Abstract:
A plasma absorption wave limiter is disclosed. The plasma absorption wave limiter comprises a limiting layer and a trigger layer. The limiting layer is transmissive in a pass band of a sensor and capable of generating a reflective and absorptive free electron plasma that will propagate and dissipate therein. The trigger layer is located aft of and in contact with the limiting layer and is capable of residually absorbing incident radiation and initiating the thermal plasma wave in the limiting layer responsive to a threat.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention pertains to optical sensors, and, more particularly, to protection of optical detectors in an optical sensor from damage by radiation in the pass band and field of view of the sensor&#39;s detector. 
   2. Description of the Related Art 
   Optical sensors are designed to receive and monitor relatively weak optical signals, whether those optical signals are natural or man-made. Thus the sensor&#39;s detectors are very sensitive and are therefore vulnerable to damage by high-level radiation, particularly if the radiation source is in the field of view and in the pass band of the sensor&#39;s focusing optics. For some applications, the optical detectors in a sensor must be protected from optical signals that are sufficiently strong to damage the detector. The most extreme example is found in military applications. Many military systems employ optical sensors for a variety of tasks. Enemy forces frequently employ counter-measures to incapacitate or damage the sensor with strong optical signals specifically designed to damage sensor(s). For instance, an enemy might illuminate an infrared imager with a high intensity laser capable of damaging the optical detector(s) in the imager. Sensors have been protected from in-band, in-view threats to some extent by mechanical shutters, reflective (notch filter) coatings, notch absorption materials, non-linear distortion and dispersion in a fluid cell, thermochromic elements, two-photon absorption materials and other techniques. 
   In the IR wavelengths, a thermoreflectance or thermochromic non-linear material (“NLM”) like Vanadium Dioxide (“VO 2 ”) can be used to modulate radiation almost 100%. This concept has been extended to optical protection and limiting by subsequent research. For example, one protection approach coats the front surface of transmissive optical elements with VO 2 . In this approach, one of two NLM coated element is placed near a focal surface—typically a plane—through which the optical energy passes on its way to a sensor&#39;s detector(s). Below the “switching threshold,” the thermochromic NLM is transmissive to optical energy in the pass band of the sensor, that is, it transmits the “normal” optical energy incident upon it. However, above this threshold of irradiance, the NLM becomes reflective; i.e., it is opaque to the potentially damaging optical irradiance. 
   In the case of VO 2 , this optical effect is due to a change in the crystal structure and optical characteristics of the material that occurs when the thin film is above a critical temperature. Since temperature is a function of, among other things, the intensity with which the incident energy impinges on the NLM, the coating acts to limit incident radiation transmitted to the sensor detector(s). This intensity is called the “switching intensity”; i.e., the intensity which produces the temperature at which the thermochromic NLM switches from high to low transmission of the incident energy. 
   In operation, the thermochromic NLM remains transmissive for the optical energy impinging upon it that is within the desired bandwidth and intensity for the optical elements associated therewith. The optical elements behind the NLM and the substrate are thereby able to receive the incident optical energy. When optical energy of dangerous intensity (e.g., a high-powered laser threat) is encountered, the NLM heats up and switches to its reflective state, whereupon the high intensity optical energy is primarily reflected. When the dangerous intensity ceases, the NLM cools down and returns to its transparent, transmissive state. Thus, by reflecting dangerous intensities of optical energy, the NLM protects downstream optical elements (e.g., sensitive detectors) from damage. 
   Such thermochromic NLM coatings are however also subject to damage from sufficiently intense radiation. If the incident energy is sufficiently intense and of sufficient duration, the energy can melt, vaporize, or delaminate the NLM from its substrate. This degree of intensity is called the “damage threshold.” Thus, a NLM protected system whose optical detector(s) remain unharmed by the damaging intensity can still be degraded. To address this issue, a second NLM switch may then placed forward of the first to protect the first element from damage (although this results in some degradation of the sensitivity of the sensor). 
   One performance characteristic used to assess an optical protection apparatus is its “dynamic range.” The dynamic range is the ratio of its switching threshold to its damage threshold. Ideally, the damage intensity should be very large relative to the switching intensity, and so a large dynamic range is desirable. The desire to improve dynamic range for these materials continues to spur efforts at improving the design of reflective limiters employing thermochromic NLMs. 
   The present invention is directed to resolving, or at least reducing, one or all of the problems mentioned above. 
   SUMMARY OF THE INVENTION 
   The invention is a plasma absorption wave limiter. The plasma absorption wave limiter comprises a limiting layer and a trigger layer. The limiting layer is transmissive in a pass band of a sensor and capable of generating a reflective and absorptive free electron plasma that will propagate and dissipate therein. The trigger layer is located aft of and in contact with the limiting layer and is capable of residually absorbing incident radiation and initiating the thermal plasma wave in the limiting layer responsive to a threat. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
       FIG. 1  is a conceptualized cross-sectional view of a plasma absorption wave limiter in accordance with the present invention; 
       FIG. 2  illustrates a portion of the plasma absorption wave limiter in  FIG. 1  in greater detail; 
       FIG. 3-FIG .  5  illustrate in conceptualized cross-sectional views of alternative embodiments of the plasma absorption wave limiter of  FIG. 1 ; 
       FIG. 6  illustrates in a conceptualized cross-sectional view one particular implementation of the embodiment of  FIG. 2 ; and 
       FIG. 7  illustrates an exemplary use for the present invention in partial cross-section, in which an optical assembly employs a plasma absorption wave limiter; and 
       FIG. 8  illustrates another embodiment of the present invention. 
   

   While the invention is susceptible to various modifications and alternative forms, the drawings illustrate specific embodiments herein described in detail by way of example. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort, even if complex and time-consuming, would be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
   In the course of studying thermochromic NLM reflective limiters described above, it was discovered that placing the thermochromic NLM behind a substrate with particular characteristics of conduction band gap energy and melting temperature would provoke a different physical mechanism resulting in a new class of limiters—the plasma absorption wave limiter (“PAWL”). In general, the incident energy passes through the substrate to a trigger layer (e.g. the thermochromic NLM). If sufficiently intense, the energy will heat the trigger layer. The heat energy then conducts from the trigger layer into the substrate. The substrate material is chosen to have a low energy band gap between its bound state electrons and the conduction band (free) electrons. However, the substrate must have a high enough band gap to allow the electrons in the substrate to be bound at normal use conditions so that the material is transmissive in the desired optical pass band of the sensor. 
   When heat conducts into the substrate, its electron population density in the conduction band will increase rapidly creating a free electron gas or “plasma.” Thermally induced conduction band electrons are free to reflect incident radiation just as the conduction band electrons do in a metal. These electrons also absorb incident radiation slightly, further heating the substrate material. The substrate material is chosen to have a high enough melting point that it is not damaged by this initial heating. As the absorbed energy heats the substrate, heat conducts from the plasma region to adjacent transparent dielectric region that is closer to the threat and begins to create plasma in front of the initial plasma region. Thus the plasma absorption region grows and propagates toward the impinging radiation like a wave from the trigger layer. 
   Since the trigger layer of the PAWL element is preferably located near a focal plane, as the absorption wave propagates toward the source, the free electron population moves into increasingly lower-intensity, less-focused position in the incident radiation pattern. This movement continues until the wave reaches the forward surface of the element or the energy absorbed from the incident radiation is balanced by the conductive heat losses from the plasma into the cooler substrate and related mounting materials. At that point the sensor is protected and the energy is distributed over sufficient material to prevent melting, vaporization or other forms of damage. Thus, the detectors are protected by the trigger layer while the substrate provides the limiting function to protect the trigger layer and thereby increase the dynamic range of the limiter. The PAWL mechanism thus reflects and absorbs the incident radiation before the trigger layer is permanently damaged. Subsequent study has shown that the “triggering” heat provided by the thermochromic NLM can also be provided in alternative ways. 
   Consider the PAWL  100  shown in  FIG. 1 .  FIG. 1  is a conceptualized cross-sectional view of a PAWL  100  in accordance with the present invention. The PAWL  100  comprises a trigger layer  103 —e.g., a thermochromic NLM layer—and a limiting layer  106 —e.g., a semiconducting substrate. If the incident energy  109  is sufficiently intense, the trigger layer  103  becomes opaque to protect the detector(s) (not shown) of the associated sensor (also not shown) then continues to absorb slightly and heat the limiting layer  106 . This heat generates a plasma wave  112  of free electrons in the limiting layer  106 . As heat is conducted away from the plasma and toward the source of radiation, the plasma wave  112  will propagate into the limiting layer  106 , which will dissipate the absorbed energy from the incident radiation  109  through conduction. Damaging levels of optical energy will thus be reflected and absorbed to prevent damage to the trigger layer that has already protected the sensitive elements of the sensor system (e.g., its detector array). 
   The absorption of the plasma wave  112  can be tailored to applications by adjusting the band gap and thermal characteristics of the limiting layer  106 . These characteristics can be adjusted by choice of materials, doping (either the bulk material or a thin layer), thermal biasing, and alloys, for example. In general, design tailoring for specific implementations will include considerations such as threat characteristics, ambient operating temperatures, desired reaction time, and sensor performance/design characteristics. Note also that the front surface  127  of the PAWL  100  may be curved to adjust refraction in some embodiments. 
   More technically, the PAWL  100  is an optical element placed at or near a focal surface  115  in a sensor not otherwise shown. In the illustrated embodiment, the focal surface  115  is a focal plane. However, in alternative embodiments the focal surface  115  may be non-planar, for example, spherical, parabolic, or cylindrical. The lines  116  illustrate the converging rays of the focused threat radiation. As used herein, “threat” means incident energy sufficiently intense to damage the detector(s) of an associated sensor. At the focus  118 , incoming energy  109  will be concentrated in a focal pattern; e.g. an Airy diffraction pattern. The trigger layer  103  (primarily transmissive) absorbs some of the incident energy  109 . Absorbed energy heats the trigger layer causing it to switch to an opaque state and protect the sensor&#39;s detector(s). 
   If the radiation continues intensely and long enough (a few milliseconds for some high power lasers), damage, such as melting and vaporization of the trigger layer  103 , will begin. However, before this occurs, heat conducts into the limiting layer  106  and thereby rapidly increases the population of charge carriers  200 , shown in  FIG. 2  (only one indicated), in the conduction band of the limiting layer  106 , making it more “metallic.”  FIG. 2  illustrates a portion  121  of the PAWL  100  in  FIG. 1  in greater detail. Some incident energy  109  absorbed by the charge carriers  200  causes further heating of the PAWL substrate near the plasma  212 . The resulting “plasma region”  203  of thermally-induced free charge carriers is highly reflective and slightly absorbing such that the previously insulating material of the limiting layer  106  becomes conducting, like a metal. The plasma region  203  then blocks the transmission of the incident radiation  109  to the focus spot  118  in the trigger layer  103 . 
   The heat absorbed in the region  206  is quickly conducted, as represented by the arrow  209 , into the adjacent, cooler volume of the PAWL  100 ; i.e., the zone  212 . This causes charge carriers  200  of the plasma  203  to increase in front of the already heated region  203 ; i.e., the heat conduction induces a free electron population density increase in the zone  212 . This newly heated zone  212  is slightly forward of the region  206  where the previous heating occurred, so the incident energy  109  is less concentrated in the newly heated region  212 . 
   This process of thermally induced absorption of the incident energy  109  in the enlarged region of plasma  203  subsequently causes heat that propagates further into the limiting layer  106  toward the source (not shown) of the incident energy  109 . The plasma  203  blocks threat transmission to the previously heated region  206 . Thus a wave  112  of thermally induced plasma  203  propagates from the triggering layer  103  into the limiting layer  106 ; i.e., away from the focus  118  and toward the threat. 
   This absorption wave  112  continues to build and propagate until it reaches the most forward face  124 , shown in  FIG. 1 , of the PAWL  100  or to a region  215  in the PAWL  100  where the threat radiation is defocused enough that heat conducted away from the plasma  203  is in equilibrium with the energy absorbed from the threat. Note that, since the limiting layer  106  absorbs the plasma, the dynamic range of the PAWL  100  can be increased by thickening the limiting layer  106 , since the threat intensity decreases away from the focal plane and there is more material to absorb threat energy loads. When the threat is removed, the PAWL  100  cools back to ambient conditions and the absorbing plasma  203  dissipates so that the sensor functions without degradation. Note that active means of cooling the PAWL  100  may be incorporated to expedite the sensor&#39;s return to full function. 
   The PAWL  100  trigger layer  103  may be implemented using, for example, an oxide of vanadium or titanium. The limiting layer  106  is a low-band-gap material that is transmissive in the pass band of the sensor at normal use temperature conditions. It may be made of any material where the band-gap energy of the conduction band is adequately above the energy of the photons in the sensor&#39;s pass band. The melting point and strength of the material is selected to be high enough to prevent damage to the PAWL  100  from threat radiation. For example if the sensor is designed for the 8 to 12 micron wavelength region like many infrared (“IR”) imagers, the PAWL  100  limiting layer  106  might be made out of Germanium (“Ge”), either pure or slightly doped to tailor its limiting properties. 
   Many materials are sufficiently transmissive to be used for refractive elements and function in the manner desired. Materials that meet these criteria are numerous and include not only Ge, but also:
         (i) for long wave infrared (“LWIR”) and medium wave infrared (“MWIR”) sensors, limiting layer materials such as GaSb, ZnSnAs 2 , InAs, InSb, CuFeS 2 , CuFeSe 2 , AgAlTe 2 , AgInTe 2 , XnSnAs 2 , CdGeAs 2 , CdSnAs 2 , Hgln 2 Se 4 , SnTe, PbSe, PbS, PbTe, BiSe, AgSbSe 2 , AgSbTe 2 , Ag 19 Sb 29 Te 52 , CdSb, ZnSb, Bi 2 Se 3 , Mg 2 Sn, Mg 3 Sb 2 , Cd 3 As 2 , TlSe, Hg 5 ln 2 Te 8 , CuAlTe 2 , CuGaSe 2 , CuGaTe 2 , CuInSe 2 , CuInTe 2 , AgAlSe 2 , ZnGeAs 2 , HgIn 2 Te 4  and Zn 3 As 2  can be considered.   (ii) for shorter wavelength sensors for near infrared (“NIR”) and visible applications, higher band gap limiting layer materials such as Si, ZnS, ZnSe, ZnTe, GaP, may be appropriate,   (iii) for millimeter wave (“MMW”) and microwave sensor applications, lower band gap materials such as InSb, Sn, Bi 2 Te 3 , HgTe, PbSe, CuFeSe 2 , and PbTe, can be considered.   (iv) for UV and X-ray applications, high band gap materials like C(diamond), BN, BP, GaN, AlN, SiC, and SrS are applicable;
 
Thus, as is implied above, the choice of materials as well as some other details will be implementation specific depending upon intended use and design constraints.
       

   Turning now to  FIG. 3 , in one particular embodiment  300 , the trigger layer  303  may be implemented as a layer of thermochromic NLM, as is implied above. In this particular embodiment, the limiting layer  306  comprises a Ge- or silicon (“Si”)-based semiconducting substrate. The triggering layer  303  may be implemented in, for example, a thermochromic coating of a vanadium oxide deposited on the surface  310  near the focus  318  and its temperature biased below but near the phase change temperature of the NLM. After slight heating, the thermochromic NLM and switches from transmissive to reflecting before the detector is damaged. Heat from the trigger layer  303  then conducts into the PAWL  100  substrate causing a plasma  203  as described above. This heat conductance then protects the trigger layer  303  from damage such as fracture, melting, vaporization, delamination, etc. 
   The trigger layer  303  may be fabricated on the limiting layer  306  using solid state material fabrication and thin film deposition techniques as are commonly known in the semiconductor and optical component fabrication arts. In general, techniques used for depositing thermochromic NLMs on the forward face of the semiconducting substrates described above for conventional reflective limiters may be readily adapted to fabricating the trigger layer  303  on the rear face of the substrate in this particular embodiment of the present invention. 
   One particular form of deposition that may be used is known as epitaxial growth, and is illustrated in  FIG. 4 . Epitaxial growth describes a process by which a film or layer of one material is “grown” on a substrate. One suitable technique for this process is known as “chemical vapor deposition,” wherein a substrate is placed in a chamber and a chemical vapor is introduced into the chamber. Over time, under proper temperature and pressure, the chemical vapor will deposit on the substrate in a crystalline film. An overview of this and other epitaxial growth techniques may also be found in any of several thin film and microchip fabrication handbooks. Any suitable epitaxial growth process known to the art may be used. 
   Alloys of silicon and germanium (“Si—Ge”) or materials doped with impurities to adjust band gap may also be used depending on the threat characteristics, required reaction time and other sensor performance or design trade issues.  FIG. 5  illustrates an embodiment  500  in which a trigger layer  503  is formed by doping a Si- or Ge-based semiconducting substrate that is the limiting layer  506 . Doping techniques are also well known in the semiconductor fabrication arts. For instance, well known ion implantation techniques are commonly used for doping purposes. An overview of this and other doping techniques may also be found in any of several thin film and microchip fabrication handbooks. Any suitable doping techniques known to the art may be employed. 
   As was mentioned above, the PAWL  100  is preferably located at or near the focal surface  115 . To block the incident energy  109  quickly (before damage to the detector) the PAWL  100  should be placed either immediately forward of the detector array or in a secondary focal plane (reimager) between the sensor&#39;s objective aperture and detector. This largely results from the desire to maximize the dynamic range in a given embodiment and the fact that the intensity of the incident energy will be highest at the focal point  118 . However, this is not necessary to the practice of the invention. All that is required is that the PAWL  100  be located at a position at which the intensity of the incident energy is strong enough to generate the plasma as described above before the sensitive elements of the sensor or the PAWL  100  trigger layer damage. 
     FIG. 6  illustrates a one particular implementation of a PAWL  600  in a conceptualized cross-sectional view. The PAWL  600  includes a trigger layer  603 —e.g., a thermochromic NLM layer—and a limiting layer  606 —e.g., a semiconducting substrate. The trigger layer  606  residually absorbs the incident energy  609 , laser radiation, for example to generate a plasma wave (not shown) of free electrons that propagates into the limiting layer  606 . The limiting layer  606  then dissipates the plasma wave through absorption. The PAWL  600  is placed at the focal plane  615 . The PAWL  600  also includes optional anti-reflective coatings  622  on the front and rear surfaces  625 ,  626  to reduce element transmission losses. 
     FIG. 7  illustrates but one exemplary use for the present invention in partial cross-section, in which an optical assembly  700  employs a PAWL limiting layer  724 . The PAWL limiting layer  724  protects the detector array  706 , which comprises an array of detector elements  709  (only one indicated), of a detector assembly  710 . Note that the PAWL limiting layer  724  is positioned so that it is in contact with the detector array  706  that also serves as the PAWL triggering layer. The detector (trigger layer)  706  is positioned at or near the focal surface  715  of the optical sensor (not shown). The assembly  710  is housed in a thermal control apparatus  718  to control the operating temperature of the assembly  710 . The thermal control apparatus  718  may be any suitable means known to the art, such as a cryogenic temperature-controlled dewar. Note that the front surface  721  of the limiting layer  724  may be configured to function as a cold window, a band-pass filter, and/or a field lens. As will be appreciated by those skilled in the art having the benefit of this disclosure, the optical assembly  700  will include additional, routine features such as support components, electronics, thermal conditioning components, and thermal isolation components. These features have been omitted for the sake of clarity and so as not to obscure the present invention. 
   Those in the art may realize further variations on the embodiments disclosed above that are also within the scope of the invention as claimed below. For example, referring now to  FIG. 8 , one particular embodiment  800  includes a trigger layer  803  and multiple limiting layers  806   a  and  806   b . The trigger layer  803  is a layer of thermochromic NLM near the focus  818  and the limiting layer  806   a  is a semiconducting substrate with a low band gap, e.g. germanium. The embodiment  800  furthermore includes a second limiting layer  806   b , which may also be a semiconducting substrate with a band gap higher than layer  806   a ; e.g. silicon. The temperature of the trigger layer  803  is biased below but near the phase change temperature of the NLM. 
   After slight heating by threat radiation  109 , the thermochromic NLM that is the trigger layer  803  switches from transmissive to reflecting before the detector is damaged. The trigger layer continues to heat but then heat from the trigger layer  803  conducts into the limiting layer  806   a  causing a plasma (not shown) as described above. The plasma protects the trigger layer from damage and if there is enough heat (from a severe threat  109 ), the plasma wave in the limiting layer  806   a  may expand to the front surface of layer  806   a . Heat from layer  806   a  then conducts into  806   b  to induce a plasma in the second limiting layer  806   b . Thus, the limiting layer  806   a  may also function as a trigger layer for the second limiting layer  806   b . Thus, a thermally induced plasma in both the first limiting layer  806   a , and subsequent limiting layers  806   b , etc. then protects its respective trigger layer from damage such as fracture, melting, vaporization, delamination, etc. 
   Thus, in its many manifestations and aspects, the present invention uses a thermally-induced conduction-band plasma wave in a solid-state material to passively block intense radiation. It thereby provides a number of benefits over and above the state of the art, including:
         it provides an automatic, low-loss means to protect optical sensors from damage by high-intensity light from a laser;   it provides sensor protection from other damaging sources within the wavelength range that the sensor is designed to detect and that would cause thermal damage to a sensitive component such as the sensor&#39;s detector or focal plane array;   it provides protection from threats in the pass band of the sensor without degrading sensor performance when a threat source is not present;   it reacts to any wavelength in the sensor pass band and is thus more robust to evolving threats than a spike filter for a specific laser wavelength;   it is passive and requires no sensors, actuators and control electronics as does a mechanical shutter;   it can be designed to work over a large range of ambient temperatures from cryogenic to refractory;   it is tolerant of wide variation in ambient acceleration, shock and vibration unlike fluid cells, resonant etalons or pellicles;   it is unobservable from outside the sensor;   it is quick reacting, compact and light weight compared to shutters;   it can be tailored to a wide range of sensor bands from the microwave to x-ray;   it does not require prior knowledge of the threat wavelength like notch filters or notch absorbers;   it protects against extreme threat levels that would damage other protection equipment like thermochromic limiters; and   it is easier to design and fabricate than many other technologies.
 
Note that not all embodiments of the present invention will necessarily exhibit all these advantages.
       

   This concludes the detailed description. The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.