• FBG
• strain gauge

Since their invention, sensors based on optical fibers are powerful and versatile devices.
Nowadays they play a fundamental role in many different fields, e.g., telecommunication, civil engineering, science research and more. Wide applications are related to the possibility of realize the so called Fiber Bragg Grating (FBG). A FBG is a fiber in which a Bragg grating is created: this is a periodic modulation of the refractive index of the fiber core, that acts like a band-rejection filter with respect to the light travelling into the fiber. This particular kind of structure allows to realize several optical devices like Fabry-P ́ erot or Circulator. High reflectivity FBG can be also used as mirrors at the ends of a doped fiber to create fiber laser. The grating period and, consequently, the reflected and transmitted wavelengths, depend on the temperature variation and fiber dilatation or contraction. Observing the wavelength shift it is possible to evaluate the temperature
changes or the fiber elongation, this make the FBG a thermometer and a strain gauge. Moreover we can measure external perturbations converting them into fiber strain by a suitable system. So FBGs can be used in devices like flow-meter, magnetic/electric field meter etc.
Relevant applications of FBGs can be found in geology and geophysics. Within these fields, in particular, there is a project that is planning to use them as strain sensors: MEDiterranean SUpersite Volcanoes (MED-SUV), an European project under the coordination of European Plate Observing System (EPOS). This project, together with FUTUREVOLC and MARsite, plans to enhance the current knowledge bout geohazards,
in order to improve the scientific institutions capacity in preventing the damage on environment and population. MED-SUV is focused on the study of Italian volcanoes: CampiFlogei/Vesuvius and Etna.
As a general framework, this thesis is involved in the MED-SUV project, which studies mount Etna. In collaboration with Marwan Technology, a spin-off company of the Department of Physics of Pisa niversity, we have realized a tri-axial strain gauge sensor and the interrogating system that controls it. The sensor is designed to measure the rock movements along the three Cartesian axes. This kind of measurement requires that the sensor is installed in hostile environment, on the slope of mount Etna. For these reasons, the project specifications for the building of sensor and interrogation system are: low cost, low energetic consumption and reduced dimension. In particular, the last parameter is due to the necessity of installing the sensor in the bottom of a 10 meter borehole, dug in the basalt rock.
The interrogation system has been realized with the cooperation of the Osservatorio Vesuviano, whose researchers realized the ”GILDA datalogger”, expressly studied to work in hostile environment and that will be use as acquisition system. To interrogate the response of the FBGs, we have assembled an opto-electronic device based on a super luminescent diode, as light source, an Array Waveguide Grating (AWG) as wavelength discriminator, and six photodiodes, to convert light signal into voltage difference. Using energy-saving components, solar panels and battery this interrogation system is designed to work unattended for months.
We successfully tested it the in laboratory, verifying that the sensor can measure strain with a sensitivity of the order of 0.5 [με ] and a dynamic range of 120 [με].