Distributed Optical Fibre Sensors for Structural Health Monitoring: Upcoming Challenges

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Mesure répartie de température et de déformations par diffusion Brillouin (de la fibre optique au capteur pour le génie civil)

Structural Health Monitoring (SHM) has been invented by aeronotic engeeniers is now an intense research topic for civil engineers. As the nervous system in the human body, a huge sensor network is detecting damages to produce health diagnostics. To do so, optical fiber sensors (OFS) took advantage of telecommunication developpments of the end of the 90's. Among them, Distributed OFS, wherein a single fibre (withoout any component) constitues the transducer, brought a real breakthrough avoiding any dead zone within the sensing chain and offering a unique multiplexing capacity. This work, performed with EDF R&D, LCPC (french laboratory on roads and bridges), and Télécom-Paristech, presents distributed OFS using Brillouin scattering for temperature and strain measurement, specifically for civil engineering application. With lab expermiments as well as field applications, it concludes on the ablilty to separate temperature and strain with a signle fiber. It also highlights caracteristic limits and difficulties linked to the host material of the sensor.Le contrôle de la santé des structures (SHM pour Structural Health Monitoring), discipline inventée par les hommes du génie aéronautique, est un sujet de recherche de plus en plus intense dans le domaine du génie civil. A l'image du système nerveux pour le corps humain, un réseau de capteur de plus en plus dense est utilisé pour détecter et évaluer l'endommagement de ces structures. A ce titre les capteurs à fibres optiques(CFO), ont profité de l'essor des télécommunications pour offrir des solutions novatrices aux industriels. Parmi eux, les CFO répartis, dont une simple fibre constitue l'élément sensible, apportent une véritable rupture technologique, en éliminant de facto toute zone morte inter-capteur et en offrant une capacité de multiplexage inédite. L'objet de ce travail, réalisé dans le cadre d'un partenariat entre EDF R&D, le Laboratoire Central des Ponts et Chaussées, et Télécom-Paristech, est l'étude des CFO répartis utilisant la diffusion Brillouin pour la mesure de température et de déformation, en vue de l'instrumentation de structures du génie civil. Les sujet s'étend de l'étude du phénomène physique dans la fibre optique jusqu'à celle de son introduction dans de grandes structures du génie civil (bâtiments, digues, ponts, etc.). Ces travaux théoriques et expérimentaux, concluent notamment sur la possibilité de séparer l'influence de la température de celle de la déformation dans ces capteurs grâce à une seule et même fibre. Enfin, des résultats obtenus lors d'expériences de terrain qui mettent en évidence les difficultés caractéristiques rencontrées avec ce type d'instrumentation.PARIS-Télécom ParisTech (751132302) / SudocSudocFranceF

Distributed Optical Fiber Sensors for Structural Health Monitoring : Upcoming challenges. In : Optical Fibre, New Developments, chapitre 9

The civil engineering structures become, on one hand more and more complicated and/or aged and on the other hand, the society seeks to increase their lifetime which require a special attention to prevent the damages: leaks, fires, landslides, earthquakes, etc... Thus to do so, a large network of sensors is necessary, and its insertion inside the structure has often to be planned before its construction. This led to a new field of research and techniques: Structural Health Monitoring (SHM). Thanks of their intrinsic small size, electromagnetic immunity and robustness, optical fibers sensors offered many advantages compared to existing electronic strain sensors (vibrating strings, linear variable differential transformers), or temperature sensors (thermocouples, resistive probes). A lot of point-like optical fiber sensors based on Bragg gratings or Fabry-Perot cavities have been developed in the nineties, and their multiplexing is still an intensive research topic nowadays, mainly on the aim of cutting down the whole sensing system costs. About ten year ago, distributed optical fiber sensing technologies appeared, constituting a breakthrough in sensor technology. Those methods, based on non-linear optical effects (Raman, Brillouin) or light backscattering, allow to measure temperature and strain (and consequently pressure, displacement ) into a high range zone, wherever the user want, with tunable spatial resolution, bringing consequently huge multiplexing possibilities. Now this technology has become quite mature and suitable for industry, thanks to the creation of many companies providing distributed optical fibers sensing devices. Nevertheless, several topics are still intensively studied. Indeed, distributed optical strain sensors are always sensitive to temperature as well. Unfortunately, in some real structures as bridges, temperature and strain can both vary significantly at the same time. We will demonstrate, with the example of a distributed Brillouin sensor located into a reinforced concrete beam, that it is very difficult to distinguish thermal and mechanical effects on measurements, even if the physical origin of the process is well known. Spatial resolution limit of these methods is tightly linked to the chosen technology. Non-linear distributed optical sensors (Raman and Brillouin) are based on optical pulse-echo technique. Then, the resolution is proportional to optical pulse time-width, and limited by the phonon lifetime (10 ns). In these cases the minimum spatial resolution is restricted to approximately 1m, and the range can reach 20km because it is only limited by the optical losses (about 0.2dB/km). For some applications, requiring high resolution (sink-hole detection), the Optical Backscattering Reflectometry (OBR) technique could be a better solution, thanks to its resolution better than 1cm. However, the range is then limited to about 100m, mainly due to the light coherence length. Thus, the sensor choice is always a trade-off between long-range and high-resolution. However, between those extreme cases, despite a few promising laboratory experiments, there is no commercial solution for both middle-range (1km) and middle-resolution (5 cm) requirements at the present time, to the best of our knowledge

Fiber optic monitoring of pipelines in permafrost context

We propose the most suitable setup to have complete integrity monitoring of buried pipelines in permafrost using optical fiber sensing. Choice of best optical fiber cables and their best location near the pipe are discussed

Two-dimensional FEM Analysis of Brillouin Gain Spectra in Acoustic Guiding and Antiguiding Single Mode Optical Fibers

The analysis of optical and acoustic properties of optical fibers is required for accurate Brillouin gain spectrum (BGS) determination. We present a full modal-analysis of the guided optical and acoustic modes based on a two-dimensional finite-element method (2D-FEM) for BGS calculation using COMSOL Multiphysics. We believe that this method will be helpful in analyzing and designing special fibers for applications, such as fiber amplifiers with significant SBS (Stimulated Brillouin Scattering) suppression or Brillouin-based fibersensors. The model is adapted for BGS evaluation of any single mode fiber (in term of optical mode) based on its profile, namely its geometry, and its doping composition. Compared to standard multi-layer methods limited to axially-symmetric fibers [1], the 2D-FEM analysis enables the BGS computation even for more complicated geometries, such as PANDA polarization-maintaining fiber where the optical index and the material stress are azimuthally dependant. The results of numerical modeling have shown good agreement with measured Brillouin spectra for different types of silica fibers. Examples are given for a standard GeO2-doped core fiber (standard fiber for telecommunication applications), Fluor-doped cladding fiber (acoustic anti-waveguide) and PANDA fiber

Self-referenced and single-ended method to measure Brillouin gain in monomode optical fibers

We present a new self-referenced and single-ended method to measure the Brillouin-gain coefficient in monomode optical fibers accurately with high reliability. Our comparative measurements on several different fibers show that a fiber with a smaller optical effective mode area can nevertheless have a higher Brillouin threshold, thus confirming the significance of acousto-optic effective mode area. © 2009 Optical Society of Americ

Soil-embedded optical fiber sensing cable interrogated by Brillouin optical-time domain reflectometry (B-OTDR) and optical frequency-domain reflectometry (OFDR) for embedded cavity detection and sinkhole warning system

A soil-embedded optical fiber sensing cable is evaluated for an embedded cavity detection and sinkhole warning system in railway tunnels. Tests were performed on a decametric structure equipped with an embedded 110 m long fiber optic cable. Both Brillouin optical time-domain reflectometry (B-OTDR) and optical frequency-domain reflectometry (OFDR) sensing techniques were used for cable interrogation, yielding results that were in good qualitative agreement with finite-element calculations. Theoretical and experimental comparison enabled physical interpretation of the influence of ground properties, and the analysis of embedded cavity size and position. A 5 mm embedded cavity located 2 m away from the sensing cable was detected. The commercially available sensing cable remained intact after soil collapse. Specificities of each technique are analyzed in view of the application requirements. For tunnel monitoring, the OFDR technique was determined to be more viable than the B-OTDR due to higher spatial resolution, resulting in better detection and size determination of the embedded cavities. Conclusions of this investigation gave outlines for future field use of distributed strain-sensing methods under railways and more precisely enabled designing a warning system suited to the Ebersviller tunnel specificities

Le réseau de fibres optiques en application sismologique et géotechnique pour ville intelligente : le projet DASARA à Lyon

International audienceLe réseau de fibres optiques en application sismologique et géotechnique pour ville intelligente : le projet DASARA à Lyo

Le réseau de fibres optiques en application sismologique et géotechnique pour ville intelligente : le projet DASARA à Lyon

International audienceLe réseau de fibres optiques en application sismologique et géotechnique pour ville intelligente : le projet DASARA à Lyo