In the last years the problem of the non invasive monitoring of the environment have gained a great attention and in particular the diagnostic techniques based on fiber optic sensors play an important role. In fact, these sensors have small dimension, low cost, are immune to electromagnetic interference, are suitable for large scale monitoring networks, and are chemically and mechanically compatible with many materials. However, a typical fiber optic sensor permits only a local measurement of the quantity of interest. This may be a drawback if a monitoring with high resolution (meter) and over a long distance (kilometres) is needed. In fact, in this case thousand sensors can be needed with complicate multiplexing networks and an increase in the cost and the complexity of the interrogation system. These limitations can be avoided using a different type of sensors called "distributed sensors" that make it possible the measure of the interested parameter with a spatial continuity along the structure under analysis. These sensors have been already used in the strain monitoring of bridges, dams, pipelines, and historical building or in the temperature monitoring of rails, lakes and rivers. The sensing principle of these sensors is the stimulated Brillouin scattering that is also present in standard telecommunication fibers and permits the strain/temperature monitoring over kilometric ranges. However, currently, the standard Brillouin techniques do not permit a quantitative evaluation of the quantity of interest. In addition, they work in time domain and this entails that the achievable resolution is related to the possibility to measure fast transient signals with strong limitation on the signal to noise ratio.
In order to overcome these limitations, innovative measurement configurations and algorithms for distributed sensor based on stimulated Brillouin scattering in frequency domain have been developed at IREA. In particular, by resorting to the exact differential formulation of the stimulated Brillouin scattering, we have developed new reconstruction algorithms for the determination of the strain/temperature profile along the fiber, starting by frequency domain measurements. Since these algorithms rely on equations that describe the physical phenomena, this approach permits to avoid the typical systematic errors that affect the classical reconstructions techniques, particularly when high resolution or/and long range monitoring are required. Furthermore, the choice of the frequency domain measurement permits to use a synchronous detection technique with a strong improvement of the signal to noise ratio and so and increase in the measurement accuracy. A prototype for the strain/temperature monitoring based on the Brillouin scattering in frequency domain has been set up at IREA. The collected measurements have permitted to experimentally validate the effectiveness of the proposed reconstruction algorithms. The reconstruction results pointed out the possibility of localizing and characterizing temperature spots with a spatial resolution of 1.5 meters and an accuracy of 1.5 °C over fiber some hundred meters long.
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