Frequency-modulated continuous wave-enabled single-ended forward sensing over all-passive fiber links for integrated sensing and communications

Distributed acoustic sensing (DAS) is being embedded in the extensively deployed fiber-optic infrastructure to extend its role beyond data transmission, forming a paradigm known as integrated sensing and communication (ISAC). Among the various DAS approaches, forward sensing is particularly appealing owing to its inherent compatibility with data transmission. Conventional implementations, however, typically require two coherent transceivers or active in-line components, which significantly increase system complexity and cost. Here, we propose and experimentally demonstrate a frequency-modulated continuous-wave (FMCW)–based single-ended forward-sensing architecture that operates over a fully passive fiber link using only one coherent transceiver. By employing an FMCW probe, the proposed approach eliminates the need for active frequency-shifting elements along the sensing link, while the single-ended interrogation design obviates the need for a secondary transceiver and time synchronization. To validate our solution, we experimentally achieve a 10-m-level localization accuracy for 10 and 20 kHz vibration events with unified performance across the entire 15 km fiber link. Furthermore, the system exhibits a dynamic range exceeding 45 dB, supporting unamplified sensing distances beyond 100 km. We also verify its coexistence with data transmission by copropagating a 20 GBaud dual-polarization 16-quadrature amplitude modulation signal, observing a negligible signal-to-noise ratio penalty of less than 0.2 dB. These results establish a practical and scalable route for integrating acoustic sensing into communication networks, paving the way for cost-effective and widespread ISAC deployment.