Apodized fibre Bragg grating acousto-ultrasonic sensor under arbitrary strain using dual Fabry-Perot filters

This paper describes a novel acousto-ultrasonic (AU) sensing system in which a fibre Bragg grating (FBG) is permanently built in a structure for in situ health monitoring. The AU wave detection using an FBG can be realized by a demodulation technique using an optical filter or light source narrower than the FBG spectrum. However, since the spectral wavelength shift induced from structural strain is much larger than the spectral oscillation induced from the AU wave, it is difficult for the demodulation wavelength to follow the spectral shift. For this work, we introduce a passive demodulation method regardless of the spectral shift based on an apodized FBG (AFBG) and dual fibre Fabry–Perot (FFP) filters. The free spectral range (FSR) of a high-finesse FFP filter is matched to the full-bandwidth of the AFBG without side lobes. Therefore, a part of the AFBG spectrum is always filtered by one of the multiple interferometric peaks. However, the wavelength regime near the maximum or minimum reflectivity of the AFBG provides insufficient sensitivity with respect to the wave. Therefore, another 1/4 FSR-biased FFP filter with the same FSR is simultaneously operated.