Abstract:
A beam steering apparatus and method for free space optical transceivers is disclosed. The beam steering function is performed internally by way of translating an internal optical fiber in the focal plane of the transceiver telescope using miniature micro-electro-mechanical systems (MEMS). The optical design of the transceiver provides a wide field of view and a pointing and tracking field of regard that is directly proportional to the translation of the optical fiber in the focal plane of the telescope. The apparatus and method can eliminate the need for external gimballing systems, and replace the gimballed free space optical beam steering function with MEMS that consumes very little power.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional patent application No. 60/609,420, entitled “MEMS-Based Optical Communications Beam Steering Apparatus” and filed on Sep. 13, 2004, and U.S. provisional patent application No. 60/609,413, entitled “Apparatus and Method for Free Space Optical Communications Beam Steering without Gimbals” and also filed on Sep. 13, 2004. The entire disclosures of these applications are incorporated herein by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract no. F29601-02-C-0021 awarded by the United States Air Force Research Laboratory. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to the field of optical communications, and in particular to the field of beam steering for free space optical transceivers. More particularly, the present invention relates to a beam pointing and tracking apparatus and method carried out internally within the respective telescopes of a transmitter and receiver, without the use of external gimbals. 
   Laser communications systems are today employed in a vast array of applications, including without limitation communication with aircraft and satellites from ground positions. A laser communication system generally consists of a transmitting terminal and a receiving terminal. A transmitting terminal typically receives an electrical signal from a signal source, and converts the electrical signal into an optical signal. The transmitting terminal 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. 
   In order for a receiving terminal to receive an optical signal from a transmitting terminal, the terminal telescopes must be properly aligned. This alignment process is known as beam steering. Generally, beam steering may be defined as changing the direction of the main lobe of a radiation pattern. In optical systems, beam steering is the manipulation of a transmitting telescope or receiving telescope to point in a desired direction. Other applications for beam steering, in addition to optical communications, include laser illumination, laser designation, laser radar, pointing and tracking, and remote optical sensing. 
   Beam steering in optical systems may be accomplished by changing the refractive index of the medium through which the beam is transmitted, or by the use of mirrors or lenses. In particular, some existing non-gimballed beam-steering solutions include acousto-optics, liquid crystals, electro-optics, micro-optics, galvanometer or magnetic mirrors, and micro-mirror arrays. These types of systems, however, have generally proven to be unwieldy, or lack the speed, precision, and reliability necessary for high-speed, long-distance free-space optical communications. Thus the most common means for beam steering in optical communications systems is by the use of a motorized gimballing system. A gimbal is a mechanical apparatus to allow a suspended object to rotate freely along two simultaneous axes, within a defined angle of view. Gimbals are well known in the art, having been used, for example, since at least as early as the sixteenth century in the suspension of maritime compasses. A gimballing system used for the alignment of an optical transmitter or receiver typically moves the entire transmitting or receiving telescope through the required field of view. Often, the transmitter and receiver telescopes are mechanically coupled so that the transmitted beam is in the exact direction of an incoming optical beam for collection by the receiving telescope, the two telescopes thereby operating with a common gimballing system. 
   Accurate alignment of the transceiver system is essential for free space laser communications systems. Therefore, gimballing systems must provide accurate alignment angular resolution in order for the receiver telescope to efficiently collect the incoming optical beam. Conversely, the transmitter telescope must be able to accurately point its beam so that a remote-receiving terminal can efficiently collect the optical signal for the photodetector. Mechanical gimballing systems have been favored in many free-space optical communications systems because they can provide very fast alignment times coupled with high angular resolution. 
   Gimballed beam-steering systems do, however, suffer from several important disadvantages. Such systems are quite heavy due to the weight of the mechanical components, motors, and servos necessary for such a system. While weight may not be as important a factor in the design of a land-based system, weight is of paramount importance in aircraft and satellite design. Gimballing systems are also quite bulky due to the required mechanical components, which is also a significant disadvantage in the design of airborne and spaceborne systems. Finally, mechanical gimballing systems require the use of a great deal of electrical power, far more power than is typically consumed by the electronics associated with an optical receiver or transmitter system. Again, while power consumption may not be as important a factor in permanent ground-based systems, it is a critically important factor in airborne and spaceborne systems, as well as in mobile ground-based systems such as may be mounted on land vehicles. 
   What is desired then is a beam pointing and tracking system that provides high speed and high angular resolution, with reduced size, weight, and power consumption as compared to traditional gimballing systems now employed in laser communications terminals. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is directed to an optical beam pointing and tracking apparatus and method that provides angular adjustment by means of the movement of an optical fiber residing in the focal plane of the transmitting or receiving telescope. Specifically, the invention comprises a micro-electro-mechanical system (MEMS) translation device residing in the focal plane of the transmitter or receiver telescope. The system accurately and rapidly moves the fiber, thereby providing, in an optical communications system, a corresponding pointing angle change in the output beam, and a corresponding relative angle change in the receiving telescope with regard to the incoming beam angle. 
   The present invention achieves very fast response times while carrying out angular pointing and tracking. Because the present invention requires only the movement of an optical fiber, it requires the consumption of far less power than the mechanical systems that rely upon gimbals. It also allows a transmitter or receiver system to be constructed that is of much smaller size and weight compared to comparable gimballed systems. Because the complex mechanical components of gimballing systems are not required, the overall cost of the transmitter or receiver system is significantly reduced. 
   It is therefore an object of the present invention to provide for an optical beam steering apparatus and method that achieves high speed and angular precision without the use of gimbals. 
   It is a further object of the present invention to provide for an optical beam steering apparatus and method that consumes relatively little electrical power during operation. 
   It is also an object of the present invention is to provide for an optical beam steering apparatus and method that is of a relatively small size and weight. 
   It is also an object of the present invention is to provide for an optical beam steering apparatus and method that has a relatively low production cost. 

   
     These and other features, objects and advantages of the present invention will become better understood from a consideration of the following detailed description of the preferred embodiments and appended claims, in conjunction with the drawings as described following: 
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view of a preferred embodiment of the present invention as employed in a Free Space Optical Transceiver Assembly (FSOTA). 
       FIG. 2  is a diagram illustrating the operation of the FSOTA for an incoming beam angle of Δφ RX     1    and outgoing optical beam at an angle of Δφ TX     1    defined as the respective angles away from the optical Z-axes of the transmitter and receiver telescopes according to a preferred embodiment of the present invention. 
       FIG. 3  is a diagram illustrating the operation of the FSOTA for an incoming beam angle of Δφ RX     2    and outgoing optical beam at an angle of Δφ TX     2    defined as the respective angles away from the optical Z-axes of the transmitter and receiver telescopes according to a preferred embodiment of the present invention. 
       FIG. 4A  is a side elevational view illustrating the translation of an optical fiber in the X-Y plane at a position that provides an input (or output) angle along the Z-Axis of the telescope, represented by the X-Y position of (X 0 , Y 0 )=(0,0) with a fiber output beam angle of Δφ=0 according to a preferred embodiment of the present invention. 
       FIG. 4B  is an end elevational view illustrating the translation of an optical fiber in the X-Y plane at a position that provides an input (or output) angle along the Z-Axis of the telescope, represented by the X-Y position of (X 0 , Y 0 )=(0,0) with a fiber output beam angle of Δφ=0 according to a preferred embodiment of the present invention. 
       FIG. 5A  is a side elevational view illustrating the translation of an optical fiber in the X-Y plane at an optical fiber position that provides an input (or output) angle from the optical fiber of either Δφ 1A  and Δφ 1B  created by the location of the optical fiber&#39;s output in the focal plane of (X 1A , Y 1A ) and (X 1B , Y 1B ), respectively, according to a preferred embodiment of the present invention. 
       FIG. 5B  is a side elevational view illustrating the translation of an optical fiber in the X-Y plane at an optical fiber position that provides an input (or output) angle from the optical fiber of either Δφ 1A  and Δφ 1B  created by the location of the optical fiber&#39;s output in the focal plane of (X 1A , Y 1A ) and (X 1B , Y 1B ), respectively, according to a preferred embodiment of the present invention. 
       FIG. 6A  is a side elevational view illustrating the translation of an optical fiber in the X-Y plane at an optical fiber position that provides an input (or output) angle from the optical fiber of either Δφ 2A  and Δφ 2B  created by the location of the optical fiber&#39;s output in the focal plane of (X 2A , Y 2A ) and (X 2B , Y 2B ), respectively, according to a preferred embodiment of the present invention. 
       FIG. 6B  is a side elevational view illustrating the translation of an optical fiber in the X-Y plane at an optical fiber position that provides an input (or output) angle from the optical fiber of either Δφ 2A  and Δφ 2B  created by the location of the optical fiber&#39;s output in the focal plane of (X 2A , Y 2A ) and (X 2B , Y 2B ), respectively, according to a preferred embodiment of the present invention. 
       FIG. 7  is an end elevation view illustrating the relationship between the actuators and the arms. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , the preferred embodiment of the present invention may be described. The preferred embodiment is a Free Space Optical Transceiver Assembly (FSOTA), but the invention is not so limited, and in fact may be put to many other applications as will be apparent to those skilled in the art. The FSOTA of the preferred embodiment includes a transmitter telescope  1  and a receiver telescope  2 . In alternative embodiments, the present invention could be applied to a transmitter-only or receiver-only arrangement, or alternatively to a system with a transceiver arrangement, that is, a single telescope used for both transmitting and receiving a signal, as are known in the art. The preferred embodiment further includes fiber optic and MEMS device power connectors  3  and  5 , for the transmitter and receiver sections, respectively. The preferred embodiment is operable to generate and send optical energy into transmitter telescope  1  for transmission into free space at transmitted beam  4 , and is operable to receive optical energy at receiver telescope  2  in the form of received beam  6 . In communication with transmitter telescope  1  and receiver telescope  2  are transmitter MEMS beam steering module  7  and receiver MEMS beam steering module  8 , respectively. Module  7  is operable to translate an optical fiber passing through connector  3  in the X-Y plane, and module  8  is operable to translate an optical fiber passing through connector  5  in the X-Y plane, as will be described hereafter. The FSOTA, in the preferred embodiment, contains the various transmit, receive, acquisition, and tracking control electronics (not shown) necessary for operation of the transceiver, which preferably are contained in a remote electronics bay. These electronic components provide control of the required azimuth and elevation range of motion and tracking slew rate for the MEMS translation devices to track and point the incoming and outgoing optical beams. 
   The preferred embodiment utilizes separated telescopes for optical noise isolation; modules  7  and  8  may, however, be utilized in other transceiver systems that require pointing and tracking. The optical receive and transmit signals are carried to the FSOTA on optical fibers  26 A and  26 B, respectively. As with most free space optical transceiver systems, the receiver telescope of the preferred embodiment is equipped with optical filters in order to filter out optical noise. In addition, the FSOTA includes an optical lens design that focuses the optical energy into the focal plane of receiver telescope  2 . Receiver optical fiber  26 A is automatically located at the focal spot in order to collect the incoming optical signal  6 . It may be noted that any selected communications band in any of the optical domains may be utilized; however, in the preferred embodiment optical filtering is utilized, providing narrow-band optical intensity, and thereby providing low noise signal detection. The system may also utilize a direction of arrival detection system, which in the preferred embodiment uses a charge-coupled-device (CCD) array that allows for the location of the angle of arrival. 
     FIG. 2  illustrates a preferred embodiment with the incoming received beam  9  and outgoing transmitted beam  17  in a first example configuration. The configuration shows the angular direction of the beams  9  and  17  with respect to the relative position of the FSOTA. Received signal  9  arrives from a remote transmitter. Received signal  9  arrives in an expanded form, such that only a portion of received signal  9  is actually captured by receiver telescope aperture  10  and focused onto receiver focal plane  11  through receiver lens system  14 . The received beam angle of arrival  12  determines the location of the focused spot at receiver focal plane  11 , as shown. Angle  12 , which may also be designated as Δφ RX1  for purposes herein, may be defined as the angle that received signal  9  makes with respect to receiver telescope Z-Axis  13 . Coordinate axis  20  of  FIG. 2  may be used as a reference for coordinates as referred to herein. 
   As already described, received beam angle of arrival  12  determines the location on receiver focal plane  11  at which the optical energy will be focused. Receiver optical fiber  26 A (shown in  FIG. 1 ) is then moved such that its end is aligned with that location on receiver focal plane  11  at which the optical energy is focused, for purposes herein designated to be at X-Y coordinates X RX1  and Y RX1 . Receiver optical fiber  26 A thus can receive the light energy being directed upon focal plane  11  at X-Y coordinates X RX1  and Y RX1 . Transmitter optical fiber  26 B is simultaneously moved to that corresponding location on transmitter focal plane  15  defined as X-Y coordinates X TX1  and Y TX1 . Light from optical fiber  26 B passes through transmitter lens system  16 , which expands and collimates transmitted beam  17  in order to produce diffraction limited beam propagation with minimal wave front distortion in the resulting transmitted beam  17 . As may be seen, this re-location of optical fiber  26 B results in transmitted beam angle  19  (also referred to herein as Δφ TX1 ) between transmitted signal  17  and transmitter telescope Z-axis  18 . Thus transmitter telescope  1  is automatically adjusted to emit a transmitted signal  17  that is directed toward the source of received signal  9 . It may be seen that in the preferred embodiment, received beam angle  12  and transmitted beam angle  19  (that is, angles Δφ RX1  and Δφ TX1 , respectively) are equal. 
     FIG. 3  illustrates a second configuration for the preferred embodiment of the present invention, with different beam angles possibly representing either a different remote transceiver terminal or a new relative location of a remove transceiver with respect to the FSOTA position. Because of the change in location, modules  7  and  8  will move optical fibers  26 A and  26 B to this corresponding new location in the focal planes of the respective telescopes. (The means by which modules  7  and  8  perform this operation will be discussed below.) 
   As in the first configuration, the second configuration accepts received signal  9  arriving from a remote transmitter and focuses received signal  9  onto receiver focal plane  11  through receiver lens system  14 . The received beam second angle of arrival  22  determines the location of the focused spot at receiver focal plane  11 , as shown. Received beam second angle  22 , which may also be designated as Δφ RX2  for purposes herein, may be defined as the angle that received signal  9  makes with respect to receiver telescope Z-Axis  13 . Receiver optical fiber  26 A (shown in  FIG. 1 ) is then moved such that its end is aligned with that location on receiver focal plane  11  at which the optical energy is focused, for purposes herein designated to be at X-Y coordinates X RX2  and Y RX2 . Transmitter optical fiber  26 B is simultaneously moved to that corresponding location on transmitter focal plane  15  defined as X-Y coordinates X TX2  and Y TX2 . Light from optical fiber  26 B passes through transmitter lens system  16 , which expands and collimates the optical signal to produce transmitted signal  17 . As may be seen, this re-location of optical fiber  26 B results in transmitted beam second angle  25  (also referred to herein as Δφ TX2 ) between transmitted signal  17  and transmitter telescope Z-axis  18 . Thus transmitter telescope  1  is automatically adjusted to emit a transmitted signal  17  that is directed toward the source of received signal  9 . It may be seen that in the preferred embodiment, received beam second angle  22  and transmitted beam second angle  25  (that is, angles Δφ RX2  and Δφ TX2 , respectively) are equal. 
     FIGS. 4A ,  4 B,  5 A,  5 B,  6 A, and  6 B illustrate the operation of the MEMS modules  7  and  8  to the preferred embodiment of the present invention , by showing the relative location and movement of optical fiber  26 . Each of the depictions represents either a receiving optical fiber  26 A or a transmitting optical fiber  26 B, both of which may be referred to generically herein as optical fiber  26 . Optical fiber  26  is fed into open fiber feed-through tube  27  through fiber support block  28 . The open tube is enclosed with the interface block  29  and the MEMS substrate  30 . The output portion of optical fiber  26  is supported by the MEMS system optical fiber support  31 , with the face of the fiber exposed for transmission and/or reception of the optical beam  32  into or out of the appropriate telescope. Optical beam  32  will diverge upon leaving the end of optical fiber  26 , forming signal cone  34 . The MEMS translation actuation devices (arms)  33  move the fiber in the X-Y plane by extending or retracting, that is, moving either toward or away from, respectively, the center of open fiber feed-through tube  27 , as illustrated in each of the cases depicted in  FIGS. 4B ,  5 B, and  6 B, thereby moving the position of fiber optic  26  and the resulting direction of signal cone  34 . Arms  33  are connected to optical fiber support  31  by means of linkages  50 . Although an infinite number of possible optical fiber  26  positions exist in order to properly align the optical beam  32  of optical fiber  26 , three positions will be shown and described for purposes of illustration. 
     FIGS. 4A and 4B  illustrate Fiber Position  0 , representing a position where optical fiber  26  lies along the instrument Z-axis  40 , and thus having a position defined as X=0 and Y=0 in the X-Y plane. The input/output angle Δφ, defined as the angle formed between instrument Z-axis  40  and the direction of radiation emitted from optical fiber  26 , is zero in Fiber Position  0 . As may be seen from  FIG. 4B , MEMS translation devices  33  are extended at equal lengths towards optical fiber  26 . 
     FIGS. 5A and 5B  illustrate Fiber Position  1 . As shown in  FIG. 5A , optical fiber  26  is now below instrument Z-axis  40 .  FIG. 5B  shows two possible sub-configurations corresponding to Fiber Position  1 , designated as Fiber Position  1 A and Fiber Position  1 B. In Fiber Position  1 A, shown in the left portion of  FIG. 5B , optical fiber  26  has moved to the lower left as viewed from the front of the device, with coordinates designated as X 1A  and Y 1A , for an input/output beam angle  41  from optical fiber  26 , designated as Δφ 1A . In Fiber Position  1 B, shown in the right portion of  FIG. 5B , optical fiber  26  has moved to the lower right as viewed from the device, with coordinates designated as X 1B  and Y 1B , for an input/output beam angle  41  from optical fiber  26 , designated as Δφ 1B . 
     FIGS. 6A and 6B  illustrate Fiber Position  2 . As shown in  FIG. 6A , optical fiber  26  is now above instrument Z-axis  40 .  FIG. 6B  shows two possible sub-configurations corresponding to Fiber Position  2 , designated as Fiber Position  2 A and Fiber Position  2 B. In Fiber Position  2 A, shown in the left portion of  FIG. 6B , optical fiber  26  has moved to the upper left as viewed from the front of the device, with coordinates X 2A  and Y 2A , for an input/output beam angle  42  from optical fiber  26 , designated as Δφ 2A . In Fiber Position  2 B, shown in the right portion of  FIG. 6B , optical fiber  26  has moved to the upper right as viewed from the device, with coordinates designated as X 2B  and Y 2B , for an input/output beam angle  42  from optical fiber  26 , designated as Δφ 2B . 
   MEMS translation devices  33  preferably provide a tracking bandwidth of up to 10,000 Hz, for closed loop control. The design and construction of MEMS translation devices  33  is set forth in U.S. Pat. No. 7,224,508, entitled “MEMS-Based Optical Communications Beam Steering Apparatus,” the entire disclosure of which is incorporated herein by reference. Specifically, and as illustrated in  FIG. 7 , actuators  15   a  drive arms  20   a , which are pivotally linked to both a corresponding actuator  15   a  and fiber yoke  25   a . Arms  20   a  move about on top of ground plane substrate  30   a . which is preferably constructed of polysilicon. Arms  20   a  have pin or flex joints  35   a  at each end to allow yoke  25   a  to move freely in the X-Y plane above ground plane substrate  30   a , including movement at non-orthogonal angles. Electrical drive signals reach each of actuators  15   a  through the corresponding bond pads  10   a  and the corresponding conducting paths  12   a . Four different types of signals are employed in the preferred embodiment: up, down, left, and right. These signals are labeled “U,” “D,” “L,” “R,” respectively, in  FIG. 7 . A ground signal is also required, which is labeled as a down arrow in  FIG. 7 . (Note that while only a single ground signal is illustrated in  FIG. 7  for clarity, the preferred embodiment would include a ground line connected to each of actuators  15   a .) Each of the “U” “D,” “L,” and “R” signals may preferably be coded as a voltage applied at the corresponding bond pad  10   a . A “U” signal causes the activation of the appropriate actuator  15   a  such that the arm  20   a  oriented in the Y-direction moves in the positive Y-direction, that is, in an upward direction, thereby causing yoke  25   a  to deflect upward. A “D” signal causes the activation of that same actuator  15   a  as activated by the “U” signal, but in this case the corresponding arm  20   a  moves in the negative Y-direction, that is, in a downward direction, thereby causing yoke  25   a  to deflect downward. An “L” signal causes the activation of each of the appropriate actuators  15   a  such that the arms  20   a  that are oriented in the X-direction move in the negative X-direction, that is, to the left, thereby causing yoke  25   a  to deflect to the left. It may be noted that this movement requires the leftward arm  20   a  to retract while the rightward arm  20   a  extends. Conversely, a “R” signal causes the activation of each of these actuators  15   a  such that the arms  20   a  that are oriented in the X-direction move in the positive X-direction, that is, to the right, thereby causing yoke  25   a  to deflect to the right. It may be noted that this movement requires the leftward arm  20   a  to extend while the rightward arm  20   a  retracts. A control system may be implemented to manipulate MEMS translation devices  33  in accordance with the preferred embodiment of the present invention. The operation of the control loop is preferably based upon a maximization of the optical power collected by the receiver version of optical fiber  26 A, and manipulation of the transmitter version of optical fiber  26 B in accordance with its position. Various such algorithms are known in the art. In the preferred embodiment, such a control system may be implemented in software using a microprocessor in communication with the LTTA. 
   The present invention has been described with reference to certain preferred and alternative embodiments that are intended to be exemplary only and not limiting to the full scope of the present invention as set forth in the appended claims.