Abstract:
An IR limiting device for a detector that is based on a micro-optomechanical cantilever array is disclosed. In the normal state, each microcantilever device in the array behaves like a mirror that reflects the infrared signal to the detector. The microcantilever device absorbs radiation outside the desired infrared region. When the radiation is stronger than a predetermined threshold, the microcantilever device bends as a result of thermo-mechanical forces, and it reflects the signal away from the detector, thereby limiting the radiation. The advantage of such a system is that each pixel in the detector can be addressed individually, and the limiting is localized.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an infrared optical limiter, and more specifically, to a passive broadband infrared optical limiter device based on an array of micro-optomechanical cantilevers that bend as a result of thermo-mechanical forces. 
   2. Description of the Related Art 
   There is a need for a broadband passive infrared optical power limiting device. In the present application, the wavelengths of the infrared spectrum will be referred to as the mid wavelength IR (mid IR, wavelength 3 to 5 microns) region and the long wavelength infrared (LWIR, wavelength 7 to 14 microns) region. Such an infrared optical limiter would be useful for protecting light sensitive items, such as the human eye, a photodetector or a camera against unexpectedly strong illumination. An ideal optical limiter is transparent under low level illumination at the wavelength of interest, but “dark” under strong incident light power. The optical signal power transmitted through an optical limiter is ideally constant, irrespective of the incident power, when the incident signal power is higher than optical limiter threshold. It is also desirable for an optical limiter to have a low initiating threshold and broad spectral range. 
   Optical limiting techniques using reverse saturable absorbers (RSA) solutions and multi-photon absorbers dyes are well known in the art. The disadvantage of these materials as limiters includes a high incident fluence threshold which the optical limiting behavior begins. More importantly, however, there are no known RSA or multi-photon solutions that can be used in the mid IR and LWIR regions. 
   Others have proposed using a vanadium oxide interference mirror for optical limiting in the infrared region. See for example, O. P. Konovalova, Al. Sidorov, “Interference systems of controllable mirrors based on vanadium dioxide for the spectral range of 0.6–10.6 μm”, J. Opt. Technol., 66(5), p. 391 (1999). The Konovalova et al. device is based on a vanadium oxide film that absorbs incident laser energy and changes phase. The main function of this limiter is laser hardening, and it is a relatively narrowband device that is not suitable for handling a broadband of wavelengths. It has an initiating threshold on the order of 1 MW/cm 2 , which is too high for the mid IR and LWIR range. A suitable mid IR and LWIR range limiter must limit a continuous wave source, having a broad band spectrum (continuously from 3 to 14 microns wavelength), but having relatively low peak power (˜1 W/cm2) radiation. 
   Others have proposed using microcantilever devices to make an infrared focal plane array sensor. See for example, P. I. Olden, and et. al., “Uncooled thermal imaging using a piezoresistive microcantilever,” Appl, Phys. Lett. 69 (21), 3277 (1996); and T. Perazzo, and et. al., “Infrared vision using uncooled micro-optomechanical camera,” Appl., Phys. Lett., 74(23), 3567 (1999). Another example of microcantilever devices is disclosed in U.S. Pat. No. 6,392,233 (Channin et al.) which is entitled “Optomechanical Radiant Energy Device”. The Channin et al. device is a radiant energy detector built from microcantilever devices. Although the use of microcantilever devices as a detector is well known in the art, the use of microcantilever devices as an infrared limiter appears to be unknown in the art. 
   SUMMARY OF THE INVENTION 
   The present invention relates to an infrared limiting device based on a micro-optomechanical cantilever array that is particularly well suited for limiting the radiation impinging upon a detector in the normal state, each microcantilever device in the array behaves likes a mirror to reflect the infrared radiation to the detector. The microcantilever device absorbs radiation outside the mid IR or LWIR region. When the radiation is stronger than a predetermined threshold, the microcantilever device bends as a result of thermo-mechanical forces, and it reflects the radiation away from the detector, thereby limiting the radiation impinging upon the detector. An advantage of the present system is that each pixel of the detector can be addressed individually, and the limiting is localized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of a microcantilever device included in the infrared limiter of the present invention; 
       FIG. 2  is a top view of an alternative embodiment of a microcantilever device having bimetallic bending arms; 
       FIG. 3  is a side view of the microcantilever device having bimetallic bending arms of  FIG. 2 ; 
       FIG. 4  is a schematic drawing of the infrared limiter system of the present invention; and 
       FIG. 5  is an illustration of the microcantilever array of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   One of the goals of the present invention is to protect an infrared device such as a LWIR camera against damage from continuous wave (CW), broadband radiation, such as radiation from the sun. The present invention, however, is not limited to LWIR devices, and it is suitable for use with mid IR devices. Because of the high temperature and brightness, continuous wave radiation can be more harmful to the detector than a pulsed, narrow bandwidth laser. On the other hand, because of its high temperature, the continuous wave radiation has a hundred times more energy in the visible and near IR spectrum than in the mid IR or LWIR region. So the approach of this invention is to use the energy from visible and near IR region to operate the limiter in the LWIR or mid IR. 
   Referring now to  FIG. 1 , the main component of the infrared limiter of the present invention is a microfabricated cantilever  10  and a mirror  11 . The microcantilever  10  includes strips  12 ,  13  of different materials that have a common anchor  15 . In normal operation, a desired radiation within the 3 to 14 microns wavelength region (primary radiation) is reflected off the mirror  11  to a detector or an infrared focal plane array (IRFPA), in the manner shown in  FIG. 3 . The mirror  11  also absorbs radiation in all wavelength regions of less than 3 microns (secondary radiation). When the secondary radiation is higher than a predetermined threshold, the cantilever  10  bends and reflects the signal away from the detector, thereby limiting detection. The cantilever bends as a result of thermo-mechanical forces. Preferably, the threshold intensity which triggers the bending is energy in the range of 100–200 watts per cm 2  that is absorbed by the cantilever  10 . The temperature sensitivity of the cantilever  10  arises due to different thermal expansion coefficients of the materials in the layered structure or a differential expansion from a temperature gradient within the vertical depth of the profile of the cantilever  10 . Exposure to radiation induces vertical deflection of the cantilever  10 . 
   Referring now to  FIGS. 2 and 3 , the same effect can be achieved using a bimetallic film. In  FIGS. 2 and 3 , a microcantilever device  20  is disposed upon a silicon substrate  21 . The microcantilever device  20  includes anchors  22  which connect thermal isolation arms  23  to the substrate  21 . The thermal isolation arms  23  are connected to bimetallic bending arms  24 . Dissimilar materials such as gold (coefficient of linear thermal expansion α=14.2×10 −6  m/m*K) and zinc (α=29.7×10 −6  m/m*K) are plated upon the bending arms  24  of the cantilever structure. Other materials of choice include copper (α=16.5×10 −6  m/m*K), iron (α=12×10 −6  m/m*K), and aluminum (α=22.2×10 −6  m/m*K) The bending arms are in turn connected to an absorbing paddle  25  having a mirror or reflective surface. In the side view of  FIG. 3 , the absorbing paddle  25  is clearly illustrated as being cantilevered. The cantilevered absorbing paddle  25  reflects the mid IR or LWIR radiation on to a detector and absorbs the visible and near IR radiation. An example material of paddle  25  is a chalcogenide glass (As x Ge y Se z ) coated on top of a reflective metal layer. The chalcogenide glass absorbs visible and near IR light, but transmits the mid IR or LWIR signal. 
   In the devices  10 ,  20  of  FIGS. 1 and 2 , the visible and near IR radiation is absorbed by the microcantilever devices  10 ,  20  and converted into a cantilever mirror deflection. The dimension of each cantilever pixel is on the order of 100 microns by 100 microns. The microcantilever devices  10 ,  20  can preferably deflect the light at an angle up to approximately 5 degrees. The total reflection by the limiter depends on the micro-device&#39;s filling factor and can be as high as 95%. 
   Referring now to  FIG. 4 , there is an illustration of a schematic system for limiting the incoming radiation. When the incoming radiation is below a predetermined threshold the radiation is reflected off the microcantilever array  41  and directed through a focusing lens  42 . The focused radiation passes a knife edge  43  disposed adjacent to a focal point FP of the focusing lens  42 , and the focused radiation is directed onto a detector  44 . However, when the radiation is above the predetermined threshold, the individual microcantilever devices of the microcantilever array  41  deflect the radiation. After passing through the imaging lens  42 , the deflected radiation will travel downward and away from the focal point FP of the lens  42 . The knife edge  43 , adjacent to the focal point, blocks the deflected radiation. The actual distance from the focal point FP of the lens  42  to the knife edge  43  depends upon the image system numerical aperture, cantilever deflection angle (˜5°) and the image lens focal length. As an example, if the lens focal length is 100 mm, the distance can be on the order of 8 mm. 
   Referring now to  FIG. 5 , the microcantilever array  41  of  FIG. 4  is illustrated in more detail. The microcantilever array  41  includes a silicon substrate  21  and a plurality of microcantilever devices  20 . Only nine microcantilever devices  20  are illustrated, but in a full array there would be an individual microcantilever device  20  for each corresponding pixel in the detector  44 . If the detector  44  includes 320×240 pixels then there would be a total of 76,800 microcantilever devices in the array  41 . Likewise, if the detector  44  includes 620×480 pixels then there would be a total of 307,200 microcantilever devices in the array  41 . 
   From the description above, it can be appreciated that the IR limiter device of the present invention is a broadband device that covers the entire mid IR and LWIR region. Local heating caused by the impinging radiation results in the cantilever pixels being independently controlled. Accordingly, high radiation is addressed locally, while enabling the detector to still find a weak target in the other portions of the detector. 
   The present invention provides several advantages over the prior art. The IR limiter is a true passive device, but it can have a relatively quick response. The signal reflected by the limiter only depends on the filling factor of the cantilever array, which can be as high as more than 95%. Moreover, the IR limiter works for continuous wave, low peak intensity incident light. 
   The present invention is described as a broadband infrared limiter. It will be understood, however, that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.