GB2499789A

GB2499789A - An optical freespace communication system which selects emitters from an array to provide beam steering

Info

Publication number: GB2499789A
Application number: GB1203363.5A
Authority: GB
Inventors: David Mark Benton
Original assignee: TRL Technology Ltd
Current assignee: L3Harris TRL Technology Ltd
Priority date: 2012-02-27
Filing date: 2012-02-27
Publication date: 2013-09-04
Also published as: GB2499693A; GB201219682D0; GB201203363D0

Abstract

This application concerns acquiring and maintaining optical alignment between a transmitter and receiver in a free-space optical communication system. The transmitter includes a plurality of optical transmitters, such as a VCSEL array or an LED array, located in the focal plane of an optical element such as a lens. Each emitter illuminates a different area vicinity of the receiver. Optical alignment between the transmitter and the receiver can be achieved by selecting the appropriate emitter to use. As either the transmitter or receiver moves, alignment can be maintained simply by switching to another emitter. This beam selection system has significant advantages over prior art mechanical alignment systems. Optionally, more than one emitter may be selected at a time. In one embodiment multiple emitters are selected to provide communication to multiple receivers. Bidirectional communication is envisioned.

Description

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A system for improving and maintaining alignment in free space optical systems.

Introduction

Free space optical comms (also known as optical wireless) is a technology which, like its optical fibre relative, is capable of delivering high bandwidth for communications. In comparison with radio frequency communications, optical wireless has benefits such as not requiring a license for usage, being covert especially when out of visible band, secure due to the well defined beams and immune to EMI and jamming. However it suffers from an image problem of being difficult to work with. The main difficulties are ensuring a line of sight and maintaining alignment. It is perhaps surprising then to realise the largest base of short range links is in fact the 'point and shoot' IRDA links that are pervasive in consumer devices [1], The difficulties for optical links tend to arise 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 detector, often this alignment must be precise, 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 are operating predominantly at short range (a few meters) or long range such as for satellite communications. The principle reason for this is a lack of intervening objects. In this document the main emphasis is on addressing the difficulty of maintaining alignment, especially in the case of communicating with a moving object and line of sight is considered an environment specific problem.

The difficulty of alignment is often overcome by widely spreading the transmitted beam to ensure a fraction of the beam footprint will overlap with the receiving aperture, thus enabling a consistent connection. This of course increases the optical power level that must be emitted. As most of the optical beam is not collected by the receiver it could pose an eye hazard as well as compromising security and covertness. So currently the options for a generic, versatile free space optical connection are:

1. A wide area beam to reduce alignment requirements but requiring a high optical power source.

2. A well directed optical beam using a lower power source but with a complex alignment system.

The idea outlined here seeks to significantly reduce the burden of complex alignment whilst utilising lower power in a well directed beam, through the use of an array of emitters.

The idea

Bright optical sources tend to be individual devices due either to the large nature of the source (bulb, laser tube) or the nature of the device fabrication (solid state laser chips must be sliced out of

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a wafer and mounted on their end). In contrast arrays of detectors are commonplace - CCD cameras are just such a device.

Accompanying this description is:

Figure 1. Showing optical beams originating from separate spatial locations behind a lens and resulting in optical beams emerging in specific and separate directions.

Figure 2. Showing a number of possible spatial locations that can be addressed using and array of emitters and a lens system.

When placed at the focal plane of a lens, different detectors see light entering the lens from different angles - the lens converts spatial to angular coordinates and vice versa. So 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-see Figure 1 where spatial separate sources (10) at the back focal plane of a lens (11) produce spatially separate beams (12).

With a single source of emission, the whole system must be realigned if the receiver moves out of the optical beam. What is clear from using an array of emitters is that realignment of the direct optical link between a transmitter and a receiver can be achieved not by physically moving the system, but simply by switching to a different emitter element. This has some significant advantages. Firstly the switching can be done very quickly without the need for slow, mechanical control which may take time to settle. Secondly, by keeping the beam as small as possible, maximum efficiency and security can be maintained. If the array were, say 10x10 emitters, the effective field of view addressable by the transmitter is 10 times that of a single element in two directions (azimuth and elevation for example), therefore any mechanical steering requirements are 10 times easier to meet, and therefore much cheaper to implement. In addition, the initial alignment of the system becomes much easier (simply switch on all the emitters ensuring the detector is within the overall field of view, then precise alignment is easily achieved through the automatic alignment process required for switching between emitters). Figure 2 shows how individual source element within the array (20) map to specific spatial locations(22), thus providing a larger effective field of view with a smaller instantaneous field of view (23). Clearly 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.

Another possibility is for simultaneous communication with several detector nodes that 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.

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 (VCSEL's), which are a more immature technology, can be fabricated in arrays. Such arrays have only recently become commercially available. They contain a small number of elements but are capable of modulation rates of 10GHz or above. Arrays with more elements

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have been fabricated as low volume specialist items. Clearly the requirements for an array based optical system are such that larger numbers of emitting elements are required - at least 10x10 - in order to see significant benefits in other areas, hence there is much development work still to 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. This may ultimately limit the number of elements on the array.

A more recent innovation has been the development of large arrays of LED's- large in terms of number of elements. These are interesting but are likely to be less useful than an array of lasers. LED's emit their light into a wide field of view 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 low and may only be usable in short range or well controlled environments. It is however possible that arrays of microlenses 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.

The main thrust of this idea is to reduce the size, weight and power requirements of optical communications system such that they can be easily deployed

Prior Art

Arrays of light emitters are a relatively recent innovation with arrays of vertical cavity surface emitting lasers (VCSEL's) being the most appropriate technology for developing into arrays. Applications of such arrays have tended to be around multichannel connections [2][3] for high bandwidth applications such as interconnections between processors. The effects of misalignment are well understood [4][5] and relevant to military free space optical applications [6][6], It has been recognised that employing an array of detectors can aid with the alignment problem [7] but to the best of the author's knowledge no one has considered the significant benefit of employing an array of emitters in such a compact and usable format.

References

[!]■ Short-Range Optical Wireless Communications, Dominic C O'Brien and Marcos Katz, http://cictr.ee.psu.edu/research/wc/Short-range-OW.pdf

[2]- 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.

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

[4]. 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,

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

[6]. 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, Editors,

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[7]- 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

Claims

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1. An optical system comprising a plurality of source element emitters, each individually addressable and controllable, positioned advantageously at, or near to the focal plane of an optical element (lens or mirror) where directional control of the light leaving the optical system is controlled through the selective choice and use of an individual source emitter.

2. An optical system as in claim 1, allowing the direction of output to be controlled in a precise fashion so as to maintain an optical connection with a separate receiver.

3. An optical system as in claim 1 allowing the remote beam area (or 'footprint') to be kept to a minimum and thus reducing light loss due to mismatch between the remote receiver aperture size and the optical beam size at the location of the receiver.

4. An optical system as in 1 allowing the optical intensity emitted from the system to be kept to a minimum for a consistent optical connection to be maintained.

5. An optical system utilising a plurality of individually addressable emitters (such as in claim 1) allowing rapid realignment with a receiver through the act of switching to an appropriate source emitter from within the plurality of emitters.

6. An optical system utilising a plurality of individually addressable emitters (such as in 1) allowing the advantageous reduction of physical realignment of the system in preference to alignment through emitter choice.

7. An optical system utilising a plurality of individually addressable emitters (such as in 1) allowing alignment with a moving receiver advantageously with a minimum of physical realignment.

8. An optical system such as in 1 where initial system alignment procedure with a separate receiver is advantageously simplified through the utilisation of all source emitters simultaneously or in sub sets to rapidly locate the optimum alignment direction.

9. An optical system as in 1 integrated with an optical receiver device

10. A system utilising a plurality of individually addressable emitters (such as in 1) where the optical element is an array of micro-lenses where the relative spacing of the lens centres and the source elements is chosen to ensure beam steering occurs as a consequence of the choice of emitter.

11. A system as in 9 used for bidirectional communication.

12. An optical system as in 1 utilising a retro reflector at the receiver location to indicate the alignment direction.

13. An optical system as in claims 5 through to 8 utilising a plurality of emitters for use in optical communications.

14. An optical system as in claims 5 through to 8 where the plurality of emitters is an arrangement of lasers.

15. An optical system as in claims 5 through to 8 where the plurality of emitters is an arrangement of light emitting diodes (LED's).

16. An optical system as in claims 5 through to 8 where the plurality of emitters is an arrangement of optical fibres.

17. An optical system utilising a plurality of emitters to enable continual illumination of a moving target.

18. An optical system as above where a number of individual emitter elements are used to form separate links with a number of spatially separate receivers.

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19. An optical system as above where a number of individual emitter elements are used to illuminate a number of separate targets.

20. A system as in 1 and 9 with an additional non-optical connection for maintaining alignment by communicating the alignment status of the opposite end of the connection.

21. A system as in any of the above claims utilising a search algorithm to establish the correct source emitter element or elements needed to ensure optical alignment.