Patent Publication Number: US-2023136379-A1

Title: Free space optical communication system with free space optical beam separator and point-ahead

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. Patent Application No. 63/273,850 filed Oct. 29, 2022, which is hereby incorporated by reference for any and all purposes. 
    
    
     BACKGROUND 
     Free space optical communications (FSOC) is a communications technology that uses light propagating in free space to wirelessly transmit data, for example, for telecommunications or computer networking. Free space is a communications medium that can include air, outer space, vacuum, or water, and contrasts with guided wave communications, such as optical fibers. In many embodiments, FSOC is carried over narrow beams of highly collimated light and modulated to carry coded information. These narrow beams may support complex high-bandwidth networks with many interconnections among moving elements, such as large networks of communication satellites. In contrast with other electromagnetic communications means, narrow optical beams can communicate independently with many terminals, even reusing the same wavelength carriers. 
     In many embodiments FSOC is bidirectional, with encoded beams transmitted in both directions. In other embodiments the communication can be unidirectional, but FSO beams may be transmitted in both directions for tracking purposes. In such embodiments, each FSO terminal, serving as an endpoint for the two beams, may have both a transmitted beam (Tx) and a received beam (Rx). In such embodiments, the two beams may need to be separated, e.g., to prevent the Tx beam from leaking into the Rx beam receiver and/or causing interference which may prevent Rx beam detection. Some Tx/Rx beam separation methods used in previous FSOC systems, such as separate Tx/Rx telescopes, add significant size, weight, power, cost, and complexity increases to the FSOC terminals. Thus, there is a need for more efficient communication devices for free space optical communication. 
     SUMMARY 
     Methods, devices, and systems are disclosed for optical communication. An example device may comprise a first optical path configured to receive and transmit signals via free space. The device may comprise an optical beam separator configured to separate the first optical path into a receiving (Rx) optical path for signals received from free space and a transmitting (Tx) optical path for signals being transmitted into free space. The Rx optical path and the Tx optical path may comprise single mode optical paths (e.g., waveguides, fibers, transport medium)). The device may comprise at least one positioner coupled to one or more of the Rx optical path or the Tx optical path. The device may comprise a controller configured to control the at least one positioner to adjust one or more of the Rx optical path or the Tx optical path with respect to the optical beam separator to facilitate communication with a remote communication device via free space. 
     An example system may comprise a remote communication device and a local communication device. The local communication device may comprise a first optical path configured to receive and transmit signals via free space. The local communication device may comprise an optical beam separator configured to separate the first optical path into a receiving (Rx) optical path for signals received from free space and a transmitting (Tx) optical path for signals being transmitted into free space. The Rx optical path and the Tx optical path may be single mode optical paths (e.g., single mode optical waveguides, single mode optical fibers). The local communication device may comprise at least one positioner coupled to one or more of the Rx optical path or the Tx optical path. The local communication device may comprise a controller configured to control the at least one positioner to adjust one or more of the Rx optical path or the Tx optical path with respect to the optical beam separator to facilitate communication with a remote device via free space. 
     An example method may comprise determining, by a local communication device, a directional parameter for free space optical communication with a remote communication device. The local communication device may comprise an optical beam separator configured to separate a first optical path into a receiving (Rx) optical path and a transmitting (Tx) optical path. The Rx optical path and the Tx optical path may be single mode optical paths (e.g., single mode optical waveguides, single mode optical fibers). The method may comprise controlling, based on the directional parameter and by a controller of the local communication device, at least one positioner to adjust a position of one or more of the Rx optical path or the Tx optical path with respect to the optical beam separator to facilitate communication with the remote communication device via free space. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to limitations that solve any or all disadvantages noted in any part of this disclosure. 
     Additional advantages will be set forth in part in the description which follows or may be learned by practice. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments and together with the description, serve to explain the principles of the methods and systems. 
         FIG.  1    shows an example free space optical communication system in accordance with the present disclosure. 
         FIG.  2    shows an example of beam propagation related to an example communication device. 
         FIG.  3    shows an example optical beam separator. 
         FIG.  4    shows an example of a point ahead offset between two communication devices. 
         FIG.  5    shows an optical configuration of an example communication device. 
         FIG.  6    shows an example positioner for offset control. 
         FIG.  7    shows example components of an example communication device. 
         FIG.  8    shows example receiver offset control. 
         FIG.  9    shows an example point-ahead control system. 
         FIG.  10    show examples of controlling pointing of an example communication device. 
         FIG.  11    shows an example method for communication. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     This disclosure relates to free space optical communication (FSO) between two similar or dissimilar communication stations, such as a FSO communication device, terminal, and/or the like. The stations may have significant separation (e.g., tens or thousands of kilometers). The motion of the stations relative to each other may be significant (e.g., tens of meters per second). The separation and relative velocity between, for example, two stations S 1  and S 2  may cause S 2  to move significantly between the time it emits a signal toward S 1  and the time when it receives a return signal from S 1 . Therefore, the direction of arrival for S 2  &#39;s signal as observed by S 1  is not the exact reciprocal of the optimal direction of transmission for signals emitted from S 1  to S 2 . This disclosure relates to methods by which S 1  may implement (e.g., optimal) transmission to S 2 . 
     The narrow beams used in some FSOC embodiments may demand accurate beam pointing between terminals and render communication unpredictable if the FSO beams miss their intended targets. For example, for communications among satellites in Earth orbit or for communication between ground-terminals and satellites this can be especially challenging. Separation between communication devices may be on the order of hundreds to thousands of kilometers, communication devices may have significant relative velocity, and atmospheric refraction may bend the optical propagation. In some cases, methods to determine time delays, apparent velocity vectors, and other sources of pointing offset from the remote signal&#39;s measured Direction of Arrival (DOA) are known. For example, a satellite in orbit at an altitude 500 km above Earth&#39;s surface experiences a point-ahead angle of 51 micro-radians (51 μrad) relative to the DOA when communicating with the ground immediately below it. The effect lessens as orbital altitude increases, through a combined effect of reduction in orbital velocity and increased propagation delay. Satellite to satellite point-ahead angles are similar in principle but depend on details of orbital parameters, such as altitude differences, relative orbital inclination, and relative orbital phase. Although aircraft-to-aircraft applications experience point-ahead effects, these may be smaller because distances and velocities are typically smaller. 
     An angle for transmission of signals may be determined and/or adjusted relative to the direction of arrival DOA (e.g., or relative to a transmission axis) associated with a remote communication device at the time a local communication device receives signal. This angle may be referred to as the transmission (Tx) offset angle α. The Tx offset angle α may be a one component or a two component angular measure. It may be convenient to decompose the Tx offset angle α into “horizontal” and “vertical” components α h  and α ν . A receiving (Rx) offset angle may also be determined and/or adjusted for purposes of communication with the remote communication device. A general system that manages the offset between Tx and Rx, for point-ahead or other purposes, may require two degrees of freedom to match α h  and α ν . It is not required to implement both degrees of freedom in either the Tx or in the Rx system. By way of example, it may be an advantage, depending on mechanical and other practical considerations, to configure the Tx so it compensates for α h  only, and to adjust the telescope axis so it compensates only for α ν , while the Rx detects signal at offset α ν . In such a system, the Rx detector may have ν-offset such that—a, and the Tx ν-offset is zero. 
     In some implementations, the satellite or other platform may rotate about the optical axis by an angle ϕ, to align the direction of the Tx offset angle α with a specific direction relative to the telescope and Tx-offset mechanism. This may offer an advantage, for example, if this makes α h =0 in the rotated frame. Then the Tx-offset mechanism may compensate for the Tx offset angle α with only a change in α ν . In this scenario the Tx-offset mechanism may require only a single degree of freedom to control α ν . In general, these techniques and other disclosed in more detail below may be used to facilitate communication between a local and remote communication device. 
       FIG.  1    shows an example free space optical communication system in accordance with the present disclosure. The system may comprise a local communication device  102  configured to communicate with a remote communication device  104 . The local communication device  102  may comprise an optical communication device, such as a terminal, station, satellite, and/or the like. The remote communication device  104  may comprise any of the features and/or components of the local communication device  102 . 
     The local communication device  102  may comprise a fore optic  106 , such as a telescope. The local communication device  102  may comprise a housing  108 . The housing  108  may be coupled to the fore optic  106 . The fore optic  106  may provide an optical path into the housing  108 . 
     The local communication device  102  (e.g., the housing  108 ) may comprise a first optical path  110 . The first optical path  110  may be configured to receive and transmit signals via free space, such as signals for communicating with the remote communication device  104 . The first optical path  110  may comprise the fore optic  106 . 
     The local communication device  102  (e.g., the housing  108 ) may comprise an optical beam separator  112 . The first optical path  110  may carry both received signals and transmission signals through the housing  108  between the fore optic and the optical beam separator. The optical beam separator  112  may be configured to separate the first optical path  110  into a receiving (Rx) optical path  114  for signals received from free space and a transmitting (Tx) optical path  116  for signals being transmitted into free space. The Rx optical path  114  may comprise a free space optical path, a fiber based optical path, or a combination thereof. The Tx optical path  116  may comprise a free space optical path, a fiber based optical path, or a combination thereof. 
     The Rx optical path  114  may comprise a single mode optical path. A physical medium in the Rx optical path, such as a waveguide and/or fiber, may support only a single optical mode. The Tx optical path  116  may comprise a single mode optical path (e.g., may be disposed, configured, dimensioned, and/or the like to only support a single mode of light). A physical medium in the Tx optical path, such as a waveguide and/or fiber, may support only a single optical mode. The Rx optical path  114  may comprise a first single mode optical fiber. The Rx optical path  114  (e.g., the first single mode optical fiber) may be optically coupled to a detector  118  for detecting received signals. The Tx optical path  116  may comprise a second single mode optical fiber. The Tx optical path  116  (e.g., the second single mode optical fiber) may be optically coupled with an optical source  120  for generating signals. The optical beam separator  112  may comprise a free space optical beam separator. The optical beam separator  112  may comprise one or more of an optical circulator (e.g., a free space optical circulator), a beam splitter, a plate beam splitter, a polarization beam splitter, or a chromatic splitter. 
     The local communication device  102  (e.g., the housing  108 ) may comprise at least one positioner  122 ,  124 . One or more of the at least one positioner  122 ,  124  may be coupled to the Rx optical path  114 . One or more of the at least one positioner  122 ,  124  may be coupled to the Tx optical path  116 . The at least one positioner may comprise a first positioner  122  configured to adjust the Rx optical path  114 . The at least one positioner may comprise a second positioner  124  configured to adjust the Tx optical path  116 . 
     The at least one positioner (e.g., the first positioner  122 ) may be configured to adjust an angle of a signal traversing the Rx optical path  114  (e.g., or adjust the Rx optical path  114  itself, or the angle of a signal as it is emitted from the Rx optical path  114 ). The at least one positioner (e.g., the first positioner  122 ) may be configured to adjust the Rx optical path  114  along a single direction, such as a direction orthogonal to a direction of signal propagation. The at least one positioner (e.g., the first positioner  122 ) may be configured to adjust the Rx optical path  114  in at least two perpendicular directions, such as two direction orthogonal to a direction of signal propagation. 
     The at least one positioner (e.g., the second positioner  124 ) may be configured to adjust an angle of a signal traversing the Tx optical path  116  (e.g., or adjust the Tx optical path  116  itself, or the angle of a signal as it is emitted from the Tx optical path  116 ). The at least one positioner (e.g., the second positioner  124 ) may be configured to adjust the Tx optical path  116  along a single direction, such as a direction orthogonal to a direction of signal propagation. The at least one positioner (e.g., the second positioner  124 ) may be configured to adjust the Tx optical path  116  in at least two perpendicular directions, such as two directions orthogonal to a direction of signal propagation. 
     The local communication device  102  (e.g., the housing  108 ) may comprise a controller  126  configured to control the at least one positioner (e.g., the first positioner  122 , the second positioner  124 ) to adjust one or more of the Rx optical path  114  or the Tx optical path  116  with respect to the optical beam separator to facilitate communication with the remote communication device  104  via free space. The local communication device  102  (e.g., the housing  108 ) may comprise an offset detector configured to determine alignment signals for communication with the remote communication device. The offset detector may be part of the detector  118  and/or a separate component (e.g., as shown in  FIG.  5   ). The controller  126  may control the at least one positioner  122 ,  124  based on the alignment signals. The controller  126  may control other alignment and positioning. The local communication device  102  may comprise a gimbal configured to adjust a direction of the first optical path  110  and/or the local communication device  102 . The controller  126  may control the gimbal to adjust the direction of the first optical path  110  and/or the local communication device  102 . The controller  126  may otherwise adjust the local communication device  102  according to a variety techniques as disclosed further herein. 
       FIG.  2    shows an examples of beam propagation related to the example communication system of  FIG.  1   . An example communication device is shown having a telescope  206  (e.g., the fore optic  106  of  FIG.  1   ) and a housing  208  (e.g., the housing  108  of  FIG.  1   ). The telescope  206  may comprise a telescope optical axis  204  extending in a direction that the telescope  206  is pointing. The telescope  206  may be configured to receive incoming optical signals  202 , transmit outgoing optical signals  210 , or a combination thereof. An incoming optical signal  202  may be at a first angle relative to the optical axis  204 . 
     The communication device may implement one or more offset angles for communication with a remote communication device, such as an Rx offset angle ξ for an Rx optical path, a Tx offset angle α for a transmission optical path, and a rotational offset angle θ. The Rx offset angle may comprise a vertical component ξ ν , a horizonal component ξ h , or a combination thereof. The Tx offset angle may comprise a vertical component α ν , a horizonal component α h , or a combination thereof. The Tx offset angle α may be with respect to the optical axis  204 . 
     By way of further explanation, the communication device may orient itself such that the optical axis  204  of the telescope  206  aligns with the incoming signal  202  from a remote communication device apparent direction of arrival. Alternatively, the communication device may orient the telescope  206  such that the optical axis  204  has an Rx offset angle ξ relative to the incoming direction of arrival. The Rx offset angle ξ may comprise a 1-component or 2-component Rx offset angle relative to the incoming direction of arrival. In practical applications, the Rx offset angle ξ may be much less than the telescope&#39;s field of view (FOV), such that both ξ and ξ+α are within the FOV. The local communication device may be designed to maintain the Rx offset angle ξ close to zero by adjusting a positioner of the Rx optical path, re-orienting the entire communication device, re-orienting the telescope, or a combination thereof. 
     In some scenarios, the disclosed communication device may implement offsets in both the Rx and Tx optical paths. For example, the communication device may implement an Rx offset ξ h  in the Rx optical path. The communication device may implement Tx offset α ν , in the Tx optical path. The communication device may implement a rotation offset ξ around a rotational axis  212  and a single offset in either the Tx or Rx optical paths, such as a, (in the Tx) or ξ ν  (in the Rx). It may be an advantage to use some combination of these approaches if pointing of a communication device has complex requirements or constraints. 
       FIG.  3    shows an example optical beam separator. The example optical beam separator may comprise an optical device configured to separate Tx and Rx optical paths. The Tx optical path comprise a path between the optical beam separator and a transmitter, such as the optical source  120  of  FIG.  1   . The Rx optical path may comprise a path between the optical beam separator and a detector, such as the detector  118  of  FIG.  1   . The optical beam separator may be optically coupled to a common optical path between the optical beam separator and a fore optic (e.g., and to a remote communication device). The optical beam separator may be a beam splitter, a part-silvered mirror. Other optical devices may be used in some embodiments. 
     The optical beam separator may comprise a free space optical circulator (FSOC) as shown in  FIG.  3   . It should be understood that the communication device is not limited to this type of optical beam separator and the one shown is only for purposes of illustration. The optical beam separator may comprise a first port  302 . The optical beam separator may comprise a second port  304 . The optical beam separator may comprise at third port  306 . 
     The first port  302  may be a common port for both the Rx path and the Tx path. The second port may optically couple the optical beam separator to the Rx optical path (e.g., to the detector). The third port  306  may optically couple the optical beam separator  330  to the Tx optical path (e.g., to the transmitter). Rx signals may enter the optical beam separator  300  via the first port  302  and exit the optical beam separator  300  by the second port  304 . Tx signals may enter the optical beam separator  300  from the third port  306  and exit the optical beam separator from the first port  302 . Port numbering is a convenience only, and any even permutation of ports one, two, and three may be substituted for ports one, two, and three. For example, it may be more convenient to use ports three, one, and two for telescope, Rx and Tx. The even permutations are: (1, 2, 3), (2, 3, 1) and (3, 1, 2). 
     Three-port devices with ports numbered 1, 2, and 3, where signal is directed primarily from ports 1-to-2, 2-to-3, and 3-to-1, are typically termed “circulators.” Circulators may be used in transceiver designs where a common antenna or telescope carries both incoming and outgoing signal. The circulator&#39;s passing signal from ports 1-2-3 rather than 3-2-1 may enable it to simultaneously direct incoming signal to the transceiver&#39;s receiver portion and to protect the receiver from the transmitter&#39;s much higher output power. The decibel ratio between signal loss in the 3-2-1 directions and the 1-2-3 directions is termed “directivity.” 
     In distinction from guided-wave circulators, the ports of a “free space” optical circulator (FSOC) may accept and emit many propagating modes with similar directions of propagation (or their reciprocal directions) over some field of view (FOV). The device ports may take the form of planar or lenticular windows. Signal beams, as described above in the background discussion, may enter a port (e.g., the first port  302 ), propagate through the circulator, and emerge, for example, from only one other port (e.g., the second port  304 ). The propagating modes of an FSOC may support multiple beams that may enter near a port&#39;s optical axis (e.g., the first port  302 ) and emerge from another port (e.g., the second port  304 ) while retaining their relative directions of propagation and other optical properties. 
     The FSOC&#39;s FOV may include all angles of propagation external to the FSOC supported in this manner, and their reciprocals. The FSOC&#39;s directivity, however, causes reciprocal-propagating signals to couple to different ports. Continuing the previous example, a received incoming signal at the first port  302  with a small offset angle ξ from the port&#39;s optical axis may propagate through the circulator and emerges ξ from the optical axis of the second port  304  (e.g., possibly with a rotation applied to ξ). At the same time, a Tx signal entering the third port  306  with a small offset angle α may emerge from the first port  302  with the offset α from the axis of the first port  302  (e.g., possibly with a rotation applied). For the case where a and ξ are both zero, the Tx signal&#39;s direction of propagation is the reciprocal of the incoming signal. 
     Thus, the free-space beam separation device supports incoming Rx beams and Tx beams with a small point-ahead offset. This may operate in the same way whether the device is a circulator or not. Both signals can propagate through the telescope (e.g., or the first optical path  110  of  FIG.  1   ) simultaneously and the telescope pointing toward the remote communication device may optimize both Rx and Tx optical paths. 
       FIG.  4    shows an example of a point ahead offset between two communication devices. The illustration shows how a local communication device  402  (e.g., a local station), which may be a satellite in orbit, may need to point ahead of an apparent location of a remote communication device  404  (e.g., remote station, another satellite in orbit) by a Tx offset angle α. The remote communication device  404  may move relative to the local communication device  402  in the direction suggested by the dashed arrow. By the time the local communication device  402  return signal arrives at the remote communication device  404 , the remote communication device&#39;s position and orientation relative to the local communication device  402  may have changed. Applying a Tx offset angle α may cause the transmitted signal to arrive at the actual location of the remote communication device  404 , rather than the location of the satellite at the time of transmission. For this reason, the local communication device  402  may offset its transmit direction (gray line) from the apparent direction of arrival from the remote (black line). As an example, the second positioner  124  of example local communication device  102  of  FIG.  1    may adjust the orientation of the Tx optical path  116  to apply the Tx offset angle α for optical signals being transmitted by the local communication device  102 . 
       FIG.  5    shows an optical configuration of an example communication device  500 . The example communication device  500  may comprise a satellite-borne station as shown. The example communication device  500  may comprise any of the features of the communication devices described elsewhere herein. A telescopic fore-optic  502  may receive an incoming light signal  504  and direct an outgoing signal  506  to a remote communication device. If the angular Rx-to-Tx offset is small, both beams may pass through a common optical path near its optical axis and with little (e.g., minimal) aberration. 
     An optical beam separator  508  (e.g., a circulator) may separate Tx and Rx paths. Along the Rx path, an offset tracker  510  may detects the Rx angle of arrival and maintains the optical axis relative to it. Following the offset tracker  510 , a receiver  512  may be configured to receiver optical signals. An Rx path positioner may be integrated into the receiver  512  and/or separate from the receiver  512 . The Rx path positioner may modify the angle of Rx signals to implement an Rx offset angle ξ. On the Tx path, a transmitter  514  may be configured to transmit optical signals. A Tx path positioner may be integrated into the transmitter  514  and/or separate from the transmitter  514 . The Tx path positioner may modify the angle of Tx signals to implement a Tx offset angle α. One or more of the Rx offset angle ξ or the Tx offset angle α may be adjusted by the corresponding positioners based on any signals, data, calculation, and/or analysis, such as any signals the offset tracker  510 . 
       FIG.  6    shows an example positioner for offset control. Top cover and alignment components are not shown in the figure to make the positioning components visible. The example positioner  600  may implement h-direction and ν-direction offset control. The positioner  600  may be implemented as an Rx positioner, a Tx positioner, or a combination thereof (e.g., if integrated into a common path with both Rx and Tx beams). In the case of a Tx positioner, the positioner may be configured to adjust a Tx horizontal offset angle α h , a Tx vertical offset angle α ν , or a combination thereof. In the case of a Rx positioner, the positioner may be configured to adjust an Rx horizontal offset angle ξ h  an Rx vertical offset angle or a combination thereof. 
     The same deflection mechanism may be used for both Tx and Rx fibers. The positioner  600  may be disposed adjacent (e.g., around) an optical fiber  602 . The optical fiber  602  is shown in relation to a focal plane  604  of the telescope of the communication device. The positioner  600  may comprise a first set of static magnets  606  and a first moving coil  608 . The first set of static magnets  606  and the first moving coil  608  may be configured to adjust the position of the fiber in the horizontal direction (e.g., to cause deflection of optical signals for a specified offset angle). An electrical signal may be applied to the first moving coil  608  to apply a force to deflect signals in the optical fiber  602 . The positioner  600  may comprise a second set of static magnets  610  and a second moving coil  612 . The second set of static magnets  610  and the second moving coil  612  may be configured to adjust the position of the fiber in the vertical direction (e.g., to cause deflection of optical signals for a specified offset angle). An electrical signal may be applied to the second moving coil  612  to apply a force to cause deflection of signals in the optical fiber  602 . The positioner  600  may comprise flexible support rods  614  to support the optical fiber  602 . 
     For configurations that allocate a single offset direction to either Tx, Rx or both, a single pair of coils and  4  static magnets may be used. Optical fibers may be single-mode fibers (SMF) for the signal wavelength. SMFs may offers benefits over other guided-wave structures for both Tx and Rx signal path processing. For optimal coupling between free-space and guided wave propagation, the positioner  600  may couple the telescope&#39;s field distribution and adjust and ξ (for Rx) or α (for Tx) for distant sources to well within the SMF core. 
       FIG.  7    shows example components of an example communication device. The components may be components stored in a housing  702  (e.g., the housing  108  of  FIG.  1   , housing  208  of  FIG.  2   ) of an example communication device. The example communication device may comprise an optical beam separator  704  (e.g., optical beam separator  112 , optical beam separator  300 ). The example communication device may comprise one or more optical redirection components  706 , such as mirrors, prisms, and/or the like. The one or more optical redirection components  706  may direct Rx signals from the optical beam separator  704  to an offset detector  708 , such as an angular offset detector. 
     The components may comprise a Tx optical assembly  710 . The Tx optical assembly  710  may comprise a Tx optical fiber. The Tx optical fiber may be mounted on a Tx adjustment stage. The Tx adjustment stage may comprise at least one positioner as disclosed herein. The Tx adjustment stage may be configured to adjust a Tx offset angle α as disclosed herein. 
     The components may comprise an Rx optical assembly  712 . The Rx optical assembly  712  may comprise an Rx optical fiber. The Rx optical fiber may be mounted on an Rx adjustment stage. The Rx adjustment stage may comprise at least one positioner as disclosed herein. The Rx adjustment stage may be configured to adjust an Rx offset angle ξ as disclosed herein. 
     In this example, the offset detector  708  (e.g., angular offset detector) may be positioned near the telescope&#39;s exit pupil. A 90-degree prisms may fold the optical path between the offset detector  708  and the optical beam separator  704  (e.g., circulator) first port (e.g., Port  1 ). The Tx signals (at Port  3 ) and Rx signals (at Port  2 ) may be carried by corresponding optical fibers and may be offset by 2-axis electromagnetic-coil deflection devices (TAECDs). With this perspective view shown in  FIG.  7   , the telescope optical axis is “down” and the telescope may be mounted beneath the optical receiver housing  702 . The angular offset detector  708  may be used to position the system (e.g., the Rx signals, the Rx optical path, the Rx optical fiber, the Rx detector, or any combination thereof), such as to reduce (e.g., minimize) the Rx offset angle ξ (e.g., with respect to a detector). Small-offset control may be accomplished using the illustrated electromagnetic deflection coils. Other implementations may, by way of example, comprise geared positioners, piezoelectric deflectors, a combination thereof, or other means to implement fine offset positioning. 
       FIG.  8    shows example Rx offset angle ξ offset. A telescope may produce the resolution and clearest signal for communication when the remote communication direction of arrival (e.g., DOA is very close to the telescope optical axis). That is, when the magnitude of the Rx offset angle |ξ| is small compared with the telescope FOV. A communication device may therefore point the optical axis to a remote communication device using a combination of these techniques, as well as other means, such as any of the following. If the communication device is coupled to a vehicle such as a spacecraft, the communication device may modify the entire spacecraft attitude with thrusters, reaction wheels, magnetic torque or other means. Movable mirrors or prisms placed outside the telescope objective element may deflect or re-orient the optical axis. This may have an advantage over spacecraft re-orientation because the bulk to be moved is much smaller. An Rx offset angle ξ detector may include of an arrangement as described in U.S. Pat. No. 9,716,549, the disclosure of which is hereby incorporated by reference. The offset detector may enable the pointing system to maintain the received signal centered on the detector, as suggested by the lighter zone of the left image below. The system may accomplish this with a closed-loop control using one of the above means to re-orient the optical axis. This may increase (e.g., maximize) receiver performance. 
     In a case where it is desired to introduce some Rx offset angle ξ, for example, to reduce Tx offset from the optical axis, the same closed-loop control may be used, but now with a bias introduced so that the control loop maintains a small ξ-offset, as illustrated in the right-side image. By this technique, a system may implement Tx-and-Rx shared point ahead as discussed further herein, offsetting for α h  in Tx only, and offsetting for α ν , in Rx detects signal at offset α ν . 
       FIG.  9    shows an example point-ahead control system. The system may comprise a laser and modulator  902 , a detector  904 , a fore optic  906 , a circulator  908  (e.g., or other optical beam separator), an offset detector  910 , and/or the like. These components may comprise any of the features of these components described elsewhere herein. The system may comprise an optical pointing and tracking controller  912 . The system may comprise a pointing and tracking target selection control  914 . The pointing and tracking target selection control  914  may receive signals from the offset detector and providing information based on the signals to the optical pointing and tracking controller  912 . 
     The optical pointing and tracking controller  912  may provide control signals and/or information to the fine position control  916 . The fine position control  916  may comprise a Tx point-head offset control  918 . The fine position control  916  may comprise an Rx fine incoming signal positioning control  920 . The Tx point-ahead offset control  918  may be configured to cause adjustments (e.g., using control signals) to a Tx positioner  922 . The Rx fine incoming signal positioning control  920  may be configured to cause adjustments (e.g., using control signals) to an Rx positioner  924 . Tx positioner  922  may cause (e.g., based on control signals from the Tx point-ahead offset control) angular adjustments to Tx signals before transmission to a remote communication device. The Rx positioner  924  may cause (e.g., based on control signals from the Rx fine incoming signal positioning control  920 ) angular adjustments to Rx signals received from a remote communication device. 
     A point-ahead control system may be integrated with coarse pointing and fine pointing toward the remote communication device&#39;s apparent position as illustrated in the  FIG.  9   . Pointing may be initialized with a priori information about the remote communication device&#39;s relative position and orientation. An offset detector  910  and a received signal strength indicator (RSSI) in a modem of the local communication device may be used to jointly optimize pointing alignment with the incoming apparent angle of arrival. The pointing system (e.g., the optical pointing and tracking controller, the fine position control  916 ) may reorient the telescope to reduce (e.g., minimize) the Rx offset angle ξ. 
     Additional processing may improve (e.g., optimize) the Tx offset angle α. For the case where the Rx positioning accepts and compensates small values of ξ (as shown in earlier illustrations of this disclosure), α optimization may take the instantaneous commanded values of ξ into account, so that the Tx positioner tracks the vectorial sum of ξ+α. 
     Once communication is established, the Tx offset processor may also take RSSI reports from the remote communication device into account and optimize the Tx offset angle α for best signal at the remote communication station. 
       FIG.  10    show examples of controlling pointing of an example communication device. The illustration shows several mechanisms a communication device may use to implement alignment to the Rx direction of arrival, and to implement Tx offset angle α. In some embodiments, the communication device may require four degrees of freedom to point the Rx and Tx correctly. As noted elsewhere herein, a communication device may implement more than four degrees of pointing freedom for Rx and Tx, to support coarse and fine pointing. 
     In some implementations, a communication device may use small controlled deviations from optimal pointing, together with assessment of signal strength, to find the optimal direction for Rx. This is termed “dithering.” In these implementations, dithering may use different motions or degrees of freedom than general pointing to reduce (e.g., minimize) Rx offset angle ξ and/or implement a Tx offset angle α. It should be noted that even though the figure shows the optical beam separator (e.g., circulator) that may produce a 90° angle between the Tx and Rx paths, the optical beam separator itself may produce a variable deflection angle between the Tx and Rx beams (e.g.: deviating from 90°) as part of the point-ahead mechanism. 
       FIG.  11    is a flowchart showing an example method for free space optical communication. At step  1102 , a directional parameter for free space optical communication with a remote communication device may be determined. The directional parameter may be determined by a local communication device. The directional parameter may comprise a transmission (Tx) offset angle as disclosed herein. The directional parameter may comprise a receiving (Rx) offset angle as disclosed herein. The directional parameter may comprise a combination of the Tx offset angle and the Rx offset angle. Determining the directional parameter may comprise determining the Tx offset angle, the Rx offset angle, any component thereof, and/or any combination thereof. The directional parameter may be determined based on offset signals from an offset detector. The directional parameter may be determined based on a point-ahead algorithm for determining that signals are transmitted to account for relative motion between the local communication device and the remote communication device. 
     The local communication device may comprise any of the components, features, and/or functionality of the communication devices disclosed herein, such as the local communication device of  FIGS.  1 - 2 ,  5 , and  9   . The local communication device may comprise an optical beam separator, such as any of the optical beam separators disclosed further herein. For example, the optical beam separator may comprise one or more of an optical circulator, a beam splitter, a plate beam splitter, a polarization beam splitter, or a chromatic splitter. The optical beam separator may be configured to separate a first optical path into a receiving (Rx) optical path and a transmitting (Tx) optical path. The Rx optical path may be a single mode optical path. The Tx optical path may be a single mode optical path. The Rx optical path may comprise a first single mode optical fiber optically coupled to a detector for detecting received signals. The Tx optical path may comprise a second single mode optical fiber optically coupled with an optical source for generating signals. 
     At step  1104 , at least one positioner may be controlled to adjust a position of one or more of the Rx optical path or the Tx optical path (e.g. with respect to respect to the optical beam separator). The at least one positioner may be controlled based on the directional parameter (e.g., to match one or more angles indicated by and/or associated with the directional parameter). The at least one positioner may be controlled by the local communication device (e.g., by a computer processor, controller, micro controller, circuit, field programmable gate array, and/or associated memory). The at least one positioner may be controlled to facilitate communication (e.g., by controlling alignment with) with the remote communication device via free space. 
     It is to be understood that the methods and systems are not limited to specific methods, specific components, or to particular implementations. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. 
     As used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. 
     “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. 
     Throughout the description and claims of this specification, the word “comprise” and variations of the word, such as “comprising” and “comprises,” means “including but not limited to,” and is not intended to exclude, for example, other components, integers or steps. “Exemplary” means “an example of” and is not intended to convey an indication of a preferred or ideal embodiment. “Such as” is not used in a restrictive sense, but for explanatory purposes. 
     Components are described that may be used to perform the described methods and systems. When combinations, subsets, interactions, groups, etc., of these components are described, it is understood that while specific references to each of the various individual and collective combinations and permutations of these may not be explicitly described, each is specifically contemplated and described herein, for all methods and systems. This applies to all aspects of this application including, but not limited to, operations in described methods. Thus, if there are a variety of additional operations that may be performed it is understood that each of these additional operations may be performed with any specific embodiment or combination of embodiments of the described methods. 
     As will be appreciated by one skilled in the art, the methods and systems may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the methods and systems may take the form of a computer program product on a computer-readable storage medium having computer-readable program instructions (e.g., computer software) embodied in the storage medium. More particularly, the present methods and systems may take the form of web-implemented computer software. Any suitable computer-readable storage medium may be utilized including hard disks, CD-ROMs, optical storage devices, or magnetic storage devices. 
     Embodiments of the methods and systems are described herein with reference to block diagrams and flowchart illustrations of methods, systems, apparatuses and computer program products. It will be understood that each block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, respectively, may be implemented by computer program instructions. These computer program instructions may be loaded on a general-purpose computer, special-purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions which execute on the computer or other programmable data processing apparatus create a means for implementing the functions specified in the flowchart block or blocks. 
     These computer program instructions may also be stored in a computer-readable memory that may direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including computer-readable instructions for implementing the function specified in the flowchart block or blocks. The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions that execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. 
     The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and sub-combinations are intended to fall within the scope of this disclosure. In addition, certain methods or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto may be performed in other sequences that are appropriate. For example, described blocks or states may be performed in an order other than that specifically described, or multiple blocks or states may be combined in a single block or state. The example blocks or states may be performed in serial, in parallel, or in some other manner. Blocks or states may be added to or removed from the described example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the described example embodiments. 
     It will also be appreciated that various items are illustrated as being stored in memory or on storage while being used, and that these items or portions thereof may be transferred between memory and other storage devices for purposes of memory management and data integrity. Alternatively, in other embodiments, some or all of the software modules and/or systems may execute in memory on another device and communicate with the illustrated computing systems via inter-computer communication. Furthermore, in some embodiments, some or all of the systems and/or modules may be implemented or provided in other ways, such as at least partially in firmware and/or hardware, including, but not limited to, one or more application-specific integrated circuits (“ASICs”), standard integrated circuits, controllers (e.g., by executing appropriate instructions, and including microcontrollers and/or embedded controllers), field-programmable gate arrays (“FPGAs”), complex programmable logic devices (“CPLDs”), etc. Some or all of the modules, systems, and data structures may also be stored (e.g., as software instructions or structured data) on a computer-readable medium, such as a hard disk, a memory, a network, or a portable media article to be read by an appropriate device or via an appropriate connection. The systems, modules, and data structures may also be transmitted as generated data signals (e.g., as part of a carrier wave or other analog or digital propagated signal) on a variety of computer-readable transmission media, including wireless-based and wired/cable-based media, and may take a variety of forms (e.g., as part of a single or multiplexed analog signal, or as multiple discrete digital packets or frames). Such computer program products may also take other forms in other embodiments. Accordingly, the present invention may be practiced with other computer system configurations. 
     While the methods and systems have been described in connection with preferred embodiments and specific examples, it is not intended that the scope be limited to the particular embodiments set forth, as the embodiments herein are intended in all respects to be illustrative rather than restrictive. 
     It will be apparent to those skilled in the art that various modifications and variations may be made without departing from the scope or spirit of the present disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practices described herein. It is intended that the specification and example figures be considered as exemplary only, with a true scope and spirit being indicated by the following claims.