IEE ELECTRONICS DIVISION: CHAIRMAN'S ADDRESS

Making measurements with lightProf. D.E.N. Davies, D.Sc, F.I.E.R.E., F.I.E.E., F.Eng.

Indexing terms: Optics, Optical fibres. Measurement and measuring, Instrumentation and measuring science

Abstract: The last decade has seen rapid developments in optical sources and low-loss optical fibres aimed atwide-bandwidth communications. The availability of these components has also encouraged the developmentof special forms of optical data highways and optical-fibre sensors as byproducts of this work. Severalinternational groups are currently working on such sensors, capable of measuring strain, temperature,acoustic signals, electric current and rotation. Examples of various types of sensor based on both coherentand incoherent light are discussed. Coherent devices have the potential for highly sensitive measurements butare often limited by problems of extraneous pick up. The optical-fibre gyroscope looks most promising as anew form of rotation sensor which should be capable of simple construction in integrated optic form.

1 Introduction

In 1966, a paper by Kao and Hockham [1] proposed the useof light propagating in thin fibres of glass as a form of wide-band electronic communication. Its viability was then limitedby both the lack of suitable optical sources and the highattenuation of light in glass fibres. Subsequent developmentsin lasers and the fabrication of low-loss fibres have trans-formed the situation, and the last decade has seen rapiddevelopments of various forms of optical-fibre communicationsystems.

The international race to produce improved fibres and low-loss glass has occasionally resembled an 'optical gold rush'. Theresults have been impressive. The figures for minimumattenuation in fibres have changed from about lOOdB/km in1966 to 20dB/km in 1970 and less than 0.25 dB/km in 1980[2].

The potential goal of all this effort is low-cost wide-band-width communications capable of handling the trunk routerequirements of PTT organisations throughout the world.However, despite the rapid technical progress, investment insuch systems still seems rather modest. This has encouragedseveral other applications of fibre optics aimed at specialisedcommunication systems and, more recently, for various formsof sensors. Such 'spin-off applications can capitalise on therange and performance of fibre-optic components beingdeveloped for PTT use.

This paper reviews these new developments and presents anumber of examples of communication systems and sensorscurrently under study. Several of the examples are taken fromresearch programmes at University College London as illus-trations of the work in progress by research groups in variouscountries. Alongside these developments in sensors andcommunications, there is growing interest in the use ofintegrated-optic techniques for the fabrication of a wide rangeof light-guiding components and optical-processing subsystems.This has led to predictions of the 'glass revolution' ultimatelyreplacing the 'silicon revolution' within the next decade. Amore realistic approach is probably to note the complemen-tary nature and interface compatibility between integrated-circuit and integrated-optic technologies. Their combinationwill represent a particularly powerful force for signalprocessing and communication.

2 Specialised optical-fibre communication systems

The availability of optical-fibre components has resulted in therapid development of a number of short-range special-purpose

Paper 1595A, delivered before the IEE Computing & Control Division,6th October 1981Prof. Davies is with the Department of Electronic & ElectricalEngineering, University College London, Torrington Place, LondonWC1E7JE

optical-fibre data links. These look particularly appropriate foraircraft and even satellite telemetry systems where theattraction of the glass fibre relates to its light weight and itsinterference-free and low-cross-talk performance. Applicationsin the chemical, petrochemical and process control industriesare now making use of fibre optics to achieve intrinsically safecommunication and telemetry systems where the electricallyinsulating nature of the link makes it more easily compatiblewith intrinsically safe designs. For military systems the factthat such lengths do not radiate stray signals makes themappropriate for highly sensitive and secure links. In electricallynoisy environments the fibre-optic link is particularlyattractive, since it is totally immune to conventional inter-ference from electric and magnetic fields. The provision ofelectrical isolation along the link and the removal of earth-loopproblems can also have advantages.

In order to meet some of the above requirements, opticaldata highways have been developed using fibre-optic T-junctions and star couplers [3]. This enables input signals toaddress multiple destinations subject to an appropriate form oftime or space multiplexing. Such schemes may be designed forwide-bandwidth or high-integrity applications where additionalredundancy may be incorporated, in order to continueoperation under various fault or damage conditions.

Many variants of such systems have been described in theliterature and can offer 2-way wide-bandwidth (lOOMbit/s)exchange of data between terminals. In such cases theattentuation of the fibres is not a dominant factor whencompared with the corresponding losses in T-junctions andconnectors. These losses tend to limit the maximum numberof access points in any such highway. Multimode fibres of stepindex or graded refractive-index profiles are, in general,adopted since they are compatible with the use of fairly simpleconnectors, and the ultra-low-dispersion performance of single-mode fibres is rarely a requirement for such data highways.

In the longer run, single-mode fibre highways will probablyemerge. Apart from the improved dispersion, they will be ableto benefit from a range of integrated-optical componentswhich are currently being studied in university and industriallaboratories. These components can include directionalcouplers, switches, modulators and spectrum analysers basedon Bragg diffraction principles. The potential fabricationproblems for such systems are being eased by the growinginterest in longer-wavelength systems operating in the infra-redband.

2.1 An experimental telemetry highwayA novel form of optical-fibre telemetry link was developed atUCL in the mid 1970s arising out of studies of the effects ofmechanical stress on optical fibres. Work started when aresearch student examined the effect of longditudinalmechanical strain on coherent light propagating in a single-

16 0143-702X/82/010016 + 08 tOl.50/0 IEEPROC, Vol. 129, Pt. A, No. 1, JANUARY 1982

Fig. 1Fig. 1 Optical-fibre phase modulator comprising a cylindrical PZTtransducer with a length of plastics-coated optical fibre wrapped aroundits circumference

mode optical fibre. It was shown that such a strain producedan electrical phase shift of the optical wave, linearlyproportional to tension [4]. This was due to three separateeffects:

(a) a change of fibre length(b) a change of refractive index(c) a change of fibre diameter.

The above effects relate to corresponding changes in thecentral core of the single-mode fibre. The dominant effect inthe above example is that due to change of length, followed bythe refractive-index effect. In most instances the change ofdiameter can be neglected.

This work resulted in the production of a simple form ofoptical-fibre phase modulator by attaching a small piece ofpiezoelectric material to a fibre and electrically exciting thecrystal to modulate the longditudinal mechanical strain of thefibre. One such design of modulator involved wrapping severalturns of fibre around a cylinder of piezoelectric material asshown in Fig. 1. The above effect is very sensitive and suchphase modulators can produce several radians of phase shiftper turn of fibre for moderate drive levels. A graph showingthe overall theoretical sensitivity of this effect is given inFig. 2.

The work on phase modulators led to the development ofan optical-fibre data highway [5]. This was produced byattaching a series of such modulators at points along the lengthof optical fibre. At each input, data was modulated onto asuitable subcarrier, and this modulated signal was fed to thepiezoelectric modulator in order to phase-modulate the opticalsignals on the fibre. The process was repeated for variousdifferent subcarriers at the different input ports along thefibre. This results in a frequency division multiplex telemetryhighway taking signals from multiple input points to onedestination.

An attractive feature of this concept is that it allows signalsto be coupled into the highway without the need to break orjoin the fibre and without the use of a separate optical sourceat each data input. A further advantage is that the use of amodulator to input the data does not produce any insertionloss in the fibre path. The system is, however, restricted toone-way operation.

Fig. 3 shows a schematic representation of such a highwaybased on single-mode fibre. The output from the laser iscoupled into the single mode fibre but a fraction of it isseparated off and subjected to a frequency shift in a Bragg cell.This reference signal is then used as an optical local oscillator.

The output from the main fibre (containing the multiplexeddata) and the local oscillator signal are combined together on aphotodetector. The resulting output is an IF signal containingthe frequency multiplexed signal from all the input datasources. This may be demodulated by conventional techniquesof a frequency discriminator followed by demultiplexing filtercircuits to recover the original data.

Experimental schemes based on this concept wereconstructed and evaluated. They provided a high-dynamic-range system but a limitation related to the use of single-modefibres, resulting in a high degree of mechanical precisionneeded for coupling light into the fibres. A modified version ofthe data highway was therefore developed based on multimodefibre [5] and this is shown in Fig. 4. The multimode schemeuses low-moded multimode fibre in which the effect of thephase modulation on the various modes propagating in thefibre results in a small degree of envelope modulation on theresultant output signal. This can be easily detected andprovides a very simple form of data highway although it doesnot achieve the linearity or dynamic range of the single-modesystem. An improved experimental scheme based on themultimode-fibre data highway has been developed jointlybetween UCL and BICC and is now being engineered andmarketed by BICC for applications associated with process-control industries [6].

Modulation frequencies for such data highways can rangefrom tens of kilohertz up to about 100 MHz. Phase modulatorscan also be designed which attach a piezoelectric crystal to theoutside of an optical fibre so that an acoustic wave passingthrough the fibre modulates the refractive index of the glass

1.2

1.0

0.8

o 0.6

0.4

photoelastic

-8 -6 -U -2longitudinal strain €(

tension

/ -1.2

Fig. 2 Theoretical phase retardation of HEU mode in single-modefibre as a function of axial strain

Interaction length L = lcm; mean refractive index n = 1.5101; wave-length A. = 0.6328jim; core radius a = 0.8Mm; normalised frequency^=2.01 Poisson's ratio i> = 0.2; effective photoelastic constantPel = 0.2; breaking strain 21 10 "2

IEEPROC, Vol. 129, Pt. A, No. 1, JANUARY 1982 17

and' hence achieves the appropriate phase modulation. Insuitable cases such a modulator may be clamped around theoutside plastic sheathing of the fibre. If this is used with thickplastic coatings it does restrict the maximum modulatingfrequency. In the case of the BICC system, modulatingfrequencies in the region of 1 MHz are adopted because of thenarrow band channels required for this telemetry application,

but laboratory data highways providing wide-bandwidthchannels up to TV capability have also been demonstrated.

An early experimental form of modulator located thepiezoelectric element within the jaws of a conventional clothespeg which could be clamped around the outside of a fibre inorder to modulate the optical signal. Fig. 5 shows a form ofmodulator engineered for wide-bandwidth operation. The

subcarrieroscillator

laser

ntegrator

Fig. 3 Schematic diagram of a two-fibre optical information highway incorporating single-mode fibres

subcarrieroscillator 1 1

datasource

laser

- f ,

optical fibre

clip-on acoustictransducer \ * fn

output 1

Fig. 4 Data-highway concept using a multimode fibre and homodyne detection

Fig. 5 Early experimental optical-fibre modulator which can attacharound the outside of a plastics-coated fibreAcoustic waves are focused to the location ofthe fibre and modulatethe refractive index of the core

Fig. 6 Clip-modulator assembly designed for use with a fullyprotected and cabled optical fibre

The cable passes along the centre of the component in the backgroundand the piezoelectric modulator disc (on top of the box in the fore-ground) is simply screwed in place over the cable fibre

18 IEEPROC, Vol. 129, Pt. A, No. 1, JANUARY 1982

Fig. 7 Photograph of an optical-fibre hydrophone consisting of anumber of turns of single-mode optical fibre

plastics-clad fibre is clipped into the unit. Fig. 6 shows a viewof the latest form of modulator used for the BICC system.This achieves modulation through the thick plastics claddingof a fibre designed for use in an industrial environment.

3 Optical-fibre sensors

3.1 Coherent sensorsThe operation of the above telemetry link has shown thatcoherent light in optical fibres can be used as a sensor since itis able to detect the acoustic signals injected into the fibre bythe piezoelectric transducers. Attention was drawn to this factin 1975 by a brief paper which indicated how a length ofoptical fibre was sensitive to mechanical strain and tempera-ture [7]. The high sensitivity of coherent fibre optics tomechanical and thermal effects has since attracted wideattention, and several authors have described many types ofsensor based on the change of phase in the fibre [8—11]. Mostsensors have used HeNe lasers as the optical source because oftheir high coherence and modest cost. Improvements in theperformance of CW solid-state lasers have now made thesedevices contenders for a range of sensor applications, althoughtheir noise levels are still rather high.

A range of such fibre sensors have subsequently beenstudied at University College but the effort has been smallcompared with the international growth of interest in thistopic, particularly in the USA where substantial research teamsnow exist at the Naval Research Laboratories, Washington,TRW, Rockwell, McDonnel Douglas, JPL and StanfordUniversity.

A m s in tjL»

One of the first areas of interest related to the study ofhighly sensitive hydrophones for application to sonar.Calculations have indicated that such optical-fibre hydro-phones can, in principle, offer an extremely high sensitivity[8] and coils of fibre can be configured to form variousconvenient shapes to achieve desired forms of directionalpattern. Such hydrophones in general use single-mode fibresbut work has also shown that multimode systems can be usedat the expense of sensitivity and linearity [12, 13].

Fig. 7 shows a photograph of an experimental hydrophoneconsisting of several coils of single-mode fibre located in awater tank, and Fig. 8 indicates a measured polar response forsuch a configuration. Fig. 9 shows a schematic diagram of suchan optical-fibre hydrophone sensor based on the use ofsingle-mode optical fibre and a coherent optical referencesignal derived via a Bragg cell. Many variants on this schemeexist, but this is a convenient arrangement since the phasemodulation is translated to IF for demodulation. The phasedeviation is proportional to the applied acoustic signal so adiscriminator plus integrator can demodulate the signal. In thecase of small phase deviations (less than about 0.1 rad) it maybe more convenient to use a form of phase-sensitive detector.

The sensor coil shown in Fig. 9 may also be used to detect

Fig. 8 Polar diagram of experimental optical fibre hydrophone asshown in Fig. 7

Diameter =100 mm; frequency = 15 kHz

T"acoustic wavef ront

laser -•" photodiode

beamcombiner

output

integrator

limiter and FMdiscriminator

IFamplifier

phase demodulator

Fig. 9 Schematic representation of optical-fibre hydrophone employing single-mode optical fibre plus a reference fibre used to complete the inter-ferometer

In some instances the reference fibre may be omitted and a short air path used for the reference beam

IEE PROC, Vol. 129, Pt. A, No. 1, JANUAR Y1982 19

changes in temperature, pressure or strain as well as acousticsignals. The subtleties of design will in general require aconfiguration which accentuates the wanted measurement andminimises or cancels the unwanted effects. In the case of theacoustic sensor it is usual to remove the slow changes of phaseshift due to thermal and stress effects by filtration. It is there-fore far easier to design such sensors to measure AC-typesignals. It can be particularly difficult to distinguish betweenwanted 'DC changes due to strain (in a strain-gaugeapplication) and corresponding phase changes due totemperature drift.

One important problem in all such sensors is that of leadsensitivity. It is necessary to ensure that the change of phase inthe optical path is that due to the stimulus being measured atthe sensor coil and not to background vibrations or changes oftemperature along the optical-fibre leads to or from thesensing element. In some cases it may be necessary to havelong fibre leads to the sensor since it may need to be locatedin a hazardous environment. This can therefore be aparticularly troublesome problem and a number of interestingapproaches to it are currently being investigated [14].

In the same way that it was possible to construct a multi-mode form of data highway, as shown in Fig. 4, we canconstruct a multimode sensor in which the effect of thestimulus is to apply different amounts of phase change to thedifferent modes propagating in the fibre, resulting in a changein amplitude at the output. This has been termed a fibredynesensor. Variations in mode propagation and the relative phaseof the mode components due to background variations oftemperature and fibre bending give rise to amplitude fluctu-ations of the output. This means that such multimode sensorsare currently only suitable for AC-type measurements suchas for vibration or acoustic signals. Although a number ofacoustic sensors have been constructed in this way, they haveso far been insensitive with a rather limited dynamic range,but the use of multimode fibres makes them far simplermechanically [12, 13, 15].

acoustic wavefront

multimode fibre-| photodiode |

-—-—I amplifier |-i

Fig. 10 Schematic diagram of optical-fibre hydrophone using multi-mode optical fibre

Demodulation of the resultant signal arises owing to the fibredyneeffect. Phase modulation of the constituent modes in the fibre resultsin a composite signal illuminating the photodiode containing a smallamount of envelope modulation in sympathy with the acoustic signal

Fig. 10 shows a simple schematic diagram of such a multi-mode fibre sensor. One example of this type of device is aflowmeter [16]. This measures fluid flow by detecting theflow noise as an acoustic signal and measuring its RMS value.This type of system has the advantage of simplicity plus theusual attractions of fibre optics, but its accuracy is ratherlimited.

To measure unidirectional changes of parameters such asstrain or temperature we need to detect changes in opticalphase shift. The simplest way is to set up an optical interfero-meter and to introduce the sensor coil into one arm as shownin Fig. 11. The relation between phase shift and temperature isgiven by eqn. 1

A0 = —2nL

where

a = linear expansion coefficientL = interaction length

— = refractive index/temperature coefficientoT

n = mean refractive index of coreXo = free-space optical wavelength

3/3— = rate of change of propagation constant with core radiusoa

Any change of phase in one arm of this interferometer givesrise to an optical-fringe shift at the photodetector. It isobviously possible to count fringes in order to measure phasechanges, but it is necessary to employ a small array of photo-detectors in order to determine the direction of movement ofthe fringes and also to interpolate between fringes for accuratemeasurement. A simple thermometer based on the above

single-mode fibre

\

splittersensor element(single-mode fibre)

array ofphotodetectorsto measurefringe movement

Fig. 11 Optical-fibre sensor based on simple interferometer in whicha change of phase (due to temperature, strain etc.) results in a phase orfringe shift on the photodector output

mass suspended between twofibres intension (axis offibre vertical)

rigid frame

Fig. 12 Balanced bridge interferometer scheme containing two fibresA and B shown as an accelerometer

Any acceleration of the structure in a direction parallel to the twotensioned lengths of fibre results in an increase of phase shift in onefibre and a corresponding decrease in the other. By using the two fibresin the balanced arms of a bridge, the resultant phase shift will bedoubled, but unwanted pick up can be partially cancelled

20 IEEPROC, Vol. 129, Pt. A, No. 1, JANUARY 1982

principles has been constructed at UCL [17] and provided asensitivity of about 85 rad/deg C per metre of fibre. The lowthermal inertia of a fibre thermometer should also offer a goodhigh-frequency performance. The optical-fibre thermometer ispotentially an extremely sensitive device [18], but again wefind that the limit of performance is set by unwanted phaseerrors and pick-up effects in the arms of the interferometer.

A scheme designed to cancel out some of these unwantedeffects is indicated in Fig. 12 which represents a proposedform of accelerometer sensor based upon optical fibre strainmeasurements. Here we have a mass supported verticallybetween two fibres which are maintained in tension. Anyacceleration in the direction parallel to the fibres will result inan increase of tension for one fibre and a correspondingdecrease for the other. If we now use these two fibres as thetwo arms of an interferometer, changes in temperature orpressure will not produce a resultant phase shift provided thatthe temperature environment of the two fibres is identical.However, the application of an acceleration will produce + 0in one fibre and —0 in the other giving a resultant phase shiftof 20. The limit of sensitivity is set by the severe problems ofphase balance between the two fibres in terms of opticalperformance and environmental conditions.

If a fibre of length L is subjected to an external strain S itslength changes by SL and its phase by A0 where A0 is givenby eqn. 2.

2-nSLn

2 nn da

n-— Oi2 -v\pn

(2)

where pn and pn are photoelastic constants and v = Poisson'sratio.

The ultimate limitation on accuracy of phase measurementin an optical system is set by noise and by bandwidth, butfigures in the region of 10~7 rad are feasible. This correspondsto an accuracy of phase measurement which is several ordersof magnitude better than the errors and drift due to theproblems of system balance and environmental effects. Clearlythis is an area where there is considerable scope for novelschemes aimed at minimising such unwanted effects.

3.2 Noncoheren t op tical-fibre sensorsAlthough fibres carrying coherent light offer extremesensitivity, in many situations this is an embarassment owingto pick-up effects plus the accuracy and alignment problems ofsingle-mode fibres. Some effort has therefore been directed atsensors which use multimode fibres (capable of jointing withstandard fibre couplers) together with simple optical sourcessuch as the LED. The latter have a low temporal coherenceand emit a noise-like spectrum covering approximately a 5%bandwidth.

Perhaps the simplest form of noncoherent sensor is onewhich detects a break in the fibre as a one-off indication thata given strain has been exceeded. Noncoherent sensorsgenerally depend on changes in the total optical power-transmission properties of the fibre in response to eitherchanges in bending loss [19] or changes in mechanicalalignment of fibre to fibre couplings. A commercial device usesa pressure sensing diaphragm to amplitude modulate the light[20] and Fig. 13 shows a form of microphone which is basedon modulated bending loss in multimode fibres.

An interesting example of a sensor dependent ontransmission loss is one designed to measure the concentrationof oil droplets in water [21]. In this case the transmission

properties of a length of bare fibre are modified by thenumber of droplets of oil in contact with the outside of thefibre core. By correct choice of core refractive index, the lightis allowed to escape at the location of the droplets owing tothe difference between the relative refractive indices of oil andwater.

Optical fibres can also represent a transmission medium toand from a remote sensing element. For example, liquidcrystals can change their reflectance with temperature. Thissensor can use fibres to carry the incident and reflected lightbetween the sensing element and the control electronics [20].

sound

light in modulated^

light out

fibre

Fig. 13 Representation of a simple optical-fibre microphone whichoperates by modulating the bending loss of a fibre mounted betweentwo corrugated surfaces

3.3 Fibre current sensors based on the Faradaymagneto-optical effect

Although fibres are in general free from unwanted pick upfrom electromagnetic fields, it is possible to use the Faradaymagneto-optic effect to detect magnetic fields and hence tomeasure current. When linearly polarised light passes throughan optically transmissive medium, the presence of a magneticfield will cause a rotation of the plane of polarisation. Therotation b'f is proportional to the line integral of the field Halong the propagation path h.

Sf = VfH-dh (3)

where Fis the Verdet constant of the material.The effect is very weak so that normal optical fibres are not

affected by stray magnetic fields.This concept has been used by Rogers and Smith to develop

a current-measuring system for the CEGB [22, 23]. The basicarrangement of the scheme is shown in Fig. 14. The light froman HeNe laser is linearly polarised and launched into a lengthof single-mode fibre which is wound round the current-carrying conductor. A Wollaston prism and a pair of photo-detectors are used to measure the polarisation rotation. Anexperimental system for field trials on a 22 kV conductor isbeing constructed at CERL.

One limitation in the above concept is the unwantedvariations of the plane of polarisation due to combinations ofimperfections and birefringence in the fibre, coupled withchanges of temperature. This problem can also arise in otherforms of sensor involving coherent light. The effect can be

stripping ofcladding modes

H--0" helium-neonlaser

/ polarisermicroscope objectives electronics

single-modelusbar optical fibre Wollaston

prismH2—

busbar current detector output

Fig. 14 A device for the measurement of current using single-modeoptical fibre and the Faraday effect

IEEPROC, Vol. 129, Pt. A, No. 1, JANUARY 1982 21

reduced by very careful control of the fibre fabrication [24].Extensions to the above measurement system have also beenproposed for the combined measurement of current andvoltage by a combination of the Faraday effect for current andelectrogyration effect for voltage [22].

4 Optical-fibre gyroscopes

In 1976, Valli and Shorthill showed that it was possible touse the Sagnac effect to produce a gyroscope for detectingrotation, by employing a length of optical fibre carryingcoherent light as the sensing element [25]. Since then, severalgroups in the USA and Europe have worked on various formsof optical-fibre gyroscope. The basic principle of the systemcan be understood from the diagram in Fig. 15. Light from an

N turns of single-modeoptical fibre

Fig. 15 Fibre-optic gyroscope employing the basic form of Sagnacinterferometer

optical source is split into two beams and focused into the twoends of a loop of optical fibre. The light propagates throughthe fibre and, emerging from the two opposite ends, tra-verses the beam splitter so that light from both beams formsan interference pattern on a photodetector.

From the symmetry of the situation the two path lengths inopposite directions around the loop must be electricallyidentical, and the pattern of the detector corresponds to a setof circularly symmetric stationary fringes. When the overallsystem is subjected to rotation, one end of the fibre isapproaching the propagating light while the other is recedingfrom the light propagating in the reverse direction. This causesan effective path difference between two directions ofpropagation which corresponds to a phase shift between thetwo waves. This leads to a fringe shift proportional to rotation.The analysis of the situation poduces the relationship shown ineqn. 4. The rotation rate Q can be seen to be proportional tothe phase difference Ad, as measured by the fringe shift:

AflApC

SirNA(4)

where TV = number of turns of fibre, A = area of coil.This type of gyroscope gives a senstivity which is dependent

on fibre length, the area enclosed by the loop and the opticalwavelength. The sensitivity is currently less than that achiev-able by laser gyroscopes where the optical path is part of thelaser cavity, and this latter system gives rise to a frequencyshift proportional to rotation rate. Despite the reduced sensi-tivity, optical-fibre gyroscopes look attractive for a range ofapplications. This is due to the potential simplicity ofmanufacture, particularly when the components can be madein integrated-optic form.

In practice, there are a number of factors which limit theideal performance of an optical-fibre gyroscope. These includethe reflections from the ends of the fibre, coupling betweenthe two possible modes of polarisation, backscatter effects and

various forms of noise [26]. A range of schemes is currentlybeing investigated to minimise these unwanted effects andimprove overall sensitivity. Some of the best results to date arethose reported from Stanford University where their gyro-scope employs 600 m of 4 jxm core fibre and uses the type ofwrap-round fibre phase modulator developed at UCL toachieve an output at an intermediate frequency. Their resultscorrespond to an RMS noise level of about 1 deg/h [27, 29].

Work by Ulrich at Hamburg has demonstrated the use ofa depolariser plus a superluminescent diode as an opticalsource for a gyroscope [28]. The identical path length in bothdirections round the ring leads to a relaxation in the coherenceconstraint in the optical source compared with other coherentsensors. There are nevertheless problems in coupling efficienciesbetween single-mode fibres and these sources.

University College is working on a modified form of Sagnacinterferometer which provides separate output of the twocounter-rotating beams at an intermediate frequency [26].This has the advantage of improving noise performance andthe configuration does not affect the basic symmetry of theSagnac ring. This system looks particularly attractive forresearch into the operation of gryoscopes since it offers thefeature of separately studying the signals from the twodirections of propagation.

It is interesting to note that the optical waves in all gyro-scopes will suffer substantial phase errors due to backgroundchanges of temperature, vibration and even ambient acousticlevel, and these changes will in general be far larger than thoseof the wanted signal. However, the symmetrical design of theinterferometer ensures that all such effects occur to an identicalextent in both counter-rotating beams, and may thus becancelled out. In practice, several second-order effects preventthis ideal situation being achieved. Improvements inperformance are therefore bound up with minimising thesesecond-order effects. Calculations indicate that there is adifference of over two orders of magnitude between thetheoretical performance limits and current results. Clearlythere is ample scope for further work.

Mention has been made earlier of the possibility of usingoptical-fibre strain-gauge measurements to make an accelero-meter. It is interesting to note that a combination of a 3-axisoptical-fibre accelerometer plus a 3-axis optical-fibre gyro-scope offers the potential for a cheap self-contained inertialnavigation system fabricated in integrated-optic form.

5 The future

Although the field of optical-fibre sensors has developed fromthe technology of wide-bandwidth optical-fibre com-munications, it is already seen as offering a range of newmeasurement capabilities from industrial instrumentation tonavigation systems. The continuing international research anddevelopment effort on optical sources and fibres will furtherextend the range and performance of sensors for many years[17]. In the longer run, it is the combination of displays,optical communications, sensors, integrated optics and opticalsignal processing that will make the running and willultimately lead to all-optical systems.

The future interrelation between microelectronics andoptical technology will be particularly interesting. The sub-stantial growth and advantages of digital electronics are notlikely to give way easily to an optical technology which isprincipally analogue in character. Perhaps the key factor is thecompatibility of the two technologies, since the optical sourcesand detectors in general interface easily to electronic systems.This suggests that it will be the optimum combinations ofthese technologies that will represent the cost-effectivesolutions.

22 IEEPROC, Vol. 129, Pt. A, No. 1, JANUARY 1982

6 Acknowledgements

The author would like to thank many colleagues at UniversityCollege London and elsewhere who have helped in thepreparation of this review. Particular mention should be madeof Dr. Brian Culshaw who has taken a leading part in severalof the recent University College developments in this field.

7 References

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2 TAKANA, S., YOKOTA, H., HOSHIKAWA, MjMIYA, T., andIGAKI, N.: 'Ultra-low-loss single-mode fibre with double syntheticcladding layers'. Proceedings of the 6th European Conference onOptical Communication, York, 1980, p. 37

3 HUDSON, M.C., and THIEL, F.L.: 'The star coupler: a unique inter-connection component for multimode optical waveguidecommunications systems', Appl. Opt., 1974,13, pp. 2040-2045

4 KINGLSEY, S.A., and DAVIES, D.E.N.: 'Method of phase-modulat-ing signals in optical fibres: application to optical-telemetrysystems', Electron. Lett., 1974,10, (2), pp. 21-22

5 DAVIES, D.E.N., and KINGLSEY, S.A.: 'An optical fibre datacollection highway'. IEE Conference on Electro Optics/Laser 16,Brighton, 1976 (Proceedings published by IPC) pp. 64-72

6 CULSHAW, B., BALL, P.R., POND, J.C., and SADLER, A.A.:'Optical-fibre data collection', Electron. & Power, 1981, 27, (2),p. 148-150

7 KINGSLEY, S.A., and DAVIES, D.E.N.: 'The use of optical fibresas instrumentation transducers'. Conference on Laser and Electron-optical systems, San Diego, 1976, Digest of technical papers, pp.24-25

8 CULSHAW, B., DAVIES, D.E.N., and KINGSLEY, S.A.: 'TheAcoustic sensitivity of optical-fibre waveguides', Electron. Lett.,1977,13, (25), pp. 760-761

9 BUCARO, J.A., and DARDY, H.D.: 'Fibre optic hydrophone', J.Acoustic Soc. Am., 1977,62, pp. 1302-1304

10 CULSHAW, B., DAVIES, D.E.N., and KINGSLEY, S.A.: 'Fibreoptic strain, pressure and temperature sensors'. 4th EuropeanConference on Optical Communications, Genoa, Italy 1978

11 BUTTER, CD., and HOCKER, C.B.: 'Fibre optic strain gauge',Appl. Opt., 1978,17, p. 2867

12 LAYTON, M.R., and BUCARO, J.A.: 'Optical fibre acoustic sensorutilising mode-mode interference', ibid., 1979,18, pp. 666-670

13 KINGSLEY, S.A., DAVIES, D.E.N., CULSHAW, B., and HOWARD,D.: 'Fibredyne systems', Fibre Opt. Commun., 1978, Sept., pp.152-158

14 CIELO, P.G.: 'Fibre optic hydrophones: improved strain configur-ation and environmental protection', Appl. Opt., 1979, 18, pp.2933-2937

15 HALL, T.J.: 'High-linearity multimode optical fibre sensor',Electron. Lett., 1979,15, (13), pp. 405-406

16 CULSHAW, B, and HUTCHINGS, M.J.: 'Optical-fibre flowmeter',ibid., 1979,15, (18), pp. 569-571

17 CULSHAW, B.: 'Fibre optic sensors', Proceedings of conference onnew frontiers in fibre optics, published by Engineers Digest,Wigmore Street, London Wl

18 HOCKER, G.B.: 'Fibre optic sensing of pressure and temperature',Appl. Opt., 1979,18, pp. 1445-1448

19 NELSON, D.F., KLEINMAN, D.A., and WECHT, K.W. 'Vibrationinduced modulation of fibreguide transmission', Appl. Phys. Lett.,1977, 30, pp. 94-96

20 JONES, B.E.: 'Optical fibres and electro-optics for instrumentation',Electron. & Power, 1981, 27,(2), pp. 143-147

21 PITT, G.D.: 'Optical techniques for oil detection'. Presented atCEGB symposium on opto-electronics, Leatherhead, 24th January1978

22 ROGERS, A.J.: 'Optical measurement of current and voltage onpower systems', IEE J. Electr. Power Appl., 1979, 2, (4), pp. 120-124

23 SMITH, A.M.: 'Optical fibre current measurement device at agenerating station'. Proceedings of European Conference on OpticalSystems and Applications, Utrecht, 1980, published by SPIE

24 SMITH, A.M.: 'Optical fibres for current measurement applications',Opt. & Laser Technol., 1980,12, p. 25-29

25 VALI, V., and SHORTHILL, R.W.: 'Fibre ring interferometer',Appl. Opt., 1976,15, pp. 1099-1100

26 GILES, I.P., and CULSHAW, B.: 'Performance limitations of opticalfibre gyroscopes', IEE Proc. H, Microwaves, Opt. and Antennas(under consideration)

27 BERGH, R., LEFEVRE, H., and SHAW, H.J.: 'AH single mode fibregyroscope', Proceedings of the 3rd International Conference onIntegrated Optics and Optical Fibre Communication, San Francisco,April 1981

28 BOHM, K., MARTEN, P., PETERMANN, K. WEIDEL, E., andULRICH, R.: 'Low-drift gyro using a superluminescent diode',Electron. Lett., 1981, 17, (10), pp. 352-353

29 BERGH, R.A., LEFEVRE, H.C., and SHAW, H.J.: 'All single modefibre optic gyroscope', Optics Lett., 1981, 6, p. 198

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