Φ-OTDR Based on Undersampling Heterodyne Detection

We demonstrate a distributed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) that employs I/Q demodulation and heterodyne detection. The proposed DAS system utilizes a 90° optical hybrid to obtain in-phase (I) and quadrature (Q) signals. By applying undersampling theory, the system significantly reduces the required analog-to-digital sampling rate. In an experimental demonstration, a 200 MHz heterodyne beat signal is successfully recovered at a sampling rate of 110 MSa/s without any loss of phase information. The system achieves a spatial resolution of 10 m, a signal-to-noise ratio of approximately 63.54 dB at a demodulation frequency of 200 Hz, and a background noise level of −52.27 dB·rad2/Hz. In addition, an amplitude-based analysis of the I/Q data is used to locate vibration events and estimate their effective length, so that an adaptive differential gauge length can be chosen to suppress common-phase fluctuations and restrict phase demodulation to a short fiber segment. This approach effectively reduces data throughput and system complexity while maintaining high sensitivity and resolution, illustrating the potential for more efficient real-time DAS implementations.