Several types of optical fibre sensors have been successfully developed into commercial products and a strong technology base has been realised to foster the development of new types of fibre sensors. In this talk, we will describe briefly the technical status of the field as of the first OFS which was held in 1983 and trace the evolution of the field (both components and concepts) and its commercialisation through the last decade. The component base developed for optical fibre sensors has found applications well beyond the sensor field and this will be examined. For example, single mode couplers developed for interferometric sensors are now being routinely used by the telecommunications industry. Low noise single mode laser studies conducted for high performance sensors also laid the foundation for many later noise studies of lasers for CATV, and single mode direct and coherent communication applications. The development of sources for fibre gyros led to the development of a new class of optical sources i.e. superluminescent diode and fibre lasers. The technology of interferometric sensor demodulation proceeded similar work for coherent communications and the pioneering studies in sensor multiplexing in its many forms has also been applied to CATV and FTTC applications. The successes by the fibre sensor community in the development of componentry however has not yet been matched with an equal success in commercialisation. Interferometric sensors such as the fibre gyro and intensity based sensors such as temperature, pressure, etc. have been successfblly marketed; however commercialisation in general has not met everyone's hopes or expectations. In this talk we will briefly also review the market growth projection made at the time of the first OFS and discuss what people project today.

In this paper, we will explore the creation of several new fiberoptic sensors based on the more sophisticated two-photon spectroscopic techniques [1] for applications, such as, chemical sensing, magnetic/electric field sensing, and for the measurement of time or frequency. The unique properties of optical fibers and fiberoptic/ integrated optic components are critical to the development of such sensors.

Optical fibres have been used to sense many different physical quantities (magnetic fields, vibrations, temperature, rotation etc.). There is however, one important exception; optical fibres are insensitive to electrical fields due to the inversion symmetry of the glass matrix, which ensures that the Pockels coefficients are zero.

Optical fiber sensors are of considerable interest to the electric power industry. Particularly attractive features as compared to conventional instrument transformers include the inherent galvanic isolation of the sensor head from ground potential, less sensitivity to electromagnetic interference, smaller size, and higher safety.

Magnetooptic current sensors are gaining interest due to the following advantages: simple potential separation, immunity against electromagnetic interference, high dynamic range and bandwidth and compact design. Magnetooptic current transformers - MOCT - offer a complete separation of energy and signal distribution in power industry.

The basic system of the optical fiber-type Faraday effect current sensor for electric power apparatus has been developed. According to the experiments, it became clear that the developed system showed excellent polarization characteristics and small tempereture dependence. Though the vibration dependent fluctuation of the output is observed, a remedy is suggested.

This paper describes polarization properties of the flint glass fiber for Faraday sensor element. Measurement results showed very small birefringence of the fiber, and good agreement with a model to describe relation between polarization and geometrical figure of the fiber.

We report a new design for a Faraday effect current sensor based on yttrium iron garnet. The improved sensor has substantially greater bandwidth than previous designs and is considerably easier to fabricate. The measured sensitivity is 0.7°/A with a -3 dB bandwidth of 500 MHz, giving a factor of 45 increase in sensitivity-bandwidth product. A noise-equivalent current of 840 nA/Hzl/2 was measured at 1.8 kHz using difference-over-sum processing. The use of newly developed turning prisms with phasepreserving coatings greatly simplifies construction, improves electrical isolation, and increases sensitivity through proximity effects.

The 1/r3 dependence of the magnetic field with distance r makes the vector magnetometer an ideal sensor in undersea systems for the localization and tracking of vessels. Since the undersea system typically spans several hundred meters to several kilometers, use of fiber optics for both telemetry and sensing offers inherent advantages of low cable weight, immunity to electromagnetic interference on the signal transmission paths, and high channel capability.

Optical-fibre current measurement, based on the Faraday magneto-optic effect, offers important advantages for use on electric power systems, owing to its potential for greatly increased bandwidth, reduction of insulation problems, lack of hysteresis and low cost [1,2].

We demonstrate a fiber optic interferometric dc and low frequency voltage sensor based on the electrostrictive effect in lead magnesium niobate (PMN) transducers. The sensor exhibits ultra-low drift and has a noise floor of 35 nV/?Hz at 10 mHz and 20 nV/?Hz at 1 Hz.

In regional hyperthermia a selective overheating of the tumour region is applied in order to avoid a growth of the tumour or at least to sensitize the tumour against other types of therapy. Selective heating can be obtained using an electromagnetic field (f = 70...110MHz), which is focussed by an appropriately driven phased array antenna. In order to adjust and, so far as nessecary, improve the steering of the dipole phased array, the knowledge of the actual Specific Absorption Rate SAR= K l£l2 /(2 p) (K: conductivity, p: specific weight) distribution is essential. This can not be circumvented by temperature measurements at the cancer region, because the latter only provide spatially piecewise informations which additionally suffer from large time constants of temperature.

We describe a novel fiber probe for monitoring a.c. high magnetic fields which is based on detecting the shift in the Bragg condition of a fiber Bragg grating due to magnetically induced circular birefringence. The technique should be usable over a wide range for fields above 1 Gauss.

Electrical discharges in gases take many forms. Coronas with currents of tens of mA occur around high voltage points at pressures in the atmospheric range; glows are low pressure discharges with currents in the mA range; and arcs are produced at high pressures with currents of amps to kiloamps. Technological applications of glow discharges include lamps, gas lasers and sources for plasma etching and processing.

This paper describes a breadboard demonstrator optical fibre sensor system that was developed as a technology evaluator suitable for generic sensing applications. The system was based on a low-coherence interferometric processing scheme that utilised a capacitive feedback-controlled fibre coupled Fabry-Perot processing interferometer. Small size pressure and temperature sensing probes were constructed suitable for minimal invasive diagnostic applications including physiological use.

Biomedical researchers and clinicians have long sought the development of small, low cost and disposable sensors for in-vivo measurements of pressure and temperature. These sensors typically measure static or quasi-static pressure and temperature and are required for direct and accurate measurements over a wide range of operational, enviromental and (especially) handling conditions.

The potential of optical fibres in monitoring water uptake from plants is considered. Applications include an optical potometer that monitors water uptake from individual roots, the detection of xylem cavitation using audio acoustic emissions with an interferometric force feedback microphone, an optical displacement transducer that detects changes in leaf thickness in relation to leaf water potential, and the use of fluorescence to monitor water uptake and translocation.

Optical detection of methane gas based on its absorption at particular wavelengths has attracted considerable attention recently. The methane detection systems reported so far use either a laser source coupled with wavelength modulation spectroscopy1,2 or a relative wide-band source (e.g., LED) together a “comb” filter3,4. Recently, the broadband source - comb filtering scheme has been sucessfully used for an evanescent wave methane sensor5,6.

The development of fiber optic sensors for medical applications originated nearly twenty years ago 1-2 and has been expanding ever since. The success of these sensors is a result of the simplicity of the sensorphysician interface and their notable degree of reliability and biocompatibility. Fiber optic sensors fully satisfy the requisite of biocompatibility, i.e., they do not adversely affect the body nor are they adversely affected by it3 .

Rare-earth doped optical fibres have received considerable attention in recent years because of their important application as in-line optical amplifiers and fibre lasers. In particular, fibre lasers are attractive as miniature, efficient and temperature-stable sources for sensors and communication applications. In the case of sensors there has been recent interest in the development of tunable fibre lasers for spectroscopic gas detection in the near IR [1].

Magnetic resonance imaging (MRI) is a non-invasive technique providing superior diagnostic information to the clinician. Unfortunately, physiological monitoring of patients in MR scanners is complicated due to the presence of radio frequency (RF) fields and static and switched gradient magnetic fields (Shellock FG1986). In the proximity of MR scanners, conventional electronic monitoring instrumentation has been found to malfunction.

We have developed a fiber-optic chemical sensor technology for the remote monitoring of various volatile solvents. The accuracy, linearity, and sensitivity of the sensor (<5 ppb by weight in water, determined by comparison with standard gas chromatographic measurements) are sufficient for environmental monitoring of at least trichloroethlyene (TCE) and chloroform.

Industrial development has led to the release of numerous hazardous materials into the environment, posing a potential threat to surrounding waters. Environmental analysis of sites contaminated by several chemicals calls for continuous monitoring of multiple analytes. Monitoring can be achieved by using imaging bundles for the fabrication of sensors. Imaging bundles are formed by melting and drawing together several thousand individual optical fibers. The fibers are drawn coherently so that the position of an individual fiber of the bundle at one end (distal) corresponds to the identical position at the other end (proximal). By coupling imaging fibers to a charge coupled device (CCD), one has the ability to spatially discriminate the distal end of the fiber.

The oscillation frequency in a Brillouin Fiber Ring Laser (BFRL) is determined mainly by the axial mode of the cold resonator located under the Brillouin gain curve that experiences the highest gain. This oscillation frequency is therefore temperature dependent since both the Free Spectral Range (FSR) and the gain curve center depend on the temperature.

Fiber laser sensors based on rare-earth doped fiber lasers have potential advantages over conventional fiber optic interferometric sensors with their simplicity in both the optical configuration and the electronic signal processing.[1,2] A ring laser gyroscope(RLG) built with rare-earth doped fiber amplifier has not been seriously investigated due to a few potential problems including gain competition between the two laser signals oscillating in opposite directions.[3]

A fiber Brillouin ring laser free from polarization fluctuation induced instability is experimentally demonstrated. Stable lasing characteristics having already been analyzed by our group are confirmed. This laser is especially applicable for Brillouin fiber optic gyros.

The mean wavelength vs. pump power,? (P), is strongly dependent on fiber length, L, in Er3+ superfluorescent sources for gyroscopes. Instabilities can be eliminated by optimizing L, reducing d ? /dT to the ? 5 ppm/°C intrinsic variation.

A twelve hundred metre long, passively mode-locked, fibre ring laser has been constructed using polarisation preserving fibre to ensure operational stability. A short length of lo-bi spun fibre has been included to exploit non-linear polarisation evolution (NLPE) as the mechanism for mode-locking the fibre laser. Pulses with durations in the range of 1 to 10ns with intensities of a few Watts centred around the 1.535? wavelength were achieved. The circulating pulse within the laser cavity generates both Rayleigh and Raman scattered light. The former may be used to measure losses within the cavity, the latter permits the distributed temperature measurements of the fibre forming the cavity.

A mode locked fiber laser gyroscope (MLFLG) [1] has a laser cavity formed with a fiber amplifier with a planar mirror at one end and a Sagnac loop interferometer at the other end, as shown in figure 1. The Sagnac interferometer functions as a loop mirror, and a phase modulator in the interferometer is used to modulate the reflectivity of the loop mirror.

This paper reports on the basic design and preliminary evaluation of an entirely novel cable configuration which enables the detection of water, pH or similar variables as a function of position along the length of an optical fibre. This sensing capability is realised through a combination of Optical Time Domain Reflectometry (OTDR) and a microbend transducer activated by chemically sensitive water swellable polymers (hydrogels). Experiments with a water sensor prototype have demonstrated the detection of wetted sections of less than 25 cm length in cables longer than 100 m and indicate that interrogation of sensors several kilometres long is possible. The present experiments have demonstrated the principal of measurement through the development of a distributed water detector. However the technique can be used to monitor various chemical parameters such as pH or ionic concentration by selecting the appropriate gel as the responsive medium.

Distributed temperature sensors which utilize Raman scattering effects in optical fibers have been widely developed and used in actual applications [1] [2]. Possible applications include temperature monitoring for power cables or pipe lines. It is highly desirable to expand measurable distance with maintaining measurement accuracy.

Brillouin gain spectrum measurement along an optical fiber has recently gained a lot of interests owing to its potentiality for strain monitoring in installed cables [1] and distributed temperature sensing [2]. This measurement requires two lightwaves propagating in opposite directions through the fiber, since Stimulated Brillouin Scattering (SBS) amplification is possible only in the backward direction.

A new scheme is proposed for sensing distributed strain and temperature in optical fibers. This scheme uses Brillouin scattering as the sensing mechanism and a branched optical fiber network as the sensing fibers. Brillouin optical time domain reflectometry (OTDR) makes it possible to distinguish Brillouin scattered lightwaves from different optical fiber branches in the network when different Brillouin frequency shifts are assigned to each branch.

In this paper we present an experimental evaluation of a quasi-distributed fiber-optic strain and temperature sensor system based on time division multiplexing. The system relies on an advanced dual-core fiber to which a set of Fabry-Perot interferometers is introduced with fiber Bragg gratings used as weak reflectors.

A novel OTDR technique to accurately monitor the optical path length to multiple reflective markers is described. The method can monitor changes in optical path length of 40 ppm in a 1 sec measurement time and is suitable for multiplexed optical-fibre strain sensing over gauge lengths of several metres.

A number of the recent developments in the field of fiber optic interferometric sensing will be discussed. The major areas in which these developments occurred will be briefly reviewed.

Recent developments of stability control in mines, essentially based on Ge-doped Fibre Bragg Gratings (FBG) are reported including results about the different aspects of the system : accurate characterisations of FBG, sensor network topology and multiplexing method, user interface design and sensor packaging.

We describe an all-fiber Fourier transform spectrometer for decoding the wavelength shifts from a series of Bragg grating sensors. The system described has a spectral resolution capability of < 0.07 cm'1, which corresponds to a Bragg wavelength shift of ~ 0.015 nm in the 1.55 ?m region.

The ability of future materials to autonomously sense and respond to environmental stimuli has been proposed for several years [1, 2, 3]. Some investigators envision Hie large-scale, "smart” integrated function structures of Fifty years from now gradually evolving from the discretely instrumented and actuated structures of today and the near future for on-line, nondestructive evaluation.

We show that a grating interferometer can be used as a receiver for quasi-distributed fibre optic sensors using white light interferometry. We report the results obtained when the sensor is based on an embedded fibre optic polarimetric interferometer for strain measurement of carbon/epoxy composites.

This paper traces microbend fiber optic sensors from the early investigative work, into intial applications to mea sure many different parameters, through advanced prototype development and commercial hardware. Advantages and disadvantages of microbend sensors are discussed.

A method to produce permanent bends in single-mode fibers is presented. The losses associated with the bends can be partly recovered when the fiber is strained. A strain sensor with a resolution of 30 ?? is demonstrated.

Spectral behaviour of strain and thermal sensitivities of two superimposed fibre gratings of two different Bragg wavelengths has been studied. This involves monitoring the Bragg wavelength as a function of strain and temperature on the gratings. The results show that the ratio of sensitivity at two different Bragg wavelengths (850 nm and 1300 nm) is different with strain and temperature, which can be used for simultaneous measurement of these parameters in fibre grating sensors.

Optical fibre interferometers have been widely used to measure strain. Most of the interferometers used are also sensitive to temperature. These two parameters can be distinguished by measuring phase for two optical modes or for two wavelengths. A linear transformation converts the two values of phase into independent values for strain and temperature [1], but this transformation is usually ill-conditioned, and so magnifies measurement errors.

A novel non-incremental interferometric fiber optic measurement method for simultaneous detection of temperature and strain is reported. The fiber cross sensitivity to strain and temperature is removed by a simultaneous reading of the optical path length and of the dispersion characteristic of the sensing fiber. The reported experimental trials confirm the effectiveness of the method to recover the static strain and temperature values with 1°C • m and 10?? • m resolution, respectively.

Advanced technology in the fabrication of submicrometer sized device leads to the realization of near field optical microscopes with super-resolution beyond the diffraction limit [l-4]. A photon scanning tunneling microscope ( PSTM ) is a type of the near field optical microscope picking-up the evanescent light around the sample surface which is generated under total internal reflection and modulated by the sample[5,6] .

Fiber-optic thermometry is advantageous for applications in electrically-hostile environments, such as in high DC electric or magnetic field, or in strong RF or microwave fields. The dielectric nature of the optical fiber is also important in safety applications, such as in measurements in an explosive environment or when monitoring patients.

Much effort has been devoted to using fiber-optic sensors to measure temperature as a function of position along the fiber. The results of this work are impressive but the equipment is expensive. In many instances the desired measurement is only the spatial average temperature over a large area; eg. ground truth calibration for a satellite imaging system, or average temperature of a farm field.

The advantages of optical fibre temperature sensors over traditional technologies in hostile environments are now well accepted with commercial application of such sensors established1. However, optical fibre temperature sensing systems with improved dynamic range, system reliability, accuracy and unit cost are desirable.

Experimental research of new optical fiber sensors for monitoring civil infrastructure systems is presented. The proposed optical sensors employ a vibrating wire whose tension can be modulated by external force, strain, or vibration and is translated into the change in the wire vibration frequency. The wire vibration frequency is detected by light sent to and reflected from the wire through an optical fiber cable. Compared to other existing optical fiber sensors which tend to suffer from the lack of reliability and robustness, the proposed sensors have two significant advantages: one is that the sensing head is a vibrating wire (rather than an optical fiber), which can sense a specific physical quantity without interference from miscellaneous effects; the other is that the wire vibration is a well understood physical phenomenon. In fact, with a high level of reliability, its frequency is optically measured and transmitted to recording and other devices through the optical fiber without attenuation or distortion. These advantages make the sensor system simple, reliable and robust, and hence moi readily deployable in civil infrastructure applications.

Photonetics S.A. was created in 1979 and was initially fueled entirely by its activity of distribution of third party optic instrumentats. After this ensured the adequate "critical mass" to the company from the economic standpoint, proprietary products were progressively introduced starting in 1984.

Results from long term testing of liber grating sensors at high temperatures are presented. Experimental test results are also presented on their high temperature strain response. Creep of the liber under load appeared to be a problem at 650 C and higher, and calculations were made to estimate die significance of Ge diffiision from die core at high temperatures.

In the framework of a project initiated in 1989 at Electricitd de France, in order to improve the thermal monitoring of the stator of 900 MW turbogenerators in nuclear power plant, its R&D Pivision has supported the development of a high performance 128 channel optical sensor network achieved by BERTIN Company.

High bandwidth unsteady temperature measurements of gas flows inside turbomachinery are required as part of a general engineering objective of increasing the efficiency of gas turbines. When combined with unsteady pressure data, temperature measurements allow the unsteady losses occurring in turbomachinery to be derived and used to validate CFD codes of the unsteady flow.

A review is presented of the development of commercially available fibre optic sensors over the past 10 years,. Whilst the number of companies and sensor types is fairly static , there has been a significant shift from development to product status

The application of laser light for a sensitive detection of environmentally relevant toxic substances is gaining more and more popularity, since the necessary light sources and the optoelectronics are now in an acceptable price/performance ratio.

Two categories of multimode fiber Bragg gratings (MMFBG) are possible, independent mode and mode coupling, each leads to novel types of fiber devices and sensors. All-mode, wave optics calculations of the reflectivity of independent mode MMFBG are presented, including the effects of individual mode power changes due to microbending transducer loss upstream from the grating. MMFBG offer added flexibility in grating design and performance characteristics compared to single mode FBG, since the reflectivity response may be tuned by the spatial periodicity, length, core size, numerical aperture, and mode coupling characteristics of the grating. New sensing and diagnostic methods that are possible with these gratings and some methods for sensor addressing and multiplexing are described.

A tunable narrow linewidth erbium doped fibre laser is reported. Wavelength definition and agility are achieved using a novel electro optically tuned "continuous fibre" bandpass filter. Linewidths down to 0. 03 nm and tunability over 20 nm have been observed.

A new integrated optical device is demonstrated for the polarisation independent spectrum analysis of in fiber optical radiation. The device exploits the heterodyne detection of the optical signals from the output of an acousto-optical tunable filter fabricated on XY LiNbO3. The device is used to analyse and multiplex the optical signals from Fiber Bragg Grating sensors.

Acousto-optics has been shown to be a powerful and versatile means of controlling and modulating laser radiation in bulk components. We discuss the fibre-optic and fibre-compatible versions of components such as the Bragg cell, together with general design rules for such components and illustrate by describing some recent work at the Optoelectronics Research Centre.

All-fibre acousto-optic frequency shifters have in the past been based on two-mode fibres1,2,3, the frequency- shifted light appearing in the second of the two modes. Mode convertors and filters are necessary to separate the residual carrier from the shifted signal and give a single-mode output.

Since its initial demonstration by Kim and Blake[1,2]. the in-fiber frequency shifter has seen several modifications including cascaded acoustic interactions,[3] new acoustic horn designs,[4] new acoustic coupling geometries,[5] and novel mode filters, [6] This device employs elliptical-core two-mode fiber, i.e., fiber which supports the LP01 and the LP11 modes.

Er doped fiber ring laser without mode instability due to polarization fluctuation is proposed and experimentally investigated. The resonator composed of a twisted Er doped fiber and a coiled fiber ?/2 element can fix the frequency-separation between two polarization-modes.

Superluminescent light-emitting diodes (SLD) and modules are now widely used for different types of fiber optic sensors, especially fiber-optic gyros and white-light interferometer sensors. The main parameters of SLDs are output power, spectrum width and residual spectral modulation (ripple).

In recent years, the huge bandwidth and low-loss transmission advantages of single-mode optical fibre have led to its incorporation in an increasing number of optical sensor systems. Traditional methods of processing the optical signals in such systems have been via interruption of the fibre and subsequent insertion of bulk- or integrated-optic components.

We demonstrate that twisting a fiber a few turns per meter before it is annealed largely eliminates the residual linear birefringence. This dramatically improves the yield of annealed coils used for current sensing and makes it possible to use fibers that previously could not be successfully annealed. It is believed that twisting is effective because the residual birefringence is associated with core ellipticity and this contribution is averaged to near zero by twisting. We also show the temperature stability of sensors made with this new technique.

The discovery of the Ortho-Conjugated Mirror and its relationship with the time-reversal concept is analysed. Practical implementation of OCM is reviewed together with its insertion in the fiber-optic circuits.

An interferometric sensor based on the use of a chirped Bragg grating reflector is described. It is shown that strain applied to the chirped grating can induce a large path length change in an interferometer, equivalent to that obtained with a multi-meter length of fiber when strained directly.

The photoacoustic process has great potential as a spectroscopic technique appropriate for opaque, highly scattering or weakly absorbing samples, all unsuitable for use with standard absorption spectroscopy. In particular, it is suited to liquids with low optical absorptions, for example the sensing of hydrocarbon pollutants in water [1].

Feedback of the interference signal of an imbalanced Michelson interferometer to the driver of the semiconductor laser source gives rise to asymmetric, sawtooth like fringes under fiber strain which is utilized for elimination of the fringe counting ambiguity.

We describe a fiber Bragg grating laser sensor which utilizes a read-out interferometer for detecting weak measurand induced wavelength shifts in the lasing wavelength. The advantages of the system include simple 'in-line' sensor design, high sensitivity, and the capability to support multiple sensors from a single pump laser using wavelength division multiplexing.

In many potential applications of optical fibre interferometric sensors the requirement is for very small sensors that perform essentially point measurements, for example, temperature, pressure, vibration and strain. For such applications low-finesse fibre Fabry-Perot interferometers (LFFFPI) have demonstrated excellent potential.

Many attempts have been made to use optical fibres to measure strain or temperature. However, since fibres are sensitive to both strain and temperature, practical strain measurements require that the effects of temperature are simultaneously measured or compensated for. This necessitates the measurement of two parameters which respond differently to changes in strain and temperature, a condition which may be met using fibres operating in the dual mode regime.

We demonstrate the feasibility of optically demodulating and processing the output produced by an interferometric fiber sensor. A technique for providing the time filtering of an optical signal in the optical domain based on the use of a doped fiber section is reported. Optical processing is a field which offers enormous potential for rapid, parallel processing of information, and is an area which will continue to experience significant growth over the coming years. Optical sensors have a natural inherent compatibility with this form of processing, which could be utilized to great advantage for processesing the data from large arrays of sensors. Many forms of fiber optic sensor, however, produce an optical output which does not vary in a uniform fashion with the measurand field, and consequently require a linearization circuit. or 'demodulator' to provide a linear transfer function before a signal could be converted back to the optical domain and operated on by an optical processor. This is particularly true with interferometric fiber sensors which exhibit a cosine squared transfer function. To demodulate this form of output, a range of demodulation schemes have been developed, such as active phase tracking homodyne [1], phase generated carrier (PGC) passive homodyne [2], and synthetic heterodyne [3J. These techniques allow the detection of measurand-induced phase shifts down to the irad/'JHz level over a wide range of frequencies with high linearity and wide dynamic range. Certain forms of interferometric sensors designed for low frequency measurands, such as magnetometers and electhc field sensors, require the additional demodulation of a 'dither' induced phase carrier signal which is modulated by the DC and low frequency measurand information. In such systems, the overall demodulation process requires several electronic processing blocks after photodetection of the interferometer signal. In applications where arrays of such sensors are of interest, a significant amount of electronics may be required to fully demodulate the sensor outputs. We have examined the practicality of using optical techniques to perform much of the demodulation process, and in this paper demonstrate the feasibility of this concept.

The application of straightforward spectral signal processing in intrinsic interferometric strain, temperature and distributed linear position fiber optic sensors based on birefringent polarization maintaining fibers is demonstrated. The method provides nonincremental absolute measurements of external physical parameter affecting the sensing fiber.

In interferometric optical-fibre sensors using conventional low-birefringence fibre, random fluctuations in the state of polarization (SOP) of the interfering beams give rise to variations in the output fringe visibility[1-4] and, consequently, to fading of the detected interferometric signal. This "polarization induced fading” (PIF) phenomenon is an important drawback which affects the practical applicability of interferometric sensors in a number of areas.

We demonstrate measurements of fundamental thermal induced phase fluctuations in the fiber of a Sagnac interferometer. Loop lengths of 1.1 and 2.2 km are considered, showing phase noise levels between ~0.2-1.4/?rad rms/?HZ in the 1-100 kHz frequency range.

The design and performance of single element fiber optic hydrophones for shallow water applications is described. Hydrophone responsivities up to -113.8 dB re rad/?Pa were measured while maintaining a usable depth capability. A system with this design of hydrophone is being used to monitor the deterioration of water carrying siphons in Arizona.

Fiber-optic sensors have been applied for measuring mechanical deformations in a variety of configurations. Fiber-optics has the potential of very high resolution measurements. However, in many situations like in detection of catastrophic events a large dynamic range rather than a high resolution is required. In the case of fiberglass reinforced composite materials the critical elongations are on the order of 0.1 -0.5%.

Fiber optic sensors used in smart structure applications currently use commercially available optical fibers manufactured for communications purposes. Typical core/cladding sizes are on the order of 9/125 ?m for single mode fibers and 50/125 or 100/140 ?m for multimode fibers. These fiber sizes cannot be considered to be unobtrusive when embedded in graphite-epoxy composites where the graphite fiber sizes are on the order of 10?m.1

Many parameters contribute to the performance of microbend sensors. These include fiber core radius, cladding radius, jacket radius, core and cladding refractive indices, core refracti"e index profile, fiber flexural rigidity, light source wavelength, light source power, spatial bend wavelength, number of bends and bend amplitude. Electromagnetic wave theory predicts that power lost from the core in a microbend is optimum when the fiber's spatial bend frequency equals the difference in propagation constants between the propagating and radiated modes.

The dependence of the performance of a compliant cylinder accelerometer upon the geometrical details and elastic properties of the transducer cylinders is examined experimentally. A simple analytical model was developed in order to explain the experimental results qualitatively.

To perform a high temperature strain measurements, the sapphire extrinsic Fabry-Perot interferometer (EFPI) sensor head was attached to a silicon carbide rod and placed in a RF induction heater mounted on an MTS load frame. The high-temperature ceramic adhesive is designed to withstand temperatures up to 4000° F. As a basis for comparison, a gold-coated silica EFPI sensor was attached along side the sapphire EFPI. Initially, strain data was recorded at room temperature as the silicon carbide rod was slowly loaded to 4500 lbs. When the temperature was stabilized at a maximum of 2000° F, the SiC rod was again loaded to determine the effects of temperature on strain. Although the sapphire sensor was capable of measuring strain at higher temperatures, die induction heater, being poorly insulated, was not able to further heat the test material. The purpose of the second experiment was to determine the survivability and repeatability of tiie sapphire EFPI due to high-cycle fatigue loading. The long-term survivability of the sensor, in both static and fatigue loading, has been demonstrated. Although the load frame was shutoff after 200,000 cycles, one would expect to be able to fatigue load tiie sensor for millions of cycles since the components of the EFPI sensing head are free to move inside the alignment tube. This is consistent widi data obtained for similar silica opdcal EFPI sensors diat were fadgue loaded to over 10 million cycles as compared to resistive strain gages that typically can be cycled only to 60,000 at 2000°F.

We describe a fiber-optic rotation sensor being developed for anti-lock braking systems (ABS). The basis of the sensor is the magneto-optic detection of the magnetic fields generated by a wheel of alternating magnetized magnets fixed to a wheel of the automobile. Highly sensitive iron garnet crystals serve as the magneto-optic sensing elements. For films with perpendicularly- magnetized domains, the domain structure produces diffraction which is magnetic-field dependent. Exploitation of this effect permits the construction of magneto-optic magnetic field sensors requiring no polarization elements or lenses. A fiber-optic wheel-rotation sensor has several advantages over present day sensors. It would be directly compatible with other optic sensors, processing computers, and equipment. It would also help solve a corrosion problem, especially common on trucks, that has prematurely ended the life of many ABS systems by destroying the wires running from the wheel sensors to the controller. Fiber optics are easily shielded and are not subject to salt damage. Operational advantages are present as well because the magnetooptic sensor detects the magnetic field strength directly, not its change in time. Thus there is no signal loss when the speed diminishes, as is the case with an inductive sensor. Since the magneto-optic sensor works all the way down to zero speed, the same sensor can be used for the speedometer, and by sensing small differences in wheel speeds it can also operate a traction control device, providing a change in coupling efficiency from the transmission to the wheels to prevent spinning. A typical ABS sensor consists of a series of magnets mounted as an integral part of the wheel bearing and a sensor in close proximity to detect the presence of those magnets as they rotate with the wheel. A Faraday effect sensor detects the change in direction of the polarization of laser light in the presence of a magnetic field within some sensitive medium, glass or a crystal. Our first ABS sensor, shown schematically in figure 1, used a crystal with a very high magneto-optic sensitivity, bismuth iron garnet(BIG). The fiber is multimoded with a core diameter of 100 jm and the laser source is at 1.3 tim. The sensor element has four components, a lens to collimate the light, a polarizing medium, the sensing element (the BIG crystal), and a reflective surface or mirror. An alternative to this arrangement is to employ a GRIN lens for the focusing but to intentionally have the fiber off axis with respect to the GRIN lens so that the reflected light does not reenter the original fiber but is captured by a second fiber adjacent to the input fiber. In this manner the fiber optic splitter(coupler) can be eliminated. With both schemes there is considerable adjustment of the optical elements required for proper operation. Our work in this field has been preceded by that of others.[1] In addition to the Faraday effect which produces a polarization rotation of the light, there are also diffraction effects arising from the iron garnet's two-dimensional magnetic domain structure. The domains in the films used in this study are naturally magnetized perpendicularly to the film surface. In the demagnetized state, domains with their magnetization vectors pointed up cover an area equal to that of domains with their magnetization vectors pointed down. Optically, this arrangement acts as a two-dimensional phase grating and produces diffraction[2}. A magnetic field applied to such a film changes the areal balance of the up and down domains (through domain wall motion) which modulates the distribution of optical intensity between the various diffracted beams. In particular, the intensity of the zeroth-.order (undiffracted) beam, which will readily couple back into the optical fiber, will be a minimum for zero applied field, and a maximum for a saturated field (of either polarity). The behavior of the higher-order (deviated) diffracted beams is less important since they will be highly attenuated when the light couples back into the optical fiber. The time scale associated with this process is typically much faster than the time scale of the moving magnets[3]. This effect can be so pronounced under certain conditions that the amount of light transmitted increases in a conventional Faraday sensor, (Fig 1), with the application of a strong magnetic field when simple Faraday effect would predict a decrease in light. Using diffraction as the sensing process allows a simplification of the sensor element shown schematically in figure 2. The use of unpolarized light allows the elimination of the polarizing element. A thin iron garnet film ~ 0.25mm thick, is attached to the end of a fiber, and the back of the crystal is coated with a reflector. Even without a lens, enough light is reflected back into a single mode fiber to provide a useable signal. In the absence of a magnetic field, much of the reflected light is diffracted away from the optical axis and therefore does not couple back into the optical fiber. When a magnet approaches the crystal, the diffraction diminishes and the return signal increases. This signal modulation provides an easy way to count the number of times a magnet passes the sensor element, giving wheel rotation. The characteristics of the fiber which are needed to optimize this response to diffraction by the sensor element are, 1) the core of the fiber should be large so that the light illuminates at least several domains, and 2) it should have a small numerical aperture so that if the light is diffracted by a small angle it will not be captured in the core upon reflection. The first requirement makes the use of single mode fibers difficult as the domain sizes are typically 10 ,?m or more. The combination of required fiber characteristics is provided in the TEC (Thermally expanded core) approach to producing sensors.[4] One of the main arguments advanced by the TEC developers is that such a procedure is specifically aimed at mass producing optical sensors in an inexpensive and simple manner. Because of this we are presently investigating the process of developing this diffraction sensor in conjunction with TEC fibers.

In the present paper an attempt has been made to immerse the micromechanical resonant structure in the gas medium and investigate the pressure and temperature sensitivities. By means of a closed loop between the exciting laser diode source,the mechanical resonator and the interrogating detector diode it is possible to design a self-oscillating system,which automatically provides the correct mechanical resonance frequency.This simple and economic data reduction system seems to operate quite efficiently.

We describe the design and performance of a novel planar ultrasonic fiber sensor for detection of air acoustic signals. The sensor is formed from a potted single layer of spirally wound optical fiber, and configured such that the static backing pressure can be varied. Experimentally we demonstrate normalized sensitivities (?Ø /Ø?P) of-295 to .265dB re. 1 µPa1.

Laser diode (LD) pumped solid state lasers are of recent concern for coherent light sources and sensing applications. It has been recently demonstrated that the LD pumped Nd:YAG laser can be operated In the single axial model1 , at orthogonal-polarization two frequencies2 and can be applied to high resolution force sensor3.

A novel fibre-optic laser vibrometer has been developed in which a single highly birefringent fibre links the launch optics with a simple passive all-glass instrument head. The instrument utilises heterodyne detection and can be made insensitive to undesired vibration effects acting on the fibre. The instrument may be greatly simplified and its performance enhanced if it is realised in all-fibre form.

Laser Doppler anemometry is a well known technique for non-invasively measuring fluid flow. Originally developed using gas lasers, the inclusion of semiconductor laser diodes and detectors [1] and optical fibres [2] have made it possible to construct compact and robust probes. To extend laser Doppler anemometry to multi-component velocity measurements, some method of distinguishing the various velocity components is essential.

We use an optical fiber Sagnac interferometer to simultaneously measure magnetic and electric fields. Phase shifts, induced by the Faraday effect and voltage-controlled birefringence, are separately retrieved by in-phase and quadrature demodulation of the interferometer output.

Laboratory and field tests have proven that magnetostrictive fiber-optic interferometric sensors can detect small low-frequency disturbances of the earth magnetic field [1]. With proper sensor design, the thermal noise limitation [2] does not rise above the interferometer baseband noise of around 0.3 µrad?Hz.

We present a magnetometer based on a new in-line configuration for the Sagnac interferometer. A magnetic field induces a phase shift between orthogonally polarized light waves that travel back and forth in a fiber bus.

Optical fibre techniques are near-ideal for use in the measurement of microwave E and H-fields, offering low volume and permittivity of intrusive material, and minimal metallic components.

The great interest in fiber optic current sensors results from their inherently high voltage isolation which eliminates the ground loop difficulty. Moreover fiber optic cunent sensors have the advaiitages of wide bandwidth, immunity to electromagnetic interference, low cost, and small size, over the traditional mutual inductance type current transducers. Most fiber optic current sensors utilize the Faraday effect.

This paper describes about a method to improve immunity from the environment of the flint glass fiber Faraday effect current sensor. Principle of the method is reciprocation of light in the fiber. From the result of experiment it was confirmed that stable properties can be realized by applying the method.

A stable ring-resonator has been fabricated by an ordinary single mode fiber. Combination of a twisted-fiber loop and a coiled-fiber ?/2 element makes the resonance-dip-separation fixed to be ? to reduce the polarization-fluctuation induced drift in Resonator-FOGs.

For years research and development in the fiber optic gyro area have been concentrated on the standard solution known as "minimum configuration with nonreciprocal phase modulation" [1]. As an alternative to that modulation scheme, S.K.Sheem published in 1980 the use of a 3x3 coupler [2], which allows to work at the quadrature point passively without using high frequency optical phase modulation schemes.

A fiber optic gyroscope with its coil axis aligned to North-South and inclined to the horizontal by the angle of Latitude (i.e. with its axis parallel to that of the Earth) measures the angular rotation of the Earth: 15.041°/hr.

A series of devices are described which blend together the wave guide properties of optical fibres and the phenomenon of electroluminescence. Each optical sensor has a pair of platinum electrodes near to the fibre face. In many conventional fibre optic sensor systems the fibre acts as a two way light guide, but in this novel system, light is generated at the anode electrode reflector plate placed opposite the fibre end, there is no need for a primary light source as in the colorimetric or photoluniinescence mode of operation. The cathode electrode is a circular ring of platinum around the fibre base. This elecirode configuration produces a symmetrical eleciric field about the fibre end, the active sensor region of the system. However, the system may be used in the two way light guide mode, in this case photo coupled electroluminescence may be generated. Another system described uses a piezoelectric ultrasound transducer coupled to the anode electrode. The operation of each system is characterised using the model hydrogen peroxide/luminol reaction.

We have reported a fibre optic evanescent wave (methane) gas sensor where a D-shaped optical fibre was used as a sensing element1 . Progress has been made in optimisation of the sensitivity of the D-fibre to methane at 1 .64µm2, in the development of signal processing techniques3, and in understanding and suppression of various noise sources4,5.

An alternative to the electrical microphone for photoacoustical detection of trace gases is presented. This microphone is made of a monomode optical fiber wound and glued on a thin plate. This fiber is one arm of a Michelson interferometer. A Sagnac interferometer has also been used and has given better results.

The ability to determine oxygen concentration is of great importance for many industhal, medical and environmental applications. Optical oxygen sensors are attractive because of their many advantages over other sensor types, in that they offer the possibility of miniaturization, have fast response times, do not consume oxygen, and are not easily poisoned.

Fibre optic chemical sensors represent a relatively new but very rapidly growing research area. This kind of sensor possesses a number of potential advantages: immunity to electromagnetic interference, low transmission loss, low cost, small size, geometric flexibility and all dielectric construction being the most obvious [1].

To evaluate the applicability of PCS optical fiber as radiation level sensor, steady state irradiation in 0.4 to 1.6 µm wavelength range is performed and radiation induced loss measured. It is found that in the visible range the response of PCS fiber is strongly linear with dose up to 103Gy and stable in time, with sensitivity 1.47x10-1 dB/km Gy at 0.6 µm, so it can be used for radiation level dosimetric purposes at 60Co ionizig source irradiations.

Fiber optic imaging bundles are comprised of thousands of individual fibers melted and drawn together in a coherent manner. Sensor arrays are fabricated by combining the optical pathways of imaging fibers with the spatial discrimination of CCD video cameras.

An Uni-diameter Tail tapered Fabry-Perot Interferometer, UTFI, based on the concept of the well known fibre optic Extrinsic Fabry-Perot Interferometer strain sensor, was geometrically developed for improved embedding properties in carbon/epoxy composites. The sensor was annealed and etched with HF-acid for improved strength and re-coated with polyimide for moisture protection. The correlation between fringe visibility and geometrical parameters of the sensor was studied to establish requirements for the manufacturing process.

For a number of years, radioactive waste produced from nuclear weapons material processing has been stored in underground storage tanks (USTs). At some government sites, these tanks were fabricated with both single and double wall construction with capacities ranging from 500,000 to 1,000,000 gal.[1] These tanks are typically 6 feet below ground level, 74 feet in diameter, 40 ft. high with a 16 ft.

Integrating fiber optic sensors offer the potential to monitor large spatial extents due to their geometric flexibility. To date, this potential has not been widely exploited. By increasing the length of an integrating fiber optic sensor, its gain and/or gauge length can be increased. In addition, by configuring the sensor to a spatial distribution ofsome parameter ofinlerest, the antenna gain ofthe sensor will provide selectivityfor that particular distribution. in this paper, the results of an experiment demonstrating these properties of integrating fiber optic sensors are presented.

This paper describes a new engineering application, in the area ofperimeter intruder detection, of an interferometric fibre optic sensor system. This system was designed originally as a multiplexed fibre optic hydrophone system, for underwater naval applications. To trace the history of the system, and the progress made to date in the area ofunderwater acoustics, see references 1 -3.

Although robots originated as machines devoid of sensoring and logical capabilities, the current generation is capable of high-speed decision-making and problem-solving. Today, thanks to the notable strides in sensoiing and artificial intelligence, fiber-optic sensors are increasingly being used to enhance robotic sensoring and logical capabilities 1,2,3,4.

Multilayer ceramic actuators (MCAs) are of interest for u in actuated materials because of their small hysterisis, small size, and durability, and the relatively large range of displacements made possible by their multilayer construction. The processing of the MCA elements requires involved detail and time, in part to ensure the removal of all organic components contained in the polymer binder and the noble metal electrode layers.1

It has become recognised of late that when some of the highly coherent laser sources that are now available are used in reflective-type interferometric systems with long connecting leads, noise due to Rayleigh back-scattering may provide a fundamental limit on the resolution of the sensor.

Signals from fiber-optic interferometric sensors fabricated with conventional communication fibers may be lost due to the random fluctuation of the polarization states of two interfering optical waves.

Optical fibres, which have been developed for long distance, high bandwidth communications, possess a very low loss, are cheap to manufacture and are readily available in long lengths.

Fibre optic Bragg gratings are attracting considerable interest as sensing elements in quasi-distributed sensors.1 ,2 The grating takes the form of a periodically varying refractive index written into germania doped fibres to reflect light at a wavelength ?=2nD where n is the mode refractive index and D is the grating period.

A variety of techniques for multiplexing fibre optic interferometric sensors (FOIS) have been described in the literature [1]. For large numbers of sensors the topology known as the matrix-array [21 using frequency division multiplexing (FDM) is attractive despite the fact that the multiplexing gain is lower than networks exploiting time division multiplexing (TDM) as it allows large numbers of sensors to be multiplexed.

Intrinsic fiber optic Fabry-Perots (FP) adhered to the surface of a cantilever beam are used as strain sensors. Low coherence demodulation with a scanning Michelson interferometer (MI) and interfringe resolution techniques allow absolute interrupt-immune measurements of static longitudinal strains with a resolution of a few microstrains. Low reflecting FPs arranged either in parallel or in series can be multiplexed by giving them a difference in their lengths.

Real world operational environments place stringent requirements upon the fidelity of the demodulator output to the acoustic signal which is present. This paper discusses the optimization of a demodulator circuit for hydrophones in time-varying hydrostatic pressure environments as well as in environments which contain large amplitude acoustic signals.

A performance improvement in the first experimental demonstration of a frequency hopping strategy for use on a passive interferometric optical fibre sensor multiplexing network is reported. The preliminary results successfully demonstrate the principle, and two interferometric sensors have been multiplexed with crosstalk levels less than -51 dB.

A hybrid interferometric sensor multiplexing strategy using spread spectrum in conjunction with wavelength division multiplexing is reported. An experimental arrangement incorporating six interferometric optical fibre sensors is demonstrated. Preliminary measurements Indicate crosstalk levels below -50dB.

A new reflectometry using delta-function-like optical coherence-function synthesized by optical frequency modulation and scanned by optical phase modulation is proposed. Basic experiments are successfully carried out with spatial-resolution of about 6 mm and measuringrange of 43.2 cm.

A new spatial multiplexing technique which uses a confocal scanning microscope to address fibre—optic sensors is proposed. Multiplexing ofmore than 1000 sensors has been achieved both in amplitude and phase sensor regimes.

Interferometric fiber-optic sensor arrays of the Mach-Zehnder type are subject to performance degradation attributed to polarization-induced signal fading1. Envfronmentally-related polarization drifts in the sensor arms change the relative polarization states of the interfering beams, leading to critical changes in the sensor scale factor and output S/N ratio up to complete signal fading2.

Polarization-maintaining(PM) couplers are essential components in coherent optical communication and fiber sensor systems. The devices are required to have high performance and low-cost. Polished type PM couplers with PANDA fibers ,for example,which show low polarization crosstalk were reported so far [1].

The performance of fiber optic interferometers constructed with 3x3 couplers is investigated. Deviations from ideal behavior are noted at low fringe visibilities. The use of Faraday mirrors in a 3x3 Michelson configuration is shown to eliminate these effects.

As single mode fibre optic sensors become more complex there is an increasing need for efficient low loss components that are compatible with optical fibres. The severe interfacing problems between fibres and the existing device technologies are well known and in-line polished fibre devices using the multimode approach [1,2] have been demonstrated to be a viable alternative since the requirement to break the fibre is removed.

The technology of so called "continuous fibre" components where active optical functions are achieved by the external perturbation of the evanescent field of the fibre waveguide is promising for the development of optical sensor systems.

Wavelength filters have applications in wavelength-division multiplexed optical communications and sensor systems. Bulk-optic birefringent filters offer sub-nanometre passband widths, but are not compatible with optical fibre systems. However, fibre analogues of the bulk-optic filters can be constructed using birefringent fibre.

Microbend losses have been used as the transduction mechanism in intensity modulated optical fiber sensors for the measurement of various physical variables such as pressure, strain, temperature, acceleration, magnetic, and electric fields 1-4. These sensors are attractive for the industrial sector because of their robust construction and simple signalprocessing techniques.

Acousto-optic interaction in fibers between optical modes and acoustic waves has been investigated recently by several groups. Coupling has been demonstrated between spatial modes in two-mode fibers1 as well as between polarization modes of high birefringence fibers2. Most publications have reported results for a single acoustic frequency without discussing the frequency response of such devices.

The impact of thermal noise in the fiber on the noise power and signal-to-noise ratio (SNR) of fiber-optic sensors is investigated, and sensors based on Sagnac and Mach- Zehnder interferometers are compared. Mach-Zehnder interferometers exhibit a slightly higher SNR at low frequencies whereas Sagnac interferometers may have a few dB higher SNR at high frequencies.

When a low coherence light source is used in a multimode fibre linked interferometer, the level of the modal noise [1] induced by the environmental variations on the fibres may be dramatically suppressed, provided the value of the coherence length, Lc, of the light source employed is less than that of the optical path differences (OPD) between the guided modes at the far end of the fibre [2, 3].

Fiberoptic sensors are employed for the measurement of many physical quantities. 1,2 Fiberoptic sensors using glass or lead glass optical fibers have been shown in radiation detectors3,4 or dosimeters.5,6 It has been shown7-9 that the main effect of the interaction of ionizing radiation (typically, ? -rays from a Co60 source) with optical fibers is the increase in the specific optical attenuation (dB/m) of the fiber waveguide due to the radio-induced additional formation of color centers.

The intrinsic thermal phase noise spectra of the Michelson, Michelson with phase conjugate mirrors, and Sagnac fiber optic interferometers are presented. The thermal phase noise is maximum at dc and zero at the loop frequency for the Michelson with ordinary mirrors. Thermal phase noise eliminates the factor of two transducer phase sensitivity advantage of the Michelson over a comparable length Mach Zehnder. The Michelson with two phase conjugate mirrors exhibits identical noise to a Sagnac interferometer with the same total fiber length. The Sagnac exhibits common mode phase noise rejection at low frequencies and a broad maximum in the phase noise at the loop frequency of ~1 .µrad rms/?Hz for a 1 km fiber ioop at 1319 nm.

We have recently demonstrated1-3 that the optical Kerr effect can be used in pulsed-pump, cw- probe configurations to detect changes in the linear birefringence of single mode optical fibres. This phenomenon can be used in different ways to detect the presence of stress in the fibres, to locate the stress and to measure the magnitude of the associated birefringence changes as a function of position along the fibre.

White light interferometry (WLI) has been proven to be an efficient technique to increase the unambiguous range and to perform absolute measurement1 . Abasic white light interferometric sensor system is generally composed of a sensing interferometer and a receving interferometer.

There is a widely recognized need in the electric power industry for a method of measuring currents which avoids the high dielectric stress which occurs in the conventional technology based on current transformers. Many attempts have been made to employ the Faraday effect in a single mode optical fibre for current measurement.

A length of PANDA fiber is cut at its midpoint and spliced again so that birefringent axes can be arranged interchangeably. At the spliced location it is folded to make a double fiber-coiled sensor to achieve immunity from temperature disturbances. The sensor sensitivity obtained is 1.9 deg/hPa.

Two fiber optic pressure gradient hydrophones have been designed, constructed and tested. These devices both exhibit good sensitivity and appropriate directionality. Their responsivities are linear in the 100 Hz to 1000 Hz frequency band.

All-fiber acousto-optic modulators have been proposed for applications in which large insertion losses or back- reflections due to fiber-air or fiber-integrated optic interfaces cause detrimental effects.[1] Many sensing applications, such as the fiber-optic gyroscope and other heterodyned interferometers have such a requirement. A component of great interest in these applications is the in-fiber frequency shifter.