Detecting vibrations by Fiber Bragg sensor interrogated with a bi-polished Silicon sample

In this paper a new method for reading out Bragg wavelength shifts experienced by fiber Bragg gratings is described. The system is based on a bi-polished silicon sample acting like a Fabry-Perot filter. The spectral response of the Silicon Fabry-Perot filter allows to convert the Bragg wavelength shift into a variation of the light intensity, which can be read by a photodiode. The method efficacy is proved monitoring dynamic strain characteristics of a simple structure. So, the vibration mode of an aluminium cantilever has been sensed by means a FBG sensor attached on the surface of the cantilever itself. In proposed demodulation method the sensitivity and accuracy depend on the spectral band width of the filter. The filter can be designed according to the range of the amplitude vibrations, assuring linear response of system, as function of the thickness of the silicon sample. Moreover, thank to great tuning capability of the Silicon Fabry-Perot filter, it is possible to place Bragg grating spectral response on the central portion of the linear region of the FP response.