Static and dynamic behavior of FBG sensors

This paper presents a static and dynamic characterization of Fiber Bragg Grating (FBG) sensors from a thermo-mechanical point of view. A proper experimental setup was built and a climatic chamber with a time constant of the order of ten minutes was employed. The output of both free and glued fibers was evaluated by varying the input stresses at constant temperature and varying the temperature at constant stress. The experimental results both confirm the presence of critical sliding effects between the cladding and coating in Acrylic FBG sensors that cause non-negligible measurement errors. These effects are not of stick-slip type, but rather they are proportional to the applied loading. At ambient temperature the sliding among the FBG layers is evident as a function of time. For example, the measured stress varies from 387+/-10 ppm to 341+/-10 ppm within one minute causing a measurement error of 12 % in Acrylic FBG sensors having a slip film. Polyamide FBG sensors and FBG sensors without coating, do not show this behavior at ambient temperature. The cut-off frequency, within 3 dB, is about 20 Hz, but the sliding among the layers causes a signal reduction of about 40% in time-varying signals with frequencies up to 30 Hz.