Phase sensitive distributed vibration sensing using double-pulse for ultra-weak FBG array

A distributed vibration sensing technique using double optical pulse based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) and ultra-weak FBG (UWFBG) array is proposed for the first time. The single mode sensing fiber is integrated with UWFBG array which has uniform spatial interval and ultra-weak reflectivity. The relatively high reflectivity of the UWFBG, compared with the Rayleigh backscattering(RBS), gains a high SNR for the signal, which can make the system achieve the maximum detectable frequency limited by the round trip time of the probe pulse in fiber. A corresponding experimental Φ-OTDR system with a 4.5 km sensing fiber integrated with UWFBG array was setup for the evaluation of the system performance. Distributed vibration sensing is successfully realized with spatial resolution of 50 m. The sensing range of the vibration frequency can cover from 3 Hz to 9 kHz.