Abstract:
A photonics sensor and array for the reception and processing of RF signals. In one embodiment, the present invention is an antenna comprising: a first electro-optically active optical waveguide; a first planar electrode substantially parallel to the first waveguide; a second electro-optically active optical waveguide; a second planar electrode substantially parallel to the second waveguide, the first and second planar electrodes being substantially adjacent and coplanar; and a third planar electrode substantially parallel to the first and second planar electrodes and disposed such that the first waveguide lies between the first and third planar electrodes, and the second waveguide lies between the second and third planar electrodes. In another embodiment, the present invention is an antenna comprising: first and second planar electrodes being substantially adjacent and coplanar; a first electro-optically active optical waveguide disposed between the planar electrodes; and a second electro-optically active optical waveguide substantially parallel to the first waveguide.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to photonic sensors, and more particularly to electro-optic antennas and sensors for wideband reception and processing of electromagnetic signals. 
     2. Related Art 
     Array antennas for reception and transmission of electromagnetic signals are well known in the art. One important objective in the refinement of this type of antenna is to increase the operational bandwidth of the antenna. 
     Traditional arrays use conventional antenna elements. One disadvantage of this type of element is that it is usually limited to a small operational bandwidth. Further, these elements require some sort of transmission line, such as coaxial cable, microstrip, or stripline, to connect to each antenna element and a feed network. For antenna arrays having many elements, this approach results in structures that are quite heavy and large. Another disadvantage that results from this approach is an increase in backplane complexity. A further disadvantage is the substantial signal losses that are incurred on the transmission lines, as the desired frequency of operation is increased. 
     One conventional approach to increasing bandwidth is to use a “spiral” antenna element within the array. One disadvantage of this approach is that such spiral elements become large as the frequency of operation is reduced. Further, the spacing of these elements increases, resulting in a physically large structure. Further, this large spacing has adverse effects on the operation of the array. A further disadvantage is the backplane complexity mentioned above. 
     Another conventional approach is to use electrically small antennas to get around the spacing issue. However, the efficiency of this class of antennas is usually very poor. 
     SUMMARY OF THE INVENTION 
     The present invention is a photonics sensor and array for the reception and processing of RF signals. In one embodiment, the present invention is an antenna comprising: a first electro-optically active optical waveguide; a first planar electrode substantially parallel to the first waveguide; a second electro-optically active optical waveguide; a second planar electrode substantially parallel to the second waveguide, the first and second planar electrodes being substantially adjacent and coplanar; and a third planar electrode substantially parallel to the first and second planar electrodes and disposed such that the first waveguide lies between the first and third planar electrodes, and the second waveguide lies between the second and third planar electrodes. 
     In another embodiment, the present invention is an antenna comprising: first and second planar electrodes being substantially adjacent and coplanar; a first electro-optically active optical waveguide disposed between the planar electrodes; and a second electro-optically active optical waveguide substantially parallel to the first waveguide. 
     In another embodiment, the present invention is an antenna comprising: a plurality of cells, each cell comprising: a first electro-optically active optical waveguide; a first planar electrode substantially parallel to the first waveguide; a second electro-optically active optical waveguide; a second planar electrode substantially parallel to the second waveguide, the first and second planar electrodes being substantially adjacent and coplanar; and a third planar electrode substantially parallel to the first and second planar electrodes and disposed such that the first waveguide lies between the first and third planar electrodes, and the second waveguide lies between the second and third planar electrodes. 
     In another embodiment, the present invention is an antenna comprising: a plurality of cells, each cell comprising: first and second planar electrodes being substantially adjacent and coplanar; a first electro-optically active optical waveguide disposed between the planar electrodes; and a second electro-optically active optical waveguide substantially parallel to the first waveguide. 
     An optical source may be coupled to a first end of each of the waveguides. An output optical waveguide may be coupled to the second end of each of the first and second waveguides. A photodetector may be coupled to the output waveguide. A coupler may electrically connect the first and third planar electrodes, whereby the first and third planar electrodes may be kept at substantially the same electrical potential. The present invention may further comprise a polymer layer in which the waveguides are formed and to which the planar electrodes are attached. The first planar electrode may be arranged so that an incident electromagnetic signal will impinge upon the first planar electrode. 
     The third planar electrode may comprise a first portion and a second portion and may be disposed such that the first waveguide lies between the first planar electrode and the first portion of the third planar electrode, and the second waveguide lies between the second planar electrode and the second portion of the third planar electrode. 
    
    
     Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE FIGURES 
     The present invention will be described with reference to the accompanying drawings. 
     FIG. 1 depicts a sampler array according to a preferred embodiment of the present invention. 
     FIG. 2 is a frontal view of a portion of a sampler array corresponding to a single sampler “cell” according to one embodiment of the present invention. 
     FIG. 3 presents a cross-section of a portion of the sampler array of FIG.  2 . 
     FIG. 4 depicts a portion of a sampler array according to another embodiment of the present invention. 
     FIG. 5 depicts a cross-section of a portion of the sampler array of FIG.  4 . 
     FIG. 6 depicts a portion of a sampler array according to an embodiment of the present invention. 
     FIG. 7 presents a cross-section of a portion of a sampler array according to another embodiment of the present invention. 
     FIG. 8 is a simplified depiction of the operation of the sampler array shown in FIG.  3 . 
     FIG. 9 is a simplified depiction of the operation of the sampler array shown in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is described in terms of the above example. This is for convenience only and is not intended to limit the application of the present invention. In fact, after reading the following description, it will be apparent to one skilled in the relevant art how to implement the present invention in alternative embodiments. 
     The present invention is a photonics sensor and array for the reception and processing of electromagnetic signals. It is especially useful for the reception of broadband signals and processing of that signal to extract information contained in the signal such as in active imaging, or as in synthetic aperture radar applications. It is also useful in bistatic and passive imaging. 
     FIG. 1 depicts a sampler array  100  according to a preferred embodiment of the present invention. Sampler array  100  includes a plurality of antenna elements  102 , a dielectric support  106 , and optical fibers  108 ,  112 . In a preferred embodiment, antenna elements  102  (also referred to as “radiators” ) are metallic strips (also referred to as “planar electrodes”) printed upon a polymer sheet, although other materials or antenna elements may be used. Sampler array  100  also includes a plurality of Mach-Zehnder modulators (not shown); each centered underneath the gap between a pair of adjacent antenna elements  102 . A metallic coupling strip (not shown) resides below each Mach-Zehnder modulator, extending underneath each arm of the Mach-Zehnder modulator, and together with a pair of antenna elements  102  forms a pair of capacitors, where each arm of the modulator lies within one of the capacitors. The sampler array  100  may can include more or less elements than depicted in FIG.  1  and may be configured to form a 2-dimensional or planar array. 
     Each Mach-Zehnder modulator is stimulated by an optical source via an input fiber  108 . In a preferred embodiment, the optical source is a laser. An electromagnetic wavefront  114 , impinging on the sampler array  100 , will generate a field across the sampler array  100  which will in turn set up a voltage across each gap between adjacent antenna elements  102  and between each antenna element  102  and a corresponding coupling strip. This voltage modulates the optical drive signal provided by input fibers  108 . Output fibers  112  are fed to a photodiode or the like, where the signal may be recovered according to conventional methods. This condition is repeated across the entire structure  100  and effectively samples the electromagnetic wavefront  114 , which can then be reconstructed. By keeping the antenna elements  102  small, the response bandwidth of the sampler array  100  can be made very large. 
     In a preferred embodiment, one antenna element  102  in each pair of antenna elements is held to the same voltage potential as the corresponding coupling strip. In addition, a DC bias can be applied to the other antenna element in the pair to bias the Mach-Zehnder modulator at its quadrature point or any other point that is desired. 
     FIG. 2 is a frontal view of a portion of sampler array  100  corresponding to a single sampler “cell”  200  according to one embodiment of the present invention. The sampler cell includes two antenna elements  208 A and  208 B, a coupling strip  214 , and a pair of optical waveguides  206  and  206 ′, which form the “arms” of a Mach-Zehnder modulator. Each arm  206  lies between one of the antenna elements  208  and coupling strip  214 , which effectively forms a pair of capacitors, where each arm  206  of the modulator lies between the plates of one of the capacitors. Other coupling configurations or schemes are contemplated. In a preferred embodiment, one antenna element  208  is tied electrically to coupling strip  214  to bring them to the same electrical potential, while the other antenna element has a DC bias applied to it, to bias the modulator at a desired operating point. The Mach-Zehnder modulator includes an optical input channel  202 , which receives the optical drive signal provided by an input fiber  108 . The optical input signal is split into two optical paths  204  and  204 ′. The optical signals pass beneath antenna elements  208 A and  208 B in optical channels  206  and  206 ′. Referring to FIG. 2, assume that antenna element  208 B is electrically tied to coupling strip  214 . The RF field that impinges on antenna elements  208  will then induce a varying voltage potential between the “floating” antenna element  208 A and coupling strip  214 . That voltage will advance or retard the optical signal in intervening optical path  206 , changing its phase relative to “tied” optical path  206 ′. The optical signals exit the modulator on paths  210  and  210 ′, and are combined, producing a modulated output optical signal  212 . 
     FIG. 3 presents a cross-section of a portion  300  of one embodiment of sampler array  100 , which corresponds to section I—I of FIG.  2 . Portion  300  includes antenna elements  308 A, B, C, D, which are mounted upon body  302 . Body  302  includes polymer layers  320 ,  322 , and  324 . Each of layers  320 ,  322  and  324  is approximately 3 micrometers thick, and has a dielectric constant of 3.4 in a preferred embodiment. Within layer  322 , optical waveguides are formed and represent the core. Polymer layer  324  adjoins a layer  326  of SiO 2  having a thickness of 2.0 micrometers and an epsilon of 3.9 preferably. Layers  320  and  324  effectively become the cladding. Layer  326  adjoins a silicon substrate having a thickness of 10-20 mils, an epsilon of 12, and a rho of 3000 ohm-centimeters. In a preferred embodiment, the electro-optic polymer is a two component material consisting of 15% (by weight) of the chromophore 4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) in the partially-fluorinated polyimide polymer ULTRADEL 4212®, available from BP Amoco Chemicals Inc., Warrensville Heights, Ohio. Although the construction has been described using polymer materials, any suitable electro-optic material may be used to form body  302 . Also in a preferred embodiment, antenna elements  308  measure approximately 1 inch on each edge and are separated from each other by a gap measuring between 100 micrometers and 2 mils. Variations on these dimensions may be made to optimize or customize the performance or operation of the present invention. 
     Layer  322  includes a plurality of optical paths. In particular, the optical paths include paths  306 B and  306 ′B, which form the branches of a single Mach-Zehnder modulator  340 . Layer  326  includes a plurality of coupling strips  314 . Coupling strip  314 B forms a part of Mach-Zehnder modulator  340 . In a preferred embodiment, portion  300  is repeated to form an array. Therefore, optical paths  306 A,  306 ′A,  306 C and  306 ′C, as well as antenna elements  308 A and  308 D and coupling strips  314 A and  314 C are shown for clarity. These elements form portions of other Mach-Zehnder modulators, as would be apparent to one skilled in the relevant art. Coupling strips  314 A and  314 C form portions of other Mach-Zehnder modulators. 
     In operation, the potential induced by electromagnetic energy  114  upon an antenna element  308  with respect to a coupling strip  314  modulates the optical signal on an intervening optical path  306 . In particular, the phase of the optical signal changes in accordance with the magnitude of the potential. Referring to Mach-Zehnder modulator  340 , when a differential potential exists between antenna element  308 B and coupling strip  314 B, and when antenna element  308 C and metallic strip  314 B are tied electrically together, such they are at the same potential, the optical signal traversing optical path  306 B is modulated to have a different phase than optical path  306 ′B. When these optical signals are again joined, an interference pattern results and thus the optical signal becomes amplitude modulated. This amplitude modulated optical signal exits Mach-Zehnder modulator  340  along an output fiber  112 . 
     FIG. 4 depicts a portion  400  of a sampler array according to another embodiment of the present invention. In this configuration, Mach-Zehnder modulators  440  have been rotated 90 degrees relative to the surface of the array, as compared to the array of FIG.  1 . Portion  400  includes four Mach-Zehnder modulators  440 A, B, C, D. Mach-Zehnder modulator  440 A is exemplary. Mach-Zehnder modulator  440 A includes antenna elements  408 A and  408 B, optical path  406 A, and optical path  406 ′A. Optical path  406 ′A is embedded within a material  430 . In a preferred embodiment, material  430  is the same polymer material used to form the optical waveguides, and loaded with a chromophore to make it electro-optic. Antenna element  408  is formed by depositing metallic strips onto material  430 . 
     A chromophore is a class of materials that exhibits an “electro-optic” effect. It is through this electro-optic effect that we can manipulate the light that passes the material, as is well known in the relevant arts. For example, an electrical voltage, when applied to an electro-optic material, will alter its optical characteristics, such as its index of refraction. In a preferred embodiment, a chromophore material is embedded in a portion of a polymer layer to create the “core” of an electro-optic waveguide. 
     FIG. 5 depicts a cross-section of a portion  500  of the sampler array of FIG. 4 corresponding to section II—II in FIG.  4 . An optical signal enters input optical path  502 , and is split into two portions. One portion traverses the “modulated” arm defined by optical paths  504 ,  506 , and  510 . The other portion traverses the “unmodulated” arm defined by optical paths  504 ′,  506 ′, and  510 ′. The optical signal in the modulated arm passes between a pair of antenna elements  508 , and so is modulated by the differential potential induced upon the antenna elements by an impinging wavefront. The optical signal traversing the unmodulated arm experiences no differential electrical potential, and so is not modulated. When the modulated and unmodulated signals are joined in output optical path  512 , an interference pattern results, producing amplitude modulation of the optical carrier. The resulting signal can be processed as described above. 
     FIG. 6 depicts a portion  600  of a sampler array according to an embodiment of the present invention. In this configuration, as in the configuration of FIG. 4, a Mach-Zehnder modulator has been rotated 90 degrees relative to the surface of the array, as compared to the array of FIG.  1 . In this embodiment, there are at least six layers. Starting from the bottom, portion  600  includes a silicon layer  628  that serves as a base onto which the other layers are deposited, a polymer dielectric layer  626 , a polymer dielectric layer  624  that is photobleached, and into which an optical waveguide  606  is formed, a polymer layer  622 , a polymer layer  620  that is photobleached, and into which an optical waveguide  606 ′ is formed, and onto which metallic strips  608 A,B are deposited; and a final polymer layer  618  that covers metallic strips  608  and forms the final waveguide. Other embodiments of the invention are constructed in a similar fashion. 
     Photobleaching is a method used to change a material&#39;s properties through the use of light. Predetermined areas of the material are exposed to light at various wavelengths and strengths to change that material properties, for example, to permanently change the index of refraction. In a preferred embodiment, a “mask” is placed over the material to allow selective photobleaching of predetermined areas of the material. In general, the section of a polymer layer that is to become the “cladding” of a waveguide is photobleached to have a lower index of refraction (for example, n˜1.60) than the core (for example, n˜1.62). This condition allows light to travel down the waveguide (through the core) without radiating out through the cladding material, as is well known in the relevant arts. 
     FIG. 7 presents a cross-section of a portion  700  of one embodiment of sampler array  100 , which corresponds to section I—I of FIG.  2 . Portion  700  is similar to portion  300 , shown in FIG.  3 . Thus, portion  700  includes antenna elements  708 A, B, C, D, which are mounted upon body  702 . Body  702  includes polymer layers  720 ,  722 , and  724 . Layer  722  includes a plurality of optical paths. In particular, the optical paths include paths  706 B and  706 ′B, which form the branches of a single Mach-Zehnder modulator  740 . Layer  726  includes a plurality of coupling strips  714 . 
     In contrast to portion  300 , in the embodiment shown in FIG. 7, each coupling strip, such as coupling strip  714 B, is divided into two portions, such as coupling strip portions  714 B- 1  and  714 B- 2 . As shown, the first optical path  706 B is disposed between antenna element  708 B and the portion  714 B- 1  of coupling strip  714 B, while the second optical path  706 ′B is disposed between antenna element  708 C and the portion  714 B- 2  of coupling strip  714 B. Coupling strip  714 B forms a part of Mach-Zehnder modulator  740 . In a preferred embodiment, portion  700  is repeated to form an array. Therefore, optical paths  706 A,  706 ′A,  706 C and  706 ′C, as well as antenna elements  708 A and  708 D and coupling strips  714 A and  714 C are shown for clarity. These elements form portions of other Mach-Zehnder modulators, as would be apparent to one skilled in the relevant art. Coupling strips  714 A and  714 C form portions of other Mach-Zehnder modulators. 
     In operation, the potential induced by electromagnetic energy  114  upon an antenna element  708  with respect to a coupling strip  714  modulates the optical signal on an intervening optical path  706 . In particular, the phase of the optical signal changes in accordance with the magnitude of the potential. Referring to Mach-Zehnder modulator  740 , when a differential potential exists between antenna element  708 B and coupling strip  714 B, and when antenna element  708 C and metallic strip  714 B are tied electrically together, such they are at the same potential, the optical signal traversing optical path  706 B is modulated to have a different phase than optical path  706 ′B. When these optical signals are again joined, an interference pattern results and thus the optical signal becomes amplitude modulated. This amplitude modulated optical signal exits Mach-Zehnder modulator  740  along an output fiber  112 . The embodiment shown in FIG. 7 increases the interaction voltage across the electro-optically active path by changing the primary direction of the voltage fields. An example of the voltage fields generated in the embodiment of FIG. 3 is shown in FIG.  8 . The voltage field  802 , which interacts with optical path  306 B is spread over a wide area and is thus significantly diffused. By contrast, the voltage field of the embodiment shown in FIG. 7, as shown in FIG. 7, is concentrated in the optical path  706 B. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be placed therein without departing from the spirit and scope of the invention. Thus the present invention should not be limited by any of the above-described example embodiments, but should be defined only in accordance with the following claims and their equivalents.