Abstract:
A free-space laser communication system having six axes of movement. The system includes a fine tracking and acquisition system comprising a low inertia steering mirror having two axes of movement which points the communication laser transmission optical apertures separately from the optical apertures for the beacon laser, a coarse tracker, and a communication receiver. A fine tracking centroider and the communication lasers have the steerable mirror in common, and thus do not experience any alignment error that might arise from use of a separate deflecting element for each. The fine tracking and acquisition system is preferably mounted on a gimbal having two axes of movement, and the gimbal in turn is preferably mounted in a housing having two axes of movement.

Description:
BACKGROUND 
     1. Technical Field 
     This invention relates to free-space laser communication systems, and more particularly to a free-space laser communication system having at least two communicating transceivers. 
     2. Background Information 
     Free-space laser communication systems transmit and receive information by means of a light beam that propagates through space or the atmosphere. Such laser systems gain their principle advantages over radio frequency based broadcast systems by being highly-directional and more difficult to jam. Compared to microwave-based systems, such laser systems usually have greater bandwidth, lower input power, smaller size and less weight. One reason a laser communication source requires less radiated power is because the angular radiated laser beam divergence is much smaller than a microwave beam. 
     When used for space-based, air-to-air or air-to-ground communications, free-space laser communication systems pose a number of challenging problems. One such problem is the fact that having a smaller beam divergence requires greater accuracy in pointing the laser beam. Microwave beam divergence is typically on the order of milliradians whereas laser beam divergence is generally less than 0.1 milliradians. This characteristic requires pointing accuracies for laser beams on the order of 1 to 10 microradians. 
     Accordingly, the first step in establishing communication for two laser communication terminals is for each terminal to acquire and track the other terminal. Typically such laser communication terminals will include a tracking beacon, a beacon receiver, and a communication transceiver, which are referred to as optical apertures. The transceiver optical aperture generally is mounted on a gimbaled platform having at least two orthogonal axes of freedom and which has been stabilized against base motion. The optical apertures are optically aligned with each other to what is called the system boresight. 
     The beacon laser of each terminal radiates a signal with much larger beam divergence than the signal of a separate communication laser, thus providing a source to acquire and track. This source must be viewable with good signal strength in the presence of other light, such as sky light, sunlight, starlight, moon light, or light reflected from the earth and other objects. 
     The acquisition and tracking system of each of a pair of laser communication terminals must be able to initially point the transmitting and receiving apertures of each terminal as close as possible to the direction of the other terminal. 
     FIG. 1 is a stylized diagram showing two vehicles communicating by means of a free-space laser communication system. A first vehicle  1  (e.g., an airplane) has a gimbal-mounted host laser communication terminal  2  mounted so as to be able to “see” a similarly mounted target laser communication terminal  3  on a second vehicle  4  (e.g., an airplane). The host terminal  2  includes a pointing system, a coarse acquisition and tracking system for generating a large field of view (FOV) “footprint”  5  to illuminate the target terminal  3 , and a fine acquisition and tracking system for generating a smaller “footprint”  6  to more precisely illuminate the target. 
     Certain aspects of the system shown in FIG. 1 are disclosed in U.S. Pat. No. 5,710,652, entitled “Laser Communication Transceiver and System” and U.S. Pat. No. 5,801,866, entitled “Laser Communication Device” and U.S. patent application Ser. No. 08/221,527, filed Apr. 1, 1994, entitled “Point-to-Point Laser Communication Device” (now U.S. Pat. No. 5,754,323); [Ser. No. 08/199,115, filed Feb. 22, 1994, entitled “Laser Communication Transceiver and System”;] and Ser. No. 07/935,899, filed Aug. 27, 1992, entitled “Voigt Filter” (now U.S. Pat. No. 5,731,585). Each of the above references is incorporated herein by reference. 
     FIG. 2 is a stylized diagram showing the angular field of view (FOV)  7  for the optical apertures of the target laser communication terminal  3  on the second vehicle  4  and the angular FOV  5  of the beacon beam from the transmitting host laser communication terminal  2 . Each laser communication terminal must be able to initially point its transmitting and receiving optical apertures as close as possible to the direction of the opposite, target terminal. The beacon beam  5  from the host terminal  2  of the first vehicle  1  must provide a large footprint  5  at the receiving target terminal  3  to give the greatest probability of illuminating the target terminal  3 . The target terminal sensor should have a large angular field of view  7  to improve the probability of seeing the host terminal&#39;s beacon beam on the first “look”. This will reduce the amount of searching time and the uncertainty in establishing the communication link. However, if the beacon beam divergence is made too large, the intensity of the received beacon signal may be so low that the tracking signal caused by the received beacon signal is obscured by system electronic noise and other illuminating light sources. 
     The platform on which each terminal is mounted must provide a means to stabilize the pointing of the transmitting and receiving optical apertures against angular disturbances of the base mount. The base mount could include a vehicle, such as an aircraft or space platform, which has a significant amount of angular motion which would cause pointing errors for the optical apertures. The ideal method of stabilization would be a totally frictionless mount which has freedom to rotate in two orthogonal directions. If a frictionless mount were possible, then system inertia would cause each terminal to stay pointed in the same direction in the presence of angular disturbances to the base mount. In practice, friction couples base motion to the optical apertures of a terminal, causing angular motion. Such angular motion should be removed with a servo system which both senses and provides opposing torques to stabilize against the base motion. 
     The frequency of base motion disturbances can vary widely. Aircraft often have base motion disturbances at propulsion system frequencies or some multiple of these frequencies. A military tank would have disturbances at the frequencies of the engine rotation. These base motion disturbances can cause large pointing errors in a laser communication system. The terminal servo system must sense these disturbances and stabilize the base mount of the terminal. In general, the servo system must have a frequency response sufficiently high to compensate for the highest frequency components of base motion that contribute to producing pointing error. 
     The tracking system also must be able to sense angular motion of a terminal and provide pointing correction. The transmitter should be pointed with greater precision than the receiver of a laser terminal, since the beam divergence angle of the transmitter is much smaller than the receiver&#39;s acceptance angle. The precision of pointing the communication laser beam is preferably a fraction of its beam divergence angle that is sufficiently small so that the received power will vary at the receiver as the beam jitters due to the exponential decay in the beam intensity from its central maximum to its edge. The system is usually configured to maintain the beam power density equal to or above one half of its maximum power density. This ensures that the received communication signal is at least one half of the maximum transmitted power density that could be received during normal motion of the transceivers and in the presence of base angular vibration. In general, the angular tracking rates are small when terminals are separated by large distances. However, satellite to satellite tracking rates can be quite large for some systems, and ground to air tracking rates can be large if smaller distances are involved. 
     To provide tracking, a beacon receiver of terminal will image an incoming beacon beam onto a pixel-imaging device of what is commonly referred to as a “centroider”. The centroider provides an error signal which is directly proportional to the angular difference between the boresight of the receiving terminal and the line of sight to the opposite transceiver. The error signal is amplified and filtered and then applied to gimbal drive motors or actuators to position the optical apertures of the receiving terminal such that the error signal is reduced to a minimum. For a free-space laser communication system with one or both terminals mounted on moving platforms to achieve a long range up to or greater than 500 km and a high data rate up to or greater than 1 GBPS, the communication laser beam must have a very small beam divergence angle in order to achieve the power density needed at the receiving terminal. This requires great precision in pointing because any errors greater than a desired beam divergence (e.g., the half angle beam divergence) would cause the footprint of the transmitted laser beam to miss the receiving aperture. 
     Some conventional designs use a common aperture configuration to achieve the above precision pointing. The transmitted communication laser beam is transmitted through the same aperture for receiving the beacon signal. Since the received beacon beam signal is used for determining the pointing direction, both apertures may be configured to have the same optical layout by using the same optical elements. This ensures that the communication laser beam is pointed in exactly the same direction as the received beacon signal. Such common aperture configuration also significantly reduces or minimizes any misalignment caused by temperature expansion, mechanical vibration or long-term drift of the mechanical misalignment. 
     One limitation of the common aperture configuration is the high background noise due to sharing of the same optical elements by a high power light source, the communication laser beam, and a sensitive receiver. Reflection or scattering of light from the optical surfaces caused by, for example, surface irregularities, optical coatings, optical element voids/inclusions, or accumulated particulate matter on the surfaces, can increase the background noise of the receiver. In a typical system, the sensitivity of the receiver is on the order of nanowatts while the transmitted power is many orders of magnitude greater (e.g., a fraction of a watt which is approximately 10 8  greater than the receiver sensitivity). Such high background noise essentially decreases the allowable maximum separation between the two transceivers, thus undesirably reducing the communication range of the system. 
     Therefore, a need exists for improving accuracy of tracking and pointing in presence of the base motion and for increasing the transceiver range for such a free-space laser communication system. One aspect of the present invention is to provide a system to meet such need. 
     SUMMARY 
     The invention is embedded in a free-space laser communication system having a fine tracking and acquisition system with six axes of movement. The tracking and acquisition system includes a low inertia steerable mirror having two axes of movement which points the communication laser transmission optical apertures separately from the optical apertures for the beacon laser, coarse tracker, and communication receiver of the system. A fine tracking receiver centroider and the communication lasers share a common steerable mirror to substantially reduce or eliminate any alignment error that might arise from use of a separate deflecting element for each. The separate apertures for the communication laser the communication receiver significantly reduces the amount of background light received by the communication receiver from its communication transmitter. The system is configured so that the steering mirror, the received beacon and the transmitted lasers use different regions of the steering mirror for reflection to prevent light from the transmitted laser from entering the fine tracking receiver. The fine tracking and acquisition system is preferably mounted on a gimbal having two axes of movement, and the gimbal in turn is preferably mounted in a housing having two axes of movement. 
     One embodiment of a free-space laser communication system having six axes of movement, includes: a housing having two axes of movement; a gimbal mounted within the housing, the gimbal having two axes of movement; a free-space laser communication transceiver mounted on the gimbal. The transceiver includes a centroiding system for determining an aiming point for the free-space laser communication system; a steering mirror for directing an incoming beacon beam to the centroiding system, the steering mirror having two axes of movement; a feedback control for aiming the steering mirror at the aiming point determined by the centroiding system; and at least one communication laser beam directed at the steering mirror and thereby directed at the aiming point. 
     The free-space laser communication system may include a fine acquisition and tracking system which comprises: a centroiding system for determining an aiming point for the free-space laser communication system; a steering mirror for directing an incoming beacon beam to the centroiding system; a feedback control for aiming the steering mirror at the aiming point determined by the centroiding system; at least one communication laser beam directed at the steering mirror and thereby directed at the aiming point. 
     One implementation of the fine acquisition and tracking system has a control bandwidth in excess of 400 Hz, which is 6 to 8 times the bandwidth that can be achieved with only a mechanical gimbal aperture stabilization system. The invention addresses the high bandwidth stabilization and tracking requirements of a free-space laser communication system capable of long range (e.g., 500 km) and high data rates (e.g., &gt;1 GBPS) and with one or both laser terminals mounted on moving platforms. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a stylized diagram showing two vehicles communicating by means of a free-space laser communication system. 
     FIG. 2 is a stylized diagram showing the angular field of view (FOV) for the optical apertures of a target laser communication terminal and the angular FOV of the beacon beam from a transmitting host laser communication terminal. 
     FIG. 3 shows one embodiment of a 2-axis turret for housing a laser communication system in accordance with the invention. 
     FIG. 4 shows one embodiment of a 2-axis gimbal on which is mounted a laser communication system in accordance with the invention. 
     FIGS. 4A and 4B show laser transmitters mounted on the gimbal of FIG. 4 for generating a wide FOV beacon and a narrow FOV beacon, respectively. 
     FIG. 5 shows a coarse acquisition and tracking system which may be used in conjunction with the invention. 
     FIG. 6 shows one embodiment of a fine acquisition and tracking system in accordance with the invention. 
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     A laser communication device may include transmitting units to produce a wide FOV beacon beam for coarse tracking, a narrow FOV beacon beam for fine tracking, and at least one laser communication beam for transmitting information. Accordingly, such a laser communication device may having a number of receiving units, including a coarse-tracking receiver for receiving a wide FOV beacon beam, a fine-tracking receiver for receiving a narrow FOV beam, and a data receiver for receiving a communication beam, from another communication device. Each optical aperture (i.e., a transmitting unit or a receiving unit) is adjustable to a desired orientation for proper operation. In a preferred embodiment, at least four axes of movement are implemented for each optical aperture. Six axes of movement may be more preferably used in directing certain optical apertures, such as the fine-tracking receiver and the transmitting unit for producing the communication laser beam, to improve the tracking accuracy and reliability in data transmission. 
     One embodiment of the laser communication device integrates all optical apertures on a common optical platform. Two steering members, a turret with two axes of rotation and a gimbal assembly with another two axes of rotation, are engaged and coupled with each other to provide four axes of rotation for the optical platform. The optical platform is mounted to the gimbal assembly and the gimbal assembly is mounted within the turret so that movement of the turret moves both the gimbal assembly and the optical platform to provide a coarse adjustment in the orientation of the optical platform. Movement of the gimbal assembly further provides a fine adjustment in the orientation of the optical platform. These features are now described in detail with reference to FIGS. 3 and 4. 
     FIG. 3 shows one embodiment of a 2-axis turret  300  for housing a gimbal-mounted laser communication device  304 . The turret  300  has a base  302  which is configured with brackets and support structures to be mounted to a base platform or a vehicle, such as the underbody of an airplane. The turret  300  is configured to rotate, at least partially, around at least two axes. In the embodiment shown, the turret  300  may revolve around a first axis  310  for azimuth control and rotate at least partially around a second axis  320  for elevation control. Such a turret can be build based on well-known turrets in the art. 
     The orientation of the turret  300 , i.e., its azimuth and elevation, must be properly pointed to the target before optical tracking and communication can be established. A radio navigation system can be used to control the actuators of the turret  300  so as to direct the turret  300 . The global positioning system (“GPS”) and an inertial navigation system (“INS”) can be implemented to achieve such operation. However, any desired gross pointing system may be used, including a purely optical system. Accordingly, the turret  300  provides a 2-axis mechanism for generally pointing the laser communication device  304  at a target terminal. 
     FIG. 4 shows one embodiment of a 2-axis gimbal assembly  400 . The gimbal assembly  400  includes a rigid gimbal ring  410  as a support member on which all optical apertures integrated on an optical platform  420  of the laser communication device are mounted. The gimbal ring  410  is engaged to the turret  300  of FIG. 3 by two orthogonal axes, an azimuth axis  411  and an elevation axis  412 , each with a limited range of rotation (e.g., a degree of freedom of about +/−5°). Hence, the optical platform  420  and the optical apertures thereon can be rotated relative to the turret  300 . The gimbal assembly  400  is positioned by an azimuth actuator  421  for rotation around the axis  411  and two elevation actuators  422 A,  422 B for rotation around the axis  412 . Two actuators  422 A and  422 B are preferably used for rotation around the axis  412  since a sufficiently large torque is needed due to the configuration of the system. The absolute position of the assembly is indicated by an azimuth resolver  431  and an elevation resolver  432 . The gimbal assembly  400  is mounted on bearings or flexures to reduce or minimize friction associated with rotation. 
     The optical apertures mounted on the optical platform  420  form an optical transceiver that receives tracking and communication beams from and transmits such beams to, another unit. The optical apertures include a telescope receiving aperture  440  for receiving a wide FOV beacon beam and communication laser beams, two communication laser apertures  450 A,  450 B, a fine tracking aperture  460 , two beacon transmitting apertures  470 A,  470 B, communication receiver detectors (not shown), a viewing camera  480 , and two tracking sensors (not shown). The two communication laser apertures  450 A and  450 B operate to produce two communication beams at a communication frequency to achieve a bandwidth twice of what is possible with a single communication beam. The beacon transmitting apertures  470 A and  470 B produce the wide FOV beacon and the narrow FOV beacon, respectively, at a beacon frequency that is different from the frequency of the communication beams. FIG. 4A shows one embodiment of the wide FOV beacon transmitting aperture  470 A. A laser diode  472 A driven by a laser driver circuit  471 A produces a linearly-polarized wide FOV beacon. A quarter-wave plate  474 A converts the linear polarization into a circular polarization. A lens  473 A is used to project the beacon to a target. The beacon transmitting aperture  470 B is similarly constructed as shown in FIG.  4 B. 
     The frequencies of these transmitting beams may be stabilized to specified frequencies with narrow linewidths so that very narrow filters can be used to receive light only at these specified frequencies and to reject light at other frequencies for suppressing noise. Such frequency stabilization is particularly important to detection of the wide FOV beacon for coarse tracking since the optical aperture  440  has a large field of view and is significantly exposed to noise such as sky light and other background light. An atomic line filter, such as a cesium vapor cell in a proper external magnetic field, can be used to lock the frequency of the wide FOV beacon and another atomic line filter can be implemented in a respective receiving unit to detect the wide FOV beacon. A semiconductor laser, when used to produce the wide FOV beacon, can also be stabilized by properly controlling the driving current without an atomic line filter. The communication lasers may also be stabilized to achieve certain operation advantages. See, U.S. Pat. Nos. 5,710,652 and 5,801,866, and U.S. patent application Ser. No. 09/123,565, entitled “METHOD AND APPARATUS FOR LOCKING THE WAVELENGTH OF A BEACON BEAM IN A FREE-SPACE LASER COMMUNICATIONS SYSTEM” and filed on Jul. 27, 1998, now U.S. Pat. No. 6,151,340, which are incorporated herein by reference. In addition, each of the transmitted beams may be circularly polarized in order to reduce noise and to separate signals from different beams in detection. 
     As the actuators  421 ,  422 A, and  422 B operate to position the optical platform  420 , all of the optical apertures are pointed accordingly. An additional steering element is implemented to provide extra two degrees of movement for the communication laser apertures  450 A and  450 B and the fine tracking aperture  460 . Hence, while other optical apertures have four degrees of movement, the communication laser apertures  450 A and  450 B and the fine tracking aperture  460  have six degrees of movement and therefore can be adjusted independently with respect to other optical apertures. This additional steering element, as described later, is operable to provide even finer adjustment than the gimbal assembly  400  in a small angle range (e.g., about +/−320 μradians) at a very high speed in comparison to the motion of the optical platform as a whole. 
     FIG. 5 is a diagram showing one implementation of the telescope receiving aperture  440 . The telescope receiving aperture  440  includes an optical receiving unit  501 , a data detection module  510 , and a coarse tracking module  520 . The optical receiving unit  501  has a primary concave spherical mirror  502 , a secondary convex spherical mirror  504  and a dichroic optical filter  506  located in an opening in the center of the mirror  502 . The mirrors  502  and  504  are positioned relative to each other to receive and direct beacon beams (both wide FOV beacon and narrow FOV beacon) and communication beams from another laser communication device to the optical filter  506 . The optical filter  506  transmits light at the beacon frequency to the coarse tracking module  520  located on the other side of the mirror  502  and reflects light at the communication frequency to the mirror  504 . An opening  505  is formed in the center of the mirror  504  to allow the communication beams to focus to the data detection module  510  on the other side of the mirror  504 . 
     Two communication beams from another laser communication device are preferably right-hand circularly polarized and left-hand circularly polarized, respectively, in order to reduce any interference therebetween since they are at the same communication frequency. The data detection module  510  is configured to separately detect the two cross-polarized communication beams. Specifically, the detection module  510  includes a spatial filter  511  (e.g., a pinhole) at or near the focal point of the communication beams, a lens  512 , a narrow bandpass filter  513 , a quarter-wave plate  514 , a polarizing beam splitter  515 , lenses  516 A,  516 B, and detectors  517 A,  517 B. The spatial filter  511  limits the field of view and the direction of light that can be received at the detectors  517 A and  517 B. The narrow bandpass filter  513  has a center transmission frequency at the communication frequency and a bandwidth (e.g., +/−3 nm) to further restrict the frequency of light that can be received by the detectors  517 A and  517 B for improving the signal-to-noise ratio of the signal detection. The quarter-wave plate  514  converts the cross circularly polarized communication beams into two linearly polarized beams that are mutually orthogonal. The polarizing beam splitter  515  separates and directs the two communication beams to the detectors  517 A and  517 B, respectively. The detection module  510  further includes two signal amplifiers  518 A and  518 B that are respectively connected to the detectors  517 A and  517 B to amplify the received signals. 
     The coarse tracking module  520  has a narrow bandpass filter  521 , a quarter-wave plate  522 , an atomic line filter formed of a polarizer  523 , an atomic vapor cell  524 , and a polarizer  525 , an intensity control unit formed of the polarizer  525 , an electrical-controllable birefringent retarder  526 , and a polarizer  527 , an imaging lens  528 , and an imaging sensing array  529  (e.g., CCD). The narrow bandpass filter  521  can be an interference filter and operate to limit the frequency of the transmitted light that passes through the filter  506 . The quarter-wave plate  522  converts circularly polarized beam into a linearly polarized beam. The polarizer  523  selects a beam of a desired polarization for detection by the sensing array  529 . The wide FOV beacon, for example, can be cross polarized with the narrow FOV beacon. Hence, these two beacon beams remain cross polarized after being converted from circular polarizations into linear polarizations by the quarter-wave plate  522 . The polarizer  523  can be orientated to select the wide FOV beacon for coarse tracking. 
     The atomic line filter is further used to select the wide FOV beacon at a selected atomic transition wavelength and reject other light. The wide and narrow FOV beacons can be produced at two beacon frequencies that are slightly different from each other by such a small frequency separation that the dichroic filter  506  and the narrow pass band filter  521  could not provide them. This can be achieved by using two single-mode diode lasers to respectively produce the beacon beams. The atomic line filter, however, can separate such a small frequency separation by transmitting one frequency while rejecting another using a sharp transition of the vapor in the vapor cell  524 . This filtering further reduces background noise and improves the precision of the coarse tracking. 
     For example, the communication beams can be at about 810 nm and the two beacon beams can be at about 852 nm. The dichroic filter  506  separates the two different wavelengths by reflecting the 810 nm and transmitting the 852 nm. Cesium vapor can be used in the vapor cell  524  to select the wide FOV beacon at a cesium transition (e.g., 852.34 nm). 
     The intensity received by the sensing array  529  can be adjusted by using the intensity control unit formed of the polarizer  525 , an electrical-controllable birefringent retarder  526 , and a polarizer  527 . Since the dynamic range of the sensing array  529  is limited, this adjustment of the received intensity allows the system to operate in a wide range of received intensity ranges and thereby extends the range of communication between two laser communication devices. A control feedback loop formed of a signal-receiving circuit  529 A, a signal processor  529 B, and a retarder controller  529 C is used to achieve this. The retarder  526  may be formed of a liquid crystal material as disclosed in U.S. patent application Ser. No. 09/033,567, filed on May 22, 1998, which is incorporated herein by reference. 
     The sensing array  529  converts the photons into pixelated electrons which are processed by the circuit  529 A. The processor  529 B calculates a centroid of the focused wide FOV beacon. This centroid gives the location of the focused energy on the sensing array  529 . The processor  529 B also calculates the signal amplitude level and adjusts the voltage on the variable retarder  526  via the circuit  529 C to keep the received beacon signal within the dynamic range of the sensing array  529 . 
     The processor  529 B receives an input reference signal which represents the boresight position of the light signal onto the sensing array  529 . The deviation of the calculated position or centroid position of the incoming light signal from the boresight position is determined and indicates the tracking error. This tracking error is applied to the two-axis gimbal  400  shown in FIG. 4 to command the azimuth and elevation actuators which align the optical platform  420  so that the focused spot of the incoming wide FOV beacon overlaps the boresight position on the sensing array  529 . 
     The accuracy of the coarse tracking is typically limited to approximately one-tenth the field-of-view of a pixel in the sensing array  529 . For example, if the sensing array  529  is an array of 256×256 sensing pixels and has an FOV of about 20.48 milliradians, each pixel will have an instantaneous FOV of about 80 microradians. The sensing array  529  will then have an accuracy of approximately 8 microradians. Therefore, once the focused point of the wide FOV beacon is locked to the boresight, the target will be tracked to about an 8 microradian rms (root mean square) error. 
     This pointing accuracy may be acceptable for some applications. However, factors other than pointing accuracy must be taken into account for a laser communication system designed for long range (e.g., 500 km) and high data rates (e.g., &gt;1 GBPS) and with one or both terminals mounted on moving platforms. For example, the turret  300  directly responds to base motion. In the preferred embodiment, such base motion is stabilized for the gimbal  400  by a stabilization system which uses a rate sensor (2-axis gyro). However, the bandwidth of this system is limited by the sensor bandwidth, gimbal mechanical inertia, and gimbal and mount compliances. In the illustrated embodiment, the mechanical systems being stabilized have inertias and mechanical compliances which typically give maximum gimbal mount servo stabilization frequencies below 100 Hz. Typical platforms for aircraft, satellites and ground based vehicles have angular vibrations which exceed 1 KHz. Such high frequency angular base motion disturbances will translate to increased errors in the communication data rate. For systems requiring small transmitter beam divergences, the pointing and tracking servo must have a larger bandwidth than can be achieved by stabilizing against base motion using a gimbal-based servo. 
     This desirable high frequency stabilization mechanism can be achieved by a fine tracking system to track the narrow FOV beacon from another laser communication device. This fine tracking system implements a single pivoting element with a low inertia to provide two axes of rotation at a high response speed for the optical apertures  450 A,  450 B, and  460 , without affecting the position of the optical platform. This pivoting element of two axes of rotation is in addition to the two axes of rotation from the turret  300  of FIG.  3  and two axes of rotation from the gimbal  400  of FIG.  4 . Hence, the optical apertures  450 A,  450 B, and  460  have a total of six axes of rotation while other elements have four axes of rotation. 
     FIG. 6 shows one embodiment  600  of the fine tracking system and communication laser transmitters which are respectively indicated as optical apertures  460 ,  450 A, and  450 B in FIG.  4 . The first communication laser transmitter  450 A includes two laser diodes  610 A and  610 B to collectively produce the first communication laser beam. The two beams from the two laser diodes  610 A and  610 B are respectively collimated by lenses  611 A and  611 B and combined by a polarizing prism combiner  612  as the linearly-polarized first communication beam. A diode laser control circuit  618  is used to control the laser diodes  610 A and  610 B and to imprint communication data on the laser beams. A quarter-wave plate  613  converts the first communication beam into a circularly polarized beam  610 . A pivoting mirror  640  receives the first communication beam  610  and directs to mirrors  614  and  615 . Projecting lenses  616  and  617  further direct the beam  610  to another laser communication device. The second communication laser transmitter  450 B is similarly constructed as shown and is also steered by the same pivoting mirror  640  except that the second communication laser beam  620  is cross polarized with respect to the first communication laser beam  610 . This polarization arrangement allows separate detection of the two communication laser beams and doubles the transmission bandwidth. The transmitters  450 A and  450 B are aligned with respect to the pivoting mirror  640  in such a way that the two communication laser beams  610  and  620  are substantially parallel to each other. 
     The fine tracking aperture  460  receives a distant narrow FOV beacon signal which is circularly polarized and at a desired wavelength (e.g., at 852 nm) by a conventional telescope with primary concave mirror  630  and a secondary convex mirror  631 . A focusing optics  632  collimates the received beacon beam into a beam of a reduced diameter and directs the beacon beam to the pivoting mirror  640 . A mirror  633  further guides the beacon beam from the pivoting mirror  640  to a fine tracking detection module. The detection module includes a narrow bandpass filter  634  which transmits the desired wavelength and rejects others, a quarter-wave plate  635  which converts the circular polarization into a linear polarization, a first linear polarizer  636 , an electrically-controllable retarder  637 , a second linear polarizer  638 , a focusing lens  639 , and a sensing array  650 . The first linear polarizer  636 , the retarder  637 , and the second linear polarizer  638  form an electrically controllable optical attenuator which adjusts the intensity of the received beacon within a dynamic range of the sensing array  650 . 
     The signal received on the sensing array  650  is amplified by sensor electronics  655  and is further processed by a processor  656  to generate the centroidal position of the focused spot on the sensing array  650 . The position of the focused spot on the sensing array  650  provides the angular information of the received narrow FOV beacon. The processor  656  compares the centroidal position with a predetermined boresight position of the optical system to generate a tracking error signal which indicates the direction and amount of deviation of centroidal position from the boresight position. A steering mirror driver amplifier  658  responds to the tracking error signal to drive the pivoting mirror  640  so as to reduce the tracking error by moving the centroidal position close to the boresight position. Similar to the coarse tracking system shown in FIG. 5, this locks the centroidal position to boresight position. 
     The sensing array  650  is configured to have fewer number of pixels than the sensing array  529  in the coarse tracking shown in FIG. 5 in order to read out in a time shorter than the readout time of the sensing array  529 . In addition, the sensing array  650  can provide higher tracking accuracy than the sensing array  529 . For example, the sensing array  650  may be a 32×32 CCD array while sensing array  529  may be a 256×256 CCD array. The readout speed of the 32×32 array  650  is faster than that of the 256×256 array  529  by a factor of about 180. Hence, the small sensing array  650  allows for correction of tracking errors of high frequencies beyond the capability of the large sensing array  529  and thereby extends the bandwidth of the tracking accuracy. The sensing array  529  may provide an accuracy of approximately 8 microradians, the accuracy of the sensing array  650  may be about 2 microradian. 
     One feature of the fine tracking system is the high response speed. This is in part due to the construction of the pivoting mirror  640 . The pivoting mirror  640  has a reflector  642  and two or more electrical actuators  644 . The actuators  644  are engaged to a support base  646  which is fixed to the optical platform  420 . The reflector  642  is pivotally coupled to the actuators  644  so as to rotate about two axes. FIG. 6 shows one embodiment which uses three piezo actuators to support and control the reflector  642 . The control signal produced by the driver amplifier  658  (e.g.,  659 A,  659 B, and  659 C) adjusts the voltages on the piezo actuators to steer the reflector  642  so that centroidal position moves towards the predetermined boresight position. The reflector  642  can be configured to be light weight and to achieve a high steering speed. A steering speed up to and greater than 1 KHz can be achieved by using the piezo actuators  644  and to correct alignment errors caused by high frequency angular disturbances of the base motion. The steering angular range of the reflector  642  preferably covers at least the field of view of the sensing array  650  and may be bigger to allow for tolerance of alignment errors. For example, if the field of view of the sensing array  650  is 640 μradians, the steering angular range of the reflector  642  may be set about 900 μradians. 
     Another feature of the fine tracking system is that all three optical apertures,  450 A,  450 B, and  460  are controlled by the same pivoting mirror  640 . The optical apertures  450 A,  450 B, and  460  are arranged relative to the mirror  640  so that the two communication laser beams become parallel to the direction of the received beacon beam incident to the reflector  642  when the centroidal position on the sensing array  650  produced by the received beacon beam overlaps the predetermined boresight position. For example, all three apertures may be co-aligned within a small fraction (e.g., 5 μradians) of the angular beam divergence (e.g., 35 μradians) of the communication lasers. Hence, the proper aiming of the communication laser beams at another laser communication device is automatically completed when the fine tracking error is maintained below a desired tolerance level. 
     In addition, the optical apertures  450 A,  450 B, and  460  are arranged to spatially separate the footprints of the two communication laser beams and the beacon beam from one another to eliminate interference between any two beams at the sensing array  650 . 
     In one implementation, the communication lasers of each laser communication terminal had a beam divergence angle of about 35 microradian (full width half maximum power), requiring an overall pointing accuracy of approximately 7 microradians rms. With this accuracy, the receiving aperture will receive a signal of greater than half the maximum power density most of the time. This pointing accuracy includes pointing errors from the centroider accuracy, fixed misalignment between the communication lasers, fixed misalignment between the communication laser and centroider imaging sensor, and the control loop positioning accuracy. 
     A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the steering mirror  640  shown in FIG. 6 may be pointed by different types of actuators. Accordingly, other embodiments are within the scope of the following claims.