Optical sensors for prognostics in aerospace applications: design and development of FBG based vibration sensors

The purpose of the thesis is to study the use of optical fibers with Bragg grating (FBG) as vibration sensors. The Bragg gratings, as invested by a beam of light, only reflect a specific wavelength. However, this response is very sensitive to variations in the length of the Bragg segment, so that it is possible to use these sensors to perceive variation in length due to vibrations. During the thesis, it was shown that a fiber, placed on a suitable support, can fulfill the task of a regular vibration sensor. It has been tried to study whether the resonance of the fiber itself or of the supports could be exploited so as to have amplification peaks only at certain frequencies. This last behavior would be applied to the dynamics of rotating objects at a constant angular velocity, since a given damage would produce a disturbance at a very precise frequency. In the thesis will be explained in detail all the tools and methodologies adopted. In particular, there is a mechanical shaker which imposes a vibration at an exact frequency, but whose amplitude must be verified with a digital sensor. Then there is the acquisition part, consisting of the interrogator and the computer in which the following software is installed: MATLAB, SmartSoft. The data coming from the interrogator are acquired and a Fourier analysis is made, in order to study the behavior of the fiber and the supports, correctly interpreting the results. It will be shown how the test bench, including some 3D printed PLA supports, has been designed and developed. Moreover, is explained how fiber with connectors and Bragg Grating are made.