Characterization of acoustic optical fiber sensors for landslides monitoring

In this thesis the experimental characterization of fiber optic sensors (FOS) for detection of precursory acoustic emissions in rockfall events is reported. All the FOSs work in interferometric configuration: the two our FOSs, namely fiber coil sensors (FCS), work on Mach-Zehnder interferometric configuration, while a third sensor, developed in VU University (Amsterdam), is a Fabry-Perot cavity working in Michelson configuration. FCSs consist in optical use a fiber coils, wounded on a aluminium cylindrical support, acting as sensing element. The other sensor, referred as ferrule top cantilever (FTC), consists in a Fabry-Perot micro-cavity created between a cantilever, carved on top of a ferrule, and the end face of a fiber housed within the same ferrule. When the cantilever vibrates due to acoustic emission waves, the length of the cavity changes, inducing an instantaneous variation of the reflectivity of the FP cavity, which is probed by a low-coherence laser, tuned at the quadrature point of the cavity. A comparison between these sensors and a classical piezoelectric transducer (PZT) respond has been also performed. In particular, a methodology of inves- tigation has been developed: the characterization have consisted in of analysing the responsivity and SNR in the frequency range of 20-100 kHz. Tests with a real block rock have also been done. The sensors were fastened with a screw on a Classic Gray Montemerlo Trachyte block and they were stimulated by the vibrations induced by a ball drop. The repeatability of the ball drop was guaranteed by the use of a steep slide. These results,yet preliminary, show the capability of such FOSs to detection and monitoring of acoustic emission generated by rockfall activities