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

Area of Science:

• Fiber optic sensing
• Distributed vibration sensing
• Natural hazard monitoring

Background:

• Conventional distributed acoustic sensors (DAS) face limitations in sensing distance and signal-to-noise ratio due to backscattered signals.
• Existing long-distance systems are constrained by backscatter noise, hindering effective monitoring of extensive infrastructure and natural phenomena.

Purpose of the Study:

• To propose and demonstrate an acousto-optic frequency-shifted forward transmission distributed vibration sensor (AFS-FTDVS) for enhanced long-distance monitoring.
• To overcome the limitations of Rayleigh backscattering and coherent noise in conventional distributed sensing systems.

Main Methods:

• Development of an AFS-FTDVS system utilizing forward transmission with acousto-optic frequency shifting to separate backscatter from forward signals.
• Experimental validation on a 272 km single-mode fiber for vibration sensing and positioning.
• Tensile testing of a 180 km submarine optical cable to simulate seabed event monitoring.

Main Results:

• Achieved a positioning accuracy of 35.6 m over a 272 km fiber.
• Successfully measured and located low-frequency (0.15 Hz) stretching events along a 180 km submarine cable.
• Demonstrated the system's capability for long-distance vibration sensing without optical amplification.

Conclusions:

• The AFS-FTDVS design effectively separates backscatter noise, enabling precise long-distance vibration sensing.
• This technology provides a robust solution for monitoring critical infrastructure like railway tracks and natural hazards, including potential applications in ocean-spanning disaster observation networks.