Abstract:
Methods and systems for providing electromagnetic protection of optical equipment are disclosed. One assembly includes an optical device and an electromagnetically shielding enclosure including a plurality of shielding surfaces, the enclosure defining an interior volume containing the optical device. The assembly further includes a waveguide beyond cutoff extending through a shielding surface of the electromagnetically shielding enclosure. The assembly also includes a first lens located on a first side of the shielding surface, and positioned and oriented to focus light through the waveguide beyond cutoff. The assembly further includes a second lens located on a second side of the shielding surface opposite the first side, positioned and oriented to receive light transmitted through the waveguide beyond cutoff.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from U.S. Provisional Patent Application No. 61/472,493, filed on Apr. 6, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present application relates generally to electromagnetic shielding systems. In particular, the present application relates to shielding of optical systems from electromagnetic events. 
       BACKGROUND 
       [0003]    Exposure to electromagnetic fields can cause interference or damage to electrical equipment, causing that equipment to malfunction or rendering it nonoperational. For example, electrical equipment can be damaged or otherwise fail in the event of a strong electromagnetic pulse (EMP) or intentional electromagnetic interference event (IEMI) is experienced. 
         [0004]    EMP/IEMI events typically take one of two forms. First, high electric field events correspond to short-duration, high voltage events (e.g., electric fields up to and exceeding 100 kilovolts per meter), and typically are of the form of short pulses of narrow-band or distributed signals (e.g., in the frequency range of 14 kHz to 10 GHz). These types of events typically generate high voltage differences in equipment, leading to high induced currents and burnout of electrical components, for example integrated circuits or solid state detector arrays. Second, low field events (e.g., events in the range of 0.01 to 10 volts per meter) are indications of changing electromagnetic environments below the high field damaging environments, but still of interest in certain applications. 
         [0005]    Enclosures designed to protect against EMP/IEMI events are generally required to have substantial shielding properties to prevent electromagnetic signals from reaching an interior of those enclosures. However, it can be difficult to transmit optical (visible or infrared) images or energy from an external source into an interior of a shielded enclosure without causing the enclosure to be susceptible to microwave or RF electromagnetic energy entering the aperture of the enclosure through which the optical energy enters the interior of the enclosure. For these and other reasons, improvements are desirable. 
       SUMMARY  
       [0006]    In accordance with the following disclosure, the above and other issues are addressed by the following: 
         [0007]    In a first aspect, an assembly for providing electromagnetic protection of optical equipment is disclosed. The assembly includes an optical device and an electromagnetically shielding enclosure including a plurality of shielding surfaces, the enclosure defining an interior volume containing the optical device. The assembly further includes a waveguide beyond cutoff extending through a shielding surface of the electromagnetically shielding enclosure. The assembly also includes a first lens located on a first side of the shielding surface, and positioned and oriented to focus light through the waveguide beyond cutoff. The assembly further includes a second lens located on a second side of the shielding surface opposite the first side, positioned and oriented to receive light transmitted through the waveguide beyond cutoff. 
         [0008]    In a second aspect, a shielding arrangement for use with optical equipment is disclosed, which includes an electromagnetically shielding enclosure including a plurality of shielding surfaces, the enclosure defining an interior volume. The arrangement further includes a waveguide beyond cutoff extending through a shielding surface of the electromagnetically shielding enclosure. The arrangement includes a first lens located on a first side of the shielding surface, positioned and oriented to focus light through the waveguide beyond cutoff, as well as a second lens located on a second side of the shielding surface opposite the first side, positioned and oriented to receive light transmitted through the waveguide beyond cutoff. 
         [0009]    In a third aspect, a method of using a shielding arrangement to shield an optical signal is disclosed. The method includes receiving an optical signal at a first lens, the first lens located on a first side of the shielding surface; the first lens positioned and oriented to focus the optical output through a waveguide beyond cutoff. The method also includes focusing the optical signal through the waveguide beyond cutoff, the waveguide beyond cutoff extending through a shielding surface of the electromagnetically shielding enclosure. The method further includes receiving the optical signal at a second lens located on a second side of the shielding surface opposite the first side, the second lens positioned and oriented to receive the optical output transmitted through the waveguide beyond cutoff. The method includes receiving the optical output at an optical receiving device located on the second side of the shielding surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates an example electromagnetically shielded system for use in connection with an optical system; 
           [0011]      FIG. 2A  illustrates an example of a protection system for use in connection with an optical system; 
           [0012]      FIG. 2B  illustrates another example of a protection system for use in connection with an optical system; and 
           [0013]      FIG. 3  illustrates another example of a protection system for use in connection with an optical system. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Various embodiments of the present invention will be described in detail with reference to the drawings, wherein like reference numerals represent like parts and assemblies throughout the several views. Reference to various embodiments does not limit the scope of the invention, which is limited only by the scope of the claims attached hereto. Additionally, any examples set forth in this specification are not intended to be limiting and merely set forth some of the many possible embodiments for the claimed invention. 
         [0015]    In general, the present disclosure describes a low cost and practical method to protect optical systems from Intentional Electromagnetic Interference (IEMI) or Electromagnetic Pulse (EMP) weapons. The invention allows for protection of the electronics and other sensitive components within imaging devices (cameras etc.) to be protected from IEMI and EMP threats. 
         [0016]    In certain embodiments, the present disclosure uses an optical system that transfers the visible or infrared camera image through a waveguide or an array of waveguides which operate beyond the cutoff frequency of the microwave and RF electromagnetic threat which thereby does not allow transmission of the microwave and radio frequency (RF) electromagnetic (IEMI or EMP) energy to pass into the camera or optical system housing. 
         [0017]    Referring now to  FIG. 1 , an electromagnetically shielded optical system  100  is shown, according to a possible embodiment of the present disclosure.  FIG. 1  illustrates a schematic block diagram of the system  100  having a shielded enclosure  102  and a waveguide beyond cutoff  114  mounted in a wall of the enclosure  102 . 
         [0018]    In general, the system  100  protects an optical system positioned within the enclosure  102  from IEMI and/or EMP threats. For example, in the present embodiment, an internal optical device  106  may receive an optical output  108  within the enclosure  102  from an external optical system  118  (e.g., an optical device, digital optical signal, image, or scene) located external to the enclosure  102 . The optical system  118  could be a variety of optical elements that are used to capture or create an image of interest. For example, the optical system could be a fixed or variable magnifying telephoto lens assembly. In another embodiment the external optical system might include an image projection device. 
         [0019]    Because it may be difficult to receive the optical output  108  at the interior of enclosure  102  from the external optical system  118  without causing the enclosure  102  to be vulnerable to undesirable IEMI and/or EMP energy (e.g., via the aperture that would normally be required for optical signals to pass through a shielded barrier), the waveguide  114  is positioned in a wall  103  of the enclosure  102  to reduce this threat by mitigating transmission of harmful energy. 
         [0020]    The internal optical device  106  and the external optical system  118  may be any optical device configured to transmit and/or receive optical energy, such as, for example, visible light (e.g., single-wavelength or broad-spectrum light, or images, or other optical signals). The devices may also transmit any visible or infrared optical images. For example, the optical device  106  may be any of a variety of camera types, such as a still camera or video camera or infrared camera, and can be configured for use in various types of systems, such as security systems, closed-circuit monitoring systems, or other integrated systems. In other embodiments, one or both of the optical device  106  may include a CCD detector array or other suitable image converter, an optical wave generator, or the like. In alternative embodiments, and facing the same challenges, the optical device  106  can be configured to transmit optical signals from within an enclosure  102 , such that the enclosure protects against EMP or IEMI damage. 
         [0021]    The enclosure  102  is generally configured to be an electromagnetically-shielding enclosure, capable of shielding the optical system of the enclosure from undesirable electromagnetic signals (e.g., electromagnetic signals exceeding a particular amplitude and frequency). In various embodiments, the enclosure  102  can be constructed from conductive materials, such as a metal (e.g., sheet metal or aluminum) having a thickness generally sufficient to attenuate electromagnetic signals to acceptable levels. Although in the embodiment shown the enclosure  102  is generally rectangular, it is understood that the enclosure  102  could be any of a variety of shapes. In general, the enclosure includes a plurality of walls, such as wall  103 , capable of substantially completely enclosing an interior volume  105 , which is sized and shaped to be capable of retaining an optical device (e.g., device  106 ), while surrounding that device with shielding surfaces (other than as may be needed to transmit optical and/or power signals, as discussed further below). 
         [0022]    The waveguide beyond cutoff  114  is mounted in a wall of the enclosure  102 . The waveguide beyond cutoff  114  can be, in various embodiments, constructed from metal or other materials. The waveguide beyond cutoff  114  may be a metal waveguide which has a suitable length to diameter ratio (L/D) to sufficiently attenuate the microwave and RF electromagnetic wave such that it cannot damage the an electronics that are stored within the enclosure  102 . For example, the L/D ratio may be on the order of four to one, for threat frequencies of up to 10 GHz. In one embodiment, filling the waveguide with a relatively high index of refraction material  116 , such as a transparent dielectric material, the field of view of the optical system  100  is significantly improved. Additional details regarding possible embodiments of the waveguide beyond cutoff  114  and the enclosure  102  is discussed in the co-pending U.S. patent application Ser. No. 13/289,861, entitled, ELECTROMAGNETICALLY SHIELDED VIDEO CAMERA AND SHIELDED ENCLSURE FOR IMAGE CAPTURE DEVICES, filed Nov. 4, 2011, the disclosure of which is incorporated by reference herein in its entirety. 
         [0023]    The waveguide beyond cutoff  114  is configured and positioned to allow optical communication between an internal area of the enclosure  102  and an area external to the enclosure  102 , for example providing an optical path, or field of view, from the internal optical device  106  to the external optical system  118 . In some embodiments, an optical path begins at optical device  106  and ends at optical device  118 . For example, the optical device  106  may transmit optical energy  108  to an internal lens  110 . Subsequently, the optical energy  108  travels through the waveguide  114  to an external lens  112 , where it is transmitted to the optical system  118 . In alternate embodiments, the optical path may begin at the optical system  118  and end at the optical device  106 , such as an image conversion device (e.g. in cases where the optical device  106  is an optical signal receiving device, such as a camera). 
         [0024]    The external and internal lenses  110 ,  112  are used to focus the image through the limited diameter waveguide  114 . In essence, the light is concentrated, and the image is shrunk to a narrow diameter images, so that it can travel through the waveguide beyond cutoff  114 . After passing through the waveguide beyond cutoff  114 , the image is captured with the internal lens  110  and directed onto an optical device such as a CCD array or other suitable image conversion device. 
         [0025]    Although lenses  110 ,  112  are discussed as being internal and external lenses, respectively, it is understood that in various embodiments, the relative positions of the lenses, or direction of travel of optical transmission, could be reversed (e.g., travelling out of an enclosure  102 , rather than into the enclosure). 
         [0026]    Referring now to  FIGS. 2A and 2B , one embodiment of a system  200  utilizing a waveguide beyond cutoff  208 , according to a possible embodiment of the present disclosure, is shown. The system  200  includes an enclosure  201 , formed by one or more shielding walls  203  and including an interior volume  205 . The system further includes first and second lenses  204 ,  206 , the waveguide beyond cutoff  208 , and an optical receiver  212 , for managing receipt or transmission of optical signals  202 . In the example embodiments, the enclosure  201  is an electromagnetically shielded enclosure. 
         [0027]    In  FIG. 2A , a dielectric  210  is shown within the waveguide  208 . In some embodiments, the dielectric  210  may be a relatively high index refraction material. In example embodiments, the dielectric  210  has a refractive index of about 1.2 or greater. In the example embodiment, the dielectric  210  has concave curvatures on the entrance and exit faces of the waveguide beyond cutoff  208 . The concave curvatures form lens elements within the material of the dielectric  210 . 
         [0028]    In  FIG. 2B , an alternative embodiment of the waveguide beyond cutoff  208  is shown. In the example, a dielectric  214  is utilized. The dielectric  214  lacks curvature on the entrance and exit faces of the waveguide beyond cutoff  208 . In still further embodiments, combinations of concave and convex curvatures could be used to form lens elements formed by dielectric materials  210 ,  214 . 
         [0029]    In general, the concave and/or convex curvatures of lens elements formed by the dielectric materials affect the focusing properties of the transmitted image. Accordingly, in still further embodiments, it may be possible to remove one or more of the external lenses  204 ,  206  entirely, relying instead on the lens elements of the dielectric material incorporated into the waveguide beyond cutoff  208 . The dielectric materials  210 ,  214  may be utilized to improve the imaging characteristics of the shielded optical system  200 . For example, to achieve an attenuation of 100 dB or greater for microwave frequencies of 10 GHz or lower, the waveguide  208  would need to have a diameter (D) of 0.2 inches and a length (L) of 0.8 inches (an L/D ratio of four or larger). The imaging optics should then be selected to match the waveguide dimensions. For example, the focusing lens at the input to the waveguide could have a one inch focal length with a one inch diameter, i.e. a lens with an f-number of one. In such an embodiment, the lens would be positioned approximately one inch in front of the entrance aperture of the waveguide. Other embodiments may employ either larger or smaller f-number optics. The lens at the output of the waveguide would need to be selected to recover the output image so similar dimensions and f-number lenses to that of the input lens could be selected. 
         [0030]    In yet further embodiments, the dielectric materials  210 ,  214  may be configured such that the entrance and exit faces of the waveguide  208  may differ in lens curvature. For example, the entrance face may utilize a concave dielectric curvature whereas the exit face may utilize a convex dielectric curvature. 
         [0031]    Referring now to  FIG. 3 , yet another embodiment of a system  300  utilizing a waveguide  308 , according to a possible embodiment of the present disclosure, is shown. The system  300  includes an enclosure  301 , formed by one or more shielding walls  303  and including an interior volume  305 . The system  300  further includes first and second lenses  304 ,  306 , the waveguide  308 , and an optical receiver  312 . In the example embodiments, the enclosure  301  is an electromagnetically shielded enclosure. 
         [0032]    The waveguide beyond cutoff  308  includes a fiber optic bundle  314 . The fiber optic bundle  314  improves a field of view of the optical path due to the index of refraction of the glass in the fiber optic bundle  314 . In some embodiments, the fiber elements may be coated with a metal layer to form a separate waveguide around each individual fiber strand in the bundle  314 . Thus, an array of waveguides “cells” may be enclosed within the waveguide  308 . The bundle  314 , therefore, transmits an optical signal  302  while the individual fiber coated waveguides prevent microwave and/or RF electromagnetic energy from entering the enclosure  301 . 
         [0033]    It is understood that in the examples above, the systems  100 ,  200 , and  300  may include components that vary in size and measurement based on known principles of optics. For example, the measurements and the materials used to construct the waveguides  114 ,  208 , and  308  may determine the corresponding measurements of the various components in the systems, such as, for example, the first and second convex lenses. Furthermore, the distance measurements between components in the systems may also be dependent on the measurements of the waveguides, as previously discussed. 
         [0034]    The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.