270 km single-span distributed fiber-optic vibration sensor without optical amplification

With our ever-expanding cities and interconnected cities, smart monitoring for long-distance railway tracks and natural hazards is imperative. Conventional distributed acoustic sensors (DAS) rely on backscattered signals along the optical fiber, which limit their maximum sensing distance and signal-to-noise ratio. Long-distance forward transmission distributed sensing systems typically use a double-ended detection strategy with bidirectional optical propagation, and the maximum sensing distance is constrained by backscatter noise rather than optical power. Rayleigh backscattering (RBS) and coherent noise arising from backscattered light mix in with the forward transmitted signals, resulting in a reduced signal-to-noise ratio. To separate backscatter from forward transmission, an acousto-optic frequency-shifted forward transmission distributed vibration sensor (AFS-FTDVS) design is proposed, which can facilitate vibration sensing over long distances without the need for optical amplification. A positioning accuracy of 35.6 m was demonstrated along a 272 km-long single-mode fiber. In addition, a tensile test of submarine optical cable was carried out to simulate the monitoring of seabed events. The experimental results reveal that the sensing system can precisely measure and locate low-frequency (0.15 Hz) stretching events along a 180 km long submarine cable, which provides the groundwork for an ocean-spanning natural disaster observation network.