Single-ended forward-transmission distributed sensing system based on chirped frequency-swept modulation

In a rapidly urbanizing world with a heavy investment in mega infrastructures, ultra-long-distance sensing enables critical structural health monitoring and natural disaster early warning. Forward-transmission distributed fiber-optic sensing offers unique advantages including hundred-kilometer-scale sensing distances and mega-hertz detection bandwidth. To meet the practical application requirements of single-ended probing and electrically passive fiber termination, we propose a single-ended, forward-transmission distributed fiber-optic vibration sensing (FTDVS) system based on single-sideband chirped frequency-swept modulation using an I/Q modulator. At the termination end, two segments of delay fibers facilitate frequency-domain separation to distinguish specific forward-propagating optical paths from unwanted paths, while digital filters eliminate Rayleigh backscattering (RBS) and extract the sensing signals. This system employs standard telecom single-mode fiber for sensing and avoids the need for wavelength-division multiplexers (WDM) and multi-wavelength laser sources, thus reducing complexity and cost. The experimental results demonstrate robust performance under extreme conditions involving RBS being 72 dB stronger than forward-propagating light, achieving 85 m in positioning accuracy over a single-span sensing distance of 120 km without inline amplification. This work paves the way for long-distance distributed mapping and ground-truthing of infrastructure and the environment, which forms the backbone of smart cities.