A fully digital MIMO-OFDM scheme for fading mitigation in coherent ∆φ-OTDR

We introduce a coherent-MIMO ∆ϕ-OTDR based sensing technique that uses digital frequency division multiplexing of probing codes in order to mitigate the coherent fading issue in ∆ϕ-OTDR. Orthogonal frequency division multiplexing (OFDM) is used to probe the optical channel on several electrical subcarriers, and their responses are then combined according to specific reliability criterion to enhance the overall sensing sensitivity. This allows for a decrease of the noise floor in sensing measurements on standard single mode fiber (SSMF) and the mitigation of false alarms. Coherent MIMO-OFDM enables sensing over telecom fibers with a measured sensitivity level below 21pε/^√Hz along 1.3km fiber over a mechanical bandwidth of 760Hz. Enhancements in the dynamic range for detection of mechanical excitation are presented with a gain over 18dB as compared to single-carrier probing.