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2 The optical fibers are filaments of glassy materials or polymeric, constructed so as to be able to lead to their internal light (guided propagation). Each single fiber is composed of two concentric layers of transparent material extremely pure: a central cylindrical core and a cladding around it. The core has a very small standard diameter of about 10 µm for the mono-modal and 50 µm for multimode, while the cladding has a diameter of about 125 µm. The optical fiber operates as a kind of tubular mirror. The light that enters the core at a certain angle (angle limit) propagates through a series of reflections on the separation surface between the two materials of the core and the cladding. Outside of the fiber there is a protective sheath polymer (jacket) which serves to give resistance to physical stress and corrosion and avoid contact between the fiber and the external environment. JACKET COATING CLADDING CORE Different types of fibers are distinguished by the diameter of the core, refractive index, material characteristics, transition profile of the refractive index and doping (addition of small quantities of other materials to modify the optical characteristics). SIMULATED BRILLOUIN SCATTERING The use of Brillouin scattering presents an interesting potential for the distributed control of the temperature and of the deformations. This system is capable of measuring the local variation of these parameters over distances up to hundreds of miles with a spatial resolution varying from 10 cm up tometers. Brillouin scattering is the result of the interaction between the optical and acoustic waves that propagate in the fiber. The thermally excited acoustic waves (phonons) produce a periodic modulation of the refractive index. The Brillouin scattering takes place when the light propagating in the fiber is diffracted by this grating in the backward movement, generating a component shifted in frequency. The acoustic waves can also be generated by injecting to the two opposite ends of the fiber two light beams with a frequency difference equal to the Brillouin shift. Through a phenomenon of electrostriction, these two waves cause an acoustic movable wave which reinforces the population of phonons. This process is called stimulated Brillouin scattering. When the incident light field is sufficiently intense, this process may become stimulated (SBS). In this case, the acoustic wave is generated by the interference pattern formed by the incident wave and the Stokes wave counter-propagated through the phenomenon of Electrostriction. The acoustic wave, in turn, produces a modulation of the refractive index in the middle, which makes possible a transfer of power from the incident wave to the Stokes wave by diffraction. It s necessary to run two signals called counter-propagating pump wave (incident wave) and probe wave (Stokes wave). The distributed sensors based on Brillouin scattering (λ) allow you to perform distributed measurements of strain and temperature by measuring the shift of the Brillouin. Each local variation of temperature and / or strain in the fiber, by acting on the acoustic velocity, produces a change in the local value of the Brillouin shift. The most used technique to measure strain and temperature consists in the application of two adjacent fibers where one of them is bound to the entity to be monitored, thus able to detect the deformations and the other one not rigidly bound or loose therefore only able to detect changes of temperature. The distinction between strain-temperature takes place by comparison.
3 Over the last few years the development of fiber optic sensors have seen greatly increase the ability to monitor even continuous variations of deformation and temperature, both within the geotechnical field and either in the engineering field. In engineering works, the use of a monitoring system for optical fiber is used to compare the actual behavior, both in terms of deformation stress, with the results of the verification calculations, and also during the various phases of construction and operation statements, indicating the presence of any discrepancies with the expected behavior and, therefore, identifying the area or areas of intervention. The system offers the following main features: Measurements of the variation of deformation and temperature profiles Wide rangemeasurementand high spatialresolution A sensitiveelementalso represented by a fiber on TLC Immunityto electromagneticinterference Minimalinvasiveness Integrabilityof the structures Measurementsovertime The peculiarities of the system bring substantial advantages in monitoring works, both in the building and working phase by giving the possibility of: IDENTIFICATION of structural abnormalities along the path where is installed the optic fiber DIAGNOSIS in real time of dangerous conditions PLANNING regular and targeted interventions The term early warning refers to the widespread alarms in a time interval between the moment in which are observed phenomena which indicate the generation of a potentially dangerous event and the time at which the event affects a given location. The DIMMS Group has acquired considerable experience in the design of early warning systems, containing a probabilistic estimation of the parameters and in setting up systems that include: high-density spatial sensor network; sturdy and redundant data transmission systems capable of working in extreme conditions and remote; ability to process data and provide real-time information; information and diffusion strategies; Legal problem solving. The monitoring system distributed in optical fiber also performs the function of fire prevention succeeding to highlight, in an extremely precise way, the areas where thermal anomalies occur. The measurements and thermal anomalies that occurred can be managed and diagnosed remotely and in realtime and can also be defined which are the warning and alarm thresholds.
4 The capacity for analysis and monitoring the behaviour of structures, even very extensive and which contain critical elements, is considered very important for the designers and is an issue of increasing importance. Usually, in nonindustrial use, structural features are a design parameter and any uncertainties are kept under control through safety factors. Typical areas where are used are: The quantitative evaluation of design parameters is commonly performed by carrying out, as for example at the time of testing, load testing; after that, the structure is controlled during the year, through periodic inspections. Some important results that can be achieved by implementing monitoring systems distributed in optical fiber are the increasing of the security level, mainly due to a continuous monitoring over time instead of a simple estimation of the evolution of the damages and the reduction of costs maintenance, due to the optimization of the interventions. May occur some improvements in terms of design and modelling, taking advantage of the knowledge of the behaviour of structures in place. The use of fiber optic sensors, permanently connected to the facilities to monitor, allows the creation of so-called 'smart structures', in practice facilities capable of providing information on their status and their integrity. The potential of a sensor consisting of a simple optical fiber, which can provide a profile of deformation or temperature for a length of several kilometers and a spatial resolution from metric to sub-centimeter, is huge when controlling large structures such as dams, bridges, large buildings, etc..
The Group DIMMS through a partnership agreement with the BRUGG CABLES, a global leader in the development, manufacture and supply of fiber optic cables can count on a wide range of types of cables with optical fiber capable of detecting changes in deformation, temperature, acoustics / vibration, pressure and humidity. The partnership between the two companies allows us to study, design and plan ad-hoc solutions for every type of work to be monitored. 5
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