Multi-Frequency Φ-OTDR DAS Using Timely-Updated Vector-Based Phase Averaging

Distributed acoustic sensing (DAS) using optical fiber cables, widely deployed for communications, can capture various information on the surrounding environment. Phase-sensitive optical time-domain reflectometry (Φ-OTDR) DAS is suitable for sensing deployed cables because Φ-OTDR has long measurement distances. However, a simple Φ-OTDR setup suffers from interference fading, in which sensitivity degradation occurs at many points. Multi-frequency Φ-OTDR is a practical solution that can remove the sensitivity degradation points effectively by averaging the signals of multi-frequency pulses. The method used to average multi-frequency signals affects the total measurement performance, including the sensitivity and the measurable vibration amplitude range. While vector-based averaging on the IQ plane is a powerful method in terms of sensitivity, calculating accurate vibration waveforms becomes difficult when vibrations become strong (> sub-μϵ). Consequently, large-scale vibration patterns are not accurately visualized, limiting potential applications. In this work, we develop a vector-based averaging method that can resolve this issue. We clarify that the known problem of the difference in phase response to strain change between multiplexed frequencies is the key matter causing the issue. To address this discrepancy, we propose a dynamically updated vector-based averaging method, enabling us to monitor strong vibrations. In the proposed method, parameters used in vector-based averaging—rotation angles and reference frequencies—that are fixed in the conventional method are updated over time in accordance with the fiber state. We demonstrate the effectiveness of the proposed method in both laboratory and field environments. We successfully visualize patterns of large-scale vibrations experienced by field-deployed communication cables, such as those caused by vehicle movements and wind blowing.