High-sensitivity chirped-pulse-pair φOTDR operated at the milli-hertz frequency band

Ultra-low-frequency acoustic wave detection has significant applications in seismic wave monitoring, oil-gas exploration, and ocean observation. However, those waves often exhibit large amplitude variations that exceed the dynamic of phase-sensitive optical time-domain reflectometry (φOTDR). Although the recently proposed chirped-pulse-pair (CPP) φOTDR scheme offers an extension of measurement range, its capability for ultra-low-frequency detection has yet to be explored. Here, we experimentally demonstrate a high-sensitivity of CPP-φOTDR operated at the milli-hertz frequency band. Initially, the CPP technique is employed to mitigate noise that accumulates during the reference trace updating. Meanwhile, an integrated adaptive filtering is applied to further reduce the noise floor, enabling the detection of acoustic signals at the milli-hertz band. As a result, the ultra-low-frequency sensitivity, after a continuous 20-hour measurement, can reach 6.6 nε/√Hz at 4 mHz, which is the highest sensitivity record at the milli-hertz frequency band that is compatible with the existing communication cables, to the best of our knowledge.