Dynamic strain response of a π-phase-shifted FBG sensor with phase-sensitive detection

Abstract

Fiber optic sensors realized with passive π–phase-shifted fiber Bragg gratings (πFBG) offer an enhanced detection limit leading to the measurement of weak strain perturbations as compared to a regular FBG sensor. This superior detection limit is due to the extremely narrow reflection notch or transmission peak of a passive πFBG that allows high resolution measurement of spectral separation. We have recently demonstrated an innovative and simple concept based on phase-sensitive detection for the efficient measurement of the wavelength-shift of the transmission peak. This phase-sensitive detection is implemented using a fiber based Mach-Zehnder interferometer (F-MZI). The Bragg wavelength-shift of the narrow transmission peak under the influence of dynamic strain due to acoustic signals is measured at the output of a designed and fabricated F-MZI. Since the bandwidth of this transmission peak is significantly smaller as compared to a regular FBG sensor of similar length, this gives the leverage of a longer optical path difference (OPD) between the two arms of an F-MZI. This new concept was initially demonstrated in our previous article through simulation and very preliminary experimental studies. Here, we present elaborate experimental studies carried out to fully demonstrate the potential of the proposed idea for low-amplitude dynamic strain sensing. We further elucidate the strain sensitivity and dynamic range of the proposed concept. In the current experimental configuration, the minimum detectable dynamic strain is estimated to be 17 pϵ over 1 Hz measurement bandwidth.