Abstract:
A free space optical communication system ( 10 ) including first and second mono-static transceivers ( 20   a,    20   b ). Each transceiver ( 20   a,    20   b ) includes a reflective assembly ( 40 ) defining a reflective surface ( 44 ) about a receiving end of a respective optical fiber ( 32 ) and configured to reflect optical signals ( 26 ) within a field of view of the transceiver ( 20   a,    20   b ) as a modulated retro-reflective signal ( 28 ). Each mono-static transceiver ( 20   a,    20   b ) includes an acquisition system ( 60 ) configured to detect a modulated retro-reflective signal ( 28 ) and adjust the alignment of the respective transceiver ( 20   a,    20   b ) in response to a detected modulated retro-reflective signal ( 28 ). A mono-static transceiver and a method of aligning a mono-static transceiver are also provided.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of optical communications, and in particular to the field of beam steering for mono-static bidirectional free space optical transceivers. More particularly, the present invention relates to a beam pointing and tracking system and method utilizing pulsed beams to assist in target acquisition. 
       BACKGROUND OF THE INVENTION 
       [0002]    Optical communications systems are today employed in a vast array of applications, including without limitation communication with aircraft and satellites from ground positions. A unidirectional optical communications system generally consists of a transmitting terminal and a receiving terminal while a bidirectional system includes a pair of transceivers, each of which acts as both a transmitting terminal and a receiving terminal. In either system, a transmitting terminal typically receives an electrical signal from a signal source, converts the electrical signal into an optical signal and then transmits the resulting optical signal using a transmitting telescope. The receiving terminal receives the optical signal through a receiving telescope, which focuses the optical signal into an optical photodetector, and then converts the optical signal back into an electrical signal. 
         [0003]    In a mono-static system, both the receiving terminal and the transmitting terminal utilize the aperture of a single telescope. An optical circulator or other bulk optical techniques are utilized to separate the transmit and receive paths such that the beams traveling in opposite directions occupy the same telescope. 
         [0004]    Accurate alignment of the transceiver system is essential for free space optical communications systems. In order for a receiving terminal to receive an optical signal from a corresponding transmitting terminal, the telescopes must be properly aligned. This alignment process is known as beam steering. In a bidirectional optical system, beam steering is the manipulation of one or both of the transceivers to point in a desired direction. Beam steering in optical systems may also be accomplished by various systems, for example, a motorized gimballing system, acousto-optics, liquid crystals, electro-optics, micro-optics, a galvanometer, magnetic mirrors, micro-mirror arrays, and micro-electro-mechanical systems. 
         [0005]    In order for an optical receiver to begin receiving a signal from a transmitter, the incoming search signal must first be located and the receiver pointed in the direction of the incoming signal. In a bidirectional system, the receiver terminal of each transceiver must be aligned with the transmitting terminal of the other transceiver. During the initial search for a signal, or if the signal is lost for some reason and reacquisition is thus necessary, a search pattern is generated by an algorithm stored in the control system. The initial search utilizes macro adjustment to locate the field of view (FOV) of the opposite transceiver, and once it is recognized that the FOV has been found, micro adjustment is utilized to align the signal precisely with the optical fiber of the receiving terminal. 
         [0006]    To more efficiently recognize when the FOV has been found and to expedite the micro adjustment, systems have been developed with a mirror or other reflective surface about the optical fiber. When the transmitted signal is within the FOV of the other transceiver, the signal is retro-reflected off the mirror along the same path back to the transmitting transceiver. Upon receipt of a retro-reflected signal, the transmitting transceiver assumes that it is aligned within the FOV and micro adjustment is implemented to achieve precise alignment. This procedure is simultaneously performed for both transceivers. (See for example U.S. Pat. No. 8,160,452 which is incorporated herein by reference). 
         [0007]    As the use of free space optical communication continues to increase, it has become desirable to use such communication systems over larger and larger distances, for example, over 10 kilometers or more. To align such long distance systems, it is necessary for the retro-reflective signal to be received and recognized by the transmitting transceiver. Since the signal is traveling from the transmitting transceiver to the receiving transceiver and then reflected back to the transmitting transceiver, the signal experiences two-way path loss. As the distance increases, there is risk that the two-way path loss will cause the signal strength to fall below the noise floor caused by other optical sources, reflections or glints. Furthermore, in a mono-static system, there is limited isolation within the optical circulator or bulk optical beam splitter. If the signal strength of the retro-reflective signal is less than the isolation, the system will not be able to differentiate between the transmitted and reflected signals 
         [0008]    It is desirable to provide a system and a method wherein the retro-reflective signals are reliably received and recognized by the transmitting terminals. 
       SUMMARY OF THE INVENTION 
       [0009]    Briefly, the present invention provides a free space optical communication system. The system includes a first and second mono-static transceivers configured to transmit and receive optical signals through an optical fiber. The first mono-static transceiver includes a first reflective assembly defining a first reflective surface about a receiving end of the first optical fiber and configured to reflect optical signals within a field of view of the first transceiver but not aligned with the receiving end of the first optical fiber as a modulated retro-reflective signal. The second mono-static transceiver includes a second reflective assembly defining a second reflective surface about a receiving end of the second optical fiber and configured to reflect optical signals within a field of view of the second transceiver but not aligned with the receiving end of the second optical fiber as a modulated retro-reflective signal. Each mono-static transceiver includes an acquisition system configured to detect a modulated retro-reflective signal and adjust the alignment of the respective transceiver in response to a detected modulated retro-reflective signal. 
         [0010]    In one aspect, the invention provides a mono-static transceiver configured to transmit and receive signals through an optical fiber. The transceiver includes an adjustable telescope through which optical signals are transmitting and received. An acquisition system of the transceiver is configured to detect a modulated signal and adjust the alignment of the telescope in response to a detected modulated signal. 
         [0011]    In another aspect, the invention provides a method of aligning a first mono-static transceiver with an optical fiber of a second mono-static transceiver. The method includes transmitting an optical signal from a telescope of the first transceiver; adjusting the alignment of the telescope of the first transceiver until the optical signal is within the field of view of the second transceiver whereby the signal is retro-reflected as a modulated signal if the signal is not aligned with the optical fiber; receiving the modulated signal through the telescope of the first transceiver; detecting the modulated signal with an acquisition system of the first transceiver; and further adjusting the alignment of the telescope in response to the detected modulated signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. In the drawings: 
           [0013]      FIG. 1  is a schematic view illustrating an exemplary free space optical communication system in accordance with an embodiment of the invention. 
           [0014]      FIG. 2  is a schematic view illustrating exemplary beam paths through one of the transceivers of  FIG. 1 . 
           [0015]      FIG. 3  is a schematic block diagram of an exemplary transceiver of the free space optical communication system of  FIG. 1 . 
           [0016]      FIG. 4  is a perspective view of an exemplary mirror in accordance with an embodiment of the invention. 
           [0017]      FIG. 5  is a partial perspective view of another exemplary mirror in accordance with an embodiment of the invention. 
           [0018]      FIG. 6  is a side elevation view of the mirror of  FIG. 5 . 
           [0019]      FIG. 7  is a side elevation view of another exemplary mirror in accordance with an embodiment of the invention. 
           [0020]      FIG. 8  is a perspective view of an exemplary mirror assembly in accordance with an embodiment of the invention with the mirror assembly in a transmit state. 
           [0021]      FIG. 9  is a perspective view of the exemplary mirror assembly of  FIG. 8  with the mirror assembly in a non-transmit state. 
           [0022]      FIG. 10  is a schematic view illustrating an illustrative path of a transmit signal through an exemplary transceiver. 
           [0023]      FIG. 11  is a schematic view similar to  FIG. 10  and illustrating the path of the corresponding retro-reflective signal. 
           [0024]      FIG. 12  is a schematic block diagram of an alternative exemplary transceiver. 
           [0025]      FIG. 13  is a schematic view illustrating the transmit signal received through the transceiver of  FIG. 12 . 
           [0026]      FIG. 14  is a schematic view similar to  FIG. 13  and illustrating the path of the corresponding retro-reflective signal. 
           [0027]      FIGS. 15A-15D  are schematic views illustrating an alignment sequence of the exemplary free space optical communication system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    In the drawings, like numerals indicate like elements throughout. Certain terminology is used herein for convenience only and is not to be taken as a limitation on the present invention. The following describes preferred embodiments of the present invention. However, it should be understood, based on this disclosure, that the invention is not limited by the preferred embodiments described herein. 
         [0029]    Referring to  FIGS. 1-3 , the exemplary free space optical communication system  10  includes a pair of mono-static transceivers  20   a  and  20   b . Each transceiver  20   a  and  20   b  includes a single telescope  24  extending from a housing  22 . The system  10  may be configured such that one or both housings  22  are adjustable in the X and Y planes, or one or both housings  22  may be fixed and the internal components adjustable in the X and Y planes to align the telescopes  24 . 
         [0030]    As illustrated in  FIGS. 2 and 3 , each telescope  24  includes one or more lenses or other optical components  25  which define the FOV  23  of the telescope. The optical components  25  focus incoming signals toward a reflective assembly  40  with the optical fiber  32  of the transceiver  20   a ,  20   b  centered therein. In the present embodiment, the reflective assembly  40  includes a mirror  30  and the receiving end of the optical fiber  32  is positioned within a through hole  31  of the mirror  30 . The receiving end of the optical fiber  32  is preferably co-planar with the reflecting surface  44  of the mirror  30 . While a mirror is described herein, other reflective structures may be utilized. 
         [0031]    Each transceiver  20   a ,  20   b  is configured to transmit optical signals  26  toward the other transceiver and to receive optical signals  29  from the other transceiver  20   a ,  20   b . The optical signal  26 ,  29  may be in the visible or invisible spectrum and is preferably in the form of a laser beam. In the illustrated embodiment, a laser diode  36  produces the transmit signals  26  and a photodiode  38  receives and converts the received signals  29 , however, other optical components may be utilized. An optical circulator  34  is provided between the optical fiber  32  and the diodes  36 ,  38  to facilitate the bidirectional signal travel. Other bulk optical techniques may alternatively be used. A beam splitting mirror  37  or the like is provided along the path of the return signal  29  such that a portion  29 ′ of the return signal  29  is directed to the acquisition system  60 . The acquisition system  60  will be described in more detail hereinafter. 
         [0032]    Once the transmit signal  26  is aimed within the FOV of the other transceiver  20   a ,  20   b , the signal  26  passes through the optics  25  and is focused on the mirror  30  of the reflective assembly  40 . If the signal  26  is not aligned with the through hole  41 , and thereby the optical fiber  32 , the signal  26  will reflect off of the mirror  42  along the same path to define a retro-reflective signal  28 .  FIG. 1  illustrates the signal  26   a  within the FOV of transceiver  20   b  such that retro-reflective signal  28   a  is generated, however, signal  26   b  outside of the FOV of the transceiver  20   a  and therefore no retro-reflective signal is generated in response to signal  26   b .  FIG. 2  illustrates the transmit signal  26 ′ and retro-reflective signal  28 ′ furthest from the optical fiber  32  and then incrementally closer thereto at signal  26 ″ and signal  28 ″. Once the signal is precisely aligned with the optical fiber  32  as indicated at  26   f , the signal passes through the through hole  41  into the optical fiber  32  and no retro-reflective signal is generated. 
         [0033]    To enhance the reliability of receipt and recognition of the retro-reflective signal  28 , the acquisition system  60  is configured to identify a modulated or pulsed signal. Since optical noise, spurious optical reflections and/or other sources of glint provide a continuous (DC) signal, by looking for a modulated signal, the acquisition system  60  can identify the retro-reflective signal  28  even if it falls below the DC noise floor. That is, the acquisition system  60  will ignore continuous optical signals, for example, optical noise, spurious optical reflections and/or other sources of glint, and instead only recognize modulated signals. The illustrated acquisition system  60  includes a high dynamic range, high speed optical power monitor  62  which receives and processes the split portion  29 ′ of the received signal  29  to stabilize the signal. The processed signal  29 ′ is then directed to a phase-sensitive detector  64  which is configured to detect signals within a definite frequency band, i.e. an anticipated modulation frequency of the retro-reflective signal  28 , thereby separating the modulated retro-reflective signal  28  from any optical noise, which will be outside the frequency band, which may have been included in the signal  29 ′. The phase-sensitive detector  64  may utilize analog processing, for example a lock-in amplifier, or digital process, for example, a fast Fourier transform device. 
         [0034]    If a modulated retro-reflective signal  28  is identified in the detector  64 , the presence of the signal  28  is communicated to a control module  66 . The control module  66  is configured to control the telescope actuator  68  in response to received data to adjust the telescope  24  and steer the beam. The telescope actuator  68  may take any form, for example, a motorized gimballing system, acousto-optics, liquid crystals, electro-optics, micro-optics, a galvanometer, magnetic mirrors, micro-mirror arrays, or micro-electro-mechanical systems. The control module  66  may utilize any desired control algorithm to steer the telescope into alignment with the opposite optical fiber  32 . While not shown, the acquisition system  60  may include other communication means to communicate with a central control and/or the other transceiver. 
         [0035]    Referring to  FIG. 4 , a first embodiment of the reflective assembly  40  configured to generate a modulated retro-reflective signal  28  will be described. As indicated above, the reflective assembly  40  includes a mirror  42  which provides a reflective surface  44  around the through hole  41 . The reflective surface  44  includes a grating  43  that modulates the retro-reflective signal  28  as the signal is translated in the X or Y direction across the surface of the mirror  42 . In the embodiment described herein, the grating  43  is a reflective grating defined by transparent strips  45  alternating with opaque strips  47 . When the signal  26  is directed at a transparent strip  45 , the signal is reflected, but when the signal is directed at an opaque strip  47 , the signal is dispersed. The strips  45 ,  47  preferably have a width greater than a beam diameter of the signal  26  such that a maximum contrast between the reflected portions of the signal  28  and the non-reflected portions is achieved. Additionally, the grating  43  preferably extends diagonally with respect to the X and Y directions such that the modulated signal will be produced whether the signal is translated in either the X direction or the Y direction. As shown in  FIGS. 10 and 11 , the transmitted continuous (DC) signal  26  is received in the opposite, receiving telescope and contacts the reflective assembly  40 . As the signal  26  is translated across the grating of the mirror, a modulated retro-reflective signal  28  exits the telescope and returns to the transceiver  20  from which it came. 
         [0036]    Referring to  FIGS. 5-9 , other exemplary embodiments of reflective assemblies  40 ′,  40 ″,  40 ′″ configured to produce a modulated retro-reflective signal  28  will be described. In the embodiment of  FIGS. 5 and 6 , the reflective assembly  40 ′ again includes a mirror  42 ′ with a reflective surface  44 ′ having a grating  43  thereon. In this embodiment, the grating  43  is a mechanical grating defined by alternating ridges  46  and grooves  48 . Again, the grating  43  is preferably diagonal and the width of the ridges  46  and grooves  48  is greater than the beam diameter of the signal  26 . 
         [0037]    The embodiment illustrated in  FIG. 7  is similar to the previous embodiment and includes a reflective assembly  40 ″ with a mirror  42 ″. The reflective surface  44 ″ again has a grating  43  thereon, however, the grating  43  is defined by alternating peaks  49  and valleys  51 . Again, the grating  43  is preferably diagonal. While the peaks  49  and valleys  51  have less defined widths, such a structure may be preferred in some applications and the acquisition system  60  may be configured to recognize the modulated signal produced by such a structure. The invention is not limited to the illustrated embodiments and other reflective and mechanical gratings may be utilized. 
         [0038]    In the embodiment illustrated in  FIGS. 8 and 9 , the reflective assembly  40 ′″ includes a mirror  42 ′″ and a liquid crystal shutter  54 . The mirror  42 ′″ includes a reflective surface  44 ′″ without any grating. A through hole  41  in the mirror  44 ′″ aligns with the optical fiber  32  as in the previous embodiments. The liquid crystal shutter  54  is positioned in front of the mirror  42 ′″ and overlies the entire reflective surface  44 ′″. While the liquid crystal shutter  54  is illustrated as a separate component, it may alternatively be formed integral with the mirror  42 ′″, e.g. as a substrate applied thereto. Power leads  55 ,  57  are connected to the liquid crystal shutter  54  and are configured to supply a modulated current. For example, the current may be provided by a high voltage driver and passed through a square wave generator to generate the modulated current. The acquisition system  60 , or another controller, may be utilized to control the generation of the modulated current. 
         [0039]    As shown in  FIG. 8 , when no current is applied to the liquid crystal shutter  54 , the shutter  54  is transparent and the transmitted signal  26  passes through the shutter  54  and reflects off of the reflective surface  44 ′″ of the mirror  42 ′″ to generate a retro-reflective signal  28 . However, when current is applied to the shutter  54 , the shutter  54  becomes opaque and the transmitted signal is dispersed before reaching the mirror  42 ′″. In this way, the retro-reflective signal  28  will be modulated in correspondence to the modulation of the current applied to the shutter  58 . In this embodiment, the mirror does not require a grating and the modulated signal  28  will be generated even when the signal  26  is not being translated relative to the mirror  42 ′″. The modulated retro-reflective signal  28  will thereafter proceed as described above with respect to the other embodiments. Once final alignment is achieved, the shutter  54  is disabled such that it does not interfere with a transmitted data signal. The shutter  54  is easily activated again if alignment is lost and the alignment procedure must be initiated. While a liquid crystal shutter is described herein, other shutters may also be utilized. 
         [0040]    While a grated mirror and a liquid crystal shutter are described herein as the modulators, other modulators may also be utilized. For example, a mechanical beam shutter, optical chopper, liquid crystal spatial light modulator, or micro-electro-mechanical system (MEMS) may be utilized. 
         [0041]    Referring to  FIGS. 12-15 , an alternative exemplary transceiver  20   a ′,  20   b ′ will be described. The transceiver  20   a ′,  20   b ′ is substantially the same as in the previous embodiments, however, the reflective assembly  40   iv  is not utilized as the modulator to generate the modulated signal. Instead, the signal transmitter, in this case the laser diode  36 , is used as the modulator to generate the modulated signal as will be described in more detail. As shown in  FIGS. 13 and 14 , the reflective assembly  40   iv  still includes a mirror  42   iv  with a reflective surface  44   iv , however, no means of modulating the signal is provided at the mirror  42   iv . 
         [0042]    Referring to  FIG. 12  again, the control module  66  of the acquisition system  60 ′ is connected to the laser diode  36  and controls the transmission of the signal therefrom. In a simplest form, the control module  66  turns the laser diode  36  on and off for predetermined periods such that the diode  36  transmits a signal  26  when on and doesn&#39;t transmit when off. In this way, the transmit signal  26   p  is a pulsed or modulated signal as it leaves the telescope  24 . The control module  66  is advantageously configured such that the laser diode  36  is on for a period less than the time of flight of the signal to the other transceiver  20   a ′,  20   b ′ such that a continuous signal does not extend between the transceivers  20   a ′,  20   b ′. Other forms of control may alternatively be utilized such that the transmitter  36  transmits a modulated signal  26 P. 
         [0043]    As shown in  FIG. 13 , the modulated transmit signal  26   p  arrives at the other transceiver  20   a ′,  20   b ′ as a modulated signal. If the signal  26   p  is not aligned with the optical fiber  32 , it reflects off of the reflective surface  44   iv  of the mirror  42   iv  as a modulated retro-reflective signal  28 . The modulated retro-reflective signal  28  will thereafter proceed as described above with respect to the other embodiments. Once final alignment is achieved, the transmitter  36  is no longer controlled to transmit a modulated signal, but instead is returned to control of the free space optical communication system  10  to transmit desired data signals. The control module  66  is easily activated again if alignment is lost and the alignment procedure must be initiated. 
         [0044]    Referring to  FIGS. 15A-15D , an exemplary acquisition sequence will be described. In  FIG. 15A , transceiver  20   a  transmits a signal  26   a  which is not in the FOV of telescope  24   b  and transceiver  20   b  transmits a signal  26   b  which is not in the FOV of telescope  24   a . The acquisition system  60  of each transceiver  20   a ,  20   b  adjusts the alignment of the respective telescope  24   a ,  24   b  in accordance with a macro alignment algorithm. 
         [0045]    Referring to  FIG. 15B , the signal  26   a  from transceiver  20   a  is within the FOV of telescope  24   b  and a modulated retro-reflective signal  28   a  is reflected back to telescope  24   a . The retro-reflective signal  28   a  may be generated in any of the manners described herein. In response to receiving the modulated retro-reflective signal  28   a , the acquisition system  60  of transceiver  20   a  begins micro adjustment of the telescope  24   a . The signal  26   b  from transceiver  20   b  is still not within the FOV of telescope  24   a  and no retro-reflective signal is generated. 
         [0046]    In  FIG. 15C , the telescope  24   a  has been precisely aligned and the transmitted signal  26   a  is received in the optical fiber of the transceiver  20   b . The telescope  24   a  locks into this alignment and this alignment may be utilized to macro adjust the telescope  24   b  such that the signal  26   b  is within the FOV of telescope  24   a . Once within the FOV, a modulated retro-reflective signal  28   b  is reflected back to telescope  24   b . In response to receiving the modulated retro-reflective signal  28   b , the acquisition system  60  of transceiver  20   b  begins micro adjustment of the telescope  24   b . Once telescope  24   b  has been precisely aligned, both telescopes  24   a ,  24   b  are fixed in alignment as shown in  FIG. 15D . The free space optical communication system  10  is now ready to transmit bidirectional communications. 
         [0047]    It will be recognized by those skilled in the art that changes or modifications may be made to the above-described embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention as defined in the claims.