Leveraging Fresnel reflection of legacy PON to enhance distributed fiber vibration/temperature sensing

Legacy fiber passive optical networks (PONs) require not only high-speed data access but also regular in-service monitoring. Optical time domain reflectometry (OTDR) is a fundamental technique for PON in-service monitoring. Traditional OTDR is limited to detecting fiber breakpoints along the feeder fiber, failing to achieve PON monitoring of the point-to-multipoint (P2MP) structure. In this work, we exploit the Fresnel reflection points in access networks as fiber-optic sensors for PON branch sensing. Linear frequency modulation (LFM)-based phase sensitive OTDR (φ-OTDR) is employed for distributed sensing and power profile estimation of the PON feeder fiber, while Fresnel reflections from fiber connectors are utilized for event detection and trace recognition in optical network unit (ONU) branches. The system achieves a spatial resolution of 1 m for distributed sensing along the feeder fiber, with a sensing sensitivity of 56 pε/Hz. The longitudinal power profile is also acquired by the averaging of the long-term φ-OTDR trace and indicates a fiber attenuation coefficient of 0.207 dB/km for the standard single-mode feeder fiber at 1550.13 nm. By adjusting the fiber lengths and the positions of the reference reflectors, traces from four distinct branches are recognized, and a spatial resolution of 0.2 m is achieved. Experimental results demonstrate the precise detection of fiber break points and bending loss, as well as the ability to monitor variations in strain and temperature. The system exhibits a vibration detection sensitivity of 93 pε/Hz, with a coefficient of 15.11 rad/mK for phase changes induced by temperature variations. In addition, we discuss the implementation of in-service PON monitoring based on the proposed scheme, comparing cost, feeder fiber sensing, and branch recognition performance with the existing OTDR-based PON monitoring schemes.