Patent Publication Number: US-7911395-B2

Title: Optically driven antenna

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optically driven plasma, or electron hole concentration, antenna that can be “turned off” when inactive, to render it electrically invisible for reducing scattering or reflecting signature and eliminating coupling and interference with other nearby antennas. The antenna can be reconfigured by geometrically changing the pattern of illumination. 
     BACKGROUND OF THE INVENTION 
     The term plasma antenna has been applied to a wide variety of antenna applications that incorporate the use of an ionized medium. In the vast majority of approaches, the plasma, or ionized volume, simply replaces a solid conductor. A highly ionized plasma is essentially a good conductor, and therefore plasmas can serve as transmission line elements for guiding waves, or antenna surfaces for radiation. The concept is not new. A patent entitled “Aerial Conductor for Wireless Signaling and Other Purposes” was already granted to J. Hettinger in 1919 (U.S. Pat. No. 1,309,031). A more recent prior art is disclosed in the U.S. Pat. No. 6,621,459 B2, “Plasma Controlled Antenna”, by Webb et al, describing a plasma controlled millimeter wave or microwave antenna where a plasma of electrons and holes is photo-injected into a photoconducting wafer, having a reflecting surface behind the wafer allowing the antenna to be generated at low light intensities and a 180 degree phase shift (modulo 360 degrees). This patent describes a way to reconfigure the antenna but it remains electrically visible due to the constant presence of the conducting reflector in the beam path. Another approach is described in the U.S. Pat. No. 5,982,334, “Antenna with Plasma Grating”, by Manasson et al. Nov. 9, 1999, where scanning antennas with plasma gratings is described. The latter includes a semiconductor slab and an electrode set or an illuminating system for injecting plasma grating, enabling beam steering. This system is not electrically invisible when not operating, and is confined to one dimension in steering. 
     There is therefore a need for an optically driven, reconfigurable, plasma antenna that can be “turned off” when inactive, to render it electrically invisible for the purpose of reducing its scattering or reflecting signature and eliminating its coupling and interference with other nearby antennas. 
     SUMMARY OF THE INVENTION 
     It is therefore a broad object of the present invention to provide a geometrically reconfigurable, optically driven, transmitting and receiving plasma antenna that can be “turned off” when inactive, to render it electrically invisible for the purpose of reducing its scattering or reflecting signature and eliminating its coupling and interference with other nearby antennas. 
     It is a further object of the present invention to provide a laser or light emitting diode-fed semiconductor antenna, where the laser or light emitting diode light impinges on a passive semiconductor wafer, serving as a microwave reflector. 
     It is still a further object of the present invention to provide a laser or light emitting diode fed semiconductor antenna, where the laser or light emitting diode light impinges on a passive semiconductor wafer, made of e.g. doped Silicon, Germanium or Gallium Arsenide, serving as a microwave reflector. 
     It is yet a further object of the present invention to provide a laser or light emitting diode fed semiconductor antenna, where the laser or light emitting diode light impinges on a passive semiconductor wafer, serving as a microwave reflector, where the spatial geometrical shape of the impinging light defines the plasma generating area and the reflector shape. 
     It is a further object of the present invention to provide a laser or light emitting diode fed semiconductor antenna, where the passive semiconductor wafer, serving as a microwave reflector, is constituted by a flat, curved or multi facet surface. 
     It is a further object of the present invention to provide a laser or light emitting diode fed semiconductor antenna, or microwave mirror, where the laser or diode light impinges on a passive semiconductor wafer, serving as a microwave reflector, where the timing of the impinging light defines the plasma generating time and the reflector on-off time. 
     It is still a further object of the present invention to provide a laser or light emitting diode-fed semiconductor antenna, where the laser or light emitting diode light impinges on a passive semiconductor wafer, serving as a microwave reflector, where the spatial pattern of the impinging light is defined by a spatial filter, between the light source and the semiconductor wafer. 
     It is yet a further object of the present invention to provide a laser or light emitting diode-fed semiconductor antenna, where the laser or light emitting diode light impinges on a passive semiconductor wafer, where the light source, light spatial filter and microwave source are confined in a microwave absorbing enclosure, thus not being electrically detectible by a microwave probe beam. 
     In accordance with the present invention there is therefore provided an optically driven, transmitting and receiving antenna transformable into an electrically invisible antenna when inactive, comprising a light source, at least one semiconductor wafer illuminatable by said light source, a microwave source or sensor, said wafer having a surface for forming optically induced plasma, or electron hole concentration, assuming a spatial and temporal shape defined by a light beam impinging on it, wherein, upon said wafer being exposed to the microwave beam having a power level sufficient for creating a dense plasma in said wafer, said wafer becomes reflective to microwaves, and returns to transparency when light from said light source is turned off. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood. 
       With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
       In the drawings: 
         FIG. 1  is a schematic, cross-sectional view of a geometrically reconfigurable, optically driven, plasma-transmitting antenna that can be turned off when inactive; 
         FIG. 2  is a schematic, cross-sectional view of a geometrically reconfigurable, optically driven, plasma-receiving antenna that can be turned off when inactive; 
         FIGS. 3   a  to  3   c  illustrate several configurations of a microwave mirror wafer utilized in  FIGS. 1 and 2 ; 
         FIG. 4  is an experimental curve of a tested antenna, and 
         FIG. 5  is another embodiment of the antenna of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     There is shown in  FIG. 1  a schematic, cross-sectional view of a geometrically reconfigurable, optically driven, plasma antenna that can be turned off when inactive. A laser or diode light source  2  emits an optical light beam  4 , shaped spatially by a spatial light filter  6  that can be one of many kinds of reconfigurable spatial filters, e.g., a liquid crystal, and directs it onto an optical mirror  8 . Mirror  8  is, e.g., a dielectric coated glass substrate that does not reflect microwaves. By means of mirror  8  the optical light beam  10  is reflected on e.g., the backside of a microwave mirror wafer  12 . This wafer  12  is a passive semiconductor wafer, made of e.g., doped Silicon, Germanium or Gallium Arsenide, serving as a microwave reflector or mirror when illuminated by beam  10  creating dense electron hole concentration, or plasma, inside the wafer, e.g. 10 18  charges/cubic cm for a 3 cm Wavelength microwave beam. When operating in transmitting mode, the microwave mirror wafer  12  deflects a microwave beam  16  emerging from microwave source  14 , into direction  18 . When not illuminated, microwave mirror wafer  12  is transparent to microwaves, and electrically invisible, reducing its scattering or reflecting signature and eliminating its coupling and interference with other nearby antennas. All the metallic components, light source, spatial filter, microwave source and electronics driving it are packed and shielded in a microwave absorbing enclosure  20 , having a window for microwave transmission, being electrically invisible. 
     In  FIG. 2 , there is shown a schematic, cross-sectional view of a geometrically reconfigurable, optically driven, plasma antenna in a receiving mode, that can be turned off when inactive. The operation is similar to the operation described with respect to  FIG. 1  but here a microwave sensor  22  replaces the microwave source  14 , and the radiation incoming into the enclosure  20  is in direction  24 , and deflected by the microwave mirror wafer  12  on path  26  into the microwave sensor  22 . When not illuminated, microwave mirror wafer  12  is transparent to microwaves, and electrically invisible, reducing its scattering or reflecting signature and eliminating its coupling and interference with other nearby antennas. All the metallic components, light source, spatial filter, microwave source and electronics driving it are packed and shielded in a microwave absorbing enclosure  20 , having a window for microwave transmission, being electrically invisible, reducing its scattering or reflecting signature. 
       FIGS. 3   a  to  3   c  illustrate possible shapes of the microwave mirror wafer  12 , showing at a) a flat geometry, at b) a curved wafer of any curvature and a multi-faceted mirror wafer  12  at c), having any desired number of facets, e.g., square, round or hexagonal shaped facets. 
       FIG. 4  is a experimental curve of a tested antenna, comprising a doped silicon wafer (having a diameter of 15 cm) operated in accordance with  FIG. 1 . When impinged upon by a multi-light emitting diode source (about 850 nm wavelength, peak power of 200 watt) in a pulsed mode, the upper curve is the RF 3 cm wavelength reflection and the lower curve is the light pulse. This curve shows the reflection when light is on and transparency, or no reflection, when light is off. 
     Referring to  FIG. 5 , there is shown a schematic, cross-sectional view of a geometrically reconfigurable, optically driven, plasma, or electron hole concentration, antenna that can be turned off when inactive, in its transmitting mode. The operation is similar to the operation described in  FIG. 1 , but here the light transmission is through a set of optical fibers or waveguides  32 . 
     Optical fibers or lightguides can just as well be used in the arrangement of  FIG. 2 . 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes, which come within the meaning and range of equivalency of the claims, are therefore intended to be embraced therein.