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The growing demand of safety awareness has stimulated, in the last few years, the development of several monitoring techniques capable of detecting early-stage events, thus preventing structures from major failures and leading to a better knowledge of the structure itself. In the field of structural and geotechnical applications such as dams, levees, bridges, buildings, landslide, sinkhole and tunnels, where both the large structure dimensions and damage location forecast represent a challenge, distributed techniques offer the capability of monitoring over several kilometers using a single Fiber Optic Sensor, (FOS). Thus, using a limited number of very long sensors it is possible to monitor structural and functional behavior of structures with a high measurement and spatial resolution at a reasonable cost (Glisic and Inaudi 2007).
DISTRIBUTED FIBER OPTIC TECHNOLOGY
Unlike electrical and localized fiber optic sensors, distributed sensor offer the unique characteristic of being able to measure physical parameters along their whole length, allowing the measurements of thousands of points using a single transducer (Inaudi and Glisic 2007). The most developed technologies of distributed fiber optic sensors are based on Raman and Brillouin scattering (Inaudi D. et al. 2012). Both systems make use of a non-linear interaction between the light and the silica material of which a standard optical fiber is made. If light at a known wavelength is launched into a fiber, a very small amount of it is scattered back at every point along the fiber. The scattered light contains components at wavelengths that are different from the original signal. These shifted components contain information on the local properties of the fiber, in particular their strain and temperature.
Raman Distributed Temperature Technology
Raman scattering is the result of a non-linear interaction between the light traveling in a fiber and silica. When an intense light signal is shined into the fiber, two frequency-shifted components called respectively Raman Stokes and Raman anti-Stokes will appear in the back-scattered spectrum. The relative intensity of these two components depends on the local temperature of the fiber. If the light signal is pulsed and the back-scattered intensity is recorded as a function of the round-trip time, it becomes possible to obtain a temperature profile along the fiber (Dakin et al. 1986). Systems based on Raman scattering are commercialized by SMARTEC in Switzerland and Sensornet in UK (Figure 1). Typically a temperature resolution of the order of 0.1°C and a spatial resolution of 1m over a measurement range up to 30 km are obtained for multimode fibers.
Brillouin Distributed Strain Technology
Brillouin scattering sensors show an interesting potential for distributed strain and temperature monitoring (Karashima et al. 1990). Systems able to measure strain or temperature variations of fibers with length up to 50 km with spatial resolution down in the meter range are now demonstrating their usefulness in field applications. Brillouin scattering is the result of the interaction between optical and sound waves in optical fibers. Thermally excited acoustic waves (phonons) produce a periodic modulation of the refractive index. Brillouin scattering occurs when light propagating in the fiber is diffracted backward by this moving grating, giving rise to a frequency-shifted component by a phenomenon similar to the Doppler shift. This process is called spontaneous Brillouin scattering. Acoustic waves can also be generated by injecting in the fiber two counter-propagating waves with a frequency difference equal to the Brillouin shift. Through electrostriction, these two waves will give rise to a traveling acoustic wave that reinforces the phonon population. This process is called stimulated Brillouin amplification. If the probe signal consists in a short light pulse and its reflected intensity is plotted against its time of flight and frequency shift, it will be possible to obtain a profile of the Brillouin shift along the fiber length. The most interesting aspect of Brillouin scattering for sensing applications resides in the temperature and strain dependence of the Brillouin shift (Niklès et al. 1997). This is the result of the change of the acoustic velocity according to variation in the silica density. SMARTEC commercializes a system based on this setup and named DiTeSt (see Figure 2). It features a measurement range of 50 km per channel with a spatial resolution of 1 m. The strain resolution is 2 με and the temperature resolution 1°C. The number of channels can be extended by a 4-20 channel Switch. The system is portable and can be used for field applications. Since the Brillouin frequency shift depends on both the local strain and temperature of the fiber, the sensor setup will determine the actual sensitivity of the system. For measuring temperatures it is sufficient to use a cable designed to shield the optical fibers from an elongation of the cable. The fiber will therefore remain in its unstrained state and the frequency shifts can be unambiguously assigned to temperature variations. Measuring distributed strains requires a specially designed sensor. A mechanical coupling between the sensor and the host structure along the whole length of the fiber has to be guaranteed. The next section will introduce different cable designs to measure strain and temperature in different applications.
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