Distributed disturbance sensor based on a novel Mach-Zehnder interferometer with a fiber-loop

摘要

A novel type of distributed fiber optic sensor is presented to detect and locate a time-varying disturbance along the whole fiber length. The sensor consists of a Mach-Zehnder interferometer (MZI) where two directional optical signals arc simultaneously traveling. This is achieved by a fiber-loop in the configuration, causing one MZI to operate as two co- and counter- propagating interferometers. The sensor is illuminated by only one continuous-wave laser, which is different from the conventional scheme. The phase shift caused by any disturbance around the fiber is detected and converted to the information on the perturbation position and amplitude, which can be determined by combining two phase signals from the above device. In addition, passive homodyne demodulation with application of a 3x3 fiber optical coupler for recovering a signal of interest from the MZI is also described. This technique can yield a large dynamic range with phase amplitudes for its symmetry. Also, high speed digital processing (DSP) technology is used in the sensor system, which can carry out all operations in real time and promote the resolution of localization. The system has capability for cross-correlation algorithm and fast Fourier transform (FFT) analysis of the detected disturbance signals, for the sake of determining the disturbance position and type. Finally, a positional resolution better than 100 m has been theoretically discussed and experimentally demonstrated in a system over 6.8 km fiber length.