Abstract:
A free space optical data communications system, including transceiver nodes employing common optics in both the transmit and receive subsystem, is disclosed. Likewise, a beacon is disclosed utilizing some of the common optics as well. In one embodiment, a double-faced mirror cooperates with a beacon, a primary mirror, and the transmit and receive subsystems. In one other embodiment, the beacon is located in front of the primary mirror. In still another embodiment, the common optics includes TEM11 mode signal separation using phase plates. A tracking system in each embodiment maintains alignment of the optics with a second node by generating a correction vector responsive to the incoming signal from its companion node.

Description:
REFERENCE TO RELATED PATENT APPLICATION  
       [0001]    This non-provisional application claims benefit of U.S. provisional patent application Serial No. 60/358,433, filed Feb. 19, 2002, and hereby claims the benefit of the embodiments therein and of the filing date thereof. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    In a free space optical communication link, two (or more) transceivers that must communicate with one another must be carefully aligned to establish and maintain a high link margin (low loss) and ultimately a high quality of service. Positional and angular errors due to, but not limited to, relative platform movement and optical path aberrations must be accurately tracked and compensated to maintain a high link margin.  
           [0003]    The current state of the art suffers from the need for a complex multi-axis optical system involving large mass optics, which in turn require expensive actuators to move in response to alignment errors and link aberrations. An automatic alignment system is essential not only to the successful operation of a free space link but ultimately will dictate the cost effectiveness of the approach.  
           [0004]    Typical single axis systems usually employ the use of separate transmit and received optics, side by side, and do not achieve any common usage of the major optical elements in both subsystems with resulting economy. Boresight scopes are often used as the alignment devices at each node and rely upon the transmitted beam spread to achieve transmission reliability.  
           [0005]    Examples of the prior art may be seen in the following United States patents:  
                                                           3,705,986   R. W. Sander et al   Dec. 12, 1972           4,330,870   T. C. Arends   May 18, 1982           4,941,205   W. R. Horst et al   Jul. 10, 1990           and           6,285,476 B1   R. T. Carlson et al   Sep. 4, 2001.                      
 
           [0006]    Examples of duplicate optical systems in the transmit and receive channels of one node are illustrated in the Sanders et al. U.S. Pat. No. 3,705,986 and the Arends U.S. Pat. No. 4,330,870, as well as U.S. Pat. No. 4,941,205 to Horst et al.  
           [0007]    In U.S. Pat. No. 6,285,476 B1 to Carlson et al., a boresight is used and the transmit and receive optics share a common housing, heated window and a dichroic beam splitter.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0008]    Given this state of the art, we have conceived and demonstrated a node or transceiver for free space optical data which combines several advances in the state of the art.  
           [0009]    First, it simplifies and reduces the cost of the transceiver through the common use of optics in the transmit branch, the receive branch, and the beacon or alignment correction subsystem.  
           [0010]    Second, we have developed a system in which common optics includes a double-faced mirror and a primary mirror within the system housing, which cooperate to transmit a beacon alignment signal reflected off of one face of the planar mirror, and on the other face thereof reflects both transmitted and received optical data signals which are reflected by the primary mirror.  
           [0011]    Third, in this preferred system, alignment is maintained with its corresponding node by movement of a lens which is common to both transmitted and received data and received beacon signals for its corresponding node.  
           [0012]    Fourth, transmit and received data signals are separated in a duplexer which comprises simple single mode and multimode optical fibers.  
           [0013]    Fifth, all the optical elements are within or secured to the enclosure whereby primary alignment using a borescope brings the transmit, receive and beacon optics into rough alignment in one step.  
           [0014]    In another embodiment, the beacon is located within the enclosure in front of the primary mirror on its optical axis where it does not interfere with either transmit or received signals, and a single faced mirror reflects signals into and out of the enclosure to a cold mirror for reflecting beacon signals for alignment correction if needed and allows received and transmitted data signals to pass. A beamsplitter of either a polarizing or nonpolarizing type separates the received and transmitted signal paths. A tracking system, which is driven by beam correction vector signals, is included.  
           [0015]    In still another embodiment, the common optics include a single faced mirror with a pinhole aperture collimating lens and a pair of phase plates to selectively pass received and transmitted optical signals. In this embodiment virtually all optical elements of both the transmit and receive channels have common optics.  
           [0016]    These inventions, enumerated above, may be applied to any free space optical (FSO) link between two nodes is shown schematically in FIG. 1 below. Each node A and node B consists of:  
           [0017]    a) front end optics assembly;  
           [0018]    b) a tracking system to maintain alignment;  
           [0019]    c) a pair of optical transceivers to interface between the FSO node and the communications network served.  
           [0020]    Given that transceivers are often commoditized off-the-shelf products at the data rates required for FSO (maximum of 2.5 Gbps, circa 2002), the first two subsystem elements, the front-end optics and the tracking system, are the most important and will set the basis of the FSO product selection.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0021]    This invention may be more clearly understood from the following detailed description and by reference to the drawing in which:  
         [0022]    [0022]FIG. 1 is a simplified block diagram of a free space optical data transmission system with one node A shown enlarged with each of its basic subsystems shown in block form;  
         [0023]    [0023]FIG. 2 is a simplified block diagram and longitudinal sectional diagram of one embodiment of a node in accordance with this invention;  
         [0024]    [0024]FIG. 3 is a side elevational view of a simple fiber optics duplexer;  
         [0025]    [0025]FIG. 4 is a side elevational view of a coated fiber optics duplexer;  
         [0026]    [0026]FIG. 5 is a block diagram of a breadboard embodiment of this invention with the optical beacon and transmitter/receiver portion shown in longitudinal sectional form;  
         [0027]    [0027]FIG. 6 is a simplified optical diagram of another embodiment of this invention; and  
         [0028]    [0028]FIG. 7 is a block diagram of the training system of this invention.  
     
    
     DETAILED DESCRIPTION  
       [0029]    The dominant source of link impairment problem in free space communication systems is alignment errors. As indicated in FIG. 1, the transmitted beam from one node must be pointed correctly at the other node (relative angular alignment); and the receiving node, in turn, must be aligned with the transmitting node to maximize the received signal. Each node transmits a divergent beacon beam that is used by the other node to acquire the signal and uses the beacon to track the transmitted beam. Our invention, as illustrated in FIG. 2, has the following key features:  
         [0030]    1) The use of some common optics for the beacon transmit and receive beams, vastly simplifying the optics, which in turn reduces assembly, component costs as well as simplifying the tracking requirements (i.e., concerned with aligning a single axis as opposed to multiple axes).  
         [0031]    2) Only a single optical element, namely, the beacon assembly fixed within an optics tube, needs to be adjusted either in two angular coordinates or the transverse positional coordinates.  
         [0032]    3) The servo system required for tracking is locally controlled in each node (i.e., the error signal is derived locally in contrast with other methods where the positional and angular errors of the transmitting node are also used in current systems which are prone to control instabilities).  
         [0033]    [0033]FIG. 2 illustrates a free space optical communications node generally designated  10  with a transmit/receive axis A, defined by node optics tube OT supporting primary mirror  11 . A small 45-degree, double-faced mirror  12  is located on the optical axis A between the primary mirror  11  and the exit opening OP of the node  10 . A beacon laser  13  is directed via an opening in the tube OT and a convex lens L at the front (facing OP) face  12 F of mirror  12 , which reflects the beacon beam out of the opening OP along axis A for detection at the matching node for node alignment.  
         [0034]    Transmitted and received optically encoded data at opening OP and the beacon signal from the remote node are reflected by the primary mirror  11 , and the rear reflective face  12 R of mirror  12  out of the tube OT through a side wall opening. Data signals are reflected 90-degrees from axis A by mirror  12  surface  12 R to or from a lens  14  mounted on a voice coil actuator  13 . The lens  14  focuses received data signals which pass through cold mirror  15  on the end of duplexer  16  for reception and detection by RX detector  17 . Transmitted optical data stream from transmitter TX  18  passing through duplexer  16 , cold mirror  15 , and lens  14  enters the optical tube OT, is reflected by surface  12 R of mirror  12  to the primary mirror  11  and out of tube OT through the opening OP.  
         [0035]    Received beacon signals from the opposite node after reflection by primary mirror  11  and surface  12 R of 45-degree oriented mirror  12  exiting the side opening of tube OT with received data signals are reflected by cold mirror  15  through a filter  18  to a CCD display  19 . Off center images of the received beacon signal at the CCD display  19  are used by tracking system  20  to feed back alignment correction signals to the voice coil actuator  13  for optimum X Y repositioning of lens  14  and optimum signal strength. The tracking system  20  of FIG. 2 is shown and described below in connection with FIG. 5.  
         [0036]    The preferred form of duplexer  16  of FIG. 2 is illustrated in FIGS. 3 and 4 produced by co-positioning a single mode fiber SMF with a multimode fiber MMF at a joining point J in FIG. 3 and suitably removing any protective or reflective coating C of FIG. 4 grinding, polishing, and fusing the fibers together to form a junction.  
         [0037]    Although different realizations of our invention are possible, we describe a particularly simple implementation that does not rely on polarization or wavelength diversity techniques to duplex the transmit and the receive beam. A more straightforward implementation that has been demonstrated in the laboratory is described by the following description of a breadboard Installation identified here as FIG. 5.  
         [0038]    The FIG. 5 breadboard system describes a common axis system that has been assembled and demonstrated. In this embodiment, a 780 nm beacon  50  that is used by each node to mutually acquire and track are arranged to function in the same way as the system described in the above system disclosure of FIG. 1. The beacon  50 , powered by an external driver  51 , is located coaxially with the transmit/receive axis A in front of a front-end, 45-degree fold mirror  52 , and in front of the concave primary beam forming mirror  53  which is enclosed along with the beacon  50  in an optics tube OT. A finder scope FS, extending parallel to tube OT, provides rough alignment of the nodes.  
         [0039]    The beacon  50  is preferably a laser with its collimating lens  54  in a small diameter, e.g., tube  55  positioned on axis A. The wavelength of the laser is chosen so that an inexpensive imaging device, such as silicon CCD arrays, can be used to sense the beacon light efficiently. For example, to operate with silicon CCD arrays, the laser wavelength is best chosen to be shorter than 800 nm. The beacon assembly  50 ,  54 , and  55  on axis A operating at 780 nm is sufficiently small in diameter, as compared with the primary mirror  53 , so as to not interfere with the data communication at 1550 nm, for example.  
         [0040]    The common optics for the transmit and receive communications data begins with the primary mirror  53 , which focuses received signals on the rear facing front end fold mirror  52  and receives transmitted data signals for the mirror  52  for reflection and transmission of a slightly spread beam to the remote receiving node, which duplicates the system of FIG. 5.  
         [0041]    The received beacon and data signals at mirror  52  are reflected at 90-degrees from the axis A to a collimating lens  56  and a 45-degree oriented scan mirror  60 .  
         [0042]    The only material difference between the breadboard system of FIG. 5 and the more general system in the disclosure is the duplexing method used to separate the transmit from the receive beam in the common axis architecture. In the breadboard system of FIG. 5, the transmit beam and the receive beams are brought together to a common axis by the use of either a polarization beamsplitter or a common non-polarization selective beamsplitter  70  after reflection by the scan mirror  60  and direct passage through cold mirror  61  as shown in the breadboard embodiment of FIG. 5.  
         [0043]    In the case of a non-polarization selective beamsplitter  70 , ½ of the transmitted light is lost, and likewise {fraction ( 1 / 2 )} of the received light is lost. In the case of a polarizing beamsplitter  70 P, each node must set up properly to send the correct polarization in the transmitted beam, which will pass through the polarizing beamsplitter  70 P in the receiving node with maximum efficiency. In this way, the theoretical efficiency can approach 100%.  
         [0044]    So, in the polarizing scenario, the position of the transmitting source and the receiving detector are switched between the two nodes, with respect to the polarizing beamsplitter. In the FIG. 5 breadboard, the single mode fiber SMF optically encoded data from brings the transmitting source via the transmitting lens  72  to the beamsplitter  70 P with a polarization state that is parallel to the plane of the paper. Transmitted data passes through the polarized beam splitter  70 P, through cold mirror, is reflected by the scan mirror  60 , through lens  56 , is reflected off the front end fold mirror  52  to the primary mirror  53  and reflected out of the opening OP via free space to the mating node.  
         [0045]    Received data reverses the path of the transmitted data, via mirror  53 , fold mirror  52 , lens  56 , scan mirror  60 , cold mirror  61  until it reaches the beamsplitter  70  or  70 P, where it is reflected to 45-degree mirror  73 , ND filter  74 , receiver lens  75  to the input end of a multi mode fiber MMF to a 3×1 demagnifier, and then to detector  76 . The received beam, therefore, must have the orthogonal polarization (i.e., perpendicular to the plane of the paper). The relative positions of the transmit and receive optics are switched in the other node.  
         [0046]    The cold mirror  61  reflects and couples the received beacon light out of the common optical system onto a 45-degree mirror  62 , focusing lens  63 , ND filter  64 , interference (780 nm) filter  65 , an imaging CCD  66  for acquisition and tracking purposes.  
         [0047]    A tracking system similar to that disclosed in FIG. 7 below is employed. The original disclosure of FIGS. 1 and 2, and demonstrated in the breadboard of FIG. 5 describes a particular method of duplexing the transmit and receive beams using a fiber coupler with single mode fiber (SMF) and multimode fiber (MMF) ports. Here, a simpler technique is described that relies on bulk optical elements that are more easily fabricated and hence lower cost in nature.  
         [0048]    The key to the system operation, as illustrated in FIG. 6, in the desired common-optics mode is to effectively couple the receive beam out of the common optical path without adversely affecting the transmit beam. This can be accomplished by requiring the transmit beam to have a null in the center of the beam in the far field. In gaussian optics terms, this means that the fundamental gaussian mode from the laser is converted into the TEM11 mode which is the lowest order doughnut mode. The far field divergence can be made approximately the same as the fundamental mode provided that a properly designed phase plate is used to perform the mode conversion.  
         [0049]    The new idea is described in FIG. 6, which shows the transmit laser source  100  as a point source behind a mirror  101  oriented at 45-degrees from the optical axis. The laser source  100  is focused through a small hole  102  at the center of the mirror to pass through and is collimated by a lens  103 . A phase plate  104  shapes the far field intensity profile of the beam into the desired TEM11 mode. The receiving node has the identical system so that the received beam is focused onto the mirror  101  plane. Because of the TEM11 mode, however, the focused distribution has a null in the center so most of the light is reflected out of the common path by the mirror  101  which can further be focused by lens  106  onto the desired photodetector or MMF  107 .  
         [0050]    The phase plates, such as plates  104  and  105 , can be designed by known computer generated hologram or optical element approaches and the preferred solution is to have either as an etched glass plate or embossed plastic plate to yield the desired phase pattern across the aperture. Since there are two such phase plates, the design procedure should take this constraint into account. Again, this is a straightforward design procedure but the overall concept of this embodiment is novel.  
         [0051]    Tracking Subsystem Description  
         [0052]    The tracking subsystem  67  of FIGS. 2, 5,  6 , and  7  analyzes the sequence of images from the CCD imager  66  to assess the current position of the other node&#39;s beacon spot and send a suitable command to the movable lens (or mirror in a different but functionally equivalent realization as described in the breadboard description of FIG. 5) to re-align the common axis optical system.  
         [0053]    As shown in FIG. 7, the image from the CCD is evaluated frame-by-frame (in he video sequence). A typical image frame is shown in FIG. 7 where the apparently misaligned beacon spot is shown off-center in the upper left quadrant with respect to the target defined by the origin, the intersection of the two X and Y axes. The angular position error is denoted by the length and direction of the correction vector CV and also by the orthogonal errors ΔY and −ΔX.  
         [0054]    As shown in FIG. 7, the desired correction vector CV is that which connects the axes center or origin and the centroid of the beacon spot. The centroid calculation can use standard curve fitting algorithms which uses the intensity distribution of the spot to yield a robust estimate of the spot position. The tracking subsystem then computes the necessary correction vector or ΔX and ΔY corrections mapped to the coordinate frame of the adjustable lens  14  of FIG. 2 or mirror  60  of FIG. 5, which is sent to the lens or mirror actuator as an alignment correction command.  
         [0055]    Altogether these embodiments show several free space communication systems employing common optics and tracking control of each node independent of the matching node.  
         [0056]    The above-described embodiments of the present invention are merely descriptive of its principles and are not to be considered limiting. The scope of the present invention instead shall be determined from the scope of the following claims including their equivalents.