Time-Delay-Interferometry-Based 600km Millihertz Fiber-optic Forward Transmission Vibration Sensor

Optical fiber vibration sensors play a vital role in applications requiring precise low-frequency disturbance detection, such as ocean seismic monitoring and structural health assessment. However, laser frequency noise, especially frequency drift, severely limits the sensitivity and signal-to-noise ratio (SNR) of conventional long-distance low-frequency sensing systems. In this work, we propose a novel forward-transmission fiber-optic vibration sensing technique based on Time Delay Interferometry (TDI), originally developed for space-based gravitational wave detection. By implementing a dual Mach-Zehnder interferometer configuration and applying time-delayed linear combinations of phase signals from both interferometers, the system effectively cancels laser frequency drift-induced phase noise. Numerical simulations demonstrate that this approach achieves over 30 dB SNR enhancement and 100 dB noise floor (NF) suppression for signals in the millihertz to hertz band over distances up to 1000 km. Experimental validation over a 600 km standard single-mode fiber link confirms a minimum SNR gain of 31.18 dB and noise suppression of 155.31 dB @ 10 mHz. In addition, the strain resolution reaches 1.5 nϵ/√Hz @ 10 mHz. Furthermore, the proposed TDI method outperforms conventional differential method in terms of SNR enhancement and NF suppression, and it enables simultaneous, independent detection of signals from both interferometers. This technique significantly advances long-range, ultra-low-frequency fiber-optic vibration sensing and holds promise for applications including submarine seismic monitoring and enhanced security in high-risk environments.