Abstract:
An antenna system for receiving both RF wave and optical wave radiation via a single antenna aperture that may be moved between stowed and deployed configurations as needed. The system includes a wavefront correction system for correcting optical wavefront distortion errors caused by anomalies in the shape of the antenna aperture itself, as well as optical wavefront distortion errors caused by atmospheric perturbations. The optical components used for optical signal conditioning are supported from the antenna aperture and form a compact, unobtrusive means for separating electromagnetic and optical wave signals received by the antenna aperture.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     This invention relates to antenna systems, and more particularly to a deployable antenna system able to receive and transmit radio frequency (RF) and optical frequency radiation, and to correct for wavefront distortion in the received optical radiation caused both by geometric anomalies in the antenna aperture itself, as well as wavefront distortion caused by atmospheric factors.  
       BACKGROUND OF THE INVENTION  
       [0002]     Optical communications are rapidly becoming the preferred approach for secure, high-bandwidth communications. The size and weight of large optical structures (lenses or mirrors, including mounts) suited for optical communications has largely precluded their mobile, individual use. The optical equipment has not supported RF communications, forcing users to also carry RF equipment, too.  
         [0003]     A compact, lightweight, field deployable antenna system of the present invention enables both optical and RF bidirectional communications (transmit and receive) from a single antenna aperture. Such an antenna may also be stowed for transport.  
       SUMMARY OF THE INVENTION  
       [0004]     Therefore, the present invention is directed to an antenna system capable of simultaneously receiving and/or transmitting both radio frequency (RF) and optical radiation signals via a single antenna aperture. The antenna aperture can be transformed from a non-operative configuration where it presents a compact, easily transported structure, to an operative configuration in which it can receive both RF and optical signals. The antenna system, in one preferred form, includes an optical wavefront correction system for correcting for wavefront distortion in a received optical signal that is caused by small static anomalies in the surface geometry of the antenna aperture itself. In another preferred form the wavefront correction system includes, for example, a Shack-Hartmann wavefront sensor to detect dynamic antenna surface shape errors and atmospheric distortion. The wavefront sensor monitors the performance of the wavefront correction system and provides feedback control signals so that the wavefront correction system can achieve near diffraction limited performance under closed loop control.  
         [0005]     In the various preferred embodiments, a narrow band optical filter is used to receive an output from the wavefront correction system and to pass only a desired bandwidth of the optical signal. In still another alternative preferred form, a wavefront division mutliplexing subsystem may be incorporated to provide a plurality of demultiplexed optical outputs from the output of the wavefront correction system.  
         [0006]     In one preferred form the wavefront correction system comprises a static wavefront corrector formed by a computer-generated holographic optical element (CGHOE). In another preferred form the wavefront correction system includes a programmable, spatial light modulator that functions as a dynamic wavefront corrector to compensate for wavefront distortion caused by dynamic changes in the shape of the antenna and/or rapidly changing atmospheric abnormalities affecting the received optical signal.  
         [0007]     In the preferred embodiments the wavefront correction system is supported closely adjacent to the antenna aperture of the antenna system. The overall antenna system forms a compact structure which can easily be carried by an individual and quickly and easily deployed for use when needed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a diagramatic view of an antenna system in accordance with a preferred embodiment of the present invention, with an antenna aperture of the system in a deployed, ready-for-use, configuration;  
         [0010]      FIG. 2  is a view of the antenna system of  FIG. 1  but with the antenna aperture illustrated in a stowed, non-usable configuration;  
         [0011]      FIG. 3  is a block diagram of the components of the wavefront correction system; and  
         [0012]      FIG. 4  is a view of an antenna system in accordance with an alternative preferred embodiment of the present invention incorporating a wavefront division multiplexing subsystem. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0014]     Referring to  FIG. 1 , there is shown an antenna system  10  in accordance with a preferred embodiment of the present invention. The antenna system  10  operates to receive both RF and optical radiation via a single antenna aperture  12 . In referring to “RF” radiation it is meant electromagnetic wave radiation having a frequency of typically between about 1 GHz and 50 GHz or lower. The antenna aperture  12  is shown in  FIG. 1  in its operative position ready for use.  FIG. 2  illustrates the system  10  with the antenna aperture  12  in a stowed configuration for transport, making the system  10  much more easy and convenient to carry.  
         [0015]     The antenna aperture  12  includes a reflector  13  made from suitable materials such as metalized mylar or stiff metalized molded plastic. The reflector  13  includes an axial center  14  at which is disposed a tubular light baffle/waveguide  16  extending perpendicularly therefrom and securely coupled to the area of the reflector  13  at the axial center  14 . A frame structure  12   a  having a plurality of spokes extending radially from the tubular light baffle/wave guide  16 , similar to that used on a conventional umbrella, supports the metalized mylar or other reflector material used to form reflector  13 . A collimator  18  is disposed at one end of the light baffle  16  and receives reflected RF and optical radiation from the aperture  12  when the antenna system  10  is receiving RF and optical signals. The collimator  18  directs received RF and optical radiation through the tubular light baffle/waveguide  16  to a signal splitter  20 . The signal splitter  20  is disposed closely adjacent to a rear surface  22  of the antenna aperture  12  and splits the RF component of the received signal from the optical component. An RF detector  24  detects the RF radiation component of the incoming signal and transmits same to appropriate RF signal processing electronics  26  for processing. An RF transmitter  24   a  generates RF energy that is applied to the aperture  12  when the aperture is used as an RF energy transmitter.  
         [0016]     The signal splitter  20  incorporates a beamsplitting mirror  20   a  for transmitting the optical radiation into a wavefront correction system  28 . The signal splitter  20  also incorporates an optically transparent conducting material. Such materials are routinely used in photonics devices. A thin film of indium-tin oxide deposited on a fused silica substrate is representative of such a material. The precise material is preferably chosen for highest RF reflectivity and highest optical transparency at the desired frequency of the antenna, which in this example is preferably at least about 1 GHz.  
         [0017]     The wavefront correction system  28  compensates for both wavefront distortion errors caused by geometric surface anomalies in the antenna aperture  12 , as well as wavefront distortion caused by changing atmospheric conditions to enable the antenna system to provide near diffraction limited performance. An output  30  of the wavefront correction system  28  is a near diffraction limited beam capable of being focused to a “spot” having a diameter of preferably less than 50 micrometers (microns), commensurate with the size of the active region of a high-speed optical detector. “Near diffraction limited” means that the size of the focused spot formed by the optical output signal is near the theoretical limit that can be produced. Optical output signal  30  may then be transmitted to a suitable optical detector  32  for further signal processing. Also, an optical emitter  32   a  could be incorporated if the antenna system  10  is to be bi-directional.  
         [0018]     In a preferred embodiment, the antenna system  10  further comprises a wavefront sensor  34  that monitors the output  30  of the wavefront correction system  28 , in real time, and which provides a feed back signal to the wavefront correction system  28  indicative of corrections needed to maintain the optical signal being output from the system  28  at the desired  50  micrometer spot size. One preferred wavefront sensor  34  is a Shack-Hartmann wavefront sensor that provides real-time monitoring of the optical output signal being generated by the wavefront correction system  28 . Such a sensor is available from Spot-Optics, inc. of Padova, Italy. In operation, a portion of the collimated beam is directed by a beamsplitter into the Shack Hartmann sensor. In a typical Shack Hartmann sensor, a two-dimensional array of micro lenses focuses the collimated beam onto a two-dimensional sensor array (typically a CCD array) located at the focal position of the microlens array. The position of each microlens within the collimated beam is representative of a similar position across antenna aperture  22 . The position of each focal spot on the detector array will vary in accordance with angular distortions within representative positions of the collimated beam due to atmospheric turbulence and/or dynamic variations of antenna aperture  22 . The deviations of the focal spots from their nominal positions can be typically measured to 1/20 th of a pixel. This information is processed and used as the feedback signal for exercising the wavefront correcting capability of the SLM, with the goal of driving the positions of the focal spots back to their nominal locations on the sensor array.    
         [0019]     The wavefront sensor  34  helps to form a closed-loop system that enables the wavefront correction system  28  to be periodically apprised of the overall quality of the optical signal which it is outputting, and to apply updated, periodic corrections as needed to ensure that the spot size of the output signal remains at the desired 50 μm spot size.  
         [0020]     Referring briefly to  FIG. 3 , the wavefront correction system  28  is shown in greater detail. The wavefront correction system  28  includes a static wavefront corrector  36  and a dynamic wavefront corrector  38  in communication with a controller  40 . The controller  40  is also in communication with an output  42  of the wavefront sensor  34 . The static wavefront corrector  36  may comprise either a computer-generated holographic optical element (CGHOE) or a film holographic optical element (HOE), both of which are lightweight and capable of correcting for many thousands of wavelengths of error. The static wavefront corrector  36  essentially removes the errors induced in the optical signal due to errors in the shape of the antenna aperture  12 . The antenna aperture  12 , being deployable from a stowed, non-operative configuration to an operative configuration, typically will have some small degree of variation in its overall shape from one deployment to the next. The static wavefront corrector  36  acts as a “coarse” wavefront distortion correction component to remove the optical distortion caused by surface contour variations from the nominal deployed shape of the antenna aperture. The remaining wavefront distortion will be that due to deployment-related variations in the overall shape of the antenna aperture  12  and/or its surface contour. After coarse correction, the remaining wavefront errors will be sufficiently small to be within the correction capability of the dynamic Wavefront corrector  38 . As a result, mechanical considerations such as simplicity, structural performance, ease of deployment/stowage, compactness, wind drag, and damage tolerance, rather than the accuracy of the reflector itself, are the predominant design considerations for the system  10 .  
         [0021]     The dynamic wavefront corrector  38  includes a programmable spatial light modulator (SLM) having an X-Y array of liquid crystal pixels, with each pixel being capable of changing its optical depth. The controller  40  controls electrical signals supplied to the liquid crystal pixels to modify the optical signal passing through the programmable SLM  38  so that small degrees of wavefront distortion caused by atmospheric anomalies and residual uncorrected antenna shape errors are either removed or substantially corrected in the optical radiation passing through the wavefront correction system  28 . The feedback signals provided by the wavefront sensor  34  enable the controller to make real time adjustments as needed to maintain the output  30  of the wavefront correction system at the desired 50 μm spot size. Alternatively, the dynamic wavefront corrector  38  could comprise a controllably deformable mirror or micro electromechanical (MEM) micromirror array device. Accordingly, the dynamic waverfront corrector  38 , in connection with the controller  40  and the wavefront sensor  34 , operates to perform a degree of “fine” wavefront distortion correction for attenuating small degrees of rapidly changing optical distortion affecting the incoming optical radiation being received by the antenna aperture  12 . In practice, the dynamic waverfront corrector  38  enables several hundred wavelengths of wavefront control. The wavefront correction system  28  further enables the control of fine pointing of the optical channel of the system  10 , as it allows a phase pattern to be imposed to correct pointing errors up to one degree in magnitude. It will also be appreciated that both MEM mirror devices and liquid crystal spatial light modulator devices are presently commercially available and capable of programmable phase modulation of an incoming optical wave signal at video frame rates.  
         [0022]     In the event that the optical signal output from the wavefront correction system  28  still cannot be focused to a suitably small spot size to be read by a 50 μm optical detector  32 , then it may become necessary to utilize an array of microlensed photo detectors, rather than a single optical detector. Such a component is available from Rockwell Scientific Company of Thousand Oaks, Calif.  
         [0023]     Referring now to  FIG. 4 , an antenna system  100  having a wavelength division multiplexing (WDM) system in accordance with an alternative preferred embodiment of the present invention is shown. The WDM system incorporates a plurality of bandpass filters  102 ,  104  and  106 . Filters  102 ,  104  and  106  each provide optical outputs of predetermined optical bandwidths at detectors  102   a,    104   a  and  106   a.  The number of bandpass filters can vary to generate any desired form and number of demultiplexed outputs.  
         [0024]     The present invention thus provides an antenna system that integrates both RF and optical wave radiation receiving and transmitting capabilities. The wavefront correction system  28  of the present invention accommodates both coarse and fine wavefront distortion correction functions without adding significantly to the bulk of the overall antenna system, and without compromising its ability to be quickly and easily deployed or stowed. Most advantageously, the antenna system of the present invention can be used to simultaneously receive and transmit both RF and optical wave radiation, thus maximizing the utility of the system  10 .  
         [0025]     While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.