  We propose and experimentally demonstrate a novel interference fading
suppression method for phase-sensitive optical time domain reflectometry
(Phi-OTDR) using space-division multiplexed (SDM) pulse probes in few-mode
fiber. The SDM probes consist of multiple different modes, and three spatial
modes (LP01, LP11a and LP11b) are used in this work for proof of concept.
Firstly, the Rayleigh backscattering light of different modes is experimentally
characterized, and it turns out that the waveforms of Phi-OTDR traces of
distinct modes are all different from each other. Thanks to the spatial
difference of fading positions of distinct modes, multiple probes from
spatially multiplexed modes can be used to suppress the interference fading in
Phi-OTDR. Then, the performances of the Phi-OTDR systems using single probe and
multiple probes are evaluated and compared. Specifically, statistical analysis
shows that both fading probabilities over fiber length and time are reduced
significantly by using multiple SDM probes, which verifies the significant
performance improvement on fading suppression. The proposed novel interference
fading suppression method does not require complicated frequency or phase
modulation, which has the advantages of simplicity, good effectiveness and high
reliability.
