Distributed Fiber-optic Acoustic Sensor with Long Sensing Range over 100 km and Sub-nano Strain Resolution

The measurement distance is one of the most important parameters for distributed acoustic sensor (DAS). In this paper, we report a long-distance and high-sensitivity DAS system based on time-gated digital optical frequency domain reflectometry (TGD-OFDR). The bi-directional distributed Raman amplification is adopted to realize long measurement distance. The interference fading and polarization fading are well suppressed, and hence phase demodulation method can be adopted, instead of intensity demodulation method. As a result, the sensitivity is enhanced and the full information (amplitude, phase and frequency) of the vibration can be obtained. In experiments, the fiber length is about 108 km, while the spatial resolution is 5 m. A weak vibration with peak-peak amplitude of $14.7 \mathrm {n}\varepsilon $ is correctly located at the distance of 98 km with a high SNR of 30 dB. It is the first time that $220- \mathrm {p}\varepsilon / \surd$ Hz strain sensitivity is realized over 100-km-level fiber and the vibration waveform is retrieved linearly without harmonics.