Optical Fiber Sensing: OTDR and Polarimetry to Design a Warning System for Debris Flows and Snow Avalanches

Optical fiber sensors are sensors that monitor variations of the physical parameters through an optical fiber that is the sensor itself, in particular Distributed Optical Fiber Sensors (DOFSs) provide spatial continuity along the whole length of the optical fiber. Sensing systems of this kind gain all the properties and all the advantages of optical fibers and are specifically indicated for monitoring of big structures and natural environments. The measurement of the backscattered light and the analysis of the evolution of changes in polarization over time are the concepts studied in this thesis, with the purpose of designing an optical fiber sensor that is able to send out an alarm signal with the occurrence of debris flows and snow avalanches in mountain environments. The focus is on the construction of an OTDR sensor that exploits a highly coherent laser as input and a coherent receiver, the phase noise characterization of the input laser in this situation, and the construction of a sensor based on a polarimeter. Performance of these experimental setups have been evaluated in laboratory and in a real site, by the acquisition of final output traces from the involved instruments and their post-processing.