Abstract:
A reconfigurable aperture includes a plurality of metallic particles confined to a volume extending across an aperture area. The metallic particles are repositioned within the volume to form opaque regions in the aperture area. The opaque regions, and transmissive regions between the opaque regions, can form a reconfigurable zone plate that can change the collimation of a microwave beam via diffraction therethrough. The zone plate can be located a fixed distance away from a microwave source and a detector in a housing, so that for any specified wavelength produced by the microwave source, the zone plate can reconfigure to have a focal length equal to the fixed distance. The reconfigurable zone plate can effectively collimate microwaves produced by the microwave source, can direct the collimated microwaves in a specified direction, can receive microwaves returning along the specified direction, and can focus the received microwaves onto the detector.

Description:
TECHNICAL FIELD 
     Examples pertain to a reconfigurable aperture for microwave transmission and detection, in which metallic particles are repositioned dynamically to form opaque regions in the aperture. The opaque regions, and the transmissive regions therebetween, can be sized and shaped as a diffractive zone plate. The reconfigurable aperture can also be applied to communications systems or systems that transmit, receive, and/or steer microwave radiation. 
     BACKGROUND 
     In radar applications, it is common to send a signal away from a radar station along a particular direction, receive a reflected signal at the radar station along the particular direction, and use the time of flight between the sent and received signals to determine a distance away from the radar station of a particular reflecting object. The sent signal can be pulsed, and can be sent with a randomly-varying wavelength, in order to avoid detection. 
     Many current radar systems use active electronically scanned arrays (AESAs). A typical AESA system includes an array of modules that can both transmit and receive signals. The AESA system can steer the outgoing signals by controlling the times at which the modules emit their respective signals. The outgoing signals effectively form a wavefront, and the sensitivity of the AESA is greatest along a direction that is perpendicular to the wavefront; this sensitivity is analogous to a condition of constructive interference for optical phenomena. 
     One potential drawback to AESAs is that they can be complex and expensive. For instance, a typical AESA system can include multiple transmission lines, phase shifter and RF front ends. 
     SUMMARY 
     In a transmission and detection system, a source directs microwaves to a reconfigurable aperture. The reconfigurable aperture can dynamically reposition small (sub-wavelength) metallic particles to appear as a zone plate, with a controllable focal length and a controllable center. The controllable focal length allows the detection system to collimate any suitable wavelength emitted by the source. The controllable center allows the detection system to direct the collimated microwaves outward in any suitable direction from the detection system. 
     In some examples, a reconfigurable aperture includes a plurality of metallic particles confined to a volume extending across an aperture area. The metallic particles are repositioned within the volume to form opaque regions in the aperture area. The opaque regions, and transmissive regions between the opaque regions, can form a reconfigurable zone plate that can change the collimation of a microwave beam via diffraction therethrough. The zone plate can be located a fixed distance away from a microwave source and a detector in a housing, so that for any specified wavelength produced by the microwave source, the zone plate can reconfigure to have a focal length equal to the fixed distance. The reconfigurable zone plate can effectively collimate microwaves produced by the microwave source, can direct the collimated microwaves in a specified direction, can receive microwaves returning along the specified direction, and can focus the received microwaves onto the detector. 
     There are potential advantages to using such a reconfigurable aperture as a collimating/focusing element in a transmission and detection system. For instance, such a reconfigurable aperture can operate over very large operating bandwidths, such as over a frequency range from 300 MHz (corresponding to a wavelength of 1 m) to 300 GHz (corresponding to a wavelength of 1 mm), or a range from 1 GHz (30 cm) to 100 GHz (3 mm). As another example, such a reconfigurable aperture can rely on the physical principles of diffraction and scattering, without using the relatively complicated multiple transmission lines, phase shifter and RF front ends of the AESA systems. As another example, such a reconfigurable aperture can optionally be curved, rather than flat, to allow for conformal geometries. Other advantages are also possible. 
     This summary is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The Detailed Description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1A  is a schematic drawing of an example of a transmission and detection system, including a reconfigurable aperture in a first configuration. 
         FIG. 1B  is a schematic drawing of the transmission and detection system of  FIG. 1A , with the reconfigurable aperture in a second configuration. 
         FIG. 2  is a schematic drawing of an example of a reconfigurable aperture. 
         FIG. 3  is an end-on schematic drawing of an example of a configuration of opaque and transmissive regions in a reconfigurable zone plate. 
         FIG. 4  is a side view schematic drawing of an example of geometry that can define the location of zone edges in a zone plate. 
         FIG. 5  is an end-on schematic drawing of another example of a configuration of opaque and transmissive regions in a reconfigurable zone plate. 
         FIG. 6  is a flow chart of an example of a method of operation of a reconfigurable zone plate. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a schematic drawing of an example of a transmission and detection system, in a first configuration. 
     The system includes a source/detector  110 , which can be optionally packaged as a single unit, or can be packaged as separate units in close proximity to each other. The source/detector  110  includes a microwave source configured to produce microwaves at a selectable specified wavelength within a specified range of wavelengths. Suitable wavelength ranges can include 3 mm to 30 cm, 3 mm to 1 m, 1 mm to 30 cm, 1 mm to 1 m, and other suitable ranges. The source/detector  110  includes a detector that is sensitive in the wavelength range of the microwave source, which can receive detected microwave radiation and convert the received microwave radiation into an electrical signal. 
     The system includes a computer  120 . The computer  120  controls an output wavelength and an output power of the source, and can provide suitable trigger signals to switch the microwave source on and off. The computer  120  receives the electrical signal from the detector. The computer  120  can optionally provide a switching mechanism between the microwave source and the detector. 
     The system includes a reconfigurable aperture  100 . The reconfigurable aperture  100  functions like a lens, which can collimate microwaves emitted by the source and direct the collimated microwaves to a particular target. The computer  120  controls the reconfiguration of the reconfigurable aperture  100 . In the first configuration of  FIG. 1A , the wavelength of the microwaves from the source is λ 1 , and the emission angle from the system is θ 1 . For microwaves emitted by the source that reflect from a particular object, the reflected microwaves retrace their paths along the emission direction, and arrive back at the reconfigurable aperture  100  at angle θ 1 . The reconfigurable aperture  100  focuses the returning microwaves onto the detector at source/detector  110 . The system uses a time of flight delay to determine the distance between the system and the particular object. 
       FIG. 1B  is a schematic drawing of an example of the transmission and detection system, in a second configuration. In this second configuration, the reconfigurable aperture  100  is configured to operate at a wavelength λ 2  different from wavelength λ 1 , and at an emission/detection angle θ 2  different from angle θ 1 . In other configurations, one or both of the wavelength and angle are varied over suitable ranges. The emission/detection angles can additionally extend out of the plane of the page in  FIGS. 1A and 1B , so that a typical range of emission/detection angles includes a range of solid angles less than 2π steradians. 
       FIG. 2  is a schematic drawing of an example of a reconfigurable aperture  200 . 
     The reconfigurable aperture  200  includes a housing  210 . The housing  210  includes an incident face  212  and an opposing exiting face  214 . The housing  210  encloses a volume  216  between the incident face  212  and the exiting face  214 . In some examples, the volume  216  is generally planar. In some examples, the volume  216  may include an overall curvature, such as an outward-bowed curvature that can accommodate a conformal geometry. It is understood that the term generally planar includes such overall curvature, and that the volume  216  has a longitudinal thickness that is less, or significantly less, than a radius or radii of curvature of the volume  216 . The incident face  212  and the exiting face  214  define a longitudinal direction  218  therebetween. The housing  210  is configured to receive and transmit microwave radiation therethrough through the incident face  212  and the exiting face  214 . 
     The reconfigurable aperture  200  includes metallic particles  220  disposed within the volume  216 . The metallic particles are smaller than a wavelength of microwave radiation, and are typically smaller than 1 mm in diameter. Each metallic particle can include a metallic coating on a dielectric core, or can be solidly metallic throughout. The metallic particles  220  attenuate microwave radiation incident thereon, such as by conducting internal electrical currents within the particles  220  that siphon off energy from a transmitted microwave beam, or by direct absorption of the microwave radiation by the metallic particles  220 . 
     The reconfigurable aperture  200  includes a controller  230  configured to reposition the metallic particles  220  within the volume  216  to form opaque regions in the aperture area. The controller  230  can include a plurality of pixels, which can be arranged in a rectilinear configuration or other suitable configuration. When activated by the controller  230 , each pixel is configured to attract metallic particles  220  in the volume  216 . The attracted metallic particles  220  can cluster around the corresponding activated pixels of the controller  230 . A cluster of metallic particles  220  can be sufficiently thick to block microwave radiation, through electrical conduction or absorption. In some examples, the clusters of metallic particles  220  are completely opaque. In other examples, the clusters of metallic particles  220  are partially opaque. The clusters of metallic particles  220  can include one or more transmissive regions therebetween. The transmissive regions can be completely or partially transparent, and can be devoid or largely devoid of metallic particles  220 . Suitable controllers  230  can be electrostatic, electromagnetic, magnetostatic, or can use other suitable technology to dynamically reposition the particles  220 . 
     One possible use for the reconfigurable aperture  200  is to arrange the metallic particles  220  as a reconfigurable zone plate. The zone plate can have an adjustable focal length, which can ensure proper collimation of the exiting beam directed out of the transmission and detection system, for a selected wavelength. The zone plate can also have an adjustable center, which can ensure that the exiting beam points in a selected direction away from the transmission and detection system. 
       FIG. 3  is an end-on schematic drawing of an example of a configuration of opaque regions  340  and transmissive regions  350  in a reconfigurable zone plate. In some examples, the opaque regions  340  and transmissive regions  350  are circular and concentric. In some examples, the opaque regions  340  and transmissive regions  350  are spaced closer together at increasing distances from their centers. In some examples, the opaque regions  340  and transmissive regions  360  have radii that determine a characteristic focal length for a given wavelength. 
     In this example, the regions  340  and  350  are centered within a circular aperture  360  of the zone plate. As a result, microwaves emitted from a source at a centered location behind the zone plate are collimated by the zone plate and emerge perpendicular to the zone plate. In other examples, the aperture can be be elliptical, rectangular, square, polygonal, or other suitable shape. 
       FIG. 4  is a side-view schematic drawing of an example of zone plate geometry, which shows the relationship among zone radii, focal length, and wavelength. The zeroth zone is a central zone that surrounds the longitudinal axis of the zone plate. The first zone annularly surrounds the zeroth zone. The second zone annularly surrounds the first zone, and so forth. For the geometry of  FIG. 4 , the Pythagorean theorem can predict the radius of the nth zone:
 
 r   n =[( n +α)λ f +( n +α) 2 λ 2 /4] 1/2 ,  (1)
 
where λ is the wavelength of the microwave radiation, f is the focal length of the zone plate (which, in the geometry of  FIG. 4 , equals the distance between the zone plate and the image point), and α is a dimensionless reference phase. In general, the zone radii ensure that microwaves from a particular zone are out of phase with microwaves from adjacent zones. In the example of  FIG. 4 , the even-numbered zones are transmissive, and the odd-numbered zones are opaque. In other examples, the even-numbered zones can be opaque, and the odd-numbered zones can be transmissive.
 
     Equation (1) provides a relationship among the zone radii r n , the focal length f, and the wavelength λ. During use in the transmission and detection system, it is desirable that the emergent beam be collimated, for each randomly-selected wavelength. As such, it is desirable to maintain a constant focal length f, for each configuration of the zones. The zone radii r n  can be selected to keep the focal length f as the wavelength λ is varied. 
       FIG. 5  is an end-on schematic drawing of another example of a configuration of opaque and transmissive regions in a reconfigurable zone plate. In this example, the regions  540  and  550  are nested and concentric, but are laterally shifted within the circular aperture  560  of the zone plate. As a result, microwaves emitted from a source at a centered location behind the zone plate are also collimated by the zone plate, but emerge at a particular angle with respect to a longitudinal axis of the zone plate, as in the geometries of  FIGS. 1A and 1B . The tangent of the angular displacement of the emergent microwave beam is given by the lateral displacement of the opaque and transmissive regions  540 ,  550 , divided by the focal length of the zone plate. By positioning the center of the concentric regions away from the center of the zone plate, the emergent microwave beam can be dynamically steered. 
       FIG. 6  is a flow chart of an example of a method of operation  600  of a reconfigurable aperture, such as the reconfigurable aperture of  FIGS. 1-5 . Step  602  produces first microwave radiation at a first wavelength at a microwave source. Step  604  repositions a plurality of metallic particles within an aperture area to form a first plurality of opaque regions in the aperture area. The first plurality of opaque regions and transmissive regions therebetween form a first zone plate. The first zone plate has a focal length at the first wavelength equal to the separation between the microwave source and the aperture area. Step  606  collimates the first microwave radiation with the first zone plate. Step  608  receives a first reflected portion of the first microwave radiation with the first zone plate. Step  610  focuses the first reflected portion onto a detector. Step  612  produces second microwave radiation at a second wavelength at the microwave source. Step  614  repositions the plurality of metallic particles within the aperture area to form a second plurality of opaque regions in the aperture area. The second plurality of opaque regions and transmissive regions therebetween forms a second zone plate. The second zone plate has a focal length at the second wavelength equal to the separation between the microwave source and the aperture area. Step  616  collimates the second microwave radiation with the second zone plate. Step  618  receives a second reflected portion of the second microwave radiation with the second zone plate. Step  620  focuses the second reflected portion onto the detector. 
     In some examples, the first plurality of opaque regions and transmissive regions therebetween have a first center in a plane of the aperture area, the second plurality of opaque regions and transmissive regions therebetween have a second center in the plane of the aperture area, and the first and second centers are laterally separated within the plane of the aperture area. 
     The examples of  FIGS. 1-6  use the reconfigurable aperture as a reconfigurable zone plate. Other uses for the reconfigurable aperture are also possible. For instance, the reconfigurable aperture can be used as a reflective tag/beacon. In a reflective tag/beacon, the pattern of opaque regions and transmissive regions adaptively reconfigures as a function of angle of arrival and frequency, so that it steers a beam back at an interrogator. The reflective tag/beacon can optionally include a spatial data exfil. As another example, the reconfigurable aperture can be used a spoofer, which adaptively responds to an interrogating radar. The spoofer can controllably move the apparent position of an aircraft. As another example, the reconfigurable aperture can be used as a reflector antenna, which can be used for beam steering, and can include piggyback spatial data for exfil. As another example, the reconfigurable aperture can be used to affect the orbital angular momentum of a beam transmitted therethrough, which can be useful for beam encoding for communications. Other uses are also possible. 
     Some embodiments may be implemented in one or a combination of hardware, firmware and software. Embodiments may also be implemented as instructions stored on a computer-readable storage device, which may be read and executed by at least one processor to perform the operations described herein. A computer-readable storage device may include any non-transitory mechanism for storing information in a form readable by a machine (e.g., a computer). For example, a computer-readable storage device may include read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices, and other storage devices and media. In some embodiments, the computer  120  may include one or more processors and may be configured with instructions stored on a computer-readable storage device.