GB2499693A

GB2499693A - An optical freespace communication system which selects emitters from an array to provide beam steering to a target based on a feedback signal

Info

Publication number: GB2499693A
Application number: GB1219682.0A
Authority: GB
Inventors: David Mark Benton
Original assignee: TRL Technology Ltd
Current assignee: L3Harris TRL Technology Ltd
Priority date: 2012-02-27
Filing date: 2012-11-13
Publication date: 2013-08-28
Also published as: GB2499789A; GB201203363D0; GB201219682D0

Abstract

This application concerns acquiring and maintaining optical alignment between a source transmitter and target receiver in a free-space optical communication system. The source includes a plurality of optical transmitters, such as a VCSEL array or an LED array. Light from each emitter passes through a lens and illuminates a different area in the vicinity of the target device. Optical alignment between the source and the target can be achieved by selecting the appropriate emitter to use. In more detail, the source emits an activation pattern in which the elements which are illuminated varies as a function of time. For example, they could be illuminated individually in sequence, or using an iterative scanning approach, such as that of Figure 4. The target device provides a wireless feedback signal to the source in response to the activation pattern, and this is used to deduce the optimum emitting element for alignment with the target. The wireless feedback signal may be RF or optical.

Description

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Optical Communications Systems

This invention relates to an optical communications source, an optical communications target, an optical communications system and a method of enabling optical communications between 5 a source and a target.

In the context of the present invention, the term "optical" is taken to encompass the ultra violet, visible and infra red ranges of the electromagnetic spectrum.

10 Background

Free-space optical communication (also known as optical wireless) is a technology which, similarly to optical fibre communication, is capable of delivering high bandwidth for communications, and presents an alternative to well-established radio frequency (RF) 15 communications techniques. In comparison with radio frequency communications, optical wireless has benefits including:

i) presently, no license is required for usage;

ii) optical wireless is covert, especially when the optical communications wavelengths used are out of the visible band; and

20 iii) security due to the well-defined beams and immunity to electromagnetic interference (EMI) and jamming.

However, free-space optical communication suffers from a problem in that it is perceived to be difficult to work with. The main difficulties are ensuring a line of sight and maintaining 25 alignment. It is perhaps surprising then to realise the largest base of short range links is in fact the 'point and shoot' infrared data association (IRDA) links that are pervasive in consumer devices. The main difficulty for optical links arises from the fact that optical sources do not 'broadcast' in the way that RF sources do, but have directionality that requires some level of alignment between source and target. Often this alignment must be precise, 30 requiring a pointing and tracking system capable of high resolution angular control. When one or both ends of the communications link are mobile, the tracking system requirements form perhaps the most expensive, power-hungry and massive part of the whole system. A combination of regular misalignments and line of sight interruptions can cause a loss of connection and be frustrating to a user. Free-space optical communications systems operate 35 predominantly at short range (for example a few metres between source and target) or long range such as for satellite communications. The principal reason for this is a lack of intervening objects for such arrangements.

The difficulty of maintaining alignment, especially in the case of communicating with a 40 moving object and maintaining line of sight, is considered an environment specific problem.

The difficulty of alignment is often addressed by widely spreading the transmitted beam to ensure a fraction of the beam footprint will overlap with the receiving aperture, thus enabling

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a consistent connection. A drawback to this approach is the increase in the optical power level that must be emitted. Additionally, since most of the optical beam is not collected by the receiver, it could pose an eye hazard as well as compromising security and covertness.

5 Currently therefore, the options for a generic, versatile free-space optical connection are:

i) A wide area beam to reduce alignment requirements but requiring a high optical power source; and ii) A well-directed optical beam using a lower power source but with a relatively complex alignment system.

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Sources of misalignment can include lateral movements of the source transmitter or target receiver, angular rotations of either the transmitter or receiver or misalignment caused by optical path irregularities, such as scintillation. Small lateral movements cause problems when the transmitter and receiver are in close proximity but are less of an issue at longer 15 distances where beams may have expanded. They can however lead to reductions in received signal intensity for small movement and complete loss of signal for movements that are a significant fraction of the optical beam size. Small angular rotations of the transmitter can lead to large lateral beam offsets at the receiver and are hence magnified by distance. Likewise, rotational movements of the receiver can result in the optical beam being steered 20 away from a detecting element. However, the use of large arrays of detectors such as in charge-coupled device (CCD) cameras, are a familiar method of capturing and defining the angle of arrival of an optical beam. Atmospheric effects, usually thermally induced, that redirect the optical path between transmitter and receiver can also lead to a reduction in received signal strength and degraded optical signals.

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Arrays of light emitting elements are a relatively recent innovation with vertical cavity surface emitting lasers (VCSELs) being the most appropriate technology for developing into arrays. Applications of such arrays have tended to be around multichannel connections for high bandwidth applications such as interconnections between processors. The effects of 30 misalignment are well understood and relevant to military free space optical applications. It has been recognised that employing an array of detectors can aid with the alignment problem. There are various examples where an array of emitting source elements has been identified as being beneficial for being able to spatially address different regions of space. These include optical fibre arrays, multi-segment LEDs, resonant cavity LEDs and arrays of lasers. 35 Developments at the Japanese NICT for example use an array of VCSELs and an array of detectors to locate the receiver to enable optical alignment. Applications of arrays of emitters for optical wireless communications have often focussed on the ability of the array to be able to address a wide field of view with each element addressing a cell within that field of view and allowing multiple connections with multiple receivers. In most known cases the optical 40 link is one-way, or is two-way via a separate channel operating at a different wavelength.

As prior art may be mentioned:

Short-Range Optical Wireless Communications, Dominic C O'Brien and Marcos Katz,

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http://cictr.ee.psu.edu/research/wc/Short-range-OW.pdf;

Scalable high-power, high-speed CW VCSEL Arrays , R. Safaisini, J.R. Joseph, G. Dang and K.L. Lear, ELECTRONICS LETTERS 9th April 2009 Vol. 45 No. 8;

Gruber, M.: 'Multichip module with planar-integrated free-space optical vector-matrix-type 5 interconnects', Appl. Opt., 2004, 43, pp. 463-470;

Mitigating angular misalignment from atmospheric effects in FSO links: Peter G. LoPresti; Hazem Refai; James J. Sluss. Proceedings SPIE Vol. 7685 Atmospheric Propagation VII, Linda M. Wasiczko Thomas; Earl J. Spillar, Editors;

"Divergence and Power Variations in Mobile Free-Space Optical Communications," Alan 10 Harris, Tayeb Giuma, icons, pp. 174-178, Third International Conference on Systems (icons 2008), 2008;

High-speed communications enabling real-time video for battlefield commanders using tracked FSO: Mouhammad K. Al-Akkoumi; Robert C. Huck; James J. Sluss. Jr. Proceedings SPIE Vol. 7685 Atmospheric Propagation VII, Linda M. Wasiczko Thomas; Earl J. Spillar, 15 Editors;

Analysis of Infrared Wireless Links Employing Multibeam Transmitters and Imaging Diversity Receivers Pouyan Djahani and Joseph M. Kahn, IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 48, NO. 12, DECEMBER 2000 2077;

"Agile multi beam free space optical communication apparatus" H. Presley et al, US Patent 20 US2002/0109884;

"Optical transmission arrangement" Jean-Claude Adam Chaimawicz et al, UK patent Application GB2221810;

"Multi segment light emitting diode", Jeffrey B. Sampsell et al, US patent application US6606175;

25 "Improvements in or relating to optical wireless communications", David John Edwards et al, International patent application WO 02/033858;

"Free space optical interconnect system" Dominic John Goodwill, US patent US 6775480; and

"Non mechanical compact optical transceiver for optical wireless communications with a 30 VCSEL array", Toyoshima M. et al, Mobile and Wireless Communications Network Layer and Circuit Level Design, ISBN: 978-953-307-042-1.

However, there are problems with such arrays that to date do not appear to have been addressed, such as expanding these ideas to permit optical communications between a source 35 and a target which are both movable, and providing techniques for quickly and effectively providing alignment, i.e. determining the optimum emitting element from the array of elements for reliable single point-to-point communications (hereinafter referred to as "operational alignment").

40 It is an aim of the present invention to overcome these problems, and so provide a fast and effective technique for enabling reliable optical communications, even for those cases where both the source and target are movable. The present invention seeks to significantly reduce the burden of complex alignment procedures and technology whilst utilising lower power in a

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well directed beam, through the use of an array of emitting elements as a means of non-mechanical beam steering, with enhanced alignment techniques. A further aim is to reduce the size, weight and power requirements of optical communications system such that they can be easily deployed in situations where the source and/or target are physically unstable and 5 would ordinarily result in significant misalignment issues making free space optical communications impractical.

These aims are achieved by providing a feedback system utilising a wireless transmission from the target to the source, such that the optimum array element for operational alignment 10 may be determined. In this way, autonomous alignment may be provided allowing optical communications between moving and unstable positions.

With the present invention, an array of emitting elements located at the focal plane of an optical system can selectively, accurately and rapidly acquire alignment with a target (or 15 receiver). As either the transmitter or receiver moves, alignment can be maintained by switching to another element.

This has significant advantages in making the optical alignment easier to acquire and maintain. Where previously realignment would be achieved by a high precision physical 20 realignment, switching between elements reduces the burden and complexity of any physical realignment system that is additionally required. This in turn will aid in reducing size, weight, power consumption and cost for optical systems. This is of particular use where the transmitter, receiver or both are in motion, such as vehicles or ships. It is also an enabling technology for hand-held optical communications. Methods of acquiring or reacquiring 25 optical alignment between source and target are presented below.

In accordance with a first aspect of the present invention, there is provided an optical communications source for sending optical data to a target, comprising:

an array of optical frequency emitting elements, each emitting element capable of 30 emitting an optical frequency output beam upon activation of said emitting element,

transmission control means for selectively activating said emitting elements of said array,

an optical element arranged to be illuminated by each said output beam and transmit a respective translated beam, the propagation direction of the translated beam being dependent 35 on the relative positioning of the activated emitting element and the optical element;

wherein the source further comprises a feedback system for controlling the selective activation of the emitting elements by the transmission control means so as to provide operational alignment between the source and the target, the feedback system comprising:

a receiver for receiving wireless signals from the target, said wireless signals providing 40 an indication of incidence of a translated beam at the target,

activation pattern generation means for causing the transmission control means to activate selected emitting elements of the array in a spatially varying activation pattern, and means for correlating the received wireless signals to the activation pattern to determine the

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optimum emitting element to be activated to provide said operational alignment.

In accordance with a second aspect of the present invention, there is provided an optical communications target comprising an optical receiver for receiving optical signals, a wireless 5 transmitter and transmission control means for causing the transmitter to output wireless signals in dependence of optical signals received by the optical receiver.

In accordance with a third aspect of the present invention, there is provided an optical communications systems comprising an optical communications source according to the first 10 aspect and an optical communications target according to the second aspect.

In accordance with a fourth aspect of the present invention, there is provided a method of enabling optical communications between a source and a target, the optical source comprising:

15 an array of optical frequency emitting elements, each emitting element capable of emitting an optical frequency output beam upon activation of said emitting element,

an optical element arranged to be illuminated by each said output beam and transmit a 20 respective translated beam, the propagation direction of the translated beam being dependent on the relative positioning of the activated emitting element and the optical element, and a receiver for receiving wireless signals from the target;

the target comprising:

an optical receiver, and 25 a wireless signal transmitter;

the method comprising the steps of sequentially activating selected emitting elements of the array in a spatially varying activation pattern,

transmitting wireless signals from the target, said signals providing an indication of incidence of a translated beam at the target,

30 receiving said transmitted wireless signals at the source, and correlating the received wireless signals to the activation pattern to determine the optimum emitting element to be activated to provide said operational alignment.

According to one specific embodiment of the present invention, a lightweight compact optical 35 source head may enable high bandwidth free-space optical communications in previously impractical situations, such as via a hand-held system, a body mounted system, vehicle to vehicle and ship-borne communications for example.

Another embodiment allows for simultaneous communication with several detector nodes that 40 are spatially separated. This would enable simultaneous optical connections with several mobile targets and could be useful if the emitter were a central hub for communications. As well as communication the optical beams could be used to illuminate one or more targets for identification.

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The use of an array of emitting elements can overcome the requirements for high precision, complex physical realignment systems, enabling much simpler, compact and general methods of coarse alignment. An example of this is one where there is no electro-mechanical 5 alignment system but coarse alignment is achieved by manual, i.e. hand-held, direction of the optical beam. A person is generally able to direct the beam towards a receiver target but will not be able to hold the alignment steady. However, when using an array of emitting elements as described, the person can hold the wider field of view of the system aligned with the receiver much more easily than an individual beam. This therefore opens up the possibility of 10 hand-held high bandwidth optical communication links, that could be integrated into other devices such as mobile phones for example.

Similar systems could be integrated into coarse pointing units with low cost and lightweight alignment systems for integration into free space communication modules for placement on 15 moving or unstable platforms. Such modules could be placed onto vehicles enabling high bandwidth communications to be established either between separate vehicles or between vehicles and fixed communication stations. Unstable platforms such as boats and ships could use these communications modules to overcome rotations and angular disturbances due to motion of water. Equally, communication with unmanned aerial vehicles (UAVs) and other 20 aircraft could be facilitated through the use of free-space optical communication modules utilising arrays of emitting elements.

Correct operational alignment is provided according to the present invention by a feedback mechanism by which the source transmitter can confirm that the beam is pointing towards the 25 target receiver and that the receiver is actually detecting the optical signal. Methods and techniques for obtaining and maintaining (reacquiring) the correct system alignment in a rapid and reliable fashion are set out below, including in particular, techniques to acquire alignment with no timing synchronisation and techniques to cope with misalignments at both ends of the connection.

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Because conventional solid state lasers require the semiconductor wafer from which they originate to be physically sliced up, fabricating arrays is not economically viable. However, vertical cavity surface emitting lasers (VCSELs), which are a more immature technology, can be fabricated in arrays. Such arrays have only recently become commercially available. They 35 contain a small number of elements but are capable of modulation rates of 10GHz or above. Arrays with more elements have been fabricated as low volume specialist items. The requirements for an array-based optical system are such that larger numbers of emitting elements are required - at least a 10 x 10 array of emitting elements is to be expected - in order to see significant benefits in other areas, hence there is much development work still to 40 be done in developing an array where the individual addressing of elements is possible without an overly complicated control system and where bandwidth and thermal control are within acceptable limits.

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A more recent innovation has been the development of large arrays of LEDs - large in terms of number of elements. LEDs emit their light into a wide field of view, i.e. their output beam is divergent, and hence only a fraction of the light can be collected by the output lens. This lack of collection efficiency means that signal strengths will be relatively low and might only 5 be usable in short range or well controlled environments. It is however possible that one or more arrays of micro-lenses could be employed to control and direct the output of the LEDs before entering the main optical system, thereby helping them to perform adequately for this function. It should be noted that such micro-lens arrays could be employed with other types of emitting element, and not just with LEDs.

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The invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 schematically shows an array of emitting elements in the focal plane of a lens and the spatially separated beams thus produced;

15 Fig. 2 schematically shows the spatial coverage of an array of emitting elements;

Figs. 3a to c schematically show three element selection search schemes in accordance with the present invention;

Figs. 4a to f schematically show 6 stages of a further element selection search scheme in accordance with the present invention;

20 Fig. 5 schematically shows an element illumination sequence in accordance with the present invention; and

Fig. 6 schematically shows an optical communications source in accordance with the present invention.

25 Bright optical sources tend to be individual devices due either to the large nature of the source (e.g. bulb, laser tube) or the nature of the device fabrication (for example solid state laser chips must be sliced out of a wafer and mounted on their end). In contrast, arrays of detectors are commonplace - CCD cameras are just such a device. When placed at the focal plane of an optical element such as a lens, different detectors "see" light entering the lens from 30 different angles - the lens effectively converts spatial to angular coordinates and vice versa. This means that each detector element receives light from a separate physical region of space. The converse would also be true, that is if light originated from a spatial coordinate in the focal plane of the lens it would be directed to a particular place in space on the opposite side of the lens. Optical elements other than lenses, for example mirrors, may be used to achieve 35 similar effects.

To illustrate this, Fig. 1 schematically shows a source array 10 of spatially separated emitting elements A, B and C in the focal plane of an optical element comprising a lens 11. Here it can be seen that the lens 11 is arranged to be illuminated by the output beams 12 originating 40 from each emitting element A, B, C and thus transmit respective translated beams 13, the propagation direction of the translated beams 13 being dependent on the relative positioning of the respective element A, B, C and the lens 11. Fig. 1 indicates the directions of translated beams 13, and it can be seen that these are spatially separated.

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With a conventional single source of emission, the whole system must be realigned if the receiver target moves out of the optical beam. What is clear from the above is that using instead an array of emitting elements enables realignment of the direct optical link between a 5 source transmitter and a receiver target not by physically tilting the system, but by switching to a different emitter element. This has some significant advantages. For example, the switching can be done very quickly without the need for slow, mechanical control which may take time to settle. Additionally, by keeping the beam as small as possible, maximum efficiency and security can be maintained. If the array were, say 10 x 10 emitters, the 10 effective field of view addressable by the transmitter is ten times that of a single element in two directions (azimuth and elevation for example), therefore any mechanical steering requirements are ten times easier to meet, and therefore much cheaper to implement. In addition, the initial alignment of the system becomes much easier (provide an initial coarse alignment, then switch on all the emitting elements virtually ensuring the detector is within 15 the overall field of view, then providing precise alignment through the automatic alignment processes described below for switching between emitting elements).

Fig. 2 schematically shows how individual source elements within an array 20 comprising sixty-four emitting elements in a square array map to specific spatial locations 22, thus 20 providing a larger effective field of view with a smaller instantaneous field of view 23. As the detector moves beyond the limits of the addressable space then the whole emitter system must be physically realigned, but the step size to achieve this realignment is significantly coarser than that required for a single element emitter system.

25 Coarse alignment of the source and target, i.e. to an extent such that activation of at least one emitting element should illuminate the target - the exact element possibly not yet being known - for example to roughly obtain initial alignment, or to re-acquire alignment after a loss of alignment, can be achieved in a number of ways, but in all cases it is required that the coarse alignment system is wireless in its operations, i.e. so that there are no physical 30 connections between the source and the target. The coarse alignment mechanism could make use for example of any of the following:

a) Any communication channel but in particular:

i) A broadcast radio frequency channel where some communication can take 35 place between the transmitter and receiver independently of the alignment status;

ii) An optical channel where the alignment status is indicated by the illumination of a light on the receiver, which is viewed and recognised by the transmitter; or iii) An optical channel that communicates data and implies a two way optical communication system between transmitter and receiver.

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b) A camera observing the beam position and providing corrective information to beam pointing/element selection control.

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c) An observation by eye of the beam position either directly, by viewing the beam itself if the beam is visible, or indirectly by viewing the position of a co-propagating visible indicator beam if the main communication channel is invisible.

5 d) Detection of a reflected signal from the output beam from a retro reflector on the receiver.

e) A motion detector or accelerometer that the source can monitor to compensate for the effects of its own motion, such as a microelectromechanical system (MEMS) device.

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Once coarsely aligned, emitting elements of the array may be selectively activated by a transmission control means at the source.

Supplementary to (in some cases integral with) the coarse alignment system outlined above, a 15 feedback system is provided for controlling the selective activation of the elements by the transmission control means so as to provide operational alignment between the source and the target, comprising a receiver for receiving wireless signals from the target, the wireless signals providing an indication of incidence of a translated beam at the target. The wireless signals may be radio frequency, optical etc, similarly to the coarse alignment system outlined 20 above, and indeed the same wireless equipment may be used for both purposes. The feedback system further requires activation pattern generation means for causing the transmission control means to activate selected elements of the array in a spatially varying activation pattern, and means for correlating the received wireless signals to the activation pattern to determine the optimum element to be activated to provide said operational alignment. In 25 practice, both the pattern generating means and correlation means could be implemented as a processor module, such as a computer for example.

An algorithm is required to make use of the information in an effective way to enable precise alignment - i.e. activation of a single emitting element to enable operational alignment - to be 30 attained. At this point, i.e. when a received wireless signal indicates that operational alignment has been made, the activation pattern may be stopped, and the correct element is kept activated until it becomes apparent that alignment has been lost.

Figs. 3a to c schematically show three element selection search schemes for providing 35 operational alignment. Each of these schemes involves activating emitting elements of the array in an activation pattern comprising a sequential activation of each element of the array and waiting for a response from the target to either confirm or decline a positive alignment status. The process of waiting at each stage can be time consuming, especially as the receiver needs time to assess whether or not it has detected a signal. Each of these figures shows a 6x6 40 array of emitting elements for the sake of example.

The simplest approach, shown in Fig. 3a, is to progress through the element selection in a linear fashion, i.e. sequentially activate each element in a row / column, then move to the next

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row / column and sequentially activate each element of that row / column and so on. While this technique is "safe", in that the correct emitting element will be found eventually, it can be the most ineffective method if the search proceeds initially in the wrong direction such that every element in the array is sequentially selected.

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An alternative approach, shown in Fig. 3b, is to infer that it is physically unlikely that the correct alignment is far from the last known alignment position. Therefore searching in a shell of nearest emitting element neighbours around the last known correct element will prove much more efficient at locating the current correct element for alignment. If alignment is not 10 found then the radial size of the shell around the last known location is increased and the search continues.

A further alternative approach, shown in Fig. 3c, employs a predictive search. By examining the recent history of which elements have resulted in alignment, it may be possible to predict 15 the next most likely element to give alignment and select that one. If this strategy fails at any time then other search schemes, such as those set out above, may be implemented.

The previous schemes involve illuminating only one emitting element at any time in order to avoid confusion. However other strategies that involve activation patterns in which a 20 plurality of elements, i.e. regions of the array, are activated at once are also possible.

Figs. 4a to f schematically show one such search strategy. This strategy involves determining if the correct alignment is to be found within a subset of the emitter array elements. Initially (stage 1, see Fig. 4a) a subset 42 of approximately half of the elements of array 40 are 25 activated. If a wireless signal indicating that the correctly aligned element 41 is within this subset is received, then a further partitioning of the subset takes place. If not, then it is apparent that the correct alignment lies within the previously non-illuminated subset and so this is then partitioned and a subset is illuminated (stage 2, see Fig. 4b). This process of continual subset partitioning, generally into groups about half the size of the previous subset 30 continues until the correct alignment is obtained (stages 3-6, see Figs. 4c to 4f). This process has the advantage of being efficient when no previous alignment data is available, such as at initial start-up connection or when a coarse alignment correction beyond the field of view has occurred. There could equally be a stage 0 (not shown) where all the elements are activated in order to confirm that the correct alignment lies within the present field of view. This 35 scheme may not be the most efficient at maintaining alignment in an operating system because it always takes a minimum number of stages to locate the correct element - for an N by N array of elements the search will take a maximum of N+l stages and a minimum of N stages, if at each stage the area is divided into approximately equal areas. However, knowledge of the expected time lag before alignment is achieved could prove useful in certain 40 situations, such as multi-tasking of processor tasks where the processor knows how long it will have to wait before engaging the optical data link.

Fig. 5 schematically shows a further method for obtaining the correct alignment channel. In

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this case, the target determines which of the emitting elements was correctly aligned based upon the arrival time of an optical signal at the target within a known sequence. This requires synchronisation between the source and target concerning the exact time at which each element was activated. However, if alignment has either been lost, or is being established for 5 the first time, such synchronisation may not exist. Assuming that the target knows the number of emitting elements and the length of time for which each element will be activated at the source, the correctly aligned "channel" can be determined as follows:

The source employs an activation pattern of sequentially activating each element in turn, 10 switching it off before proceeding to the next element and not waiting for a response from the target. At the end of a "forward" sequence 50, the source activates each element in a "reverse" sequence 51, i.e. in the reverse order to the previous sequence 50. Following this is a blank sequence 52 where no emissions are produced in order to prevent ambiguity in the interpretation of the detected signals.

15 Since the target is informed of the total number of emitting elements, it "knows" the total time that the series plus its reverse should take. By measuring the time difference 57 between a detected optical signal in both the forward 53 and reverse 56 series, the sequence number (i.e. the position of the element in the sequence) of the correct element can be ascertained using the simple relationship:

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AT = 2(n-x)t

Where AT is the time difference between signal detections, n is the total number of emitting elements, x is the sequence number of the correctly aligned element and t is the dwell time per 25 element. Because there is no synchronisation the first detected signal could belong to the reverse sequence. This will result in the time difference being greater than the time for a whole sequence and the correct time difference should be adjusted to AT-n.t if AT>n.t and is the reason that a blank sequence is required. In the case where there is more than one signal detection within each sequence (because more than one element beam overlaps with the 30 receiver aperture) then the best alignment occurs for the channel with the strongest signal and therefore the measured time difference should be between the largest peaks in both the forward and reverse sequences. In cases where the peak sizes are similar or variable, the positions of the signal peaks could be matched by the use of a convolution between one directional portion of the sequence and the reverse of other directional portion, thus linking 35 peak positions with sequence positions and allowing corresponding positions and sequence numbers to be identified. The source should keep repeating the forward sequence - reverse sequence - blank sequence series until a response is obtained from the target.

Once the sequence number of the correctly aligned element has been determined the target 40 must send the number to the source. This necessitates that the feedback system must use a communication channel with sufficient bandwidth to communicate the element number in a time at least commensurate with the time-scale of the sequence.

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Fig. 6 schematically shows an embodiment of an optical communications source. An optical head assembly 61 includes of an array 62 of emitting elements, some array control electronics 64 acting as the transmission control means, a three axis rotation sensor 63 and a beam-forming lens system 65, which acts to produce a transformed optical output beam 66. The 5 optical head assembly also includes a wireless radio frequency receiver 69 as part of the feedback system. Connected to the optical head assembly 61 by a connecting cable 67 is an electronics control unit 68, which embodies the pattern generation and correlation means. This arrangement allows for gross misalignments due to instability in the head assembly pointing to be compensated or pre-empted through the use of the three axis rotational motion 10 sensor. Thus the optical head assembly 61 can be relatively small and suitable for use as a hand-held optical communications system. The electronics control unit 68 contains the means of providing the data to be transmitted as well as the processing of the feedback signals from the target, in this case a radio frequency communications channel, and the processing for implementing the search algorithm for establishing optical alignment with the target. The 15 separate optical head 61 can be mounted in a variety of ways to allow applications such as head-mounted optical communications, where the wearer controls the pointing of the beam towards the target through the movement of their head. The optical head 61 could alternatively be placed on a vehicle where the pointing towards the receiver is achieved through the directing of the vehicle.

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The above-described embodiments are exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art. For example, for two-way communication, the target could be provided with transmitting and feedback equipment similarly to the source. In Fig. 6, the optical head assembly includes a 25 wireless radio frequency receiver, this receiver does not need to be located in the optical head assembly, and may alternatively be located elsewhere in the system.

Alternative embodiments could include a camera for viewing the beam position, either in addition to, or instead of, other alignment monitoring means.

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While the optical element used in each of the embodiments outlined above comprises a lens, other elements capable of translating a beam, for example mirrors, may alternatively be employed.

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Claims

Claims

1. An optical communications source for sending optical data to a target, comprising:

5 an array of optical frequency emitting elements, each emitting element capable of emitting an optical frequency output beam upon activation of said emitting element,

transmission control means for selectively activating said emitting elements of said array,

an optical element arranged to be illuminated by each said output beam and transmit a 10 respective transformed beam, the propagation direction of the transformed beam being dependent on the relative positioning of the activated emitting element and the optical element;

wherein the source further comprises a feedback system for controlling the selective activation of the emitting elements by the transmission control means so as to provide 15 operational alignment between the source and the target, the feedback system comprising:

a receiver for receiving wireless signals from the target, said wireless signals providing an indication of incidence of a transformed beam at the target,

activation pattern generation means for causing the transmission control means to activate selected emitting elements of the array in a spatially varying activation pattern, and 20 means for correlating the received wireless signals to the activation pattern to determine the optimum emitting element to be activated to provide said operational alignment.

2. An optical communications source according to claim 1, wherein the activation pattern comprises a sequential activation of each emitting element of the array until a received

25 wireless signal indicates that operational alignment has been made.

3. An optical communications source according to claim 2, wherein the first emitting element activated within the activation pattern comprises that emitting element which last provided operational alignment.

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4. An optical communications source according to claim 3, wherein the second and subsequent emitting elements activated within the activation pattern are activated in decreasing order of spatial separation from the first emitting element.

35 5. An optical communications source according to either of claims 2 and 3,

wherein the activation order of emitting elements within the activation pattern is determined based on the history of emitting elements providing operational alignment.

6. An optical communications source according to claim 1, wherein the activation 40 pattern comprises initially simultaneously activating a plurality of emitting elements, and subsequently activating smaller or equal numbers of emitting elements in dependence on the wireless signal received after each activation, until a single emitting element is located which provides operational alignment.

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7. An optical communications source according to claim 1, wherein the activation pattern comprises a sequential activation of each emitting element of the array until a received wireless signal indicates the identity of that emitting element that provides operational

5 alignment.

8. An optical communications source according to any preceding claim, comprising alignment control means for receiving an indication of incidence of a translated beam at the target and causing the transmission control means to selectively activate at least

10 one emitting element in dependence on the indication received.

9. An optical communications source according to claim 8, wherein the alignment control means comprises the wireless receiver.

15 10. An optical communications source according to claim 8, wherein the alignment control means comprises a second wireless receiver.

11. An optical communications source according to either of claims 9 and 10, wherein the alignment control means wireless receiver comprises a radio frequency receiver

20 and the data comprises radio frequency data.

12. An optical communications source according to either of claims 9 and 10, wherein the wireless receiver comprises an optical receiver and the data comprises optical data.

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13. An optical communications source according to claim 12, wherein the alignment control means further comprises an optical transmitter for transmitting optical data to the target.

30 14. An optical communications source according to claim 12, wherein the optical receiver is configured to detect a light emitted from the target.

15. An optical communications source according to claim 8, wherein the alignment control means comprises a camera for observing the translated beam position.

35

16. An optical communications source according to claim 8, wherein the alignment control means comprises an optical receiver for detecting a portion of the translated beam reflected from the target.

40 17. An optical communications source according to claim 8, wherein the alignment control means comprises a motion detector for monitoring motion of the source.

18. An optical communications source according to any preceding claim, wherein

15

each emitting element comprises a laser.

19. An optical communications source according to claim 18, wherein each laser comprises a VCSEL.

5

20. An optical communications source according to any of claims 1 to 17, wherein each emitting element comprises an LED.

21. An optical communications source according to any preceding claim, further 10 comprising an array of micro lenses located in the optical path between the emitting element array and the optical element, for directing the output beams from the emitting element array to the optical element.

22. An optical communications source according to any preceding claim, wherein 15 the transmission control means is operable to activate more than one emitting element simultaneously such that each activated emitting element provides an individual communications channel.

23. An optical communications source according to any preceding claim, suitable 20 for hand-held operation.

24. An optical communications source according to any preceding claim, wherein the optical element comprises a lens.

25 25. An optical communications target comprising an optical receiver for receiving optical signals, a wireless transmitter and transmission control means for causing the transmitter to output wireless signals in dependence of optical signals received by the optical receiver.

30 26. An optical communications target according to claim 25, specifically for optically communicating with the optical communications source of any of claims 1 to 24.

27. An optical communications systems comprising an optical communications source according to any of claims 1 to 24 and an optical communications target according to 35 either of claims 25 and 26.

28. A method of enabling optical communications between a source and a target, the optical source comprising:

an array of optical frequency emitting elements, each emitting element capable of 40 emitting an optical frequency output beam upon activation of said emitting element,

transmission control means for selectively activating said emitting elements of said array,

an optical element arranged to be illuminated by each said output beam and transmit a

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respective translated beam, the propagation direction of the translated beam being dependent on the relative positioning of the activated emitting element and the optical element, and a receiver for receiving wireless signals from the target;

the target comprising:

5 an optical receiver, and a wireless signal transmitter;

the method comprising the steps of sequentially activating selected emitting elements of the array in a spatially varying activation pattern,

transmitting wireless signals from the target, said signals providing an indication of

10 incidence of a translated beam at the target,

receiving said transmitted wireless signals at the source, and correlating the received wireless signals to the activation pattern to determine the optimum emitting element to be activated to provide said operational alignment.

15 29. An optical communications source as herein described with reference to the accompanying figures.

30. An optical communications target as herein described with reference to the accompanying figures.

20

31. An optical communications system as herein described with reference to the accompanying figures.