Patent Description:
Free Space Optical (FSO) communications systems are well known for their ability to provide high data rate communications links. A FSO communication system typically consists of a pair of nodes or communication devices. Each node typically comprises an optical source, for example a laser or light emitting diode (LED), and an optical receiver. In use, the optical source of each node is aligned with the optical receiver of the other node. Modulation of optical signals emitted by the optical sources allows for the bidirectional transfer of data between the two nodes. Thus, there is a datalink between the nodes.

Most current FSO systems are mounted in fixed positions on the Earth and are manually aligned with each other. Commercial systems are available which can offer data rates of several Giga-bits per second (Gbps) over a range of several kilometres.

It is desirable to have communications systems that allow for underwater communications. Radio Frequency (RF) signals tends to be heavily attenuated by seawater, and hence the range of RF communications systems tends to be severely limited. Acoustic systems can offer low data rate transmission (~kbps) over long ranges, but are typically overt which is undesirable for certain applications.

Optical communications systems have also been developed for underwater applications. Such underwater optical communications systems tend to provide relatively high data rate communications over short to medium ranges, for example up to a <NUM> or so.

Like land-based FSO communication systems, an underwater optical communication system typically comprises a pair of nodes, each node will comprising an optical source and detector. However, unlike land-based FSO communication systems (where node positions are typically fixed), underwater modes tend to be mobile. Hence, in underwater applications, the position of each node, and hence the range and angular separation between the nodes, is not fixed. Thus, in underwater applications, alignment between opposing optical sources and detectors tends to be required in order for the communications system to function. In addition, many underwater nodes are unmanned nodes (e.g. unmanned vehicle), and hence manual alignment between a pair of nodes tends not to be possible. Furthermore, while each underwater node may have some estimate of the relative location of the opposing underwater node (e.g. a pre-programmed location and navigation using GPS/inertial systems, or through use of a separate data link) there may be a large range and angular uncertainty in its position.

Hence, a strategy is required for each node to accurately acquire the location of the opposing node before beam alignment (and hence optical communications) can occur. In addition, since the platforms may still be mobile, active beam alignment may be required during data transfer (tracking).

The present inventors have realised that it would be beneficial for optical communication nodes or devices to be able to reliably and accurately acquire the location of the opposing node, for example, before beam alignment and optical communications occur. The present inventors have further realised that it would be beneficial for nodes to be capable of active beam alignment (i.e. tracking) during data transfer, particularly for mobile nodes.

The present inventors have further realised that using an optical beam with large divergence, for example to illuminate the area surrounding the transmitting node to ensure that the other each node receives some optical power, tends to be very power inefficient since most of the transmitted optical power does not reach the detector of the opposing node. The present inventor have realised that such a strategy tends to only be suitable in clean water and/or at short range.

The present inventors have further realised that using a lens to focus the transmitted light to produce a focussed "spot" on the communications detector tends to be highly dependent on angle of incidence of the incoming light. Thus, large incidence angles may result in all or part of the spot being displaced from the centre of the communications detector, thereby reducing the communication link margin.

The present inventors have further realised that scanning a narrow/collimated light beam across an angular region of uncertainty may be inefficient. For example, each node in a pair of nodes may comprise an angle of arrival (AoA) light sensor. A first of the nodes may scan a narrow light beam across the angular search area. At some point during the scan, the light beam from the first node is incident on the AoA sensor of the second node, thereby allowing the second node to infer the angular location of the first node. The second node may then direct a narrow light beam towards the inferred location of the first node. The AoA sensor of the first node may then detect the beam from the second node, and, using measurements of the beam, infer the location of the second node. The first node may then stop scanning, and direct its narrow light beam in the inferred location of the second node. Communications between the nodes may then begin. However, the present inventors have realised that, for such a system and method, scanning a large angular search area with a narrow beam may take significant amount of time. Thus, acquiring the communication link tends to use increased time and/or power. The present inventors have further realised that the link margin may not be known (for example, since the water conditions and/or the range between nodes may not be known). Hence, if conditions happen to be favourable (for example, the distance between nodes is relatively short range, and/or the water through which communications are to occur is relatively clear), time may be wasted scanning the search area with a collimated beam of very high irradiance, when a scan with a larger beam (with lower irradiance) may provide the AoA sensors of the nodes with sufficient power to enable location of the other node.

The present inventors have realised it would be beneficial to provide a system and method that overcome the above mentioned deficiencies of using optical beams having large divergence and also the above mentioned deficiencies of using narrow/collimated optical beams.

In a first aspect, the present invention provides a method of transmitting an optical signal by a Free Space Optical, FSO, communication system. The method comprises: transmitting, by an optical signal transmitter, a first optical signal towards a vehicle present within a volume of water, such that the optical signal covers a given search area in the volume of water; and in response to not receiving an incident optical signal from the vehicle after transmitting the first optical signal; transmitting, by the optical signal transmitter, a second optical signal into the volume of water, the second optical signal having a reduced beam divergence than the first optical signal; and controlling, by a controller, the optical signal transmitter, to scan the volume of water using the second optical signal having the second beam divergence in a sequence of non-overlapping closed loops wherein each of the sequence of non-overlapping closed loops crosses itself at least once.

When scanning the volume, to scan a loop, the controller controls the transmitter such that at least part of the transmitter moves along a path that defines a loop, i.e. a path that crosses itself at least once. For example, when scanning the volume, the controller controls the transmitter to move along a path that has its start point the same point as its end point. Thus, during the scanning of the volume, a path over a plane onto which the optical signal is projected, defines a plurality non-overlapping loops on that plane, i.e. a plurality of non-overlapping paths, each having their start points be the same as their end points. Each of loops in the sequence may be a substantially circular loop.

The step of controlling may comprise controlling the optical signal transmitter to scan the at least part of the volume by repeating at least part of one or more of the loops at least twice. At least part of one or more of the loops may be repeated at least twice. At least part of one or more of the loops may be repeated at most twice. At least part of one or more of the loops may be repeated exactly twice.

Transmitting the optical signal may comprise: generating an optical signal; modifying a divergence and an irradiance of the generated optical signal such that at least one of the divergence and the irradiance is equal to a preselected value; and transmitting the modified optical signal.

The method may further comprise: transmitting, by the optical signal transmitter or a further optical signal transmitter, a further optical signal into at the least part of the volume, the further optical signal having a different beam divergence and irradiance than the optical signal; and controlling, by a controller, the optical signal transmitter or the further optical signal transmitter, to scan the at least part of the volume using the further optical signal in a further sequence of non-overlapping loops.

The method may further comprise: responsive to transmitting the optical signal into the at least part of the volume, receiving, by an optical signal detector, a response optical signal; and determining, using measurements of the response optical signal by the optical signal detector, by one or more processors, a location of a source of the response optical signal.

A field of view of the optical signal transmitter may be directed in a same direction as a field of view of the optical signal detector.

The method may further comprise: the FSO communication system transmits a location signal wherein the location signal at least operates to enable the source of the response optical signal to identify the position of the FSO communication system.

The location signal may be transmitted in substantially the same direction as the field of view of the optical signal detector. This may be to enable the source of the response optical signal to use the location signal to manoeuvre to a position where effective communication may be performed, for example, the bit error rate of a communication signal is at an acceptable level for the current mission. Manoeuvring may be required due to while the source of the response signal has identified the position of the FSO communication system, the communication signal levels may be too low to achieve the required communication data rate between the FSO communication system and the source of the response signal.

The location signal may act as a beacon and may undertake Identification Friend or Foe (IFF) so that the FSO communication system may avoid an unknown acquisition or a known undesirable acquisition.

The location signal is preferably an optical location signal. The location signal may be an acoustic signal, which may have a greater range than an optical signal.

In a further aspect, the present invention comprises a Free Space Optical, FSO, communication system comprising: an optical signal transmitter configured to transmit a first optical signal towards a vehicle within a volume of water such that the optical signal covers a given search area in the volume of water; and a processor configured to determine whether an optical signal detector has received an incident optical signal from the vehicle after transmitting the first optical signal; and in response to not receiving an incident optical signal; the optical signal transmitter configured to transmit a second optical signal into the volume of water, the second optical signal having reduced divergence than the first optical signal; and a controller configured to control the optical signal transmitter to scan the volume of water using the second optical signal having the second beam divergence in a sequence of non-overlapping closed loops wherein each of the sequence of non-overlapping closed loops crosses itself at least once.

The sequence of non-overlapping loops may be a sequence of non-overlapping, concentric circular loops. The controller may be further configured to scan the at least part of the volume by repeating at least part of one or more of the loops at least twice.

The FSO communication system may further comprise an optical signal detector configured to, responsive to the optical signal transmitter transmitting the optical signal into the at least part of the volume, receive a response optical signal. The FSO communication system may further comprise one or more processors configured to determine, using measurements of the response optical signal by the optical signal detector, a location of a source of the response optical signal. A field of view of the optical signal transmitter may be directed in a same direction as a field of view of the optical signal detector.

In a further aspect, the present invention comprises a vehicle comprising an FSO communication system according to any preceding aspect. The vehicle may be a submersible vehicle.

<FIG> is a schematic illustration (not to scale) showing an example vehicle <NUM> in which an embodiment of an optical communications system <NUM> is implemented.

The vehicle <NUM> is an unmanned, submersible (or underwater) vehicle, i.e. a vehicle that is configured to operate while submerged, for example, in water.

In this embodiment, the optical communications system <NUM> comprises an optical signal transmitter <NUM>, an optical signal detector <NUM>, and a processor <NUM>.

The optical signal transmitter <NUM> is configured to transmit an optical signal (such as a laser beam) from the vehicle <NUM>, as described in more detail later below with reference to <FIG>. In this embodiment, the optical signal transmitter <NUM> comprises an optical signal generator <NUM> and lens <NUM>.

The optical signal generator <NUM> is configured to generate an optical signal, and send the generated optical signal to the lens <NUM> for transmission from the vehicle <NUM>. The optical signal generator <NUM> may comprise, for example, a laser or light emitting diode (LED). The optical signal generator <NUM> may be configured to modulate the generated optical signals to encode data.

In this embodiment, the lens <NUM> is arranged to focus the generated optical signal from optical signal generator <NUM>, and direct the focussed optical signal away from the vehicle <NUM>. The lens <NUM> is controllable to vary the divergence of the transmitted optical signal. For example, the lens <NUM> may be a translating lens, a zoom lens, a fluidic lens, a programmable liquid crystal lens, or a programmable holographic lens (such as a switchable Bragg Element or a Digilens). The lens <NUM> is controlled by the processor <NUM>.

The optical signal transmitter <NUM> is operatively coupled to the processor <NUM> such that the processor <NUM> may control operation of the optical signal transmitter <NUM>, i.e. of the optical signal generator <NUM> and the lens <NUM>.

The optical signal detector <NUM> is configured to detect an optical signal (such as a laser beam) incident on the optical signal detector <NUM>, and to generate an output corresponding to the received optical signal. In this embodiment, the optical signal detector <NUM> comprises an Angle of Arrival (AoA) sensor having a relatively narrow Field of View (FoV).

The optical signal detector <NUM> is operatively coupled to the processor <NUM> such that the processor <NUM> may receive an output of the optical signal detector <NUM>. The processor <NUM> is configured to process the received output of the of the optical signal detector <NUM> as described in more detail later below with reference to <FIG>.

In this embodiment, the optical signal transmitter <NUM> is steerable such that the direction, relative to the vehicle <NUM>, in which an optical signal is transmitted by the optical signal transmitter <NUM> may be varied. Thus, an optical signal transmitted by the optical signal transmitter <NUM> may be scanned over an area. The steering of the optical signal transmitter <NUM> is controlled by the processor <NUM>. Also, in this embodiment, the optical signal detector <NUM> is steerable such that the FoV of the optical signal detector <NUM> may be varied. The steering of the optical signal detector <NUM> is controlled by the processor <NUM>.

The optical signal transmitter <NUM> and the optical signal detector <NUM> may be steered in any appropriate way. For example, in some embodiments the optical signal transmitter <NUM> and/or the optical signal detector <NUM> are mounted to a pan/tilt unit which is controlled by the processor <NUM>. In some embodiments, one or more steering mirrors is used to steer the optical signal transmitter <NUM> and/or the optical signal detector <NUM>. The one or more steering mirrors may be controlled by the processor <NUM>.

In this embodiment, the optical signal transmitter <NUM> and the optical signal detector <NUM> are directed in substantially the same direction.

Apparatus, including the processor <NUM>, for implementing the above arrangement, and performing the method steps to be described later below, may be provided by configuring or adapting any suitable apparatus, for example signal amplifiers, one or more computers or other processing apparatus or processors, and/or providing additional modules. The apparatus may comprise a computer, a network of computers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine readable storage medium such as computer memory, a computer disk, ROM, PROM etc., or any combination of these or other storage media.

<FIG> is a process flow chart showing certain steps of an embodiment of an optical communications process. In this embodiment, the optical communications process by which the submersible vehicle <NUM> communicates in an underwater environment with a further submersible vehicle.

At step s2, the processor <NUM> sets the value of an iteration index, i, to be equal to one, i.e. i = <NUM>.

In this embodiment, the optical communications process comprises iteratively performing steps s4 to s8. The iteration index i is indicative of an iteration number of the process.

At step s4, the processor <NUM> selects a value for the divergence of the ith light beam to be transmitted from the vehicle <NUM>.

In this embodiment, in the first iteration of steps s4 to s8 (i.e. for / = <NUM>), the value for the divergence of the first light beam to be transmitted from the vehicle <NUM> is selected to be as large as possible, i.e. the largest possible beam divergence that is achievable by the lens <NUM>.

At step s6, the processor <NUM> controls the optical signal transmitter <NUM> to transmit a light beam having the selected beam divergence.

<FIG> is a schematic illustration (not to scale) showing transmission of the first light beam at the (i = <NUM>)th iteration of step s6.

In this embodiment, the vehicle <NUM> transmits the first light beam in a direction of the further vehicle <NUM>, through an optical medium <NUM> which, in this embodiment, is water (for example, seawater).

The first light beam is indicated in <FIG> by two solid arrows and the reference numeral <NUM>. The first light beam <NUM> is bounded by these arrows. In this embodiment, the first light beam <NUM> has a divergence of θ<NUM>, where θ<NUM> is the maximum divergence achievable by the optical signal transmitter <NUM>.

In this embodiment, the first light beam <NUM> substantially covers the entirety of a search area or search volume, which is indicated in <FIG> by dotted lines and the reference numeral <NUM>. The search area <NUM> is an area in which the vehicle <NUM> is to transmit light beam to attempt to establish a communications link with the further vehicle <NUM>. In this embodiment, since, in the first iteration i = <NUM>, the first light beam <NUM> covers all of the search area <NUM>, the first light beam <NUM> is not scanned across the search area <NUM>. In particular, in this embodiment, the search area <NUM> is defined by the value θ<NUM>.

In this embodiment, the further vehicle <NUM> comprises an optical communications system capable of detecting optical signals incident on the further vehicle <NUM>, and further configured to transmit optical signals. For example, in some embodiments, the further vehicle may comprise the optical communications system <NUM> described in more detail earlier above with reference to <FIG>.

In this embodiment, the communications conditions (for example the distance between the vehicle <NUM> and the further vehicle, and/or the turbidity of the water <NUM>) are such that an optical detector on board the further vehicle <NUM> does not receive sufficient power from the first light beam <NUM> to enable it to detect the first light beam <NUM>. In other words, the first light beam <NUM> is attenuated by the water <NUM> to such a degree that, in effect, the first light beam <NUM> is not received by the further vehicle <NUM>.

Because, in the first iteration / = <NUM>, the further vehicle <NUM> does not detect the light beam transmitted by the vehicle, the further vehicle <NUM> does not transmit a response optical signal towards the vehicle <NUM>.

However, in other embodiments, the communications conditions may be more favourable and such that the wide first beam <NUM> is received and detected by the optical detector of the further vehicle <NUM>. In this case, the further vehicle will transmit a response signal back to the vehicle <NUM>, which is received at the vehicle <NUM>, as described in more detail later below with reference to <FIG>.

At step s8, the processor <NUM> determines whether or not the optical signal detector <NUM> has detected an incident optical signal. In particular, in this embodiment, the processor <NUM> determines whether or not the optical signal detector <NUM> has detected a response optical signal from the further vehicle <NUM>.

If at step s8, the processor <NUM> determines that the optical signal detector <NUM> has detected an incident optical signal, the method proceeds to step s12. Steps s12 to s14 will be described in more detail later below after a description of step s10 and subsequent iterations (i = <NUM>, <NUM>,. ) of steps s4 to s8.

However, if at step s8, the processor <NUM> determines that the optical signal detector <NUM> has not detected an incident optical signal, the method proceeds to step s10.

At step s10, the processor <NUM> increase the value of the iteration index, i, by one, i.e. i = i + <NUM>.

After step s10, the method proceeds back to step s4 for a next iteration of steps s4 to s8.

In this embodiment, in the subsequent iterations of step s4, the processor <NUM> selects successively decreasing values for the divergence of the light beam to be transmitted from the vehicle <NUM>. In other words, in each iteration of step s4, the processor <NUM> selects a value for the divergence of the light beam that is lower than the value selected in the previous iteration.

In particular, in this embodiment, at each iteration of step s4, the processor <NUM> selects the value for the divergence of the light beam to be transmitted from the vehicle <NUM> using the following formula: <MAT> where:.

Thus, for example, in a second iteration (i = <NUM>) of steps s4 to s8, the selected beam divergence is θ<NUM>/<NUM>; in a third iteration (i = <NUM>) of steps s4 to s8, the selected beam divergence is θ<NUM>/<NUM>; in the fourth iteration (i = <NUM>) of steps s4 to s8, the selected beam divergence is θ<NUM>/<NUM>; and so on.

In this embodiment, the divergence of the transmitted light beams in the subsequent iterations of step s6 (i.e. iterations i = <NUM>, <NUM>,. ) is less than that in the first iteration, i = <NUM>. Thus, in subsequent iterations of step s6, the transmitted light beam will not cover the entirety of the search area <NUM> at the same time. Thus, in this embodiment, in the subsequent iterations of step s6, the processor <NUM> controls the optical signal transmitter <NUM> to scan the transmitted light beam across all of the search area <NUM>.

In particular, in this embodiment, at each iteration of step s6, the processor <NUM> controls the optical signal transmitter <NUM> to scan the transmitted light beam in a sequence of non-overlapping concentric circles, as described in more detail below with reference to <FIG>.

In this embodiment, the power used to generate the light beam is substantially the same for each iteration. Thus, decreasing the divergence of the light beam increases its irradiance. In other words, in each iteration of step s6, the irradiance of the light beam is higher than the irradiance in the previous iteration. Thus, at subsequent iteration, the light tends to penetrated through the water <NUM> to a greater extent, in effect increasing the acceptable attenuation of the signal through the water path.

In particular, in this embodiment, at each iteration of step s6, the irradiance of the transmitted light beam tends to be in accordance with the following formula: <MAT> where:.

<FIG> is a schematic illustration (not to scale) showing transmission of the light beam at the (i = <NUM>) iteration of step s6.

In this embodiment, the vehicle <NUM> transmits the second light beam (bounded by solid arrows <NUM>) in a direction of the further vehicle <NUM>, through the optical medium <NUM>. The second light beam <NUM> has a divergence of θ<NUM>, which, in this embodiment, is equal to θ<NUM>/<NUM>.

By decreasing the divergence of the beam (from θ<NUM> to θ<NUM>), the effective range of the optical communication system <NUM> is increased.

In this embodiment, to cover the entirety of the search area <NUM>, the second light beam <NUM> is scanned in a pattern indicated in <FIG> by arrows and the reference numerals <NUM> and <NUM>, and described in more detail below with reference to <FIG>.

<FIG> is a schematic illustration (not to scale) showing a scan pattern implemented by the optical communications system <NUM> to scan the second light beam <NUM> over the search area <NUM>.

<FIG> shows the search area <NUM> from the point of view of the vehicle <NUM>. In this embodiment, from the point of view of the vehicle <NUM>, the search area <NUM> is substantially circular.

In this embodiment, the search area <NUM> is scanned as follows.

Firstly the processor <NUM> controls the optical signal transmitter <NUM> to transmit a light beam at the centre <NUM> of the search area <NUM>. In particular, the optical signal transmitter <NUM> is centred with respect to the search area <NUM>.

Secondly, after scanning at the centre <NUM> of the search area <NUM>, the processor <NUM> controls the optical signal transmitter <NUM> to move its FoV towards the edge of the search area <NUM> through an angular distance of θ<NUM>/<NUM> (i.e. the divergence of the light beam <NUM> in this iteration). In particular, the optical signal transmitter <NUM> is centred at point <NUM> shown in <FIG>. This movement is indicated in <FIG> by a straight arrow and the reference numeral <NUM>.

Thirdly, the processor <NUM> controls the optical signal transmitter <NUM> to scan an outer portion of the search area <NUM> by moving its FoV in a circular loop about the centre <NUM>. This movement is indicated in <FIG> by an arrow and the reference numeral <NUM>. Thus, an annulus surrounding the scanned central portion is scanned.

Advantageously, the scanned circular loop <NUM> does not overlap with the originally scanned central portion. Thus, scanning of the search area <NUM> tends to be efficient in term of both time and power.

In this embodiment, the processor <NUM> controls the optical signal transmitter <NUM> to scan the circular loop <NUM> twice. This double scanning of the circular loop advantageously tends to facilitate the detection of a return signal by the vehicle <NUM>, as described in more detail later below.

In this embodiment, the communications conditions are such that an optical detector on board the further vehicle <NUM> does not receive sufficient power from the second light beam <NUM> to enable it to detect the second light beam <NUM>. In other words, the second light beam <NUM> is attenuated by the water <NUM> to such a degree that, in effect, the second light beam <NUM> is not received by the further vehicle <NUM>.

Because, in the second iteration i = <NUM>, the further vehicle <NUM> does not detect the light beam transmitted by the vehicle <NUM>, the further vehicle <NUM> does not transmit a response optical signal towards the vehicle <NUM>. Thus, in the second iteration of step s8, the processor <NUM> determines that no response optical signal is received at the optical signal detector <NUM>, and a third iteration (i = <NUM>) of steps s4 to s8 is performed.

In this embodiment, the vehicle <NUM> transmits the third light beam (bounded by solid arrows <NUM>) in a direction of the further vehicle <NUM>, through the optical medium <NUM>. The third light beam <NUM> has a divergence of θ<NUM>, which, in this embodiment, is equal to θ<NUM>/<NUM>.

In this embodiment, to cover the entirety of the search area <NUM>, the third light beam <NUM> is scanned in a pattern indicated in <FIG> by arrows and the reference numerals <NUM>-<NUM>, and described in more detail below with reference to <FIG>.

<FIG> is a schematic illustration (not to scale) showing a scan pattern implemented by the optical communications system <NUM> to scan the third light beam <NUM> over the search area <NUM>. <FIG> shows the search area <NUM> from the point of view of the vehicle <NUM>.

Firstly the processor <NUM> controls the optical signal transmitter <NUM> to transmit the third light beam <NUM> at the centre <NUM> of the search area <NUM>. In particular, the optical signal transmitter <NUM> is centred with respect to the search area <NUM>.

Secondly, after scanning at the centre <NUM> of the search area <NUM>, the processor <NUM> controls the optical signal transmitter <NUM> to move its FoV towards the edge of the search area <NUM> through an angular distance of θ<NUM>/<NUM> (i.e. the divergence of the light beam <NUM> in this iteration). This movement is indicated in <FIG> by a straight arrow and the reference numeral <NUM>. The optical signal transmitter <NUM> is centred at point <NUM> shown in <FIG>.

Thirdly, the processor <NUM> controls the optical signal transmitter <NUM> to scan an annular portion of the search area <NUM> by moving its FoV in a circular loop about the centre <NUM>, maintaining the separation between the centre <NUM> and the transmitted light beam. This movement is indicated in <FIG> by an arrow and the reference numeral <NUM>. Thus, an annulus surrounding the scanned central portion is scanned. Advantageously, this scanned circular loop <NUM> does not overlap with the originally scanned central portion. Thus, scanning of the search area <NUM> tends to be efficient in term of both time and power.

In this embodiment, the processor <NUM> controls the optical signal transmitter <NUM> to scan the circular loop <NUM> twice.

Next, after scanning the circular loop <NUM>, the processor <NUM> controls the optical signal transmitter <NUM> to move its FoV towards the edge of the search area <NUM> through an angular distance of θ<NUM>/<NUM> (i.e. the divergence of the light beam <NUM> in this iteration). This movement is indicated in <FIG> by a straight arrow and the reference numeral <NUM>. The optical signal transmitter <NUM> is centred at point <NUM> shown in <FIG>.

Lastly, the processor <NUM> controls the optical signal transmitter <NUM> to scan an outer annular portion of the search area <NUM> by moving its FoV in a circular loop about the centre <NUM>, maintaining the separation between the centre <NUM> and the transmitted light beam. This movement is indicated in <FIG> by an arrow and the reference numeral <NUM>. Thus, an annulus surrounding the scanned central portion and the circular loop <NUM> is scanned. Advantageously, this scanned circular loop <NUM> does not overlap with the originally scanned central portion or the scanned circular loop <NUM>. Thus, scanning of the search area <NUM> tends to be efficient in term of both time and power.

More generally, the relationship between the iteration number and the number of concentric circular loops to be scanned is given by the following formula: <MAT> where Ni is number of concentric circular loops to be scanned at the ith iteration of steps s4 to s8. Thus, for example, at the second iteration, i = <NUM>, there is one loop <NUM> scanned about the central portion of the search area <NUM>. Also, at the third iteration, i = <NUM>, there are two loops <NUM>, <NUM> scanned about the central portion of the search area <NUM>.

Also, the relationship between the iteration number and the radii of concentric circular loops to be scanned is given by the following formula: <MAT> where ri is the radius (i.e. a distance between the centre of the search area <NUM> and the centre of the FoV of the optical signal transmitter <NUM>) of a loop at the ith iteration of steps s4 to s8. Thus, for example, at the second iteration, i = <NUM>, the radius of the loop <NUM> scanned about the centre <NUM> of the search area <NUM> is θ<NUM>/<NUM>. Also, at the third iteration, i = <NUM>, the radii of the two loops <NUM>, <NUM> are θ<NUM>/<NUM> and 2θ<NUM>/<NUM> respectively.

It should be noted that <FIG> and <FIG> show the plane of the scan pattern and in order to create the scan pattern shown in the figures a pointing angle of the optical signal transmitter <NUM> is changed where the pointing angle defines the direction of the scan.

In this embodiment, the communications conditions and the increased irradiance of the third beam <NUM> are such that an optical detector on board the further vehicle <NUM> receives sufficient power from the third light beam <NUM> to enable it to detect the third light beam <NUM>.

As shown in <FIG>, in this embodiment, the further vehicle <NUM> detects the third light beam <NUM> transmitted by the vehicle <NUM>. A processor of the further vehicle <NUM> then determines a location of the vehicle <NUM> using the measured third light beam <NUM>. The further vehicle <NUM> then uses the determined location to transmit a response light beam <NUM> back towards the vehicle <NUM>.

In this embodiment, when the third beam <NUM> from the vehicle <NUM> scans over the optical detector of the further vehicle <NUM> with sufficient irradiance, there is a finite time before the further vehicle <NUM> can direct a response light beam back towards the vehicle <NUM>. By the time the further vehicle <NUM> does this, the vehicle <NUM> may have moved the third light beam <NUM> away from the further vehicle <NUM>. Thus, the response light beam from the further vehicle <NUM> may be outside the FoV of the optical signal detector <NUM> of the vehicle <NUM>. In this embodiment, as described above, at iterations i = <NUM>, <NUM>, and so on, the search area <NUM> is scanned in a sequence of none overlapping concentric circular loops (e.g. loops <NUM>, <NUM>, <NUM>). Each of these loops is repeated twice. Thus, advantageously, if the FoV of the optical signal detector <NUM> is moved away from the response light beam from the further vehicle <NUM> in the first performance of a scanning loop, the optical signal detector <NUM> is directed towards the response light beam from the further vehicle <NUM> in the second performance of that scanning loop.

Thus, the response optical signal tends to be received by the vehicle <NUM>.

Thus, in the third iteration of step s8, the processor <NUM> determines that a response optical signal is received at the optical signal detector <NUM>.

Returning now to the description of <FIG>, in response to the processor <NUM> determining that a response optical signal is received at the optical signal detector <NUM> at some iteration of step s8, the method proceeds to step s12.

At step s12, the processor <NUM> stops the optical signal transmitter <NUM> scanning the search area <NUM>.

At step s14, the processor <NUM> determines a location of the further vehicle <NUM> using the measured response light beam <NUM>. The FSO communication system <NUM> may transmit a location signal in the form of an optical signal and/or an acoustic signal to enable the further vehicle <NUM> to identify the position of the FSO communication system and manoeuvre to a position where effective communication can take place. The location signal may act as a beacon and may undertake Identification Friend or Foe (IFF) so that the FSO communication system <NUM> may avoid an unknown acquisition or a known undesirable acquisition. The processor <NUM> then uses the determined location of the further vehicle <NUM> to transmit a communication optical signal to the further vehicle.

<FIG> is a schematic illustration (not to scale) showing the communication optical signals <NUM>, <NUM> being transmitted between the vehicles <NUM>, <NUM>.

Thus, a two-way communication link between the two vehicles <NUM>, <NUM> is provided.

In this embodiment, a tracking process is performed to actively align the light beams <NUM>, <NUM> during data transfer between the vehicles <NUM>, <NUM>.

Thus, an optical communications process between submersible vehicles in an underwater environment is provided.

Advantageously, the above described method and apparatus allow one or more nodes of an optical communications system to efficiently and robustly acquire the location of one or more other nodes in a relatively short period of time.

The above described system and method tends to facilitate the acquisition of a datalink between two nodes.

The above described system and method is particularly useful in environments in which certain parameters (including, for example, range between nodes, turbidity of optical medium, locations of nodes, etc.) are unknown.

It should be noted that certain of the process steps depicted in the flowchart of <FIG> and described above may be omitted or such process steps may be performed in differing order to that presented above and shown in <FIG>. Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally.

In the above embodiments, the optical communication system is implemented on board an unmanned submersible vehicle. Also, optical communications is performed between two submersible water vehicles. However, in other embodiments the optical communication system is implemented on a different entity, such as a building, a static underwater sensor node, or a different type of vehicle such as a manned submersible vehicle, a land-based vehicle, or an aircraft. In some embodiments, optical communications is performed between a different number of entities, such as more than two entities. Also, optical communications may be performed through more than one type of optical medium, such as through both air and water, e.g. if only one of the entities is underwater while the other is not underwater.

In the above embodiments, the optical signal transmitter and the optical signal detector are mechanically steerable. However, in other embodiments one or both of the transmitter and the detector is electronically steerable. In some embodiments one or both of the transmitter and the detector is not steerable relative to the vehicle. For example, in some embodiments, the optical signal transmitter and the optical signal detector are fixed relative to vehicle, and the vehicle is moved to vary the directions of the FoVs of the transmitter and the detector.

In the above embodiments, the light beam is not scanned during a first iteration of steps s4 to s8. However, in other embodiments, for example in embodiments in which the search area is larger than the maximum beam divergence, the first light beam is scanned over an area. In some embodiments, the first iteration of steps s4 to s8 may be, in effect, omitted, and the method may begin with iteration number <NUM>, or higher.

In the above embodiments, the loops in which the optical signal transmitter is moved across when scanning the search area are substantially circular. Also, each loop is followed twice. However, in other embodiments, one or more of the loops is a different shape, i.e. not circular. Also, in some embodiments, one or more of the loops is scanned a different number of times, for example, once (e.g. if the vehicle is equipped with an AoA sensor having a wide FoV), or more than twice.

In the above embodiments, the divergence of a transmitted beam is varied for the purpose of establishing a communications link between the vehicle and the further vehicle. However, in other embodiment, the beam divergence may be varied for a different purpose, for example for data transfer after the communications link is established.

What will now be described is a further embodiment of an optical communications system that transmits wide divergence optical beams to establish or acquire a communications link with a different entity, and then transmits a narrow divergence beam for data transfer to and/or from that entity.

<FIG> is a schematic illustration (not to scale) showing an embodiment of an optical communications system <NUM>.

In this embodiment, the optical communications system <NUM> is submersed in water <NUM>, which may be sea water. The optical communications system <NUM> is fixed to a surface <NUM>, which may be, for example, a sea bed.

The optical communications system <NUM> comprises three subsystems, namely a central subsystem <NUM>, and two further subsystems <NUM> positioned at opposite sides of the central sub-system <NUM>.

In this embodiment, the optical communications system <NUM> further comprises one or more optical detectors (not shown) for detecting incident optical signals. For example, transmit and receive modules may be combined into a single module, thereby providing a system of reduced size. For example, in some embodiments, a light source is located at the centre of one or more (e.g. each) optical detector elements. This light source may be, for example, an LED, Laser diode, VCSEL (Vertical Cavity Surface Emitting Laser), or an optical fibre coupled to a laser, an LED, or a VCSEL. In some embodiments, a light source is located between the optical detectors. This tends to reduce the impact of adding a light source to the detector element. However, in some embodiments, the optical communications system <NUM> does not include an optical detector, and may, for example, be used together with a separate receive module.

In this embodiment, the central subsystem <NUM> comprises an array of optical signal transmitters <NUM>, a first lens <NUM>, and a fluidic lens <NUM>. The array of optical signal transmitters <NUM> comprises a plurality of optical transmitters which may be controlled, e.g. by a controller (not shown) coupled to the array <NUM>, to electronically steer a light beam transmitted by the array <NUM>. Three examples of steered light beams transmitted by the array <NUM> are shown in <FIG> as bounded by respective pairs of dotted lines and indicated by the reference numerals 922a, 922b and 922c respectively. In this embodiment, the array of optical transmitters <NUM> is arranged to transmit optical signals 922a-c to the first lens <NUM>. The first lens <NUM> focusses the optical signals 922a-c onto the fluidic lens <NUM>, from which the optical signals 922a-c are emitted into the water <NUM>.

In this embodiment, each of the further subsystems <NUM> comprises a respective optical signal transmitter <NUM> and a respective lens <NUM>. For each further subsystem <NUM>, the optical transmitter <NUM> of that subsystem <NUM> is arranged to transmit respective optical signals <NUM> to the lens <NUM> of that subsystem <NUM>, from which those optical signals <NUM> are emitted into the water <NUM>.

In other embodiments, the optical communications system <NUM> comprises a different number of subsystems, e.g. more than three. For example, in some embodiments, the optical communications system comprises a central subsystem and a plurality (e.g. <NUM>) further subsystems arranged around the periphery of the central subsystem, each further subsystem facing in a different respective direction. In some embodiments, multiple subsystems comprise fluidic lenses.

The optical communications system <NUM> is shown in <FIG> performing a process of acquiring a communications link with the further vehicle <NUM>. During performance of this process of acquiring a communications link, the fluidic lens <NUM> is controlled such that the divergence of the transmitted beam 922a-c is relatively large (e.g. greater than or equal to <NUM>°, e.g. <NUM>° to <NUM>°). For example, the fluidic lens <NUM> may be controlled such that the beam divergence of an optical signal 922a-c transmitted by the central sub-system <NUM> is a maximum achievable beam divergence.

Also, the lenses <NUM> are configured such that the divergences of the light beams <NUM> transmitted by the further subsystems <NUM> are relatively large (e.g. greater than or equal to <NUM>°, e.g. <NUM>° to <NUM>°).

During the process of acquiring a communications link, the transmitted light beams 922a-c <NUM> may be, for example, continuous wave (CW) beams, alternating current (AC) modulated beams, low data rate optical signals for performing an Identification Friend or Foe (IFF) process, or a combination thereof.

In this embodiment each of the subsystems <NUM>, <NUM> has a respective different facing. In other words, the central sub-system <NUM> and each of the further sub-systems <NUM> is configured to transmit optical signals into the water <NUM> in a different direction.

Advantageously, transmission of wide divergence light beams in a plurality of different directions by the optical communications system <NUM> tends to increase the likelihood that the further vehicle <NUM> receives an optical signal.

After receiving a wide divergence light beam transmitted by the optical communications system <NUM>, the further vehicle <NUM> transmits a response optical signal to the optical communications system <NUM>. The optical communications system <NUM> receives the response optical signal from the further vehicle <NUM> and performs tracking and communication, for example, as described in more detail earlier above with reference to step s14.

<FIG> is a schematic illustration (not to scale) showing the optical communications system <NUM> tracking and communicating with the further vehicle <NUM>.

In this embodiment, the optical communications system <NUM> determines the relative location of the further vehicle <NUM> using the response optical signal received from the further vehicle. The array <NUM> is then controlled (e.g. by the controller) to transmit a further optical signal <NUM> to the further vehicle <NUM>. In this embodiment, the further optical signal <NUM> encodes data that is being transferred (for example, at a higher rate compared to the transfer of any data occurring during the communication acquisition process shown in <FIG>) between the optical communications system <NUM> and the further vehicle <NUM>. The further optical signal <NUM> has a higher signal level than the optical signals 922a-c, <NUM> transmitted during the communication acquisition process shown in <FIG>.

In this embodiment, the array <NUM> is controlled (e.g. by the controller) so that the further optical signal <NUM> is electronically steered towards the further vehicle <NUM>.

Also, the fluidic lens <NUM> is controlled (e.g. by the controller) so that the further optical signal <NUM> has a relatively narrow beam divergence compared to the divergences of the beam 922a-c, <NUM> transmitted during the communication acquisition process shown in <FIG>. The further optical signal <NUM> may also have increased irradiance compared to the irradiances of the beam 922a-c, <NUM> transmitted during the communication acquisition process. In some embodiments, the beam divergence and/or the irradiance of the further optical signal <NUM> is determined (e.g. optimised) based on a determination of a distance between the optical communications system <NUM> and the further vehicle <NUM>.

Thus, a further embodiment of an optical communications system is provided.

Advantageously, a smart transmit system which uses electronic beam steering for acquisition, tracking and data transfer is provided. Wide divergence beams are transmitted during the acquisition phase (with optional IFF), which are then switched to a narrower beams for the main transfer of information. This advantageously tends to reduce the power used to transmit at a set data rate. Also, data transfer tends to be more covert.

In some embodiments, the optical communications system <NUM> comprises a mechanical beam steering module, for example a pan/tilt unit, instead of or in addition to the electronic beam steering means.

In some embodiments, the optical communications system <NUM> comprises a different type of means for varying the divergence of a transmitted light beam instead of or in addition to the fluidic lens. For example, the optical communications system <NUM> may comprise a translating lens, a zoom lens, a programmable liquid crystal lens, or a programmable holographic lens (such as a switchable Bragg Element or a Digilens).

In the above embodiments, the optical communications system comprises means for varying beam divergence. However, in other embodiments, the optical transmitter is not configured for selectively varying the divergence of a beam. For example, in some embodiments, the optical communications system comprises a plurality of optical signal transmitters, each of which is configured to produce a beam having a different divergence and irradiance.

Claim 1:
A method of transmitting an optical signal by a Free Space Optical, FSO, communication system, the method comprising:
Transmitting, by an optical signal transmitter (<NUM>), a first optical signal towards a vehicle (<NUM>) present within a volume of water (<NUM>), such that the optical signal covers a given search area in the volume of water (<NUM>); and
in response to not receiving an incident optical signal from the vehicle (<NUM>) after transmitting the first optical signal;
transmitting, by the optical signal transmitter (<NUM>), a second optical signal into the volume of water (<NUM>), the second optical signal having a reduced beam divergence than the first optical signal; and
controlling, by a controller, the optical signal transmitter (<NUM>), to scan the volume of water (<NUM>) using the second optical signal having the second beam divergence in a sequence of non-overlapping closed loops (<NUM>, <NUM>) wherein each of the sequence of non-overlapping closed loops (<NUM>, <NUM>) crosses itself at least once.