Abstract:
A free space optical communication system is disclosed whereby a transmit telescope and a receive telescope are aligned such that the point of maximum power of the received light beam is incident upon the optical fiber located at a specific point on the focal plane of the receive telescope. Such incidence is achieved by causing the transmitted beam to diverge by moving the transmit optical fiber to a point in front of the focal plane of the transmit telescope until at least a portion of the beam is incident upon the receive optical fiber. The transmit telescope and/or the receive telescope (or, alternatively, only the respective optical fibers of those telescopes) are then moved such that the point of maximum received power for that degree of divergence is incident upon the receive optical fiber. The transmit optical fiber, in response to received power measurements at the receive telescope, then incrementally moves back toward the focal plane of the transmit telescope thereby reducing the degree of divergence and achieving a greater signal per unit area at the receive telescope. As the divergence is reduced, the position of the either the transmit telescope and/or the receive telescope (or, respectively, the optical fibers) is incrementally adjusted to maximize the received power of the received signal which is incident upon the receive optical fiber for each degree of divergence. By monitoring received signal power this iterative process continues until the transmit and receive telescopes are aligned such that the transmitted beam is focused so as to maximize the received power that is incident upon the receive optical fiber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims subject matter that is described in United States Patent Application filed simultaneously herewith and entitled “Method and Apparatus for Controlling Received Power Levels Within A Free Space Communication System,” Ser. No. 09/680,165, and United States Patent Application filed simultaneously herewith and entitled “Telescope for Free Space Optical Communication System,” Ser. No. 09/679,159. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related generally to data communication systems and, in particular, to free-space optical data communication networks. 
     BACKGROUND OF THE INVENTION 
     Traditional telecommunication systems that connect two or more sites with physical wire or cable are generally limited to relatively low-speed, low-capacity applications. In order to address these limitations, recently developed systems utilize optical fibers. Yet, fibers still require a physical cable connection. To remove this limitation, systems utilizing the free-space transmission of one or more light beams modulated with data have been developed. Systems using such beams greatly improve data speed and capacity rates, up to 10 Gbps, over traditional wire-based systems and, at the same time, avoid the traditional communication system infrastructure cost of laying fiber cable to physically connect one site in the system to another. Instead of cables, free-space optical communications systems consist, in part, of at least one transmit telescope and at least one receive telescope for sending and receiving information, respectfully, between two or more communications sites. Each of these telescopes contains optics, consisting of at least a primary mirror and a secondary mirror or a lens. The transmit telescope uses its optics to transmit the light beam to the receive telescope. The receive telescope uses its optics to focus the incoming light beam onto the focal plane of the telescope. Generally, each telescope is attached to a communications network or other source/destination of information. In operation, the transmit telescope receives information from its respective network via cable or wireless transmission, and then transmits a light beam modulated with this information to one or more destination receive telescopes. Each receive telescope then relays data to its intended destination in its respective network via a cable or wireless transmission. 
     The aforementioned free-space communications systems would, therefore, appear to have the benefits of reducing costs associated with installing and maintaining physical hard-wired portions of networks while, at the same time, increasing transmission capacity. However, free-space optical communications may be hampered by a variety of factors. For example, since the transmit and receive telescopes may be located a great distance from each other, initial alignment of the telescopes, to insure that the transmitted light beam is incident upon the focal plane of the receive telescope, may be difficult to achieve. Additionally, even if initially aligned, misalignment of the transmit and receive telescopes may result from any displacement of the light beam during transmission or any movement of either the transmit or receive telescopes or their respective physical mounting platforms. As a result of such misalignment, the transmitted light beam may not be incident upon the focal plane of the receiving telescope, or may only be partially incident thereupon, leading to a loss or degradation of communications connectivity. 
     Another problem with free-space optical communications results from the variation in atmospheric conditions. Specifically, since conditions like fog or snow can interfere with the transmitted light beam in such systems, the transmit telescope must produce a light beam with power sufficient to maintain communications connectivity in such variable conditions. In the absence of such signal-degrading conditions, however, the power of the received light beam may overload the electronics of the receive telescope. While the power to the laser or laser amplifier can be reduced to compensate, this may mean operating the devices at gains where they do not operate efficiently. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems related to initial alignment and to potential loss of communications connectivity due to misalignment occurring during communications operations are essentially eliminated with the present invention. 
     In accordance with the present invention, during initial alignment of the transmit and receive telescopes, the cross-sectional area of the transmitted light beam is increased beyond the normal cross-sectional area of a focused (parallel) beam at the point where the receive telescope is or should be. The cross-section of the transmitted beam is thus physically larger at the receiving end, thereby increasing the likelihood that the received light beam will be incident upon the focal plane of the receive telescope. This increase in the cross-sectional area of the transmitted beam is effected by varying the divergence of the transmitted beam. By monitoring at the receive telescope a measurable signal parameter such as, for example, received signal power, and providing some type of feedback to the transmit telescope, the divergence of the transmitted light beam can be varied until the beam is incident upon the optical fiber at the focal plane of the receive telescope. Once the transmitted beam is detected at the receive optical fiber by measuring a detectable level of receive signal power, the transmit optical fiber and/or the receive optical fiber are aligned with each other to maximize the received signal power by, in a first embodiment, physically moving the entire transmit telescope until maximum transmitted power is incident upon the receive telescope. Alternatively, in accordance with a second embodiment, instead of moving the entire telescope apparatus to achieve alignment, certain telescope mirror designs, such as those defined by a non-standard conic constant, allow for the movement of only the optical fiber located at the focal plane of the transmit telescope such that the transmitted light beam is incident upon the receive telescope. 
     In further accordance with the present invention, after the transmitted beam is diverged, the receive telescope is physically moved so that maximum power of the transmitted beam is incident upon the receive optical fiber. 
     Once alignment is accomplished, by moving the transmit telescope and/or the receive telescope, or by moving the optical fiber at the focal plane of either or both of those telescopes, maximum receive power is achieved by decreasing the divergence of the transmitted light beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an optical communication system using a prior art telescope apparatus during normal communications conditions; 
     FIG. 2 shows an optical communication system using a prior art telescope apparatus wherein interference such as turbulence or attenuation between the receive and transmit telescopes causes the degradation or loss of communications; 
     FIG. 3 shows an optical communication system using a prior art telescope apparatus wherein relative movement between the transmit and receive telescopes causes the loss or degradation of communications; 
     FIG. 4 shows an optical communication system consisting of a telescope with moveable optics to facilitate alignment of the transmit and receive telescope; 
     FIG. 5 shows the transmit telescope in the system of FIG. 4 that is capable of diverging the transmitted beam and, thus, maintaining received signal power below a specified threshold; and 
     FIG. 6 shows the receive telescope in the system of FIG. 4 that is capable of maintaining received signal power below a specified threshold. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a diagram of two prior art telescope optical communication telescopes,  101  and  102 , during normal aligned operating conditions in a free-space optical communications system. Laser  130  produces an optical light beam that is modulated with data received from network  110  and transmitted on optical fiber  106 . The transmit telescope  101  receives the modulated optical signal via optical fiber  106 . Then, the primary mirror  120  and secondary mirror  121  optically shape and transmit the modulated light beam such that the beam is incident upon the focal plane  125  of receive telescope  102 . Receive telescope  102  utilizes its optics, consisting of a primary mirror  122  and a secondary mirror  123 , to focus the incident transmitted modulated light beam  103  onto the receive optical fiber  112  at the focal plane  125  of the receive telescope  102 . The receive optical fiber then transmits the modulated optical signal to receiver  129  which converts the optical signal to an electrical signal, demodulates the data, and forwards the data to network  109 . 
     In certain situations, however, the light beam transmitted by transmitting telescope  101  may not be incident upon the optical fiber at the focal plane of the receive telescope  102 . For example, installation of the telescopes will require an initial alignment to insure that the transmitted beam is incident upon the focal plane of the receive telescope. This is necessary to establish initial communications connectivity between the transmit and receive telescopes. Also, as shown in FIG. 2, atmospheric interference, such as turbulence  204  or attenuation  205 , anywhere along the path between telescopes  201  and  202 , may cause the transmitted beam  203  to deviate from the direct path between the transmit and receive telescopes such that it partially or entirely misses the receive telescope  202 . The result is the loss or degradation of communications between the two telescopes. In this case, realignment may be necessary to possibly reestablish communications and to maximize received signal power. 
     FIG. 3 shows another prior art situation where the light beam transmitted by the transmit telescope is not incident upon the receive telescope&#39;s focal plane. In this case, transmit telescope  301  transmits light beam  303 , which is not incident upon the receive telescope  302  due to initial misalignment or relative movement between the transmit and receive telescopes during communications operations. Such movement during communications operations could result from high wind conditions, movement of the underlying support structure, or any number of other potential causes. 
     FIG. 4 shows one embodiment of the present invention that utilizes variable divergence of the transmitted light beam to initially align the transmit and receive telescopes and/or to realign a misalignment of the transmit and receive telescopes during communications, as discussed above. In the free-space telecommunications system in FIG. 4, laser  419  produces a light beam that can be modulated by modulator  418  with data from network  410 . This modulated light beam is then transmitted to telescope  401  which shapes the beam  403  so that it is incident on the focal plane of receive telescope  402 , which is connected to network  409 . Although there may be a physical, hard-wired, relatively low-bandwidth connection  417  between networks  409  and  410 , the free-space system provides a high-speed and high-capacity link between these networks. 
     In accordance with the present invention, prior to initial alignment, the transmitted light beam may not be incident upon the focal plane of the receive telescope  402 . Thus, referring to FIG. 5, during initial alignment of the telescopes the optical fiber  406  at the focal plane of the transmit telescope is moved in the z-direction to a point in front of the focal plane along the transmit telescope&#39;s longitudinal axis of its primary mirror  420  thereby causing light beam  504  to diverge as it is transmitted from the transmit telescope. Thus, rather than being transmitted with a uniform cross sectional area as shown by beam  103  in FIG. 1, the light beam diverges with an increasing cross-section area as the distance from the transmit telescope  401  increases. This, therefore, increases the likelihood that the diverging transmitted beam  403  will be incident upon the optical fiber,  412  in FIG. 4, located at the focal plane of the receive telescope. This increase in the beam&#39;s cross-sectional area results, however, in a reduced signal power for any given cross-sectional area at the receive telescope,  402  in FIG. 4, as compared to a focused beam with a uniform cross-sectional area, since maximum received signal power is achieved with a focused non-diverged beam. Therefore, once the divergent transmitted beam is detected as being incident upon the receive telescope, and the transmit and receive telescopes are aligned in a manner as described below, the transmitted beam can again be focused by reducing the divergence through the movement of the optical fiber  406  in the z-direction back toward the focal plane of the transmit telescope. The smaller cross section of the transmitted beam thus results in more power per unit area of the transmitted beam at the receive telescope. Such variable divergence, followed by realignment, allows for achieving the greatest signal power possible by correcting any initial misalignment. This same method can also be used to correct any subsequent misalignment that occurs during communications operations between the transmit and receive telescopes. 
     In order to align the transmit and receive telescopes, a feedback mechanism is desirable to communicate information there between. Referring once again to FIG. 4, at receive telescope  402 , the incoming light beam is converted by photo-detector  432  to an electrical signal, which is passed to receiver  429 . Power meter  408  measures the signal power of the received signal and passes this measurement to controller  430 . During initial alignment stages this received signal power may be zero or below a specified minimum level. Controller  430  then sends a signal to controller  415  at the transmit telescope to initiate divergence of the transmitted light beam. When controller  415  receives this signal, it outputs a signal to motorized stage  416  that effects the movement of optical fiber  406  to the front of the focal plane of the transmit telescope  401  along the longitudinal axis of it&#39;s primary mirror (z-axis). It should be noted that any mechanism for moving the optical fiber at the focal plane of the transmit telescope in the z-direction, such as a calibrated indent using finite steps, may be used. Such devices are well known in the art. As previously described, this movement of the optical fiber to a point in front of the focal plane of the transmit telescope results in the divergence of transmitted light beam  403  such that it presents an increased cross-section at the receive telescope  402 . 
     In order to realign the telescopes, it may be necessary to redirect the transmitted beam so that it is incident upon the receive telescope when the beam is not diverged. Therefore, in one embodiment of the present invention, the entire transmit telescope is moved using gimbals, or any well-known equivalent device, to redirect the transmitted beam so that it is incident upon the receive telescope. In another embodiment of the present invention, mirrors defined by a non-standard conic constant may be used in the optics of the transmit and receive telescopes  401  and  402 , respectively. Such non-standard conic constant mirrors are the subject of the copending Patent application filed simultaneously herewith entitled “Telescope for a Free-Space Optical Communication System,” having Ser. No. 09/679159, and which is incorporated by reference herein. The use of such mirrors in free-space optical communications is the subject of the aforementioned copending patent applications which are incorporated herein by reference. Mirrors with a non-standard conic constant are advantageous in that their use in free-space optical communications obviates, within limits, the need to move the entire transmit telescope structure to achieve initial alignment or to correct alignment during communications operations. Rather, by using such mirrors, the focal plane of the transmit and receive telescope is enlarged. By moving the transmit optical fiber within that enlarged focal plane, the resultant beam is transmitted in a different angular direction relative to the transmit telescope. Therefore, as opposed to moving the entire telescope apparatus  401  itself, as is necessary with prior art free-space communication telescopes, a transmit telescope utilizing mirrors with a non-standard conic constant may be aligned by moving optical fiber  406  in the x-y plane within the focal plane of the transmit telescope so that the transmitted beam is incident upon the receive telescope. 
     Any misalignment due to movement of the transmit telescope may be corrected by moving either the transmit telescope or the transmit optical fiber, as described above. However, in cases where the receive telescope is not aligned with the transmit telescope, or where there are multiple cells of turbulence or attenuation in the atmosphere, it may be necessary to also move the receive telescope to achieve alignment. Therefore, in another embodiment of the present invention, the entire receive telescope  402  is moved using gimbals, or any well-known equivalent devices, to achieve alignment between the transmit and receive telescopes and, accordingly, maximize received signal power. Once again, however, in another embodiment of the present invention utilizing mirrors with a non-standard conic constant in the receive telescope, moving the entire receive telescope may be unnecessary. Rather, as in the transmit telescope, a receive telescope utilizing mirrors with a non-standard conic constant may be aligned with the transmit telescope by moving optical fiber  412  in the x-y plane within the focal plane of the receive telescope so that the transmitted beam is incident upon the receive optical fiber. 
     As the entire receive transmit telescope  401  or receive telescope  402  (or, respectively, the optical fibers  406  or  412  of those telescopes) is moved such that the light beam  403  is incident upon the receive optical fiber  412 , the divergence of the transmitted beam  403  may be decreased to achieve a more focused transmitted beam  403 . To effect this increased focus, controller  430  at the receive telescope sends a signal to controller  415  at the transmit telescope that will cause it to reduce the divergence of the transmitted beam. This is achieved through motorized stage  416 , which moves optical fiber  406  to a point along the longitudinal axis of the primary mirror  420  (z-axis) of the transmit telescope that is closer to its focal plane. Iterative power measurements, followed by variations of the degree of divergence of the transmitted beam  403  and movements of the receive telescope  402  or the receive optical fiber  412  in response to those measurements, may be necessary to achieve alignment of the transmit and receive telescopes  401  and  402 , respectively. 
     Once alignment has been achieved as described above, it may be determined that the resulting maximum power level is not the optimum level for communications operations. For example, in free-space telecommunications systems, the maximum transmitted power level must be such that communications are possible even in foggy or snowy atmospheric conditions. This means that in clear atmospheric conditions, for example, the maximum received power level may overload the receiver electronics. Therefore, if the received power exceeds a specified threshold, the power received by the receive optical fiber can be reduced by moving the optical fiber of either the transmit or receive telescope  401  and  402 , respectively, along the z-axis of the primary mirror of the respective telescope. 
     In the first case, this reduction in received power is accomplished by moving the transmit optical fiber  406  to a point in front of the focal plane of the transmit telescope along the z-axis of its primary mirror  420 . During operation, if the received signal power measured by power meter  408  exceeds the specified threshold, controller  430  sends a signal to controller  415  at the transmit telescope to reduce the transmitted power. Controller  415  then sends a signal to motorized stage  416  to effect the movement of optical fiber  406  to a point in front of the focal plane along the axis of the transmit telescope&#39;s primary mirror (z-axis). With reference to FIG. 5, such a movement of the transmit optical fiber  406  causes the transmitted light beam  504  to diverge and, correspondingly, increases the cross-sectional area of the transmitted beam at the focal plane of the receive telescope. Also as previously described, this increase in the beam&#39;s cross-sectional area reduces the power per unit area of the cross section of the received signal, resulting in less power being incident upon the receive optical fiber,  412  in FIG. 4, at the focal plane of the receive telescope  402 . Iterative measurements of received signal power by power meter  408  followed by feedback to the transmit controller  415  allow adjustment of the position of transmit optical fiber  406  to maintain the received power levels below the specified maximum power threshold. A similar reduction in received power may be achieved by moving the transmit optical fiber to the rear of the focal plane of the transmit telescope. However, such a movement would result in the transmitted beam converging at a point along the transmitted beam&#39;s path prior to being incident upon the receive telescope. A beam with a smaller cross section is more susceptible to distortion due to atmospheric disturbances than a beam with a larger cross section. As a result, absent corrective measures (e.g., adaptive optics) that reduce or eliminate such distortion, a reduction in received signal power by moving the transmit optical fiber to the rear of the focal plane may possibly lead to inferior signal quality received at the receive optical fiber. 
     In the second case, when the received signal power exceeds the specified threshold, a reduction in received power is accomplished by moving the receive optical fiber in the z-direction along the axis of the receive telescope&#39;s primary mirror  422 . In this case, the fiber optic may be moved either to a point in front of the focal plane of the receive telescope or, alternatively, to a point to the rear of the focal plane of the receive telescope. Either movement will result in a reduced signal power incident upon the receive optical fiber with no degradation in the received signal quality. FIG. 6 shows the case where the receive optical fiber is moved to a point to the rear of the receive focal plane. In this case, receive telescope  402  utilizes its primary and secondary mirrors  422  and  423 , respectively, to focus the incoming light beam on point  601  within the focal plane of primary mirror  422 . This focal point is the point of maximum received power. Once the incoming light beam passes focal point  601 , however, it begins to diverge and, accordingly, the power per unit area of the beam decreases as the beam&#39;s distance past the focal point increases. If the power meter,  408  in FIG. 4 measures a received signal power that exceeds the specified threshold, controller  430  sends a signal to XYZ positioner  411  to effect the movement of optical fiber  412  to a point which is some distance from the rear of focal point  601 , such as distance  603 , thereby reducing the power incident upon the receive optical fiber. As previously discussed, a similar reduction of power may be achieved by moving the fiber to a point in front of the receive focal plane. By iteratively measuring the received power level at power meter  408 , controller  430  may send signals to XYZ positioner  411  to adjust the location of receive optical fiber  412  to maintain optimum received power levels that do not exceed a specified power threshold. 
     The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included in its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting aspects and embodiments of the invention, as well as specific examples thereof, are intended to encompass functional equivalents thereof. 
     Thus, for example, it should be apparent to one skilled in the art that diagrams herein represent conceptual views of optical telescopes and light beams modulated with data for the purposes of free-space optical communications. Diagrams of optical components are not necessarily accurate in dimension or in relationship to other components but are, instead, merely representative of possible physical arrangements of such components. Also, mirrors represent only one possible type of optical system that could be used in a free-space optical communication system. Any other structure that would provide the same function as the mirrors, such as, without limitation, lenses, could be used. Additionally, it will be apparent to one skilled in the art that all optical fibers depicted in the diagrams represent only one method of transmitting data between telescopes and network destinations. Any other communication method for passing data from the telescopes to network destinations is intended as an alternative to the method shown in the diagram, without limitation. Similarly, in the above described embodiments of the present invention, an optical fiber is located at or near the focal plane of the transmit and receive telescopes. One skilled in the art will recognize that any other device, such as, without limitation, a photo-detector, may be used as an alternative to accomplish the same function 
     Additionally, in the above described embodiment of the present invention, the transmitted beam is made to diverge by moving the transmit optical fiber to a point in front of the focal plane of the primary mirror of the transmit telescope along the longitudinal axis of that mirror. One skilled in the art will recognize that the same effect would result by moving the fiber to a point behind the focal plane. The result of this movement of the fiber would be that the beam would first converge at a point between the transmit and receive telescope along the path of the beam. Then the beam would diverge which would allow the same alignment function to take place as in the case where a divergent beam was initially transmitted by the transmit telescope. While such beams of smaller cross sectional area are more susceptible to atmospheric disturbances, any such disturbance could be corrected through the use of adaptive optics or other corrective measures. 
     In the above embodiment, the controller at the receive telescope communicates the need to diverge the transmit light beam via traditional network connections. However, it should be obvious to one skilled in the art that this feedback information could be accomplished by many different methods including, without limitation, wireless radio frequency transmitters and receivers or even free-space light beam communication. This latter method could be accomplished using a second pair of telescopes, co-located with the transmitting and receiving telescopes. Such telescopes would not be required to communicate with high bandwidth and could, therefore, be less complex and less costly than the primary communications telescopes. Alternatively, the already existing transmitting and receiving telescopes could transmit the feedback signal, in addition to their primary communications function. In this case, the communications system could use a different frequency for the feedback signal to avoid interfering with the primary communications signal. 
     Other aspects of this invention are also merely representative in nature. For instance, while the embodiment presented utilizes traditional network connections to deliver information to and from the telescopes, wireless methods of communication could alternatively be used. Additionally, an XYZ controller is shown as the mechanism for moving receive telescope to bring it into alignment with the transmit telescope. There are many other functional equivalents for accomplishing this function, including without limitation, piezeo-electric drivers and voice-coil devices. The same holds true for the motorized stage used to move the optical fiber at the focal plane of the transmit telescope. Any device that is the functional equivalent of the drivers is intended to be encompassed by the present invention. 
     Finally, it will also be apparent to one skilled in the art that the different aspects of this invention may be implemented together in one communications system or, alternatively, any aspect of this invention may be implement as a “stand-alone” measure to increase communications availability. Nothing herein is intended to require that all aspects of the present invention be included in any single embodiment.