Abstract:
This invention is the Optically Powered and Controlled Non-Foster Circuit (OPCNFC) that is electrically floating; i.e., it does not have any metallic electrical/conductive connection to a power supply, ground, or control signal. Rather power and control signals are applied to the OPCNFC using optical energy. The Non-Foster Circuit (NFC) synthesizes negative inductance, negative capacitance, and/or negative resistance between metallic patches disposed in an array of an Artificial Impedance Surface (AIS).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following U.S. patent applications: (i) U.S. patent application Ser. No. 13/441,659,” entitled “Wideband Tunable Impedance Surfaces” filed Apr. 4, 2012, (ii) U.S. Provisional Patent Application Ser. No. 61/537,488 entitled “Wideband Tunable Impedance Surfaces”, filed Sep. 21, 2011; (iii) U.S. Provisional Patent Application Ser. No. 61/473,076 entitled “Wideband Adaptable Artificial Impedance Surface”, filed Apr. 7, 2011; (iv) U.S. Provisional Patent Application Ser. No. 61/505,037 entitled “Differential Negative Impedance Converters and Inverters with Tunable Conversion Ratios” filed Jul. 6, 2011; (v) U.S. patent application Ser. No. 12/768,563 entitled “Non-Foster Impedance Power Amplifier”, filed Apr. 27, 2010; and (vi) U.S. patent application Ser. No. 13/441,730 entitled “Differential Negative Impedance Converters and Inverters with Tunable Conversion Ratios”, filed Apr. 6, 2012, each of which is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     This disclosure relates to the use of optical sources to provide power and control to Non-Foster Circuits (NFCs). An NFC provides negative inductance, negative capacitance, and/or negative resistance. Non-Foster circuits enable tunable impedance surfaces having improved bandwidths. The term, “tunable impedance surface” is meant to refer to a class of surfaces called Artificial Impedance Surfaces (AISs), Artificial Magnetic Conductors (AMCs) and Frequency Selective Surfaces (FSSs), and this disclosure relates to the use of optical sources to power and control circuits with variable negative inductance in order to provide not only tunability but also a wider bandwidth than known in the prior art. In the tunable impedance surface, the impedance which a wave sees, either a free-space plane wave or an attached surface wave, is variable and has wider bandwidth performance than traditional passive artificial impedance surfaces and prior art passive artificial impedance surfaces loaded with varactors. In particular, this disclosure relates to the use of optical sources to provide power and control for NFCs which provide loading of a traditional passive AIS/AMC/FSS with tunable negative inductors realized with NFC technology. 
     BACKGROUND 
     Conformal and hidden antennas are desirable on many mobile platforms for reasons of aerodynamics and styling, among others. Such antennas have been implemented as or on Artificial Impedance Surfaces (AIS) and have been associated with Frequency Selective Surfaces (FSS). AIS can also be referred to as Artificial Magnetic Conductors (AMC), particularly when a separate antenna is disposed on it. AMC, AIS and FSS are all well known in the art and look very similar to each other which means that persons skilled in the art have not always maintained bright lines of distinction between these terms. AMC, AIS and FSS are generically referred to as impedance surfaces and if they are tunable using active circuits (to generate negative capacitance or negative inductance for example) they are referred to as Active Artificial Impedance Surfaces herein. 
     AIS and AMC tend to have a ground plane which is closely spaced from an array of small, electrically conductive patches. The AIS can serve as an antenna itself whereas an AMC tends to have, in use, a separate antenna disposed on it. Other than the manner of use (and where an antenna is specifically mounted on one), an AIS and a AMC are otherwise basically pretty much identical. The FSS, on the other hand, tends to have no ground plane and therefor it can be opaque (reflective) at certain frequencies and transmissive at other frequencies, much like an optical filter. The FSS looks much like a AMC or an AIS, except that there is typically no ground plane as noted above. All of these devices (AMC, AIS and FSS) operate at RF frequencies and have many applications at UHF and higher frequencies. All of these devices (AMC, AIS and FSS) include two dimensional arrays of metallic patches spaced in a subwavelength periodic grid compared to the RF frequencies at which the devices are designed to operate. The metallic patches come in numerous possible geometric shapes. 
     At VHF and UHF frequencies, however, many relevant platforms which might use AIS/FSS antenna technology are on the order of one wavelength or less in size, which dictates that the antennas be electrically small. Therefore, the performance is limited by the fundamental bandwidth-efficiency tradeoff given by the Chu limit when passive matching is employed. 
     A wideband artificial magnetic conductor (AMC), a special case of an AIS, can be realized by loading a passive artificial magnetic conductor structure with NFCs (i.e. negative inductance and negative capacitance) as suggested by D. J. Kern, D. H. Werner and M. J. Wilhelm, “Active Negative Impedance Loaded EBG Structures for the Realization of Ultra-Wideband Artificial Magnetic Conductor”, in  Proc. IEEE Antennas and Propagation Society Int. Symp.,  2003, pp 427-430. Only simulation results were presented in this paper with ideal NFCs; no details are provided of how to realize the stable NFCs needed in such an application. 
     NFCs (non-foster circuits) are so named because they violate Foster&#39;s reactance theorem and overcome these limitations by canceling the antenna or surface immittance over broad bandwidths with negative inductors or negative capacitors. See the article by Kern mentioned above and also S. E. Sussman-Fort and R. M, Rudish, “Non-Foster impedance matching of electrically-small antennas, “ IEEE Trans. Antennas and Propagat .”, vol. 57, no, 8, August 2009. These non-passive reactive elements are synthesized using Negative Impedance Converters (NICs) or Negative Impedance Inverters (NIIs). NICs are feedback circuits that convert a passive capacitor to a negative capacitor while NIIs are feedback circuits which convert a passive capacitor to a negative inductor. It is also possible to use passive inductors to make negative capacitors and negative inductors using these circuits, but since a passive capacitor is easier to make using semiconductor fabrication techniques, it is assumed herein that a passive capacitor is preferably used to generate a negative inductance (using a NII) or a negative capacitance (using a NIC) as needed herein. 
     The main challenge in realizing NFCs is stability; NICs and NIIs are conditionally stable, and the stability margin typically approaches zero as immittance cancellation becomes more complete. For this reason, few stable demonstrations are reported in the literature at and above VHF frequencies. Sussman-Fort and Rudish noted above and K. Song and R. G. Rojas, “Non-Foster impedance matching of electrically small antennas,”  Proc. IEEE Ant. Prop. Int. Symp ., Jul. 2010 have reported negative-capacitance circuits and measured improvement in the realized gain of electrically small monopole antennas. 
     A well-known class of AIS consists of printed metallic patterns on an electrically thin, grounded dielectric substrate. They can be used to synthesize narrow-band Artificial Magnetic Conductors (AMC) for the realization of low profile antennas as well as suppress surface waves over a narrow bandwidth. They can be made tunable. See, for example, U.S. Pat. No. 6,538,621 to Sievenpiper et al mentioned above. Furthermore, HRL Laboratories of Malibu, Calif. has shown that they can be used to build directional antennas with arbitrary radiation patterns and direct incident energy around obstacles using conformal surfaces with a holographic patterning technique. See the paper noted above by B. H. Fong, et al. entitled “Scalar and Tensor Holographic Artificial Impedance Surfaces”. One issue with the use of NFCs in these arrays is that the power and control wiring to the NFCs can affect the electromagnetic properties of an active AIS system. Furthermore, it can be challenging to run the wires over a distance more than about an inch. Most importantly, this approach does not extend to bulk metamaterials or metasurfaces with no ground plane. 
     Power to the NFCs can be provided by batteries: S. D. Stearns, “Non-Foster circuits and stability theory,” in proceedings, 2011  IEEE Antennas and Propagation Intl. Symposium , pp. 1942-1945. However, batteries are large so that integration into smaller areas such as 1 square millimeter is not practical. In addition, batteries are heavy and are not practical in extreme temperatures and in high shock applications. More importantly, a battery powered NFC cannot be controlled remotely, either to turn it on/off or to vary the circuit parameters. Furthermore, it would be undesirable for an operator to control NFCs manually in an array/AIS environment which may have hundreds of NFCs. 
     In A. Adonin, et al. “Monolith Optoelectronic Integrated Circuit With Built-In Photo-voltaic Supply For Control and Monitoring,” 1998  IEEE International conference on electronics, circuits and systems , vol. 2, pp. 529-531, a low power IC is powered by an integrated PV cell network. This is a low power digital circuit, not an RF circuit. The goal seems to be that it is powered by ambient light. 
     Schaffner, James H. and Jones, Dennis C., “Single fiber optical links for simultaneous data and power transmission,” U.S. Pat. No. 7,941,022, May 10, 2011 describes how to use double core fiber to send power on one optical wavelength and data on another optical wavelength to a remote receiver. In this invention, the use of single mode double core fiber was necessary because of the long length of fiber needed for the application described in the patent. In the present invention, the fiber length is much shorter and the data rates needed for logic control (in some embodiments) is low enough that the double core, single mode fiber is not needed. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This invention is the Optically Powered and Controlled Non-Foster Circuit (OPCNFC) that is electrically floating; i.e., it does not have any electrical/conductive connection to a power supply, ground, or control signal. The Non-Foster Circuit (NFC) generates negative inductance, negative capacitance, and/or negative resistance. The preferred method for generating negative inductance, negative capacitance, and/or negative resistance is using Negative Impedance Converters (NICs) or Negative Impedance Inverters (NIIs) preferably comprising transistor feedback circuits. Alternatively, the NFC may be enabled by diodes or other negative resistance elements. 
     Whereas state of the art NFCs require conductive (e.g. metallic) connections to a power supply and control lines, the present invention is both powered and controlled using electromagnetic energy, preferably over one or more optical fibers or integrated optical waveguides. One or more light sources at some distance from the NFC generate sufficient power to operate the NFC and transmit it to the NFC preferably over an optical waveguide. The power is then converted to Direct Current (DC) using photovoltaic cells. Preferably, control signals are also transmitted to the NFC over optical waveguides; these signals are used to vary the synthesized negative capacitance, inductance, etc. produced by the NFC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    shows an exemplary application of the Optically Powered and Controlled Non-Foster Circuit (OPCNFC) wherein NFCs are connected between metallic patches on a periodic grid on the Active Artificial Impedance Surface. 
         FIG. 1 b    shows a top level schematic of the OPCNFC. The light source module sends both high power and data to the NFC module over the waveguide bus (typically a bundle of optical fibers) to power and control the NFC. RF 1  and RF 2  represent the two connections to metallic patches on the Active Artificial Impedance Surface. 
         FIG. 1 c    shows a single light source module powering and controlling multiple NFC modules. 
         FIG. 1 d    is a plan view of the OPCNFC disposed between two of the metallic patches of shown in  FIG. 1 a   , with the connections to the OPCNFC being depicted in greater detail. 
         FIG. 1 e    is a side elevational view taken along section line  1   e - 1   e  depicted on  FIG. 1   d.    
         FIG. 1 f    depicts an embodiment with a hybrid OPCNFC. 
         FIG. 2  shows an exemplary NFC. 
         FIG. 3 a    shows an optical module providing power to an NFC module through an optical waveguide. 
         FIG. 3 b    shows an NFC module being provided constant power supply voltage and current. A single light source supplies power through an optical waveguide. The light source is modulated with information to control the NFC. 
         FIG. 3 c    shows an NFC module being provided constant power supply voltage and current from a continuous wave light source and being controlled by a separate modulated light source. Both light sources couple to the NFC module by separate optical waveguides. 
         FIG. 3 d    shows an NFC module being provided constant power supply voltage and current from a continuous wave light source and being controlled by a separate modulated light source. Instead of multiplexing the control light, each control signal has its own waveguide in an optical waveguide bus. 
         FIG. 3 e    shows an NFC module being provided power from a continuous wave light source and being controlled by a separate intensity modulated light source. The two light sources, at different wavelengths, are multiplexed (MUX) together into a single optical fiber and demultiplexed at the NFC module to provide power and control signal. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary application of OPCNFCs is the Active Artificial Impedance Surface (AAIS)  8  of  FIG. 1 a    in which NFCs  10  which are preferably set up to serve as negative inductors (see, for example,  FIG. 2 ) are connected between patches  12  which are arranged in a subwavelength periodic grid (which may have a pitch—or a spacing between centers thereof—equal to about λ/10 where λ is the wavelength of impinging electromagnetic (EM) radiation  9  with which the AAIS is meant to interact). The patches  12  of  FIG. 1 a    are depicted as square shaped but those skilled in the art making Artificial Impedance Surfaces know that the patches  12  may assume other geometric shapes (such as triangular shaped patches, hexagonal shaped patches, rectangular shaped patches, etc.). The patches  12  are disposed on a dielectric substrate  11  and are typically (but not necessarily) coupled to a ground plane  13  (disposed on an opposing surfaces of the dielectric substrate  11 —see  FIG. 1 e   ) by a centrally located metallic connection  15  in each (or some) of the patches  12  which connection  15  penetrates and the dielectric substrate  11  and contacts the underlying ground plane  13 . The patches  12 , dielectric substrate  11  and ground plane  13  (if used) may be formed using printed circuit board technologies. 
     Preferably, the NFCs  10  are floating, meaning that they have exactly two RF circuit nodes RF 1  and RF 2  (one of which is connected to one patch  12  and the other of which is connected to a neighboring patch  12 ) with preferably no additional connections to the ground plane or to any lengthy metallic control lines. This is because any additional metallic control wires or bias lines may will tend to adversely affect the electromagnetic properties of the AAIS  8 . For example, it is well known to those of ordinary skill in this art that bias wires that are not perpendicular to the electric field E (see  FIG. 1 d   ) impinging the AAIS  8  will scatter EM waves  9 . One may assert that the NFCs  10  can be biased through the RF nodes, but this not practicable in most cases; first, it is often difficult to realize a potential difference between neighboring patches  12  and, second, attempting to power the NFC  10  itself this way necessitates chokes and DC blocks that may lead to undesirable oscillations. 
     If Y L  is a pure capacitance, then Y NII  is a negative inductance, scaled by R 1 ·R 2 , in parallel with variable resistor R 4 . This tunability requires that at least one additional voltage be applied to the circuit. 
     If Y L  is a pure capacitance, then)(NH is a negative inductance, scaled by R 1 *R 2 , in parallel with variable resistor R 4 . This tunability requires that at least one additional voltage be applied to the circuit. 
     For these reasons, the inventors propose an OPCNFC, an embodiment of which is depicted by  FIG. 1 b   , where the NFC  10  thereof is both powered and controlled by optical energy instead of using relatively conventional metallic wires on the AAIS  8  for power and control functionality. The OPCNFC is preferably formed by a module  16  which in this embodiment includes an NFC  10  (preferably of the type shown in  FIG. 2 , for example) and a photovoltaic (PV) device  14  that converts an received EM wave to DC current at the voltage required to power the NFC  10  preferably mounted on a common substrate  17  (which may be a printed circuit board). Preferably, the OPCNFC module  16  is associated with means to apply control signals to the NFC  10 . These optical power and control signals may be coupled through free-space, but preferably are coupled through a waveguide bus  18 , which is preferably composed of one or more optical waveguides or optical fibers that connect the OPCNFC module  16  with one or more light-source modules  20 . The ground connection shown in  FIG. 2  preferably floats with respect to the ground plane  13 , but is connected to a ground associated with the photovoltaic device  14 . If bus  18  is implemented as an integral waveguide bus then it is preferably embodied as an integrated optical waveguide network disposed on top of or in substrate  11  preferably just below or laterally adjacent the layer defined of the 2D array of patches  12 . 
     The light-source module  20  comprises or connects to a conventional power supply and generates the power and control signals to operate each NFC  10  in each OPCNFC module  16 , as well as an interface  22  for a user to control the NFCs  10  in each OPCNFC module  16 . One of ordinary skill in the art will appreciate that that this interface  22  may be analog voltages or currents or may be digital (e.g. USB) such that it can be controlled by computer. The light source(s) in light-source module  20  may be, for example, lasers or LEDs. Single mode diode lasers are commercially available that put out  100 &#39;s of mW into a fiber or other waveguide in bus  18  and solid state lasers are available commercially that can provide a few watts of power into a fiber or other waveguide in bus  18 . Thus, assuming no loss in the fibers or other waveguides in bus  18  or in a fiber/waveguide to PV cell  14  interface, if it takes about 30 mW to power a single NFC  10 , then a single laser diode can fiber multiplex to about ten NFCs  10  (see  FIG. 1 c   ), while a solid state laser can fiber multiplex to approximately sixty NFCs. Superluminescent LEDs are another option; an LED could provide up to 30 mW into a fiber and could provide power to a single NFC  10 . 
       FIG. 1 d    is a plan view showing the OPCNFC module  16  disposed between two neighboring patches  12  in greater detail. The NFC  10  thereof has two metallic leads RF 1  and RF 2  each of which couple to one of the neighboring metallic patches  12  preferably in a direction parallel to the E field as noted by the arrow on  FIG. 1   d.    
     The OPCNFC module  16  may be implemented as a hybrid circuit where the NFC  10  and optoelectronics  14  may be two or more separate components packaged together on a single printed circuit board (PCB)  17  or other small package or as an optoelectronic integrated circuit (OEIC), where the entire circuit is preferably combined on a single semiconductor die for both the NFC  10  and the optoelectronics  14 . If the NFC  10  and the optoelectronics  14  are disposed on separate semiconductor dies then relatively short metallic interconnections J (formed by metallic traces, leads or jumpers) will need to be made between the two dies. Metallic interconnections J are preferably disposed perpendicular to the direction of E field as noted by the arrow. If the NFC  10  and the optoelectronics  14  are disposed on a single semiconductor die then relatively short metallic interconnections J occur on or within the die and a separate substrate  17  may then not be needed, rather the single die bearing the NFC  10  and the optoelectronics  14  then becomes the aforementioned OPCNFC module  16  as is depicted by the embodiment of  FIG. 1   f.    
       FIG. 1 e    is a side sectional view taken along line  1   e - 1   e  depicted on  FIG. 1 d   . Holes H may be optionally formed through substrate  11  (and through the underlying ground plane  13  if used and layer  11   a  discussed below) adjacent each OPCNFC module  16  (and between neighboring patches  12 ) as a passage for each fiber of a fiber embodiment of the waveguide bus  18  so that the fibers of bus  18  do not significantly impair interaction between the impinging EM radiation and the AAIS  8 . Alternatively the fibers or waveguide(s)  18  may be simply disposed on or over the top surface of the AAIS  8  but in that case then preferably arranged to fall or lie upon (or be affixed to the top surface of the AAIS  8 ) and preferably between the patches  12  to limit any interference with EM radiation  9  (see  FIG. 1 a   ). And as noted above the fibers or waveguide(s)  18  may be implemented as an integrated optical waveguide network disposed on top of or in substrate  11  preferably just below or laterally adjacent the layer defined of the 2D array of patches  12 . 
     Dielectric substrate  11  may be formed by a printed circuit board with patches  12  being formed by patterning one metallic surface of such a printed circuit board. The distance t between patches  12  and the ground plane  13  is preferably about λ/40 where λ is the wavelength of impinging EM radiation  9 . If the dielectric layer  11  of a printed circuit board (for example) is not sufficiently thick, than an additional layer of dielectric foam (or any other dielectric material including air or even a vacuum)  11   a  may be employed so that the desired thickness t of all of the dielectric region between the patches  12  and the ground plane  13  is preferably attained. 
     Several embodiments of the NFC module  16  are shown in  FIGS. 3 a -3 e   , where common reference numerals are used to identify common or similar components used in the described embodiments. 
     In the embodiment of  FIG. 3 a   , the NFC  10  is designed such that its parameters are a function of the NFC bias current. An exemplary NFC  10  is depicted by  FIG. 2  where the resistors are all fixed and the load is a pure capacitance C. The NFC  10  is then generates a negative inductance given by Y NII =−K 2 /Y L  where K 2 =g m   2 /[(2+g m R 1 )(2+g m R 2 )] and g m  is the transconductance of the transistors which is directly proportional to the bias current. In this embodiment, the waveguide bus  18  can be a single optical waveguide and the light source is a single source with variable output power. Thus the NFC  10  of  FIG. 3 a    is controlled by the power of light source  24 . 
     In the embodiment of  FIG. 3 b   , the NFC  10  has a constant power supply voltage and current and is controlled by separate voltages or currents. A single light source  24  supplies power and is modulated by MUX/MOD  26  with information to control the NFC  10 . Control signals for multiple parameters may be multiplexed on the data stream supplied via optical waveguide bus  18 . In the NFC module  16  of this embodiment, the modulated signal supplied via the optical waveguide is filtered by a filter  30  to supply power to the NFC  10  and is also sent to a demodulator  32  where it is demultiplexed. Finally, one or more digital to analog converters (DACs)  34  generate the physical control signals applied to NFC  10  to control, for example, R 1  and R 2 . 
     In the embodiment of  FIG. 3 c   , the NFC  10  has a constant power supply voltage and current and is controlled by separately generated voltages or currents. A continuous wave light source  24 - 1  transmits a relatively high power EM wave over a first optical waveguide or fiber  18 - 1  to power the NFC module  16 , which high power EM wave is converted to DC by PV cell  14  and filter  30 . A separate light source  24 - 2  (which may be a low power communications laser or LED) transmits the relatively low power control signals over a separate waveguide or fiber  18 - 2 , which are then detected by detector  36  and demodulated, demultiplexed, and converted into analog control signals by DeMod/DeMUX  32  and DAC  34  for controlling R 1  and R 2  of the NFC  10  of  FIG. 2 . 
     The embodiment of  FIG. 3 d    is similar to the architecture of the embodiment of  FIG. 3 c    except that instead of multiplexing the control signals, each control signal has its own waveguide or fiber in bus  18 - 3 , which eliminates the need for a DeMod/DeMUX in module  16  but adds the need for a plurality of relatively low power light sources  24 - 3  in module  20  and associated detectors  36  in module  16  (one for each desired unique setting of R 1  and R 2  in the NFC  10  of  FIG. 2 ). 
     Oftentimes a high power source has no modulation capability. In the embodiment shown in  FIG. 3 e   , two light sources are used, one high power ( 24 - 1 ) and the other low power ( 24 - 2 ) but capable of intensity modulation. Intensity modulations can be achieved by directly modulating a semiconductor laser current or by using an external modulator with a fixed wavelength laser. These two light sources  24 - 1  and  24 - 2 , at different wavelengths, are multiplexed (MUX) together by an Optical MUX  28  into a single optical fiber  18 - 3 , which can be of a double core design for high data rates, or of a multi-mode design for lower data rates. Data rates using multi-mode fiber range from 10 Mbps to 10 Gbps, depending upon the length of fiber (for example 1 Gbps can be transmitted through 1 km of graded index fiber with little dispersion—John Gower,  Optical Communications Systems,  2 nd  edition, Prentice Hall, 1993). At the NFC module  16  the high power and data signals are demultiplexed (DEMUX) by an Optical DEMUX  38 ; the high power signal goes to a PV cell  14  and the data signal goes to a high speed photodetector  36  for demodulation of the control signals. The optical isolator  29  at the source and the optical filter  40  at the NFC Module  16  are optionally used to protect the data laser  24 - 2  and detector  36  from damage from any high power light that leaks through the OMUX  28  and ODEMUX  38 . 
     This concludes the description including preferred embodiments of the present invention. The foregoing description including preferred embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible within the scope of the foregoing teachings. Additional variations of the present invention may be devised without departing from the inventive concept as set forth in the following claims.