Pico-strain-level dynamic perturbation measurement using {π}FBG sensor

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 demonstrate that high resolution weak dynamic strain measurement down to 4 p{\epsilon}/sqrt(Hz) is easily achievable.