Patent Publication Number: US-9413469-B2

Title: Apparatus and method supporting covert communications using terahertz imaging camera

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND PRIORITY CLAIM 
     This application claims priority under 35 U.S.C. §120 as a continuation-in-part of U.S. patent application Ser. No. 13/926,767 filed on Jun. 25, 2013, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/690,600 filed on Jul. 2, 2012. Both of these applications are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to communication systems. More specifically, this disclosure relates to an apparatus and method supporting covert communications using (in part) a terahertz imaging camera. 
     BACKGROUND 
     Since the last decade of the twentieth century, there has been increasing interest in building cameras that utilize the shortest wavelengths of the radio frequency (RF) portion of the electromagnetic spectrum that still penetrate materials. The range of interest typically starts at the “W band” (about 75 GHz to about 110 GHz) and extends into higher terahertz frequencies. The general concept is to perform imaging to detect and recognize objects of interest concealed behind cover materials such as clothing. This is passive imaging in that natural radiation from a scene is used to perceive differences in the apparent temperatures of objects. 
     SUMMARY 
     This disclosure provides an apparatus and method supporting communications using terahertz imaging. 
     In a first embodiment, an apparatus includes a link receiver having a detector array configured to receive radiation from a remote communication device, where the detector array includes multiple detectors. Each detector includes a horn receiver configured to capture the radiation using an antenna positioned in or proximate to a throat of the horn receiver, where the antenna is coupled to an antenna load. Each detector also includes a bolometer electrically isolated from the antenna load and in thermal contact with the antenna load. 
     In a second embodiment, a system includes a first communication device having a link receiver, a link transmitter, and a communication transmitter/receiver. The link receiver includes a detector array configured to receive first radiation from a second communication device, where the detector array includes multiple detectors. Each detector includes a horn receiver configured to capture the first radiation using an antenna positioned in or proximate to a throat of the horn receiver, where the antenna is coupled to an antenna load. Each detector also includes a bolometer electrically isolated from the antenna load and in thermal contact with the antenna load. The link transmitter is configured to transmit second radiation to the second communication device. The communication transmitter/receiver is configured to communicate with the second communication device. 
     In a third embodiment, a method includes identifying, at a first communication device, angular coordinates to a second communication device using a link receiver at the first communication device. The method also includes communicating with the second communication device using a communication transmitter/receiver of the first communication device. The link receiver includes a detector array configured to receive radiation from the second communication device, where the detector array includes multiple detectors. Each detector includes a horn receiver configured to capture the radiation using an antenna positioned in or proximate to a throat of the horn receiver, where the antenna is coupled to an antenna load. Each detector also includes a bolometer electrically isolated from the antenna load and in thermal contact with the antenna load. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a perspective view of a single horn receiver with incident radiation entering from above and with a bow tie antenna positioned at its throat; 
         FIG. 2  shows a magnified view of the throat area of the horn receiver including bow tie antenna posts, irises in a ground plane, and lateral signal traces; 
         FIG. 3  shows a top view of the horn receiver and the bow tie antenna; 
         FIG. 4  shows a two-dimensional array of the horn receiver/bow tie antenna combinations of  FIG. 3  with some details removed for simplicity; 
         FIG. 5  shows the two-dimensional array with only the horn receivers fabricated in one piece and referred to as a director; 
         FIG. 6  shows a layer of crystalline quartz separating the bow tie antenna from the ground plane; 
         FIG. 7  shows a highly magnified front view of the antenna posts and the signal traces; 
         FIG. 8  shows a highly magnified side view of the antenna posts and the signal traces; 
         FIG. 9  shows a top view of the signal traces and a twin lead; 
         FIG. 10  shows the twin lead in more detail with a terminating load; 
         FIG. 11  shows a structure of the twin lead; 
         FIG. 12  shows the inclusion of vanadium oxide as a bolometer material; 
         FIG. 13  shows the inclusion of amorphous silicon as a bolometer material; 
         FIG. 14  shows a dual hook-up structure for a bolometer and a matching resistor; 
         FIG. 15  shows a circuit for extracting a signal from the bolometer and matching resistor; 
         FIG. 16  shows a chart illustrating noise-equivalent power (NEP) of the bolometer versus bias voltage; 
         FIG. 17  shows a chart illustrating receiver figure-of-merit versus frequency; 
         FIG. 18  shows a staggered version of the array from  FIG. 4 ; 
         FIG. 19  shows a chart illustrating capture diameter versus frequency of the horn receiver/bow tie antenna combinations in the array; 
         FIG. 20  shows the staggered array of  FIG. 18  with image sampling centers; 
         FIG. 21  shows a manifold of thin traces that connect outputs from members of the array to inputs of readout integrated circuits (ROICs); 
         FIG. 22  shows a highly magnified view of a middle section of the manifold with the dual hook-up structures included; 
         FIG. 23  shows a highly magnified view of output pads that match a pitch of the integrated circuit inputs; 
         FIG. 24  shows outer parts of the manifold that are folded together and placed on a backside of a bolometer substrate, with outlines of the ROICs seen on the side; 
         FIG. 25  shows traces for biasing the bolometers using two contacts; 
         FIG. 26  shows a highly magnified view of bias lines with hook-up structures included; 
         FIGS. 27 through 39  represent a sequence of steps for fabricating an antenna array part of a terahertz focal plane array; 
         FIG. 40  shows a side view of a vacuum enclosure into which a complete terahertz focal plane array can be mounted with a window at the top; 
         FIG. 41  shows the window separately; 
         FIG. 42  shows a chart illustrating transmission of the window versus frequency; 
         FIG. 43  shows a side view of principal elements of a modified Cassegrain imaging system with a primary mirror, a secondary mirror, and a focal plane detector array; 
         FIG. 44  shows a layout of the Cassegrain imaging system and cage with an embedded infrared camera; 
         FIG. 45  shows a block diagram of a dual-mode terahertz imaging system; 
         FIG. 46  shows an illustration of a pixel tracking-based enhancement of a terahertz signal; 
         FIG. 47  shows parts of a terahertz signal enhancement operation; 
         FIG. 48  shows two stations communicating via a communication link; 
         FIGS. 49 and 50  show a station supporting communications using terahertz cameras; 
         FIGS. 51 through 54  show details of communications involving the station of  FIGS. 49 and 50 ; 
         FIG. 55  shows another station supporting communications using terahertz imaging; 
         FIGS. 56 through 59  show details of communications involving the station of  FIG. 55 ; and 
         FIGS. 60 through 63  show a band pass filter that can be used in a station and related details. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 63 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the invention may be implemented in any type of suitably arranged device or system. 
     In this document, terahertz radiation is defined for the frequency range from about 0.1 THz to about 0.5 THz. This radiation sees through many materials, such as clothing, tarps, camouflage, foliage, and some walls. It also enables the detection of concealed weapons, explosives, and other objects. Some conventional imaging systems operate at these frequencies, but these types of conventional systems suffer from high cost, limited utility, and high power consumption. 
     This disclosure provides a passive terahertz imaging system that can be lightweight, low cost, and portable while using very little power. The imaging system here includes a high-density staring terahertz focal plane detector array. The imaging system can be deployed in a wide variety of locations. Angular resolution can be high, and a detectable change in scene temperature can be very small, such as a few degrees Kelvin at 7.5 and 30 Hz. An embedded infrared camera, such as a long-wave infrared (LWIR) camera, or visible camera with special image processing can help to facilitate an additional reduction in terahertz detectable scene temperature changes, such as to below 0.5 Kelvin at 7.5 and 30 Hz. LWIR or other infrared imagery or visible imagery also provides additional high-resolution situation awareness. Operation can occur at normal ambient temperatures, and the same design can be used for a variety of receiving apertures and frequency bands. 
     As described in more detail below, in some embodiments, an integrated imaging system includes a terahertz camera and an infrared or visible camera, such as a commercially-available or other uncooled LWIR camera. The nominal center of the imaging system in the terahertz band could be about 0.3 THz. The terahertz camera can include a two-dimensional (2D) antenna-coupled micro-bolometer detector array, with a ground plane separating an antenna array and a bolometer array. This allows substantial independence between the two arrays, meaning the arrays can be designed and fabricated separately and joined later. Improved detector sensitivity can be achieved using low noise-equivalent power (NEP) for the bolometers and wide-reception bandwidth for the antennas. Low NEP can be achieved by taking advantage of micro-bolometer array technology and manufacturing developed for infrared devices. The bolometer configuration and bias can be uniquely applicable to an antenna input. The antenna design and layout can be aided by three-dimensional (3D) finite-element computer aided design (CAD) simulation or other types of simulations. 
     With various design features described below, high antenna gain and wide bandwidth can be achieved. Also described below are techniques for fabrication and packaging of the antenna array. High angular resolution of the system comes in part from being able to conveniently over-sample the array. This can be accomplished through the arrangement of the elements in the array and an option afforded by modified Cassegrain optics. The system also contains one or more processing devices that use the output of an infrared/visible camera to control a terahertz camera and drive a terahertz display. This capability is used to create a high frame rate stream of terahertz images where each frame benefits from an extended dwell time, but the extended dwell may appear only at the beginning of the stream. This creates an enhanced terahertz video delayed in time, and the delay can be dependent on the operator&#39;s choice based on the situation. The infrared/visible camera can also produce its own high-resolution video for additional situation awareness. 
     This system is capable of producing imagery that reveals objects hidden behind or under clothing, tarps, camouflage, some building materials, or other materials. This is accomplished using a system that can be affordable, lightweight, low power, portable, and battery operated. A variety of different receiving apertures and frequency bands may be derived from the staring array design. The overall system and its operational modes constitute a very versatile tool for a wide variety of applications. 
     A terahertz focal plane detector array includes an antenna array and a bolometer array. Processing of the bolometer array is not specifically described here because, apart from spacing and pretreatment of a substrate upon which the bolometers are fabricated, the process can be essentially the same as for uncooled infrared (IR) arrays. In general, each detector in the terahertz array has incoming radiation collected by a receiving horn and captured by an antenna (such as a bow tie antenna) positioned at the throat of the receiving horn. Output posts of the antenna are fed to a bolometer below a ground plane through irises in the ground plane. 
     This disclosure further provides communication devices that operate using terahertz radiation. The communication devices employ passive terahertz cameras (with or without embedded infrared cameras) as imagers to establish and maintain highly-covert communication links. This functionality could find use in a variety of instances, such as covert support of forward military units for force protection, airborne surveillance with communications to ground, ship-to-ship or ship-to-shore communications, and relaying information using multiple covert links. A link can maintain lock for continuous communications between moving platforms. Link operation is extremely simple, and the overall system is highly flexible with low weight, volume, power and cost. 
       FIG. 1  shows a perspective view of a single horn receiver  1  with incident radiation entering from above and with a bow tie antenna positioned at its throat  1   a . The incident radiation is indicated by the arrow at the top of  FIG. 1 . The horn receiver  1  is shown as a block, rendered in wireframe to allow the interior details to be seen. A horn receiver generally denotes a structure having an opening that receives radiation and directs the radiation to at least one antenna, and the opening narrows from a larger area (where the radiation is received) to a smaller area (where the antenna is located). The antenna at the throat  1   a  of the horn receiver  1  in  FIG. 1  is a bow tie antenna with arms  3   a - 3   b , and output posts  4   a  and  4   b  from the antenna are shown entering a ground plane  2  directly below. One advantage here is that the ground plane can function as a shield, giving design independence between the horn receiver/antenna array on top and the bolometer array below. The arrays can therefore be fabricated separately and joined later if desired. 
     In the following discussion, any dimensions and materials cited are for illustration only, and other selections may lead to different behaviors of the system. These different choices can be made using more precise modeling and incremental improvements in design. The horn receiver  1  and the bow tie antenna could have any suitable dimensions. For example, the horn receiver  1  could have a height of about 2 millimeters, and the throat  1   a  of the horn receiver  1  could be about 0.9 millimeters by about 0.9 millimeters. Also, the total span of the bow tie antenna could be about 0.8 millimeters 
       FIG. 2  shows a magnified view of the throat area  1   a  of the horn receiver  1  including bow tie antenna posts, irises in a ground plane, and lateral signal traces. Here, irises  7   a  and  7   b  in the ground plane  2  are shown more clearly. Each iris  7   a  and  7   b  could have any suitable shape and size, such as circles having a diameter of about 90 microns. Below the ground plane  2 , the antenna posts  4   a  and  4   b  are connected to conducting traces  6   a , which converge into an extension  6   b  representing a twin lead. Circles  5   a - 5   b  represent additional holes in the ground plane  2  and are described below. The circles  5   a - 5   b  could have any suitable diameter, such as about 150 microns. A middle support  10  is used to support the conducting traces  6   a.    
       FIG. 3  shows a top view of the horn receiver  1  and the bow tie antenna. The ground plane  2  is omitted here for clarity. As can be seen in  FIG. 3 , the horn receiver  1  may be asymmetric. For example, a mouth  1   b  of the horn receiver  1  could have dimensions of about 2 millimeters by about 1 millimeter, while the throat  1   a  of the horn receiver  1  could have dimensions of about 0.9 millimeters by about 0.9 millimeters. 
       FIG. 4  shows a two-dimensional array  8  of the horn receiver/bow tie antenna combinations of  FIG. 3  with some details removed for simplicity. This is a “crisscross” array in which adjacent horn receivers/bow tie antennas are oriented orthogonally, which allows tight packing of elements and reduces or minimizes cross coupling and interference. As a result, each individual array element and its nearest neighbors are cross-polarized. The horn receivers  1  could have any suitable spacing(s) in the array  8 , such as vertical and horizontal spacings of about 1.5 millimeters. Details of each bow tie antenna  3  have been emitted, and  FIG. 4  shows only a portion of a large number of detectors (such as up to 8,000 detectors or more) that can be placed on a tile. 
     One advantage of using a horn receiver  1  to collect radiation is that a substantially consistent high gain can be obtained over a wide bandwidth for a horn receiver/antenna combination, together with a greater decoupling between elements in an array. Additional flexibility can come from the fact that the array&#39;s performance is approximately the same whether the antennas  3  are oriented parallel with or perpendicular to the long mouth dimension of the horn receiver  1 .  FIG. 5  shows the two-dimensional array with only the horn receivers  1  fabricated in one piece and referred to as a “director.” 
       FIG. 6  shows a layer of crystalline quartz  9  separating the bow tie antenna from the ground plane  2 . The crystalline quartz  9  here provides a mechanism by which the antenna arms  3   a - 3   b  and the ground plane  2  can be maintained at a precise separation. The crystalline quartz  9  could have any suitable dimensions, such as a thickness of about 147.5 microns. The ground plane  2  is the sketched area at the bottom surface of the quartz  9 . It can be seen here that the holes  5   a - 5   b  penetrate both the ground plane  2  and the crystalline quartz  9 . These holes  5   a - 5   b  allow space around bolometers below the ground plane  2  to be evacuated so that the bolometers can operate in a vacuum. Note that the use of crystalline quartz is an example only and that other material(s) could be used to separate a bow tie antenna from a ground plane. 
       FIG. 7  shows a highly magnified front view of the antenna posts  4   a - 4   b  and the signal traces  6   a .  FIG. 8  shows a highly magnified side view of the antenna posts  4   a - 4   b  and the signal traces  6   a . Both views are cropped to show only the immediate area around the antenna posts and the signal traces.  FIG. 7  shows a front view with the two antenna posts  4   a - 4   b  visible, and extensions  4   a   1 - 4   b   1  of the posts  4   a - 4   b  are also shown. These extensions  4   a   1 - 4   b   1  could be laid down as solder bumps as described below. The antenna posts  4   a - 4   b  can have any suitable size and shape, such as generally cylindrical structures having a diameter of about 25 microns. 
     The crystalline quartz  9  is shown here as separating the antennas  3  and the ground plane  2 . At the bottom is an upper surface of a substrate  11  upon which bolometers can be fabricated. The traces  6   a  from the antenna posts  4   a - 4   b  are positioned on or above the substrate  11 , such as about 4 microns above the substrate  11 . The traces  6   a  are supported by conducting pedestals  4   a   2 - 4   b   2  under the antenna post extensions  4   a   1 - 4   b   1  and by the middle support  10 . In some embodiments, the support  10  may include two metal posts about 3.5 microns high topped by about 0.5 microns of insulating material. In particular embodiments, the traces  6   a  may be supported only at the position of the support  10  and at the antenna post locations. The gap between the substrate  11  and the ground plane  2  can have any suitable size, such as about 30 microns. There may be no intervening material placed in the gap, other than more widely spaced solder bumps described below. The substrate  11  could be formed from any suitable material(s), such as high-resistivity silicon or quartz glass. 
       FIG. 8  shows a side view with only the antenna post  4   a  visible. Here, the length of the traces  6   a  from the antenna posts  4   a - 4   b  are shown with the support  10  at the junction between the traces  6   a  and the extension  6   b . The extension  6   b  is a twin lead terminated by an antenna load. 
       FIG. 9  shows a top view of the signal traces  6   a  and a twin lead (the extension  6   b ). A circle around the extension  6   b  represents the boundary of a thermal bridge  12 .  FIG. 10  shows the twin lead in more detail with a terminating load. In this example, the load represents an antenna load  13  terminating the twin lead, but note that the antenna load  13  may be configured differently than shown. The twin lead forming the extension  6   b  could have any suitable size, shape, and design, such as a layered design that is about 50 microns long.  FIG. 11  shows a structure of the twin lead. Each lead includes an upper layer  6   b   1 , such as a layer of titanium about 0.075 microns thick. Each lead also includes a bottom layer  6   b   2 , such as a layer of silicon oxide or silicon nitride that is about 0.1 microns thick. Both leads can be about 0.3 microns wide. The resistivity of titanium is about 4×10 −5  ohm-centimeters and the resistivity of silicon oxide is about 1×10 15  ohm-centimeters, so the resistance of each lead may be about 889 ohms. The center-to-center separation of the twin leads can be about 2.8 microns. 
       FIGS. 12 and 13  show two bolometer options. In particular,  FIG. 12  shows the inclusion of vanadium oxide as a bolometer material, and  FIG. 13  shows the inclusion of amorphous silicon as a bolometer material. Bolometers can be formed on the substrate  11  and coupled to the antenna array. In  FIG. 12 , the bolometer layout is shown in both a top view and a side view. The antenna load  13  is shown as terminating the ends of the extension  6   b . As particular examples, the load  13  can be about 2.5 microns long, about 0.075 microns thick, and about 0.3 microns wide. The material forming the load  13  could be nickel-chrome, and the load  13  can have a nominal resistance of about 500 ohms. 
     A bolometer  14   a  in  FIG. 12  is formed using vanadium oxide. In the top view, it can be seen that the bolometer material is laid out in a meandering pattern. Additional layers  6   b   3 ,  14   c  in the side view are transparent in the top view to more clearly reveal this pattern. Electrical contacts and leads  15  provide biasing and extraction of a signal from the bolometer  14   a . In the side view, it can be seen that the layer  6   b   3  covers the length of the bolometer material. It also covers the width of the bolometer material and can match the bolometer material in both dimensions. The layer  6   b   3  is an extension of the twin lead&#39;s lower layer  6   b   2 . Also, in the side view, layer  14   c  represents the support material upon which the vanadium oxide is grown. The layer  14   c  could represent a layer of silicon oxide or silicon nitride. 
     In one aspect of operation, signal power captured by a bow tie antenna  3  is routed to the extension  6   b  and the load  13 , which can be warmed (such as through Joule heating). The layer  6   b   3  helps to keep the bolometer  14   a  in electrical isolation from the extension  6   b  and load  13  but also provides close thermal contact. Because of good lateral thermal conduction, the layers  6   b   3 ,  14   a ,  14   c  can be warmed over their entire areas substantially equally with the extension  6   b  and the load  13 . Assuming that vanadium oxide has a nominal resistivity of about 5 ohm-centimeters and is about 0.2 microns thick, the meander pattern leads to a total resistance between the leads  15  of about 10×10 6  ohms. 
     In  FIG. 13 , the bolometer material is formed by a layer  14   b  of amorphous silicon. The extension  6   b , load  13 , and additional layers  6   b   3  and  14   c  in  FIG. 13  may be the same as or similar to the corresponding components in  FIG. 12 . The layer  14   b  here is formed in a continuous sheet, and the conducting contacts and leads  15  for the output signal leads extend along opposite sides of the bolometer material. The contacts and leads  15  are shown in the top view but are transparent in the side view. The operation of the structure in  FIG. 13  can be the same as or similar to the operation of the structure in  FIG. 12 , except the property values of amorphous silicon are different. Assuming that the nominal resistivity of amorphous silicon is about 800 ohm-centimeters and that it is about 0.4 microns thick, the resistance between the leads  15  is about 20×10 6  ohms. 
     Referring back to  FIG. 9 , the bolometer, load, and other layers can be suspended in the middle of the thermal bridge  12  and supported by the extension  6   b  and the contacts and leads  15 . The supporting contacts and leads  15  also allow heat to flow between the bolometer/load and the boundary. The nominal thermal conductivities of the materials in the extensions  6   b  can include about 0.17 watts/centimeter-K (for titanium) and about 0.014 watts/centimeter-K (for silicon oxide). The effective thermal conductivity for each extension  6   b  could equal about 0.081 watts/centimeter-K. Each extension  6   b  can therefore have a thermal conductance G L  of about 8.49×10 −9  watts/K. Assume a thermal conductance G bl  for the remainder of the signal lead/bolometer combination of  FIG. 13  to be about 4.35×10 −9  watts/K. The total G T  therefore equals about 2.133×10 −8  watts/K (where K stands for degrees Kelvin). Titanium can be used as the conducting material for the twin lead, although other materials may be used. The material forming the upper twin lead layer  6   b   1  can have a minimum thermal conductivity and a maximum electrical conductivity, and the material can be chosen to minimize the ratio of these two quantities (the Wiedemann-Franz ratio). 
       FIG. 14  shows a dual hook-up structure for a bolometer and a matching resistor. Note, however, that arrangements other than that shown could also be used. On the right in  FIG. 14  is a bolometer  14   a  on a thermal bridge (not shown). The bolometer  14   a  is connected to the antenna posts  4   a - 4   b  via the traces  6   a  and the extension  6   b . On the left in  FIG. 14  is a resistor  14   d , which can have a resistance that substantially matches the resistance of the bolometer  14   a . However, the resistor  14   d  is not located on a thermal bridge or connected to the antenna posts  4   a - 4   b . Instead, the resistor  14   d  is used to provide balance against a large bias current flowing through the bolometer  14   a . A larger structure  16   a - 16   b  is a conductor (such as one formed from gold) that allows (i) biasing of the bolometer  14   a  and the resistor  14   d  through two lower pads B and (ii) additional circuitry to be connected to a top pad S, such as a capacitive trans-impedance amplifier (CTIA) that extracts a difference in outputs from the resistor  14   d  and the bolometer  14   a . This arrangement can help to cancel unwanted drift and other non-signal fluctuations. In some embodiments, under static conditions, the impedance of the resistor  14   d  and the bias are chosen to produce a zero potential at the junction between the resistor  14   d  and the bolometer  14   a.    
       FIG. 15  shows a circuit for extracting a signal from the bolometer  14   a  and matching resistor  14   d . The bolometer  14   a  has a resistance R b , a constant bias voltage V o , and a current I b . Below that is a resistor R v  with a constant bias voltage −V l  and current −I v . In the middle of  FIG. 15  is a trans-impedance amplifier with capacitive feedback. The output of the amplifier goes to a holding capacitor C hold , which couples into readout circuitry. In this arrangement, the trans-impedance amplifier represents a current reading device. The resistor R v  here is used for balancing current from the bolometer  14   a  and represents the resistor  14   d.    
     The following mathematically describes example operations of components in a particular implementation of a terahertz camera with antenna and bolometer arrays. Other implementations could differ from this mathematical description. The total current input into the trans-impedance amplifier in  FIG. 15  can be expressed as:
 
 I   v +( I   b   o   +ΔI   b   1   +ΔI   b   2 )
 
where I b   o  is the bias value of the bolometer current without external input from a scene, ΔI b   1  is due to input from a background in the scene, and ΔI b   2  is due to a change in the scene features. The peak value of the amplifier&#39;s voltage output can be expressed as 1/C int  times the charge accumulated on the hold capacitor over a time t int . When I b   o =−I v , this can be expressed as:
 
               V   out   peak     =       1     C   int       ⁢       ∫   0     τ   int       ⁢       (       Δ   ⁢           ⁢     I   b   1       +     Δ   ⁢           ⁢     I   b   2         )     ⁢           ⁢     ⅆ   t                 
Only a change in R b  manifested by input from a scene appears in the peak value of the output. This serves the performance of the bolometer  14   a  as described below.
 
     The bolometer  14   a  can have a high impedance to allow for a substantially constant voltage bias. Assume that the bolometer  14   a  is in a biasing circuit and placed on the thermal bridge  12 . The static heat balance equation can (dropping the subscript  b  for the current I) be given by:
 
 IV+P   r   =G [( T−T   Joule )+( T   Joule   −T   o )]
 
The temperature of the bolometer  14   a  can be dictated, at least in part, by Joule heating (IV) from the circuit, the collected (signal) power P r  absorbed by the bolometer  14   a , and the thermal conductance G (in watts/K) to the local environment. Here, T is the equilibrium temperature of the bolometer  14   a , T Joule  is the temperature brought about by Joule heating, and T o  is the reference or local environment temperature. Also, V is the voltage spanning the bolometer  14   a , and I is the current. The contribution to T from static absorbed power only can be isolated as follows:
 
 P   r   =G ( T−T   Joule )
 
The temperature rise divided by the absorbed power can be expressed as:
 
                 T   -     T   Joule         P   r       =       1   G     .           
The consequences of a change in absorbed power ΔP r  can be considered using the derivative of the static heat balance equation:
 
Δ V=IΔR   b   +R   b   ΔI  
 
plus the expression for the thermal responsivity (α):
 
             α   =       1   ⁢   Δ   ⁢           ⁢     R   b           R   b     ⁢   Δ   ⁢           ⁢   T             
where R b  is the bolometer&#39;s resistance. With ΔV=0 (constant voltage bias), the change in temperature ΔT of the bolometer  14   a  per unit change in the absorbed power ΔP r  becomes:
 
                 Δ   ⁢           ⁢   T       Δ   ⁢           ⁢     P   r         =       1     G   +     α   ⁢           ⁢   IV         =     1     G   eff               
where G eff  is the effective thermal conductance for a non-static case.
 
     A current responsivity, which is the change in current ΔI per unit change in the absorbed power ΔP r , may be derived as follows: 
                 Δ   ⁢           ⁢   I       Δ   ⁢           ⁢     P   r         =     =         -   α     ⁢           ⁢   I       G   eff               
where   represents the current responsivity. As a practical matter, for a source to maintain a constant voltage (ΔV=0) under changing loads, the following may be used:
 
                 NR   int       R   b       ⁢     &lt;&lt;   1           
where N represents the number of bolometers  14   a  served, and R int  is the internal resistance of the source. When R b =9.7×10 6  ohms (for amorphous silicon) and N=7,000, R int &lt;&lt;1386 ohms. For vanadium oxide, it would be approximately half that value. For a battery source, this can be easily achieved. Note that with a negative α (as is the case with most bolometer materials),   not only rises directly with the current I but also with the diminishing value of G eff .   theoreticlly becomes infinite when G+αIV=0.
 
     In one approach, advantage is taken of G eff  in the performance of the bolometer  14   a . This approach and the exploitation of G eff  are not used in conventional uncooled IR imagers since direct illumination of a bolometer material and other factors impose restrictions that make it generally impractical. For detectors designed in accordance with this disclosure, the noise-equivalent power NEP and the noise-equivalent temperature change NEDT detected in a scene are related: 
                 ΔΦ     Δ   ⁢           ⁢     T   sc         ⁢     NEDT   ′       =     NEP   ′           
where
 
               Δ   ⁢           ⁢   Φ       Δ   ⁢           ⁢     T   sc             
is the change in power (ΔΦ) from the scene with respect to its change in temperature (ΔT sc ). To a good approximation,
 
                 Δ   ⁢           ⁢   Φ       Δ   ⁢           ⁢     T   sc         ≅     η   ⁢           ⁢     kB   eff             
for a single polarization captured by an antenna at terahertz frequencies, where k is Boltzmann&#39;s constant (1.38×10 −23  J/K), B eff  is the effective terahertz bandwidth, and η is the optics-to-antenna receiver throughput efficiency. Values for B eff  and η are based on the antenna design. Therefore, as a principal performance parameter of the detector:
 
               NEDT   ′     =       NEP   ′       η   ⁢           ⁢     kB   eff               
The “prime” symbols are used here as a reminder that these quantities refer to changes in the scene (their static quantities are different).
 
     To minimize NEDT′, a minimum NEP′ and a maximum B eff  can be used. There are two noise sources commonly associated with the properties of conventional bolometer materials, namely Johnson noise and 1/f noise. A third noise source also comes from the exchange of heat (phonons) between the bolometer and its surroundings, which is referred to as thermal or temperature fluctuation noise. Both Johnson noise and thermal noise are white noise sources, meaning their noise spectra are substantially flat over a large frequency range. Using the responsivity  (T) previously derived, the NEP′ for Johnson noise (where the dependence of R b  and α on T is acknowledged) can be given by: 
               NEP   noise   Johnson     =         -     G   eff           α   ⁡     (   T   )       ⁢   I       ⁢         4   ⁢           ⁢   k   ⁢           ⁢   T   ⁢           ⁢   Δ   ⁢           ⁢   f         R   b     ⁡     (   T   )                   
The NEP for thermal (phonon exchange) noise can be given by:
 
NEP noise   phonon =√{square root over (4 kT   2   G   eff   Δf )}
 
where Δf is the signal bandwidth. 1/f noise is power or “colored” noise, meaning its noise spectrum is not substantially flat over a large frequency range. The NEP for 1/f noise can be given by:
 
               NEP   noise     1   /   f       =         -     G   eff         α   ⁡     (   T   )         ⁢           K   ′     v     ⁢     ln   ⁡     (       f   2       f   1       )                   
where v is the volume of the bolometer, and f 1  and f 2  span the frequency band Δf. The noise spectrum is not flat but rises as the frequency decreases. K′ has the dimension of volume but is independent of volume. For both vanadium oxide and amorphous silicon:
 
 K′≅ 7.5×10 −23  cm 3  
 
     The dependence of T, R b  and α on the bias current I can be expressed as follows. For semiconductors like amorphous silicon: 
             T   =       T   o     +           I   2     ⁢     R   o       G     ⁢     ⅇ       α   o     ⁢       T   o     ⁡     (         T   o     T     -   1     )                           R   =       R   o     ⁢     ⅇ       -     α   o       ⁢       T   o     ⁡     (         T   o     T     -   1     )                         α   =         α   o     ⁡     (       T   o     T     )       2           
Here, R o  and α o  are the initial values of R b  and α at zero bias, and T o  is the temperature of the environment (room temperature). Total NEP from the contributions described above plus those from the receiving circuitry can be expressed as follows:
 
               NEP   noise   total     =       [         (     NEP   noise   Johnson     )     2     +       (     NEP   noise     1   /   f       )     2     +       (     NEP   noise   phonon     )     2     +       (     NEP   noise   circuit     )     2       ]       1   /   2                     NEP   noise   circuit     =     5   ×     10     -   13       ⁢       G   eff       α   ⁢           ⁢     I   b         *           
(* from CTIA-equivalent input noise current).
 
     In the above, the resistor R v  (resistor  14   d  in  FIG. 14 ) is included in the 1/f noise and the Johnson noise. Assume the bolometer material is amorphous silicon and the following input numbers are selected:
         T o =295K   R o =20×10 6  ohms   R v =9.7×10 6  ohms   α o =−0.04K −1      G=2.133×10 −8  W/K   K′=7.5×10 −23  centimeters 3      v=2.9×10 −11  centimeters 3      Δ f= 30 Hz, τ int =33 milliseconds   f 1 =1 Hz and f 2 =30 Hz       

       FIG. 16  shows a chart illustrating noise-equivalent power NEP noise   total  and its components of the bolometer  14   a  versus bias voltage. Although not plotted, the bolometer temperature rises slowly with bias voltage until a threshold is reached (such as about 2.08 volts), at which point it rises very rapidly and the bolometer self-destructs. Near the destruction point, the total NEP′ becomes limited by the thermal NEP′. Prior to that, all NEP′components may fall with bias and fall more rapidly as the destruction point is approached. The value of the total NEP′ immediately prior to the destruction point may be about 5×10 −13  watts. Noise from the various sources (thermal, 1/f, and Johnson) may rise with bias, but the responsivity can rise faster, leading to a reduction in the noise sources&#39; NEP′. If choosing a bias of 2 volts, for example, the bolometer temperature T rises from its initial value of about 295 K to about 314 K, the bolometer resistance R b  falls from its initial value of about 20 MΩ to about 9.7 MΩ, and α falls from its initial value of about −0.04 K −1  to about −0.0352 K −1 . 
     This is therefore a tightly coupled thermal/electrical circuit that exhibits positive feedback. For the feedback to be effective, the total time between samplings can equal or exceed the time for the thermal/electrical circuit to respond. One measure of this time is the bolometer/bridge thermal time constant τ th , which can be expressed as: 
               τ   th     =     C     G   eff             
where C is the effective thermal mass of the bolometer and bridge support. Referring back to  FIG. 13 , the calculated total effective thermal mass for the twin lead, load, bolometer, and signal leads could be estimated at just under 4×10 −11  Joules/K. At a bias of 2 volts, the effective thermal conductance G eff  can be about 6.78×10 −9  Watts/K, and the thermal time constant τ th  is therefore about 5.9×10 −3  seconds.
 
     Besides being independent of changes in load, the bias voltage V remains substantially stable during operation of a bolometer. Note that the bias voltage V can be varied to different settings based on desired operation of the bolometer, and the bias voltage V can remain substantially constant at each setting (independent of changes in the bolometer). From  FIG. 16 , if any operating bias near but short of the destruction point is chosen, analysis shows that (for a specific implementation) any instability in V may be kept below 2×10 4  volts in order to keep the fractional change in the responsivity  (T) at less than 1%. Referring again to  FIG. 16 , at an operating bias of 2 volts, the total NEP′ is about 1.5×10 −12  watts. At a slightly higher bias value of 2.06 volts, the total NEP′ is about 1.0×10 −12  watts, and the bias voltage stability requirement remains at 2×10 −4  volts. Here, G eff  is about 3.82×10 −9  W/K, and the thermal time constant τ th  is about 10.5×10 −9  seconds. How close to the destruction point one operates can be determined using routine experimentation, although non-uniformity of detectors in an array can affect that. 
     The structure described in  FIGS. 1 through 11  can be inherently broadband. For one specific implementation of this structure, a 3D finite-element CAD simulator was used to simulate the delivery of input radiant power to antennas and their loads via f/0.95 to f/0.8 imaging systems over a terahertz spectral range from about 150 GHz to about 450 GHz. Solutions were obtained at discrete frequencies throughout the range. A figure-of-merit was derived for each frequency.  FIG. 17  shows a chart illustrating receiver figure-of-merit versus frequency. In particular,  FIG. 17  is a plot of the figure-of-merit versus frequency for the case of a quartz glass bolometer substrate. 
     From this, an effective bandwidth can be derived corresponding to a unity figure-of-merit and power delivered to the loads with an efficency η of 50% (the maximum for an imaging system feeding a receiving antenna in its focal plane). The effective bandwidth could be about 125 GHz with a nominal center frequency of about 300 GHz. Referring back to  FIG. 16  and the choice of a 2 volts bias, NEP noise   total  from the chart is about 1.5×10 −12  watts as previously stated. Combined with an effective bandwidth of about 125 GHz and a throughtput efficiency η of 50%, this gives an NEDT′ of about 1.74 K at 30 Hz. If 2.06 volts can be used successfully, the NEDT′ would be about 1.163 K at 30 Hz. 
     In the following, assume a bias of 2 volts. Referring back to  FIG. 4 , the spacing between antenna centers can be about 1.5 millimeters as defined by the horn receivers&#39; dimensions. For a staring array, the angular resolution can be set by the detector spacing, provided a diffraction limit is compatible.  FIG. 18  shows an array  17  representing a staggered version of the array from  FIG. 4 . The array  17  has a somewhat tighter arrangement of horn receivers and antennas. For simplicity, the horn receivers are shown as blocks, which are at ±45° angles and staggered. In some embodiments, the spacing between receivers is about 1.414 millimeters both horizontally and vertically. 
       FIG. 19  shows a chart illustrating capture diameter versus frequency of the horn receiver/bow tie antenna combinations in an array. The dashed line in  FIG. 19  is a 1.414 millimeter spacing. At lower frequencies, the capture diameter is much larger. Despite that, there is little mutual interference because of the cross-polarization of adjacent receivers. As might be expected, for frequencies above about 230 GHz, the capture diameter is less than the spacing. One can use this as it is and take a loss in filling factor for the higher frequencies, or one can take advantage of the staggered array in  FIG. 18  and step the image.  FIG. 20  shows an array  18  with the same arrangement, where bow tie antennas are used as site markers and small circles  19  indicate the desired sampling of the image. The circles  19  could be about 0.707 millimeters apart in some embodiments. 
     Now consider the following dither sequence. With the staggered antenna array  18  of  FIG. 20  fixed, the image is sampled, moved up two steps relative to the antenna array (described below), and sampled at each step. The image is then re-sampled at two steps going down and back to the original position and then sampled again. This can be repeated, moving down and back up, except the original position is not sampled again. When this is done, the image has been sampled eight times, but each circle has been sampled twice (by three antenna sites). This scales to four steps needed to sample all circles once. One advantage here is that the dithering need only be along one direction. An alternative would be to move the image horizontally one step and back and then vertically one step and back (or vice versa) for dithering in multiple directions. In some embodiments, it may take about 0.133 seconds to completely sample an image for an NEDT′ of 2.18K. 
     Referring again to  FIG. 19 , a camera aperture can be used that supports a diffraction-limited resolution at least comparable to the capture diameter. For example, by choosing an f/0.8 optical system and a 610-millimeter imaging aperture (24 inches), the nominal diffraction-limited resolution at the focal plane for 430 GHz is about 0.71 millimeters (˜Rayleigh). This can be compared with the capture diameter at 430 GHz of about 0.7 millimeters. This gives an angular resolution of about 1.75×10 −3  radians. 
     As additional options and with additional manipulation of the structure (such as by using a finite-element CAD simulator), it is possible to narrow the figure-of-merit response over that shown in  FIG. 17  and keep the same effective bandwidth. For example, a first step could include creating a greater representation in the frequency range of about 200 GHz to about 300 GHz and reducing or eliminating the response in the range of about 400 GHz to about 450 GHz. The upper value may be confined to about 370 GHz. In this case, as a modification of the arrangement in  FIG. 18 , the nearest-neighbor horn receivers could be aligned horizontally and in parallel, the vertical spacing could be about 1 millimeter, and the horizontal spacing could be about 2 millimeters. To achieve a 1-millimeter sampling in both directions, a two-step horizontal dither can be used. Selecting a 20-inch aperture and f/0.95 optics can result in an angular resolution of about 2×10 −3  radians. The weight of the system could decrease (such as to around 10 pounds). 
     A second step could include further increasing the representation in the range from about 200 GHz to about 300 GHz and confining the upper frequency to about 320 GHz. The pitch can again be about 1 millimeter in both directions using a two-step vertical dither. With f/0.83 optics, this can allow the aperture to shrink to 12 inches. A blur diameter of 1 millimeter at the focal plane becomes equal to the capture diameter at about 320 GHz (see  FIG. 19 ), and the angular resolution becomes about 3.9×10 −3  radians, which is again dictated by the pitch of the horn receivers. In some embodiments, it could take about 0.066 seconds to completely sample an image with an NEDT′ of 2.18K. This system could have further reductions in weight (such as to about 3-5 pounds). However, there can be a lower limit to the pitch. For example, whatever the pitch, the size of the thermal bridge  12  that supports the bolometer  14   a  and load  13  and the size of vacuum ports (described below) may remain the same. These are offset from beneath the antenna arms  3   a - 3   b , so there is less available space as the pitch is lowered. 
     Standard or custom readout circuits can be used to extract data from these types of arrays. For example, readout integrated circuits (ROICs) developed for infrared imaging systems could be used.  FIG. 21  shows a manifold of thin traces that connect outputs from members of the array to inputs of ROICs. The approach here is to route outputs of the bolometers to a common area using the manifold of signal traces, where the manifold converges to match the pitch of the ROIC(s). The biasing of the bolometers can be done similarly, leading to single contacts for plus and minus applied voltages. In other words, one manifold can be used to connect the bolometers to the ROICs, and another manifold can be used to connect the bolometers to bias inputs. The different manifolds can be shielded from one another by a second ground plane. 
     In this example, the signal manifold includes three sections  20   a - 20   c . The sections  20   a - 20   c  are electrically connected but are slightly separated from each other in the drawing for clarity. For the particular case shown, the traces in section  20   a  extend from 2,048 individual detector sites in the middle to the edges of the section  20   a  on either side.  FIG. 22  shows a highly magnified view  21  of the middle section  20   a  of the manifold with the dual hook-up structures  16  included. The middle section  20   a  of  FIG. 21  can have any suitable dimensions, such as about 2 inches by about 3 inches. 
     In sections  20   b - 20   c  of  FIG. 21 , the traces are picked up from the middle section  20   a  and routed to four groups of ROIC pads, two on the left and two on the right. In  FIG. 21 , the individual horizontal pads are unresolved.  FIG. 23  shows a highly magnified view of output pads  22  that match a pitch of the integrated circuit inputs. The pitch could have any suitable value, such as about 25 microns. There could be four linear ROICs, each with 512 inputs positioned over the four sets of output pads. 
     Referring again to  FIG. 21 , it can be seen that sections  20   b - 20   c  take up considerable room. To help confine the overall profile to that of the array itself, other configurations can be used. For example,  FIG. 24  shows outer parts of the manifold that are folded together and placed on a backside of a bolometer substrate, with outlines of the readout integrated circuits seen on the side. The sections  20   b - 20   c  have been folded 180° and are placed on the back of the bolometer substrate. Small vias (not shown) can be inserted through the substrate to connect the sections  20   b - 20   c  with the section  20   a . There may be 2,048 vias (1,024 on each side). In  FIG. 24 , four blocks  23  on the right represent four ROICs. In the drawing, they are displaced from their correct position in the center of the folded sections  20   b - 20   c  so they can be seen more clearly. The markers [x] represent common center positions. In particular embodiments, a suitable linear ROIC with CTIA inputs is an ISC9802 standard  512  linear array from INDIGO SYSTEMS CORP. (with the ROIC outputs reordered for display). 
       FIG. 25  shows traces for biasing the bolometers using two contacts. In particular,  FIG. 25  shows an overall layout of bias lines  24 . The (+) and (−) pads are the bias voltage inputs.  FIG. 26  shows a highly magnified view  25  of bias lines  24  with hook-up structures  16  included. The traces manifold and the bias lines can be at different levels on a substrate, and a ground plane (not shown) can separate them for shielding purposes. The traces can have relatively low resistances (such as up to several hundred ohms), which lends the system to negligible cross-talk and interference. 
       FIGS. 27 through 39  represent a sequence of steps for fabricating an antenna array part of a terahertz focal plane array. Note, however, that other approaches could be used to fabricate the same or similar structure. 
       FIG. 27  shows a side view of a z-cut wafer of crystalline quartz  9  (the z direction is vertical here and is parallel with the thickness dimension of the wafer). Terahertz feed holes  44  are etched in the wafer, such as through chemical or laser etching. The wafer of crystalline quartz  9  could be about 147.5 microns thick, and the holes  44  could be about 25 microns in diameter. A mask material for etching the quartz  9  could include nickel/gold, and DUPONT photolithographic film or its equivalent could be used for patterning. The film could be a negative resist material and bridge over gaps in the quartz  9 . To etch the holes  44 , chrome/gold can be deposited (such as a few thousand Angstroms thick) on the wafer and patterned to mask all but the prospective hole areas. The wafer can be etched in a mixture of hydrofluoric acid (HF) and ammonium fluoride (NH 4 F). The etch rate can be determined by the temperature of the bath, and a fast etch can occur in the z direction. Z-cut crystalline quartz may be selected for use since it may have no observable loss in the z direction over the frequency range being used and its asymmetric etching properties facilitate micro-machining and segmentation. 
     As shown in  FIG. 28 , surfaces (including hole surfaces) are plated with conductive material  45 . The material  45  could include nickel/gold and have a thickness of about 10 microns. The material  45  can be deposited in any suitable manner, such as by using electro-less plating. The pre-existing chrome/gold mask may be removed or left on the structure. The material  45  can serve as a subsequent mask. It can also constitute the ground plane  2  with its irises and the antennas and their terahertz vias. 
     As shown in  FIG. 29 , vacuum ports  5  are created. For example, holes of suitable diameter (such as about 150 microns) can be opened in the nickel/gold, and the z-cut quartz  9  can be etched. As shown in  FIG. 30 , dicing slots  46  are partially created in the structure. Any suitable etching or other operations could be used to form the vacuum ports  5  and the dicing slots  46 . 
     As shown in  FIG. 31 , the antennas  3  are formed on the structure. This could be accomplished, for example, by patterning and etching the material  45  on top of the quartz  9 . As shown in  FIG. 32 , a support wafer  47  is attached to the antenna side of the quartz wafer. Any suitable attachment could be used here, such as wax bonding. As shown in  FIG. 33 , formation of the dicing slots  46  through the wafer is completed, such as by performing another etching or other operation. As shown in  FIG. 34 , irises  7  are etched in the material  45  on bottom of the wafer (where that material represents the ground plane  2 ). 
     As shown in  FIG. 35 , solder bumps  48  are added on the material along the bottom of the quartz wafer.  FIG. 36  is a magnified view of a pair of solder bumps (forming the extensions  4   a   1 - 4   b   1 ) under the antenna posts. The gold plating or other material  45  can fill any suitable amount of each hole through the wafer, such as between 96%-100% of the hole&#39;s volume. The solder bumps here can provide good RF contact while reducing or preventing lateral expansion from encroaching into the irises  7 . In some embodiments, with reasonably high tin content in the solder bumps, there may be no appreciable degradation in performance. Additional solder bumps (used away from the output posts  4   a  and  4   b ) can be used for securing the bond. 
       FIG. 37  shows the overall structure at this point. Twelve diced blocks are shown in  FIG. 37 , although there could be any number of diced blocks (including an array of thousands). Each block can be about 1 millimeter by about 2 millimeters, and the dicing slots  46  can be about 50 microns wide. The partial dicing from the top can be done in the event that the support wafer bonding material (such as special-purpose wax) is attacked by the quartz etchant. The dicing is done in two steps here (as shown in  FIGS. 30 and 33 ). The dicing could also be done in a single step, such as during the step shown in  FIG. 33 . During the dicing of the blocks, the vacuum holes  5  can be left uncovered, and only the slowly etched lateral faces (x and y directions) of the crystalline quartz  9  may be exposed. The hole widths may not be critical, and any later widening of the holes  5  can be anticipated and corrected for when they are first made. The material of the support wafer  47  can be chosen to match a bolometer substrate, and the wax bond is not shown in the figures. 
     As shown in  FIG. 38 , the structure is bonded (such as using the solder bumps  48 ) to a completed bolometer substrate  49 . Thermal expansion of crystalline quartz in the x and y directions is different from either silicon or quartz glass. The solder-bonding can be done at high temperature, and dicing the crystal quartz  9  into blocks allows them to expand independently and to move with the expansion of the support wafer  47  and bolometer substrate  49 . At least two solder bumps  48  can be assigned to each block. Thermal expansion within a block can be very small. In some embodiments, there may be metal pedestals (elements  4   a   2 - 4   b   2 ), such as about 4 microns high, already placed on the bolometer substrate  49  to accommodate the solder bumps  48 . The pedestals  4   a   2 - 4   b   2  go under the antenna posts and have traces that connect to the twin leads and the bolometers. Whatever the initial height of the solder bumps  48 , the separation between the ground plane  2  and the bolometers  14   a  after bonding can have a desired distance, such as about 30 microns. As shown in  FIG. 39 , the upper support wafer  47  is removed, and the wax bonding is cleaned up. 
     The fabrication approach described above, using crystalline quartz and metal plating, provides a path for creating the array with residual materials that do not outgas. This can be important when the array is used in a vacuum. Moreover, this approach provides relatively wide passages for removing air from the array, which is why at least one vacuum hole  5  is placed in each antenna site. These holes  5  also provide access for freeing bolometer bridges and for ashing and clean up. In the bolometer fabrication, polyimide (or suitable substitute) can be left under the bolometer bridges, and removal of the polyimide constitutes releasing the bridges. The array of horn receivers ( FIG. 5 ) or the staggered version ( FIG. 18 ) sits over the antenna array. 
     As previously stated, the director/array can be fabricated in a single piece of material. Small feet (not shown) at the bottom of the horn receivers  1  can maintain a desired separation, such as about 12.5 microns, from the upper surface of the crystalline quartz  9 . The director can be all metal, with submicron accuracy for the horn interiors. This can be made, for example, by any vendor who specializes in precision machining (such as OWENS INDUSTRIES INC., Oak Creek, Wis.). 
       FIG. 40  shows a side view of a vacuum enclosure  28  into which a complete terahertz focal plane array can be mounted with a window at the top. The vertical dimensions are greatly exaggerated to show details. The overall thickness may have any suitable value, such as about 10 millimeters. The upper part of the enclosure  28  can be made of quartz glass. A window  26   a  in the enclosure  28  can be fabricated separately and bonded or otherwise attached to the enclosure  28 . Layers  26   b  represent two antireflection coatings. The bottom part  29  of the enclosure  28  can be made of metal with shelves supporting the bolometer substrate  11 , which in turn supports the z-cut crystalline quartz  9 . The bow tie antennas  3  and the antenna posts  4  are also depicted. The ROICs  23  are shown attached to the bottom of the bolometer substrate  11  with control, signal, and bias cables  27  extending outside the vacuum enclosure  28 . The mechanism for evacuating the enclosure  28  is not shown, although any suitable mechanism could be used. 
     The window  26   a  in  FIG. 40  is shown in  FIG. 41 . The vertical dimension is greatly exaggerated. The window  26   a  could be formed from quartz glass that is about 0.72 millimeters thick with antireflection coatings  26   b  on top and bottom. The antireflection coatings could be about 0.175 millimeters thick. The index of refraction is about 1.944 for the quartz glass and about 1.394 for the antireflection coatings.  FIG. 42  is a plot of the window transmission versus frequency. Note that the vertical scale starts at 80%. Transmission is substantially 100% from about 250 GHz to about 350 GHz, and transmission stays substantially above 90% out to about 450 GHz. 
     The interior or lower antireflection coating  26   b  in  FIG. 40  can be made by etching a grid into the quartz surface with a pitch smaller than the shortest wavelength of interest. In some cases, the shortest wavelength could be about 0.7 millimeters (about 430 GHz). The depth of the grid can be about 0.175 millimeters, and the ratio of material removed to that remaining could produce an equivalent index of refraction of about 1.394. The original thickness of the quartz block can be chosen to accommodate this etching and leave a final body thickness of about 0.72 millimeters. The exterior or upper antireflection coating  26   b  in  FIG. 40  could optionally be a material with the right thickness and index. 
     The position of the window  26   a  can affect the performance of the entire array. In  FIG. 40 , the separation between the top of the array  8  and the bottom of the window  26   a  could be about 0.655 millimeters. When placed at that position, a high-frequency finite-element CAD simulator can yield a somewhat better performance with the window  26   a  in place than without it. This may seem counterintuitive, but it can be explained by remembering that far field reception is influenced by “everything” in the near field. The structure may be represented by a complex equivalent circuit. For maximum reception, this circuit&#39;s impedance can match the radiation impedance of free space as closely as possible over as wide a frequency range as possible. The window  26   a  is part of that circuit and can be included in fine-tuning the structure. If necessary, at least one dielectric column may be inserted between the window  26   a  and the array  8  to maintain the correct spacing while the space in the gap is under vacuum. 
     A number of different imaging systems can utilize the focal plane array described above.  FIG. 43  shows a side view of principal elements of a modified Cassegrain imaging system, which is suitable for terahertz imaging. Radiation is incident from the right and is first reflected from a primary mirror  30  and converged onto a secondary mirror  31 , which in turn focuses the radiation back onto a focal plane array  35 . The radiation arriving at the focal plane array  35  represents an image of a scene/object. Table I gives example specifications for the elements of the system in  FIG. 43 . 
                                             TABLE I               Description   PM diameter   PM hole diameter   SM diameter   SM pivot-drive   P-S separation   S-FP separation                  Millimeters   508.0   178.0    243.0   79.6   292.5   181.2       Other   Radius/curve       Radius/curve           ** adjustable-       millimeters   872.9   *aspheric, e = 0.8   1154.0           focus                    
In Table I, PM (or P) represents the primary mirror, SM (or S) represents the secondary mirror, and FP represents the focal plane occupied by the detector array. In this example, the primary mirror  30  is aspheric with an eccentricity of 0.8, and the secondary mirror  31  is spherical. Note, however, that other optical components could be used to direct radiation onto the detector array.
 
       FIG. 44  shows a layout of the Cassegrain imaging system and cage with an embedded infrared/visible camera. The system in  FIG. 44  is a dual-mode imaging system in that it supports both terahertz and infrared/visible imaging. On the left in  FIG. 44  is a front view of the primary mirror  30 . Four struts  34   a  support the primary mirror  30 , and a focusing mechanism  36  is included. In the center of  FIG. 44  is a side view of the complete terahertz camera and cage. The four struts  34   a  on the primary mirror  30  are connected to four bars  34   b  that maintain a fixed standoff between the primary mirror  30  and a retaining ring  32 . A cowling  33  encircles the cage. This arrangement forms a rigid frame for the terahertz optics. Here, the primary mirror  30  can have an aperture of about 510 millimeters, and the secondary mirror  31  obscures the incident radiation by about 20% (although these values are examples only). The modulation transfer function and the point-spread function for this annular aperture are negligibly different from an open aperture. 
     In the center of the figure, the focal plane array  35  is shown as being supported by a bar along with the focusing mechanism  36 . To focus, the mechanism moves the detector array  35  closer to or further away from the secondary mirror  31 . In the center of the figure, to the right of the terahertz camera cage, is an infrared/visible camera  39 , such as an uncooled IR camera. The center-lines of the terahertz and infrared/visible cameras can be bore-sighted, and their fields of regard can be mapped. The terahertz field of regard for the design shown can be about 10°, which could be expanded up to about 14° with modification. Since the throughput can be about 80%, the NEDT of the array can increase from a value of 1.74 K to an effective value of 2.18 K. 
     A front view of the retaining ring  32  is shown on the right of  FIG. 44 . It holds the secondary mirror  31  in place. A mechanism  38  fixes the xyz position of the secondary mirror  31  relative to the primary mirror and allows rotation around two orthogonal axes of rotation (through a center of the secondary mirror  31  and perpendicular to a center line of the optical system). Through a linkage  38   a , a small back and forth tilt (dither) of the secondary mirror  31  can be produced by a miniature servo-controlled voice coil  37 . This results in the image being dithered up and down past the detector array. Specifications for a suitable voice coil are shown in Table II. 
                                     TABLE II                          Electrical time constant   70   microseconds           DC resistance   3.5   Ohms           Coil inductance   0.2500   mH           Force constant   0.7200   Lbs/Amp           Peak force   1.5   Lbs           Linear stroke   0.1500   inches                        
One example of a suitable voice coil is the NCMO2-05-005-4JB from MICROMO ELECTRONICS.
 
     To accomplish the dither described in connection with  FIG. 20 , the total forward and backward thrust of the voice coil could be very small, such as about 0.62 millimeters. The mirror  31  can be lightweight (such as about 200 grams) and can therefore be moved rapidly. For example, if a continuous version of the sequence described for  FIG. 20  is executed, there would be a turnaround twice in eight samples. The average power over a full cycle (based on the voice coil parameters in Table II) would be about 24 milliwatts. For 20-inch and 12-inch apertures, the average power can be greatly reduced. For the staggered case of  FIG. 20 , it could take about 0.133 seconds to record each image. For the two-step dither, it could take about 0.066 seconds to record an image. All power and control lines can be fed to the back of the imaging system, and the power used by the detector array could be less than about 3 milliwatts (not including the ROICs). The cage and mirrors can be made of carbon fiber reinforced plastic (CFRP), such as from COMPOSITE MIRROR APPLICATIONS, Tucson, Ariz. The mirror surface accuracy could be within about 2 microns to about 5 microns. The estimated weight for a camera cage and mirrors with a 20-inch aperture could be about 12 pounds. The structure can be made collapsible for transport and storage. 
       FIG. 45  shows a block diagram of a dual-mode terahertz imaging system. The imaging system includes a terahertz detector array and an infrared/visible camera, such as an uncooled infrared camera. The terahertz detector array is formed using an antenna array and a bolometer array. The antenna array includes the director (array  8 ) described above. The display presents images captured by the terahertz detector. The signal enhancement block refers to a device (such as a signal processor) that performs image processing, such as by using the output of the infrared/visible camera to control the terahertz detector and to drive the display. The antenna array block, bolometer array block, and packaging block have been discussed above. The components of the terahertz detector array can be placed within a suitable packaging, such as the vacuum enclosure  28 . 
     There are also methods that can be used to support frame-to-frame tracking of pixels associated with objects of interest in a scene. The application of this is referred to as “optic flow” or “image flow.” Referring back to  FIGS. 44 and 45 , these systems are dual-mode systems since each contains terahertz and infrared/visible imagers. Both imagers can share the same frame rate and bore sight, and their fields of regard can be mapped. Using an NEDT of 2.18 K per frame, the terahertz imager can have adequate sensitivity for many scenarios. However, in those cases where an object either has a very low signal contrast, is moving rapidly, or both, the terahertz imager may not be able to follow. One purpose of the infrared/visible camera (with its high sensitivity) can be to perform continuous high precision pixel tracking. A live record of the results is provided to the terahertz imager, its appropriate detector sites are identified, and their outputs are accumulated in a buffer. This is done repeatedly over multiple frames. Multi-frame accumulation can take place, and the signal-to-noise ratio grows as if the object was stationary. Since the instantaneous position and orientation of objects in the scene are tracked in real time by the infrared/visible camera, terahertz video may be constructed after a small initial lag. 
     In the following example, assume there is no dithering for simplicity.  FIG. 46  depicts an open rectangular object moving left to right and rotating in time. Its positions are labeled  1 ,  2 ,  3 , and so on. Above the rectangles are the sample times denoted t 1 , t 2 , t 3 , and so on for successive images taken by the infrared/visible and terahertz cameras. At time t 1 , the pixels in the infrared/visible recorded image of the rectangle at position  1  are mapped into larger terahertz pixels that form the terahertz image of the rectangle at position  1 . The identification of the corresponding terahertz detectors is made, and the output image of these detectors is placed in a first storage buffer shown in the figure below the rectangle at position  1 . 
     At time t 2 , the pixels in the infrared/visible recorded image of the rectangle at position  2  are mapped into larger terahertz pixels that form the terahertz image of the rectangle at position  2 , and the new terahertz detectors are identified. The output image from these detectors is first copied to the second storage buffer shown in the figure below the rectangle at position  2 , then rotated back to the orientation the object had at time t 1  and added to the first storage buffer. At time t 3 , the pixels in the infrared/visible recorded image of the rectangle at position  3  are mapped into larger terahertz pixels that form the terahertz image of the rectangle at position  3 , and the new terahertz detectors are again identified. The output from these detectors is first copied to the third storage buffer shown in the figure below the rectangle at position  3 , then rotated back to the orientation the object had at time t 2  and copied to the second storage buffer. The output is further rotated back to the orientation the object had at time t 1  and added to the first storage buffer. 
     At times t 4  and t 5 , the sequence is repeated for the pixels in the infrared/visible recorded images  4  and  5 , except three back rotations are performed. The image at position  4  is the last contributor to the first storage buffer, and the image at position  5  is the last contributor to the second storage buffer. The arrows show the progression. At time t 4 , the terahertz image in the first storage buffer is made up of four accumulated images as depicted in the figure. It has a signal-to-noise ratio twice as large as it had at time t 1  (signal-to-noise improves as the square root of the number of images in the buffer). At time t 5 , the terahertz image in the second storage buffer is made up of four accumulated images as depicted. It has a signal-to-noise ratio twice as large as it had at time t 2 . 
     This process is repeated at times t 6 , t 7 , and so on as the infrared/visible camera performs additional image sampling and hand off to the terahertz camera. When the data in a given storage buffer is no longer needed, it can be cleared and used again. As time goes on, the revolving storage buffers provide time-dependent image data for the terahertz display, complete with the proper pixel positions. The rectangular object, as portrayed on the terahertz screen, travels across the display with its true speed and rotation, exhibiting twice its single-sample signal-to-noise ratio, but with a real-time delay of four sample times. 
     The pixel tracking and signal enhancement sequences described above can be extended to a whole scene being viewed. In that case, instead of undoing a rotation, vector displacements of pixels from one frame to the next can be measured and reversed. The number of terahertz images accumulated in the buffers can be chosen in any suitable manner (such as by an operator) and can range from a value of 1 to N (such as based on the scene conditions). This value can be changed, such as by an operator, during operation of the system. As mentioned above, the open rectangular object in  FIG. 46  is seen by both the infrared/visible and terahertz cameras. If the object has something inside it, such as under a cover that the infrared/visible camera cannot penetrate, the terahertz camera can reveal and image it with an enhanced signal-to-noise ratio based on the chosen delay. 
       FIG. 47  shows parts of a terahertz signal enhancement operation. In this example, the terahertz camera is again stripped down to the primary mirror  30 , the secondary mirror  31 , and the focal plane array  35 . There are two displays shown for illustration purposes, though a single display could be used. A display  42  is the infrared/visible display, and a display  43  is the terahertz display. The infrared/visible camera  39  is in front. Two additional components are shown in  FIG. 47 . A block  40  performs a master/slave function between the infrared/visible camera and the terahertz detector array. Using tracked pixels from the infrared/visible camera, the block  40  instructs the terahertz array which of its detectors to use for imaging. Another block  41  represents a buffer and driver for the terahertz display. 
     The thin lines in  FIG. 47  connecting components indicate the flow of information. The infrared/visible display  42 , which is fed by the infrared/visible camera  39 , shows a person in heavy garb walking past the camera. The terahertz display  43 , fed by the block  41 , shows the same person and a concealed weapon being carried. Object space can be used to compare the pixel sizes of the infrared/visible and terahertz imagers. Assume that an uncooled LWIR camera has a focal length of about 100 millimeters and a pitch of about 25 microns. This translates to an angular resolution of about 2.5×10 −4  radians. For the staggered array, the effective terahertz pitch (after dithering) can be about 0.707 millimeters, and the focal length can be about 408 millimeters (510×0.8). This translates to an angular resolution of about 1.75×10 −3  radians. The ratio of resolution footprints in object space is therefore about 7:1. 
     Note that there are often uncertainties in the pixel tracking. It is therefore often advantageous to have the pixel tracking done by the infrared/visible camera with its higher initial precision. With the horizontal and vertical dithering mentioned previously, it may take a longer period of time (such as four 1/30th second exposures) to fully record a terahertz image. At the end of the first 0.133 seconds, four superposed recordings (each with lower resolution) can be obtained, and together they can constitute a full-resolution image. 
     To calculate the NEDT over extended times, assume an update every 0.133 seconds (a picture frame rate of 7.5 Hz) and accumulate using the single sample system NEDT of 2.18 K as the base. The NEDT lowers as the inverse square root of time. For example, if accumulation is performed for 0.632 seconds, the NEDT can equal 1 K. When accumulation is performed for a little over 2.5 seconds, the NEDT can equal 0.5 K. 
     For each case, potential 1/f noise and bias drifts can be reduced or minimized because successively different detectors are used for accumulating the signal from each pixel. Optic flow and pixel handoff can allow dwell time accumulation as long as the object(s) of interest stay within the field of regard or until the object(s) become too distorted through change of direction or adverse rotation. At that point, the system can be reset and the count continued with the new perspective. It may not be unreasonable to expect an NEDT down to about 0.5 K or below in many or most cases. 
     The terahertz imager sees through cover that normally hides concealed weapons, explosives, or other objects. If the cover and the object(s) being concealed are stationary and the cover is transparent at these wavelengths, a high signal-to-noise image of the concealed object(s) may be acquired with the terahertz camera alone by dwelling longer. If a concealed object (such as a suicide vest, weapon, or explosive) is fixed with respect to a moving carrier (such as a vehicle, human, or animal) and the carrier is not concealed, a high signal-to-noise terahertz image may be acquired using dual mode and hand-off. If a concealed object is moving behind a fixed cover, the infrared/visible camera may be unable to assist, and the terahertz camera acquires images unaided. As an added capability, two or more terahertz cameras as described above may be set up in a binocular or other arrangement to view people and objects at close range (such as out to 10 feet or more) in high-resolution 3D. 
     As noted above, various communication devices can employ passive terahertz cameras (with or without embedded infrared cameras) as imagers to establish and maintain highly-covert communication links. In some embodiments, the initial linking frequency of the communication devices may range from a low of about 90 GHz up to about 300 GHz, and advantage is taken of the very low noise threshold for the antenna-coupled micro-bolometer receivers in the terahertz cameras. At least one wide band communication channel can be incorporated that is freely chosen to fit the application, and there may be no technical requirements beyond those found in conventional communication systems. The communication systems described below can represent hybrid systems that use both RF components and optics. 
       FIG. 48  shows two stations  4802 - 4804  communicating via a communication link  4806 . Assume that the stations  4802 - 4804  are identical and separated by a distance R. Also assume that the stations  4802 - 4804  are unaware of each other&#39;s location except for a general direction. Each station  4802 - 4804  has an optical field-of-regard defined by an angle  4808 , and the optical fields-of-regard overlap. Each station  4802 - 4804  performs link transmit and receive operations through the angles  4808 . 
     The stations  4802 - 4804  shown here denote any suitable fixed or portable communication devices that engage in communications. Communications between the stations  4802 - 4804  could be unidirectional or bidirectional. Note that while only two stations  4802 - 4804  are shown here, any suitable number of stations could communicate. For instance, a series of stations could be used to support communications between devices separated by one or more intermediate devices. 
       FIGS. 49 and 50  show a station  4802  supporting communications using terahertz cameras.  FIG. 49  shows the elements of an optional configuration for accomplishing linking and communication in the station  4802 . Linking refers to the stations  4802 - 4804  identifying one another and each station determining the angle to the other station. As described below, this can be accomplished using terahertz cameras. Communication refers to the stations  4802 - 4804  exchanging information, which could occur using infrared, RF, or other communication technologies. 
     There are two optical systems shown in  FIG. 49 . One optical system is a link receiver  4902  that includes a modified Cassegrain imaging system with a primary mirror  4904 , a secondary mirror  4906 , and an antenna-coupled micro-bolometer detector array  4908  at the focus position between the two mirrors  4904 - 4906 . This link receiver  4902  is a terahertz camera with a field of regard of Ω steradians. 
     To the right of the secondary mirror  4906  in  FIG. 49  is a link transmitter (LT)  4910 , which can operate at the same or similar frequency as the link receiver  4902 . The link transmitter  4910  also has a field-of-regard of Ω steradians. The link transmitter  4910  may contain a Gunn diode or other source facilitating transmissions. The link transmitter  4910  can be bore-sighted with the optical axis of the link receiver  4902 . 
     The link transmitter  4910  and the link receiver  4902  perform linking operations with a remote station, such as the station  4804 . When receiving, link-frequency radiation from the remote station  4804  is incident from the right, and the position of the radiation&#39;s focal point on the detector array  4908  allows a determination of the remote station&#39;s relative angular coordinates. When transmitting, the link transmitter  4910  broadcasts through its solid angle  4808  to be received by the remote station  4804 , allowing the remote station  4804  to identify the relative angular coordinates to the station  4802 . 
     The second optical system is positioned on the left in  FIG. 49  and represents a conventional Cassegrain optical system. The second optical system includes a primary mirror  4912  and a secondary mirror  4914 . The secondary mirror  4914  here denotes the backside of the primary mirror  4904  that is resurfaced to serve this purpose. The Cassegrain optical system is used for wide band communications with the remote station  4804  having a similar wide band system. The primary mirror  4912  has an aperture, which can match the directionality of the link receiver  4902 . A wide band communication transmitter/receiver (COM) unit  4916  is located at the focal point of this system behind the primary mirror  4912 . The COM unit  4916  could represent an infrared, RF, or other suitable wireless communication unit. The COM unit  4916  and the mirrors  4912 - 4914  form the wide band communication system, which can be bore-signed with the linking communication system. An absorber  4918  at the center of the secondary mirror  4914  is used to reduce or eliminate reflected radiation directed back at the COM unit  4916  when transmitting. In some embodiments, both Cassegrain systems have annular apertures, and the link frequency (used for linking) should be chosen to be at least twice the communication frequency (used for communication) in order to limit obscuration of the Cassegrain system&#39;s aperture. 
     In one aspect of operation, each station  4802 - 4804  can transmit a signal using its link transmitter  4910  to the other station, and each station  4802 - 4804  can receive the other station&#39;s signal using its link receiver  4902 . This allows each station  4802 - 4804  to identify angular coordinates where to direct wide band communications to the other station via the second optical system. At each station, the detector or detectors in the detector array  4908  that receive a signal from a remote station can be identified, and the angular origin of the incoming signal relative to a central axis of the optical system can be determined. 
       FIG. 50  shows a 3D perspective view of the station  4802  implemented as shown in  FIG. 49 . The link receiver  4902  is at the right front with a cowling  5002 . Radiation is incident from the right front. The link transmitter  4910  is integrated with the link receiver  4902 , and the COM unit  4916  is also shown here. Note that the form factors of the various components are examples only. 
     Table III provides example parameters for specific implementations of the link receiver  4902  and link transmitter  4910  (referred to as a link system) and of the second optical system (referred to as a com system). Note that these parameters are examples only. 
                                                     TABLE III                                   Focal   # of   Resolution               Aperture   Weight   F/#   length   Detectors   Angle   Frequency                                                                    Link system    6 inches   1-1.5 lbs.   1    6 inches   350   9 milliradians   275 GHz       Com system   20 inches    7-10 lbs.   1.6   32 inches   N/A   9 milliradians    80 GHz                    
The weight values here assume that mirrors and cages for the two systems are made of carbon fiber reinforced plastic (CFRP), which is a very strong, rigid, and light-weight material with low thermal expansion. One source of CFRP is COMPOSITE MIRROR APPLICATIONS of Tucson, Ariz. The detector array  4908  here contains 350 antenna-coupled micro-bolometer detectors, which represents the number of discrete positions that can be resolved in two dimensions at the array&#39;s surface for this particular choice of camera parameters.
 
       FIGS. 51 through 54  show details of communications involving the station  4802  of  FIGS. 49 and 50 . These details are for illustration only and are based on simulation results for a specific implementation of the station  4802 . Other implementations of the station  4802  could also be used.  FIG. 51  is a chart  5100  showing a broadcast link footprint versus range for a 10° field of regard (Ω=0.024 steradians). This “search” footprint is about 1,750 feet across at a range of 10,000 feet (just under 2 miles). 
       FIG. 52  is a chart  5200  showing a resolution footprint versus range after a link has been established (for both optical systems). The positional accuracy is about 90 feet across at a range of 10,000 feet. Referring back to  FIG. 48 , a simple expression for the power P received at the station  4804  with the station  4802  transmitting at power P 0  is: 
             P   =         P   0     ⁢   A   ⁢           ⁢     ⅇ       -   α     ⁢           ⁢   R           Ω   ⁢           ⁢     R   2               
where A is the receiving aperture at the station  4804  and α is the attenuation of the atmosphere. If the noise-equivalent-power of the receivers in the detector array  4908  at the station  4804  is NEP, the requirement for P 0  to meet that threshold is:
 
     
       
         
           
             
               P 
               0 
             
             ≥ 
             
               
                 
                   NEP 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Ω 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     R 
                     2 
                   
                 
                 
                   A 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       
                         - 
                         α 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       R 
                     
                   
                 
               
               . 
             
           
         
       
     
     As a particular example of an implementation, each station  4802 - 4804  sends out low-level link pulses at a nominal interval, such as an interval of about 30 milliseconds. These pulses can be broadcast through the solid angle Ω at a carrier frequency, such as about 275 GHz. The procedure can begin transmissions at some initial low power level and increase the power in steps to some maximum level. This would correspond to a sweep through a range looking for the noise threshold of the target receiver. An outside interceptor would need time to locate the transmitters, given its own remote location and provided it can even detect the pulses at this frequency. Once the target receiver threshold has been reached, the receiving station can send confirming pulses, and the determination of mutual angular coordinates could take place quickly. The two stations  4802 - 4804  could then be pointed to bring their angular offsets to substantially zero and remain locked in that orientation by continuing mutual link pulses. 
     Wide band two-way communications could occur between the stations  4802 - 4804 , such as at a carrier frequency of 80 GHz. The two-way communications could operate over a very narrow solid angle, which would reduce or eliminate interception for anyone not in the direct line between the stations  4802 - 4804  as can be seen in  FIG. 52 . In the following example, assume a link receiver noise threshold (NEP) of 1.5×10 −12  watts at 30 Hz and an atmospheric attenuation of 9 dB/Km. This attenuation value corresponds to the nominal high attenuation in water vapor density at 275 GHz, which is the principal driver of attenuation when the sky is clear. Most of the time, atmospheric attenuation is much lower (not accounting for rain). 
       FIG. 53  is a chart  5300  showing transmitter power required to meet the noise threshold of a receiver as a function of range. If there is somebody trying to intercept a link,  FIG. 54  shows the low power that the interceptor would have to detect versus interceptor range. Assume that an interceptor is in a broadcast zone of the transmitting receiver that is attempting to establish a link. In this case, the transmitter power is made equal to the receiver threshold at a range of 10,000 feet. The carrier frequency could be about 275 GHz and unknown to the interceptor. The chart  5400  illustrates the challenge for the interceptor even if close to the transmitting station. The link pulses could be about 20 to 30 milliseconds long. The pulses could include bursts of pulses and may be sent at random intervals. The transmitter and receiver solid angles need not be the same or even symmetric as assumed here. If the positions of the stations  4802 - 4804  are fixed, continuing link pulses to maintain lock could be at a very slow rate with reduced power levels or possibly not at all. If the positions of one or both stations  4802 - 4804  are dynamic, the rate of link pulses could be dictated by that dynamic. The link pulses might also be used for simple coded transmissions between the stations  4802 - 4804 , which could be determined by the specific application. The best arrangement for all communications may be to rely on a very narrow solid angle through which the COM unit operates. 
       FIG. 55  shows another station  4802  supporting communications using terahertz imaging. In particular,  FIG. 55  shows a 3D perspective view of this implementation of the station  4802 . Here, a communication transmitter/receiver  5502  and a link receiver  4902 /link transmitter  4910  are placed side-by-side on a common platform. The dashed arrow indicates the direction in which the components are looking, and this arrangement can allow the transmitters/receivers to operate at similar frequencies. Again, the link transmitter  4910  and the COM unit  4916  are shown, and the basic elements of the link receiver  4902  are those of a terahertz camera. Table IV provides example parameters for specific implementations of the link receiver  4902  and link transmitter  4910  (the link system) and of the communication transmitter/receiver  5502 . These parameters are examples only. 
                                                     TABLE IV                                   Focal   # of   Resolution               Aperture   Weight   F/#   length   Detectors   Angle   Frequency                                                                    Link system   18 inches    7-10 lbs.   0.85   15 inches   350   9 milliradians   94 GHz       Com system   20 inches   10-15 lbs.   1.6   32 inches   N/A   9 milliradians   80 GHz                    
The weight values here assume that mirrors and cages are made of CFRP. A link frequency of about 94 GHz could be selected for lower atmospheric attenuation, but it could range up to about 130 GHz and still give a similarly low value. The number of discrete positions that can be resolved at the array surface for this camera again comes out to 350, which is the number of antenna-coupled micro-bolometer detectors in the focal plane.
 
       FIGS. 56 through 59  show details of communications involving the station  4802  of  FIG. 55 .  FIG. 56  is a chart  5600  showing a broadcast link (search) footprint versus range for a 10° field of regard (Ω=0.024 steradians). This chart  5600  is substantially identical to the chart  5100  shown in  FIG. 51 .  FIG. 57  is a chart  5700  showing a resolution footprint versus range for the link and communications units. The positional accuracy for both systems is about 90 feet at a range of 10,000 feet.  FIG. 58  is a chart  5800  showing the link transmitter power versus range. Assume a link receiver noise threshold (NEP) of 1.5×10 −12  watts at 30 Hz but with a reduced atmospheric attenuation of 1 dB/Km. This attenuation value corresponds to the nominal high attenuation in water vapor density at 94 GHz. In the previous case shown in  FIG. 53 , the transmitter power required at 10,000 feet was several hundred microwatts, while here it is about 30 microwatts.  FIG. 59  is a chart  5900  that shows the interceptor link power versus range. In  FIG. 54  the potential signal to be intercepted is 1×10 −10  watts at 3,500 feet from the transmitter, while in  FIG. 59  it is an order of magnitude smaller (1×10 −11  watts) at 4,000 feet from the transmitter. 
       FIGS. 60 through 63  show a band pass filter that can be used in a station  4802 - 4804  and related details. The terahertz camera (link receiver) can accept radiation over a wide terahertz band.  FIG. 60  is a plot  6000  of antenna response versus frequency for an example terahertz camera. Natural radiation from the Earth over this band typically constitutes an input that is several hundred times the noise threshold (NEP) being used here. The radiation is made up of two parts. The major part is fixed background radiation from the Earth&#39;s surface emitting at a nominal blackbody (gray body) temperature of 295 Kelvin, which can be removed electronically (as described above). The smaller part is from time and space variations due to different features of the surface and various objects on it. Most variations involve lower emissions and negative apparent temperatures, but rare positive variations can come from absorbent things warmed by the sun, running engines, heating systems, fire, etc. If these things are ignored while searching (and they can be distinguished by having no time signature and appearing at discrete locations on the detector array), a background level can be obtained that is as low as or lower than the noise threshold (NEP), provided the acceptance band is restricted. 
       FIG. 61  represents a band pass filter  6100  that can be used by the stations  4802 - 4804 . The band pass filter  6100  is implemented as a “holey plate” here. If the hole dimensions is chosen to be 0.1 mm by 0.1 mm and the hole spacing is 1.0909 mm, the resultant reflection versus frequency for this plate can be as shown in  FIG. 62 . As the chart  6200  in  FIG. 62  shows, this looks to be a very narrow filter. However, closing in on the frequency range around the zero point is shown in  FIG. 63 . There are two curves in  FIG. 63 , one for normal incidence and one for 5° off-normal (which is the maximum for a 10° field of regard). For the off-normal incidence, the curve shifts to a higher frequency, in this case by about 0.8 GHz. For each incident angle, the curve can shift accordingly. With this filter  6100 , the effective acceptance bandwidth is about 6.5 GHz. If a positive temperature variation overlaps the position of the transmitting station but does not exceed 20 Kelvin, its background power level would not exceed 60% of the threshold noise floor (NEP). The filter  6100  may be fabricated in any suitable manner, such as by depositing and patterning a thin metal film on a substrate, which could be a thin film of dielectric material. It can be placed over the forward aperture of a terahertz camera/link receiver as a dust cover or other structure. 
     In summary, different designs for communication stations (including a compact design and a side-by-side design) are provided that support a wide range of link and communication frequencies. The two configurations are illustrated here to show flexible choices in linking frequency and may be used in many different scenarios. These systems may be scaled up or down for different resolutions and ranges, and they may be mounted on fixed or moving platforms. Multiple links may be strung together for relaying information. Also, the “search” solid angle Ω may be adjusted to be wider or narrower. As previously stated, atmospheric attenuations chosen in the illustrations were nominally the highest that would typically be found throughout the world (absent rain and storms), and atmospheric attenuation is much less most of the time and in most places. This method of linking and communicating can be highly covert. When establishing a link, the needed transmission power levels can be extremely low, and the flexible linking frequency can be higher than is normally accessible and unknown by potential interceptors. Once established, the link may be maintained with high angular resolution. When communicating, the transmitting solid angle can be very narrow, thereby reducing or prohibiting interception for anyone not accidently in the beam itself. The communication bandwidth may be independently chosen. Also, as previously stated, the operation of the system is simple and straightforward. 
     Although the figures and description above have shown and described particular implementations and details of dual-mode terahertz imaging systems and communication systems using terahertz imagers, this disclosure is not limited to the specific examples shown and described above. Any other suitable dual-mode infrared/visible and terahertz imaging system or communication system that operates in the same or similar manner as those described above fall within the scope of this disclosure. Moreover, all numerical values given above (such as dimensions, frequencies, efficiencies, time constants, and NEP and NEDT values) are approximate values only and are provided as examples only. Other dual-mode infrared/visible and terahertz imaging systems or communication systems could operate using different values or ranges of values. Similarly, all materials described above (such as vanadium oxide, amorphous silicon, crystalline quartz, gold, and nickel) are provided as examples only. Other dual-mode infrared/visible and terahertz imaging systems or communication systems could be fabricated using other or additional materials. 
     In some embodiments, various functions described above are implemented or supported by a computer program that is formed from computer readable program code and that is embodied in a computer readable medium. The phrase “computer readable program code” includes any type of computer code, including source code, object code, and executable code. The phrase “computer readable medium” includes any type of medium capable of being accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disc (CD), a digital video disc (DVD), or any other type of memory. A “non-transitory” computer readable medium excludes wired, wireless, optical, or other communication links that transport transitory electrical or other signals. A non-transitory computer readable medium includes media where data can be permanently stored and media where data can be stored and later overwritten, such as a rewritable optical disc or an erasable memory device. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more elements, whether or not those elements are in physical contact with one another. The terms “application” and “program” refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or a portion thereof adapted for implementation in a suitable computer code (including source code, object code, or executable code). The terms “receive” and “communicate,” as well as derivatives thereof, encompass both direct and indirect communication. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrase “associated with,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like. The phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B, and C” includes any of the following combinations: A, B, C, A and B, A and C, B and C, and A and B and C. 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.