Impact of I/Q Amplitude Imbalance on Coherent $\Phi $ -OTDR

In this paper, the quadrature amplitude imbalance which may consist in I/Q demodulated phase-sensitive optical timedomain reflectometry (Φ-OTDR) is analyzed, which may happen in both ac and dc components of received signals. Based on the analysis of imperfect coherent detection module, the impacts of each imbalance factor on the demodulated sensing results of ΦOTDR are discussed qualitatively and quantitatively. The amplitude mismatch of ac and dc components may incur the distortion of demodulated signals, while the effect caused by residual dc parts is significantly severer than the impacts of ac imbalance. When both factors are present simultaneously, the ac amplitude imbalance can further degrade the demodulated signals distorted by residual dc components. Furthermore, this paper proposes a corresponding method to characterize and compensate the amplitude imbalance dynamically, using the real-time data of Φ-OTDR traces. This method is also feasible for other fiber-optic sensing/communication systems with coherent detection.