Unrepeatered remote Φ-OTDR system with single-ended amplifier using hollow-core leading fiber

Phase-sensitive Optical Time Domain Reflectometry (Φ-OTDR) is widely used for monitoring critical infrastructure. However, conventional Φ-OTDR systems are limited by fiber nonlinear effects, which restrict the probe pulse energy and thus limit the sensing distance. To overcome this bottleneck, this paper innovatively proposes a Φ-OTDR scheme that uses a hollow-core fiber as a leading fiber (LF-HCF). The design leverages the low nonlinearity and low transmission loss of HCF to transmit high-power probe pulse light to a remote fiber under test (RFUT) and to return the backscattered signal generated by the RFUT back to the detection end. In this way, it overcomes the power threshold limitation of conventional Φ-OTDR systems and achieves ultra-long-distance unrepeatered sensing. Moreover, the extremely low transmission delay of the HCF further broadens the system’s vibration response range. Experimental results demonstrate that, over a total transmission distance of 280 km (consisting of 230 km of LF‑HCF and 50 km of RFUT), the system maintains a 1‑meter spatial resolution at the RFUT. Vibration signals at the far end of the fiber are clearly detected and recovered, with a system noise floor of −42 dB rad 2 /Hz, a signal-to-noise ratio exceeding 30 dB before the optical signal enters the 50 km RFUT, and a 37 % improvement in vibration response range compared with conventional solid-core silica fiber. Benefiting from the combination of the excellent transmission properties of HCF and the high-resolution demodulation capability of LFM technology, this scheme achieves ultra-long sensing distance and extremely high spatial resolution. Notably, this scheme represents a unrepeatered remote Φ-OTDR scheme with a single-ended amplifier, offering a promising technical solution for applications such as deep-sea or deep-earth exploration, where construction conditions are harsh and both long sensing distance and high spatial resolution are strictly required.