Table of contents

The aim of this special issue of Measurement Science and Technology is to provide the reader with an overview of the recent progress and current state-of-the-art in `Optical Fibre Sensors'.

The 41 papers appearing in this issue contain detailed presentations of some of the articles presented at the 14th International Conference on Optical Fibre Sensors, which was held at the Giorgio Cini Foundation on the Island of San Giorgio Maggiore in Venice, Italy, from 11 to 13 October 2000. This series of Conferences began in London in 1983, travelling around the world with a meeting held every one and a half years, in rotation, in the three geographic areas of the USA, Asia and Europe, always in a different location. While the term `optical fibre' is associated by the man in the street with telecommunications, this special issue offers an insight into the heterogeneous community of scientists dealing with research, new concepts and new applications for the field of optical fibre sensors. Their efforts have been rewarded by the success of these sensors in many applications that demand very high resolution measurements or impose very difficult working conditions.

The range of topics and issues addressed in this collection reflects the general trends in optical fibre sensing technology and applications. Half of the papers deal with optical fibre Bragg grating and long-period grating technology for many varied applications ranging from aerospace to medicine, from refractometry to torsion and bending monitoring. In addition to grating-based sensors, original ideas are presented for applications in which optical fibres continue to confirm themselves as unique tools for overcoming the drawbacks and limitations of sensors based on other technologies, especially in extremely high or extremely low temperature conditions, in safe environmental monitoring or in smart structure applications.

Besides sensors exploiting technologies and interrogation schemes which are becoming consolidated concepts, there is still basic research in progress both on emerging fibre-based sensors, such as those that make use of microstructured fibres, and on new interrogation schemes, such as those that make use of artificial neural network processing.

We hope that this feature will stimulate new directions for an ever wider instrumentation community and will continue the drive to more intelligent, precise, autonomous and durable sensing devices.

A note of thanks is addressed to all the contributing authors and reviewers who have made this special issue on Optical Fibre Sensors possible, and also to Institute of Physics Publishing and their staff for all their hard work and support.

An opto-chemical in-fibre Bragg grating (FBG) sensor for refractive index measurement in liquids has been developed using fibre side-polishing technology. At a polished site where the fibre cladding has partly been removed, a FBG is exposed to a liquid analyte via evanescent field interaction of the guided fibre mode. The Bragg wavelength of the FBG is obtained in terms of its dependence on the refractive index of the analyte. Modal and wavelength dependences have been investigated both theoretically and experimentally in order to optimize the structure of the sensor. Using working wavelengths far above the cut-off wavelength results in an enhancement of the sensitivity of the sensor. Measurements with different mode configurations lead to the separation of cross sensitivities. Besides this, a second FBG located in the unpolished part can be used to compensate for temperature effects. Application examples for monitoring fuels of varying quality as well as salt concentrations under deep borehole conditions are presented.

We investigate the changes in the transmission spectrum of long period fibre gratings and tilted short-period fibre Bragg gratings versus the refractive index of the surrounding medium. The metrological characteristics of tilted short-period fibre Bragg gratings and an analytical method enabling their potential use in accurate refractometry are discussed.

This work describes a new technique for fibre Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning Fabry-Perot (SFP) bandpass filter used to wavelength-multiplex multiple gratings in a single fibre, and an unbalanced Mach-Zehnder fibre interferometer made with a 3×3 coupler to detect strain-induced wavelength shifts. A passive technique for interferometer drift compensation using non-sensing FBGs is included in the system. A prototype complete system interrogates four gratings in a single fibre at a Nyquist sampling rate up to 10 kHz, with a noise floor measured near 4 n Hz^-1/2 above 0.1 Hz. The inclusion of the interferometer drift compensation technique is shown to make quasi-static measurements feasible.

We will discuss fibre-optic sensing applications with high resolution and sensitivity that are based on a pair of long-period fibre gratings (LPFGs) characterized by a fine interference fringe pattern and a narrow bandwidth. The dependence of the shift in resonance wavelength and change of peak depth of a pair of LPFGs pair on changes in the external environment such as the bending curvature, transverse load and surrounding index are investigated experimentally. The temperature sensitivity of the LPFGs was suppressed to 0.002 nm °C^-1 in order to reduce the effect of temperature on the performance of the sensor.

The results of an experimental and theoretical investigation of the noise and signal-to-noise levels associated with the interferometric detection of fibre-Bragg-grating- (FBG-) reflected light are presented.

Curved long-period fibre gratings are described in terms of a new model, based on a matrix method. The variations in the formulation of the main equations, which define the spectral behaviour of a long period fibre grating, are presented. Good agreement between experimental data that had previously been reported and results from the proposed model are obtained.

Based on the photoelastic effect, the strain distribution within a corrugated long period fibre grating (LPFG) subjected to tensile stress and twisting will cause a periodic index variation, which may result in resonant couplings between fibre core and cladding modes. The resonance wavelength corresponding to the peak loss is directly sensitive to the torsional angle per unit length; thus a novel torsion sensor can be made from the corrugated long period fibre grating. We also present a phenomenological theory based on the scattering matrix formalism to qualitatively explain the mode coupling behaviour in the corrugated grating.

A method for measuring the sensitivity of Bragg gratings to external parameters for different spatial and polarization modes is presented together with the results of measurements of sensitivity to temperature and elongation for LP^x₀₁, LP^y₀₁, LP^x₁₁ and LP^y₁₁ modes. The difference in response according to the type of mode shows the possibility of performing simultaneous temperature and elongation measurements using two-mode Bragg gratings.

A fibre optic Bragg grating sensor to measure the strain in the chest of patients is described. The sensor provides a new method to measure ventilatory movements and it can also determine the respiratory frequency spectrum. The apparatus uses a fixed optical filter reference scheme to reduce the overall cost and is able to detect respiratory movements with frequency components up to 10 Hz. Ventilatory signals were acquired from slow breathing, with 500 ml inspiration volume, to fast, with a reduced volume of 60 ml, respiration. The sensor described here can be used to trigger the pulse burst of electrically assisted ventilation processes or to monitor high frequency oscillatory ventilation.

The control of robots in electromagnetically noisy environments may benefit from the use of EMI-insensitive multi-component force sensors. Multi-component force sensing is usually done with local strain measurements on an elastic transducer. We propose to use fibre Bragg grating (FBG) strain sensors to perform these local strain measurements, taking advantage of their multiplexing capabilities and their immunity to electromagnetic interference. In this paper, we discuss the design and the calibration of a compact multi-component force sensor using an elastic transducing body and eight multiplexed fibre Bragg gratings. We demonstrate, for the first time, that multi-component force sensors based on multiplexed FBG strain sensors can be constructed.

We propose a sub-carrier phase detection scheme for interrogation of a chirped Bragg grating strain sensor. The phase delay of the envelope of amplitude modulated light upon reflection from a chirped Bragg grating provides information on the strain applied to the grating. The concept was demonstrated experimentally with a linearly chirped grating with a bandwidth of approximately 1 nm. For a 12 cm long apodized grating and modulation frequency of 1 GHz strains of up to 800 micro-strain could be measured with a standard deviation of 7.3 micro-strain.

In this paper, we report the growth behaviour of long-period gratings (LPGs) in a hydrogen-loaded fibre during and immediately after 193 nm UV inscription. The LPG's peak loss follows a sinusoidal function of the amplitude of the index modulation in the fibre core, thus the peak loss increases first and then decreases with further increase of writing pulses during UV inscription. After UV inscription has stopped, the LPG continues to grow with time owing to the hydrogen diffusion, and an increase of as much as 14 dB in the LPG peak loss was measured. We offer a qualitative explanation for the growth dynamics of the LPGs immediately after 193 nm UV inscription and discuss the effects of temperature and initial grating strength on the growth behaviour. We also investigate the impact of post-UV irradiation on the LPGs after annealing. Our study shows that post-irradiation trims not only the peak loss intensity but also the peak wavelength of the LPGs. This provides an opportunity to control the resonance peaks of the LPGs.

Phosphosilicate fibre gratings can be stabilized at temperatures in excess of 500 °C for sensor applications by optimizing thermal and UV presensitization recipes. Furthermore, the use of 193 nm presensitization prevents the formation of OH absorption bands, extending the use of fibre gratings across the entire wavelength spectrum. Gratings for operation at 700 °C retaining up to 70% reflectivity after 30 min are demonstrated.

We present a new sensor configuration based on the intrinsic bend sensitivity of Bragg gratings written in D-type fibres. This sensor can be embedded in any layer of a composite material to evaluate curvature in a way independent of axial strain and temperature. It can also be used to measure other parameters such as acceleration, angle and acoustic pressure.

Simultaneous optical fibre distributed strain and temperature measurements have been obtained, by measuring the spontaneous Brillouin intensity and frequency shift, using the technique of microwave heterodyne detection. The enhanced stability from using a single coherent source combined with optical preamplification results in a highly accurate sensor. Using this sensor, distributed temperature sensing at 57 km and simultaneous distributed strain and temperature sensing at 30 km were achieved, the longest reported sensing lengths to date for these measurements. As a simultaneous strain and temperature sensor, a strain resolution of 100 µε and temperature resolution of 4 °C were achieved.

We present the design and initial investigation of a fibre optical system which may be used both for intra-cavity and for ring-down measurements of absorption losses. The system consists of a fibre loop containing a length of erbium-doped fibre pumped at 980 nm, with gain adjustment below or above threshold for the two types of operation. The fibre loop is constructed from standard fibre optical components and includes a micro-optical gas cell. The intended application is for measurement of levels of trace gases which possess near-IR absorption lines within the gain bandwidth of the erbium fibre amplifier. We discuss the key issues involved in operation of the system and the level of sensitivity required. Our initial experimental investigations have demonstrated that ring-down times of several microseconds can be obtained, which can be altered through adjustment of the attenuation or gain factor of the loop. Gain control is one of the most important issues and we explain how this may be achieved.

A new and unusual type of Faraday cell/ring-core optical current transformer characterized by a transverse configuration of the light beam and current-induced magnetic field is developed. A fibre-linked current sensor system is constructed using a small bulk yttrium iron garnet as the Faraday cell. It is shown that the sensor has the highest ever reported sensitivity of about 20% A^-1 in modulation depth of the output light power with a very good linearity for currents below 1 A and has a very good isolation from surrounding currents.

The optical and geometrical properties of microstructured optical fibres present new alternatives for a range of sensing applications. We present the design criteria for achieving significant overlap between the light guided in the fibre and the air holes and hence for producing efficient evanescent field devices. In addition, the novel dispersive properties combined with the tight mode confinement possible in holey fibres make ultra-broadband single-mode sources and new source wavelengths a possibility. Microstructuring technology can be readily extended to form multiple-core fibres, which have applications in bend/deformation sensing. Finally, fibre-based atom waveguides could ultimately be used for rotational or gravitational sensing.

A fibre optic two-dimensional array sensor for sound field visualization is presented. The proposed system consists of an optical fibre network and sensor element installed at every node of the network. The sound field is measured as an intensity modulation of the optical coupling between pairs of fibres in the network. In this paper, demonstration of the temporal and spatial sampling of the distributed sound field by matrix-style access to the sensor elements with the switching of the light sources is described. A 64-element array was used for stationary field measurement with `slow-mode' switching, while a 16-element array was made for transient field measurement with `fast-mode' switching. The present system eliminates the need for electrical multiplexers to switch the signals, and reduces the number of cables required to connect the array with the post-processing unit.

A simple sensor for measuring the air density at atmospheric pressure has been developed by the author using common optical fibre cable and a radioactive alpha-ray source. A silver activated zinc sulphide scintillator film and the photomultiplier tube for alpha-ray detection were separated by an optical fibre cable and the scintillator film was connected with the optical fibre cable in order to transmit the light caused by alpha-rays. The effect of the temperature on the photomultiplier tube could be removed in the temperature range from 0 to 60 °C. From the relationship between the attenuation of alpha-rays passing through the air and the air density, the air density could be measured with an accuracy of ±4×10^-6 g cm^-3 in the range from 1.07×10^-3 to 1.309×10^-3 g cm^-3. Also, a new-type absolute hygrometer sensor has been devised by coupling the two air density sensors. One of the two is for measuring the dry air density of absolute humidity zero and the other is for measuring the humid air density. Absolute humidity could be measured with an accuracy of ±2 g m^-3 in a range of 0 to 66 g m^-3 by taking into account the differences in counting rates of alpha-rays from the two sensors.

Three-phase volume-fraction measurement can be obtained by a combination of reflectance and fluorescence methods. Fluorescence from oil can be detected by an optical fibre probe placed in the flow, while reflectance at the end of the same fibre probe is also detected. The method is demonstrated by preliminary experiments.

New plastic optical fibre sensors for detecting alcohol vapour have been studied. A certain kind of polymer such as a Novolac resin causes swelling when it is exposed to alcohol vapour. This effect produces a change in the polymer refractive index. Based on this principle, the plastic optical fibre (POF) type sensor head was fabricated by coating Novolac-resin and Novolac/Fe:SO complex film as a cladding layer on the plastic fibre core. When this sensor head was exposed to ethanol and methanol vapour, the light intensity passing through the sensor head changed remarkably depending on the vapour pressure. The sensor response was also found to be fast, stable and reproducible.

A quasi-distributed pH sensing system was developed to utilize information derived from evanescently excited fluorescence signals originating from sensor sites fabricated on an optical fibre. The sensor sites comprise a section of fibre core exposed by polishing upon which is laid down a sensing film. The film comprises a fluorescent indicator dye immobilized covalently within a hydrogel matrix which is then photopolymerized and attached covalently to the exposed fibre core. Position information is determined from the propagation delay of the returning signals. A system comprising eight sensors spaced at 10 m intervals along an optical fibre has been constructed and characterized. The sensors operate in the region pH 6-pH 8 with a response time of 500 s. The properties of the immobilized indicator dye are found to be similar to those of the dye in aqueous solution. Furthermore, the sensing films so created are found to be resistant to dye leaching.

One of the main challenges of the Large Hadron Collider (LHC), a new particle accelerator currently under construction at CERN (the European Organization for Nuclear Research) in Geneva, resides in the design and production of the superconducting dipoles used to steer the particles around a 27 km underground tunnel. These so-called cryodipoles consist of an evacuated cryostat and a cold mass containing the particle tubes and the superconducting dipole magnet. The latter is cooled by superfluid helium at 1.9 K. The particle beams must be centred in the dipole magnetic field with a sub-millimetre accuracy. This requires that the relative displacements between the cryostat and the cold mass must be monitored with great accuracy.

Because of the extreme environmental conditions (the displacement measurements must be made in vacuum and between two points at a temperature difference of about 300 degrees) no adequate existing monitoring system was found for this application. It was therefore decided to develop an optical sensor based on low-coherence double interferometry, which measures with micrometer precision the distance between a mirror welded to the dipole cold mass and an optical head attached in the inner wall of the cryostat.

This contribution describes the development of this novel sensor and the first measurements performed on the LHC cryodipoles.

We report a new concept for interrogating and multiplexing large numbers of fibre Bragg grating sensors for dynamic measurands. The series combination of an AOTF and a frequency-modulated MZI forms the basis of a system, which allows simultaneous interrogation of eight sensors, and near-simultaneous interrogation of 64. A minimum strain resolution of 2.23 nε Hz^-1/2 at a frequency of 10 kHz was achieved.

A fibre-optic system based upon fibre Fabry-Perot strain sensors, capable of measuring deformations and modes of vibration of a composite panel, is presented. Wavelength-division signal demultiplexing allows for the simultaneous interrogation of the strain sensors.

The performance of the ordinary fibre Bragg grating sensor strain measurement system, which uses a Fabry-Pérot filter for scanning wavelength, has limitations in application due to the hysteresis characteristics of the PZT element in the filter, the slow scanning rate of the filter and the high cost of the system. The system proposed here uses a LED as a light source and adapts the method of code division multiple access (CDMA). Without a tunable laser source or Fabry-Pérot filter, the price of the system is very low and the speed of response of the system is very fast. CDMA is based on correlation techniques implemented in order to separate out individual reflected sensor signals from a multiplexed signal.

An in-line single mode optical fibre reference beam laser Doppler anemometer that utilizes the reflectivity of a Bragg grating as the source of the reference beam has been demonstrated. Tuning the emission wavelength of a laser diode across the bandwidth of a fibre Bragg grating (FBG) provides control of the amount of power in the reference beam, and thus a means for optimizing the signal to noise ratio.

Low-temperature sensing was studied using a fibre Bragg grating fixed on Teflon substrate that has a large thermal expansion coefficient. The wavelengths of reflected light from 1.55 µm fibre Bragg gratings were measured in the temperature range from 77 to 300 K. By bonding the fibre Bragg grating in between two Teflon substrates, the temperature sensitivity of 39 pm K^-1 was obtained at 77 K, which is 1.5 times higher than that of the sensor using a poly(methyl methacrylate) substrate. At room temperature, the sensitivity increased to 150 pm K^-1. For a more compact structure using one substrate only, the temperature sensitivity decreased by 5% compared with the sensor using two Teflon substrates. Comparisons with theoretical values are also discussed.

In this article, we report an implementation of distributed sensors that employ compact-grating-array structure, offering advantages of arbitrary dynamic range, high spatial resolution and simple interrogation by utilizing the established wavelength-division-multiplexing technique. The sensors have been demonstrated for measuring both linearly and non-linearly distributed strain with high spectral and spatial resolutions.

A uniform period fibre Bragg grating (FBG) can be chirped by applying an axial strain gradient to it. We have demonstrated a novel technique for making chirped fibre Bragg gratings (CFBGs) which involves bonding a FBG to the surface of a tapered stainless steel plate that is strained by loading. Four kinds of tapered plate profiles (convex, gradient, concave and rectangular) and two kinds of loading procedures (bending and direct tension) were used as the means for shifting the absolute centre wavelength in situ and broadening the reflection bandwidth of a CFBG. With this method the centre wavelength of a CFBG was tuned from 1548.92 nm to 1550.99 nm, resulting in a maximum adjustable change in bandwidth of 2.00 nm for a 5 cm long FBG.

This paper proposes and demonstrates the use of a fibre optic Mach-Zehnder interferometer to determine the average bulk moisture content of concrete. The principle of operation is based on the moisture dependence of the propagation velocity of shock waves in the concrete. A spring-loaded gun with stainless steel balls of various diameters created the shock waves. The time delay of the shock waves between the two arms of the interferometer embedded in an experimental concrete block could be determined from the autocorrelation function of the interference signal obtained at the optical detector. The average velocity at room temperature measured in dry concrete was 3.54±0.34 km s^-1, and for moist concrete it was 5.00±0.45 km s^-1. These values indicate that the discrimination between moist and dry concrete is large enough, and the standard deviations in the measurements small enough, to make this a feasible approach to determine the moisture content in concrete.

We present an analysis of the spatial resolution achievable with a new fibre-optic reflectometric technique, contrived by the authors for the automatic detection of cracks in the inner sides of steam generator tubes of small diameter (≃20 mm). An analytical expression of the modulation transfer function (MTF) of our measurement system is obtained, from which the resolution limit of the technique is defined. Experimental values of the MTF, measured with a first laboratory prototype, are compared with the theoretical predictions and the range of validity of the theoretical expressions is discussed. Also, some of the results obtained with a second-generation prototype are presented, it being demonstrated that parallel cracks with a spacing of 50 µm between adjacent edges can be resolved in practical inspection tasks.

Both the position and velocity of reflecting targets can be measured simultaneously using a fibre-coupled, highly coherent, frequency-modulated laser. By using an asymmetric triangular frequency modulation, the direction of the target's motion is unambiguously determined.

Polarization properties of elliptical-core liquid crystal optical fibres have been investigated and the experimental results on the liquid crystal fibres in the polarimetric configuration subjected to the influence of hydrostatic pressure and temperature are presented.

An optical preamplifier system consisting of a 27 dB-gain erbium-doped fibre amplifier, a three-port circulator and a 47 GHz bandwidth in-fibre grating has improved the optical signal-to-noise capabilities of a Brillouin-based fibre-optic distributed temperature sensor by 17 dB. Using this system and operating at 1533 nm, a sensor with a range of 23 km, spatial resolution of 1.8 m, measurement time of 9 min and temperature resolution of 6 K was implemented. The temperature resolution was attributed to contamination of the backscattered spontaneous Brillouin signal by the residual Rayleigh signal after optical filtering.

An optical fibre sensor system for monitoring contamination in water supplies is presented. The sensor comprises a number of individual sensor elements on a single fibre loop. It is addressed using optical time domain reflectometry so that the required spatial resolution of 1 metre is achieved. Analysis of the signals at the receiving end is performed using artificial neural networks coupled with pattern recognition techniques, thus allowing external influences such as the degree of sensor fouling to be detected. In this investigation limescale build-up in hard water is investigated as the interfering parameter.

In order to apply a Raman distributed temperature sensor (RDTS) to the monitoring of nuclear facilities, some correction techniques for radiation-induced errors were developed and investigated. One is a simple correction technique with two thermocouples, when uniform radiation-induced loss distribution can be assumed. The other is a special technique with a loop-type arrangement. In this technique, even when the loss distribution is not uniform, radiation-induced errors can be cancelled.

To demonstrate these techniques, RDTS has been installed at the experimental fast reactor JOYO. During continuous measurements with a total dose of more than 7.7×10³ C kg^-1 (3.0×10⁷ R), the radiation-induced error showed a tendency to saturate and the feasibility of the correction techniques was demonstrated. These results show that RDTS could be useful as a temperature distribution monitor in nuclear plants.

Close meshed instrumentation or sensor networks applying conventional sensors for temperature and strain monitoring may result in excessive penalties in terms of weight constraints, reliability and sensitivity to environmental conditions, and complex interfaces. The fibre-optic sensor network described in this paper is a multiplexed system of fibre Bragg grating (FBG) strain and temperature sensors and was designed and developed for a demanding space environment, but it can also be emphasized as a promising sensor technology with high potential for non-space applications.

The FBG sensor network measures both strain and temperature at the measuring conditions of the structural core of the X-38 spacecraft, by means of wavelength shifts due to tensile stress on a Bragg grating. Dependent on the fixation of the fibre, either isolated from or mechanically coupled to the structure, local thermal or mechanical loads can be determined in the temperature range from -40 to +190 °C, and in the strain range from -0.1% to +0.3%. Short-term resolution and repeatability of the strain measurement amount to 5 µε and 25 µε, respectively.

The FBG sensor network is very suitable for structural health monitoring of large structures, i.e. to determine thermal and mechanical load profiles during operation, to assess residual strength of structural elements or to detect irregular working conditions. In comparison to conventional sensors like thermocouples and strain gauges, an FBG sensor network significantly reduces the amount of required front end electronics (FEE) and harness.

In this work, a study of a fibre optic ruby crystal probe exposed to high temperatures has been carried out for the first time, and `annealing' effects have been considered and reported upon. The work has enabled the device to be studied at temperatures up to 1500 °C to test its `survivability': the results show that a single-crystal probe is able to operate continuously at temperatures up to 1400 °C, and can reliably cover a wide temperature measurement range from 0 °C to 1400 °C. Uncertainties in the measurement of, at worst, approximately ±7 °C result after many hundreds of hours of temperature cycling of the probe.

This paper presents the experimental construction of a liquid-crystalline Bragg filter. The filter is designed as a low-cost WDM decoupler for fibre-optic networks, but can also be used as a demultiplexer in polarization-coded transmission. The theoretical basis, principle of operation and results of experimental examination of the device are given.