Characterization and Compensation of Phase Offset in Φ-OTDR With Heterodyne Detection

Characterizing and real-time compensating the time-skew and phase-mismatch of the in-phase and quadrature (I/Q) signals, for phase-sensitive optical time-domain reflectometry (Φ-OTDR) with heterodyne detection, are the aims of this paper. After amplitude imbalances are compensated, there are still some intrinsic distortions in demodulated signals, which are generated from the time-skew and phase-mismatch of I/Q signals. A novel method, proposed in this paper, can separate the impacts from RF cable length-difference, fiber pigtail length-difference, and the non-orthogonality between I/Q signals, providing the basis for precise and real-time compensation. Both the theoretical analysis and the experimental demonstration are presented. It should also be noted that this method also fits for other heterodyne demodulation systems.