Recent advances in fibre optic sensors

Recent advances in fibre optic sensors K. T. V . G r a t t a n Measurement and Instrunnentation Centre, School of Electrical Engineering and Applied Physics, The City University, Northampton Square, London EC1V OHB, UK The subject area of fibre optic sensing is one in which there has been shown a very rapid expansion of interest over the last few years. Many novel techniques are appearing in the literature and some products are available to the industrial user. The background to fibre optic sensing and some recent developments will be reviewed in this paper, which includes an assessment of developments in the field in various countries of the world. Keywords:

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fibre optic, sensor, sensor schemes, sensor applications.

Introduction

The scientist and engineer in industry today has the continued need for instruments which enable him to make accurate and reliable measurements in the environment in which his work is carried out. A wide range of instrumentation is available and recently this has been supplemented by the development of fibre optic instrumentation which can offer certain advantages to make it the best or indeed the only economic solution to some sensing needs. However, in order to compete with well established instrumentation on a broad front, fibre optic devices must show all the qualities of such instrumentation in terms of accuracy, convenience of use, etc, as well as offering those additional advantages, which make the retraining of technical staff to use this new technology worthwhile. The use of fibre optic techniques for sensor purposes has expanded from the original development of the technology for use in vocal communication and data transmission, now a well established field. The confidence of major telephone operating companies throughout the world in this type of technology is seen in the rapid blossoming of the fibre optic trunk networks and their long-term future in this field is assured. Maximum advantage is taken of the high data transmission rates available with low materials costs through the very high bandwidlhs available. The low loss of such systems, with the consequent extension of the distance between 'repeaters' to amplify the signal, has tremendous potential for undersea cable systems. The possibility exists of a fibre optic Atlantic cable with only one or two repeaters if certain impending developments come to fruition. In addition, since the invention of the laser and the recent rapid developments in solid-state lasers which have been seen, ideal sources for fibre optic light transmission systems are available, at reasonable cost and ease of use. The initial drive for the development of fibre optic sensors came from military and aerospace applications where cost factors were less rigid and environments more hazardous to conventional instrumentation. The adaptation of this technology to ready availability for various sectors of industry has been much slower, and articles proclaiming the benefits to industry of the range of fibre optic sensors have been seen in the popular technical press for a number of years. As a result, the industrial sphere has not experienced the deployment of fibre optic sensors in the numbers envisaged some years ago; but steady 122

progress, stretching into areas of industry where their use is somewhat unexpected, has been made and is continuing to be made. The parallel with the slow widespread use of the laser, for example, may be seen but the situation is more favourable due to the installation of fibre optic data transmission systems. In order to be fully acceptable, fibre optic techniques must show all the accuracy, sensitivity and physical robustness of the conventional sensor, at comparable cost. Coupled to this there must be some additional advantage in the use of a new technique to overcome the problems of the introduction of such new technological methods to the workplace. Further, as the electronics industry is moving rapidly towards the use of digital techniques, systems which rely upon intensity-based analogue information can only be expected to have a short working lifetime and fibre optic sensors must incorporate or strive to incorporate these digital data output schemes. Optical transducers may be classified either in terms of those devices where the fibre itself is the sensing medium intrinsic sensors - or those where the fibres merely act to carry light to and from the transducer element, termed extrinsic sensors. In the former case, the transmission characteristics of the fibre are modified by the action of the external effect, while in the latter the interaction occurs only in the transducer element (ideally). The latter type tend to be simpler and require less complex signal processing techniques. A variation on the latter is the use of a non-optical primary transducer, the output of which serves as an input to an optical transmission system. In spite of initial conservatism by some users, there are a number of very positive reasons why optical fibre sensor systems have advantages over conventional sensors. They are intrinsically free from interference by radio frequency fields and other electromagnetic radiation present, and offer excellent electrical isolation properties. These two reasons alone account for extensive research in these systems being carried out by the UK Electricity Supply Industry. They are safe to use in biologically and chemically hazardous areas - there being no currents flowing and no risk of sparks. Tests have shown that even the leasing effect of accidentally broken fibres will not cause a sufficient local light intensity outside the fibre to cause such a risk, for normal levels of transmitted optical signal. Further, the material is inexpensive, with the development costs having been met by the telecommunications indusMeasurement Vol 5 No 3, Jul—Sep 1987

Grattan try, and additionally unique sensing opportunities are afforded by fibre optic sensors (eg, distributed sensing of temperature in a working environment in a single optical fibre loop has been demonstrated.) By taking advantage also of mainstream developments in electronics and signal processing at the receiving end of the system, there is the possibility of a major drive into the sensor market by fibre optic devices in the near future.

structure, in a fibre termed a 'multimode' fibre. For many sensor applications this can be ignored; however, some sensors depend upon the change in the modes propagating in the fibre as a result of an external influence to sense the magnitude or presence of that influence. The geometry of a fibre may be such that only one mode is allowed to propagate - so-called 'single mode' fibre. Hence a phenomenon seen in multimode fibres where the modes interact due to the fact that they travel at slightly different speeds in the fibre cannot occur and consequently that type of signal distortion that would be experienced in multimode fibre is removed. However, to achieve this, the core diameter is typically ^10/zm, with the consequent difficulty of launching light into the fibre and joining of fibres. However, if it is needed (as in communication applications), there is available a very high signal bandwidth. The use of this guiding phenomenon has been known for some time and was demonstrated by Tyndall as early as 1870. The cladded fibre was proposed in 1954 and only in 1966 were fibres first constructed which began to overcome the serious attenuation problem faced in transmission through considerable distances. For example, early fibres had an attenuation of ~100dBkm~' but today figures of ~0.2dBkm~' are routinely achievable for infrared transmission. As a consequence, multikilometre data transmission links are being constructed, with repeater spacings of > 100 km. Fundamental limits of intrinsic losses in the material are thus being reached, and Fig 2 illustrates the wavelength dependence of these losses for a typical silica fibre. The fundamental limit is Rayleigh scattering from disorder in a glass material, and this may be structural or compositional in origin. In the former case, the basic molecular units are connected in a random way, whereas in the latter, the composition may vary from place to place. The net effect is a refractive index change and if each irregularly is of size ~A/10, it will act as a scattering centre, with the absorption coefficient for Rayleigh scattering having a A"* dependence. Absorption losses are due to the presence of inpurity ions, eg, OH~, Cu^"^,

2 Optical fibres A brief description of the properties of optical fibres is given. An optical fibre is a thin glass filament, a dielectric material used for guiding light. Fig 1 illustrates the construction of a typical optial optical fibre, where a central core of material of one refractive index, nj, is surrounded by a material of slightly different refractive index Wj, called the cladding. Two types of fibre are generally available, one whose refractive index obeys a square law in the radial direction and the more common type where the index change occurs abruptly. Here the relationship Hj >n2 's obeyed and, if the angle of incidence is shallow, total internal reflection will occur at the media boundary, which acts like a mirror. Hence the light propagates along the fibre. As a result of interference effects of various rays propagating in the fibre, an interference pattern is formed which is stable and reproduces itself down the fibre - producing a so called 'mode'

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Grattan conventional light sources (eg, discharge lamps, fragile glass bulbs, etc) for production of the light to input to the fibre. Additionally, it may be difficult to direct light into fibres from these sources and stable, well aligned lens systems are needed. In many cases, a modulation of the light at frequencies ^ 1 kHz is required and mechanical shutter-type modulation systems are limited in response time and mechanically unstable when operated at high speed. Hence the acceptability of optical fibres as sensors will be greatly increased if the light sources are compact and readily compatible. Fortunately, the development of laser sources at a range of wavelengths (660 nm ^ / < 1500nm) to suit the peak transmission characteristics of silica and polymer fibres has continued and light from such sources can readily be coupled into optical fibres. Additionally, high-power and inexpensive LEDs hold open the possibility of some very cheap and simple optical sensor schemes being employed. An illustration of the readily available sources is shown in Fig 3, for LED and laser diode devices. Also, the He-Ne laser, operating at ~633nm and 1.15/im, is a very convenient additional source at comparatively low cost and can be modulated efficiently by electro-optic techniques.

Fe^ *. The water (OH ") absorptions are the most significant, having a number of distinctive peaks. Beyond ~ 1.6 nm, losses are due to transitions between vibrational states of the lattice. However, there exists a region in the visible through into the near-infrared, where longdistance transmission can occur. In extrinsic sensor applications, therefore, the transducer can be placed at a considerable distance from the optical source and detector, and with intrinsic sensors, long fibre lengths and sensing regions are possible, if losses induced by the sensing process are small. Polymer fibres offer advantages over silica in terms of cost and flexibility, although at the expense of higher losses, in particular in the infrared. However, even with typical figures of ~ 150-200 dBkm " ' at 660 nm (1500 dBkm " ' at 820 nm), there is considerable scope for applying these fibres to short-range sensing applications. One promising future development is that of fibres constructed from various fluorides which have a high transmission at long wavelengths where Rayleigh scattering is less (due to the / " * dependence) (/ > 1.5 //m), with predicted minimum losses of 10"^ or 10"-'dBkm''. Work is actively continuing in this field.

High-voltage photomultipliers, whilst being excellent in sensitivity, are fragile and bulky devices to use which also require > 1 kV power supplies. With the availability of high intensities of light from sources described. Si and Ge p-i-n diodes may be used to sense in the visible/near-ir

3 Light sources and detectors There seems, in many cases, little to be gained from the use of fibre optic techniques if there is a reliance upon Emitters

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Grattan (to ~ 1.2/jm and beyond 1.6^m to ~ 1.9nm respectively), with adequate overall sensitivity. Being low-voltage small devices, these are ideally matched to LEDs/laser diodes to produce a low voltage, directly modulated, portable, robust emitter and detector system. With such devices in mind, the use offibre optics sensors outside the laboratory environment is much less of a problem than would otherwise be envisaged.

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4 Optical sensing techniques In this paper, a number of the recent developments in optical fibre sensors will be considered and discussed. A number of reviews of optical fibre sensors for specialist applications (Rogers, 1984) and wider and more general applications (Pitt et al, 1985), or concentrating on fibre optic techniques (Medlock, 1986) and specific measurands (Grattan, 1984), have been produced already. Consequently, this work will attempt to supplement those reports, rather than duplicate them and indicate both general principles and directions of future advancement for fibre optic sensing, and assess developments in various regions of the world. Table 1 lists a number of basic sensor types with a range of applications of the specific sensor technique on which they are based. 4.1 Interruption sensors These are the simplest of all fibre optic sensors, where the intensity of the light transmitted from one fibre to another is reduced or decreased to zero as a result of an interruption - e g, a shutter passing between an input and output fibre. Variations on this theme include modulation of the received light due to displacements to one of the fibres, either in an angular, longitudinal or transverse mode, as shown in Fig 4. Delta Controls (UK) market an optical microswitch based on this principle, for incorporation in the housings of their existing range of switches for

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ease of market acceptance. Additionally, an electro-optic relay has been produced by the same firm to interface with other fibre optic sensors. Alternatively, light may be reflected from one fibre to another parallel to it and the intensity transmitted be modulated by the movement of a mirror. This device can be seen in a more sophisticated mode with the use offibre bundles to sense the movement of the reflector in a variable area flowmeter or with a CCD array in a level sensor for changing liquid levels. A prismbased liquid level alarm, operating by the change in light path with the change in refractive index of the surrounding medium (air or the liquid to be sensed) has been marketed, also by Delta Controls (UK), and is illustrated as Fig 5.

TABLE 1: Summary of some sensor types Sensor scheme

Typical application

Interruption of light passing between fibres Light receiving sensors (ie, light to and from the sensing region by optical fibres) Sensing element connected to fibres; multlmode fibres

On/off switches Spectroscopy, fluorimeters. Spectrophotometers. LDV-laser Doppler velocimetry

Photoelastic effect (stress), photoluminescence, liquid crystals for pressure, temperature applications Wavelength-dependent and Temperature, pressure sensors, etc transient phenomena, etc Polarisation properties Current measurement. Low birefringence fibre for Faraday effect. Special high birefringence fibres for polarisation preservation Continuous monitoring of long fibre Distributed sensors lengths. Positional information obtained Unique applications for temperature, liquid leaks, etc Many parameters including Interferometers Mach-Zehnder or Michelson displacement, temperature, pressure Fibre optic gyroscope Sagnac effect Microbend sensors

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4.2 Light receiving sensors In these sensors, fibre optic techniques are used to couple light to and from a region where an interaction of the light with matter will occur. A fibre optic adaptation of bulk optical techniques and laboratory instruments (eg, spectrometers, fluorimeters and laser-Doppler anemometry velocity meters) results. Such devices enable measurements to be taken under adverse circumstances and in a small interaction volume, and a number of such systems are marketed currently (Photonetics, France) as shown in Fig 6. 4.3 Optical sensing elements This is one of the most widely applicable types of optical sensor, where a transducer element is connected to optical fibre(s) and the properties of the light are altered at the transducer itself. Illustrations of this technique are the changes in absorption, transmission and fluorescence properties of the transducer, or in its effects upon the phase of the light or its polarisation, discussed in detail in the next section. 125

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4.4 Polarisation properties As has been noted, the propagation properties of an optical fibre have a polarisation characteristic, depending on the interaction of the electric field of the incident light with the preferred direction of the fibre-ie, that lying in a plane tangential to the core/cladding bounflary at the point where reflection takes place. Conventional 'communications' type optical fibre will not preserve the polarisation properties of, for example, laser light launched into the fibre. However, polarisation perserving fibres have been produced and are marketed, where a stress is introduced into the fibre in its production, inducing birefringence, and as a result the input polarisation of the light is perserved. Such fibre, however, is expensive by comparison with 'communications' fibre. If other forms of directionality are imposed upon the fibre, the polarisation of the propagating light will be altered. External magnetic and electric fields can cause such eflects in bulk optical materials and in optical fibres, and the Faraday magneto-optical effect can be used to measure magnetic fields. The most important specific application is in measurements in the electricity industry, where bulk optical and fibre measuring systems are being tested. The 126

use of these fibres in this context is discussed in a paper by Rogers (1983). A detailed discussion of the use and measurement of the optical polarisation state in optical fibre sensors is given in a recent paper by Jones etal(\986). 4.5 Distributed sensors This technique is one for the continuous interrogation of an optical fibre which is subjected to a specific stimulus, and the magnitude and position of that stimulus can be measured. This unique sensing property of optical fibres is being exploited for security purposes for temperature sensor applications and for voltage measurement through sensing distributed polarisation changes in fibres. Techniques used for distributed sensing include optical timedomain reflectometry (OTDR), polarisation OTDR and frequency-domain reflectometry. In each case the position along the fibre where the perturbation of the optical signal occurs can be identified and this has significant advantages for sensing. A review of these techniques has been published by Kingsley (1985). The most important use is in the thermometer marketed by York Technology, and discussed in the next section. Measurement Vol 5 No 3, Jul—Sep 1987

Grattan 4.6 Interferometer sensors The Mach-Zehnder interferometer sensing technique is one that is widely applicable and has been used for the sensing of many physical parameters (Giallorenzi, 1981). Light propagates in a monomode fibre which splits into two separate arms of the interferometer. A stimulus to be sensed is applied to one arm and the two fibres recombine. The difference in the phase relationship of light in the two arms produces an interference pattern which is itself affected by the presence of the external stimulus. Pressure, for example, will alter the density of a material and hence the refractive index and thus the interference pattern produced. Frequently the measurand is converted to a displacement, measured as a shift in an optical fringe pattern. A special case is the optical fibre gyroscope (Bergh et al, 1981) which has been the subject of extensive research, both in university laboratories and those of major aerospace companies. Interferometric sensors are not discussed in detail in this paper, but are reviewed in depth in a recent work by Watanabe et al (1985).

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5 Specific sensor applications - Recent advances 5.1 Displacement measurement The sensing of displacement is one of the most basic measurement applications as a temperature change, pressure change, flow change, etc, can cause a displacement which may be measured using fibre optic systems. Early work concerned the use of the simple techniques described previously using intensity modulation to produce a displacement (Fotonic sensor) (Fig 7). Apart from the use of interferometric techniques to sense very small displacements (and hence magnetic and electric fields via magnetostrictive or piezoelectric effects) through the stretching of a fibre in one arm of the instrument (with associated problems of a non-desirable sinusoidal amplitude function), new displacement measuring techniques have been described. The use of wavelength modulation opens up a number of possibilities through the rotation of a prism or diffraction grating to allow a different wavelength to enter the receiving fibre, as a function of the position of the optical element. This is illustrated by Fig 8. An alternative, new commerical device (Place, 1986) is a displacement device configured into a pressure transducer utilising a zone plate, consisting of a reflective surface with a series of concentric grooves at controlled spacing. These allow it to act as a spherical concave mirror with radius of curvative inversely proportional to wavelength. As it moves, the transmitted wavelength of light varies and this causes a change in the ratio of light at two photocells. Developments of the principle could extend to the indicating of valve 'on/off' states, in a device which does not rely upon an intensity measurement (thus removing the corresponding referencing problems).

5.2 Temperature measurement

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Many approaches to the measurement of temperature through fibre optic systems have been made and a wide

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Fig 8 Wavelength modulation techniques for optical fibre sensors 127

Grattan range of extrinsic sensors have been proposed. In addition, in this measurement regime, new intrinsic sensors have both been described and marketed very recently.

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(i) Extrinsic sensors In the temperature sensing field, there are at least two well known commercial fibre sensors - the Luxtron type 1000/2000 device (as shown in Fig 9A) (Wickersheim et al, 1982) and the Asea type 1010 sensor (Ovren et ah 1983) (Fig 10). Both are sensors where internal intensity referencing is provided. Light is transmitted in the former case to a rare-earth phosphor sample and light re-emitted as fluorescence is detected. The changing ratio of the emission on two wavelength bands is monitored. In the Asea system, a GaAs sample is the tranducer, irridated by directly modulated light from an LED (as opposed to a UV lamp source - Luxtron) and the temperaturedependent quantity again measured is the wavelength shift of the fluorescence. Recent work by Grattan et al (1985a) described a sensor using Nd^* doped material, excited by light from an LED, measuring the change in decay time of the material to sense changes in temperature. Mechanically modulated systems had been proposed previously (McCormack, 1981) yielding results somewhat at variance with laboratory measurements on the active material (Sm^ *) in a bulk optical set-up. In 1985, Luxtrop (Wickersheim et al, 1985) announced a new commerical temperature measurement system (Model 750 Fluoroptic Sensor Fig 9B) using the decay-time characteristics of magnesium fluorogermanate as the measurand, again with UV lamp excitation and slow data acquisition. Recently, in Japan, Omron (Hirano, 1586) announced a new fibre optic temperature sensor using the fluorescent decay-time principle to measure in the range — 30° to 200''C, using a phosphor transducer excited by light from a visible LED. An independent assessment of a number of commercial optical fibre sensors has been undertaken by Harmer (1986) and this has shown that some of these sensors perform less well in practice than the manufacturers would suggest, due to interference from insertion losses, microbending, etc. Also, recently proposed are the use of thermochromic substances such as cobalt chloride solution in water/alcohol (Brenci, 1984) which show a marked change in colour between 25° and 75°C, and this is measured

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by a two-wavelength detection principle using white light irradiation of the sample. Wavelength modulation in transmission is also a feature of material used for optical filters - eg, interference filters, semiconductors and doped glasses. A 'two-wavelength' system of this type was demonstrated by Theocharous (1983) using a filter which absorbs differentially with temperature at one wavelength and does not absorb a second wavelength. The ratio of the output intensities is a function of temperature, with the second wavelength acting as a reference signal. Time division multiplexing was used. Alternatively, Grattan et al (1985b) have recently proposed a 'self-referenced' system which uses, for a second wavelength, fluorescence generated at the sensor head from a fluorescent glass bonded to the transducer. The time Measurement Vol 5 No 3, Jul—Sep 1987

delay in obtaining thefluorescencesignal results in a self time-division multiplexed system. The sophisticated schemes described appear to illustrate that temperature sensing is one area where optical techniques are becoming important, even if only for specialist applications, as yet, due to the cost of the devices. In the high-temperature region (^ 5(X)°C), a variation on the standard technique of optical pyrometry has been developed into a fibre system. The black body radiation from a film on a sapphire fibre is transmitted along a fibre and the temperature computed from two narrow bands of the spectrum received. A commercial system (Accufibre Corp) has been developed, with a high performance claimed - resolution of ~ 10" ^°C and an accuracy of ~ 0.0025%, in the range 500°C-2000°C. 129

Grattan (ii) Intrinsic sensors Such sensors have recently been developed using either ordinary silica fibre or doped fibre. In the former case, using optical time-domain reflectrometry (OTDR) (a system which uses the time-resolved information on the propagation of a short pulse in a fibre to determine positional information over a fibre length), the use of non-linear interactions results in a temperature measurement being made from Raman scatter data. The ratio of the Antistokes to the Stokes intensity of the backscatter signal is a function of temperature, and measurements with a temperature resolution of ~5°C and spatial resolution of ~ 5 m have been reported (Dakin et a/, 1985). The signal obtained as a function of intensity is illustrated by Fig 11. A commercial system using similar principles has recently been marketed by York Technology which uses the intensity of the Antistokes Raman line; this is a well behaved characteristic function of the temperature. The scheme is attractive as it uses solid-core fibre and carefully designed signal processing to amplify the small returned signal. The resolution claimed is ~ TC and over 1 km of fibre can be used to give 200 measurement sites. Doped fibres are more expensive to manufacture than regular communications type silica fibre, but using Nd^ * doped fibre, United Technologies (Morey et al, 1983) have described a sensor which relies upon the differential spectral absorption characteristic of the material. An absorption band centred at ~ 840 nm decreases while there is a corresponding increase in a band centred on ~ 860 nm. The ratio is measured as the temperature-dependent function. Very recently Parries et al (1986) announced a Nd^* doped fibre OTDR system. Early results are less satisfactory than those from silica fibre, but future developments could be promising. In conclusion, whilst many sensors have in the past been described, the recent developments include commercial development of successful prototypes and new ideas to overcome referencing difficulties.

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5.3 Current/voltage As discussed earlier, electrical parameters can be measured using the change in polarisation effects in optical fibres through, for example, the Faraday magnetooptical effect to determine the magnetic field produced by a current-carrying conductor. If a fibre is looped around such a conductor and linearly polarised light is launched into the (monomode) fibre, the direction of polarisation will be rotated in proportion to the current magnitude. Detection of this effect can be by means of a polarisation beam-splitter which analyses the incoming light. Rogers (1986a) has described an experimental demonstration of this technique to measure currents of ~ 10-15 k A in a fastresponse system (~0.2s). Difficulties exist with vibration effects but methods are being sought to overcome these through reference channels. Voltage may be measured through the electrogyration effect, which is the electrical counterpart of the magneto-optical effect. It can be observed when linearily polarised light propagates in certain crystalline materials (eg, quartz). Hence such a sensor using quartz crystals on either side of a highvoltage bar can be constructed (Rogers, 1979). This system can also measure current simultaneously as the electrogyration effect is the same for both crystals but the magneto-optic effect is reversed. Hence, using sum and difference techniques, the current and voltage effects can be separated. With the construction of crystalline fibres currently a matter of research investigation, there is the possibility in the future of such an all-fibre sensor being constructed. The use of presently available and possible future developments in optical fibre technology is discussed in some detail (Rogers, 1986b) in a recent paper, to remove the interference of vibration effects from fibre optic current sensors. Alternatively, the Pockels effect has been exploited to measure an electric field. A phase retardation, linearily proportional to the magnitude of the electric field, is produced in linearly polarised light propagating in an isotropic medium. BijjSiOij and Bi4Ge30,2 have been used in such a sensor (Shibata, 1983) because of their lowtemperature coefficient, yielding a phase shift inversely proportional to the wavelength of light used and proportional to the third power of the material refractive index. Sumitomo Electric (Japan) have recently marketed a Bismuth silicon oxide electric-field intensity (voltage) meter type EOS-02 or magnetic field intensity (or electric current with a magnetic core) device type EOS-03 with wide bandwidth. Additionally, National/Panasonic are marketing a fibre optic magnetic field sensor using the polarisation change principle with a 1.3 ^m LED source. Further, Toshiba of Japan are marketing optical voltage and magnetic field sensors based on polarisation changes in LiNbOj (voltage) and RjFesOjj (magnetic field). Hence it can be seen that considerable commercial activity exists in this field in Japan, and devices are available in the market place. A review of recent work is given by Kanemaru (1986). 5.4 Chemical and biological sensors

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One of the most exciting recent developments in optical fibre sensing technology is in the field of sensing of chemical parameters (eg, pH), the presence of certain gases and, in certain instances, being able to do this within the human body for clinical purposes. Apart from the Measurement Vol 5 No 3, Jul—Sep 1987

Grattan facility to use the sensors described in the chemical industry for continuous measurements (to which they are well suited due to their passive nature), chemical sensing devices where the optical property of the transducer element changes with the chemical parameter under investigation are being studied. Extrinsic devices have been developed so far which use the colour change of an indicator solution (phenol red) as a pH sensor in liquids (Benaim et al, 1986) and immobilised on polyacrylamide microspheres (Narayanaswarmy, 1986) addressed by visible light carried by conventional fibres. The indicator converts from one tautomeric form to another with an accompanying colour change. High precision is reported (~0.01 pH) with a low temperature offset effect (Narayanaswarmy, 1986). There is the possibility of using fluorescence techniques as well to measure pH changes of solutions which can alter the fluorescence characteristics of certain dyes (Wolfbeis et al, 1986). Additionally, reflectance techniques can be used to monitor the ratio of reflectances of an indicator, bromothymol blue, at two wavelengths, one in the visible and one at infrared wavelengths. Hence a wide range of optical techniques can be applied to monitoring the colour changes seen. The main problems arise from the long-term stability of the immobilised dyes - some tendency exists for the dyes to leave the immobilised base with time. Recent work to sense the presence of various gases by optical means has been reported. Techniques employed include the use of an optical fibre coated with palladium which expands on exposure to hydrogen (Butler, 1984) and is one arm of a Mach-Zehnder interferometer. The behaviour of the palladium hydride is sensed and it is envisaged that the range 1 to 30000 ppm Hj can be detected. An alternative approach is the measurement of O2 by its effect on the mean lifetime of fluorescent dyes (Gehrich et al, 1986), quenching, as it does, in a nonradiative manner. Detection of CO2 can be achieved using a pH sensitive dye encapsulated in a suitable matrix to allow the gas to change the pH of the sensor material (Gehrich et al, 1986). These techniques have been used in the medical field in a cleverly engineered pH, COj and O2 probe which is small enough to be inserted into the body as an intravascular blood monitoring system as shown in Fig 12. Non-invasive blood oxygenation monitoring has also been achieved looking through the skin by using two wavelengths, ~600nm and 805 nm. The latter wavelength corresponds to an isobestic point for blood (same transmission characteristics whether the blood is oxygenated or not) whilst the red wavelength is transmitted in a significantly different manner for oxygenated and deoxygenated blood. Measurements which compare well with those of conventional techniques have been made by Yoshija ei al (1980). With the development of

solid state devices, the potential for the further development of such an instrument is greatly increased. As yet, there has been little penetration of the commercial marketplace by devices to sense these parameters, yet it is consistently reported as an area where there is much need for systems showing the advantages that fibre optics possess. It is expected, however, that a wider choice of devices will be available commercially soon. 5.5 Optical fibre gyroscopes The use of the Sagnac effect to measure rotation has been exploited successfully in the development of the optical fibre gyro. Much progress has been made and there is considerable commercial interest in the product due to the large aerospace market in Europe, Japan and the USA. This work requires detailed discussion in its own right and is thus beyond the scope of this paper. Reviews of the subject have been presented by Ezekiel (1985) and Dankowych et al (1985).

5.6 Novel applications of fibre optic sensors In certain areas, the presence of fibre optic sensors is creating new markets and novel applications where conventional sensor schemes are either very cumbersome or inappropriate. The intrinsic temperature sensor described earlier (York Technology) is a device that is comparable to many hundreds of thermocouples joined in series and is inherently much simpler. A cryogenic liquid detection system has been described and marketed by Pilkington in the UK (Pinchbeck et al, 1985). The device operates using a fibre which becomes liable to loss when the cladding is exposed to severe sub-zero temperatures due to the presence of cryogenic fluids. Hence, if this fibre is looped round a cryogenic installation and the signal transmitted through the fibre monitored, a drop in signal level will indicate a leak of fluid causing that loss. A 'fault condition' will also give a drop in signal, but this only triggers human intervention to examine the storage plant. Such a device is intrinsically safe for use with flammable materials, reliable, maintenance free and cost effective, thereby superseding the complexity of a battery of traditional thermocouples which may be an alternative. The BMT optical crack detection system (British Maritime Technology - OptiCAT Crack Detection Scheme) is a simple application of optical fibres where the growth of a crack in a structure is determined by three parallel sensing optical fibres, placed 2 mm apart, to enable the rate of crack length growth to be determined over a range of 5 mm. The actual path of the crack is determined by monitoring visible fight escaping from the

T L D COdtlnri

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Fig 12 Intravascular blood multisensor using optical fibres 131

Grattan fractures in the fibres immediately over the crack. The fibres are bonded to the structure and thereby the high strain associated with the cracic formation is transmitted to the fibres. Conventional instruments (eg, the Bourdon gauge) can be converted to an optical readout by monitoring the strain induced in a fibre by its operation, and this may be advantageous for remote monitoring. Further, the use of optical fibres in security systems extends to the development by Hergalite of a pressuresensitive mat. A spiral is wrapped round the fibre and when pressure is applied to the fibre, losses are induced in the light transmitted in the fibre, thereby sensing the presence of the individual causing that pressure. A further system, described by Leung (1986), uses a fibre underground at a depth of 10 cm in gravel, to sense the presence of intruders in an installation through causing intermodal interference in the laser signal transmitted. Normal fibre is used and thus can be laid quite cheaply, yet provide an efl'ective sensor. Commerical sensors have been developed to measure the void-fraction of liquids (gas volume to total volume ratio) using the fact that when gas bubbles are present in the liquid, light transmitted into the liquid will be scattered and returned via a fibre to a photodiode. Such a device is marketed by Photonetics (France) as shown in Fig 13 and Kanomax Inc (Japan). A similar principle has been used to detect the presence of oil in liquids, eg, in appHcations to monitor the discharges from oil tanks

using the presence of scattering induced by a foreign body (oil droplets). This is marketed by Monitek (USA) and illustrated by Fig 14. Hence there exist several applications of fibre optics using the unique properties of fibres themselves to achieve the desired sensing result in a simple way. Finally, the versatility of fibre optic sensors is seen in the development of a multi-sensing system, marketed by TDI (USA) (Saaski et al, 1986). Using a series of sensors based on the measurand modifying the characteristric of an optical cavity, a single control system is used in the sensing of pressure and temperature over a number of ranges. The development of such flexible sensor systems is encouraging for the future of fibre optic sensors.

6 Optical fibre sensor markets The wide range of applicability of optical fibre sensors is seen in Table 2, listing the potential uses in various areas.

7 World activity in fibre optic sensors 7.1 Europe There has been considerable activity in the field of research and development in fibre optic sensors in Europe, especially in the UK, France and Germany. The

liquid present no r e f l e c t i o n fibre optic sensor

gas presentj light reflected

Fig 13 Schematic of operation of void fraction sensor (Photometries, France)

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USA)

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Grattan TABLE 2: Optical fibre sensor market Field

Methods

Features necessary

industrial

Flow, pressure, temperature, level

Easy to install, inexpensive. Distributed use for specialised tasks Specialised applications. Higher cost sensors acceptable

Utilities and Gas detection Specialised Current detection Transformer temperatures Industries Water processing and monitoring Chemical parameter/colour Medical changes surface absorption, immobilisation. Total internal reflection. Laser Doppler velocimetry Aerospace Encoders, switches, oil-inwater, particulates in fuels, oil, etc. Pressure displacement. System monitoring Aerospace applications plus Defence acoustic, navigation magnetic, fire, radiation, temperature Platform gas detection, local Offshore and over the area. Fire, smoke Well head controls. Pollution control. Well pressure, temperature

Disposable. Small devices for insertion into body. Blood parameters Light. Shock, vibration resistant. Large temperature range. Highly reliable Mamy types of sensor required Possible operation in very adverse conditions and underwater

British woric is the leading European effort, largely due to the impetus provided by the formation in April 1982 of OSCA, the Optical Sensors Collaborative Association. This body, which is part-funded by the UK Government and part by British industry, finances and co-ordinates, on a voluntary basis, research projects on a precompetitive level in universities or research institutes. Thereby the 30 industrial organisations and 17 affiliates (universities and the like) who are members have access to a considerable body of research information for a fraction of the cost required for any one of them to obtain such information by conventional in-house research efforts. It has provded an excellent opportunity to assess the feasibility of optical sensor systems rather than for each individual manufacturer to do the necessary background work to launch a commercial range of such sensors, and its existence explains much of the UK effort in the field. In France, seven universities and institutes and eight industrial companies are engaged in fibre optic sensor development, including major utiUties such as the nuclear industry, gas industry, electricity and ELF, the petrochemical company. Plans are being developed for the launch of a grouping of similar constitution to the UK OSCA. In Germany, nine industrial companies and two universities and institutes are engaged in fibre optic sensor research. Additionally, in Europe, there are efforts in Sweden, where one of the more successful commercial sensors was developed by Asea Innovation (the fibre temperature sensor), and in Italy and Switzerland there are research institutes engaged in fibre optic sensor development.

7.2 Japan The level of organisation of the Japanese fibre optical sensor development programme by the government is seen in the range of such sensors offered on the commerical market by Japanese companies. The Agency of Measurement Vol 5 No 3, Jul—Sep 1987

Industrial Science and Technology of the Ministry of International Trade and Industry (MITI) has presently a 15 billion yen (SIOOM) programme over seven years on a project entitled 'Optical Measurement and Control System' to develop the technology for fibre optic measurement and control in large industrial plants, where the specific advantages of optical methods are particularly valuable. Major industrial companies participating were Oki Electricity Industry, Sumitomo, Toshiba, NEC, Hitachi, Fuji Electric, Matsushita and Mitsubushi Electric, amongst others. Details of the scheme are summarised in a recent paper by Yajima (1986). A major plan of this programme was the development of optical fibre sensors for the following applications: temperature (point and distributed), pressure, acceleration, flow, gas leak, oil leak, level, on-off sensors, wavelength scanning sensors, optically controlled thyristors and a holographic image scanner. Tests have been carried out at the Mizushura Oil Refinery of the Nippon Mining Company using integrated systems in 1986, and further developments are continuing.

7.3 United States of America

A number of research groups in the United States are working in the fibre optic sensor field and devices are available commercially for process control and similar applications. In general, these are sensors for pressure, temperature, liquid level and flow. In addition, a major research effort has been undertaken by the Naval Research Laboratories in Washington DC to develop highquality, state-of-the-art sensors for military applications (Giallorenzi et al, 1982). A significant part of this work has been directed towards the use of interferometric sensors. As yet, no fibre interferometer sensors have been marketed. A limitation on the development of such sensors is the high cost of the major building block - the fibre optic coupler - which is only now available at ~$100 per unit; so these devices have so far been limited largely to specialised and laboratory applications. A number of successful sea trials of acoustic sensors have been carried out to date by the US Navy, indicating that the interferometer can operate in the field. However very specific and rigorous packaging of these devices is required for use out of the laboratory and this difficulty, coupled with the fact that most devices are sensitive to more than one parameter at a time, makes their prospects for general field use somewhat limited. A laser-Doppler velocimeter, using an interferometric configuration, is marketed in the US but its price makes it a research rather than a common industrial device. Many US manufacturers supply fibre optic components which could be configured into sensors and, as already described, TDI have recently launched a multi-sensor system using a resonant cavity system to sense different parameters with similar sensor heads and single control system. Research is continuing at Stanford University (Byer, 1986) on crystalline optical fibre development. With this process there exists the possibility of their use in the field of electrical measurements, to replace bulk optical components. There appears not to be the same level of co-ordination of research nationally as is seen in Japan and to a lesser extent in Europe, but the military drive of the maket is significant and much of the most important work has been done by military research establishments or under military contracts. The funding 133

Lirattan of the Strategic Defence Initiative (SDI) will give impetus to the development of these technologies with their inherent insensitivity to certain types of interference with, it is hoped, a spin-off into the civilian marketplace.

8 Conclusion This paper has illustrated the fundamental nature of fibre optic sensor systems, illustrated some recent developments in areas where progress has been rapid and discussed the activity in the field in various parts of the world. By definition it cannot be comprehensive but aims to illustrate that this field is still one where there is significant development with the availability of commercial devices of high quality for some sensor purposes. It is expected that in the near future this market will continue to expand as the new ideas described in the literature result in commercial developments.

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