Phase Demodulation under Free Undersampling in Heterodyne Coherent Φ-OTDR

Heterodyne coherent detection phase-sensitive optical time-domain reflectometer
$(\mathbf{\Phi}-\mathbf{OTDR})$
is widely used in structural health monitoring of large facilities due to the ease of achieving stable phase demodulation, but it usually requires a high sampling rate. The sampling rate can be reduced effectively by undersampling, but the sampling rate of undersampling is strictly limited by spectrum aliasing effect. To solve this problem, this paper proposes a free undersampling phase demodulation method in the heterodyne coherent
$\mathbf{\Phi}-\mathbf{OTDR}$
. The method changes from traditional “distance” direction demodulation to “time” direction demodulation of the recombination signal. Therefore, the spectrum aliasing in the “distance” direction caused by undersampling does not affect the demodulation in the “time” direction, and the sampling rate of undersampling can be freely selected. In the experiment, we proposed method can accurately demodulate the phase in sampling rates of 100 MSa/s and 10 MSa/s that within the sample rate restricted area of conventional undersampling demodulation method.