Interferometric interrogation of π-phase shifted fiber Bragg grating sensors

Interferometric interrogation technique realized for conventional fiber Bragg grating (FBG) sensors is historically known to offer the highest sensitivity measurements, however, it has not been yet explored for pi-phase-shifted FBG (pi FBG) sensors. This, we believe, is due to the complex nature of the reflection/transmission spectrum of a pi FBG, which cannot be directly used for interferometric interrogation purpose. Therefore, we propose here an innovative as well as simple concept towards this direction, wherein, the transmission spectrum of a pi FBG sensor is optically filtered using a specially designed fiber grating. The resulting filtered spectrum retains the entire characteristics of a pi FBG sensor and hence the filtered spectrum can be interrogated with interferometric principles. Furthermore, due to the extremely narrow transmission notch of a pi FBG sensor, a fiber interferometer can be realized with significantly longer path difference. This leads to substantially enhanced detection limit as compared to sensors based on a regular FBG of similar length. Theoretical analysis demonstrates that high resolution weak dynamic strain measurement down to 4 p epsilon / root Hz is easily achievable. Preliminary experimental results are also presented as proof-of-concept of the proposed interrogation principle. (C) 2017 Elsevier B.V. All rights reserved.