Distributed vibration sensor based on space-division multiplexed reflectometer and interferometer in multicore fiber

The maximum detectable vibration frequency by phase-sensitive optical time-domain reflectometer (Φ-OTDR) is limited by the repetition rate of the pump pulse. Additionally, the intensity-demodulation based Φ-OTDR sensor suffers from the generally nonlinear dependency of the backscattered optical intensity on vibration. This makes quantitative measurement of vibration frequency difficult. In order to mitigate these limitations, we propose and demonstrate a multicore fiber (MCF) based space-division multiplexed (SDM) Φ-OTDR and Mach-Zehnder interferometer (MZI) hybrid sensor, enabling truly uninterrupted distributed vibration sensing with broad vibration frequency response range and high spatial resolution. By taking advantage of highly integrated multiple spatial cores offered by an MCF, Φ-OTDR implemented in one of the cores is used to locate the vibration. Meanwhile, an MZI is constructed using another two parallel spatial cores with sufficient continuous-wave optical power from a coherent narrow linewidth laser, which is used to retrieve the vibration frequency by processing the acquired interference signal. Compared with the single mode fiber based hybrid systems, the proposed SDM configuration allows implementation of Φ-OTDR and MZI independently without any crosstalk between them. Consequently it offers many unique advantages, such as simple data processing procedure, good signal-to-noise ratio of the demodulated vibration frequency spectrum, no frequency dead zone, and single-end access. This cost-effective SDM hybrid sensor provides significant potential for distributed and accurate monitoring of vibrations.