Abstract:
A free-space laser communication system and method for compensating for the atmospheric effects and target motion of a target that may occur during free-space laser communication between of a pair of the systems. Each system makes use of a plurality of narrow infrared (IR) laser beams, a means for pointing and tracking the laser, an adaptive optics system and a communications transceiver. Optionally turbo coding techniques may be used to encode data transmitted by each of the systems. The laser communication system is less susceptible to adverse weather effects that could otherwise negatively influence the operation of an optical communication system.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates in general to a free-space laser communication system and, more particularly, to a free-space laser communication method and apparatus designed to compensate for the atmospheric effects and target motion that may occur during free-space laser communication between a pair of such communication systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Laser communication systems have shown great promise to replace currently used radio-frequency (RF) communication systems in many applications. Laser communication systems offer more than an order of magnitude improvement in data bandwidth over conventional RF systems. Separate from the growth of fiber optic communication, free-space laser communication requires transmission of directed laser signals through either the atmosphere (terrestrial) or space (extraterrestrial).  
           [0003]    Free-space laser communication presents numerous challenges that have limited the growth of such systems in commercial markets. Two of the most significant challenges include compensating for atmospheric affects and maximizing line-of-sight pointing accuracy. The result has been the limited development of large commercial systems that are typically mounted on fixed structures and transmit broad laser beams.  
           [0004]    It would therefore be desirable to provide a free-space laser communication system that is smaller than previously designed systems and that is capable of propagating narrow laser beams to and from both fixed and moving platforms. More specifically, it would be desirable to provide a laser communication system that propagates narrow infrared laser beams; compensates for atmospheric effects; and provides for accurate pointing and tracking of the system links in spite of adverse weather conditions that would otherwise negatively impact the performance of the system.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the present invention, a preferred embodiment of a laser communication system is disclosed which comprises a laser source for producing a plurality of narrow infrared wavelength laser beams, a means for pointing and tracking the laser beams, an adaptive optics subsystem, and a communication transceiver. Preferably, two such laser communication systems are provided to form a two-way free-space laser communication link.  
           [0006]    The pointing and tracking subsystem is capable of performing micro-radian class pointing and tracking that is required to take advantage of the benefits that narrow infrared wavelength laser beams provide. The pointing and tracking capability is also an important element in enabling the communication links to be deployed on moving platforms.  
           [0007]    The adaptive optics subsystem performs adaptive correction of phase (i.e., path length) variations in the path of an optical communication system. Unlike conventional systems, the present invention does not utilize a separate beacon to measure the phase variations in the beam path, but instead uses the communication channel as the beacon. The adaptive optics subsystem senses aberrations in the beam&#39;s path using wavefront sensors located at the receiving end of the communications link. The wavefront information is placed on the communications channel for transfer back to the transmitting system, which uses the information to adjust the properties of the transmitted laser beams to compensate for the beam path phase variations. The adaptive optics subsystem employs a closed loop system that continuously corrects for atmospheric aberrations in the path of the transmitted laser beams.  
           [0008]    The communication transceiver preferably utilizes Turbo Code algorithms for data encoding and decoding to partially compensate for signal fades caused by atmospheric variations. Turbo Codes enable the laser communication system to achieve lower bit error rates in the presence of signal fades than is achievable using conventional data encoding/decoding techniques.  
           [0009]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0011]    [0011]FIG. 1 is a simplified diagram showing a plurality of applications for the laser communication system of the present invention;  
         [0012]    [0012]FIG. 2 is a functional block diagram of a preferred embodiment of the laser communication system of the present invention;  
         [0013]    [0013]FIG. 3 is a schematic of the transmitter subsystem of the present invention, including the optics and electronics that produce a modulated, multi-beam link between two transceiver systems;  
         [0014]    [0014]FIG. 4 is a functional block diagram of the beam pointing and tracking system of the present invention;  
         [0015]    [0015]FIG. 5 is a diagram of the scanning process performed by each pair of transceivers of the present invention as a means for acquiring a transmitted laser beam;  
         [0016]    [0016]FIG. 6 shows the re-alignment and re-calibration process each transceiver of the present invention goes through periodically during operation;  
         [0017]    [0017]FIG. 7 is a simplified block diagram of a prior art laser communication system that uses a beacon separate from the communication laser to detect atmospheric aberrations in the transmitted beam&#39;s path;  
         [0018]    [0018]FIG. 8 is a simplified block diagram of a preferred embodiment of the present invention, wherein the communication laser is also used as a beacon; and  
         [0019]    [0019]FIG. 9 is a simplified block diagram of another preferred embodiment of the present invention, wherein the wavefront sensor at one end of the communication link is eliminated. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    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.  
         [0021]    The present invention relates to various aspects of an improved laser communication system. As will become apparent from the remainder of this detailed description, the present invention more particularly relates to the features of a laser communication system that preferably includes a laser source for producing a plurality of narrow beam infrared lasers, an autonomous pointing and tracking subsystem, adaptive optics, and Turbo Coding for data processing. However, while a preferred embodiment of a laser communication system is shown and described herein as a single cooperative apparatus, it will be understood that the various features may be utilized independent from one another.  
         [0022]    Referring to FIG. 1, there is shown a simplified diagram of the various high data rate communication links that can be established with the apparatus and method of the present invention. For example, communication can be supported for fixed line-of-sight (LOS) points between two or more fixed structures, such as buildings  10 , without the need to run fiber optic cables between the buildings. Building-to-building non-LOS communications can be accommodated via one or more airborne or space based relays (i.e., transponders)  12 . These relays  12  can also be used to effect non-LOS communication links between mobile platforms such as trains  14 , airships  16 , aircraft  18 , ships  20 , and land-based vehicles  22 . The present invention can further be used to effect LOS communication links between the mobile platforms  14 - 22 .  
         [0023]    Referring now to FIG. 2, a free-space laser communication system  24  (hereinafter referred to as a transceiver) in accordance with a preferred embodiment of the present invention is illustrated. It will be appreciated that a transceiver  24  will be employed at each end of a communication link (hereinafter, each end shall be referred to separately as  24   a  and  24   b ), such as, for example, on aircraft  18  and one of buildings  10 , to effect a free-space laser communication system.  
         [0024]    Transceiver  24  generally includes a conventional network interface  26  that connects transceiver  24  to various data sources that are well known to those skilled in the art. Network interface  26  produces an output that is communicated to data encoding/decoding electronics  28  via a conventional fiber optic cable  30  or other suitable connecting device. The encoded data is transferred from encoding/decoding electronics  28  to transmitter board  32  through a suitable conductor, for example, a conventional 50-ohm coaxial cable  34 . Transmitter board  32  superimposes the data upon a plurality of laser beams  36   a - 36   c  for transmission to the other end of the communication link. The laser beams  36   a - 36   c  are generated by a laser beam generating subsystem  33 , which is comprised of the transmitter board  32  and laser beam generating sources  35   a - 35   c . Before exiting transceiver  24 , the optical characteristics of the laser beams are modified by means of deformable mirrors  38   a - 38   c . These adjustments are necessary to correct for fluctuations in beam intensity due to scintillation caused by atmospheric variations in the path of the laser beams, thereby producing optically corrected laser beams  39   a - 39   c . Finally, laser beams  39   a - 39   c  pass through an aperture  42  formed by the body of telescope  40 , whereupon the laser beams travel along substantially parallel paths through the atmosphere to the other end of the communication link.  
         [0025]    In addition to providing a means for aiming laser beams  39   a - 39   c , telescope  40  also provides a means for receiving laser beams transmitted from the other end of the communication link. Received laser beams  44  pass through aperture  42  of telescope  40  and are directed onto beam steering mirror  46  by means of a series of mirrors not shown, but which are well known to the skilled artisan. Beam steering mirror  46  focuses the received laser beams  44  onto a deformable mirror  48 . Deformable mirror  48  performs a similar function as deformable mirrors  38   a - 38   c , in that deformable mirror  48  adjusts the optical characteristics of received laser beams  44  to correct for fluctuations in beam intensity due to scintillation caused by atmospheric variations in the path of the laser beams. Deformable mirror  48  generates an optically corrected laser beam  50 .  
         [0026]    Laser beam  50 , the properties of which have been previously adjusted using deformable mirror  48 , is split into two separate beams  56  and  54  by means of a conventional beam splitter  52 . Beam splitter  52  does not alter the optical characteristics of beams  54  and  56 . Laser beam  54  is directed towards a wavefront sensor  58  which is used to analyze the optical characteristics of the received laser beam. Wavefront sensor  58  produces an output that is sent to a wavefront processor  62 . Wavefront processor  62  compares the optical characteristics of the received beam to those of a reference laser beam that has no atmospherically induced aberrations (i.e., a theoretically perfect laser beam). If wavefront processor  62  determines that the optical characteristics of the received laser beams  44  differ from those of the reference laser beam, wavefront processor  62  sends a signal to deformable mirror  48  instructing the mirror to adjust the optical properties of the received laser beams  44  in order to produce laser beam  50 , which is free of atmospherically induced aberrations. Essentially, components  48 ,  52 ,  58 , and  62  form a closed loop system that continuously corrects, in real time, for atmospheric effects that could optically distort laser beams  44 . Deformable mirrors  38   a - 38   c  and  48 , wavefront sensor  58 , and wavefront processor  62  comprise the adaptive optics subsystem, the features of which are discussed in greater detail below.  
         [0027]    With further reference to FIG. 2, beam  56  passes through a second conventional beam splitter  60 . Beam splitter  60  divides beam  56  into two optically equivalent beams  64  and  66 , which are utilized by a communication receiver  68  and a position tracker  70 , respectively. Communication receiver  68  extracts the embedded data from laser beam  64  using a known method and transfers the information to the appropriate communication system components. In a preferred embodiment of the present invention, beam  64  will not only carry communication data, but will also carry wavefront and pointing and tracking information. Communication receiver  68 , utilizing a suitable known extraction method, extracts the wavefront information from the received laser beam  64  and sends the information electronically to wavefront processor  62  for processing. Wavefront processor  62  uses the information to determine the proper setting for deformable mirrors  38   a - 38   c . Communication receiver  68  also extracts the pointing and tracking data from received laser beam  64  and communicates the information to a pointing and tracking processor  72 . Finally, communication receiver  68  extracts the communication data, which is sent to data encoding/decoding electronics  28  for decoding. The decoded data is sent to network interface  26 , which is preferably coupled to one or more known processors that have been adapted to handle the data and place the same in a usable form or format.  
         [0028]    Continuing to refer to FIG. 2, after leaving beam splitter  60 , beam  66  impinges upon position tracker  70 , which produces a calibrated signal that is communicated to pointing and tracking processor  72 . Based on the calibrated signal, the pointing and tracking processor  72  determines the amount of correction, if any, that needs to be made to the pointing angle of telescope  40  and beam steering mirror  46 . The pointing position of telescope  40  is adjusted by means of pointing gimbals  74 , which are controlled by the pointing and tracking processor  72 . Position tracker  70 , pointing and tracking processor  72 , beam steering mirror  46 , pointing gimbals  74 , and telescope  40  comprise the pointing and tracking subsystem, the features of which are discussed in greater detail below.  
         [0029]    With the foregoing description of a preferred embodiment of transceiver  24  as background, the various specific features of the present invention will now be described in greater detail.  
         [0030]    I. Narrow Beam Infrared Laser Source  
         [0031]    Preferably, as indicated in FIG. 2, a minimum of three laser sources  35   a - 35   c  will be utilized to generate the transmitted laser beams  36   a - 36   c . Each laser source  35   a - 35   c  generates a single infrared laser beam with a wavelength of preferably about 1.55 micrometers. Laser sources  35   a - 35   c  are preferably spaced apart by some predetermined distance that will allow the emitted laser beams  36   a - 36   c  to be separated upon leaving their respective source. Although each laser beam  36   a - 36   c  will initially be separated, all of the beams will preferably overlap to some extent upon reaching the receiver at the other end of the communication link. Using multiple transmitters operating at a wavelength of about 1.55 micrometers wavelength allows for averaging of the signal power at the receiving end of the communication link, thereby increasing the overall signal strength. Moreover, using multiple transmitters versus a single transmitter increases the probability that the receiver will have enough signal power to decode the transmitted data.  
         [0032]    Referring now to FIG. 3, there is shown a preferred embodiment of a communication transmitter  80  for generating multiple infrared laser beams used to transmit data between the ends of a communication link. As described herein, encoding/decoding electronics  28  receives data from network interface  26  (shown in FIG. 2) by means of the conventional fiber optic cable  30 . The data is encoded by means of Turbo Coding algorithms that are stored in a memory of the encoding/decoding electronics  28 . Although Turbo Codes are well known and commonly used within the communication industry, adapting turbo codes for use in connection with a free-space laser communication system is believed to be a novel application of this technology. Turbo Coding is discussed in greater detail below.  
         [0033]    Encoding/decoding electronics  28  produces an output signal that is communicated to the transmitter board  32 , preferably by means of the 50-ohm coaxial cable  34 . The encoded data acts as an input signal for a laser diode driver  81 , which produces a control signal that directly modulates a laser diode  82 . Although the light beam is preferably generated using a compact semi-conductor diode, it shall be appreciated that there are other equally acceptable methods for generating a beam of light in accordance with the present invention. Laser diode  82  produces an infrared light beam that passes through a conventional single mode fiber optic cable  83  to a known fiber optic variable attenuator  84 . Variable attenuator  84  provides a means for adjusting the intensity of the light beam produced by laser diode  82 . Conventional FC/PC connectors  85  are used to connect cable  83  to components  82  and  84 .  
         [0034]    The infrared beam travels from variable attenuator  84  through a conventional single mode fiber optic cable  86  to optical converter  88 . Fiber optic cable  86  is connected to variable attenuator  84  using conventional FC/PC connector  85 . Optical converter  88  splits the single infrared beam into multiple beams, each of which has a different wavelength. Lastly, optical converter  88  amplifies all of the infrared beams to the same power level.  
         [0035]    The infrared beams travel from optical converter  88  through conventional single mode fiber optic cables  92   a - 92   c  to fiber collimators  94   a - 94   c . Fiber collimators  94   a - 94   c  are of a conventional design and function by expanding and collimating the infrared beams. Upon exiting fiber collimators  94   a - 94   c , the infrared beams pass through a divergence setting lens  96  that is used to set the beam divergence to some predetermined level, which is preferably in the range of 100 TO 500 microradians. The infrared beams then pass through a conventional cored fold mirror  98 . Upon exiting core fold mirror  98 , the beams are directed upon a set of deformable mirrors  38   a - 38   c , which adjust the optical properties of the beams based upon inputs received from wavefront processor  62 . Deformable mirrors  38   a - 38   c  and wavefront processor  62  are discussed in greater detail in the Adaptive Optics section found below. The infrared beams then pass through a conventional collimating lens  100 , which produces very narrow laser beams with diameters preferably in the range of 1 to 3 cm. The laser beams exit the system by passing through aperture  42  of telescope  40 .  
         [0036]    II. Autonomous Pointing and Tracking  
         [0037]    To benefit from the link margin advantages of a narrow beam laser communication system, it is strongly preferable that the system be able to achieve micro-radian class pointing accuracy. Referring again to FIG. 2, pointing and tracking is preferably performed by means of the conventional gimbals  74  and the telescope  40 , which provides coarse, large-angle pointing capabilities, and beam steering mirror  46 , which provides high-bandwidth control of small angular motions and tilt correction. Pointing and tracking processor  72  controls the movement of telescope  40  and beam steering mirror  46  based on information received from position tracker  70  and communication receiver  68 .  
         [0038]    Referring to FIG. 4, the pointing and tracking subsystem is implemented via pointing and tracking processor  72 . Pointing and tracking processor  72  determines the received signal strength, the position of the received laser beam on the detector, and the communication signal transforms. Pointing and tracking processor  72  utilizes conventional serial interfaces to connect the processor to the various communication system components, including position sensing device  70 , a temperature controller  112 , a motion controller  110 , and a wireless communication apparatus  114 . Motion controller  110  controls the position of telescope  40  and beam steering mirror  46 , both of which are shown in FIG. 2.  
         [0039]    The pointing and tracking subsystem&#39;s functions include initially aligning transceivers  24   a  and  24   b , maintaining the maximum laser beam energy on the detector (not shown) of communication receiver  68  (see FIG. 2) during system operation, and performing periodic realignment and recalibration of the pointing and tracking subsystem (i.e., components  40 ,  46 ,  70 ,  72 , and  74 ). Initial alignment is performed during installation as part of the set-up process and includes manually pointing transceivers  24   a  and  24   b  at each other. When transceivers  24   a  and  24   b  are pointing towards one another the automated acquisition process can begin. The process commences with the pointing and tracking subsystem of transceivers  24   a  and  24   b  each performing a nested pair of conical scans  120  and  140  as shown in FIG. 5. The scans are performed using the transmitted laser beams from transceivers  24   a  and  24   b . The pointing and tracking processors  72  (see FIGS. 2 and 4), one of which is included as part of each transceiver  24   a  and  24   b , are time-synchronized so that the nested pair of conical scans are performed at the appropriate time.  
         [0040]    Still referring to FIG. 5, transceivers  24   a  and  24   b  begin their respective scans at some pre-synchronized time. Transceiver  24   a  holds its pointing gimbals  74  (see FIG. 2) at position  122  of conical scan pattern  120 , while transceiver  24   b  uses its pointing gimbals to scan through positions  142 ,  144 ,  146 , . . . , n, of conical scan pattern  140 , where n is some pre-determined number of steps. After transceiver  24   b  completes conical scan  140 , transceiver  24   a  then moves to position  124  of conical scan pattern  120 , and transceiver  24   b  repeats conical scan pattern  140 . When transceiver  24   a  detects a laser beam from transceiver  24   b  on its receiver, transceiver  24   a  stops and holds its position. Since the transmitter and receiver subsystems of transceiver  24  share the same optical telescope  40 , and the laser beams produced by transceivers  24   a  and  24   b  are the same diameter, when transceiver  24   a  detects a beam from transceiver  24   b  on its communication receiver (element  68  of FIG. 2), transceiver  24   b  will also detect a beam from transceiver  24   a  on its communication receiver (element  68  of FIG. 2). Accordingly, transceiver  24   b  will also hold at the detected beam position. Initial acquisition for transceivers  24   a  and  24   b  will then be established.  
         [0041]    Once initial acquisition is established, transceivers  24   a  and  24   b  automatically transition to a closed loop tracking mode as a means for maintaining the beam from each transceiver on the detector of communication receivers  68  of its system  24  (see FIG. 2). Transceivers  24   a  and  24   b  store in memory their respective position pointing angles as established during the initial acquisition procedure. While operating in the closed loop tracking mode, each transceiver maintains its respective position pointing angle despite motion caused by building sway, wind, vibration, or atmospheric effects.  
         [0042]    Each transceiver  24   a  and  24   b  also maintains in memory a pointing angle time history. If acquisition is lost, automatic re-acquisition can begin at the last known valid pointing angle. The time history provides an improved re-initialization pointing angle that will enable the system to reestablish a communication link after long periods of signal loss (which may be caused, for example, by very dense fog). Typically, the optimum pointing angle is a historical function of temperature and/or time of day. Finally, while operating in the closed loop tracking mode, each transceiver will periodically re-synchronize in time so if alignment and tracking are lost, the time synchronization protocol will dictate when reacquisition starts.  
         [0043]    During operation, each transceiver may require periodic in-system realignment and recalibration. Realignment and recalibration equates to making the outgoing beam path the same as the incoming line-of-sight zero-error track reference. The new offset track reference maximizes the output energy to the other transceiver&#39;s receiver.  
         [0044]    Referring to FIG. 6, realignment and recalibration consists of transceiver  24   a  performing a peak power scan with its laser  39   a - 39   c  while transceiver  24   b &#39;s position sensing device  172  (part of communication receiver  68 ) measures received power for each pointing angle of transceiver  24   a &#39;s scan. Transceiver  24   b  transmits its received power back to transceiver  24   a , where the information is used to determine transceiver  24   a &#39;s peak transmitting power as a function of pointing angle, a sample of which is shown graphically at  176 . After transceiver  24   a  completes its peak power scan, transceiver  24   b  then commences the same peak power scan while transceiver  24   a &#39;s position sensing device  162  (part of communication receiver  68 ) measures received power for each pointing angle of transceiver  24   b &#39;s scan. Transceiver  24   a  transmits it received power back to transceiver  24   b  where the information is used to determine transceiver  24   b &#39;s peak transmitting power as a function of pointing angle, a sample of which is shown graphically at  166 . The updated offsets are saved in memory as new zero-error track references on transceiver  24   a  and  24   b &#39;s position sensing devices  162  and  172  respectively. The received power measurement taken at each step of a peak power scan is time averaged long enough to provide an overall power measurement that is independent of short-term atmospheric effects.  
         [0045]    III. Adaptive Optics  
         [0046]    The adaptive optics subsystem of the laser communication system of the present invention provides adaptive correction of phase (path length) variations in the path of the laser beam. Referring back to FIG. 2, the adaptive optics subsystem is comprised of the wavefront sensor  58 , which is used to sense the aberrations in the transmitted beam&#39;s path, the wavefront processor  62 , which is used to process the data produced by wavefront sensor  58 , and the deformable mirrors  48  and  38   a - 38   c , which are used to correct for the aberrations in the beam&#39;s path. The conventional beam splitter  52  provides a reference beam for wavefront sensor  58 . The adaptive optics subsystem and the known reconstruction algorithms used to control deformable mirrors  38   a - 38   c  and  48  are used to correct for the aberrations in the beam path between the two ends of the communication link.  
         [0047]    Wavefront sensor  58  is used to sense the perturbations in the beam path between the ends of the communication link. The skilled artisan will appreciate that there are many types of known wavefront sensors that can be used to perform this function, including interferometric and Hartmann approaches. Furthermore, those skilled in the art will appreciate that wavefront sensor  58  can be used to either directly observe the phase tilt of the beam path or indirectly observe an affect caused by the aberrations, such as a change in the transmitted beam&#39;s intensity or image sharpness.  
         [0048]    Although conventional adaptive optic systems have the capability to correct for significant atmospheric aberrations, it will also be understood by persons skilled in the art that the signal-to-noise ratio of conventional wavefront sensors, as well as the speed of the camera and processors used in connection with the wavefront sensors, effectively limit the amount of correction that can be performed by an adaptive optics system. However, by using one of the techniques shown in FIGS. 8 and 9, it is possible to alleviate the signal-to-noise limitation.  
         [0049]    Referring to FIG. 7, a simplified diagram of a laser communication system is shown that uses a conventional method of wavefront correction. In this method, beacons  180  and  182  produce reference laser beams  181  and  183 , respectively, used to sense the atmospheric aberrations in the paths of the beams. Beams  181  and  183  are never corrected before being transmitted and are therefore susceptible to the full depth of fading (aberrations) caused by the atmosphere. On the other hand, if the beams  181  and  183  were corrected prior to being transmitted, or, more economically, a transmit laser beam  184  is used as a beacon, the fading effects of the atmosphere can be progressively corrected and the signal-to-noise ratio requirement placed on the wavefront sensor can be reduced.  
         [0050]    Now referring to FIG. 8, there is shown a simplified diagram of a laser communication system in which transmit laser beams  190  and  192  are used as beacons, in accordance with a preferred embodiment of the present invention. This scheme, however, requires that wavefront information from one end of the communication link be transmitted at a high data rate to the other end of the link. Since this is a high-speed communications system, the wavefront information will preferably be placed on a SONET service channel or pilot tone.  
         [0051]    Continuing to refer to FIG. 8, although the beacon used in a conventional adaptive optics system (see FIG. 7) is typically separate from the communication beam that is being corrected, it is nevertheless assumed that most atmospheric effects are “seen” by the beacon. However, because the beacon is not corrected prior to being transmitted, the amount of correction that the adaptive optics subsystem can achieve is limited by its own signal-to-noise ratio. To overcome this limitation, transmit lasers  190  and  192 , which are used as beacons in a preferred embodiment of the present invention, are pre-corrected by the transmitter&#39;s adaptive optics subsystem using deformable mirrors  38   a - 38   c , which are shown in FIG. 2. Since the transmit beams  190  and  192  have been pre-corrected before reaching the wavefront sensor  58  of the receiving transceiver  24 , the fading effects that limit the use of a conventional wavefront sensors are minimized. To ensure that the entire atmosphere is sampled though, the wavefront information must be made available to the transceiver  24  at the other end of the communication link to allow it to fully correct its beam. Allowing the wavefront information to be placed on the communications channel (i.e., beams  190  and  192 ) for use by the communication receiver  68  (see FIG. 2) reduces the effects of wavefront sensor sensitivity and dynamic range and allows transmitted lasers  190  and  192  to be used as beacons for the adaptive optics subsystem.  
         [0052]    Referring again to FIG. 2, deformable mirrors  48  and  38   a - 38   c  cooperate with one another to correct for atmospheric aberrations in the transmitted beam&#39;s path. Deformable mirror  48  provides phase delays in the optical path to compensate for the aberrations sensed by wavefront sensor  58 . If the aberrations become too large for deformable mirror  48  to completely correct, wavefront processor  62  instructs deformable mirrors  38   a - 38   c  to precondition transmit lasers  36   a - 36   c  with a phase profile. This results in the corrected laser beams  39   a - 39   c , which possess near-diffraction limited beam divergence upon arriving at the transceiver  24  at the other end of the communication link.  
         [0053]    Although the adaptive optics subsystem preferably employs miniature electro-mechanical systems (MEMS) deformable mirrors due to their performance capability and potential for low cost, it will be appreciated that there are other alternative apparatuses that may be used to achieve the same result. However, by using deformable mirrors in a free-space laser communication architecture, a cost-effective solution can be achieved to adaptive-optics correction of the communication path.  
         [0054]    For those applications where one end of the communication link is located on a moving platform, for example where the communication link is established between an aircraft  18  and a building  10  (see FIG. 1), the wavefront sensor on the stationary end of the communication link (i.e., building  10  of FIG. 1) is preferably eliminated. Referring to FIG. 9, there is shown, in accordance with another preferred embodiment of the present invention, a simplified diagram of a laser communication system in which only the transceiver employed on the mobile end of the communication link  200  (i.e., on aircraft  18  of FIG. 1) utilizes a wavefront sensor  58 . System  202  is identical to transceiver  24 , except that it does not include a wavefront sensor  58 . System  202  is disposed at the stationary end of the communication link (i.e., on building  10  of FIG. 1). All wavefront information is transmitted from the mobile transceiver  200  to the stationary transceiver  202  via transmit laser  206 .  
         [0055]    IV. Data Encoding and Decoding  
         [0056]    A preferred embodiment of the present invention incorporates Turbo Code type algorithms for encoding and decoding the transmitted data as a means to partially compensate for the signal fades caused by atmospheric scintillation. Although adapting Turbo Codes for use in telecommunication devices is not new per se, these codes are not believed to have been adapted for use with a laser communication system. The Turbo Codes are preferably incorporated as part of the data encoding/decoding electronics  28  (see FIGS. 2 and 3).  
         [0057]    A Turbo Code is a parallel concatenation of two or more systematic codes that can reduce bit error rates (BERs) in the presence of signal fades. Turbo Codes allow a communications system to achieve lower BERs in the presence of fading. Turbo Codes, which were introduced by Berrou, Glavieux and Thitimasjshima in 1993, offer large block code lengths, while keeping complexity of the decoder to a minimum. The key to the encoder is a pseudo-random interleaver followed by recursive encoders. The parallel output from each is concatenated to form a Turbo Code.  
         [0058]    Many kinds of fading are assumed in the literature for radio transmission such as Rayleigh, Nakagami, and Rician. The simplest model, Rayleigh, assumes that the channel has multiple paths whose magnitude is Gaussian distributed and phase is uniformly distributed. None of these models, however, are accurate for a laser transmitted through the atmosphere. Since the index of refraction structure function has Kolmogorov statistics, the power received is usually assumed to have a lognormal distribution. It should be noted that the lognormal distribution degrades the signal in a channel by increasing the standard deviation as the signal mean decreases. Application of the Turbo Coder to propagation through the atmosphere will improve the BER of the communication link, which will thereby improve the overall communication link efficiency.  
         [0059]    The detailed description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.