Continuously Variable Optical Beam Splitter

Abstract

The continuously adjustable optical beam splitter includes a circular-to-linear polarization converter, a polarization rotator, and a polarization beam splitter that distributes light between two outputs. The polarization converter can include a quarter-wave plate. The polarization rotator can include a mechanically rotated half-wave plate or an electronically controlled liquid crystal variable retarder. The light source can be an astronomical object, reflected LIDAR light, or a laser beam from a remote laser communication node. Also disclosed is a free-space laser communication terminal implementing the disclosed optical beam splitter, where the optical beam splitter redirects substantially all received laser light into an alignment channel during initial alignment, and then, during message communication, maintains alignment by redirecting more than 90%, and in embodiments up to 98%, of the received light into the communication channel while continuing to redirect the remaining laser light into the alignment channel.

Description

Related Applications

[0001]

[0001] This application is related to U.S. Patent Application No. 16 / 694,082, filed November 25, 2019, now U.S. Patent No. 10,637,572, and U.S. Patent Application No. 16 / 720,362, filed December 19, 2019, now U.S. Patent No. 10,841,007, both of which are by the present inventor and assigned to the present applicant, both of which are incorporated by reference in their entirety for all purposes. [Technical field]

[0002]

[0002] The present disclosure relates to optical detection devices, and more particularly to optical beam splitters implemented in devices configured to receive and process light received over free space from a remote source. [Background technology]

[0003]

[0003] Devices configured to receive and process light over free space from remote sources often need to transition between operational phases that may include an "alignment" phase, in which light from the remote source is detected and the system optics are aligned with the remote source, and a processing phase, in which the light received from the remote source is processed. It is often necessary to continue to "track" the remote light source during the processing phase, i.e., to maintain alignment with the remote light source. Non-limiting applications of such devices include free-space laser communications (ground, airborne, and space), laser ranging and LIDAR, and certain astronomical observations.

[0004] For example, a free-space laser communications receiver is required to detect laser light transmitted by a remote communications node, align itself with the remote node, and then receive and process the laser communications transmitted by the remote node while maintaining alignment. A LIDAR device must locate a remote target, direct a laser beam at the target, and then maintain application of the beam to the target while the reflected laser light is analyzed to determine the direction and distance to the target. Similarly, astronomical observations of distant light sources, such as planets or stars, require detection of the remote light source and alignment of a telescope or other observation optics with the source, followed by receiving and processing the light received from the remote light source, typically over extended periods during which precise alignment with the remote light source must be maintained.

[0005]

[0005] The various types of equipment used to perform these tasks typically include separate subsystems or "channels" dedicated to functions that must be performed during different phases of operation. For example, a laser communications terminal typically includes separate alignment and communications channels.

[0006]

[0006] The implementation of multiple channels in a light receiving device requires that the received light must be distributed among the channels. In many cases, it is necessary to simultaneously allocate the received light among two or more channels during at least one phase of operation. For example, co-pending U.S. patent application Ser. No. 16 / 720,362 by the present inventor, which is assigned to the present applicant and incorporated herein by reference in its entirety for all purposes, discloses a free-space laser communication terminal that implements a "receive beam splitter" that splits the received light between an alignment channel and a communication channel. Rather than implementing a separate "beacon laser," the laser terminal disclosed by application Ser. No. 16 / 720,362 uses a receive beam splitter to redirect a small portion of the received message-bearing laser light into the alignment channel during the communication phase, which is used by the alignment channel to maintain optical alignment with the remote node during reception of the laser communication.

[0007]

[0007] The received light may be distributed between two or more channels of the device by a fixed beam splitter, such as a partially silvered mirror or a dichroic beam splitter. However, this approach often results in a compromise that is not ideal for any of the operation phases. For example, ideally, the laser communication terminal of application Ser. No. 16 / 720,362 redirects all of the received light to the ATS during the alignment phase, and then redirects only a small percentage of the received light to the alignment channel during the communication phase once the node is aligned and the divergence of the received beam is reduced. In such a case, if a fixed beam splitter is implemented, for example a splitter that directs 20% of the received light to the alignment channel and 80% of the received light to the communication channel, the result is a compromise that is far from ideal for both the alignment phase and the communication phase.

[0008]

[0008] Another approach is to provide two or more beam splitters with different splitting ratios and switch between them as the device transitions between operational phases, but this approach wastes space and energy and at best supports only a few selectable splitting ratios.

[0009]

[0009] Therefore, what is needed is an optical beam splitter and a method for using it that allows light to be received from a remote source and flexibly distributed among two or more channels of a device without consuming excessive space and energy. Summary of the Invention

[0010]

[0010] Disclosed is an optical beam splitter and method of use that allows light to be flexibly allocated between two or more channels of a device without consuming excessive space and energy. More specifically, disclosed is an optical beam splitter and method of use that allows light to be received from a remote source and allocated between two channels of a device according to a continuously adjustable split ratio.

[0011]

[0011] According to the present disclosure, light enters the device in a circularly polarized state. Depending on the implementation, this can be ensured, for example, by the LIDAR transmitting circularly polarized light and detecting its reflection, or by requiring that all of the nodes in the free-space laser communication network transmit their beacons and messages using circularly polarized laser light. In the case of astronomy, the present disclosure is applicable to the observation of light from remote sources that is naturally circularly polarized, for example, when studying strong magnetic regions on the surface of the sun.

[0012]

[0012] The disclosed optical beam splitter converts received laser light from circularly polarized light to linearly polarized light, where the direction of the linear polarization is well defined and fixed. In an embodiment, this conversion from circular to linear polarization is performed by passing the received light through a quarter wave plate (QWP).

[0013]

[0013] The linearly polarized light is then directed to a variable retarder that can rotate the direction of the linear polarization of the light through any adjustable rotation angle. Finally, the light enters a polarizing beam splitter (PBS), which distributes the light between the two channels according to the relative angle between the linear polarization direction of the light and the polarization direction of the PBS. Additional retarders and polarizing beam splitters can be included if it is necessary to distribute the light to more than two channels.

[0014] In some embodiments, the variable retarder is a half-wave plate (HWP) and the angle of the linear polarization is adjusted by physically rotating the HWP, in other embodiments, the variable retarder is an electronically controlled device such as a liquid crystal variable retarder (LCVR).

[0015] Also disclosed herein is a free-space laser communication terminal similar to the laser communication terminal of co-pending U.S. patent application Ser. No. 16 / 720,362, in which the disclosed optical beam splitter is implemented as a receive beam splitter. According to this exemplary embodiment, rather than implementing a separate beacon laser, the communication signal wavelength is used by the laser communication terminal to maintain optical alignment during the communication phase. During the alignment phase, the disclosed beam splitter redirects substantially all of the received light to the alignment channel. Once alignment with the remote node is complete and there is a transition to the communication phase, the disclosed beam splitter is reconfigured to redirect only a small percentage of the received light to the alignment channel, while the majority of the received light is directed to the communication channel. The disclosed beam splitter implementation thereby enables the disclosed laser communication terminal to adjust and optimize the distribution of received light between the two channels during each phase of operation.

[0016] In addition to receiving circularly polarized laser light, embodiments of the disclosed laser communication terminal are also configured to transmit circularly polarized light having orthogonal properties. In some embodiments, the disclosed laser communication terminal receives right-handed circularly polarized laser light and transmits left-handed circularly polarized laser light, or vice versa, so that any transmitted laser light that inadvertently enters the receive input can be filtered out of the receiver channel. Further details regarding these features are provided in U.S. Patent Application No. 16 / 694,082, now U.S. Patent No. 10,637,572, and U.S. Patent Application No. 16 / 720,362, both of which are incorporated herein by reference in their entireties.

[0017]

[0017] A first general aspect of the invention is an optical beam splitter configured to receive light via free space from a remote light source, the light being circularly polarized, and to distribute the received light between two outputs according to a continuously adjustable splitting ratio. The optical beam splitter includes a polarization converter configured to convert the received circularly polarized light into linearly polarized light having a defined initial light polarization direction, a retarder configured to rotate the light polarization direction from the initial light polarization direction to an adjusted polarization direction, and a polarizing beam splitter having a splitter polarization direction, the polarizing beam splitter configured to distribute the received light between the first and second outputs according to a difference between the adjusted polarization direction and the splitter polarization direction.

[0018] In an embodiment, the received light is circularly polarized laser light.

[0019] In any of the above embodiments, the polarization converter may include a quarter wave plate.

[0020]

[0020] In any of the above embodiments, the retarder may include a half-wave plate, and the retarder may be configured to change the adjusted polarization direction by mechanically rotating the half-wave plate.

[0021] In any of the above embodiments, the retarder may include a liquid crystal variable retarder (LCVR) configured to electronically change the tuned polarization direction.

[0022] In any of the above embodiments, the distant light source may be a celestial light source that emits light having a naturally occurring circular polarization.

[0023] In any of the above embodiments, the optical beam splitter may be implemented in the LIDAR device, and the remote light source may be a remote target reflecting the circularly polarized light transmitted by the LIDAR device.

[0024] In any of the above embodiments, the remote light source may be a remote node in a free space laser communications network.

[0025]

[0025] A second general aspect of the present invention is a free space laser communications terminal configured to establish optical alignment with a remote node during an alignment phase and to receive laser communications transmitted by the remote node during a communication phase, the free space laser communications terminal including: an alignment channel configured to establish and maintain optical alignment of the laser communications terminal with the remote node using received laser light received from the remote node, the received laser light being circularly polarized; a communication channel configured to derive communication information from the received laser light during the communication phase; and a receive beam splitter as claimed in claim 1, the receive beam splitter configured to adjustably allocate the received laser light between the alignment channel and the communication channel.

[0026] In an embodiment, the polarization converter of the receive beam splitter comprises a quarter wave plate.

[0027]

[0027] In any of the above embodiments, the retarder of the receive beam splitter may include a half-wave plate, and the retarder may be configured to adjust the adjusted polarization direction by mechanically rotating the half-wave plate.

[0028]

[0028] In any of the above embodiments, the retarder of the receive beam splitter may include a liquid crystal variable retarder (LCVR), which may be configured to electronically change the adjusted polarization direction.

[0029] Any of the above embodiments may further include a transmit channel configured to generate and transmit circularly polarized laser light to the remote node.

[0030]

[0030] A third general aspect of the present invention is a method for aligning a free-space laser communications terminal with a remote node and maintaining the alignment while receiving a laser transmission message from the remote node without requiring receipt of a separate alignment beacon from the remote node, the laser communications terminal comprising an alignment channel and a communications channel. The method includes implementing an optical beam splitter as claimed in claim 1 as a receive beam splitter in the laser communication terminal; receiving laser light from a remote node, the received laser light being circularly polarized; causing the receive beam splitter to redirect substantially all of the received laser light into an alignment channel; aligning the laser communication terminal with the remote node in accordance with the laser light received by the alignment channel; causing the receive beam splitter to reallocate the received laser light between the alignment channel and the communication channel such that at least 90% of the received laser light is allocated to the communication channel while no more than 10% of the received laser light is allocated to the alignment channel; and simultaneously maintaining alignment of the laser communication terminal with the remote node in accordance with no more than 10% of the received laser light allocated to the alignment channel while a laser transmit message encoded in the received laser light is processed by the communication channel.

[0031] In an embodiment, less than 5% of the received laser light is allocated to the alignment channel during the communication phase.

[0032]

[0032] In any of the above embodiments, the retarder of the receive beam splitter may include a half-wave plate, and causing the receive beam splitter to redistribute the received laser light may include physically rotating the half-wave plate.

[0033]

[0033] In addition, in any of the above embodiments, the retarder of the receive beam splitter may include a liquid crystal variable retarder (LCVR), and causing the receive beam splitter to redistribute the received laser light may include electronically rotating the linear polarization direction of the received laser light in the LCVR.

[0034]

[0034] The features and advantages described herein are not all-inclusive, and in particular many additional features and advantages will become apparent to those skilled in the art in light of the drawings, specification, and claims. Furthermore, it should be noted that the language used in this specification has been selected primarily for readability and instructional purposes, and not to limit the scope of the inventive subject matter. [Brief description of the drawings]

[0035] [Figure 1A] FIG. 1A is a block diagram of an optical beam splitter in one embodiment of the present disclosure. [Figure 1B]

[0036] FIG. 1B is a block diagram of a very specific example of a free-space laser communications terminal according to one embodiment of the present disclosure that implements the continuously variable configurable beam splitter of FIG. 1A. [Figure 1C]

[0037] FIG. 1C is a block diagram illustrating the interaction between the transmitted laser light and the input / output polarizing beam splitters and wave plates in one embodiment of the disclosure. [Figure 1D]

[0038] FIG. 1D is a block diagram illustrating the interaction between received laser light and input / output polarizing beam splitters and wave plates in one embodiment of the present disclosure. [Diagram 2]

[0039] FIG. 2 is a flow diagram illustrating a method for causing the beam splitter of FIG. 1A to allocate light between two outputs in one embodiment of the present disclosure. [Diagram 3]

[0040] FIG. 3 is a flow diagram illustrating a method for causing the free-space laser communications terminal of FIG. 1B to redirect substantially all received light into an alignment channel during an alignment phase, and then redistributing the received light between the alignment channel and the communications channel during a communications phase. Detailed Description

[0036]

[0041] Disclosed is an optical beam splitter and method of use that allows light to be received over free space from a remote source and distributed between two channels of a device according to a continuously adjustable splitting ratio.

[0037]

[0042] 1A, circularly polarized light 112 received over free space from a remote source passes through a polarization converter 132, which converts the received circularly polarized light 112 into linearly polarized light 156 with a well-defined polarization direction. In the embodiment of FIG. 1A, the polarization converter is a quarter-wave plate (QWP).

[0038]

[0043] The linearly polarized light 156 then enters the variable retarder 146, which can rotate the direction of the linear polarization of the light 154 by any adjustable rotation angle. The light 154 then enters the polarizing beam splitter (PBS) 120, which distributes the light between two outputs 128, 158 according to the relative angle between the linear polarization direction of the incident light 154 and the polarization direction of the PBS 120.

[0039]

[0044] In some embodiments, the variable retarder 146 is a half wave plate (HWP), and the linear polarization angle of the laser light 154 is adjusted by physically rotating the HWP 146. In other embodiments, the variable retarder 146 is an electronically controlled device, such as a liquid crystal variable retarder (LCVR).

[0040]

[0045] The two outputs 128, 158 of the laser beam splitter 152 in FIG. 1A are labeled in the figure as a communication channel 158 and an alignment channel 128. However, this labeling in FIG. 1A should not be considered limiting, and it should be understood that the scope of the present disclosure extends to all applications in which an optical beam splitter receives circularly polarized light 112 from a remote source and distributes the received light 112 between the two outputs 128, 158 according to an adjustable split ratio. It will be apparent to one skilled in the art that the disclosure of FIG. 1A can be extended to implementations requiring distribution of the received light between more than two channels by directing at least one of the output channels 128, 158 to an additional retarder and PBS.

[0041]

[0046] 1B, also disclosed herein is a free space laser communication terminal similar to the laser terminal of co-pending U.S. patent application Ser. No. 16 / 720,362 in which the disclosed optical beam splitter 152 is implemented as a receive beam splitter. The disclosed terminal is able to align itself with a remote node during an alignment phase and then maintain alignment with the remote node during a communication phase without implementing a separate beacon laser.

[0042]

[0047] In these embodiments, once alignment with a remote node is achieved and the communication phase begins, a portion of the communication laser signal itself is used for spatial tracking to maintain optical alignment between the terminals. In particular, the disclosed optical beam splitter 152, implemented as a receive beam splitter 152 in the disclosed laser communication terminal, distributes the received laser light 112 between the communication channel 158 and the alignment channel 128, directing substantially all of the received laser light 112 to the alignment channel 128 during the acquisition and alignment phase, while directing only a small percentage of the received laser light 112 to the alignment channel 128 during the communication phase.

[0043]

[0048] The disclosed functionality of the laser communication terminal of FIG. 1B requires that the received laser light must be circularly polarized. Typically, this requirement is met by requiring that all nodes in a free space laser communication network or subnet be configured to transmit and receive circularly polarized laser light. Thus, the laser communication terminal of FIG. 1B is also configured to transmit circularly polarized laser light 112. The laser communication terminal of FIG. 1B may be further configured to receive right circularly polarized laser light and transmit left circularly polarized laser light, or vice versa, so that any transmitted laser light that inadvertently enters the receiver input may be filtered out of the receiver channel. Further information regarding these features may be found in U.S. Patent Application No. 16 / 694,082, now U.S. Patent No. 10,637,572, and U.S. Patent Application No. 16 / 720,362, both of which are incorporated herein by reference in their entireties.

[0044]

[0049] 1B embodiment, the transmit and receive wavelengths are configurable on command, and laser communication terminal receive module 114 includes a filter assembly 116 that can transition, on command from controller 148, between directing a "red" laser light into communication channel 158 and directing a "blue" laser light into communication channel 158 while excluding other extraneous light. In an embodiment that includes only one transmit laser and is limited to receiving a communication laser beam at only one wavelength, filter assembly 116 is fixed at the receive wavelength.

[0045]

[0050] 1B embodiment distributes laser light 124 received from a remote laser communication node between a preamplifier 130 of the communication channel 158 and an "acquisition and tracking sensor" (ATS) of the alignment channel 128 used to establish and maintain optical alignment with the remote node. Light emerging from the communication channel preamplifier 130 is directed to a tunable optical filter (TOF) 118 and ultimately to a detector (not shown).

[0051] 1B embodiment, receive beam splitter 152 in receive module 114 includes a variable retarder 146 that is a half-wave plate 146, and the percentage of received light 124 that is directed to alignment channel 128 is adjusted by mechanically rotating half-wave plate 146, for example, using a stepper motor (not shown). In other embodiments, variable retarder 146 is an electronically controlled device, such as a liquid crystal variable retarder (LCVR), which can adjust the percentage of received laser light that is allocated to alignment channel 128 and communication channel 158 without mechanical manipulation.

[0052] 1B, the disclosed laser communication terminal comprises a transmit laser module 100 that includes two transmit lasers 102, 104, namely, a "first" transmit laser 102 and a "second" transmit laser 104, referred to herein as a "red" laser 102 and a "blue" laser 104. Similar embodiments include only one transmit laser.

[0046]

[0053] During the alignment phase, one or both of the two transmit lasers 102, 104 in FIG. 1B are used to generate a beacon laser output. Either of the transmit lasers 102, 104 can be selected to generate the beacon by a 2×1 switch 106, or the switch 106 can switch between the two transmit lasers 102, 104 to generate a “ping-pong” modulated output beacon. The light from the 2×1 switch 106 is then amplified by a high-power optical amplifier (HPOA) 108.

[0047]

[0054] The laser communication terminal of FIG. 1B is configured to transmit a circularly polarized laser beam in a free space path to a remote node and direct and differentiate the laser light received from the remote node according to its polarization state. The embodiment of FIG. 1B includes an input / output diplexer 150 that imposes a circular polarization on all of the laser light 112 transmitted by the laser communication terminal, including both the beacon transmitted during the acquisition phase and the communication beam transmitted during the communication phase. In the illustrated embodiment, the transmitting laser module 100 is configured to emit a linearly polarized beam. The input / output diplexer 150 includes an input / output polarizing beam splitter 122 followed by an input / output polarization converter (i.e., a quarter wave plate) 132 that converts the linearly polarized light from the beam splitter 122 to a circularly polarized output beam 112. In an embodiment, the quarter wave plate 132 can mechanically or electronically transition between imposing a right-hand circular polarization and a left-hand circular polarization on the output beam 112. The input / output quarter wave plate 132 also converts the received circularly polarized light into linearly polarized light 124 that is directed by the input / output polarizing beam splitter 122 to the receiver module 114 .

[0048]

[0055] For example, referring to FIG. 1C , if the transmitting laser module 100 is configured to emit vertically polarized “red” light 134, the input / output polarizing beam splitter 122 is horizontally polarized, and the vertically polarized red light 134 from the transmitting laser module 100 is reflected 136 by the input / output polarizing beam splitter 122 to the input / output quarter-wave plate 132, thereby being converted to an output beam 112 that is right-handed circularly polarized 138 (according to the illustrated example).

[0049]

[0056] Referring to FIG. 1D , in this example, communication 112 is received as left-handed circularly polarized light 140, which is converted by quarter-wave plate 132 to horizontally polarized blue light 142 and then transmitted through polarizing beam splitter 122 into receiver module 114 as received light 124.

[0050]

[0057] Referring to FIG. 2, an embodiment of the disclosed method applicable to the optical beam splitter of FIG. 1A includes receiving circularly polarized light via free space from a remote source 200, converting the circularly polarized light to linearly polarized light having a well-characterized polarization direction 202, using a retarder to rotate the polarization direction of the linearly polarized light, thereby determining a splitting ratio 204, and then using a polarizing beam splitter to allocate the laser light between two channels of the device 206.

[0051]

[0058] Referring to FIG. 3, an embodiment of a method applicable to the free space laser communications terminal of FIG. 1B includes adjusting 300 the receive beam splitter 152 to redirect all of the received light into the alignment channel 128 during the alignment phase, capturing 302 the circularly polarized light through free space from the remote node, aligning 304 the optics of the terminal with the light received from the remote node, and then, upon transition to the communications phase and after the remote node narrows its transmitted laser beam to increase its brightness 306, reconfiguring 308 the receive beam splitter 152 to direct most of the light received from the remote node into the communications channel 158 while continuing to direct a small percentage of the received light into the alignment channel, and processing 310 received messages while the alignment channel continues to maintain alignment between the terminal and the remote node.

[0052]

[0059] Thus, once the acquisition and alignment phase is complete and the communication phase begins, the communication signal itself is used to maintain optical alignment between the terminals. In an embodiment, substantially all, e.g., 98%, of the received laser light is directed by the laser beam splitter 152 to the alignment channel 128 during the alignment phase 300, while only a small percentage, e.g., 5-20%, of the received laser light 124 is directed to the alignment channel 128 during the communication phase 306, with the remainder being directed to the communication channel 158.

[0053]

[0060] The disclosed approach of requiring that laser communication beams in a free space laser communication network must be circularly polarized, converting the circularly polarized light received from remote nodes to linearly polarized light having a well-characterized polarization direction, and using a retarder 146 in combination with a polarizing beam splitter 120 to continuously vary the amount of light redirected to the ATS 128 provides significant performance improvements to embodiments of the disclosed laser communication terminal compared to conventional approaches. For example, the disclosed apparatus and method can direct five times more light to the alignment channel during the alignment phase compared to conventional approaches that use a fixed ratio beam splitter to redirect 20% of the light received to the alignment channel during all phases of operation.

[0054]

[0061] The foregoing description of embodiments of the present disclosure has been presented for purposes of illustration and description. Each and every page of this submission and all of its contents, no matter how characterized, identified, or numbered, regardless of form or arrangement within this application, are considered to be a substantial part of this application for all purposes. This specification is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Many modifications and variations are possible in light of this disclosure.

[0055]

[0062] Although the present application is presented in a limited number of forms, the scope of the present disclosure is not limited to only these forms, and various changes and modifications can be accepted without departing from the spirit of the present disclosure. The disclosure presented herein does not explicitly disclose all possible combinations of features within the scope of the present disclosure. The features disclosed herein for various embodiments can generally be interchanged and combined in any combination that is not self-contradictory without departing from the scope of the present disclosure. In particular, the limitations presented in the following dependent claims can be combined with their corresponding independent claims in any number and in any order without departing from the scope of the present disclosure, as long as the dependent claims are not logically inconsistent with each other. The invention as described in the claims of the original application is set forth below. [C1] 1. An optical beam splitter configured to receive light via free space from a remote light source, the light being circularly polarized, and to distribute the received light between two outputs according to a continuously adjustable splitting ratio, the optical beam splitter comprising: a polarization converter configured to convert the received circularly polarized light into linearly polarized light having a defined initial light polarization direction; a retarder configured to rotate the light polarization direction from the initial light polarization direction to a tailored polarization direction; a polarizing beam splitter having a splitter polarization direction, the polarizing beam splitter configured to distribute the received light between the first and second outputs according to a difference between the adjusted polarization direction and the splitter polarization direction; and An optical beam splitter comprising: [C2] The optical beam splitter of claim 1, wherein the received light is circularly polarized laser light. [C3] The optical beam splitter of claim 1, wherein the polarization converter comprises a quarter wave plate. [C4] 2. The optical beam splitter of claim 1, wherein the retarder comprises a half-wave plate, the retarder configured to change the adjusted polarization direction by mechanically rotating the half-wave plate. [C5] The optical beam splitter of claim 1, wherein the retarder comprises a liquid crystal variable retarder (LCVR), the retarder configured to electronically change the adjusted polarization direction. [C6] 2. The optical beam splitter of claim 1, wherein the remote light source is an astronomical light source that emits light having naturally occurring circular polarization. [C7] 2. The optical beam splitter of claim 1, wherein the optical beam splitter is mounted to a LIDAR device and the remote light source is a remote target reflecting circularly polarized light transmitted by the LIDAR device. [C8] The optical beam splitter of claim C1, wherein the remote light source is a remote node in a free space laser communications network. [C9] 1. A free space laser communications terminal configured to establish optical alignment with a remote node during an alignment phase and to receive laser communications transmitted by said remote node during a communication phase, said free space laser communications terminal comprising: an alignment channel configured to establish and maintain optical alignment of the laser communications terminal with the remote node using received laser light received from the remote node, the received laser light being circularly polarized; and a communication channel configured to derive communication information from the received laser light during the communication phase; 2. The receive beam splitter of claim 1, wherein the receive beam splitter is configured to adjustably allocate the received laser light between the alignment channel and the communication channel; A free space laser communication terminal comprising: [C10] The laser communication terminal of C9, wherein the polarization converter of the receiving beam splitter comprises a quarter wave plate. [C11] The laser communication terminal of C9, wherein the retarder of the receiving beam splitter comprises a half-wave plate, and the retarder is configured to adjust the adjusted polarization direction by mechanically rotating the half-wave plate. [C12] The laser communication terminal of C9, wherein the retarder of the receiving beam splitter comprises a liquid crystal variable retarder (LCVR), the LCVR configured to electronically change the adjusted polarization direction. [C13] The laser communications terminal of C9, further comprising a transmit channel configured to generate and transmit circularly polarized laser light to the remote node. [C14] 1. A method for aligning a free space laser communication terminal with a remote node and maintaining said alignment while receiving laser transmission messages from said remote node without requiring receipt of a separate alignment beacon from said remote node, said laser communication terminal comprising an alignment channel and a communication channel, said method comprising: C1 is mounted on the laser communication terminal as a receiving beam splitter; receiving laser light from the remote node, the received laser light being circularly polarized; causing the receive beam splitter to redirect substantially all of the received laser light into the alignment channel; aligning said laser communication terminal with said remote node according to said laser light received by said alignment channel; causing the receive beam splitter to reallocate the received laser light between the alignment channel and the communications channel such that at least 90% of the received laser light is allocated to the communications channel while no more than 10% of the received laser light is allocated to the alignment channel; maintaining said alignment between said laser communication terminal and said remote node according to no more than 10% of said received laser light allocated to said alignment channel while a laser transmit message encoded in said received laser light is processed by said communication channel; A method comprising: [C15] The method of claim 14, wherein no more than 5% of the received laser light is allocated to the alignment channel during the communication phase. [C16] The method of claim 14, wherein the retarder of the receive beam splitter comprises a half-wave plate, and causing the receive beam splitter to redistribute the received laser light includes physically rotating the half-wave plate. [C17] The method of claim 14, wherein the retarder of the receive beam splitter comprises a liquid crystal variable retarder (LCVR), and causing the receive beam splitter to redistribute the received laser light includes causing the LCVR to electronically rotate a linear polarization direction of the received laser light.

Claims

1. 1. An optical beam splitter configured to receive light via free space from a remote light source, the light being circularly polarized, and to distribute the received light between two outputs according to a continuously adjustable splitting ratio, the optical beam splitter comprising: a polarization converter configured to convert the received circularly polarized light into linearly polarized light having a defined initial light polarization direction; a retarder configured to rotate the light polarization direction from the initial light polarization direction to a tailored polarization direction; a polarizing beam splitter having a splitter polarization direction, the polarizing beam splitter configured to distribute the received light between first and second outputs according to a difference between the adjusted polarization direction and the splitter polarization direction; Equipped with 13. An optical beam splitter, wherein the optical beam splitter is mounted to a LIDAR device and the remote light source is a remote target reflecting circularly polarized light transmitted by the LIDAR device.

2. 10. The optical beam splitter of claim 1, wherein the received light is circularly polarized laser light.

3. The optical beam splitter of claim 1 , wherein the polarization converter comprises a quarter wave plate.

4. 10. The optical beam splitter of claim 1, wherein the retarder comprises a half-wave plate, the retarder configured to vary the adjusted polarization direction by mechanically rotating the half-wave plate.

5. 10. The optical beam splitter of claim 1, wherein the retarder comprises a liquid crystal variable retarder (LCVR), the retarder configured to electronically change the adjusted polarization direction.

6. 1. A free space laser communications terminal configured to establish optical alignment with a remote node during an alignment phase and to receive laser communications transmitted by said remote node during a communication phase, said free space laser communications terminal comprising: an alignment channel configured to establish and maintain optical alignment of the free space laser communications terminal with the remote node using received laser light received from the remote node, the received laser light being circularly polarized; and a communication channel configured to derive communication information from the received laser light during the communication phase; 2. The receive beam splitter of claim 1, wherein the receive beam splitter is configured to adjustably allocate the received laser light between the alignment channel and the communication channel; A free space laser communication terminal comprising:

7. The free space laser communications terminal of claim 6 , wherein the polarization converter of the receive beam splitter comprises a quarter wave plate.

8. 7. The free space laser communications terminal of claim 6, wherein the retarder of the receive beam splitter comprises a half wave plate, the retarder configured to adjust the adjusted polarization direction by mechanically rotating the half wave plate.

9. 7. The free space laser communications terminal of claim 6, wherein the retarder of the receive beam splitter comprises a liquid crystal variable retarder (LCVR), the LCVR configured to electronically change the adjusted polarization direction.

10. The free space laser communications terminal of claim 6 , further comprising a transmit channel configured to generate and transmit circularly polarized laser light to said remote node.

11. 1. A method for aligning a free space laser communications terminal with a remote node and maintaining said alignment while receiving laser transmission messages from said remote node without requiring receipt of a separate alignment beacon from said remote node, said free space laser communications terminal comprising an alignment channel and a communications channel, said method comprising: Implementing the optical beam splitter of claim 1 as a receiving beam splitter in the free space laser communication terminal; receiving laser light from the remote node, the received laser light being circularly polarized; causing the receive beam splitter to redirect substantially all of the received laser light into the alignment channel; aligning the free space laser communication terminal with the remote node according to the laser light received by the alignment channel; causing the receive beam splitter to reallocate the received laser light between the alignment channel and the communication channel during a communication phase such that at least 90% of the received laser light is allocated to the communication channel while no more than 10% of the received laser light is allocated to the alignment channel; maintaining the alignment between the free space laser communication terminal and the remote node according to no more than 10% of the received laser light allocated to the alignment channel while a laser transmission message encoded in the received laser light is processed by the communication channel; A method comprising:

12. The method of claim 11 , wherein no more than 5% of the received laser light is allocated to the alignment channel during the communication phase.

13. 12. The method of claim 11 , wherein the retarder of the receive beam splitter comprises a half-wave plate, and causing the receive beam splitter to redistribute the received laser light comprises physically rotating the half-wave plate.

14. 12. The method of claim 11 , wherein the retarder of the receive beam splitter comprises a liquid crystal variable retarder (LCVR), and causing the receive beam splitter to redistribute the received laser light comprises causing the LCVR to electronically rotate a linear polarization direction of the received laser light.

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