Introducing Coherent MIMO Sensing, a fading-resilient, polarization-independent approach to $phi$-OTDR

Nowadays, long distance optical fibre transmission systems use polarization diversity multiplexed signals to enhance transmission performances. Distributed acoustic sensors (DAS) use the same propagation medium: single mode optical fibre, and aims at comparable targets such as covering the highest distance with the best signal quality. In the case of sensors, a noiseless transmission enables to monitor a large quantity of mechanical events along the fibre. While long-haul transmission systems are equipped with the latest transceivers, DAS interrogators remain minimalist, mainly for cost concerns. What if the focus was more on performances than cost ? To answer this question, we developed a sensor interrogation technique based on coherent phase-sensitive optical time domain reflectometry ($phi$-OTDR), with dual polarization multiplexing at the transmitter and diversity at the receiver. That technique is called Coherent-MIMO sensing. A study is performed from a dual-polarization numerical model to compare several sensor interrogation techniques, including Coherent-MIMO. We demonstrate that dual-polarization probing of a fibre sensor makes it insensitive to polarization effects, decreases the risks of false alarms and thus increases its sensitivity. The simulations results are validated with an experiment, and finally quantitative data are given on the performance increase enabled by Coherent-MIMO sensing.