Real-Time Low Noise Distributed Acoustic Sensing In 171 Km Low Loss Fiber

We demonstrate distributed acoustic sensing (DAS) by interrogation of Rayleigh backscattering from fibers with long linearly frequency modulated pulses and coherent detection. This system provides sustained real-time phase demodulation without inline amplification over a range of 148 km in standard single mode fiber and up to 171 km in low-loss OFS TeraWave SCUBA 125 fiber. This is the longest reported range for DAS measurements. The optical dynamic range of the recording is 57 dB. With a 10 km fiber, we obtain a record-low interrogation noise above 50 Hz (rms average over position) of 134 and 89 ?rad/?Hz with gauge lengths (equal to spatial resolution) of 10 and 34 m, respectively. A total harmonic distortion of ?42 dB (rms average over position) is demonstrated with a gauge length of 10 m.