Abstract:
A free-space optical lasercom system is disclosed which includes an optical subsystem for receiving an Rx signal and sending a Tx signal, wherein said optical subsystem establishes a focal plane. 
     A first optical fiber with an end coupled to the optical subsystem for directing the Rx signal to an RX detector. A second optical fiber is also included with an end coupled to the optical subsystem for directing the Tx signal from a Tx laser source. A linear X-Y shifting apparatus is used for positioning the respective ends of the first and second optical fibers in the focal plane of the optical subsystem so as to maintain a desired coupling efficiency.

Description:
BACKGROUND OF THE INVENTION 
     The present invention pertains to the field of free-space optical communication, particularly of the type used for two-way satellite communication, i.e. with an incoming Rx signal and an outgoing Tx signal. Effective communication between lasercom terminals requires precision pointing and tracking of the signal beams, and fine beam steering control is significant. In previous systems, rotating deflecting mirrors were the only reliable means of providing the required speed, range, accuracy and repeatability of beam steering. 
     The use of rotating mirrors in previous systems necessitates a complex optical subsystem, including at least one telescope with an eyepiece to form an afocal reducer, at least one afocal relay (or two relays for a telescope without a field lens) and a focusing lens. Large transverse magnifications are required for an afocal reducer to produce a large field of view in the image space. Also, to achieve high pointing accuracy, the entrance and exit pupils of the system must be well defined, and they must coincide with the respective steering mirrors. Thus, multi-element lens groups must be employed for relays, eyepieces and detector optics. Such systems are costly to manufacture and also have high mass which is undesirable for deployment and operation in satellite applications. 
     In addition to the above, the optical subsystem for the previous lasercom terminal design requires a large number of optical surfaces. Each optical surface contributes deformation to the optical wavefront due to optical material imperfections, manufacturing errors, assembly and alignment errors and environmentally-introduced errors, such as temperature gradients and vibrations. However, satellite lasercom systems must propagate efficiently over thousands of kilometers, and must provide efficient coupling to the internal detector components. Thus, wavefront error must be minimized. A state-of-the-art optical terminal must be diffraction-limited with a very small wavefront deformation budget (i.e. total cumulative RMS error smaller than 0.07 of a wavelength of laser light). It is a technically challenging task to meet such a requirement in a system involving so many optical surfaces. 
     Further, each optical surface inherently scatters a portion of the beam, further reducing efficiency and elevating the stray light level. This can result in optical crosstalk between the transmission and receiving channels, in addition to other errors such as lower than expected BER, burst errors, and failure to acquire and track a signal in the presence of background radiation (such as from celestial sources, i.e. the Sun, Earth, Moon and planets, and also reflections and scatter from the satellite). Typical previous optical architectures require 50 optical surfaces or more, resulting in significant contributions to the above-noted problems. 
     There are other sources of optical crosstalk, and several design strategies are typically implemented, such as carefully designing baffles and stops, and using narrowband spectral filters. In addition, there are several known approaches at the optical subsystem level for reducing crosstalk. FIG. 1 shows a previous system that provides complete spatial separation of the Rx and Tx signals by using two telescopes  10 ,  12 . While this design offers excellent control of stray light, it creates other problems such as significantly increasing mass, difficulty in alignment increased optomechanical complexity, and numerous optical surfaces in the subsystem. 
     Another approach is shown in FIG. 2, in which partial spatial separation is accomplished with a single telescope  20  and a shearing aperture  22 , where Rx and Tx signals share the same optical path. The Tx source  24  is drawn off a separate optical path from the Rx detector  26  by e.g. a reflecting mirror  28 . While lighter in weight as less complex than the two-telescope design, the shearing aperture approach has poor stray light control since it is impossible to isolate the Tx beam from the Rx beam. 
     FIG. 3 illustrates polarization isolation, in which a single telescope  30  is used, and the Rx and Tx signals are distinguished as perpendicular polarized beams separated by a polarizing diplexer  32 , which sorts the Rx and Tx beams along separate optical paths. This method requires the highest quality polarization components and extremely tight control of the state of polarization as the Rx and Tx beams propagate through the subsystem. 
     FIG. 4 illustrates dichroic isolation, in which a single telescope  40  is used, and different laser wavelengths are used for the Rx and Tx channels. The separate optical paths are defined by a dichroic beamsplitter  42 . As a result, the two communicating terminals are precluded from operating at identical wavelengths. 
     In previous systems, as shown in FIG. 5, track detection is accomplished using a fast steering mirror (FSM)  50  which shifts in position to correct for mispoint in Rx channel beam. A beam splitter  52  directs the Rx signal to an Rx detector  54 , and directs a portion of the Rx signal to a track detector  56 . Since the Rx detector  54  and the track detector  56  are aligned, the FSM  50  cancels out mispoint on both simultaneously. However, the physical size of the track detector  56  limits the field of view of track detection. Thus, previous tracking systems are only sensitive to small angular mispoints, which makes signal acquisition and tracking more difficult. 
     In all previous designs, boresight alignment also poses a major technical challenge. Typically, for communication distances greater than 4000 km, the communication Rx and Tx beams should be aligned within 1-3 microradians in free space. The corresponding misalignment requirement between channels may be even less than one microradian. While such boresight alignment is difficult to achieve during manufacture, it is even more difficult to maintain during launch deployment and on-orbit operation. Due to the long optics required for previous systems, the channel inputs must be separated by a great physical distance, which requires an extremely stable and expensive mechanical structure with tight controls over environmental variables. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In view of the drawbacks and disadvantages of previous systems, there is therefore a need for a lasercom system with reduced optical crosstalk. 
     There is also a need for a lasercom system with reduced mass. 
     There is also a need for a lasercom system with a reduced number of optical surfaces. 
     There is also a need for a lasercom system with a simplified optical and mechanical structure. 
     There is also a need for a lasercom system with reduced wavefront deformation. 
     There is also a need for a beam steering and tracking system that does not use rotating mirrors. 
     There is also a need for a lasercom system that provides active alignment compensation. 
     These needs and others are satisfied by the free-space optical lasercom system of the present invention in which a free-space optical lasercom system is disclosed including an optical subsystem for receiving an Rx signal and sending a Tx signal, wherein said optical subsystem establishes a focal plane. A first optical fiber is provided with an end coupled to the optical subsystem at a focal plane for directing the Rx signal to an Rx detector. A second optical fiber is provided with an end coupled to the optical subsystem at a focal plane for directing the Tx signal from a Tx laser source. A linear X-Y shifting means is disclosed for positioning the respective ends of the first and second optical fibers in the focal plane of the optical subsystem so as to maintain a desired coupling efficiency and also a required precision in pointing the Tx laser beam. Both fibers move in telecentric space so that a linear X-Y shift of the fiber is transformed into an angular shift of the beam in free space. 
     As will be appreciated, the invention is capable of other and different embodiments, and its several details are capable of modification in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The embodiments of the invention will now be described by way of example only, with reference to the accompanying figures wherein the members bear like reference numerals and wherein: 
     FIG. 1 schematically illustrates a previous optical subsystem that provides complete spatial separation of the Rx and Tx channels. 
     FIG. 2 schematically illustrates a previous optical subsystem that provides partial spatial separation of the Rx and Tx channels. 
     FIG. 3 schematically illustrates a previous optical subsystem that provides polarization isolation of the Rx and Tx channels. 
     FIG. 4 schematically illustrates a previous optical subsystem that provides dichroic isolation of the Rx and Tx channels. 
     FIG. 5 schematically illustrates a previous track detection assembly. 
     FIG. 6 schematically illustrates the lasercom system as according to a first embodiment of the present invention. 
     FIGS. 7A and 7B schematically illustrate the linear positioning arrangement for the Rx and Tx optical fibers, as according to the present invention. 
     FIGS. 8A,  8 B,  8 C and  8 D are respective top, side, oblique and exploded views of an exemplary linear positioning arrangement, as according to the present invention. 
     FIG. 9 schematically illustrates the track detection assembly according to the present invention. 
     FIGS. 10A and 10B illustrate the lasercom terminal package with gimbal mount of the present invention. 
     FIGS. 11A and 11B are side-sectional views illustrating the configuration of the present lasercom terminal package. 
     FIGS. 12A and 12B illustrate the spatially-isolated optical subsystem as according to the first embodiment of the present invention. 
     FIGS. 13A and 13B schematically illustrate the dichroically-isolated optical subsystem as according to second and third embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 6 illustrates the optical structure of the lasercom terminal of a first embodiment of the present invention. An optical subsystem is used for receiving an Rx signal from a remote terminal, e.g. a satellite, and sending a Tx signal toward the same or a different remote terminal. The optical subsystem includes a dual telescope, i.e. an Rx telescope  100  and a Tx telescope  102 , which form a respective pair of afocal reducers, and cooperate respectively with an Rx telecentric focusing lens  104  and a Tx telecentric focusing lens  106 . In the preferred embodiment, the beams are collimated beams in the space between the telescopes  100 ,  102  and the focusing lenses  104 ,  106 . In the preferred embodiments, the focusing lenses  104 ,  106  have a focal length effective to establish a focal plane for respective Rx optical fiber  110  and a Tx optical fiber  112 . 
     As is also seen in FIG. 7A, the present invention includes a precision X-Y linear shifter which is used to precisely position the optical fibers  110 ,  112  in the focal plane of the optical subsystem, so as to align the optical fibers with the respective beams. The Rx signal, after entering the Rx optical fiber  110 , passes through an Erbium Doped Fiber Amplifier (EDFA)  118  where it is detected by a lasercom detector  120 , preferably a PIN diode, where it is transduced into an electrical signal which is processed by the terminal electronics. The Tx signal is generated by a laser diode  122 , which in the illustrated embodiment is a 1552 nm infrared laser source. The Tx signal propagates through an EDFA  124  and outward through the optical system. An acquisition quadcell detector  108  is used with a beamsplitter and appropriate optics to provide proper signal acquisition for the terminal. 
     The present invention uses a novel alignment system. The focusing lenses  104 ,  106  are telecentric, and thus have the property that the central ray of the optical beam passing through the lenses  104 ,  106  will be parallel to the optical axis of the lenses, regardless the angular orientation of the incident beam. Thus, the Rx telecentric lens  104  defines a flat focal plane for beams arriving from any angle of incidence. In this way, as shown in FIG. 7B, it is only necessary to linearly shift the Rx optical fiber  110  in an X-Y plane without rotation to align with the Rx signal beam, irrespective of the angle of incidence. Similarly, a linear X-Y shift in the Tx fiber  112  results in an angular transformation of the propagating Tx beam through the Tx focusing lens  106 . This linear-to-angular transformation is expressed as the “travel gain” G for effecting a fine alignment function of the system. Depending on the parameters of various working embodiments, the system can have a travel gain of about 1-6 microns of linear shift per microradian of angular displacement. (The fiber travel design parameters of a working embodiment is given below in Table I.) 
     Accordingly, a travel stage, i.e. an X-Y shifter  114  is used to linearly vary the position of both the Rx and Tx fibers  110 ,  112 . A separate X-Y shifter  116  is added to the Tx stage to make minor adjustments to Tx fiber. In this way, a large measure of active control is enabled over the Rx and Tx channels, which is useful for communication, signal acquisition, point-ahead and tracking functions. Additionally, this active alignment capability also permits active correction of boresight misalignment due to environmental changes, both long and short term, such as temperature changes, operation-induced deformations, and aging of the optomechanical structure. Also, with the present invention, the channels can be located physically closer together reducing sensitivity to misalignment. Additionally, the traveling fiber arrangement is significantly lightweight compared to the previous tracking mirror structures, resulting in considerable savings in payload delivery and reduced inertia during operation. 
     As is shown in FIG. 8, the traveling stage  114  also carries a track detector  126 , which moves together with the Rx fiber  110 . The track detector  126  is preferably a multi-element detector, e.g. a single quadrant photodiode. The present X-Y shifter  114  can be realized by many different technologies, such as with piezoelectric, voice coil and electrostrictive actuators. An exemplary embodiment is shown in FIGS. 9A,  9 B,  9 C and  9 D, which is given for purposes of illustration alone, and is in no way intended to limit the scope of the invention. 
     The travelling stage  114  includes a housing  180  which is held stationary against displacement. An X-motion fiber stage  182  is provided, and is displaced in an X-direction with a pair of X-motion actuators  184 . The magnitude of displacement is measured with respective X-motion differential impedance transducers  186 . Similarly, a Y-motion fiber stage  188  is provided with respective Y-motion actuators  190  and transducers  192 . A printed circuit assembly  194  is mounted on the Y-motion stage, which provides a mounting surface for the optical fiber tip  196 , along with an electrical connection for the tracking detector  126 . 
     As shown in FIG. 8, the track detector  126  and the Rx fiber tip are mounted on the same stage, with the Rx signal being divided by a beamsplitter. This results in the canceling out of mispoint in both elements simultaneously. Due to the telecentricity of the forward optics, angular mispoint of the incoming Rx signal is transformed into a linear shift that is compensated by the traveling stage  114 . In this way, the field of view is not limited by the physical size of detector  126  itself, but only by the range of travel of the stage. In this way, the present invention is not limited as were previous systems, and a greater range of control is provided. 
     The traveling stage  114  is used to effect fine positional adjustment of the fiber tips for pointing and tracking. As is shown in FIGS. 10A and 10B, course adjustments in pointing and tracking capability are carried out in the present invention using a gimbaled mount for carrying the complete lasercom terminal  130 . The gimbaled mount includes an azimuth actuator  132  and an elevation actuator  134  for pointing the laser com terminal  130  in a desired direction for communicating with a particular remote terminal. The actuators  132 ,  134  are sufficiently accurate so as to position the entire terminal in such a way as to align the traveling stage  114  with the remote terminal, in order to effect communication. The precision of the system must be greater than the total field of view of the track detector  126 , so as to permit signal acquisition. The precision can vary with the requirements of the system, but for most configurations, an accuracy of within 10&#39;s of arcseconds is sufficient. 
     A first embodiment of the optical subsystem of the present invention is shown in FIGS. 11A,  11 B,  12 A and  12 B. In this embodiment, the telescopes  100 ,  102  are in fact a single telescope system in which a common objective is used for both the Rx and Tx signal optical paths, to couple with the respective focusing lenses  104 ,  106 . Mersenne telescope is preferably used, in which a pair of confocal parabolic mirrors  140 ,  142  are used to create an afocal telescope having a folded optical path, thereby reducing the size of the telescope. This system preferably provides an angular magnification of about 5×. A baffle  144  is preferably used to separate the optical paths, providing excellent control of stray light. Such a “two-in-one” telescope provides the benefits of the two-telescope approach while providing the reduced mass and optomechanical simplicity of previous single telescope designs. 
     FIGS. 11A and 11B illustrate the lasercom terminal  130  with the optical subsystem of the first embodiment. The telecentric focusing lenses  104 ,  106  include a positive lens group  146  and a negative lens group  148 , properly configured in a telephoto arrangement to allow compact and efficient packaging of the various modules. The optical path can be folded by using a flat folding mirror  150  (as shown in FIGS. 11A,  11 B,  12 A and  12 B) to direct the light at a desired angle β, where β is preferably 90 degrees to the axis of rotation of the primary  140  and secondary  142 . In this way, the physical size of the system is further reduced, resulting in considerable savings of mass and size, permitting the package to be gimbal mounted, eliminating the need for a complex coude mount as seen in previous systems. Of course, the invention is not limited to the above configuration, and many configurations may be contemplated without departing from the invention. 
     The basic parameters of the optical subsystem are determined as follows: The entire afocal reducing system has a magnification M, and the focusing lenses  104 ,  106  have focal lengths f, and so the equivalent focal length of the complete optical system is Mf. If the required field of new in the object space is W, the required maximum linear shift in the direction of interest will be Mf tan W, assuming no aberration from the optical elements. This value must match the maximum linear shift available with the linear actuator  114  for the required bandwidth. The numerical aperture NA of the optical fibers  110 ,  112  must correspond to the diameters of the collimated axial beams  152 ,  154  of the Rx and Tx signals as they exit the respective telescopes  100 ,  102  and impinge upon the focusing lenses  104 ,  106 . These diameters are given by f (2NA), and so the entrance pupil diameter of the optical front-end is Mf (2NA), which will usually be the diameter of the primary mirror. Thus, the product Mf can be determined from the required field and aperture. Magnification of the telescope can be further determined from the requirements of its length, obscuration (or optical clearance) and aberration correction. In the dual telescope configuration illustrated at present, it is highly advantageous if the values of M and f are the same for both telescopes. The values of NA and pupil diameter can be different as long as both have the same angular field of view in the object space. The above considerations do not take into account the additional angular offset for point-ahead correction of the Tx channel, which only requires an additional offset of the field of view for that channel. Numerical values of these parameters for a working embodiment are shown in the following Tables I: 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 FIBER TRAVEL DESIGN PARAMETERS 
               
             
          
           
               
                   
                 Symbol 
                 Nominal 
               
               
                   
               
               
                 Given Parameters 
                   
                   
                   
                   
               
               
                 Wavelength 
                   
                 1.552 
                 microns 
               
               
                 Telescope Aperture 
                   
                 130 
                 mm 
               
               
                 Telescope Magnification 
                   
                 7 
               
               
                 Fiber NA 
                   
                 0.0563 
               
               
                 Fiber Core Diameter 
                   
                 10 
                 microns 
               
               
                 Track FOV 
                   
                 200 
                 microrads 
                 in Free Space 
               
               
                 ACO FOV 
                   
                 20000 
                 microrads 
                 in Free Space 
               
               
                 Max Point-Ahead Angle 
                   
                 150 
                 microrads 
                 in Free Space 
               
               
                 Point-Ahead Accuracy 
                   
                 1 
                 microrads 
                 in Free Space 
               
               
                 Calculated Parameters 
               
               
                 Equivalent Focal Length (FFL) 
                   
                 1155.2 
                 mm 
               
               
                 Focal Length of the Telec Lens 
                   
                 165.0285714 
                 mm 
               
               
                 Telecentric Lens F-number 
                   
                 8.886153846 
               
               
                 Travel Gain 
                 G 
                 1.1552 
                 microns/microrad 
               
               
                 Travel Range for Track 
                   
                 231.04 
                 microns 
               
               
                 Travel Depth of focus (axial) 
                   
                 245.1034185 
                 microns 
                 Total Range for 1 dB Loss 
               
               
                 Travel Range for P-A 
                   
                 173.28 
                 microns 
               
               
                   
               
             
          
         
       
     
     The spatial isolation embodiment particularly depicted in FIGS. 11A,  11 B and  12 A,  12 B is very useful for closely-positioned terminals. However, this design requires a Tx aperture that is much smaller than the Rx aperture. Consequently, spatial isolation is not practical for separations greater than 1000 km since too much laser power is required for the Tx channel. For long-distance communication ranges of 4000-6000 km or more, the minimum laser power requirement is satisfied when the sizes of the Rx and Tx apertures are equal. For these distance requirements, a dichroic isolation embodiment is employed, as shown in FIGS. 13A and 11B. As seen in FIG. 13A, a common Tx/Rx telescope  100  is used to define a single aperture. A dichroic beamsplitter  160  is used to separate a portion of the Rx signal for use as a beacon for detection by the acquisition sensor  108  and tracking sensors  126 . A Fabry-Perot etalon  162  is used to direct the remaining Rx signal to the Rx optics. A fold mirror  164  received a Tx signal (differing in wavelength from the Rx signal) and directs the beam back through etalon  162  and beamsplitter  160 , or through the telescope  100  toward the remote terminal. In a variant embodiment shown on FIG. 11B, a common telecentric lens  166  can be used for the track detector  126 , along with Rx and Tx fibers  110 ,  112 , thereby further reducing the mass and the number of optical surfaces. The respective signal wavelengths are optically processed through a pair of dichroic beamsplitters  168 ,  170 . Also, in this embodiment, two separate X-Y stages are implemented instead of a single satge  114  (see FIG.  13 A). 
     The present invention can also include active repositioning of the optical fibers in the Z-direction (i.e., along the optical axis) to compensate for longitudinal defocuss, to actively defocus the system in an emergency situation. Also, such positioning allows a highly divergent Tx beam with a significantly enlarged field of view during the phase of signal acquisition between satellite terminals. For example, a 10 microradian beam may typically be used to communicate between terminals, but a divergence on the order of hundreds of microradians may be necessary to acquire a signal during the acquisition phase. Such change in divergence can be easily achieved by actively defocusing the fiber. This feature is also useful for compensating for mechanical fluctuations that may occur in space, especially for the Rx fiber. 
     The present invention significantly reduces the number of optical elements resulting in a diffraction-limited, high-optical-performance system. The reduced mass of the system facilitates steering of the satellite and reduces payload upon delivery, thus reducing deployment and operating expenses. By eliminating the steering mirrors, the present optical system is much less complex, permitting a modular structure with clearly defined, independent modules. The present invention is more flexible, permitting precise beam steering and tracking at least one order of magnitude better than that required while preserving the required range. In addition to the above, the present system is easier and less expensive to manufacture, which reduces the overall expenses associated with satellite communications. 
     As described hereinabove, the present invention provides a simplified lasercom terminal with greatly improved efficiency. However, it will be appreciated that various changes in the details, materials and arrangements of parts which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.