Improved free-space optical transceiver

Abstract

A free-space optical transceiver comprises a wide field-of-view lens for receiving an incoming beam, and a sensing system comprising a photodetector for reading data from the incoming beam, a coarse tracking sensor configured to indicate a coarse degree of alignment with the incoming beam, and a fine tracking sensor configured to indicate a fine degree of alignment with the incoming beam. The coarse tracking sensor comprises a central aperture behind which the photodetector and the fine tracking sensor are situated such that the incoming beam is directed to the photodetector and the fine tracking sensor via the central aperture. The optical transceiver further comprises a coarse beam-steering device intermediate the wide field-of-view lens and the sensing system, the coarse beam-steering device being for coarsely aligning the incoming beam with the photodetector and a centre of the coarse tracking sensor based on the indicated coarse degree of alignment.

Description

[0001] IMPROVED FREE-SPACE OPTICAL TRANSCEIVER

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an improved free-space optical transceiver. More particularly, but not exclusively, it relates to a crosslink, full duplex free-space optical transceiver for communication between satellites.

[0004] BACKGROUND OF THE INVENTION

[0005] Optical communication refers to the use of light for communication at a distance, where "light" refers to electromagnetic radiation at wavelengths in the infrared range or smaller (i.e. 1 mm or smaller, equivalently 300 GHz or higher). It is thus distinguished from radio communication, which uses higher wavelengths / lower frequencies.

[0006] Because of the longer wavelengths of radio waves, they are suitable for broad beam transmission and can diffract more easily around obstacles. In contrast, the short wavelengths / high frequency of optical communication allows for much higher bandwidth and hence much higher data rates than radio communication, but a narrow beam transmission is needed. High-performance optical communications links thus typically use lasers as a transmission source.

[0007] Optical communication can utilise optical fibers to carry the light, allowing for routing of fiber optic cables across long distances without the need for alignment between the transmitter and receiver. Alternatively, the transmitter can align with the receiver and transmit directly through the open space between them - this is known as "free-space" optical communication. Free-space optical links are primarily applied to communication between satellites in orbit (crosslink communication, for which the transmission medium is the vacuum of space), and between satellites and the ground (downlink communication, for which the light is transmitted through both atmosphere and vacuum). However, purely terrestrial applications such as communication with and between moving vehicles or drones can also utilise free-space optical communications, in which case the medium of transmission would generally only be air.

[0008] Crosslink free-space optical links are typically established between like transceivers as a bi-directional link allowing for simultaneous communication in both directions (full duplex). However, other configurations include one-way (simplex) links and links allowing communication in only one direction at a time (half duplex). The transmitter and receiver may also be separated into different units. Although high data rates make free-space optical links an appealing option for satellite communication, achieving the alignment needed to establish a link presents technical challenges. Typically, mechanical gimbals are used for coarse alignment while optical components may be used for fine alignment, and control may be achieved via tracking sensors that detect when an incoming beam is off-centre. European patent 607906 illustrates one such example of an optical transceiver configured to provide intersatellite communication.

[0009] However, establishing an optical link with such a transceiver may require the attitude control system of the spacecraft to be operated to supplement the coarse pointing capabilities of the gimbal - this means that the optical transceiver cannot operate as an independent module. Additionally, mechanical gimbals are generally bulky, consume high power, are relatively slow in changing orientation, and are prone to failure in the space environment.

[0010] Furthermore, due to the increase in commercial orbital launch availability it has become more feasible in recent times to launch and maintain 'constellations' of numerous small satellites. Such constellations have the potential to be used as ad hoc communications networks, in which each satellite acts as a node to dynamically transmit data through the network. Although free-space optical links could support very high data rates between nodes, the overall data rate through the network would be bottlenecked by acquisition and alignment time associated with establishing each link in the series.

[0011] In addition to angular rate constraints imposed by the gimbal (and potentially also the spacecraft), one obstacle to achieving faster acquisition / alignment time is that connecting to the next node requires information about where to point, generally calculated by propagation from known orbital data. Such orbital data is generally based on intermittent measurements by ground-based radar stations, and is commonly stored in the simple data format of two-line element sets (TLEs). Measurement error and orbital perturbations over time lead to uncertainty in all such orbital data and propagations therefrom - thus, each transceiver will generally need to follow a search pattern before it can acquire the next node. Given the potentially large search space and the degree of alignment needed to establish a link, the worst-case search performance may take considerable time.

[0012] It is therefore desirable to develop new optical communications systems which can achieve faster acquisition and alignment times, such that the high bandwidth and data rates offered by the optical spectrum can be more effectively exploited - in particular in the context of an ad hoc network of satellites. It is an object of the present invention to provide a free-space optical transceiver which overcomes or at least partially ameliorates some of the abovementioned disadvantages or which at least provides the public with a useful choice.

[0013] BRIEF DESCRIPTION OF THE INVENTION

[0014] According to a first aspect, the invention broadly comprises a free-space optical transceiver comprising: a wide field-of-view lens for receiving an incoming beam; a sensing system comprising: a photodetector for reading data from the incoming beam; a coarse tracking sensor configured to indicate a coarse degree of alignment with the incoming beam; and a fine tracking sensor configured to indicate a fine degree of alignment with the incoming beam, wherein the coarse tracking sensor comprises a central aperture behind which the photodetector and the fine tracking sensor are situated such that the incoming beam is directed to the photodetector and the fine tracking sensor via the central aperture; a coarse beam-steering device intermediate the wide field-of-view lens and the sensing system, the coarse beam-steering device being for coarsely aligning the incoming beam with the photodetector and a centre of the coarse tracking sensor based on the indicated coarse degree of alignment; and a fine beam-steering device intermediate the wide field-of-view lens and the sensing system, the fine beam-steering device being for more finely aligning the incoming beam with the photodetector and a centre of the fine tracking sensor than with the coarse beam-steering device based on the indicated fine degree of alignment; and a light transmission source for transmitting an outgoing beam along the same path as the incoming beam toward the fine beam-steering device finely aligning the incoming beam with the photodetector and the centre of the fine tracking sensor, the coarse beam-steering device coarsely aligning the incoming beam with the photodetector and the coarse tracking sensor, and the wide field-of-view lens for transmitting the outgoing beam.

[0015] According to another aspect, the coarse tracking sensor is an image sensor comprising a pixel array. According to another aspect, the coarse tracking sensor is a CMOS sensor.

[0016] According to another aspect, the incoming beam is directed to the photodetector and the fine tracking sensor only when the incoming beam is approximately centred on the coarse tracking sensor and hence passes through the central aperture.

[0017] According to another aspect, the photodetector is shielded by the coarse tracking sensor comprises until a sufficient degree of alignment with the incoming beam is achieved via the central aperture.

[0018] According to another aspect, the coarse tracking sensor further comprises four independent sensor segments arranged to form the central aperture therebetween.

[0019] According to another aspect, the optical transceiver further comprises a beam splitter intermediate with the coarse tracking sensor and the photodetector, the beam splitter being configured to split the incoming beam between the photodetector and the fine tracking sensor.

[0020] According to another aspect, the beam splitter is configured to direct at least 70% of the incoming beam to the photodetector.

[0021] According to another aspect, the optical transceiver further comprises a beam combiner intermediate the light transmission source and the fine beam-steering device, the beam combiner being configured to allow the incoming beam finely aligned with the photodetector and the centre of the fine tracking sensor, by the fine beam-steering device, to pass through the beam combiner to the coarse tracking sensor, and direct the outgoing beam transmitted by light transmission source along the same path as the incoming beam toward the fine beamsteering device finely aligning the incoming beam with the photodetector and the centre of the fine tracking sensor, the coarse beam-steering device coarsely aligning the incoming beam with the photodetector and the coarse tracking sensor, and the wide field-of-view lens for transmitting the outgoing beam.

[0022] According to another aspect, the fine tracking sensor is a quadrant detector.

[0023] According to another aspect, the wide field-of-view lens is a fisheye lens.

[0024] According to another aspect, the fisheye lens is circular and has a field of view of between 150 and 195 degrees.

[0025] According to another aspect, the wide field-of-view lens has an external aperture of between 50 mm to 200 mm.

[0026] According to another aspect, the optical transceiver further comprises a divergence control device connected to the light transmission source. According to another aspect, the divergence control device is a variable focus liquid lens.

[0027] According to another aspect, the coarse beam-steering device and the fine beamsteering device are each steering mirrors.

[0028] According to another aspect, the optical transceiver further comprises a relay lens intermediate the wide field-of-view lens and the coarse beam-steering device.

[0029] According to another aspect, the optical transceiver further comprises a beam expander intermediate the coarse beam-steering device and the fine beam-steering device.

[0030] According to another aspect, the optical transceiver further comprises an optical bandpass filter intermediate the wide field-of-view lens and the sensing system.

[0031] According to another aspect, the light transmission source has a transmission wavelength of between 380 nm to 1 mm.

[0032] According to another aspect, the light transmission source has a transmission wavelength of approximately 1550 nm.

[0033] According to another aspect, the light transmission source is a laser diode.

[0034] According to another aspect, the optical transceiver further comprises a housing within which at least the light transmission source, the photodetector, the tracking sensors, and the beam-steering devices are contained.

[0035] According to another aspect, the optical transceiver is configured for crosslink, full duplex communication with like optical transceivers.

[0036] According to another aspect, the optical transceiver further comprises: one or more processors; and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: receiving sensing data from the sensing system; adjusting the coarse beam-steering device and the fine beam-steering device based on the sensing data to prevent direct sunlight from hitting the photodetector; checking data from the coarse tracking sensor against known objects; and switching to an acquisition mode if a new incoming beam is detected.

[0037] According to another aspect, the invention broadly comprises a satellite comprising a plurality of optical transceivers of any one of the preceding claims, the plurality of optical transceivers being oriented to face different directions such that together they provide full or nearly full directional coverage of free space.

[0038] According to another aspect, the plurality of optical transceivers are a pair of optical transceivers oriented to face opposing directions.

[0039] According to another aspect, the satellite further comprises retroreflecting plates configured to reflect the incoming beam when the incoming beam is not yet aligned with the wide field-of-view lens of one of the optical transceivers.

[0040] According to another aspect, the invention broadly comprises a free-space optical transceiver comprising: a first beam angle amplifier for receiving an incoming beam; a photodetector for reading data from the incoming beam; a first sensing system comprising: a first coarse tracking sensor configured to indicate a first coarse degree of alignment with the incoming beam; and a first fine tracking sensor configured to indicate a first fine degree of alignment with the incoming beam, wherein the first coarse tracking sensor comprises a first central aperture behind which the photodetector and the first fine tracking sensor are situated such that the incoming beam is directed to the photodetector and the first fine tracking sensor via the first central aperture; a first beam steering system comprising: a first coarse beam-steering device intermediate the first beam angle amplifier and the first sensing system, the first coarse beam-steering device being for coarsely aligning the incoming beam with the photodetector and a centre of the first coarse tracking sensor based on the indicated first coarse degree of alignment; and a first fine beam-steering device intermediate the first beam angle amplifier and the first sensing system, the first fine beam-steering device being for more finely aligning the incoming beam with the photodetector and a centre of the first fine tracking sensor than with the first coarse beam-steering device based on the indicated first fine degree of alignment; a light transmission source for transmitting an outgoing beam; a second beam steering system comprising: a second coarse beam-steering device for coarsely aligning the outgoing beam with a receiver; and a second fine beam-steering device for more finely aligning the outgoing beam with the receiver than with the second coarse beam-steering device; and a second beam angle amplifier for amplifying an angle of the outgoing beam.

[0041] According to another aspect, the free-space optical transceiver further comprises a second sensing system comprising a second coarse tracking sensor configured to indicate a second coarse degree of alignment with the incoming beam, and a second fine tracking sensor configured to indicate a second fine degree of alignment with the incoming beam. The second coarse tracking sensor comprises a second central aperture behind which the second fine tracking sensor is situated such that the incoming beam is directed to the second fine tracking sensor via the second central aperture. The second beam angle amplifier amplifies an angle of the incoming beam, the second coarse beam-steering device is intermediate the second beam angle amplifier and the second sensing system, the second coarse beam-steering device being for coarsely aligning the incoming beam with a centre of the second coarse tracking sensor based on the indicated first and second coarse degrees of alignment, and the second fine beam-steering device is intermediate the second beam angle amplifier and the second sensing system, the second fine beam-steering device being for more finely aligning the incoming beam with a centre of the second fine tracking sensor than with the second coarse beamsteering device based on the indicated first and second fine degrees of alignment.

[0042] According to another aspect, the first coarse beam-steering device is for coarsely aligning the incoming beam with the centre of the first coarse tracking sensor based on the indicated first and second coarse degrees of alignment, and the first fine beam-steering device is for more finely aligning the incoming beam with the centre of the first fine tracking sensor than with the first coarse beam-steering device based on the indicated first and second fine degrees of alignment.

[0043] According to another aspect, the free-space optical transceiver further comprises an optical crossover switch intermediate the second beam steering system and the second beam angle amplifier, the optical crossover switch being configured to selectively allow the outgoing beam to pass therethrough to the first sensing system via the first beam steering system. The first coarse tracking sensor is configured to indicate a third coarse degree of alignment with the outgoing beam, the first fine tracking sensor is configured to indicate a third fine degree of alignment with the outgoing beam, and is situated behind the first central aperture of the first coarse tracking sensor such that the outgoing beam is directed to the first fine tracking sensor via the first central aperture, the second coarse beam-steering device is for coarsely aligning the outgoing beam with the receiver based on the indicated third coarse degree of alignment, and the second fine beam-steering device is for more finely aligning the outgoing beam with the receiver than with the second coarse beam-steering device based on the indicated third fine degree of alignment.

[0044] According to another aspect, the optical crossover switch allows the outgoing beam to pass therethrough, the second coarse beam-steering device coarsely aligns the outgoing beam with the receiver based on the indicated third coarse degree of alignment, and the second fine beam-steering device more finely aligning the outgoing beam with the receiver based on the indicated third fine degree of alignment, before the incoming beam is sensed by the first sensing system.

[0045] According to another aspect, the invention broadly comprises a sensing system for a free-space optical receiver or transceiver, the sensing system comprising: a coarse tracking sensor comprising a central aperture, the coarse tracking sensor being configured to indicate a coarse degree of alignment with an incoming beam; a photodetector mounted behind the coarse tracking sensor, the photodetector being configured to receive the incoming beam via the central aperture and to read data from the incoming beam; and a fine tracking sensor mounted behind the coarse tracking sensor, the fine tracking sensor being configured to receive the incoming beam via the central aperture and indicate a fine degree of alignment with the incoming beam, wherein the incoming beam is directed to the photodetector and the fine tracking sensor only when the incoming beam is approximately centred on the coarse tracking sensor and hence passes through the central aperture.

[0046] According to another aspect, the coarse tracking sensor is in image sensor comprising a pixel array.

[0047] According to another aspect, the coarse tracking sensor is a CMOS sensor.

[0048] According to another aspect, the coarse tracking sensor further comprises four independent sensor segments arranged to form the central aperture therebetween.

[0049] According to another aspect, the fine tracking sensor is a quadrant detector. According to another aspect, the sensing system further comprises a beam splitter mounted behind the central aperture of the coarse tracking sensor and configured to split the incoming beam between the photodetector and the fine tracking sensor.

[0050] According to another aspect, the beam splitter is configured to direct at least 70% of the incoming beam to the photodetector.

[0051] According to another aspect, the invention broadly comprises a method of operating a satellite comprising at least one free-space optical transceiver to act as a node in an ad hoc satellite network, the method comprising: receiving data comprising a target destination; predicting positions of other nodes in the network by propagating orbits from a satellite almanac; computing an ideal path through the ad hoc network to the target destination based on the predicted positions; acquiring a subsequent node in the computed ideal path by using the at least one free- space optical transceiver; and forwarding data to the subsequent node.

[0052] According to another aspect, computing an ideal path through the ad hoc network comprises validation or updating of a pre-computed ideal path if one is present in the received data, and wherein the ideal path computed is included with the data forwarded to the subsequent node.

[0053] According to another aspect, while computing the ideal path, the predicted positions of other nodes are updated after each hop based on an estimated latency between nodes.

[0054] According to another aspect, the satellite almanac used in predicting positions of other nodes is whichever is more recent between a satellite almanac stored on the satellite and a satellite almanac present in the received data, and wherein the more recent satellite almanac is included with the data forwarded to the subsequent node.

[0055] According to another aspect, the invention broadly comprises a non-transitory computer-readable storage medium comprising instructions which, when executed by one or more processors, cause the processors to execute instructions comprising any of the above methods.

[0056] According to another aspect, the invention broadly comprises a satellite comprising: at least one free-space optical transceiver; one or more processors; and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising any of the above methods.

[0057] According to another aspect, the invention broadly comprises a free-space optical transceiver comprising: a wide field-of-view lens for receiving an incoming beam; a sensing system comprising: a photodetector for reading data from the incoming beam; a coarse tracking sensor configured to indicate a coarse degree of alignment with the incoming beam; and a fine tracking sensor configured to indicate a fine degree of alignment with the incoming beam; a coarse beam-steering device intermediate the wide field-of-view lens and the sensing system, the coarse beam-steering device being for coarsely aligning the incoming beam with the photodetector and a centre of the coarse tracking sensor based on the indicated coarse degree of alignment; and a fine beam-steering device intermediate the wide field-of-view lens and the sensing system, the fine beam-steering device being for more finely aligning the incoming beam with the photodetector and a centre of the fine tracking sensor than with the coarse beamsteering device based on the indicated fine degree of alignment.

[0058] According to another aspect, the coarse tracking sensor comprises a central aperture behind which the photodetector and the fine tracking sensor are situated such that the incoming beam is directed to the photodetector and the fine tracking sensor via the central aperture.

[0059] According to another aspect, the a free-space optical transceiver further comprises a light transmission source for transmitting an outgoing beam along the same path as the incoming beam toward the fine beam-steering device finely aligning the incoming beam with the photodetector and the centre of the fine tracking sensor, the coarse beam-steering device coarsely aligning the incoming beam with the photodetector and the coarse tracking sensor, and the wide field-of-view lens for transmitting the outgoing beam. Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings.

[0060] As used herein the term "and / or" means "and" or "or", or both.

[0061] As used herein "(s)" following a noun means the plural and / or singular forms of the noun.

[0062] The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in the same manner.

[0063] In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.

[0064] For the purpose of this specification, where method steps are described in sequence, the sequence does not necessarily mean that the steps are to be chronologically ordered in that sequence, unless there is no other logical manner of interpreting the sequence.

[0065] BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The invention will now be described by way of example only and with reference to the drawings in which:

[0067] Figure 1 shows a schematic of an optical transceiver according to the invention;

[0068] Figure 2A shows a perspective view of the optical transceiver in a housing;

[0069] Figure 2B shows a perspective view of two optical transceivers integrated together with an integral housing;

[0070] Figure 3 shows a perspective view of internal components of the optical transceiver;

[0071] Figure 4A shows a perspective view of a sensing system of the optical transceiver;

[0072] Figure 4B shows a rear perspective view of the sensing system of the optical transceiver; Figure 5A shows a perspective view of a satellite equipped with optical transceivers and having retroreflecting panels; Figure 5B shows a diagram of an ad hoc network of satellites equipped with optical transceivers;

[0073] Figure 6 shows a process diagram of a networking algorithm of the optical transceiver;

[0074] Figure 7 shows a process diagram of an acquire mode of the optical transceiver;

[0075] Figure 8 shows a process diagram of a standby mode of the optical transceiver;

[0076] Figure 9 shows a block diagram of the electronics of the optical transceiver; and

[0077] Figure 10 shows a block diagram of a dual path Sync-on-Received-Beam (SoRB) optical transceiver according to another example of the invention;

[0078] Figure 11 shows a process diagram of a controlling algorithm of the dual path SoRB optical transceiver of figure 10;

[0079] Figure 12 shows a block diagram of a dual path Sync-on-Transmitted-Beam (SoTB) optical transceiver according to a further example of the invention;

[0080] Figure 13 shows a process diagram of a controlling algorithm of the dual path SoTB optical transceiver of figure 12; and

[0081] Figure 14 shows a block diagram of a controller of each of the dual path SoRB optical transceiver of figure 10 and the dual path SoTB dual path optical transceiver of figure 12.

[0082] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0083] According to various aspects of the various embodiments of the present invention as illustrated in figures 1-9, there is provided a free-space optical transceiver 100 which will now be described.

[0084] As shown in figure 1, the optical transceiver 100 comprises a wide field-of-view lens 102, a light transmission source 104, and a sensing system 105. The light transmission source 104 transmits an outgoing beam of light. The sensing system 105 is configured to receive an incoming beam of light and determine a degree of alignment with the source of the incoming beam, as well as to read data encoded therein. The wide field-of-view lens 102 ('wide' meaning a field of view of at least 63 degrees) allows for a wider cone of space to be covered by the optical transceiver 100 compared to a conventional lens, which can assist in reducing acquisition time and facilitate attitude-independent operation (in conjunction with at least one other optical transceiver 100) as will subsequently be described.

[0085] The light transmission source 104 is configured to encode data into the outgoing beam, with the optical transceiver 100 being thereby configured to communicate with like optical transceivers - the light transmission source 104 corresponding to the transmit (TX) channel and the sensing system 105 corresponding to the receive (RX) channel. The light transmission source 104 is preferably a laser diode, and preferably has a transmission wavelength of between 380 nm to 1 mm (violet to infrared). The exact transmission wavelength is preferably selected based on absorption properties of the atmosphere, with the wavelengths of 1050 nm and 1550 nm being examples that help to reduce atmospheric attenuation.

[0086] Preferably the optical transceiver 100 is configured as a single-aperture transceiver, such that both the incoming beam and the outgoing beam both pass through the wide field- of-view lens 102. However, the optical transceiver 100 could alternatively be configured as a dual-aperture transceiver, such that a separate lens is provided for the outgoing beam. The dual-aperture configuration may improve isolation between transmit and receive channels, but will add mass and make maintaining alignment more complex.

[0087] The sensing system 105 comprises a photodetector 106 for reading data encoded in the incoming beam. Preferably the photodetector 106 is a photodiode or a photodiode array. The photodetector 106 may have an extremely narrow field-of-view to maximise the receivable optical power and thus the maximum possible data rate, which will in turn require a correspondingly fine degree of alignment between the photodetector 106 and the incoming beam before data can be read.

[0088] The sensing system 105 further comprises a coarse tracking sensor 108 for indicating a coarse degree of alignment with the incoming beam, and a fine tracking sensor 110 for indicating a fine degree of alignment. Preferably the coarse tracking sensor 108 is an image sensor comprising a pixel array, most preferably a CMOS sensor. The fine tracking sensor 110 may be a less complex sensor, for example a quadrant detector capable of detecting light intensity in only four quadrants.

[0089] The sensing system 105 further comprises a coarse beam-steering device 112 for coarsely aligning the incoming beam with the photodetector 106, and a fine beam-steering device 114 for more fine alignment. Preferably the beam-steering devices are both steering mirrors, however the coarse beam-steering device 112 may be configured to provide beam steering in the range of degrees (e.g., ± 25° of optical angle) without strict resolution or accuracy requirements, whereas the fine beam-steering device 114 is preferably configured to provide a resolution of microradians or smaller. Alternatively, one or both of the beam-steering devices may be variable focus liquid lenses configured to provide similar beam steering capabilities. During operation, the sensing system 105 provides sensing data used for closed loop control of the beam-steering devices 112, 114. The coarse tracking sensor 108 may be able to resolve the incoming beam at a particular coordinate on its surface, corresponding to a coarse angular error of the incoming beam. Steering the incoming beam towards the centre of the coarse tracking sensor 108 may be achieved primarily by the coarse beam-steering device 112, and once centred the incoming beam may be directed to the photodetector 106 and the fine tracking sensor 110. The fine tracking sensor 110 may give an indication of fine angular error based on relative intensity between its quadrants, and steering the incoming beam towards the centre of the fine tracking sensor 110 may be achieved primarily by the fine beam-steering device 114. When the incoming beam is centred on the fine tracking sensor 110, this generally corresponds to alignment with a partner optical transceiver and facilitates a two-way lock.

[0090] Transmitting the outgoing beam simultaneously with tracking of the incoming beam facilitates simultaneous co-alignment between the partner optical transceivers. Preferably, the optical transceiver 100 is configured for crosslink, full duplex communication such that data can also be simultaneously transferred in both directions once the optical transceivers are aligned. Preferably a beacon signal with a larger beam width is used during initial acquisition, but a narrower beam width is used to transmit data once sufficiently aligned.

[0091] It will be appreciated that the optical transceiver 100 may comprise various other optical components which facilitate proper functioning or improve performance, the selection of which may depend on the specific configuration of the optical transceiver 100 e.g. the specific beam-steering devices and light transmission source used, the transmission wavelength utilised, the desired form factor, the intended maximum link range, and whether the system is single aperture or dual aperture. Various examples of such secondary optical components will herein be described.

[0092] The optical transceiver 100 may further comprise a beam combiner 118 which directs the outgoing beam along the same path as the incoming beam and allows them to both utilise the same beam-steering devices and wide field-of-view lens 102. The beam combiner 118 thus facilitates a single-aperture arrangement, and may be unnecessary for a dual-aperture arrangement. The beam combiner 118 may be a polarised beam combiner, such that the incoming and outgoing beams are polarised differently after passing through the beam combiner 118. This may assist with isolation between the transmit and receive channels. It will be understood that because the incoming and outgoing beams are travelling in opposite directions, the beam combiner 118 does not 'combine' the beams in a traditional sense of producing a combined beam travelling in a common direction - rather it combines the beams only in the sense of facilitating a common path.

[0093] The optical transceiver 100 may further comprise an optical bandpass filter 120 intermediate the wide field-of-view lens 102 and the sensing system 105. The range of the optical bandpass filter 120 is preferably centred on the transmission wavelength, for example 1550 nm, and may have a relatively narrow passband width of 30 nm or less (for example approximately 17 nm). The optical bandpass filter 120 serves to attenuate incoming light outside of the wavelength of interest and to protect sensitive components such as the photodetector 106 from damage, in particular from the wider solar spectrum. Preferably the optical bandpass filter 120 is situated immediately behind the wide field-of-view lens 102, such that filtering occurs before incoming light reaches any other optical component.

[0094] The optical transceiver 100 may further comprise a relay lens 122 situated between the wide field-of-view lens 102 and the beam-steering devices 112, 114 and configured to interface therebetween. The relay lens 122 may have a diameter of between 35 to 65 mm, for example approximately 50 mm. However, it may be unnecessary depending on the characteristics of the wide field-of-view lens 102 and the beam-steering devices used.

[0095] The optical transceiver 100 may further comprise a beam expander 124 intermediate the coarse beam-steering device 112 and the fine beam-steering device 114. The light transmission source 104 will typically produce a small beam diameter, and the beam expander 124 acts to increase this beam diameter for the outgoing beam. This facilitates easier alignment with like transceivers. In reverse, the beam expander 124 reduces the beam diameter of the incoming beam, allowing it to be easily steered onto the photodetector 106 (which is of relatively small size). Although for simplicity the beam expander 124 is illustrated as a transmissive type in figure 1, preferably it is of a reflective type to avoid chromatic aberration.

[0096] The optical transceiver 100 may further comprise a divergence control device 126 connected to the light transmission source 104. The divergence control device 126 has larger diameter than the beam produced by the light transmission source 104, and may for example be a variable focus liquid lens. The divergence control device 126 may facilitate dynamically changing the beam width of the outgoing beam, which could for example facilitate switching between a beacon signal of high beam width used during acquisition, and a signal of low beam width used for transferring data once aligned. It may also be used to adjust the range of the light transmission source 104.

[0097] As shown in figure 2A, the optical transceiver 100 preferably comprises a housing 200 which mounts the wide-field-of-view lens 102 and contains the other optical components. The 'single' configuration shown in figure 2A may be approximately 1.5U in size according to the CubeSat standard. As shown in figure 2B, two optical transceivers 100 may have their housings 200 conjoined such that the wide field-of-view lenses 102 face in opposite directions, the optical transceivers 100 together forming an integrated free-space optical communications system 202. This integrated 'double' configuration may be approximately 3U in size according to the CubeSat standard.

[0098] Preferably, the wide field-of-view lens 102 is a fisheye lens (generally meaning a field of view of between 100 and 280 degrees), which is preferably circular. Preferably it has a field of view of between 150 and 195 degrees, for example approximately 180 degrees. Its external aperture may for example be between 50 mm to 200 mm, but can be selected based on the specific configuration of the optical transceiver 100. An approximately 180-degree field of view allows a pair of optical transceivers 100 facing in opposite directions to achieve a combined 360-degree field of view, and therefore full directional coverage of the free-space environment.

[0099] Depending on the size of the host satellite, full directional coverage can therefore be achieved either by mounting two 'single' optical transceivers 100 at different locations on the satellite facing opposite directions, or by mounting one 'double' optical communications system 202 at an appropriate location on the satellite. The same arrangements may achieve nearly full directional coverage if the field of view of the fisheye lens is less than 180 degrees.

[0100] A field of view above 180 degrees would cause oppositely faced optical transceivers 100 to overlap in their directional coverage, which may improve the reliability of beam detection at extreme angles but may be redundant to some extent. It will be appreciated that various arrangements of optical transceivers 100 may be suitable depending on their individual field of view and the desired overall directional coverage.

[0101] Containing the coarse-steering device 112 within the housing 200 makes the optical transceiver 100 (or the optical communications system 202) a self-contained module that can be directly mounted to a satellite without the need for a gimbal system. By coarsely steering the beam inside the housing 200, the coarse beam-steering device 112 can achieve higher speeds, consume less power, add less mass, and be more reliable than an external motorised gimbal system used to steer the entire transceiver.

[0102] Furthermore, achieving full (or nearly full) directional coverage of the free-space environment by facing optical transceivers 100 with wide field-of-view lenses 102 in different directions on a satellite allows for attitude-independent operations without the need for any gimbal systems. Systems utilising conventional lenses may require the operation of the host satellite's attitude control system to facilitate full directional coverage of the free-space environment, or otherwise one or more gimbal systems to rotate transceivers as needed. Achieving full (or nearly full) directional coverage with one or two self-contained modules greatly simplifies the mounting of the system to the satellite in addition to achieving the aforementioned benefits.

[0103] As shown in figure 3, the internal optical components of the optical transceiver 100 may be physically arranged in a relatively compact manner. The largest component may be the wide field-of-view lens 102. The other 'large' components may be the coarse beam-steering device 112 and the beam expander 124 (shown here as the preferred reflective type). The sensing system 105 may be provided as a unit with an overall size considerably smaller than any of the aforementioned components. Various arrangements may be possible other than that shown, but it will be appreciated that optically suitable arrangements of the 'large' components may be the primary limiting factor in making the optical transceiver 100 more compact.

[0104] As shown in figures 4A and 4B, the coarse tracking sensor 108 preferably comprises a central aperture 402. The coarse tracking sensor 108 may for example comprise four rectangular sensor segments (which may each be independent sensors) arranged so as to form the central aperture 402 therebetween. The photodetector 106 is situated behind the central aperture 402 such that the incoming beam is directed to the photodetector 106 via the central aperture 402. This means that the photodetector 106 will largely be shielded by the coarse tracking sensor 108 until a sufficient degree of alignment with the incoming beam is achieved, which helps to prevent bright objects (in particular the sun) from damaging the photodetector 108 during standby operations of the optical transceiver 100.

[0105] The fine tracking sensor 110 is likewise preferably situated behind the central aperture 402 such that the incoming beam is directed to the fine tracking sensor 110 via the central aperture 402. In order to direct light to both the fine tracking sensor 110 and the photodetector 106, a beam splitter 116 may be used. To improve signal strength the beam splitter 116 is preferably configured with an uneven split ratio that directs more light to the photodetector 106 than to the fine tracking sensor 110, preferably at least 70%. For example, the split ratio may be 90:10. However, in an alternative embodiment the fine tracking sensor 110 could comprise a secondary central aperture concentric with the central aperture 402, such that the incoming beam is only directed to the photodetector 106 when centred on the fine tracking sensor 110. This would mean the beam splitter 116 is not needed, but it may require a greater degree of alignment before data can be successfully read from the photodetector 106.

[0106] It will be appreciated that instead of providing the central aperture 402 in the coarse tracking sensor 108, the central aperture 402 could alternatively be provided in a separate sun shield situated in the same place relative the photodetector 106 and the fine tracking sensor 110 (i.e. in front of them), and the coarse tracking sensor 108 could be situated elsewhere with light directed to it via an additional beam splitter. However, the use of an additional beam splitter would necessarily reduce the optical power delivered to the photodetector 106, and this alternative arrangement would be less compact than having the coarse tracking sensor 108 serve a dual purpose as a sun shield.

[0107] As shown in figure 5, at least one optical transceiver 100 may be mounted on a satellite 500 - figure 5 shows a configuration where two 'single' optical transceivers 100 are mounted on opposite sides of the satellite 500 facing opposite directions to achieve full directional coverage.

[0108] The satellite 500 may further comprise one or more retroreflecting panels 502, configured to reflect the incoming beam when it is not yet aligned with the wide field-of-view lens 102. This allows the satellite 500 to passively acknowledge receipt to the source of the incoming beam, indicating that the satellite 500 has been successfully located. If retroreflecting panels 502 are used, they are preferably as large as possible and mounted on as many locations of the satellite 500 as possible to maximise reflectiveness. This may reduce the time taken for a like optical transceiver on another satellite to achieve a lock when executing a search pattern. However, sufficiently fast acquisition times may be achievable via active acknowledgement alone, such that the additional cost and difficulty of mounting the retroreflecting panels 502 is unnecessary.

[0109] The configuration of the optical transceiver 100, and optionally the use of retroreflecting panels 502 on the host satellite 500 as described above, facilitate generally faster acquisition and alignment time between like transceivers 100. By using a wide field-of- view lens 102, the optical transceiver 100 is able to regard a wider cone of space when in standby and awaiting an incoming signal. When the optical transceiver 100 is executing a search pattern for another satellite with a like optical transceiver, the wide field-of-view lens 102 and the coarse beam-steering device 112 likewise allow for a reasonably wide cone of space to be searched much faster than with a gimbal that rotates the whole transceiver. Passive acknowledgement from the retroreflecting panels 502 may facilitate detection of the target satellite even before it has actively detected the search beam, and the combination of coarse and fine tracking sensors 108, 110 facilitate rapid alignment after acquisition.

[0110] Utilising the features as summarised above, it may take one second or less to fully sweep the search space and acquire a target satellite, assuming that a prediction of that satellite's position was sufficiently accurate. As shown in figure 5B, reducing acquisition time in such a way enables an ad hoc network 504 of satellites 500 to be formed where each satellite 500 is provided with at least one optical transceiver 100 and acts as a node in the network. The term "ad hoc" in this context may mean a connection that is not preplanned and that can be created by a single party without forewarning of the other party until the moment the connecting beam first arrives. Such an ad hoc network 504 could be used to relay data between different ground stations 506 (or other terrestrial locations), especially across distances where cable-based connections are unavailable and no single satellite has sufficient range to act as a relay. The ad hoc satellite network 504 could likewise be used to relay satellite transmissions either to a ground station 506 or between one another.

[0111] Various methods of operating the satellite 500 and its optical transceiver(s) 100 will now be described, suitable for use in such an ad hoc network 504, however such methods may be relevant to other applications of the optical transceiver 100.

[0112] As shown in figure 6, the satellite 500 may utilise a networking method to facilitate routing of data through the ad hoc network 504. In a first step 600, data is received with a target destination i.e. a target node that data packets are to be transmitted to. For the first satellite node in a transmission through the ad hoc network 504, this data may have been received from a ground station or another spacecraft either by optical transmission or by conventional radio communication to the host satellite 500 - or it may have been generated by the satellite 500 itself, e.g. data produced from its instruments. For an intermediate node, the data is received from a like optical transceiver 100 of the previous node in the ad hoc network 504 via optical transmission. In a second step 601 of the networking method, new positions of satellites in the ad hoc network 504 are predicted by propagating orbits from a satellite almanac. The satellite almanac may store satellite orbital data in the form of two-line element sets (TLEs) or an equivalent data format, allowing the orbits to be propagated to predicted positions at an arbitrary time using a standard algorithm (e.g. using one of the simplified perturbations models such as SGP4). The orbits can be propagated to the current time to obtain current predictions of position. Predictions of position will be less accurate the longer it has been since the epoch of the orbital data in the satellite almanac. The satellite almanac may be stored in persistent memory and updated intermittently from a ground station with new orbital data to ensure it does not become outdated. Preferably a satellite almanac is also received along with the data from the previous node, such that the stored satellite almanac can be updated if the received almanac is newer, and the more recent almanac can be used for propagation.

[0113] In a third step 602 of the networking method, an ideal path through the ad hoc network 504 to the target destination is computed based on the predicted positions of the satellites. The path computation preferably takes into account an ideal path transmitted from a previous node, which avoids repeating the computation from scratch at every node. For example, the pre-computed ideal path may be checked for validity and / or updated based on the newly predicted satellite positions.

[0114] Preferably, while computing the ideal path the predicted positions are also updated after each node hop in the computation, based on an estimated latency between nodes. 'Latency' includes the time for one node to acquire the subsequent node, for the nodes to achieve a two-way lock, and for data to be transferred. Latency estimation may take into account the distance between the nodes (which may be the dominant factor), the size of the data to be transferred, the configuration of the optical transceivers 100, and other such information. Taking this latency into account in the predictions of position helps to improve accuracy when considering overall transfer time for a candidate path, and makes it more likely that the computed ideal path will remain valid.

[0115] The ideal path will generally be the one expected to take the least time to traverse all nodes, i.e. the sum of all latencies between nodes in the path. Direct paths will generally be fastest, i.e. those where for each hop the subsequent node is expected to be in acquisition range immediately. However, if no direct paths are found then an indirect ideal path may be computed which involves one or more nodes waiting for a subsequent node to be in acquisition range.

[0116] Especially for large ad hoc networks, computational constraints may prevent the truly ideal path from being uniquely identifiable within a sufficiently short time period. Thus, the 'ideal path' computed may simply be the best solution found within a fixed time, or the first solution found that is expected to transfer data to the target destination in an acceptable time. It will be appreciated that various heuristics could be used to avoid considering candidate paths that are unlikely to be ideal.

[0117] In a fourth step 603 of the networking method, the appropriate optical transceiver 100 is switched to an acquire mode (which will subsequently be described) to forward data to the subsequent node based on the computed ideal path. Preferably the computed ideal path is also transmitted, to speed up computation at the subsequent node as previously described.

[0118] Carrying out this networking method at each node helps to improve the speed and reliability of data transmission through the ad hoc network 504. Use of a satellite almanac allows for each node to independently carry out ideal path calculations, while transmission of the ideal path alongside the primary message data avoids unnecessary duplication of computation to the extent possible. Re-checking the ideal path at each node and performing dynamic adjustments prevents the use of a path that has become invalid or suboptimal due to deviations from estimated node latency, or from other unpredictable factors.

[0119] As shown in figure 7, the optical transceiver 100 may utilise an acquisition method (or 'acquisition mode') to facilitate acquisition of another node in the ad hoc network 504. The acquisition mode may be entered upon receipt of data from a previous node, in which case it begins at a first step 700 of executing a search pattern for the subsequent node. The search pattern may for example involve steering the outgoing beam in a spiral pattern diverging outwardly from the expected location of the subsequent node. However, other search patterns may also be effective. The search pattern is primarily executed by the coarse beam-steering device 112 and continues until passive acknowledgement (via retroreflecting panels 502) or active acknowledgement (via a return beam) is received from the subsequent node. The search may also be aborted if it fails to locate the subsequent node in time.

[0120] In a second step 701 of the acquisition method, the coarse and fine beam-steering devices 112, 114 are adjusted to align the nodes until a two-way lock is achieved - i.e. alignment is sufficient to allow transmission and reception of light beams in both directions. This step may be entered after a search (i.e. first step 700) in the case of the transmitting node, however it is also entered directly from standby in the case of the receiving node.

[0121] In a third step 702 of the acquisition method, the optical transceiver 100 is switched to transmitting / receiving. Given that the link is two-way, communication will generally occur in both directions, but the primary purpose of the communication is to transfer data from one node to the next. Transmitted data (at least the primary message) may be encrypted for security during transit.

[0122] During transmitting / receiving, the current node sends headers to the subsequent node including secondary data such as satellite ID, the satellite almanac, and the computed ideal path. The current node also forwards the primary message data to be sent to the target destination, and the subsequent node may actively acknowledge receipt. During this process, the sensing system 105 and beam-steering devices 112, 114 of both optical transceivers 100 continue to be operated to maintain the two-way lock. Once the message data has been forwarded, the current node may return to standby. The subsequent node may then execute the networking method if the message data still requires additional hops, and proceed to forward the data in the manner described.

[0123] As shown in figure 8, the optical transceiver 100 may utilise a standby method (or 'standby mode') to facilitate readiness to relay data when called upon. In a first step 800 of the standby method, sensing data is received from the sensing system 105.

[0124] In a second step 801 of the standby method, the coarse and fine beam-steering devices 112, 114 are adjusted based on the sensing data to ensure that the sun does not shine directly on the photodetector 106 (which may cause damage). The coarse tracking sensor 108 (or a separate sun shield as previously described) acting to shield the photodetector 106 ensures that only a small change in angle is needed to keep the sun clear.

[0125] In a third step 802 of the standby method, the sensing data is checked against known objects. Known objects may include the sun, moon, or other bright objects to be distinguished from an incoming beam which encodes transmission data. With the coarse tracking sensor 108 being an image sensor, it can resolve multiple distinct objects simultaneously. Thus, the known objects may be identified via suitable computer vision algorithms or the like.

[0126] In a fourth step 803 of the standby method, the optical transceiver 100 is switched to the acquisition mode if a new object (i.e. an incoming beam) is detected. Specifically, it is switched to the second step 701 of the acquisition method to achieve two-way lock. Otherwise, the standby mode continues in a loop.

[0127] The methods described above can be combined as appropriate, and may together be considered a method of operating a satellite 500 comprising at least one free-space optical transceiver 100 to act as a node in an ad hoc satellite network 504 as described. However, such methods may be applicable to optical transceivers more generally, and are not necessarily limited to use with the optical transceiver 100 as described.

[0128] As shown in figure 9, the methods described above may be executed by a processor or processors 900 of the optical transceiver 100, by a processor or processors 902 of the host satellite 500, or by some combination thereof. Thus, such methods may be stored as instructions in at least one non-transitory computer-readable storage medium i.e. memory 904, operably connected to the relevant processor(s). During execution, the methods may make use of RAM 906 and an encryption chip 908 as needed. A power supply 910, which may be provided by the host satellite 500, can power the processor(s) 900 in addition to any optical components requiring independent power supply e.g. the light transmission source 104.

[0129] It will be appreciated that the diagram of figure 9 is simplified, and the optical components shown may be connected to the processor(s) 900 via intermediate driver circuits or other peripherals rather than directly. Such driver circuits may perform functions such as closed loop control for the beam-steering devices 112, 114 for example. The sensors may be connected via ADCs or circuits configured to process their raw data, as appropriate.

[0130] Although satellite communication is the primary use case described throughout the specification, the optical transceiver 100 and ad hoc networks utilising such transceivers may also be used in other applications e.g. communication between vehicles or drones. Certain aspects described may not be relevant to other applications, for example the use of satellite almanacs in predicting position - however, some other applications may still involve nodes with positions that change over time in a predictable way, and another suitable almanac could therefore be used.

[0131] Figure 1 describes a single path device in which an incoming beam and an outgoing beam share the same optical path. However, there may be benefits of having a dual path device in which an incoming beam and an outgoing beam respectively have two completely separate optical paths. For example, such a dual path device may avoid the possibility of any light from the powerful outgoing beam reaching the path of the relatively weaker incoming beam. This means that all components shared in the single path device must be duplicated, such that each path has its own separate set.

[0132] This poses a challenge, however, as any difference between the optics of the two paths could cause a mismatch between the paths, resulting in the dual path device being unable to point its outgoing beam at a source of an incoming beam precisely enough for connections over extreme distances. The differences could come from imprecision in manufacturing, but also gradual drift over time and complexities in the dual path device's dynamics.

[0133] The solution for the above is to synchronise the two paths of the dual path device while in operation, by detecting and compensating for any mismatch. This can be done by having both paths observe the same beam, compare their responses, and make adjustments until the two paths are compensated to match. Doing this right before connection and then maintaining it during connection are ideal as the real mismatch would be best captured with as little time as possible for additional errors to be accumulated.

[0134] As there are two beams involved in the dual path device, there are two architectures that can be implemented to make this system work. Sync-on-Transmitted-Beam (SoTB) uses an outgoing beam of the dual path device to synchronise its two halves, while Sync-on- Received-Beam (SoRB) uses an incoming beam to do so.

[0135] According to various aspects of the various embodiments of the present invention as illustrated in figures 10-14, there is provided a dual path SoRB optical transceiver 1000 and a dual path SoTB optical transceiver 1200, respectively, which will now be described.

[0136] As shown in figure 10, the dual path SoRB optical transceiver 1000 includes a first beam angle amplifier 1001, a first beam steering system 1002, a first sensing system 1003, and a photodetector 1004 on a "receive side" of the dual path SoRB optical transceiver 1000. The dual path SoRB optical transceiver 1000 further includes a light transmission source 1005, a divergence control device 1006, a second beam steering system 1007, a second beam angle amplifier 1008, and a second sensing system 1009 on a "transmit side" of the dual path SoRB optical transceiver 1000.

[0137] The first beam angle amplifier 1001 is configured to receive and amplify an incoming beam, which is shown as a solid line. A relatively large beam angle of the incoming beam outside the optical transceiver 1000 is decreased to a relatively small beam angle inside the optical transceiver 1000. This allows for a relatively wide angular range for detecting incoming beams, while maintaining a relatively narrow beam path inside the optical transceiver 1000. Advantageously, extremely off-axis beams may be detected by the optical transceiver 1000 without pre-planning of communication connections, and the optical transceiver 1000 may be more compact as the incoming beam path is relatively narrow across a steering range compared to the outgoing beam path. In examples, the first beam angle amplifier 1001 may include the wide field-of-view lens 102, the optical bandpass filter 120, and / or the relay lens 122 of figure 1, although other optical elements are contemplated.

[0138] The first beam steering system 1002 is configured to, based on a control signal from a controller (e.g., of figure 14), adjust an angle of the incoming beam to steer it onto the first sensing system 1003 and the photodetector 1004. In examples, the first beam steering system 1002 may include the coarse beam-steering device 112, the beam expander 124, and the fine beam-steering device 114 of figure 1, although other optical elements are contemplated.

[0139] The first sensing system 1003 is configured to sense a presence and an angle of the incoming beam. In other words, the first sensing system 1003 determines that the optical transceiver 1000 is being attempted to be acquired and determines where the acquiring incoming beam is coming from. The sensed presence and angle are transmitted as a control signal to the controller, the control signal being later used to determine how to steer the incoming beam to the photodetector 1004 and to steer an outgoing beam to a receiver or target device. In examples, the first sensing system 1003 may include the coarse tracking sensor 108, the fine tracking sensor 110, and / or the beam splitter 116 of figure 1, although other optical elements are contemplated.

[0140] The photodetector 1004 is configured to detect and demodulate the incoming beam once it is steered correctly to the photodetector 1004. In detail, the photodetector 1004 converts the incoming beam as an incident optical signal back into an electrical signal resembling that which it was originally modulated with. In an example, the photodetector 1004 may include the photodetector 106 of figure 1, although other optical elements are contemplated.

[0141] The light transmission source 1005 is configured to generate and transmit an outgoing beam, which is shown as a dotted line. In detail, the light transmission source 1005 includes a laser, a power source, and hardware to align the outgoing beam and set its base focus. The light transmission source 1005 may further modulate the transmitted outgoing beam and / or the power source of the light transmission source 1005. In detail, the outgoing beam is an already-modulated electrical signal, the light transmission source 1005 converts the already- modulated electrical signal into a modulated electric power or optical signal, and an amplitude of this signal may be modulated. In an example, the light transmission source 1005 may include the light transmission source 104 of figure 1, although other optical elements are contemplated.

[0142] The divergence control device 1006 adjusts, based on a control signal from the controller, a focus of the transmitted and / or modulated outgoing beam to adjust its coverage area in space and to account for distortion that is applied by other optical elements of the optical transceiver 1000, particularly, the second beam angle amplifier 1008. This allows for quicker acquisition of a receiver or target device. In an example, the divergence control device 1006 may include the divergence control device 126 of figure 1, although other optical elements are contemplated.

[0143] The second beam steering system 1007 is configured to, based on a control signal from the controller, adjust an angle of the outgoing beam to steer it toward a receiver or another optical transceiver in space. In examples, the second beam steering system 1007 may include the coarse beam-steering device 112, the beam expander 124, and the fine beam-steering device 114 of figure 1, although other optical elements are contemplated.

[0144] The second beam angle amplifier 1008 is configured to amplify and transmit the outgoing beam. A relatively small beam angle of the outgoing beam inside the optical transceiver 1000 is increased to a relatively large beam angle outside the optical transceiver 1000. This allows for a relatively wide angular range for transmitting outgoing beams, while maintaining a relatively narrow beam path inside the optical transceiver 1000. In examples, the second beam angle amplifier 1008 may include the wide field-of-view lens 102, the optical bandpass filter 120, and / or the relay lens 122 of figure 1, although other optical elements are contemplated.

[0145] The incoming beam is redirected and / or split towards the second beam angle amplifier 1008. Thus, the second beam angle amplifier 1008 is further configured to amplify and transmit the incoming beam to the second beam steering system 1007. In this case, a relatively large beam angle of the incoming beam outside the optical transceiver 1000 is decreased to a relatively small beam angle inside the optical transceiver 1000. In examples, the incoming beam may be redirected and / or split by one or more beam splitters and / or other optical elements (not shown). The second sensing system 1009 is configured to sense a presence and an angle of the incoming beam passing through the second beam angle amplifier 1008 and the second beam steering system 1007, which is shown as a dashed line. In other words, the second sensing system 1009 determines that the optical transceiver 1000 is being attempted to be acquired and determines where the acquiring incoming beam is coming from. The sensed presence and angle are transmitted as a control signal to the controller, the control signal being later used to determine how to steer the incoming beam to the photodetector 1004 and to steer the outgoing beam to a receiver or target device. In examples, the second sensing system 1009 may include the coarse tracking sensor 108, the fine tracking sensor 110, and / or the beam splitter 116 of figure 1, although other optical elements are contemplated.

[0146] The optical transceiver 1000 performs its synchronisation upon reception of the incoming beam entering through both beam angle amplifiers 1001 and 1008. In detail, the incoming beam is observed on both sides of the optical transceiver 1000 through the identical sensing systems 1003 and 1009, and both beam steering systems 1002 and 1007 attempt to centre the incoming beam. During this process, any errors between the two sides of the optical transceiver 1000 are observed and characterised, and corrections are applied and tested. Advantageously, the optical transceiver 1000 can reap the benefits of the optical transceiver 100 of figure 1, while not requiring additional technology development and while ensuring that all optics dynamics are taken into account during the sync process. This process will now be described more in detail with respect to figure 11.

[0147] As shown in figure 11, a controlling algorithm 1100 of the dual path SoRB optical transceiver 1000 of figure 10 includes an idle state 1101, a plan state 1102, a steer state 1103, a sweep state 1104, a wait state 1105, a maximum timeout determination state 1106, an acquisition failure state 1107, a lock state 1108, a re-plan state 1109, a track state 1110, and a lock failure state 1111. The controlling algorithm 1100 is performed by the controller of the dual path SoRB optical transceiver 1000.

[0148] In the idle state 1101, the controlling algorithm 1100 includes observing the sensing systems 1003 and 1009 for an incoming beam. In response to the incoming beam being detected by at least one among the sensing systems 1003 and 1009, the controlling algorithm 1100 moves to the steer state 1103. In response to receiving a connection request for connecting to a receiver or target device, the controlling algorithm 1100 moves to the plan state 1102. In the plan state 1102, the controlling algorithm 1100 includes determining an approximate target location of the receiver for which the connection request is received. The controlling algorithm 1100 continues to the sweep state 1104.

[0149] In the steer state 1103, the controlling algorithm 1100 includes controlling or synchronizing the beam steering systems 1002 and 1007 to steer the incoming beam to a centre of an optical element (e.g., the central aperture of the coarse tracking sensor 108) of each of the sensing systems 1003 and 1009. The controlling or synchronizing step includes performing path compensation on the beam steering system 1002 or 1007, based on a behaviour (i.e., beam angles) of the incoming beam that is sensed by the sensing systems 1003 and 1009. This path compensation will be described in further detail below with respect to figure 14. In response to the incoming beam being steered to the centre of the optical element of each of the sensing systems 1003 and 1009, the controlling algorithm 1100 moves to the lock state 1108. In response to the incoming beam disappearing, the controlling algorithm 1100 moves to the re-plan state 1109.

[0150] In the sweep state 1104, the controlling algorithm 1100 includes controlling the light transmission source 1005 to transmit an outgoing beam and to sweep in a pattern (e.g., a spiral) that is centered on the determined target location of the receiver for which the connection request is received. In response to an incoming beam from the receiver being detected by at least one among the sensing systems 1003 and 1009, the controlling algorithm 1100 moves to the steer state 1103. In response to the sweep being complete, the controlling algorithm 1100 moves to the wait state 1105.

[0151] In the wait state 1105, the controlling algorithm 1100 includes waiting for an incoming beam from the receiver. In response to an incoming beam from the receiver being detected by at least one among the sensing systems 1003 and 1009, the controlling algorithm 1100 moves to the steer state 1103. In response to a wait time reaching a timeout without receiving an incoming beam, the controlling algorithm 1100 moves to the maximum timeout determination state 1106.

[0152] In the maximum timeout determination state 1106, the controlling algorithm 1100 includes incrementing a timeout count and determining whether the timeout count reaches a predetermined maximum timeout count. In response to the timeout count being determined to reach the maximum timeout count, the controlling algorithm 1100 moves to the acquisition failure state 1107. Otherwise, the controlling algorithm 1100 returns to the sweep state 1104. In the acquisition failure state 1107, the controlling algorithm 1100 includes reporting a connection failure of the optical transceiver 1000, to any party or device of interest including a user.

[0153] In the lock state 1108, the controlling algorithm 1100 includes controlling the light transmission source 1005 to transmit an outgoing beam that is directed at the receiver or target device. In response to an incoming beam from the receiver being detected by at least one among the sensing systems 1003 and 1009, the controlling algorithm 1100 moves to the track state 1110. In response to a wait time reaching a timeout without receiving an incoming beam, the controlling algorithm 1100 moves to the lock failure state 1111.

[0154] In the re-plan state 1109, the controlling algorithm 1100 includes updating the determined target location with new data based on the behaviour of the incoming beam that is measured during the path compensation. The controlling algorithm 1100 returns to the plan state 1102.

[0155] In the track state 1110, the controlling algorithm 1100 includes controlling the beam steering systems 1002 and 1007 to maintain their positions to maintain the connection of the optical transceiver 1000 to the receiver or target device. The controlling step includes steering only the first beam steering system 1002, based on the behaviour of the incoming beam that is sensed through only the first sensing system 1003 of the receive side. This is because, on the transmit side, the large outgoing beam is being transmitted, and thus, the path of the incoming beam through the second beam angle amplifier 1008 and the second beam steering system 1007 to the second sensing system 1009 (i.e., the dashed line) is turned off, disabled, and / or not sensed. Therefore, to perform path compensation on the second beam steering system 1007, the controlling algorithm 1100 uses other input information. This path compensation will be described in further detail below with respect to figure 14.

[0156] In the lock failure state 1111, the controlling algorithm 1100 includes reporting a lock failure to any party or device of interest including the user. The controlling algorithm 1100 may further include receiving, from the party of interest, instructions on next steps to be able to detect the incoming beam from the receiver.

[0157] As shown in figure 12, the dual path SoTB optical transceiver 1200 includes, on a "receive side," the first beam angle amplifier 1001, the first beam steering system 1002, the first sensing system 1003, and the photodetector 1004 of the dual path SoRB optical transceiver 1000 of figure 10. The dual path SoTB optical transceiver 1200 further includes, on a "transmit side," the light transmission source 1005, the divergence control device 1006, the second beam steering system 1007, and the second beam angle amplifier 1008 the dual path SoRB optical transceiver 1000. For brevity, the elements of the dual path SoTB optical transceiver 1200 that are the same as the dual path SoRB optical transceiver 1000 will not be further described.

[0158] In contrast to the dual path SoRB optical transceiver 1000 of figure 10, the dual path SoTB optical transceiver 1200 additionally includes an optical crossover switch 1201 being configured to selectively allow the outgoing beam to pass therethrough to the first sensing system 1003 via the first beam steering system 1002. This path of the outgoing beam is shown as a dashed line and is enabled and disabled by the optical crossover switch 1201. In examples, the optical crossover switch 1201 may include one or more beam splitters and / or one or more relay lenses (not shown) for redirecting, splitting, and / or passing the outgoing beam toward the first beam steering system 1002.

[0159] The first sensing system 1003 is configured to sense a presence and an angle of the outgoing beam passing through the first beam steering system 1002. The sensed presence and angle are transmitted as a control signal to the controller, the control signal being later used to determine how to steer the outgoing beam to a receiver or target device.

[0160] The optical crossover switch 1201 enables the path of the outgoing beam toward the first beam steering system 1002 before the optical transceiver 1200 attempts to acquire a target device, or during an initial acquisition sweep, so that the optical transceiver 1200 may observe a behaviour of the outgoing beam having traversed both beam steering systems 1002 and 1007. Through this, the optical transceiver 1200 may determine appropriate path compensation to apply to the second beam steering system 1007 to have the beam steering systems 1002 and 1007 match. This path compensation will be described in further detail below with respect to figures 13 and 14.

[0161] Advantageously, the optical transceiver 1200 can reap the benefits of the optical transceiver 100 of figure 1, while having two completely separate optical paths and ensuring any light from the powerful outgoing beam does not reach the optical path of the relatively weaker incoming beam during a control process. This process will now be described more in detail with respect to figure 13.

[0162] As shown in figure 13, a controlling algorithm 1300 of the dual path SoTB optical transceiver 1200 of figure 12 includes the idle state 1101, the plan state 1102, the sweep state 1104, the wait state 1105, the maximum timeout determination state 1106, the acquisition failure state 1107, the lock state 1108, the re-plan state 1109, the track state 1110, and the lock failure state 1111 of the controlling algorithm 1100 of figure 11. The controlling algorithm 1300 is performed by the controller of the dual path SoTB optical transceiver 1200. For brevity, the steps of the controlling algorithm 1300 that are the same as the controlling algorithm 1100 of figure 11 will not be further described.

[0163] In contrast to the controlling algorithm 1100 of figure 11, the controlling algorithm 1300 additionally includes a path compensation state 1301 and alternatively includes a steer state 1302 different from the steer state 1103 of figure 11.

[0164] In the path compensation state 1301, the controlling algorithm 1300 includes controlling to open or enable a path for an outgoing beam to pass through the optical crossover switch 1201 of figure 12, and controlling the light transmission source 1005 of figure 12 to transmit the outgoing beam to the determined target location of the receiver for which the connection request is received. Because the opening of the path for the outgoing beam allows it to reach the receive side of the optical transceiver 1200 of figure 12, namely, the first beam steering system 1002 and the first sensing system 1003, in the path compensation state 1301, the controlling algorithm 1300 further includes performing path compensation on the second beam steering system 1007 of figure 12, based on a behaviour (i.e., a beam angle) of the outgoing beam that is sensed through the first sensing system 1003. This path compensation will be described in further detail below with respect to figure 14. In response to the path compensation being complete, the controlling algorithm 1300 further includes controlling to close or disable the path through the optical crossover switch 1201, and continues to the sweep state 1104.

[0165] In the steer state 1302, the controlling algorithm 1300 includes controlling the first beam steering system 1002 to steer the incoming beam to a centre of an optical element (e.g., the central aperture of the coarse tracking sensor 108) of the first sensing system 1003. In contrast to the controlling algorithm 1100 of figure 11, the controlling algorithm 1300 does not include controlling the second beam steering system 1007 to steer the outgoing beam to a centre, because the path compensation is performed on the second beam steering system 1007 in the path compensation state 1301. Accordingly, the controlling algorithm 1300 also does not include performing path compensation on the beam steering system 1002 or 1007 as described with respect to the steer state 1103 of figure 11. Because of the two separate optical paths, which might have slightly different dynamics and errors, a controller of each of the optical transceivers 1000 and 1200 of figures 10 and 12, respectively, is modified in comparison to the controller of the optical transceiver 100 of figure 1, to generate a different control signal to drive a second beam steering system. This modified controller will now be described below with respect to figure 14.

[0166] As shown in figure 14, a controller 1400 of each of the optical transceivers 1000 and 1200 of figures 10 and 12, respectively, includes an adder 1401, a steering controller 1402, an adder 1403, a path compensation controller 1404, and an adder 1405.

[0167] The adder 1401 subtracts, from a beam target (i.e., a central point (0,0) of a central aperture of a coarse tracking sensor of the sensing system 1003 or 1009 of figures 10 and 12), a beam angle A of a beam A (i.e., an incoming or outgoing beam) that is detected by the sensing system 1003 or 1009, to determine a beam angle error.

[0168] The steering controller 1402 is configured to control, using an actuation command, the beam steering system 1002 or 1007 of figures 10 and 12 to steer the beam A towards the beam target, based on the determined beam angle error. This actuation command is also transmitted to the adder 1405. Based on the actuation command, the beam steering system 1002 or 1007 changes the beam angle A and outputs a new beam A of which the beam angle A is changed. The sensing system 1003 or 1009 receives the new beam A and senses the changed beam angle A.

[0169] The adder 1403 subtracts, from the beam angle A, a beam angle B of a beam B (i.e., a corresponding incoming or outgoing beam) that is detected by the corresponding sensing system 1003 or 1009, to determine a path error.

[0170] The path compensation controller 1404 is configured to generate a compensating actuation command based on the determined path error. This compensating actuation command is transmitted to the adder 1405 to compensate, for the path error, the actuation command transmitted from the steering controller 1402 to the adder 1405. The compensating actuation command is generated to drive the path error to zero.

[0171] The adder 1405 adds the compensating actuation command to the actuation command to generate a combined actuation command for the corresponding beam steering system 1002 or 1007 to steer the beam B towards the same point at which the beam A is steered. Based on the combined actuation command, the corresponding beam steering system 1002 or 1007 changes the beam angle B and outputs a new beam B of which the beam angle B is changed. The corresponding sensing system 1003 or 1009 receives the new beam B and senses the changed beam angle B.

[0172] For the dual path SoRB optical transceiver 1000 of figure 10, in the steer state 1103 of figure 11, the steering controller 1402 may control the first beam steering system 1002 to steer an incoming beam toward the first sensing system 1003, based on the beam angle error, while the path compensation controller 1404 may perform path compensation on the second beam steering system 1007 to steer the incoming beam toward the second sensing system 1009. Alternatively, this arrangement may be flipped, and the steering controller 1402 may control the second beam steering system 1007 to steer an incoming beam toward the second sensing system 1009, based on the beam angle error, while the path compensation controller 1404 may perform path compensation on the first beam steering system 1002 to steer the incoming beam toward the first sensing system 1003.

[0173] However, for the dual path SoTB optical transceiver 1200 of figure 12, in the path compensation state 1301 of figure 13, only the steering controller 1402 controls the second beam steering system 1007 to steer an outgoing beam toward the first sensing system 1003, based on the beam angle error, while the path compensation controller 1404 does not operate. This is because the outgoing beam is very bright in comparison to the incoming beam, making it difficult for the path compensation controller 1404 to use this incoming beam to perform path compensation on the first beam steering system 1002. This is why the controlling or synchronizing of the first beam steering system 1002 is done in the steer state 1302 of figure 13.

[0174] For each of the dual path SoRB optical transceiver 1000 of figure 10 and the dual path SoTB optical transceiver 1200 of figure 12, in the track state 1110 of figures 11 and 13, respectively, the outgoing beam is being transmitted, and the path compensation controller 1404 will no longer be able to receive useful information from, e.g., the second sensing system 1009. In these cases, the path compensation controller 1404 will continue to output a compensating actuation command, but this will be generated by other inputs, such as a) recorded beam behaviour during the last steer state (i.e., a compensation model that was recently constructed), and / or b) information from the receiver or target device (i.e., the optical transceiver 1000 or 1200 and the receiver exchange information about behaviours of each other's beams, particularly warnings if a beam begins to slip off target). Alternatively, short compensation pulses may also be performed during the track state 1110, namely, the outgoing beam is switched off for a short time to allow an incoming beam to be observed by both sensing systems 1003 and 1009. This would be a process in which both the optical transceiver 1000 or 1200 and the receiver or target device would agree to perform, to ensure they can return to connectivity after the process is finished.

[0175] In examples, aspects of the SoRB optical transceiver 1000 of figure 10 and the SoTB optical transceiver 1200 of figure 12 may be implemented at the same time. In one example, the SoRB optical transceiver 1000 may operate with the second sensing system 1009 for sensing an incoming beam and with the optical crossover switch 1201 for transferring an outgoing beam to the first sensing system 1003 for sensing the outgoing beam. Similarly, the SoTB optical transceiver 1200 may operate with optical crossover switch 1201 for transferring an outgoing beam to the first sensing system 1003 for sensing the outgoing beam and with the second sensing system 1009 for sensing an incoming beam. That is, aspects of the SoRB optical transceiver 1000 and the SoTB optical transceiver 1200 are not mutually exclusive.

[0176] Where optical components are described as being 'intermediate' or 'between' other components throughout the specification and in the appended claims, it will be understood that in context this generally refers to relative position in the optical path - and not necessarily to relative physical position. Although for some components these may be equivalent, changes in beam direction (e.g. via mirrors or beam splitters) may cause certain components to be intermediate others optically but not physically. It will further be understood that there may be further intermediate components in the optical path even if not explicitly mentioned.

[0177] To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims.

[0178] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Claims

CLAIMS1. A free-space optical transceiver comprising: a wide field-of-view lens for receiving an incoming beam; a sensing system comprising: a photodetector for reading data from the incoming beam; a coarse tracking sensor configured to indicate a coarse degree of alignment with the incoming beam; and a fine tracking sensor configured to indicate a fine degree of alignment with the incoming beam, wherein the coarse tracking sensor comprises a central aperture behind which the photodetector and the fine tracking sensor are situated such that the incoming beam is directed to the photodetector and the fine tracking sensor via the central aperture; a coarse beam-steering device intermediate the wide field-of-view lens and the sensing system, the coarse beam-steering device being for coarsely aligning the incoming beam with the photodetector and a centre of the coarse tracking sensor based on the indicated coarse degree of alignment; a fine beam-steering device intermediate the wide field-of-view lens and the sensing system, the fine beam-steering device being for more finely aligning the incoming beam with the photodetector and a centre of the fine tracking sensor than with the coarse beamsteering device based on the indicated fine degree of alignment; and a light transmission source for transmitting an outgoing beam along the same path as the incoming beam toward the fine beam-steering device finely aligning the incoming beam with the photodetector and the centre of the fine tracking sensor, the coarse beam-steering device coarsely aligning the incoming beam with the photodetector and the coarse tracking sensor, and the wide field-of-view lens for transmitting the outgoing beam.

2. The optical transceiver of claim 1, wherein the coarse tracking sensor is an image sensor comprising a pixel array.

3. The optical transceiver of claim 2, wherein the coarse tracking sensor is a CMOS sensor.

4. The optical transceiver of any one of the preceding claims, wherein the incoming beam is directed to the photodetector and the fine tracking sensor only when the incoming beam is approximately centred on the coarse tracking sensor and hence passes through the central aperture.

5. The optical transceiver of claim 1, wherein the photodetector is shielded by the coarse tracking sensor until a sufficient degree of alignment with the incoming beam is achieved via the central aperture.

6. The optical transceiver of claim 1, wherein the coarse tracking sensor further comprises four independent sensor segments arranged to form the central aperture therebetween.

7. The optical transceiver of any one of the preceding claims, further comprising a beam splitter intermediate with the coarse tracking sensor and the photodetector, the beam splitter being configured to split the incoming beam between the photodetector and the fine tracking sensor.

8. The optical transceiver of any one of the preceding claims, further comprising a beam combiner intermediate the light transmission source and the fine beam-steering device, the beam combiner being configured to: allow the incoming beam finely aligned with the photodetector and the centre of the fine tracking sensor, by the fine beam-steering device, to pass through the beam combiner to the coarse tracking sensor; and direct the outgoing beam transmitted by light transmission source along the same path as the incoming beam toward the fine beam-steering device finely aligning the incoming beam with the photodetector and the centre of the fine tracking sensor, the coarse beam-steering device coarsely aligning the incoming beam with the photodetector and the coarse tracking sensor, and the wide field-of-view lens for transmitting the outgoing beam.

9. The optical transceiver of any one of the preceding claims, wherein the fine tracking sensor is a quadrant detector.

10. The optical transceiver of any one of the preceding claims, wherein the wide field-of-view lens is a fisheye lens.

11. The optical transceiver of any one of the preceding claims, further comprising a divergence control device connected to the light transmission source.

12. The optical communication transceiver of claim 11, wherein the divergence control device is a variable focus liquid lens.

13. The optical transceiver of any one of the preceding claims, wherein the coarse beam-steering device and the fine beam-steering device are each steering mirrors.

14. The optical transceiver of any one of the preceding claims, further comprising a relay lens intermediate the wide field-of-view lens and the coarse beam-steering device.

15. The optical transceiver of any one of the preceding claims, further comprising a beam expander intermediate the coarse beam-steering device and the fine beam-steering device.

16. The optical transceiver of any one of the preceding claims, further comprising an optical bandpass filter intermediate the wide field-of-view lens and the sensing system.

17. The optical transceiver of any one of the preceding claims, wherein the light transmission source has a transmission wavelength of between 380 nm to 1 mm.

18. The optical transceiver of claim 17, wherein the light transmission source has a transmission wavelength of approximately 1550 nm.

19. The optical transceiver of any one of the preceding claims, wherein the light transmission source is a laser diode.

20. The optical transceiver of any one of the preceding claims, further comprising a housing within which at least the light transmission source, the photodetector, the tracking sensors, and the beam-steering devices are contained.

21. The optical transceiver of any one of the preceding claims, wherein the optical transceiver is configured for crosslink, full duplex communication with like optical transceivers.

22. The optical transceiver of any one of the preceding claims, further comprising: one or more processors; and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising: receiving sensing data from the sensing system; adjusting the coarse beam-steering device and the fine beam-steering device based on the sensing data to prevent direct sunlight from hitting the photodetector; checking data from the coarse tracking sensor against known objects; and switching to an acquisition mode if a new incoming beam is detected.

23. A satellite comprising a plurality of optical transceivers of any one of the preceding claims, the plurality of optical transceivers being oriented to face different directions such that together they provide full or nearly full directional coverage of free space.

24. The satellite of claim 23, wherein the plurality of optical transceivers are a pair of optical transceivers oriented to face opposing directions.

25. The satellite of claim 23 or 24, further comprising retroreflecting plates configured to reflect the incoming beam when the incoming beam is not yet aligned with the wide field-of-view lens of one of the optical transceivers.

26. A free-space optical transceiver comprising: a first beam angle amplifier for receiving an incoming beam; a photodetector for reading data from the incoming beam; a first sensing system comprising: a first coarse tracking sensor configured to indicate a first coarse degree of alignment with the incoming beam; and a first fine tracking sensor configured to indicate a first fine degree of alignment with the incoming beam, wherein the first coarse tracking sensor comprises a first central aperture behind which the photodetector and the first fine tracking sensor are situated such that the incoming beam is directed to the photodetector and the first fine tracking sensor via the first central aperture; a first beam steering system comprising: a first coarse beam-steering device intermediate the first beam angle amplifier and the first sensing system, the first coarse beam-steering device being for coarsely aligning the incoming beam with the photodetector and a centre of the first coarse tracking sensor based on the indicated first coarse degree of alignment; and a first fine beam-steering device intermediate the first beam angle amplifier and the first sensing system, the first fine beam-steering device being for more finely aligning the incoming beam with the photodetector and a centre of the first fine tracking sensor than with the first coarse beam-steering device based on the indicated first fine degree of alignment; a light transmission source for transmitting an outgoing beam; a second beam steering system comprising: a second coarse beam-steering device for coarsely aligning the outgoing beam with a receiver; and a second fine beam-steering device for more finely aligning the outgoing beam with the receiver than with the second coarse beam-steering device; anda second beam angle amplifier for amplifying an angle of the outgoing beam.

27. The free-space optical transceiver of claim 26, further comprising a second sensing system comprising: a second coarse tracking sensor configured to indicate a second coarse degree of alignment with the incoming beam; and a second fine tracking sensor configured to indicate a second fine degree of alignment with the incoming beam, wherein the second coarse tracking sensor comprises a second central aperture behind which the second fine tracking sensor is situated such that the incoming beam is directed to the second fine tracking sensor via the second central aperture, wherein the second beam angle amplifier amplifies an angle of the incoming beam, the second coarse beam-steering device is intermediate the second beam angle amplifier and the second sensing system, the second coarse beam-steering device being for coarsely aligning the incoming beam with a centre of the second coarse tracking sensor based on the indicated first and second coarse degrees of alignment, and the second fine beam-steering device is intermediate the second beam angle amplifier and the second sensing system, the second fine beam-steering device being for more finely aligning the incoming beam with a centre of the second fine tracking sensor than with the second coarse beam-steering device based on the indicated first and second fine degrees of alignment.

28. The free-space optical transceiver of claim 27, wherein the first coarse beamsteering device is for coarsely aligning the incoming beam with the centre of the first coarse tracking sensor based on the indicated first and second coarse degrees of alignment, and the first fine beam-steering device is for more finely aligning the incoming beam with the centre of the first fine tracking sensor than with the first coarse beam-steering device based on the indicated first and second fine degrees of alignment.

29. The free-space optical transceiver of claim 26, further comprising an optical crossover switch intermediate the second beam steering system and the second beam angleamplifier, the optical crossover switch being configured to selectively allow the outgoing beam to pass therethrough to the first sensing system via the first beam steering system, wherein the first coarse tracking sensor is configured to indicate a third coarse degree of alignment with the outgoing beam, the first fine tracking sensor is configured to indicate a third fine degree of alignment with the outgoing beam, and is situated behind the first central aperture of the first coarse tracking sensor such that the outgoing beam is directed to the first fine tracking sensor via the first central aperture, the second coarse beam-steering device is for coarsely aligning the outgoing beam with the receiver based on the indicated third coarse degree of alignment, and the second fine beam-steering device is for more finely aligning the outgoing beam with the receiver than with the second coarse beam-steering device based on the indicated third fine degree of alignment.

30. The free-space optical transceiver of claim 29, wherein the optical crossover switch allows the outgoing beam to pass therethrough, the second coarse beam-steering device coarsely aligns the outgoing beam with the receiver based on the indicated third coarse degree of alignment, and the second fine beam-steering device more finely aligning the outgoing beam with the receiver based on the indicated third fine degree of alignment, before the incoming beam is sensed by the first sensing system.

31. A sensing system for a free-space optical receiver or transceiver, the sensing system comprising: a coarse tracking sensor comprising a central aperture, the coarse tracking sensor being configured to indicate a coarse degree of alignment with an incoming beam; a photodetector mounted behind the coarse tracking sensor, the photodetector being configured to receive the incoming beam via the central aperture and to read data from the incoming beam; and a fine tracking sensor mounted behind the coarse tracking sensor, the fine tracking sensor being configured receive the incoming beam via the central aperture and to indicate a fine degree of alignment with the incoming beam,wherein the incoming beam is directed to the photodetector and the fine tracking sensor only when the incoming beam is approximately centred on the coarse tracking sensor and hence passes through the central aperture.

32. The sensing system of claim 31, wherein the coarse tracking sensor is in image sensor comprising a pixel array.

33. The sensing system of claim 32, wherein the coarse tracking sensor is a CMOS sensor.

34. The sensing system of any one of claims 31 to 33, wherein the coarse tracking sensor further comprises four independent sensor segments arranged to form the central aperture therebetween.

35. The sensing system of any one of claims 31 to 34, wherein the fine tracking sensor is a quadrant detector.

36. The sensing system of any one of claims 31 to 35, further comprising a beam splitter mounted behind the central aperture of the coarse tracking sensor and configured to split the incoming beam between the photodetector and the fine tracking sensor.

37. A method of operating a satellite comprising at least one free-space optical transceiver to act as a node in an ad hoc satellite network, the method comprising: receiving data comprising a target destination; predicting positions of other nodes in the network by propagating orbits from a satellite almanac; computing an ideal path through the ad hoc network to the target destination based on the predicted positions; acquiring a subsequent node in the computed ideal path by using the at least one free-space optical transceiver; and forwarding data to the subsequent node.

38. The method of claim 37, wherein computing an ideal path through the ad hoc network comprises validation or updating of a pre-computed ideal path if one is present in the received data, and wherein the ideal path computed is included with the data forwarded to the subsequent node.

39. The method of claim 37 or 38, wherein while computing the ideal path, the predicted positions of other nodes are updated after each hop based on an estimated latency between nodes.

40. The method of any one of claims 37 to 39, wherein the satellite almanac used in predicting positions of other nodes is whichever is more recent between a satellite almanac stored on the satellite and a satellite almanac present in the received data, and wherein the more recent satellite almanac is included with the data forwarded to the subsequent node.

41. A non-transitory computer-readable storage medium comprising instructions which, when executed by one or more processors, cause the processors to execute instructions comprising the method of any one of claims 37 to 40.

42. A satellite comprising: at least one free-space optical transceiver; one or more processors; and at least one storage medium operably connected to the one or more processors and storing instructions that, when executed by the one or more processors, perform operations comprising the method of any one of claims 37 to 40.

Images