Experimental analysis of FBG sensors thermal calibration under different loading conditions

In recent decades, the use of the optical fiber has revolutionised several technological sectors, starting of course from communications. In addition to this field, however, its use has now become typical in even very different areas, from structural monitoring to medicine. This growth has largely been caused by the possibility of also using this technology for sensor applications. This aspect, together with its physical characteristics, has also led optical fiber to be appreciated in the aerospace sector. In particular, among the advantages there are the high sensitivity of optical sensors, the electrical passivity, the small cables' size and immunity to electromagnetic interference. Among the various optical fiber-based sensors, those used in the present work are the Fiber Bragg Gratings (FBG). They are realised by laser photoengraving directly into the fiber itself and they work like a filter. In fact, each FBG is able to reflect a specific wavelength, known as Bragg wavelength, which is proportional to the geometry of the sensor itself. In this way, it becomes possible to measure physical parameters acting on it, such as temperature, strain, etc. The FBG function mechanism, however, generates only one optical output: in order to be able to distinguish the various contributions, a detailed knowledge of the proportionality between the reflected wavelength and the physical parameter considered is essential. The test campaign described in this work quantifies in detail the sensitivity of the FBG to temperature variation. More specifically, the sensor's thermal response is analysed as a function of the preload condition applied to the fiber. The results showed a variation of proportionality over the various measurement cycles. This leads to a twofold conclusion: on the one hand the need to carry out a specific calibration of the FBG in the case of pre-load; and on the other hand the possibility of exploiting this variation to decouple the thermal and mechanical contributions actina on the sensor.