Fiber optic in-line distributed sensor for detection and localization of the pipeline leaks

When a gas or liquid pipeline leaks, it will generate broadband acoustic signal. This acoustic pressure will induce an optical phase signal of the optical fiber fixed on the surface of pipes. In this paper, we propose a hybrid configuration of Mach-Zehnder and Sagnac interferometer as sensing frame. In this interferometer, there are two light paths that have the same optical length but travel different sequence paths. Because the propagation lights of the two light paths pass through the leaking point at different times, the resulting phase signals differ respectively. After interference, we demodulate the sensing phase signal by a broadband phase generated carrier (PGC) circuit and then the leaking point can be acquired from null frequency of the output spectrum. This system has many advantages, such as its in-line configuration, all fiber structural design and polarization-insensitive for sensing fiber. This system also has very wide dynamic range which can be greater than 76 dB with the minimum detectable phase signal about 3.3 × 10^{- 4} (rad / sqrt(Hz)).