Patent Description:
Free-space optical communication uses a beam of light to transport data (either analog or digital) from a transmitter terminal to a receiving terminal. Often a bidirectional pair of links is desirable. Such links are particularly effective in space and airborne systems communicating between platforms or between the platform and ground. Compared to RF signals, optical signals have very short wavelengths meaning the required aperture is considerably smaller than similar speed RF terminals.

Many optical terminals require precision pointing to the opposite terminal, and very high precision in fabricating the optics and the mechanical systems that hold the terminal. To achieve optimal performance, diffraction-limited optical beams are desirable. Additionally, the optical system in these terminals must be able to efficiently couple photons from free-space into (and conversely out of) small area devices such as single mode fibers, high bandwidth optical detectors, and small output facet lasers.

Free-space optical systems can be used in a wide variety of applications. Bidirectional free-space optical links can offer power-efficient communication between spacecraft, between spacecraft and ground stations, between spacecraft and aircraft and also between underwater vehicle and/or terminals. In bidirectional links, the partner terminal's optical signal, either the main communication signal, or a parallel beacon signal, is often used establish pointing. For some situations, an additional "point ahead" correction must be applied to compensate for relative movement and flight time of the signals.

Unidirectional links can be used in the same scenarios describe above but are often aided by a return direction unmodulated or slowly modulated optical beacon signal. This return signal is used to establish and maintain alignment.

The core objective of any free-space optical communication system is to point the outgoing beam at the partner receiver. The partner's transmit beam, or beacon signal, informs the terminal precisely where the partner is located. In almost all free space optical communication applications, this pointing arrangement is highly dynamic and control systems must be used to maintain pointing over time. Pointing adjustments arise not only from relative movement of terminals, but also from local jitter imparted by platform vibration, thermal warpage, and many other factors. Many space-to-space applications require <NUM> microradian pointing precision with control loop bandwidths extending to several hundred hertz.

In principle, if you know your terminal's attitude relative to the stars to extreme precision, your terminal's location to within meters, and the position of your communication partner's terminal to within meters, one could blindly point in the right direction. Practically, this is all but impossible. The platform vibrates, creeps with temperature changes and sun angle, and shifts during launch and maneuvers. The resulting knowledge of the optical system pointing is quite poor, even if the star orientation system is exquisite. <CIT> discloses a system for active co-boresight measurement including a detector, a steering mirror and a controller. The detector detects a portion of a transmission beam emitted by a transceiver and a portion of a received beam that is received from a remote terminal. The controller measures an offset between the detected portions of the received beam and the transmission beam, and controls a position of the steering mirror to align the portion of the received beam with a defined position on the detector, which position is based on the offset. <CIT> discloses an optical atmospheric link system for transmitting an information signal modulated light beam between a transmitter and a remote receiver. The receiver includes a detector for generating a position error signal, and a second light source for generating a position error signal modulated light beam. The transmitter includes a detector for receiving and demodulating the position error signal modulated light beam to obtain the position error signal and to control the transmitter position such that the information signal modulated light beam id directed toward the receiver. <CIT> discloses an adaptive communications focal plane array to be implemented in cameras adapted to receive information in the form of optical beams.

Embodiments of single-aperture optical transceivers disclosed herein use the receive beam or beacon to provide a precise pointing reference. By keeping a common optical path to a fully differential tracker. The tracker precisely measures the difference in angle between the transmit and receive beams. In embodiments, the system maintains sub-microradian pointing with better than <NUM>-microradian precision in <NUM> of bandwidth.

In a first aspect a method for aligning a first optical transceiver according to claim <NUM> is provided, said method including steps of splitting, directing, recording, and actuating. The splitting step includes splitting a light beam into a) a reference beam that propagates along a common optical path within the first optical transceiver and b) a transmit beam that that propagates away from the first optical transceiver and toward a second optical transceiver. The directing step includes directing, with a beam director, a receive beam from the second optical transceiver onto the common optical path. The recording step includes recording, with a tracking focal-plane array (FPA) that intersects the common optical path, a reference-position of the reference beam and an initial-received-position of the receive beam on the tracking FPA. The actuating step includes actuating the beam director based upon the initial-received-position to achieve a subsequent position of the receive beam on the tracking FPA.

In a second aspect, a single-aperture optical transceiver according to claim <NUM> is provided, said optical transceiver including a tracking focal-plane array (FPA), a beam splitter, and a retroreflector. The beam splitter that includes a first port, a second port, a third port opposite the first port, a fourth port opposite the second port, and a beam-splitting interface. The beam splitter and the tracking FPA define a common optical path for a receive beam and a reference beam that are respectively received and generated by the single-aperture optical transceiver. The retroreflector that retroreflects the reference beam exiting the third port back to the third port. The beam splitter splits a light beam incident on the first port into a transmit beam and the reference beam. The tracking FPA receives both the reference beam and the receive beam via the common optical path.

<FIG> is a schematic of a single-aperture optical transceiver <NUM>, which includes a transceiver optical assembly <NUM> and a beam director <NUM>. Herein, single-aperture optical transceiver <NUM> is also referred to as transceiver <NUM>. Transceiver optical assembly <NUM> provides a collimated interface to beam director <NUM> for both a transmitted beam <NUM> and a receive beam <NUM>. Herein, TX and RX refer to transmitted beams and received beams respectively, examples of which include beams <NUM> and <NUM> respectively. Beam director <NUM> may include a gimbal. In embodiments, transceiver optical assembly <NUM> includes beam director <NUM>.

Transceiver optical assembly <NUM> may also provide angle estimates for both transmitted beam <NUM> and receive beam <NUM> by way of a tracking focal plane (FPA) <NUM>, hereinafter tracking FPA <NUM>. Tracking FPA <NUM> receives transmitted beam <NUM> and receive beam <NUM>, which are parts of transmitted beam <NUM> and receive beam <NUM> respectively. These estimates are provided to a pointing, acquisition and tracking algorithm which generates correction commands for the beam director.

In embodiments, optical transceiver <NUM> includes a controller <NUM>, which includes a processor <NUM> and a memory <NUM>. Memory <NUM> stores machine readable instructions, e.g., a pointing acquisition and tracking algorithm, that when executed by processor <NUM>, control processor <NUM> to change a pointing direction of beam director <NUM>.

A technical benefit of transceiver optical assembly <NUM>'s design is its robustness to implementation imperfections. These imperfections can arise from mechanical tolerances, thermal stresses, launch loads, etc. The imperfections can be broadly divided into two categories: those which compromise beam quality (e.g., imprecise focus leading to reduced transmit irradiance) and those which alter beam pointing (e.g., a tip/tilt bias affecting either the transmit or receiver beam). Aspects of transceiver optical assembly <NUM> address both categories of imperfections; however, the design's largest advantage over other approaches is its ability to maintain pointing performance.

In embodiments, a primary tenet of the design is to maximize length of a common optical path, relative to non-common (shared) optical paths, for the outgoing (TX) and incoming (RX) optical signals propagating from and to transceiver <NUM>. Imperfections in components along this common optical path affect both signals equally. <FIG> is a schematic of a transceiver optical assembly <NUM>, and is an example of transceiver optical assembly <NUM>. <FIG> is not comprehensive of components used to realize a real system (e.g., stops for stray light, multi-element collimation systems, polarization components, etc.) but is rather intended to convey operation at a high level.

In <FIG>, components marked with a diamond are common-optical-path components that carry both incoming and outgoing signals along the common optical path. In the example of <FIG>, the common optical path traverses beam splitter <NUM> and ends at tracking FPA <NUM>.

Transceiver optical assembly <NUM> includes a beam splitter <NUM> and tracking FPA <NUM>. In embodiments, transceiver optical assembly <NUM> also includes at least one of a lens <NUM>, a beam sampler <NUM>, a beam displacer <NUM>, a retroreflector <NUM>, a light source <NUM>, and a photodetector <NUM>. Transceiver optical assembly <NUM> may also include at least one of optical filters <NUM> - <NUM>. When optical assembly <NUM> includes each of the following components, the included component intersects the common optical path such that the common optical path traverses the component: filter <NUM>, lens <NUM>, beam sampler <NUM>, beam displacer <NUM>, and filter <NUM>.

In embodiments, at least part of the common optical path is within an optical fiber, or multiple optical fibers of transceiver optical assembly <NUM>. The multiple optical fibers may be coupled together. (coupled together. In such embodiments at least one of filter <NUM>, beam splitter <NUM>, lens <NUM>, beam sampler <NUM>, beam displacer <NUM>, and filter <NUM> is an inline optical fiber component optically coupled to the one or more optical fibers of the common optical path.

Light source <NUM> generates a light beam <NUM>, which beam splitter splits into a transmitted beam <NUM> and a reference beam <NUM>. Reference beam <NUM> is transmitted by beam splitter <NUM>, retroreflected by retroreflector <NUM> back to beam splitter <NUM>, which reflects reference beam <NUM> to tracking FPA <NUM>. Beams <NUM>-<NUM> are respective examples of beam <NUM>-<NUM>. Retroreflector <NUM> may be a corner cube retroreflector.

In embodiments, light beam <NUM> has a spectral range that is in one or more of the following regions of the electromagnetic spectrum: x-ray, ultraviolet, visible, near-IR, mid-IR, and far-IR.

Transceiver optical assembly <NUM> receives a receive beam <NUM> at port <NUM> of beam splitter <NUM>. Receive beam <NUM> is, for example, an instance transmitted beam <NUM> transmitted by an additional transceiver <NUM> toward optical assembly <NUM>. <FIG> denotes a tracked receive beam <NUM>, which is at least part of receive beam <NUM> incident on tracking FPA <NUM>. Additional transceiver <NUM> is an example of transceiver <NUM>. In embodiments, optical assembly <NUM> includes a beam director <NUM> that directs receive beam <NUM> toward port <NUM>. Beam director <NUM> is an example of beam director <NUM>.

Filter <NUM>, beam splitter <NUM>, lens <NUM>, beam sampler <NUM>, beam displacer <NUM>, filter <NUM>, and tracking FPA <NUM> are all common-optical-path components (marked with diamonds). Imperfections in these components largely affect both transmitted beam <NUM> and receive beam <NUM> in an identical manner. For example, if lens <NUM> for tracking FPA <NUM> is inadvertently tilted, both reference beam <NUM> and tracked receive-beam 282T are translated an equal amount on tracking FPA <NUM>. Non-common-optical-path components are used sparingly in the system and only in situations where tolerances can be well-controlled or where tolerances are loose. "Tolerance" is being used loosely here to refer to both tip/tilt imperfection as well as positioning imperfections (e.g., centering on beam or focal length) and other assembly or mounting errors.

The various filters in the design are either wavelength or polarization sensitive devices depending on the orthogonality/diversity scheme that is used (see <NUM>. The function of filter <NUM>, which may include a plurality of filters, serves to reduce stray light into the system but efficiently passes both transmitted beam <NUM> and receive beam <NUM>. Filter <NUM> prevents sampled TX light from reaching photodetector <NUM>, thereby reducing background noise. Filter <NUM>, another common-optical-path component, is optional in the system but can be helpful for balancing the irradiance of the tracking TX and RX beams on tracking FPA <NUM>. This can help relax the dynamic range requirements of tracking FPA <NUM>. Filter <NUM> may also be placed between beam splitter <NUM> and retroreflector <NUM>.

A second tenet of the design is to use a common tracking FPA <NUM> for sensing the angle of reference beam <NUM> and tracked receive-beam 282T via positions of a reference spot <NUM> and a RX spot <NUM> respectively on tracking FPA <NUM>. In embodiments, along the common optical path of transceiver <NUM>, a distance between lens <NUM> and tracking FPA <NUM> differs from a focal length of lens <NUM> by less than a depth of focus of lens <NUM>. As such, lens <NUM> forms reference spot <NUM> and RX spot <NUM> on tracking FPA <NUM>, thereby converting propagating angles of beams <NUM> and <NUM> to positions on tracking FPA <NUM>.

Beam displacer <NUM> applies a tightly controlled displacement to one or both of the signals so that the beams converge onto independent spots <NUM> and <NUM> on tracking FPA <NUM>. This may seem counterintuitive given the first design tenet of maximizing the common optical path; however, components that perform this function are monolithic and extremely stable over temperature and other factors (e.g., angle, wavelength, etc.). Beam displacer <NUM> is used so reference beam <NUM> and tracked receive-beam 282T may both be tracked by tracking FPA <NUM> without overlapping. Alternate approaches, e.g., when optical assembly <NUM> does not include beam displacer <NUM>, would be to use the point-ahead angle of the system, chromatic dispersion in a wavelength division multiplexed system, or timing in a time division multiplexed system.

The tracking approach of optical assembly <NUM> differs from many traditional free space optical systems which use quadrant or position sensitive diode (PSDs) detectors to measure the position/angle of an optical signal. Quad-cells and PSDs also have a relatively narrow field of view, which puts them at a disadvantage relative to tracking FPAs, such as tracking FPA <NUM>. Notably, they can only be used to track a single optical signal. With this limitation, many designs will include separate RX and TX tracking sensors which must then be aligned to one another. PSDs also provide a limited range/resolution ratio of approximately <NUM>: <NUM>, tracking FPA <NUM> and centroiding algorithm can perform at <NUM>,<NUM>: <NUM> or greater.

To transmit and receive at the same time, the two beams (mostly) travel down the same optical path and must not interfere with each other. The general quality we are describing is orthogonality of the transmit and receive beams. Orthogonality is necessary to separate the transmit and receive beams in the system and can be accomplished in a variety of ways. In general, transmit beam power is enormous compared to the receive beam power. As a consequence, the orthogonality scheme must provide high selectivity. Any cross-talk between the paths can easily swamp the receive detector with transmit beam.

Spatial diversity (e.g., separate apertures) is one way to accomplish this orthogonality, but it requires precision alignment of the two apertures. For a single aperture designs (which relieve alignment issues) one must use other orthogonal characteristics of the optical beams. These might include wavelength, polarization, angular orbital momentum, or time interleaving. Angular isolation is in principle possible, but given the long ranges and limited apertures, this is impractical for most systems.

This section describes of key optical components of embodiments of core optical assembly <NUM>. Their role in the system as well as suitable commercial options for each of them are given.

Transceiver optical assembly <NUM> accepts a collimated transmit beam from an optical source such as a laser. A bare laser diode could be used in conjunction with suitable collimation optics. Alternatively, a fiber coupled laser could be connected into the design by way of a fiber collimator. The exact approach is not critical so long as the beam is well collimated.

The transmit source may also incorporate additional filtering to improve the purity of the signal in the chosen diversity scheme. For a wavelength diversity system, this could be accomplished with wavelength selective filters. For a polarization diversity system, polarizers could be used to attenuate cross-polarized signal.

Beam sampler <NUM> may be a polarizing or dichroic beam splitter. Beam sampler <NUM> and retroreflector <NUM> combine the TX and RX signals into a common collimated beam space. A small fraction of the TX signal power is sampled and retroreflected onto the RX signal path, which is critical for the self-referenced optics approach. Both the sampled TX signal and the RX signal overlap in position and angular space when exiting beam splitter <NUM> but are orthogonal (e.g., in polarization or in wavelength).

In embodiments, beam splitter <NUM> is a polarizing beam splitter or dichroic beam splitter. When beam splitter <NUM> is a polarizing beam splitter, retroreflector <NUM> may be polarization preserving (e.g., metalized coating, or "cat's eye" retro). Retroreflector <NUM> is not on the common RX/TX optical path.

For some applications, optical assembly includes, along an optical path other than the common-optical path, an active steering mechanism <NUM> can change the relative alignment between reference beam <NUM> and receive beam <NUM>. Examples of steering mechanism <NUM> include fine steering mirrors, electro-optic nutators, a wedge prism, and Risley prisms. Steering mechanism <NUM> may be used to align receive beam <NUM> with photodetectors <NUM>, or to an optical fiber coupled to photodetector <NUM>. Self-calibration or post-factor calibration of system alignment is possible with this added mechanism.

<FIG> is a schematic of beam splitter <NUM>, retroreflector <NUM>, and light source <NUM>. Beam splitter <NUM> has ports <NUM>, <NUM>, <NUM>, and <NUM>, which may be respective planar surfaces of beam splitter <NUM> when beam splitter <NUM> is a beam splitter cube. Beam splitter <NUM> combines reference beam <NUM> and receive beam <NUM> into a common collimated space at port <NUM>. Receive beam <NUM> passes through beam splitter <NUM> (port <NUM> to <NUM>) undisturbed except for an insignificant amount of signal that is lost from port <NUM>. Light beam <NUM> enters at port <NUM> and small portion is sampled, as reference beam <NUM>, via port <NUM> where it is reversed in direction and joins receive beam <NUM> exiting at port <NUM>.

Beam sampler <NUM> divides incoming optical power between the high bandwidth communications detector and tracking FPA <NUM>. When optical assembly <NUM> includes beam sampler <NUM>, beam sampler <NUM> reflects part of receive beam <NUM> as a receive beam 282R, and transmits part of receive beam <NUM> as a tracked receive-beam 282T. In embodiments, tracked receive-beam 282T co-propagates with sampled Reference beam <NUM>. In embodiments, the optical intensity of beam 282R exceeds that of 282T, e.g., by a factor of between ten and ninety-nine.

Photodetector <NUM> converts the optically modulated receive beam <NUM> back in to an electrical signal for electronic demodulation. Photodetector <NUM> may include at least one of a photodiode, avalanche photodiode, silicon photomultiplier, or other similar device. Silicon photomultiplier detectors tend to have relatively large sensitive areas (<NUM> diameter or more, and hence a relatively large field of view) so mechanical positioning is not a challenge relative to lens <NUM> (<FIG>). In embodiments, photodetector <NUM> is located such that it receives uniform illumination (e.g., out of focus or in a deliberately altered beam such as collimated or simply aberrated).

In the simplified optics diagram (<FIG>) some of sampled reference beam <NUM> is also diverted toward photodetector <NUM> as a noise beam <NUM>. With the exception of some built-in-self-test use cases, noise beam <NUM> is unwanted as it contributes background noise in the detection of RX beam 282R. For this reason, additional filtering may be implemented here to attenuate noise beam <NUM>, e.g., by filter <NUM>. The approach would depend on the diversity scheme (e.g., polarization, wavelength, etc.) used in the system. For example, filter <NUM> may be either a polarizer or a spectral filter.

Beam displacer <NUM> imparts a fixed spatial displacement to one or both of the signals so that they arrive at different positions on tracking FPA <NUM>. This ensures that the signals can be sensed independently. In embodiments, beam displacer <NUM> is one of a ytterbium orthovanadate beam displacers and Wollaston prism. These components are extremely stable over temperature and have very modest alignment requirements.

Without this component, both Reference beam <NUM> and RX beam <NUM> would overlap and fall on the same pixel(s) of tracking FPA <NUM>. This situation would complicate simultaneous differential tracking of both signals, hence the need for beam displacer <NUM>.

Tracking FPA <NUM> is used to measure the far-field position (determined by the angle of arrival in the far field) of both Reference beam <NUM> and RX beam <NUM>. In embodiments, the position of each signal is estimated with a centroiding algorithm (or similar), which can provide sub-pixel accuracy. This accuracy can achieve sub-microradian track accuracy in angular space of angular position estimates stored in memory <NUM>.

The angular position estimates are used to correct system pointing with an external beam director, which forms a pointing control loop, which may be executed by controller <NUM>. The pointing control loop executed controller <NUM> uses platform data <NUM> received from beam director <NUM>. Data <NUM> includes at least one of platform attitude, platform position, and time.

Note as the beam director moves, RX spot <NUM> moves on the focal plane, but TX spot <NUM> remains fixed. The bandwidth of this control loop is limited by the frame rate of tracking FPA <NUM>. For some applications, control bandwidths in excess of <NUM> may be necessary which requires a tracking focal plane capable of frame rates exceeding a few hundred frames per second. Some focal planes can provide these frame rates using "region of interest" readout features.

Dynamic range is another important characteristic of tracking FPA <NUM> since it is being used to simultaneously image sampled reference beam <NUM> as well as tracked receive-beam 282T. The degree to which these signals must be matched in power/irradiance is dictated by the dynamic range of tracking FPA <NUM>. Focal plane dynamic range arises from factors such as well-depth, ADC bit depth, and exposure control. An external polarization or wavelength selective filter may also be used to help "balance" the power of the two signals.

The exact choice of a tracking focal plane solution also depends on the wavelength of operation. The approach we describe can be generalized to any wavelength. For example, silicon detectors can be used for x-rays through the near infrared, InGaAs and quantum dot detectors can be used in the mid-infrared and InSb or HgCdTe can be used in the mid- and long-wave infrared.

Operation of the tracking focal plane may also employ other common calibration techniques such as dark frame subtraction and "flat fielding" to mitigate unwanted device characteristics.

It is also possible to use two tracking focal planes instead of one. Since a separate focal plane is used for the transmit and receive beams, the dynamic range requirements can be relaxed and additional flexibility in frame rate selection is possible. A device similar to beam splitter <NUM> (Section <NUM>. <NUM>) is needed to split the two signals, but beam displacer <NUM> is no longer necessary. Additional alignment requirements stem from the non-common beam paths leading to each of the focal planes.

In this section the acquisition and tracking process is described through example scenarios that are illustrated by the image "seen" by tracking FPA <NUM>.

<FIG> illustrates a result of calibration and/or alignment of optical assembly <NUM>, which places both TX spot <NUM> and RX spot <NUM> (when aligned with photodetector <NUM>) near the center of a field of view of tracking FPA <NUM>. This calibration is considered a "coarse" adjustment as the field of view is relatively wide (order of <NUM>×<NUM> degree). Note that the separation between TX spot <NUM> and RX spot <NUM> is purely a result of the effects of beam displacer <NUM>, and is very stable over mechanical/thermal loading.

In addition to coarse alignment, focus and power level adjustments may also be performed to establish the desired signal irradiance and spot size on the focal plane. A simulated receive beam (or externally reflected transmit beam) will be used to identify the position of RX spot <NUM> on FPA array <NUM>.

Full frame readout of the focal plane, likely at a relatively low frame rate, may be used to support the above calibration.

During launch and also between operating sessions, transceiver <NUM> will invariably be subjected to various mechanical and thermal stresses which will perturb the factory alignment. Due to design of optical assembly <NUM>, the result is equal movement of both TX spot <NUM> and RX spot <NUM> on the focal plane, at FPA <NUM>. <FIG> shows this perturbation as double-headed arrows <NUM>. Crucially, their relative position remains fixed due to the properties of beam displacer <NUM>.

Before each operating session, full frame images are collected with Reference beam <NUM> enabled in order to measure any drift (i.e., system alignment changes that arise due to mechanical and thermal loading). Note that receive beam 282T is not needed to perform this measurement; simply measuring the position of TX spot <NUM> centroid is sufficient.

In embodiments, this perturbation measurement is performed continuously during operation of transceiver <NUM> to compensate for thermal transients that arise from terminal duty cycling or solar loading.

In embodiments, the pointing, acquisition and tracking algorithm of controller <NUM> receives information from the host to assist with coarse acquisition. These priors can include the position and velocity of both terminals (transmit and receive), the attitude of the local platform, as well as the time of day. From these priors, the terminal can compute the relative look angle of the partner terminal and command beam director <NUM> accordingly. <FIG> illustrates this process.

Ideally, these priors are accurate enough to place the partner terminal within the field of view (FOV) of tracking FPA <NUM> (e.g., order of <NUM> degree) with high probability. When the solution is not accurate enough, then other well-known acquisition methodologies may be employed such as spiral scan or raster scan patterns. These alternate strategies require additional acquisition time, but enable the use of focal planes with smaller FOV.

One additional complexity is that of probability of detection, which depends on the quality of the priors, the path length, and the beam width. The communication beam from the partner spacecraft is typically very narrow (microradians) so the probability of detection is very low when the acquisition process begins. The simplest solution is for each terminal to employ a secondary acquisition beacon or intentional, configurable, defocus of the TX beam. This beacon laser has a much wider beam width than the communication beam and is only enabled during the acquisition process.

In a subsequent stage of acquisition, which may be a final stage, pointing is further improved to bring the RX signal within the field of view of photodetector <NUM>, as shown in <FIG>. Fine tracking is established when receive beam <NUM> is brought into the FOV of photodetector <NUM>. The tracking focal plane also shifts to a region of interest (ROI) readout scheme, which increases frame rate and control loop bandwidth. Although the <FIG> shows two ROIs, the spots may be close enough to each other for a single ROI to be sufficient.

Note that the field of view of photodetector <NUM> is generally much smaller than that of the tracking focal plane. Additionally, the tracking bandwidth is also increased during the stage by switching from full frame readout of tracking FPA <NUM> to faster region of interest (ROI) based readout.

Once fine pointing is established, continuous tracking is needed to compensate for platform jitter and other system stresses (e.g., thermal transients). In all cases, the goal of the tracking algorithm is to maintain the relative position of the TX and RX centroids.

In embodiments, transmitted beam <NUM> is orthogonal to receive beam <NUM> in at least one of polarization and wavelength. As an example, assume transmit beam <NUM> is in state A and receive beam <NUM> is in state B. That means the partner must transmit in state B and receive from state A. For A and B, you may substitute orthogonal polarizations (left circular fright circular, horizontal/vertical, for example), red and blue wavelength, odd or even seconds (time multiplexing), any other orthogonality approach. Conversely, when two terminals are constructed identically, they will both transmit A, which they will be unable to receive at their detector. Therefore, it is necessary for the terminal to be able to switch between the two types of systems.

This is referred to as the "handedness problem", as there must be two complementary types of terminals or terminal configurations. Only two are needed as all links are simple point-to-point, with two ends. If it is desirable for any terminal to talk to any other terminal, then at least one of them needs a capability to switch between handedness configurations.

In polarization, this is simply accomplished. The polarization to/from the terminal will be left- or right-circular polarization. Typically, circular polarization is used to avoid sensitivity to platform orientation. A quarter wave plate oriented with its fast axis <NUM> degrees between horizontal and vertical will switch vertical to right circular, and horizontal to left circular. The same waveplate can take vertically polarized TX light and make it right circular, and left circular from the other terminal entering the spacecraft and make it horizontal. With a few exceptions, none relevant here, the direction of light can be "time reversed" and behavior remains the same. To switch the conversion of vertical from right circular to left circular, simply rotate the fast axis of the quarter-wave plate from +<NUM> degrees to -<NUM> degrees. This can also be achieved without moving the quarter waveplate by moving a half waveplate into the path. A half-wave plate will exchange left and right circular polarizations regardless of the orientation of its fast axis. Finally, note that the quarter waveplate may be made from an electro-optic polymer allowing one to electronically control the fast axis direction without moving parts.

The techniques of wavelength-division multiplexing are well developed in the telecom industry. These techniques will allow one to switch between two sets of wavelengths to resolve the handedness problem. Gratings, etalons, even fiber switches can be employed to change states.

Time interleaving is perhaps the simplest to implement. At any one time, the link is half-duplex so there is no contention or interference between transmit and receive paths. The only requirement is that terminals must stay synchronized. Two approaches are to use an absolute time reference (e.g., GPS) or simply coordinate using the optical signal. There are subtleties to consider as the round-trip delay ranges from several milliseconds to tenths of a second for geosynchronous satellites. There are data latency penalties associated with time interleaving that depend on the turnaround interval and path propagation delay.

Using polarization-based orthogonality simplifies the common-path approach and precision tracking while allowing simultaneous transmit and receive. This also leaves open the addition of wavelength division multiplexing to increase data rate. receive beam.

This section contains alternate implementation options and additional design features.

Data rates in optical communication systems can be increased through the use of larger apertures, higher power transmitters, more sensitive receivers or shorter path lengths. A practical limit on scalability arises from the performance of the electronics and electro-optics in the system. Modulators and detectors have finite bandwidth, and digital electronics have upper limits on clock rates, etc. For this reason, optical systems with very high data rates often utilize wavelength division multiplexing (WDM). In essence, multiple orthogonal channels are operated in parallel with each channel operating at a rate that is within the electronics' capabilities.

Transceiver optical assemblies disclosed herein may be adapted to handle WDM signaling. Some notable embodiments include:.

In embodiments, photodetector <NUM> is a fiber-coupled receiver, as shown in <FIG> is a schematic of a single-aperture optical transceiver <NUM>, which includes an optical assembly <NUM>. Transceiver <NUM> and optical assembly <NUM> are respective examples of transceiver <NUM> and optical assembly <NUM>, <FIG>. Optical assembly <NUM> includes a detector <NUM> and an optical fiber <NUM> coupled thereto for augmenting pointing, acquisition, and tracking. Detector <NUM> is an example of photodetector <NUM>, <FIG>. RX beam <NUM> is coupled from free-space to an end facet <NUM> of optical fiber <NUM>.

A benefit of detector <NUM> being coupled to optical fiber <NUM> is that either or both detector <NUM> and <NUM> may be components developed for the fiber telecommunication industry. Optical fiber <NUM> may include an optical preamplifier (e.g., an erbium-doped fiber amplifier) and/or a filter for accomplishing WDM.

In embodiments, optical fiber <NUM> is a single mode fiber. For example, optical fiber <NUM> may be a single mode fiber at <NUM>, and have a core diameter of roughly ten microns, which leads to a much narrower field of view than a photodiode (e.g., <NUM> bandwidth APDs have a sensitive area ~<NUM> microns in diameter).

In embodiments, a power peaking algorithm is used for tracking (co-aligning) receive beam 282T and optical fiber <NUM> (end facet <NUM>) or photodetector <NUM>. The coarse position of optical fiber <NUM> is well established by the common optical path design of optical assembly <NUM>. This knowledge reduces the space over which transceiver <NUM> needs to search and the power received by the fiber detector can be used as a metric to assess tracking state.

In embodiments, the pointing, acquisition and tracking (PAT) algorithm is executed by controller <NUM> and uses the receive power of beam <NUM> as an input parameter,.

The PAT algorithm uses its priors to position RX beam <NUM> in the approximate location of optical fiber <NUM>, then a raster (or similar) scan can be used in conjunction with the power measurement to identify the precise position. A perturb-and-observe approach may be used to maintain tracking.

Another approach for tracking the position of optical fiber <NUM> (e.g., of end facet <NUM>) is to add a counter-propagating signal to the fiber that can be detected by the tracking focal plane, as shown in <FIG> is a schematic of a transceiver optical assembly <NUM>, which is an embodiment of transceiver optical assembly <NUM> with the addition of optical fiber <NUM> for launching a metrology signal <NUM>. <FIG> depicts end facet <NUM> as an ellipse to illustrate that optical fiber <NUM> both receives signal 282R and launches metrology signal <NUM> from end facet <NUM>.

In embodiments, transceiver optical assembly <NUM> includes a light source <NUM> coupled to optical fiber <NUM> for generating metrology signal <NUM>. Light source <NUM> may be light source <NUM>, such that metrology signal <NUM> is part of light beam <NUM> redirected to optical fiber <NUM>.

Metrology signal <NUM> propagates along common optical path to allow tracking FPA <NUM> to measure the position of reference beam <NUM>, tracked receive-beam 282T and end facet <NUM> all in a common collimated beam space. In embodiments, metrology signal <NUM> differs from each of Reference beam <NUM> and tracked receive-beam 282T in some property such that it can be distinguished upon detection. Examples of this property include wavelength diversity and temporal amplitude variation.

An alternate optical configuration for this approach is illustrated in <FIG> is a schematic of a transceiver optical assembly <NUM>, which is an embodiment of transceiver optical assembly <NUM> with the addition of optical fiber <NUM> for launching metrology signal <NUM>, and a retroreflector <NUM> for directing metrology signal <NUM> to tracking FPA <NUM> as a metrology signal <NUM>. While transceiver optical assembly <NUM> has one fewer optical component than transceiver optical assembly <NUM>, a benefit of this additional component, retroreflector <NUM>, is less attenuation of metrology signal <NUM> as it propagates from optical fiber <NUM> to tracking FPA <NUM>.

In embodiments, photodetector <NUM> is a photon counting detector. Some of these detectors are fiber-coupled, while others are directly coupled. Examples include superconducting nanowire arrays, Geiger mode APDs, silicon photomultipliers (SiPM), and conventional photomultiplier tubes. Some of these devices demand uniform illumination for optimal operation, and the coupling scheme may need to accommodate this requirement.

<FIG> is a schematic of a transceiver optical assembly <NUM> with a reference beam <NUM> and a common optical path <NUM>. Transceiver optical assembly <NUM> is an example of transceiver optical assembly <NUM>, and includes a beam splitter <NUM> and tracking FPA <NUM> at respective ends of a common optical path <NUM>. Light beam <NUM> is split by beam splitter <NUM> to form reference beam <NUM>, which is directed onto common optical path <NUM>, and transmit beam <NUM>. In embodiments, transceiver optical assembly <NUM> includes at least one of filter <NUM> and beam director <NUM>, in which case common optical path <NUM> extends to intersect each of these elements. In embodiments, at least part of common optical path <NUM> is within an optical fiber.

Transceiver optical assembly <NUM> receives a receive beam <NUM> that travels along common optical path <NUM>. In an embodiment, light beam <NUM> is generated by a laser <NUM> that is included in transceiver optical assembly <NUM>. In embodiments, optical assembly <NUM> includes retroreflector <NUM>, which may be a corner-cube mirror that redirects reflected light while maintaining collinearity. As a result, reference beam <NUM> and transmit beam <NUM> are collinear. In an embodiment, retroreflector <NUM> is a flat mirror. In an embodiment, transceiver optical assembly <NUM> includes a beam director <NUM> that directs one or both of transmit beam <NUM> and receive beam <NUM>. Beam director <NUM> is positioned such that transmit beam <NUM> strikes beam director <NUM> after interaction with beam splitter <NUM>. Beam director <NUM> is used to direct transmit beam <NUM> toward a target (not shown), which may be another single-aperture optical transceiver or another member of a communication network (not shown). In embodiments, beam director <NUM> is, or includes, a mirror.

In an embodiment, transceiver optical assembly <NUM> includes a beam displacer <NUM> configured to separate reference beam <NUM> and receive beam <NUM> such that reference beam <NUM> and receive beam <NUM> are distinctly detected by tracking FPA <NUM>. Beam displacer <NUM> may use a range of physical parameters to de-multiplex receive beam <NUM> and reference beam <NUM>. Examples of physical parameters used by beam displacer <NUM> include linear polarization of light, circular polarization of light, orbital angular momentum of light, wavelength of light, or time interleaving.

In an embodiment, beam displacer <NUM> is formed of a birefringent material. In such embodiments, when reference beam <NUM> is linearly polarized in a first direction and receive beam <NUM> is linearly polarized in a second direction that is substantially perpendicular to the first direction, beam displacer <NUM> spatially separates reference beam <NUM> and receive beam <NUM> (such that both reference beam <NUM> and receive beam <NUM> are detected by tracking FPA <NUM> at different physical locations on a focal plane of tracking FPA <NUM>).

Without departing from the scope hereof, beam displacer <NUM> may use other methods to separate receive beam <NUM> from reference beam <NUM>, such as time or wavelength of light without departing from the scope herein.

In an embodiment, transceiver optical assembly <NUM> of <FIG> includes a beam sampler <NUM> to direct a portion of receive beam <NUM> toward photodetector <NUM>.

In an embodiment, optical assembly <NUM> includes a polarizing filter <NUM> that attenuates components of light beam <NUM> that are polarized in a direction other than the second direction. In an embodiment, transceiver optical assembly <NUM> includes filter <NUM> to prevent noise beam <NUM> from reaching photodetector <NUM>.

In an embodiment, transceiver optical assembly <NUM> includes a lens <NUM> configured along common optical path <NUM> that conditions reference beam <NUM> and receive beam <NUM>. In an embodiment, lens <NUM> brings both reference beam <NUM> and receive beam <NUM> to a focus at a focal plane of tracking FPA <NUM>. By bringing reference beam <NUM> and receive beam <NUM> to a focus, lens <NUM> allows for more sensitive detection of reference beam <NUM> and receive beam <NUM> by tracking FPA <NUM>. Lens <NUM> affords information about the incoming angle of reference beam <NUM> and receive beam <NUM>, as well.

<FIG> is a schematic of a transceiver optical assembly <NUM>, which is an example of transceiver optical assembly <NUM> that includes a quarter-wave plate <NUM> and a polarizing beam splitter <NUM>, which is an example of beam splitter <NUM>. In this embodiment, beam splitter <NUM> is a polarizing beam splitter that splits light beam <NUM> into transmit beam <NUM> and a reference beam <NUM> based on the linear polarization of the two. Beams <NUM> and <NUM> are respective examples of beams <NUM> and <NUM>.

Quarter-wave plate <NUM> converts (a) transmit beam <NUM> to a circularly-polarized output beam 1281C and (b) a circularly-polarized input beam 1282C to a linearly polarized receive beam <NUM>, which is directed by polarizing beam splitter <NUM> onto common optical path <NUM> and subsequently to a tracking FPA <NUM>. Receive beam <NUM> is an example of receive beam <NUM>.

In this configuration, when light beam <NUM> is physically overlapped with receive beam <NUM> when incident on polarizing beam splitter <NUM>, receive beam <NUM> and reference beam <NUM> will remain spatially overlapped (both in position and angle) while traversing common optical path <NUM>. This overlapped condition is still satisfied when polarizing beam splitter <NUM> becomes misaligned.

<FIG> is a schematic of a transceiver optical assembly <NUM>. Optical assembly is an example of optical assembly <NUM>, and includes a re-directing mirror <NUM> along a common optical path <NUM>, which is an example of common optical path <NUM>. Mirror <NUM> allows for a more compact physical size of transceiver optical assembly <NUM>. Transceiver optical assembly <NUM> includes beam splitter <NUM> and tracking FPA <NUM>. In an embodiment, transceiver optical assembly <NUM> includes beam sampler <NUM> and photodetector <NUM>. The orientation angles of re-directing mirror <NUM> and beam sampler <NUM> may be chosen to alter the aspect ratio of transceiver optical assembly <NUM> to be more favorable for certain manufacturing processes. In an embodiment, more than one re-directing mirrors are used to alter overall size and shape of transceiver optical assembly <NUM>. It should be understood that the physical orientation of the single-aperture optical transceivers <NUM>, <NUM>, and <NUM> may vary with respect to the planar angles or torsional angles. Rotations about any of the light axes do not depart from the scope herein.

<FIG> is a flowchart illustrating a method <NUM> for aligning an optical transceiver. Method <NUM> may be implemented by part or all of any of transceiver optical assemblies <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. Method <NUM> includes steps <NUM>, <NUM>, <NUM>, and <NUM>.

Step <NUM> including splitting a light beam into a) a reference beam that propagates along a common optical path within the first optical transceiver and b) a transmit beam that that propagates away from the first optical transceiver and toward a second optical transceiver. In example of step <NUM>, beam splitter <NUM> splits light beam <NUM> into reference beam <NUM> and transmit beam <NUM>.

Step <NUM> includes directing, with a beam director, a receive beam from the second optical transceiver onto the common optical path. In an example of step <NUM>, beam director <NUM> directs receive beam <NUM> onto common optical path <NUM>, <FIG>.

Step <NUM> includes recording, with a tracking FPA that intersects the common optical path, a reference-position of the reference beam and an initial-received-position of the receive beam on the tracking FPA. In an example of step <NUM>, tracking FPA <NUM> records a position of TX spot <NUM> and an initial position of RX spot <NUM>.

Step <NUM> includes actuating the beam director based upon the initial-received-position to achieve a subsequent position of the receive beam on the tracking FPA. In embodiments, the tracking FPA includes a sensor array having a sensor-array center, and a distance between the initial-received-position and the sensor-array center exceeding a distance between the subsequent position and the sensor-array center. In an example of step <NUM>, controller <NUM> actuates beam director <NUM> based on the initial position of RX spot <NUM> determined in step <NUM>.

In embodiments, the desired position may incorporate the position of the reference beam and thereby the desired position is, in effect, a desired relative position. In embodiments where the receive beam originates from a moving object, it may be necessary to include a so-called point-ahead correction to the desired position of the signals, which results from the relative velocity of the single-aperture optical transceiver and the moving object.

In embodiments, method <NUM> also includes at least one of steps <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. In step <NUM>, the light beam is generated within the first optical transceiver by a laser. In an example of step <NUM>, light source <NUM> generates light beam <NUM>.

Step <NUM> includes converting the transmit beam to a circularly polarized output beam and converting the receive beam to a linearly polarized receive beam. In an example of step <NUM>, quarter-wave plate <NUM> converts transmit beam <NUM> to beam 1281C, and converts receive beam 1282C to receive beam <NUM>, as shown in <FIG>.

Step <NUM> includes spatially separating the reference beam and the receive beam according to an attribute value of the reference beam that differs from an attribute value of the receive beam. The attribute value is a value of an attribute, wherein the attribute is one of polarization, wavelength, temporal amplitude variation, and orbital angular momentum.

Step <NUM> includes directing a portion of the receive beam out of the common optical path and toward a detector. In an example of step <NUM>, beam sampler <NUM> directs receive beam 282R to photodetector <NUM>.

In embodiments, step <NUM> includes directing a portion of the reference beam toward the photodetector. Such embodiments may include step <NUM>, which includes at least partially attenuating the portion of the reference beam. In an example of step <NUM>, filter <NUM> at least partially attenuates noise beam <NUM>, hence preventing part or all of noise beam <NUM> from reaching detector <NUM>.

Claim 1:
A method for aligning a first optical transceiver, comprising:
splitting a light beam (<NUM>) into a) a reference beam (<NUM>) that propagates along a common optical path within the first optical transceiver and b) a transmit beam (<NUM>) that propagates away from the first optical transceiver and toward a second optical transceiver;
changing the polarization state of the transmit beam (<NUM>) from linear to circular with a quarter wave plate (<NUM>);
changing the polarization state of a receive beam (<NUM>, <NUM>) from circular to linear with said quarter wave plate (<NUM>),
directing, with a beam director (<NUM>, <NUM>), the receive beam (<NUM>, <NUM>) from the second optical transceiver onto the common optical path to yield a tracked-receive beam (282T) that propagates along the common optical path, wherein said changing the polarization states is performed between said splitting and said directing.
recording, with a tracking focal-plane array (FPA) that intersects the common optical path, a reference-position of the reference beam and an initial-received-position of the tracked-receive beam on the tracking FPA; and
actuating the beam director based upon the initial-received-position to achieve a subsequent position of the tracked-receive beam on the tracking FPA.