  Nowadays, long distance optical fibre transmission systems use polarization
diversity multiplexed signals to enhance transmission performance. Distributed
acoustic sensors (DAS) use the same propagation medium ie. 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. This
paper aims at extending the perspectives of DAS systems with regard to
technology breakthroughs introduced in long haul transmission systems over the
last decade. We recently developed a sensor interrogation method based on
coherent phase-sensitive optical time domain reflectometry ($\phi$-OTDR), with
dual polarization multiplexing at the transmitter and polarization diversity at
the receiver. We name this technique 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 strongly
enhances 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.
