Abstract:
A millimeter wave module for providing one pixel having a pixel resolution in a millimeter wave focal plane array includes a horn antenna having a first cross section area less than or equal to the pixel resolution, a detector for detecting the millimeter wave signals received by the horn antenna, the detector mounted in a recess in the horn antenna and having a second cross section area less than or equal to the first cross section area, and a video output adapter connected to the horn antenna and electrically connected to the detector for providing a connection from the detector, the video output adapter having a third cross section area less than or equal to the first cross section area.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The present invention was made with support from the United States Government under contract W911QX-04-C-0127 awarded by the DARPA. The United States Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to millimeter wave imaging arrays and in particular to a modular and scalable millimeter wave imaging array. 
     BACKGROUND 
     In the past, very few millimeter wave imaging cameras have been produced and millimeter wave detectors were typically machined as individual units and then hand assembled. The need for millimeter wave imaging arrays has increased, because of the need to detect concealed weapons and contraband hidden under clothing. Yngvesson, K. S.; Korzeniowski, T. L.; Kim, Y.-S.; Kollberg, E. L.; Johansson, J. F., “The tapered slot antenna-a new integrated element for millimeter-wave applications,” IEEE Trans. Microwave Theory Techniques,”, Vol. 37, No. 2, February 1989, pp. 365-374 describes a tapered slot antenna for millimeter wave applications. B. Kane, S. Weinreb, E. Fisher, and N. Byer, “High Sensitivity W-Band MMIC Radiometer Modules,” IEEE 1995 Microwave and Millimeter-Wave Monolithic Circuits Symposium Digest, 1995, pp. 59-62 describes a W-band module. Each of these references describes antennas or millimeter wave modules that are not modular. The designs in both these papers are not scalable to large arrays and are also not suitable for volume manufacturing. 
     U.S. Pat. No. 7,135,848 to D. F. Ammar describes a radiometer sensor cell for a scanning millimeter wave scanning imaging camera. The cell of Ammar is only suitable for building scalable imaging arrays having dimensions of 2×M, where M may be an arbitrary integer number. The cell of Ammar is quite large, so pixel resolution is low and the Ammar design is also not suitable for volume manufacturing. 
     What is needed is a modular scalable imaging array, which allows an arbitrarily large N×M array to be built. There is a need for a low-cost module design so that commercial quantities of imaging arrays can be produced in order to lower the cost of millimeter wave imaging cameras. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a millimeter wave module for providing one pixel having a pixel resolution in a millimeter wave focal plane array includes a horn antenna for receiving millimeter wave signals and having a first cross section area less than or equal to the pixel resolution, a detector for detecting the millimeter wave signals received by the horn antenna, the detector mounted in a recess in the horn antenna and having a second cross section area less than or equal to the first cross section area, and a video output adapter connected to the horn antenna and electrically connected to the detector for providing a connection from the detector, the video output adapter having a third cross section area less than or equal to the first cross section area. 
     In another embodiment disclosed herein, a scalable millimeter wave focal plane array for providing a plurality of pixels each having a pixel resolution includes a frame, a plurality of millimeter wave modules, each for providing one pixel having the pixel resolution and each held within said frame, wherein each module comprises a horn antenna for receiving millimeter wave signals and having a first cross section area less than or equal to the pixel resolution, a detector for detecting the millimeter wave signals received by the horn antenna, the detector mounted in a recess in the horn antenna and having a second cross section area less than or equal to the first cross section area, and a video output adapter connected to the horn antenna and electrically connected to the detector for providing a connection from the detector, the video output adapter having a third cross section area less than or equal to the first cross section area. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of an imaging array in accordance with the present disclosure; 
         FIG. 1B  is another perspective view of the imaging array of  FIG. 1A  in accordance with the present disclosure; 
         FIG. 2A  is a perspective view of a horn antenna for an imaging array in accordance with the present disclosure; 
         FIG. 2B  is another perspective view of the horn antenna of  FIG. 2A  in accordance with the present disclosure; 
         FIG. 2C  is another perspective view of a horn antenna in accordance with the present disclosure; 
         FIG. 3A  shows a substrate with circuit components in accordance with the present disclosure; 
         FIG. 3B  is a perspective view of a back short on top of the substrate in accordance with the present disclosure; 
         FIG. 3C  is a perspective view of a back short on top of the substrate mounted in the horn antenna in accordance with the present disclosure; 
         FIG. 4  is a sectional view showing the coupling of the video output using a ball-grid array in accordance with the present disclosure; 
         FIG. 5  is a perspective sectional view of a module for an imaging array with cable connectors in accordance with the present disclosure; and 
         FIG. 6  is a perspective view of a module for an imaging array in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A , the millimeter wave receiving side of an imaging focal plane array  10 , in accordance with the present disclosure, is shown.  FIG. 1B  shows the output side of the focal plane array. Millimeter wave modules  11 , which form the imaging focal plane array, are held in a frame  12  fastened together by screws  15 . Each millimeter wave module  11  is configured to provide one pixel of the imaging focal plane array and each module includes a horn antenna  14  to collect millimeter wave signals, a detector to detect the video signal in the millimeter wave signals, and a video output adapter, which provides for connecting the detected video signal to a post processor (not shown). In  FIG. 1B  the video output adapters  16  are shown as cable connectors; however, there are other embodiments of video output adapters as further described below. The detector is located within the millimeter wave module between the input to the horn antenna and the video output adapters, as described further below. 
     For operation at millimeter wave frequencies, or between 70 and 150 GHz, the overall size of the exemplary 8×8 imaging array shown in  FIGS. 1A and 1B  can be approximately 4.8 cm×4.8 cm, not including the frame  12 . The individual millimeter wave modules  11  may have a horn antenna cross section area that is about 6 mm×6 mm, which corresponds to two wavelengths at about 100 GHz. This dimension provides acceptable pixel resolution for millimeter wave imaging. Smaller size modules with smaller horn antenna cross section areas are also possible. Although  FIGS. 1A and 1B  show an 8×8 array, much larger arrays, such as 1024×1024 arrays are possible. 
       FIGS. 2A and 2B  show a detailed perspective view of the front and back of an exemplary horn antenna  14 , respectively. The horn antenna can be made using low-cost manufacturing techniques such as electro-deposition or metal injection molding. 
     The input cross-section area of the horn antenna is the dimension of side  21  times the dimension of side  23 . As described above, in one embodiment sides  21  and  23  may each be 6 millimeters. Sides  21  and  23  can also be less than 6 millimeters. As shown in  FIG. 2A  the sides of the horn antenna, such as side  25 , are configured so that the cross section area of the horn antenna at any section taken along the side, never exceeds the cross section area defined by sides  21  and  23 . The cross section area of millimeter wave module  11 , which as described above includes the horn antenna, a detector, and a video output adapter, also never exceeds the cross section area defined by sides  21  and  23 . This ensures that the millimeter wave module  11  can be used to build any size focal plane array  10 . 
     In  FIG. 2A  the horn antenna  14  is shown to have ridged waveguide transitions  24 , which are raised portions that provide for wider bandwidth operation. A millimeter wave signal received by the horn antenna is focused by the horn antenna into opening  20 . The opening  20 , shown in  FIG. 2A , is generally circular; however, it is also ridged as shown by ridges  22 , which are located on either side of the opening near the center of opening  20  and are aligned with the ridged waveguide transitions  24 , which connect to the ridges  22 . The ridges  22  together with the ridged waveguide transitions  24  enhance the wideband operation of the module. 
       FIG. 2B  shows the rear side of an exemplary horn antenna  14 . A recess  35  is located on the rear side of the horn antenna and the recess is configured so that a detector substrate piece  40 , shown in  FIG. 3A , can be inserted into the recess  35 . The recess  35  ensures that the detector substrate piece, which includes a detector  60 , is properly aligned relative to the signal input from the opening  20 . The tabs  34  on the horn antenna are configured to mate to a video output piece described below. 
       FIG. 2C  shows another embodiment of the horn antenna  14  with an essentially rectangular opening  36  rather than essentially circular opening  20  shown in  FIG. 2B . The opening  20  of  FIG. 2C  also has ridges  22  to provide for wider bandwidth operation. 
     The detector substrate piece  40 , shown in  FIG. 3A  includes a substrate  51  that contains thin-film printed circuit probes  42  and  44  that receive the signal from the horn antenna  14  opening  20 . The circuit probes  42  and  44  provide the signal to a diode  60 , which can be flip-chip mounted onto the substrate  51  or can be wire-bonded to the substrate  51 . The diode detects a video signal and, as shown, video output lines  48  and  50  on the substrate  51  carry the video output from the diode  60  to two video output contact pads  52  and  54  to provide a differential video signal output. Some applications can also have a single video output rather than differential. 
     In one embodiment the diode  60  can be a Sb-heterostructure diode, which allows for unamplified detection of millimeter wave frequencies from 70 GHz to 150 GHz. This reduces the volume otherwise required by a monolithic microwave integrated circuit (MMIC) low-noise preamplifier. The cost of a low-noise preamplifier is also saved. Other diodes that require amplification may also be used. 
     The substrate  51  is laser machined to a shape that matches the shape of recess  35  on the rear of the horn antenna  14 . The dimensions of recess  35  and substrate  51  are well within the cross section of the horn antenna. Thus the substrate that contains the detector has a cross section area that is less than or equal to the cross section area of the horn antenna. 
     The substrate  51  can typically be alumina, quartz, or other millimeter wave substrate materials of the appropriate thickness. The thin-film printed circuit on the substrate  51  is fabricated using commercially available techniques. Plated holes  53  provide electrical interconnects between the top and the bottom of the substrate  51 . 
       FIG. 3B  shows a back short  66  placed over the detector substrate piece  40 , and  FIG. 3C  shows this combination mounted in the recess  35 . The back short  66  is put on top of the detector substrate piece  40  to provide an impedance match and termination for the signal from the horn antenna  14 . The back short  66  can be fabricated as a separate piece of metal and then is attached to the detector substrate piece  40  on plated holes  53 , which provide alignment for the back short  66 , as well as the electrical interconnect discussed above between the top and the bottom of the substrate  51 . The detector substrate piece  40  with the back short piece  66  is then attached into the recess  35  using either solder or conductive epoxy, as shown in  FIG. 3C . This assembly process can be performed using automated pick-and-place machines. 
       FIG. 4  shows one embodiment for a video output adapter and is a half of a cross section along the video output line  48  on the substrate  51  that leads to the video output contact pad  52 . In this embodiment a video output piece  30  is configured to mate with the horn antenna  14  and is placed over the rear of the horn antenna  14 . The cross section area of the video output piece  30  is less than or equal to the cross section area of the horn antenna. 
     A hole  75  is fabricated in the video support piece  30  and is centered on the center of video output contact pad  52 . Teflon or another non-conducting material is formed into a sleeve  78  that is inserted into the hole  75  and then a conductive pin  76 , which can be metal, is placed in the sleeve. The pin can slide in the sleeve and the sleeve  78  insulates the pin  76  from the video output piece  30 . Gentle contact is made between the pin  76  and the video output contact pad  52  by tiny commercially available springs  74 , such as fuzz-buttons. These springs  74  keep constant pressure on between the video output contact pad  52  and the pin  76  to insure a good electrical contact. 
     As shown in  FIG. 4 , the other end of the pin is in contact with a ball  80  of a ball grid array on board  82 . The board  82  contains a ball for each millimeter wave module  11  in the array and printed circuits to route the signals from each ball  80  to a post processor (not shown). 
     Also shown in  FIG. 4  is the connection of the diode  60  to the video output line  48  via flip-chip connection  62 . The opening  20  guides the signal to the circuit probes  42  and (not shown) and the signal is terminated by back short  66  to provide an impedance match. 
       FIG. 5  shows a perspective cross-sectional view of another embodiment of the video output adapter using cable connectors. The video output is similar to that shown in  FIG. 4  in that a spring  74  is between pin  76  and video output contact pad  52 . However, in  FIG. 5 , instead of the pin  76 , insulated by sleeve  78 , contacting a ball in a ball grid array, the pin  76  contacts a cable connector  90 , which is installed in a shroud  96 . The video cable output piece  94  can be mounted on the rear of the horn antenna  14  by screws  98 . The cross section area of the video cable output piece  94  is less than or equal to the cross section area of the horn antenna. 
     For an embodiment with differential video outputs from the detector  60 , two cable connectors  90  and  92  are provided. The cable connectors  90  and  92  can be commercially available snap-on connectors. 
       FIG. 6  shows a perspective view of the assembled module using the video output adapter with cable connectors. The video output as described above can also be via a ball grid array and other connection techniques can also be employed. Since each module is entirely self-contained from millimeter wave input via the horn antenna  14  to the video output, focal plane imaging arrays can be built and scaled to any array size. 
     The module enables low-cost millimeter wave focal plane arrays for millimeter wave imaging cameras. Although the bandwidth of the modules as described by the embodiments above is for millimeter wave frequencies or from 70 GHz to 150 GHz, other operational frequencies can be implemented by scaling the dimensions of this module, along with making appropriate modifications to the waveguide and circuitry. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . . ”