Abstract:
An RF/Optical shared aperture is capable of transmitting and receiving optical signals and RF signals simultaneously. This technology enables compact wide bandwidth communications systems with 100% availability in clear air turbulence, rain and fog. The functions of an optical telescope and an RF reflector antenna are combined into a single compact package by installing an RF feed at either of the focal points of a modified Gregorian telescope.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/076,885, titled “RF/Optical Shared Aperture for High Availability Wideband Communication RF/FSO Links,” filed Mar. 31, 2011, incorporated herein by reference. U.S. patent application Ser. No. 13/076,885 claims the benefit of U.S. Provisional Patent Application No. 61/320,017, titled “RF/Optical Shared Aperture for High Availability Wideband Communication RF/FSO Links,” filed Apr. 1, 2010, incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to wireless communications, and more specifically, it relates to radio-frequency (RF) and free-space optical (FSO) communications. 
         [0005]    2. Description of Related Art 
         [0006]    The availability of free-space optical links is limited by atmospherics and weather conditions. RF and microwave systems are less susceptible to these problems. Hybrid RF/Optical communications systems that can switch back and forth between RF and Optical transmissions in order to optimize the overall availability of the link and maximize communication performance would be a solution. The use of a terminal with shared RF/Optical aperture could provide the overall link availability advantage described above in the smallest form factor possible. This becomes increasingly important in deployment scenarios where space is at a premium and the use of multiple RF and optical apertures is not desirable, for example on small aircraft, satellites, and in certain ground vehicles and ground deployments. 
         [0007]    As discussed in U.S. Pat. No. 6,667,831, incorporated herein by reference,  FIG. 1  illustrates a traditional Gregorian telescope  100  according to the prior art. The Gregorian telescope  100  has a concave primary mirror  102  and a concave secondary mirror  104 . In many traditional Gregorian telescopes, the primary mirror has a parabolic curvature and the secondary mirror has an elliptical curvature. The secondary mirror  104  is disposed outside the focal plane of the primary mirror  102 , and the mirrors share a common optical axis  106 . The primary mirror  102  reflects light from a far field and directs the light towards the secondary mirror  104 . The secondary mirror  104  is appropriately sized and positioned so that light reflecting off the primary mirror  102  is incident on the secondary mirror  104 . The secondary mirror  104  reflects light and directs it through an aperture  108  in the primary mirror  102  that is centered about the optical axis  106 . The light is thereafter imaged at the focal plane  110  of the compact telescope for advantageous use. 
         [0008]      FIG. 2  illustrates an embodiment of a compact telescope as discussed in U.S. Pat. No. 6,667,831. The compact telescope  200  comprises a first reflecting surface  202  and a radially defined second reflecting surface  204 . The first reflecting surface  202  includes an annular outer portion  206 , a radially defined inner portion  208 , and a radially defined aperture  210 . Other shapes may be used for these elements of the compact telescope, however, alternative shapes may increase the complexity of the optics. 
         [0009]    The outer portion  206  of the first reflecting surface  202  is the functional equivalent of the primary mirror in a traditional Gregorian telescope, while the inner portion  208  is the functional equivalent of the secondary mirror. Therefore, hereinafter, the term “primary mirror,” as it relates to a compact telescope, is used interchangeably with the outer portion  206  of the first reflecting surface. Likewise, the term “secondary mirror”, as it relates to a compact telescope, is used interchangeably with the inner portion  208  of the first reflecting surface. The primary and secondary mirrors  206 ,  208  are both concave, with the curvature of the secondary mirror  208  being greater than the curvature of the primary mirror  206 . In  FIG. 2 , both the primary mirror  206  and the secondary mirror  208  have elliptical curvatures (i.e., conic between −1 and 0). Those skilled in the art will recognize that with both mirrors having elliptical curvatures, correcting for both spherical and coma aberrations is facilitated without the need for additional optical elements. In an alternative embodiment, the primary mirror  206  may have a parabolic curvature (i.e., conic equal to −1) and the secondary mirror  208  may have an elliptical curvature. Other curvatures may also be used for the primary and secondary mirrors  206 ,  208  of the compact telescope. 
         [0010]    The optical axes  212  of the primary and secondary mirrors  206 ,  208  are coincidental. Additionally, the aperture  210  and the second reflecting surface  204  are centered upon the coincident optical axes  212 . Non-coincidental and/or off-axis optics may be employed, however, coincident optical axes reduce complications in aligning the optical elements and simplify the optics of the compact telescope. 
         [0011]    In the embodiment of  FIG. 2 , the primary and secondary mirrors  206 ,  208  form the integral first reflecting surface  202 . Such a double-curved mirror facilitates manufacturing and optical axis alignment of each curvature on the first reflecting surface  202 . This is important because greater errors in axis alignment result in greater optical aberrations. For example, a double-curved mirror may be Manufactured using diamond turning or other appropriate equipment that is frequently used to create high quality mirrors. With the appropriate manufacturing equipment, the primary and secondary mirrors may be manufactured sequentially using a single piece of equipment without realigning the equipment to obtain coincidental axes. 
         [0012]    Alternatively, in lieu of a double curved mirror, the compact telescope may comprise a first reflecting surface having an annular shape (the primary mirror), with a third reflecting surface (the secondary mirror) disposed within the inner radius of the first reflecting surface. The curvatures of this alternative embodiment for the first and third reflecting surfaces are the same as the curvatures for the aforementioned outer and inner portions, respectively. 
         [0013]    Returning to  FIG. 2 , the second reflecting surface  204  is a planar surface, hereinafter referred to as the “folding mirror”. The folding mirror  204  optically couples the primary mirror  206  to the secondary mirror  208 . The folding mirror  204  is disposed between the first reflecting surface  202  and the focal plane of the primary mirror  206 . Thus, light from a far field may enter the primary aperture of the compact telescope  200  and reflect off the primary mirror  206  towards the folding mirror  204 . The folding mirror  204  reflects such light towards the secondary mirror  208 , and the secondary mirror  208  reflects the light back towards the folding mirror  204 . Upon this second reflection from the folding mirror  204 , the light passes through the aperture  210 . Light emerging from the aperture  210  creates an upright image at the focal plane  214  of the compact telescope that may be advantageously used. 
         [0014]    Alternative embodiments of the compact telescope may include a curved folding mirror. A curved folding mirror preferably has a high radius of curvature, such as a radius of 1 meter or more. Smaller curvatures may also be employed. In another alternative embodiment, the folding mirror comprises a steering mirror. The steering mirror may have a planar or curved reflective surface. A steering mirror having a curved reflective surface may help improve the optics of a compact telescope when the optical axes of the primary and secondary mirrors are imprecisely aligned. 
       SUMMARY OF THE INVENTION 
       [0015]    It is an object of the present invention to combine free space optical and RF communications into a single networked system to provide compact, robust, high bandwidth mobile communications for commercial, military and government applications. 
         [0016]    This and other objects will be apparent based on the disclosure herein. 
         [0017]    The invention provides embodiments of an RF/Optical shared aperture for high availability wideband communication RF/FSO links that have a communications terminal capable of transmitting and receiving optical signals and RF signals simultaneously. RF/Optical hybrid technology enables communications systems with 100% availability in clear air turbulence, rain and fog. RF and optical communications transmitters have different sensitivities to atmospheric perturbations; consequently selective switching between systems provides an optimal solution. In embodiments of the current invention, a RF/millimeter wave (RF/MM) beam is fed into an optical telescope at one of two focal points in a manner that does not perturb the optical beam. In some embodiments, a planar patch fixed beam array antenna with a through hole in the array center is placed at the co-location of the optic focal plane of a compact telescope to create the optical and radio frequency (RF) shared aperture terminal. The patch array antenna is the feed for RF frequency. This feed is designed and configured so that it does not disturb the path of the original optical rays. Both optical and RF share the same aperture without interfering with the performance of one another. In other embodiments, a RF/millimeter-wave feed is installed at the focal point of the primary reflector, which is in front of the secondary mirror. A properly designed dichroic plate replaces the standard aluminum secondary mirror of the modified Gregorian system. This dichroic plate reflects the optical signal and transmits the RF signal. This invention enables extremely compact implementation of RF/Optical hybrid technology for high data capacity and high availability communications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0019]      FIG. 1  shows a prior art Gregorian telescope. 
           [0020]      FIG. 2  shows a prior art modified Gregorian telescope. 
           [0021]      FIG. 3  shows an embodiment of a modified Gregorian telescope with a dichroic folding mirror, an RF signal compensating element, a light source at focal point  1  and an RF feed at focal point  2 . 
           [0022]      FIG. 4  shows a picture of a millimeter-wave scalar feed horn antenna which can be used in the embodiment of  FIG. 3 . 
           [0023]      FIG. 5  shows an embodiment of a modified Gregorian telescope with an aluminum folding mirror and both a light source and an RF antenna at focal point  1 . 
           [0024]      FIG. 6  shows a black-and-white line illustration of a 16×10 slot array feed antenna. 
           [0025]      FIG. 7  shows the measured antenna radiation pattern of a 16×10 slot array feed antenna. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    To achieve a high-availability, high throughput data link, it is advantageous to employ a hybrid communication system architecture capable of both RF and optical transmission. This approach enables high capacity data transfer over the optical channels during good atmospheric and weather conditions and provides a smooth transition to a lower data rate RF channel in bad weather. For applications where limited space is available, such as an airborne platform, it is desirable to combine the functions of the optical telescope and the RF antenna into a single compact package. Such a device, referred to herein as an RF/optical shared aperture, is a terminal that is capable of transmitting and receiving optical signals and RF signals simultaneously to and from the same aperture. The present invention utilizes recent developments in precision-machined telescopes and customized patch array antenna feeds. See, e.g., U.S. Pat. No. 6,667,831, titled Compact Telescope,” incorporated herein by reference. 
       Dichroic Folding Mirror and RF Feed 
       [0027]      FIG. 3  shows an embodiment of the present invention having an RF feed  300  located at the focal point of the primary mirror  306 . The embodiment includes a dichroic folding mirror  304 , a primary mirror  306  (shown in cross section) and having an inner perimeter  350  and an outer perimeter  352 , a secondary mirror  308  shown cross section) and disposed within said inner perimeter and having an aperture  310 . The embodiment further includes a light source  312  at a second focal plane  314  of the system and a compensating element  316 . If the folding mirror is made of material, such as solid aluminum, that will not transmit an RF signal, it would block the RF energy that is directed from RF feed  300  toward the primary and secondary mirrors. In the present invention, the solid disc-shaped metallic folding mirror  204  of the prior art is replaced with folding mirror  304  made of dichroic material that is able to efficiently reflect the incident optical beam from light source  312 , often having a wavelength of 1550 nm, while maintaining transparency to RF energy. Due to the design of the telescope, the direction of the incident optical beam ranges between normal to the surface and about 40° from normal. 
         [0028]    The dichroic folding mirror  304  achieves high reflective performance in the optic band by acting as synthetic Bragg crystals. This effect is created by layers of materials deposited in a periodic stack. The thickness of each layer is often less than one tenth of a micrometer. The materials used for the layers are dielectrics, such as pyrex. Through constructive interference of the reflected light, this structure can efficiently reflect the incident optical signal according to Brags law, which relates the reflected wavelength to the angle of the incident light. The dichroic folding mirror  304  is transparent to the RF signal because the thickness of the layers that construct the plate is typically less than three orders of the RF wavelength. This produces no wave phenomena as the RF signal passes through the plate; therefore, the Bragg crystal is transparent to the RF signal. There is no constructive reflection. Furthermore, the conductivity of the layer material is very low in the RF band. The insertion loss is negligible. A portion of the RF signal transmitting through folding mirror  304  will impact onto the primary mirror  306  and a portion will impact onto the secondary mirror  308 . Because the secondary mirror has a different curvature than the primary mirror, the secondary mirror will reflect incident RF signal in a different direction than the direction of RF rays that reflect from the primary mirror. In order to compensate the RF signal direction for the different curvature of the secondary mirror, a dielectric lens  316 , having a properly designed thickness and curvature, is inserted between the folding mirror  304  and the RF source  300 . As shown in  FIG. 3 , an embodiment of the invention places dielectric lens element  316  in contact with folding mirror  304 . In this embodiment, the diameters of the dielectric lens, the folding mirror and the secondary mirror  308  are about the same. Dielectric lens element  316  has a thickness and curvature that will cause the transmitted RF signal to reflect from the secondary mirror  308  such that the signal will have the same direction as the signal reflecting from the primary mirror  306 . The RF signal reflecting from the secondary secondary mirror will be collimated as it propagates out of the telescope. Based on the teachings herein, those skilled in the art will be able to properly design dielectric lens element  316 . 
         [0029]      FIG. 4  shows a picture of an exemplary millimeter-wave scalar feed horn antenna which can be used as RF feed  300  as depicted in  FIG. 3 . This type of feed horn has a highly symmetrical antenna pattern and very low side lobe characteristics. It can be designed to uniformly illuminate the primary mirror, while occupying a small volume (25 mm diameter, 70 mm length). Scalar teed horn antennas are commercially available. 
       Aluminum Secondary Mirror and Back-Mounted RF Feed 
       [0030]      FIG. 5  illustrates a device that provides the RF signal and the optical signal from the same aperture. Elements common to the embodiment of  FIG. 3  are identically numbered. The apparatus places a RF feed  420  and the light source  312  at focal point  1  of  FIG. 5 . In this configuration, RF energy that is transmitted through the aperture  310  of the secondary mirror  308 , is reflected by aluminum folding mirror  404  and is emitted from the primary mirror  306 . Since both RF and optical feeds share this focal point, the RF feed antenna must be planar and include a central hole through which the optical beam can pass. A slot-array feed antenna, as known in the art, may be designed to include a central hole, as illustrated in  FIG. 6 . This slot array design was taken through manufacturing at Kyocera America. The actual antenna radiation pattern of the Kyocera 16×10 slot-array feed is shot in  FIG. 7 . 
         [0031]    The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments disclosed were meant only to explain the principles of the invention and its practical application to thereby enable others skilled in the art to best use the invention in various embodiments and with various modifications suited to the particular use contemplated. The scope of the invention is to be defined by the following claims.