Ultra-Low-Noise MIMO Distributed Acoustic Sensor Using Few-Mode Optical Fibers

Distributed optical fiber acoustic sensing technology (DAS) based on Rayleigh backscattering (RBS) is experiencing a rapid development because of the unique capability of measuring acoustic waves along long-distance optical fiber with high spatial resolution, that has broad prospects of applications, including geophysical prospecting, seismology, pipeline security, and train tracking. Despite the numerous applications, DAS currently suffers from high noise level and insufficient signal-to-noise ratio (SNR) due to inherent low RBS coefficient (∼ −70 dB/m) in fiber, and cannot meet the application requirements of weak signal detection. Here, an ultra-low-noise MIMO DAS system using few-mode fibers (FMF) is proposed. FMF is characterized by higher non-linear effect threshold and larger capture fraction of RBS light, and through MIMO technology, the system can realize higher diversity scale and obtain more SNR gain. In addition, 'frequency label' method is proposed to address the signal aliasing problem in transmission modes. In preliminary experiments, a 4×4 MIMO Φ-OTDR system is realized, where the minimum noise floor is −88 dB rad ² /Hz with corresponding strain resolution of 0.15 $p\varepsilon /\sqrt {Hz} $ . These results pave the way for advanced acoustic applications of DAS, such as acoustic emission and marine acoustics.