WO1988003730A1

WO1988003730A1 - Fibre optic communication system for use in hazardous environments

Info

Publication number: WO1988003730A1
Application number: PCT/GB1987/000774
Authority: WO
Inventors: Roger John Hicks; Leonard Edward Virr
Original assignee: The Secretary Of State For Defence In Her Britanni
Priority date: 1986-11-03
Filing date: 1987-11-03
Publication date: 1988-05-19
Also published as: GB8626224D0; GB2219166B; GB8909039D0; AU8156787A; GB2219166A

Abstract

A fibre optic communication system for communication between a main station and a remote station includes a downlink for communication from the main station to the remote station and an uplink to enable to remote station to communicate with the main station. The downlink includes one or more optical sources (12) at the main station to provide one or more light beams (11) which are modulated to carry a signal which is then transmitted to the remote station and displayed. The uplink is formed by modulation at the remote station of a light beam (2) transmitted from a source (24) at the main station, along an optical fibre (25), to the remote station. The modulated beam is transmitted back to the main station and decoded by comparison with the unmodulated beam. The communication system has zero or very low magnetic and electrical signatures at the remote station.

Description

£ij_£ _2Eti£ Communication System for Use in Hazardous Environmenf-F..

The invention relates to fibre optic communication systems for use particularly, but not exclusively, for communication between amain station and a remote station where it is^'desirable for there to be no electrical or magnetic signals at the remote station. Such situations include the co πunicationbetweenasurface shipand adiver orbetween acontrol station and a person in a hazardous environment.

Fibre optic comπunication systems are well known but in all cases electrical signals are involved in converting acoustic signals to optical signalsandviceversa. Thereare, however, somesituationswherefor safety or other reasons, there is a requirement for a ∞πinunication systemwith no electrical or magnetic signals within a particular area. In potentially explosive atmospheres the major requirement may be the elimination of the possibilityofanelectrical spark occuring, while in other situations, such as in the presence of underwater mines, the prime objective may be the eliminationofanymagneticsignature. Forsecurespeech itisdesirableto eliminate any signals that can be intercepted and decoded.

The object of the invention,is toprovide a systemfor communicating between a main station and a remote station in which any electrical and magnetic signals at the remote station are minimised and in which the possibility of an electrical spark occurring is eliminated.

The invention provides a two-way optical ccmmunications system including: a) a downlink for transmission from a main station to a remote station; and b) an uplink for transmission from the remote station to the main station; wherein the downlink comprises at least one optical source at the main station to provide a light beam, means tomodulate thelightbeamwith a signal, an optical fibre for transmission of each modulated light beam from the main station to the remote station, and means to display the signal carried by the light beam at the remote station; and theuplinkcomprisesatleastonelightsourceatthemainstationfor providing a light beam, an optical fibre for transmission of the light beam from the main station to the remote station, modulationmeans at the remote station consisting of transducer means responsive to an acoustic signal for modulating the lightbeam, means to transmitthemodulated lightbeamviathe same or a second optical fibre back to the main station, signal processing means at the main station to compare the modulated light beam with the unmodulated light beam and to decode the signal carried by the modulated light, and means to display the signal.

The uplink and downlink light beams may be provided by the same or different optical sources.

The uplink and downlink optical sources, the downlink modulation meansandthemainstationsignalprocessinganddisplaymeanscompriseamain station transceiver unit. The remote stationdisplaymeans andmodulation means together comprise a remote station transceiver unit.

Preferablythemeanstodisplaytheuplinksignal atthemainstation comprises means to reproduce the signal acoustically or in some other suitable form. Thus speech at the remote station canbeunderstood at the main station.

Preferablythe transducer means at the remote stationcomprises an optical microphone. The arrangement is such that when a person at the remote station speaks, the microphone modulates the unmodulated lightbeam received from the main station and the modulated light beam is transmitted back to the transceiver unit at themain station. The transceiverunit at the main station then compares the modulated and unmodulated light and processes the optical signal, decoding it and reproducing it acoustically.

Advantageously the source of the unmodulated light is a laser. Preferably there is provided means to produce a collimated beam of laser illumination. Conveniently a laser diode is used as the light source.

Preferably the optical microphone comprises a light-intensity modulating diaphragmarranged such that the unmodulated input light beam is focussed in the plane of the diaphragm by a lens. The diaphragm is reflective and ispreferablysilvered. Anacousticsignalcausespressure changes at the diaphragm which cause it to move. i_ny movement of the diaphragm shifts it away from the focal point of the input lightbeam, thus causing changes in the intensity ofthelight reflectedfromthediaphagmas it is moved out of focus. These changes modulate the reflectedlight beam which is then transmitted back to the main station. The modulation is a functionof thepositionof thediaphragmrelativetothepointof focus of the input light beam.

The intensity modulated light beam transmitted back to the main station is focussed on a light sensitive detector. The output signal from the light sensitive detector is connected to one input of a comparato"r and a signal representative of the intensity of the unmodulated light beam is connected to the second input of thecomparator, such thattheoutputfromthe comparator is a direct analogue of the acoustic signal.

Advantageously the unmodulated beam of light is split by a beam splitter and onepartof thebeam is thenmodulatedbytheacousticsignal, the other part acting as a reference signal. The modulated beam can then be compared to the unmodulated beam and the acoustic signal can be derived from the optical signal.

Thebeam splitter may be either inthemain transceiver unitor inthe remote microphone unit. In the preferred arrangement thebeam splitter is contained in the main transceiver unit. In this arrangement a single, common fibre maybeused to transmit thelaser light tothe remote stationand to return the intensitymodulated lightwhich is reflectedfromthediaphragm back to the main station.

Alternatively the microphone includes two optical fibres placed end to end and arranged such that changes in the alignment of the fibres modulate the optical signal.

The downlink preferably includes an opto-acoustic transducer at the remote station to convert the modulated optical signal sent from the main station into sound so that speech from the main station can be heard at the remote station. Apossibleopto-acoustic transducer is a simplephotophone earpiece.

Where it is difficult to hear the output of an opto-acoustic transducer at the remote station as in a very noisy environment, downlink ccarπmunication other than by voice can be used. In some cases it may be advantageous to use a simple signalling system of lights instead of an acoustic system. Such a signalling systemadvantageouslycomprises one or more light sources at themain transceiverunit eachconnectedvia respective optical fibre cables to the remote station. Ifmore thanone lightsource is used, they are preferably of different colours. In one convenient form, there are three light sources: one green, one yellow and one red. Advantageously information is transmitted to the remote station in the form of colour coded light signals, singly or in coπfoination. For improved recognition the lights are preferably flashed on change of combination.

Advantageouslyamechanical shutterisusedtosettherequiredlight signal cσπfcination in the main station transceiver unit. Alternatively electronic switching of the coloured light sources is possible. External electronic controls may also be used to flash the light output on change of coπbination. Preferably a check of the lights being transmitted is included. This check of the transmitted signal may be made using a beam splitterto reflect someof thetransmittedlightontoadiffuserwhichcanbe viewed by the operator.

The signal displayed at the remote stationmay be in the form of a light display unit where the lights preferably show up on small, easily visible displays.

Conveniently the fibre is ferrule tera nated at the remote station and the transmitted light is transmitted through and internally reflected ontothediffusefaceofa rightangledprism. Thisconfigurationoftheunit at the remote station occupies only a very small space.

Preferably super luminescent LEDs provide the sources of light. Each source is advantageously collimatedbyan aspheric condensing lens and focussed into the corresponding output fibrebymeans ofan achromatic lens.

The ferrule terminations of thefibreopticscarryingtherespective light signals are fixed into position at the remote station such that each fibre end is adjacent to a side face of a 90° prismofwhich theother right angled facehas a lightly diffusing finish. Light emitted fromeach fibre is internally reflected fran the hypotenuse face onto the diffuse surface.

An alternative systemuses an opto-electrical-acoustic earphone to convertoptical signals to sound. Thesoundfromsucha systemcanbeheard even if the environment at the remote station is noisy. These earphones have very low electrical and magnetic signals and can thus be used at the remote station where it is necessary only to elzuninate electrical sparking and/orhighelectrical signals. This systemisnothoweversuitableforuse withmines, wherethemagnetic signaturemustbeas near to zero aspossible. A further possibility for conversion of the optical signal to sound at the remote station is theuse ofopto-acoustic induction inhelium. This could be used in underwater diving applications where helium is used.

Preferably the system configuration is such that the downlink and uplink lightbeams aretransmittedto andfromthe remote stationbymeansofa fibre optic cablehaving an appropriatenumber of optical fibres at itscore. This cable couldbeup to several kilometres longbefore losses of the signal become such that signal boosting is necessary.

In order that the invention may be more fully understood one embodiment thereof will now be described, by way of example only, with reference to the accompanying drawings of which:

Figure 1 shows the basic elements of a communication uplink for a communication system according to the invention; Figure 2 shows comparative voltage outputs at points in the communication uplink shown in Figure 1 for an acoustic signal input; Figure 3 shows the basic elements of a single channel of a communication downlink for the communication system; and Figure^"4 is a schematic block diagram of the configuration of the coπrounication system incorporating the uplink of Figure 1 and the downlink of Figure 3.

Figure1 shows thebasic elements of acommunicationuplink according to the invention including an optical microphone 1. An input beam of light 2, consisting of a narrow collimated beam of laser illumination, is passed through a 50/50 beam splitting cube 3 and part of the beam 2a is transmitted via an optical fibre (not shown) to the remote station. Here it is focussed in the plane of the microphone diaphragm 1 at 4 bymeans of a lens 5. Light reflected from the surface of the diaphragm 1 is collected by the lens 5 and transmitted back to the beam splitter 3 at the main station where it is reflected to a second lens 6 which focusses it in the plane of a light sensitivedetector7. The intensityof thelightatthispoint is a function ofthepositionof thediaphragm1 relativetothepointoffocus 4 of the input beam 2. The diaphragm must be set up very accurately at the focal point of the light to maximise the intensity changes of the reflected lightwhich are very small. A displacement d of the diaphragm 1 normal to its surface, of the order of lum results in a variation of approximately 1% in the level of light detected by the photodetector 7. The electrical signal from the photodetector 7 ispassedthrough afilter 8, amplifiedbyan a plifier9 and is converted to an audio signal at a speaker 10.

The lens 5 and diaphragm1 are located within a transceiver at the remote station, whilealightsource (notshown),- thebeamsplitter3 andthe signal processing electronics are located in a main station transceiver. Thelightsourcecanbe alaser or alaserdiode. Alaserdiodewithapower output of the order of litW is suitable. It will be obvious that any microphone which converts sound signals to optical signals can be used.

Figure 2 shows the variations in voltage outputs for the system in Figure 1 for changes in pressure at the diaphragm 1. Graph (a) shows the pressurechangesatDcausingthediaphragm1tovibrateinresponsetoasound signal P(t) . The variation in the voltage output from the detector 7 at P about themean level <V>, thevoltage signal fromtheunmodulatedlightbeam, is adirect analogueof the sound signal atDandis shown ingraph (b). The output at P is filtered bythe filter 8 to remove thebasevoltagesignal <7> andgives anoutputatFas shown inGraph (c) . This signal isthenamplified and converted to sound.

Figure 3 shows the basic elements of a single channel of a downlink whichtransmits informationto the remotestation intheformofcombinations of colour-coded light signals. Such a downlink is of particular use where theremote station is inaverynoisyenvironmentsuchthathearingtheoutput of an acoustic earphone isverydifficult. Asinglechannel canbeprovided for Morse code or other similar signalling or a simple code of two or more coloured light signals can be used. A versatile but still very simple version has 3 coloured lights of red, yellowand green respectively. In the signalling downlink, a beam of light 11 from a super luminescent LED 12 is collimated by a lowf/number (ieabout 0.8), colliπvating, aspheric lens 13. Thecollimatedbeamoflightisthencoupled intotheoptical fibre14bymeans of an F/2.8 achromatic lens 15.

At the remote station end the fibre 14 is ferrule terminated 16 and thelight is transmitted into aside face ofa rightangleprism17. Thebeam is reflected fromthehypotenuse face of theprism17 onto itsother sideface which has a lightly diffusing surface 18 where it can be viewed. Mechanical shutters 20 areinterposedbetweenthelenses13 and15 to shut off the beams 11. Each shutter 20 on the respective channels of the downlink can be used to set the required light signal combination.

The light beam 11 passes through a beam splitter 21 before passing through the optical fibre 14. This reflects some of the transmitted light onto a diffuser 22 which can be viewed by an operator 23. In this way the operator 23 can check the transmitted signal.

Ihe LED 12 can be coloured if required or the light can be passed through a coloured filter (not shown) to provide a coloured light signal. Another system which is not shown in the drawingswouldbe for a single light source tobe split into 2 or morebeamswhich arethenpassedthroughcoloured filters to provide coloured light signals.

Figure 4 shows the configuration of thewhole comπunication system. The uplink (Fig 1) and downlink (Fig 2) light sources, optics and signal processing electronics are included in a main station transceiver unit with optical fibres connected to a remote station transceiver unit. The main station transceiver unit has three main functions:

1) thesupplyandcoding ofalightdisplayfor avisual downlink, or the supply of a speech modulated light source for an acoustic downlink;

2) the supply of a light source for the uplink microphone;

3) the detection of the intensity modulated light signal derived f omthemicrophonediaphragmand its conversion to an audio signal. This is carried out by the signal processing electronics. The remainder of the electronicsdrive theLEDs, lasers, laser diodes or other light sources as required.

In the uplink (Fig 1) a laser light source 24 provides the beam of light 2 which ispassed throughthebeamsplitter 3. Fromthebeamsplitter 3 part of the beam 2 is transmitted down an optical fibre 25 by means of an optical coupling unit 26. The fibre25 is connectedby a connector 27 at the remote station to a microphoneunit 28, Themicrophone unit 28 contains the focussing lens 5, focussing the receivedlightbeam 2 on tothediaphragm1 as shown in Fig 1. The modulated beam reflected from the diaphragm 1 is transmittedback tothemain transceiverunitthroughtheoptical fibre25 and is passed together with the unπϋdulated portion of the beam 32, to the light sensitive detector 7 where the optical signal is converted to an electrical one. The electrical signal is thenpassedthrough a filter 8 andamplifier9 and is converted to an audio signal by a speaker 10.

C rmunicationfromthemain stationtotheremotestation isbymeans ofa threechannel downlink, onechannelofwhichisshowninFigure3. Three light sources 12providebeams oflight11whicharethen transmittedor shut offbya signal coding unit 29 toprovideacodedlightcombination signal to theremotestation. Thesignalcodingcanbeachievedbymeansofmechanical shutters as shown in Fig. 3. Alternatively the light sources could be switched on and off as required. When the signal is changed its noticeability is increased if the light is flashed. This can be done by momentarily increasing the intensitybyopeningandclosingtheshutterorby an appropriate switching waveform applied to the light sources. A connection 33 from the signal coding unit 29 can be used to activate electronicswitching (notshown) toflashthelightsource12whentheshutter 20 (Fig. 3) is opened. Each beam 11 is passed down a respective optical f e30 totheremotestationviatheconnector27. Thelightcombinationis displayed on a light displayunit31which is visible at the remotestation.

Thus cαununication can be.carried outbetween a main station and a remote station where the systemat the remote stationis totallyoptical and no electrical or magnetic signals occur.

If a voice downlink is required a photophone earpiececanbeusedto convert optical signals to sound. The light beam shines onto an absorber mediumwhich produces movementof adiaphragmand hence an acoustic signal. Such a photophone earpiece has been described by Bell Laboratories. The photophone earpiece uses charred cotton fibre as the absorber medium and a laser light source is used to drive thephotophone. This may be a Helium- Neon laser or an Argon-ion laser, but is preferably a laser diode.

In situations where low magnetic and or electrical signals are acceptable an opto-electrical-acoustic earpiece can be used to convert the optical signals to sound at the remote station. ϊwo alternative downlink systems are suitable for use. In the first, a laser beamtransmitted from the main station down an optical fibre drives a laser diode at the remote station end. Speaking into the microphone at the main station creates an electrical signal which is used to modulate the lightbeamwhich drives the laser diode. This produces a very lew electrical signal which drives a transducer so thatthesoundcanbeheard. Thesecondsystemuses lesslaser power at themain station andneeds abatterypower source at the transducer. Because of this, it is not suitable for underwater use, but is useful for surface applications.

A photocell which converts light to e f can be used in an opto- electrical earpiece in either of these systems. The systems consist of a source of speech-modulated lightwhich is transmittedbymeans of an optical fibre to theopto-electrical earpieceunit. This comprises a conventional electrical earpiececonnectedtoaphotodetectortoconvertlighttoemf. The photocell may be a silicon photodiode. The most appropriate operating condition of such a diode is photoamperic operation, and a passive system without an amplifier is preferable.

The system can provide secure or safe communications, as it incorporates a totally optical solution at the remote station. For use in diving applications, the use of optical fibres increases the information carrying capacity of the cables thus saving on size and weight. Optical systems have good noise immunity from other signals and are secure from outside interference and interception.

Claims

£L Ii_£

1. A two-way optical cα mnications system including: a) a downlink for transmission from a main station to a remote station; and b) an uplink for transmission from the remote station to the main station; wherein the downlink comprises atleastoneoptical source12 atthe main stationtoprovide alightbeam11, means29 tomodulatethelightbeam11 with a signal, an optical fibre 30 for transmission of eachmodulated light beam 11 fromthemain station to the remote station* and means 31 to display the signal carried by the light beam 11 at the remote station; and theuplink comprises atleastonelightsource24 atthemainstation forprovidingalightbeam2, anoptical fibre25 fortransmissionofthelight beam 2 from the main station to the remote station, modulation means at the remote station consisting of transducer means 28 responsive to an acoustic signal formodulatingthelightbeam2, meanstotransmitthemodulatedlight beamviathesame or asecondopticalfibre25backtothemainstation, signal processing means 7 at the main station to compare the modulated light beam with the unmodulated light beam and to decode the signal carried by the modulated light, and means 10 to display the signal.

2. Anoptical communications systemaccording toclaim1wherein themeans 10 to display the uplink signal at the main station comprises means to reproduce the signal acoustically or in some other suitable form.

3. Anoptical communications systemaccordingtoclaim1orclaim2wherein the transducer means 28 at the remote station comprises an optical microphone.

4. An optical coi tunications system according to claim 3 wherein the optical microphone 28 comprises a light-intensity modulating diaphragm 1 arranged such that theunmodulated input lightbeam is focussed in theplane of the diaphragm 1 by a lens 5.

5. An optical communications system according to claim 4 wherein the diaphragm 1 is reflective.

6. An optical communications system according to claim 3 wherein the optical microphone 28 includes two optical fibres placed end to end and arranged such that changes in the alignment of the fibresmodulate the input light beam.

7. An optical communications system according to any one of the previous claims wherein the downlink includes an opto-acoustic transducer at the remote station to convert the modulated optical signal sent from the main station into sound.

8. An optical communications systemaccordingtoclaim7 whereintheopto- acoustic transducer is a photophone earpiece.

. An optical communications system according to any one^"of. claims 1 to 6 wherein the downlink comprises a signalling system of lights.

10. An optical communications system according to claim 9 wherein the signalling system comprises one or more light sources 12 at themain station unit each connected via respective optical fibre cables 30 to the remote station.

11. An optical communications system according to claim 9 or claim 10 wherein the information is transmitted to the remote station in the form of colour coded light signals, singly or in combination.

12. An optical communications system according to claim 11 wherein the lights 12 are flashed on change of signal.

13. An optical communications system according to any one of claims 1 to 6 wherein the downlink includes an opto-electrical-acoustic earphone to convert optical signals to sound.

14. An optical communications system according to any one of claims 1 to 6 wherein the downlink includes means for conversion of the optical signal to sound at the remote station which uses opto-acoustic induction in helium.

15. An optical communications system according to any one of the previous claims wherein the systemconfiguration is such thatthedownlink anduplink lightbeams aretransmittedtoand fromtheremotestationbymeansofafibre optic cable having an appropriate number of optical fibres at its core.