operation of the components,  13 ,  14 ,  15 , and  17  are essentially described in the earlier patent applications. For example, a low/intensity ray  10 , representing ambient IR levels at which the FLIR detector is designed to operate is transmitted undeviated and substantially unattenuated through the protective limiter device. The former limiters were composed of two solid elements  13  and  15  between which is a space or a layer of liquid  14  sealed by a sheath  14 A. An anti-reflective coating  17  is placed on the output face and usually will be also placed on the input face, as well. All of these elements are transparent to IR, but some exhibit optical transmission changes under very intense IR radiation, such that a ray  11  of such radiation is redirected to an absorbing dump  18 . Dump  18  may be a bulk absorbing material mounted on the housing of the FLIR. The present invention improves this device by adding a low power limiter element  16  to the output face before applying the anti-reflective coating. This limiter element consists of layers  16 A,  16 B and  16 C,  16 B being the active chalcogenide layer. Sheath  14 A is extended to cover the edges of these added layers. Layers  16 A and  16 C are merely windows transparent to far-infrared which, with sheath  14 A or some equivalent, encapsulate the toxic chalcogenide. If the input intensity is increased to a much higher level, represented by ray  11 , it is reflected by the FTIR device because of the nonlinearity of components  13 ,  14 , and  15  and is absorbed by the dump device  18 . 
   As the input intensity is increased above the level represented by ray  10  but held below the level for ray  11 , there is a window of vulnerability for the detector damage. Namely, the input intensity is high enough to damage the detector material, but low enough not to trigger the FTIR. The device  16  is triggered at a characteristic threshold below the level of the detector damage. The exact level of this threshold is not critical and many chalcogenide materials with suitable thresholds are now available. The device absorbs almost all the input radiation, other than the small portion of the radiation which passes through the device before the onset of the energy limiting. It will continue to function, as long as the level of the input radiation remains higher than threshold or is increased, until the FTIR is triggered. 
     FIG. 2  shows a somewhat simpler device for the same purpose. Again, a low power limiter  20  is mounted on the output face before applying the anti-reflective coating  17 . This limiter has only two layers  20 A and  20 B,  20 B being the active layer formed from vanadium oxide. The characteristic threshold may be varied by varying the metal to oxide ratio, pure vanadium dioxide being a suitable choice for current FLIRS. Layer  20 A is germanium or other material transparent to far-infrared having sufficient strength to support the vanadium oxide. The anti-reflection layer  17  may be deposited on the vanadium oxide, as before. When layer  20 B is formed from vanadium oxide or vanadium dioxide, instead of the chalcogenide, there is provided another type of low energy switching device. In this device, the input radiation is reflected by the vanadium oxide above the characteristic threshold. In other words, the device changes from a transmissive device into a reflective device, as the level of input radiation is increased, above ambient levels. It reflects most of the input radiation, except for the very small portion of radiation that passes through the device before the onset of the energy reflection. The reflected radiation from layer  20 B also facilitiates the switching of elements  13 ,  14 , and  15 . The characteristic threshold can be normally too high and may be lowered, if desired, by heating either the chalcogenide or the vanadium dioxide by means of an electrical heating element in, on or around the window  16 A or  20 A, respectively. This would allow the FLIR to operate to its fullest potential. A thin resistive structure or coating  19  between the window and the protection layer  16 B or  20 B and, transparent to IR is preferable as the heating element. 
   The detector element is protected by the chalcogenide or vanadium dioxide device, which in turn is protected by the FTIR device. In this configuration, the window of vulnerability of the FTIR device is closed by the addition of the low energy switching device, such as the chalcogenide device and the vanadium dioxide device. 
   The protective elements may be bonded together and mounted in the imaging module of FLIRs using ordinary mounting brackets, materials and techniques well known in the optical art.