In this paper we report both theoretical and experimental evidence that equal MPD is extremely unlikely in few mode fiber, and next to impossible to achieve in highly multimode fiber. In one approach, we simply add together the intensity distributions of the various possible modes for a given V number fiber, and compare with what is observed in the laboratory under a variety of conditions. This is done for the incoherent case using a one hundred meter length of a similar fiber. In another method, Gloge's theory for the number of propagating modes is used to determine the approximate MPD in a highly multimode fiber. Both techniques point to the conclusion that the most likely distribution of power among the modes is one that decreases exponentially with mode, despite various mode-scrambling efforts.

The theoretical description of linearly polarized (LP) modes in weakly guiding fibers includes an amplitude parameter which relates to source intensity and input conditions. As such, this parameter has direct bearing on the amount of power carried by individual modes. This paper derives the mathematical relationship between the power in the first two LP modes and their respective amplitude parameters. In subsequent experiments, this parameter is employed in conjunction with a measurement of the amplitude parameter to yield the modal power distribution in a dual mode fiber. Alternatively, if the power distribution is known by empirical means, the amplitude parameters can be determined to allow for accurate visualization of the propagating modes. This work allows for a better understanding of the operation of dual mode fibers in preparation for structural sensing.

Supraconductive installations are now commonly used in large facilities, such as power plants and particle accelerators. This requires a permanent temperature control at very low temperature, but cryogenic temperature measurements in the 1-77K range requires expensive calibrated temperature probes. We report here the possibility to use stimulated Brillouin scattering in optical fibers for temperature sensing down to 1K. Such a technique offers the additional advantage to make possible distributed measurement, so that very large structures and systems can be controlled using a single fiber and a single analyzing instrument. In addition only one by-pass for the fiber is required as input to the cryogenic vessel, that is definitely a key advantage for the design and the energy loss. Brillouin scattering in optical fibers has never been investigated so far at temperature below 77K (nitrogen boiling point). This absence of interest probably results from the constant decrease of scattering efficiency that was observed while cooling the fiber down to 77K. Our measurements show the unexpected feature that scattering efficiency is significantly raised below 50K and is even much better than observed at room temperature. The relevance and the feasibility of the technique is demonstrated in real scale on the supraconductive magnets for the future world largest particle accelerator, namely the large hadron collider (LHC) at CERN Laboratory in Geneva.

This paper introduces a new type of fiber-optic acoustic emission (AE) sensor, based on a fused tapered 2x2 coupler. Laser light is injected into one input fiber and variations in the splitting ratio, caused by the passage of an ultrasonic pulse through the coupling region, are measured via a differential detection scheme. The sensor was designed to be directly compatible with an existing AE instrumentation system (the MISTRAS system from Physical Acoustics). Sensors were mounted on the surface of and embedded within glass-reinforced polyester laminates. They were excited with simulated AE signals and their performance was compared to that of a conventional piezoelectric AE transducer. The characteristics of the response of the fiber-optic sensor were similar to those of a commercial piezoelectric transducer. A signal-to noise ratio of 45 dB was achieved.

The introduction of long-gauge fiber optic sensors has opened new opportunities for the monitoring of large structures. Using these sensors it is possible to observe the local elongation of a structure and calculate its global deformations. The use of internal sensors detecting integrated strain is however unable to detect the rigid-body motion of a structure with external reference points. Another possibility is to use gravity as a reference to measure vertical rotations of the structure. A gravity- referenced rotation sensor is conventionally called an inclinometer and measures the angle variations between a nominally vertical reference line in the structure and the direction of gravity. It is possible to use uni-axial inclinometers of bi-axial ones, measuring rotations in the two perpendicular vertical planes. Today's inclinometers are based on a variety of techniques and can achieve sensitivities in the micro-radian range. When using fiber optic deformations sensors in a given structure, it would however be interesting to use the same technology to obtain inclination reading. This prompted us to develop a fiber optic-based inclinometer compatible with the SOFO system.

Optical fiber-based extrinsic Fabry-Perot interferometric (EFPI) sensors have been extensively deployed for sensing a number of measurands including temperature, strain, vibration and pressure. Their circular cross-section has made it relatively simple and attractive to embed them in advanced fibre reinforced composites (AFRCs) such as glass and carbon fibers. However, a typical construction of an EFPI consists of two optical fibers that are positioned and secured within a precision bore capillary. The relative outer diameters of the various key components are as follows: capillary = 300 micrometers ; optical fibre = 125 micrometers ; carbon and glass fibers = 8 and 14 micrometers respectively. This mismatch in relative diameters of the reinforcing and the sensor fibers can result in significant spatial distortion of the former. The location of the embedded sensing fibre in relation to the reinforcing fibre layers can also lead to the formation of resin-rich regions in the AFRC. These factors can have a detrimental effect on the compressive properties of the material. Therefore, there is significant attraction in reducing the overall diameter of the sensor. In this current paper, the feasibility of reducing the diameter of EFPI sensor design to that of the optical fibre is demonstrated via two techniques. The first technique involved the use of hydrofluoric acid to etch and create the Fabry-Perot cavity. In the second technique, the feasibility of using laser ablation to fabricate the Fabry-Perot cavity in silica and sapphire substrates is presented. The optical fibre-based Fabry-Perot cavity produced via acid etching was interrogated using white light interferometry.

There are currently available many software tools for modeling the processing of composite materials, that help designers to evaluate the process constraints and the feasibility of different concepts. Nevertheless, several manufacturing tests are still required for adjustment of the control parameters before production may start. Real time monitoring is the only way to validate the numerical results and to get a deeper knowledge on the process evolution. Final objective would be a closed loop known as 'Intelligent Material Processing'.: process model - in situ sensors - predictive control, able to react on real time to small disturbances, adapting the process parameters for optimal results. This paper concentrates on the sensor development for two aerospace processes, autoclave curing and RTM, and it present the results obtained on a real aircraft structural part, a five meter diameter frame for the fuselage of Airbus A380 . An optical fiber system has been implemented to monitor the movement of the resin flow front during the injection and the internal residual strains. The procedure has the advantage of being very robust, and it may be used for complex geometry of the part. It has been demonstrated the feasibility of the procedure to work at an industrial environment; the results are being used to refine the data on the material properties, as the preform permeability, and to improve the process control.

The application of PZN-PT single crystal piezoceramic in low-cost, high-resolution strain sensors was examined using an ACX QuickPack PZT strain sensor as a basis for the new design, with active structural control of precision machinery as the target application. A study was performed to determine the influence of mechanical feedthrough on the sensor signal. Mechanical feedthrough appears when an actuator is close to the sensor and near-field effects dominate the actuator-to-sensor transfer function. Both voltage follower and charge amplifier signal conditioning electronics were tested, and the benefits of each on the nonlinear characteristics of PZT and PZN-PT were analyzed. Finally, the coherence between the sensor characteristics was analyzed. The results show that the QuickPack strain sensor compares favorably with some higher cost off-the-shelf products in terms of noise and resolution, at a significantly lower cost. The single crystal sensors show significantly higher sensitivity than their conventional multi-crystalline counterparts, though less than expected due to the softer base material.

Many components in hard disk drives (HDDs), when in operation, are subjected to vibration due to out of balance of rotating components, inertial impacts under servo driving and dynamic interactions between components. These vibrations have been found to have significant effect upon the servo performance of drive systems. In order to improve the servo performance by reducing the effect of mechanical resonance in HDDs, this paper seeks to detect and suppress mechanical resonance of the head actuator using smart sensors and multi-sensing control techniques. In this regard, sensitive and miniature piezoelectric elements from the polymer-based piezoelectric materials PVDF (polyvinylindin fluoride) or the ceramic-based piezoelectric materials PZT 9lead zircornate titanate) are built in the head actuator for sensing the mechanical vibration. In the experiment, the multi-sensing signals by the piezoelectric sensors and the laser Doppler vibrometer (LDV) are transferred into a voice coil motor (VCM) through a feedback controller so as to actively suppress structural resonance. Numerical simulation and experimental results indicate that the piezoelectric sensors provide an effective way in monitoring the HDD actuator resonance, and the active vibration control strategy is capable of suppressing main mechanical resonance in the head actuator effectively.

A method for early failure detection for elastic diaphragms during their operation is described in this paper. The method is based on frequency analysis of the acoustic signal obtained from an air-driven pump. A fully digital system for frequency analysis based on a personal computer was developed for practical implementation of this method. The system was tested in the laboratory and in plant conditions. In laboratory, diaphragms with different artificial flaws were mounted in the pump and the corresponding spectra recorded. The results helped to set the parameters for in-plant experiments. These experiments were between three and fourteen days. During this time the system performed acquisition and data processing at 10 second apart. Three in-plant experiments have shown that a failure initiation event can be detected from frequency behavior analysis for each test and that a subsequent failure occurs. All failures indicated by the system have been confirmed by visual inspection.

Fiber reinforced composites offer increased resistance to fracture as compared to isotropic materials. In addition, they have demonstrated great potential to support embedded sensor systems. However, to develop a truly reliable, embedded sensor for composites, the failure modes of such materials, including the influence of the embedded fiber sensor, must be known. Crack bridging by intact fibers is considered to be one of the most efficient mechanisms to slow down transverse crack propagation in a fiber reinforced composite. This paper presents non-invasive, direct measurements of bridging fiber stresses in a model epoxy/glass composite, using long gage length optical fiber Bragg gratings. Several central crack specimens, containing artificially bridged cracks, were fabricated and tested. The Bragg grating gage length of 12 mm permitted measurement of the force distribution in the reinforcing fiber extending from the crack surface to the far field region. A T-matrix simulation was used to model the grating response. Results from specimens involving both a strong and mixed interface are presented. The measured strain distribution in the bridging fibers compared well with previous analytical models. Discussion of the application of these results to structurally embedded sensors for damage detection is also presented.

Chirped fiber Bragg grating (FBG) sensors were applied for the identification of damages in carbon fiber reinforced plastic (CFRP) laminates since the reflection spectrum from a chirped FBG was expressed as a function of the position along the grating. First, for the identification of crack locations in the 90 degree(s) ply of cross-ply laminates, chirped FBG sensors, whose grating length and spectrum width were 50mm and about 5nm respectively, were embedded in CFRP cross-ply laminates, and the reflection spectra were measured after tensile loadings to the laminates. As a result, the spectra had dips corresponding to locations of transverse cracks. This change in the form of the spectrum was also confirmed by a theoretical calculation. Hence, the crack locations could be identified from the form of the spectrum. Secondly, this technique was applied to the identification of the delamination originating from a tip of a transverse crack in a cross-ply laminate. Since theoretical calculations indicated that the reflection spectrum changed its form corresponding to the progress of the delamination, it was also found that the chirped FBG sensor could be applied to the identification of the length and position of the delamination.

Airbus Espana, INTA and UPM are collaborating in a joint research directed to increase their experience and the knowledge in the use of fiber Bragg gratings as strain and damage sensors in advanced composite structures for aerospace applications. In certain conditions, fiber Bragg gratings are able to detect damage by measuring sudden changes in the strain distribution of composite monolithic structures due to delaminations or debonding of stiffening elements. However, strain measurement of FBGs can be compromised by spurious phenomena like birefringence promoted by manufacturing induced residual stresses, with the subsequent spectral peak splitting, and spectral chirping induced by local non uniform strain fields. These effects add a new issue to the difficult task of demodulating the spectral response of fiber Bragg gratings when employed as strain sensors in composite structures. However, photoelastic models already developed allow obtaining considerable information about this phenomena, and relate them with local changes in the stress field promoted by structural damage. Thus, it is possible to think in FBGs not only as load monitoring sensors, but as a part of SHMS for composite structures designed under damage tolerance criteria.

This investigation concerns the application of different techniques, including optical fibre strain sensing, pulsed Digital Speckle Pattern Interferometry (DSPI), traditional modal analysis and finite element modeling to the study of vibrating composite structures. A prototype system for condition monitoring of composite structures is being developed which relies on the on-line measurement of dynamic strains in order to detect any deterioration in performance due to the accumulation of damage. A range of carbon-fibre reinforced composite specimens that incorporate innovative Fabry-Perot interferometric long gauge-length strain sensors have been produced and tested. The optimal design of fibre sensor network configurations for the identification of different damage parameters is being aided through the development of a software simulation tool and the use of a p-version FEM package. Vibration modes of the excited structures have also been determined using an out-of-plane pulsed-DSPI system, employing a dual-cavity frequency-doubled Nd:YAG pulsed laser with a 25 Hz repetition rate. In general, experimental results compare favorably with finite element predictions. The data derived from the strain sensors is used to update a parameterized FE model of the composite structure, allowing the determination of the position and extent of damage present.

This paper presents delamination monitoring techniques in composite laminates using small-diameter fiber Bragg grating (FBG) sensors that was developed for embedding in the laminates without inducing any structural defects. First, this FBG sensor was embedded in carbon fiber reinforced plastic (CFRP) cross-ply laminates. The reflection spectra from the FBG sensor were measured at various delamination sizes through a static four-point bending test. From experimental results, the spectrum was found to have two peaks due to the initiation of the delamination. The intensity ratio of these two peaks changed as the delamination size increased. In order to confirm the deformation of the measured spectra, the spectra were calculated theoretically. The calculated result reproduced the change in the measured spectrum very well. Thus, we proposed the intensity ratio of the two peaks as an effective indicator for evaluation of the delamination size. Secondly, detection of the edge delamination in quasi-isotropic CFRP laminates was conducted using the FBG sensor. The reflection spectra were measured at various delamination sizes through fatigue test. The form of the spectrum changed sensitively by growth of the edge delamination. Hence the small-diameter FBG sensor could also be applied to detect the edge delamination in quasi-isotropic composite laminates under cyclic loading.

This work considers the dominant current methods for fiber Bragg grating wavelength interrogation. In addition, a new interrogation method, based on hybridizing a scanning Fabry-Perot filter for selecting individual reflection wavelengths, and an unbalanced Mach-Zehnder interferometer for high-resolution conversion of wavelength shifts to phase changes, is presented. The method utilizes a 3x3 fiber optic coupler in such a way that a completely passive demodulation algorithm is implemented. The current version interrogates multiple FBGs at frequencies from DC to near 10 kHz with nanostrain resolution across the full band. Low-frequency resolution is maintained with an interferometer drift compensation technique. We describe the system design and operation in detail and present key performance metrics with comparison to other primary FBG interrogation architectures.

Multi-axis fiber grating strain sensors have been used to quantitatively measure axial strain and temperature; transverse strain and transverse strain gradients in composite weave structures. This paper overviews the multi-axis fiber optic grating strain sensors and how they can be applied to measuring multidimensional strain fields interior to composite parts with complex composite weave structures. Experimental results are given for the case of a bi-axially woven composite coupon as well as for an E-glass/epoxy composite sample.

Several experimental demonstrations of a unique Fiber Bragg Grating (FBG) based transducer for strain and temperature have been made. The proposed technique is based on the inscription of an unique uniform FBG what presents first- and second-order diffraction wavelength response. The measurement of the wavelength change at both wavelengths allow the design of a simple and efficient fiber optic based transducers. Although strain-temperature discrimination feasibility have been proved previously, the errors associated to the transfer matrix must be improved to achieve a similar performance than other FBG based discrimination techniques. Up to our knowledge, theoretical analysis which allows an optimized transducer design have not been made. In this communication a generalized study of the behavior of two wavelength measurement based transducers is going to be made. Physical parameters which are involved in the transducer construction are going to be analyzed and a suitable technique for optimal transducer design is going to be proposed. Several conclusions about the relation among each FBG based transducer parameter and the transfer matrix condition are going to be presented. The measurement errors associated to this physical parameters will be derived allowing the design of optimized specific transducers.

Experimental, analytical and finite-element-simulation approaches are presented for the characterization of fibre Bragg grating sensors written in conventional monomode and polarization-maintaining fibers subjected to transverse loading. Firstly, a diametrical-load configuration is considered. Numerical simulations show the behavior to be nonlinear as a function of the applied load when an appropriate analytical model for the opto-mechanical response is employed. Secondly, experiments are carried out with the sensors embedded in epoxy specimens, when the latter are subjected to transversal biaxial loading. The response is monitored as a function of the vertical/horizontal load ratio. A finite-element model of the specimen with the embedded fibre and the previous analytical procedure are used to calculate the strain distributions in the fibre core resulting from loading, and predict the corresponding Bragg wavelength shifts. Experimental results are then compared to numerical predictions.

Bragg grating sensors have been proved to be well suited for strain sensing, but they are restricted to give only one-dimensional strain information. In the present study, embedded interferometric Bragg grating sensors are arranged in a rosette configuration to measure the in-plane strains. The rosette is based on three sensors, which are embedded along the reinforcement in three different plies. An analytical and experimental work is carried out to demonstrate how in-plane strains can be determined for arbitrary in-plane loading. Considering the three-dimensional strain effect of embedded sensors, the phase output was related to the laminate strain. The relationship between the optical response from each sensor and the strains in the laminate was obtained. Experimental results obtained from the embedded fibre optic strain rosette in tension, compression and shear are described. The obtained strains were compared to those measured by surface mounted resistive strain gauges. It was shown that embedded optical sensors provide an effective way to measure in-plane strain components in laminates.

Fiber Bragg gratings (FBGs) written into polarizations maintaining (PM) fibers have been employed for multi-axis strain and temperature sensing. In this paper we report the development of a FBG interrogation system that is capable of detecting independently the two orthogonally polarized signals reflected from a PM FBG. The interrogation system imposes no limitation on the measurable strain range. This relaxes the requirements on the bandwidth of the grating spectrum, allowing the use of a shorter PM FBG (larger bandwidth) for sensing applications where a higher spatial resolution is required. In particular, this system is useful in the cases where the spectra of the reflected polarized signals are broadened or split, resulting from strain-induced structural changes of the PM fibre, or caused by the application of a non-uniform strain on the FBG.

We have been studying optical sensing technologies using fiber Bragg gratings (FBGs) for health-monitoring systems in the fields of constructions, civil engineering, aerospace and so on. In these fields, various kinds of sensing techniques such as strain, temperature, vibration and crack detection are required. To meet these needs, we have fabricated the FBGs by precisely controlling photo-induced refractive index modulation or the grating period along a fiber. Also, as embedded sensors, we developed small-diameter FBGs which are embedded in fiber reinforced plastics (FRP) composite materials without inducing any mechanical deterioration. In this paper, we present the progress of our small-diameter FBGs and propose a new wavelength detection technique for FBG sensors using a wavelength division multiplexing (WDM) coupler. The reflected light is divided at the ratio depending wavelength through the WDM coupler. Especially, we adopted a PLC-type WDM coupler which has the advantage in low polarization sensitivity and integration compared with a fused-optical fiber-type WDM coupler. This technique is useful in high-speed detection for vibration or impact damage. Another application is a multipoint measurement system by using together with optical time domain reflectometry (OTDR) method. In this system, pulses of light incident into an optical fiber return from the FBGs to a detector with different arrival time. As a result, we can use FBGs with the same Bragg wavelength, because measuring delay time of the pulses enables to distinguish each FBG location. In addition, using together with wavelength division multiplexing within wavelength region for optical communications can increase the number of FBG sensors. The results of the basic performance in this system showed that it is very promising for the multipoint measuring system. These wavelength detection techniques will expand sensing applications using small-diameter FBGs.

Blue Road Research is developing moisture-sensing technology that is capable of precision measurements of relative humidity and soil moisture content. Based on optical fiber sensor technology, the sensors are highly corrosion and EMI resistant and can be installed in locations where it would be impractical to insert a bulky conventional sensor. While the sensors are currently in the research and development phase, Blue Road Research has demonstrated the performance of the technology and is moving toward a commercially available product.

In this study, adhesively bonded optical fibers and Bragg grating sensors were investigated. The primary objectives were to assess the durability of the optical fiber-to-composite bond during loading and how that load was transferred from the structure to the optical fiber sensor. Using a two-phase epoxy and a highly reproducible bonding technique, different specimen configurations were fabricated and tested under cyclic fatigue loading. The parameters that were considered were the orientation of the top ply of the composite and the loading direction relative to the optical fiber. In addition, the load transfer from the structural composite to the optical sensor was studied numerically and experimentally. In all the specimen configurations studied, cyclic fatigue to 0.3% strain did not lead to cracking of the bond up to 500,000 cycles. The bonded sensor performance was unchanged during these experiments. A bonded optical sensor was proven functional up to 0.55% strain, where the composite failed. The top ply orientation of the laminate relative to the optical fiber did not seem to have a significant effect, although the stress analysis showed that the stresses in the adhesive and the part of the fiber nearest to the substrate did vary significantly. The stress analysis further indicated that loading transverse to the bonded optical fiber should be most critical in terms of bond durability. The analysis showed that strain is primarily transferred along the fiber axis, not in the transverse direction. Thus, the strain state in the bonded fiber may be approximated to that in an axially loaded free fiber.

There is a general tendency in systems for environmental monitoring towards ever more automatic and autonomous operation. Moreover, technologies and instruments are available to reliably interconnect distributed, disparate components. This allows the measurement, logging, data processing and interpretation activities to be carried out by separate units at different locations in near real-time. Building on the results of a previous research and development project at SUPSI, which focused on movement monitoring with GPS, the system has been generalized to accommodate a range of other sensors, thus rendering it even more interesting for geotechnical applications. In particular a laser distance meter and a robotized theodolite have been integrated. First results confirm an expected increase in robustness of the combined measurement network, which is particularly important in unfavorable stand-alone GPS reception conditions. Due to the modular architecture of the system, other sensor types, ranging from simple analog or digital sensors to complex measuring instruments may be supported with minimal effort. Measurements are transmitted via cellular or point-to-point radio links to a control station, which provides for post-processing and system management. The control station may be remotely accessed via an Internet connection. The system takes advantage of a standard and flexible database structure which has been tailored to measurement and monitoring projects using different sensors. The system represents an architecture for remote monitoring tasks requiring a high degree of autonomy, reliability and automation. The solution can be advantageously applied to remote, near real-time measurements of low dynamics movements.

A new central railway station - Lehrter Bahnhof - is being built in Berlin. Because of construction activities in immediate vicinity and because of difficult soil conditions, different vertical displacements have to be expected. In order to avoid damage to the bridges and to a widely spanned glass roof which will be supported by two concrete bridges these two bridges have to be monitored with regard to their deformation performance right from the beginning of construction until commissioning as well as later on for several years. For this purpose, a monitoring concept has been developed and sensors with excellent long-term stability have been chosen. This paper describes the system for monitoring settlements and heaves by means of laser-based optics and hydrostatic leveling. Additionally, strain and inclination of the prestressed concrete bridges are redundantly monitored by embedded long-gage length fiber-optic strain sensors as well as resistive strain gages, and inclinometers. Measurements on-site are referenced by measurements on two test beams well-defined loaded under laboratory and field conditions. The paper also describes the measuring concept and the sensor techniques as well as installation of the sensor system and first results.

This paper describes a prototype system being developed for on-line structural monitoring of the cathedral of Como, Italy. Monitoring is based on relative displacement and temperature measurements employing optical fibre Bragg grating transducers. System features include an edge-filter demodulation scheme with wavelength multiplexing, palmtop-based control with customised software, automated data acquisition and analysis and GSM-based data transmission via email. The system also offers the option to be solar powered, allowing it to be located in areas with limited access, such as on the cathedral roof. Initial results obtained over an eight-month demonstration period are also presented. The intention is to develop this prototype into a complete system for on-line structural monitoring of the Cathedral, offering real decision support to those responsible for its maintenance.

The bond behavior of textiles embedded in concrete and consisting of multi filament yarns (rovings) differs from that of homogenous materials such as steel. Test results published in the literature as well as own investigations revealed that, for textile reinforced concrete, it has to be distinguished between external and internal bond of the fibers (filaments). The outer filaments which have direct contact to the cement matrix show a good bonding performance. In contrast to this, the inner filaments (core filaments) of a roving transfer forces only by friction, resulting in a less bonding to the surrounding matrix. In order to confirm such a bonding model, strain and slip measurements at single filaments are necessary in pull-out-samples. However, such measurements are not possible with strain gauges usually used in structural engineering. Therefore, strains in outer and inner filaments as well as in the cement matrix of selected samples are measured by using flexible Fabry-Perot fiber interferometer sensors.

In a German slab track system (Feste Fahrbahn FF, system Boegl) for speeds up to 300 km/h and more different fiber optic sensors have been embedded in several levels and locations of the track system. The track system consists of prestressed precast panels of steel fiber concrete which are supported by a cat-in-situ concrete or asphalt base course. The sensors are to measure the bond behavior or the stress transfer in the track system. For that, tiny fiber-optic sensors - fiber Fabry-Perot and Bragg grating sensors - have been embedded very near to the interface of the layers. Measurements were taken on a full scale test sample (slab track panel of 6.45 m length) as well as on a real high speed track. The paper describes the measurement task and discusses aspects with regard to sensor design and prefabrication of the sensor frames as well as the embedding procedure into the concrete track. Results from static and dynamic full scale tests carried out in the testing laboratory of BAM and from measurements on a track are given.

Although several Fiber Bragg Grating (FBG) based transducers have been successfully applied to the measurement of strain and temperature on civil engineering applications, important research work must be done to obtain transducers able to be used out of the laboratory. One of these research topics are the study of good interfaces between the optical fiber and the structural part to be measured. In this work a wide range of fixing techniques and interfaces have been investigated for fiber Bragg grating based transducers. Several transducers were constructed and fixed to a steel girder. Conventional strain gauges and professional displacement metering devices was added to the test specimen to make comparisons. A complete characterization setup using a laboratory test machine to test the transducers was designed. The test included several static and dynamic cycles and was applied to the steel girder while all the transducers were monitorized simultaneously. Results obtained with traditional and with the optical fiber mentioned technologies show a very good agreement.

The use of Fiber Bragg Grating (FBG) technology for the measurement of strain and temperature in civil engineering structures have been previously well demonstrated. A lot of work have to be done to achieve the new transducer technique to be able to compete with well established strain gauges. The measurement of strain and temperature within concrete beams is a niche of application where fiber optic transducers have advantages over conventional techniques. Fiber optic sensors can be inserted in a concrete beam for this purpose and debt to the known characteristics of fiber optic a higher durability can be achieved. In order to prove the ability of this type of transducers to make the measurements a set of FBG were fixed to the steel bars within the concrete beam and other set of long gauge FBG based transducers were embedded in the beam. The former were used for steel bar strain measurement and the later were employed for structural concrete beam sensing. During the concrete curing process, temperature of the beam were monitorized. After that, a complete flexural test was made while the beam was monitorized using traditional laboratory measurement techniques. Interesting conclusions have been obtained and will be presented.

On the steel production industry, a mixture of selected scrap and pellets of DRI is put into the furnace according to the desired quality of steel. The quality of the iron steel bars, among other, depends on the temperature profile during the cooling and solidifying process. A new multipoint high temperature transducer based on the optical power collected by a group of multimode optical fibers has been developed for measuring the transversal cooling profile. The optic fibers are adequately sheathed by a ceramic mixture in order to achieve its mechanical and thermal protection. Besides in order to use the transducer in pollution environments, the probe is putted inside of a close chamber with a window made with a vitreous ceramic material. The use of silica fiber optic limits the spectral range to analyze, and the measured temperature range. The behavior of the transducer from the angle, the distance and the change of emissivity have been measured, and the transducer has been optimized. The transducer was calibrated in the Photonics Engineering Group laboratory and it was installed in a iron-steel bar manufacturing plant in Spain where it is working high relative humidity and high temperature environment.

Fiber Bragg gratings have demonstrated great potential as sensing elements for the structural health monitoring (SHM) of composites. However, the complexity of stress and strain fields due to one or multiple damages makes the health-monitoring problem extremely difficult and sensitive to errors. Current approaches consider only the Bragg grating averaged axial strain as input to the health-monitoring algorithm. This paper presents a strategy to improve SHM information. In previous experiments, Bragg grating sensors embedded to measure the strain field near a crack demonstrated significant loss in their reflected spectrum at high strain. This loss was not a constant but a function of wavelength and is shown to be due to bending. The effect of shear loading is shown to be negligible. In order to demonstrate the technique an axially loaded unidirectional composite with a transverse crack is considered. The axial stress distribution is calculated using a modified shear lag theory suitable for multiple fibers and cracks. The bending stress at the crack tip is calculated using linear elastic fracture mechanics and a FEM model. From the calculated stress field a grating response simulation is performed taking into account the local bending of the fiber.

The monitoring of slope deformations is often required for management of construction or for disaster prevention. Although various kinds of electrical monitoring systems have been developed, they still have problems on shielding electric noises, waterproofing and so on. An optical monitoring system is expected to solve those problems because it is basically unaffected by electric noises and waterproofed by itself. Borehole inclinometer is one of the typical transducers for slope deformation monitoring. By electrical ones, it is hard to take continuous data of many points in a small diameter borehole. To improve these weak points, we developed the monitoring system with borehole inclinometers using Fiber Bragg Gratings (FBG) as a strain sensor. The transducer consists of series of unit-bars connected each other with elastic joints. FBGs were embedded on the elastic joints, and bending angles of each joint were measured by FBGs. The transducer was set in a borehole, and underground deformations were measured as bend of each joint at a remote monitoring center. We had applied this monitoring system for artificial slope under construction for 4 months. The data obtained by the system indicate the deformation according to a progress of land banking.

For accurate strain measurement by fiber Bragg grating (FBG) sensors, it is necessary to compensate the influence of temperature change. In this study two devices using FBG sensors have been developed for temperature-compensated strain measurement. They are named hybrid sensor and laminate sensor, respectively. The former consists of two different materials connected in series: carbon fiber reinforced plastic (CFRP) and glass fiber reinforced plastic (GFRP). Each material contains an FBG sensor with a different Bragg wavelength, and both ends of the device are glue to a structure. Using the difference of their Young's moduli and coefficients of thermal expansion (CTEs), both strain and temperature can be measured. The latter sensor is a laminate of two 90 degree(s) plies of CFRP and an epoxy plate, and an FBG sensor is embedded in the epoxy plate. When the temperature changes, the cross section of the optical fiber is deformed by the thermal residual stress. The deformation of the fiber causes the birefringence and widens the reflection spectrum. Since the temperature can be calculated from the spectrum width, which changes in proportion to the temperature, the accuracy of the strain measurement is improved. The usefulness of these sensors were experimentally confirmed.

Most interferometric optical fiber sensors, including the extrinsic Fabry-Perot interferometric (EFPI) sensor, measure strain or temperature with only interferometric fringes. Thus the measurement direction such as tension/compression of strain or increase/decrease of temperature is difficult to be distinguished. The EFPI optical fiber sensor has been constructed with the transmission-type structure to compensate this disadvantage. In the transmission-type EFPI (TEFPI) optical fiber sensor, the additional signal level variation in the fringe signal presents the measurement direction information. However, the TEFPI optical fiber sensor has low visibility of the interferometric fringes, and requires somewhat sophisticated signal processing to distinguish the measurement direction changes. In this study, the hybrid extrinsic Fabry-Perot interferometric optical fiber sensor is presented, from which the transmission-type and reflection-type sensor signals can be simultaneously acquired. To detect peaks and valleys in the interferometric fringes, the reflection-type signal with relatively higher visibility is processed. The measurement direction is determined by the linear combination of the transmission-type and reflection-type signals, which presents the signal level variation with minimum fringe amplitude. Because the hybrid sensor uses the separated signals containing measurement quantity and direction information, its signal processing is more robust than one of the TEFPI sensor.

The direct observation was made of the stress concentration around circular holes as a cause of fatigue failure by cyclic loading. The change of surface conditions of carbon steel plates with 3 circular holes subjected to cyclic loading with the load ratio 0 was observed. In this time, Lueders' bands were used as a visible smart sensor. Generation and propagation of the yielding zone that appeared around circular holes by cyclic loading were clarified. A clear difference was demonstrated between stress concentration factors and experimental values of stress concentration by cyclic loading. It was confirmed that the interference of stress concentration generated in monotonic loading was similar to that of cyclic loading. Comparison of the yielding zone that appeared around circular holes with the stress distribution that was obtained by the FEM analysis, confirmed a difference between monotonic and cyclic loading.

The authors have applied newly developed small-diameter fiber Bragg grating (FBG) sensors, whose cladding is 40 micrometers in diameter, for the detection of transverse cracks in carbon fiber reinforced plastic (CFRP) laminates. In the previous research, the small-diameter FBG sensors were embedded in CFRP cross-ply laminates. When the transverse cracks occurred, reflection spectra from the FBG sensors became broad with an increase in the crack density. Thus, the authors revealed that the small-diameter FBG sensors have a potential to detect the occurrence of the cracks. In the present research, this technique was applied to the detection of the cracks in CFRP quasi-isotropic laminates, whose laminate configuration was more suitable for the practical use. Through the experiment and the theoretical calculation, it was found that the small-diameter FBG sensor could also detect cracks in the quasi-isotropic laminates. Furthermore, the identification of the crack locations was attempted using an inverse scattering algorithm. The profiles of the grating period and the average refractive index were inversed from a complex reflection spectrum with this algorithm. From the profiles, the cracks could be identified, because the positions of the peaks in the profiles corresponded to the locations of the cracks.

A novel type of distributed fiber optic sensor for petroleum hydrocarbon leak detection based on the measurement of transmitted and Rayleigh backscattered power is presented. The sensor structure includes a sensitive polymer, which reversibly expands in hydrocarbon presence and induces the bending losses in the fiber. The location of the loss region is determined from unique relationships between normalized transmitted and Rayleigh backscattered powers for different positions of the disturbance along the test fiber. The localization of a strong disturbance with an estimated accuracy of +/- 1 m along a 2.844 km-length single-mode fiber was demonstrated.

For some applications there is no necessity in precise measurement of external influence strength. Sometimes, for early warning and alarm systems, it is sufficient to detect appearance of the monitoring variable within some predetermined interval. A system must generate an alarm signal when the monitored parameter approaches near or crosses the limit of the dangerous zone. For such applications, we propose a simple distributed sensor based on a large array of equal Bragg gratings. An interrogation of the sensor is performed at a fixed wavelength. We discuss a principle of operation of the sensor and present results of experimental demonstration of this technique for distributed detection of gasoline leak.