Hybrid Pulse-Compression $\varphi$-OTDR and Forward Vibration Sensing Using Residual Carrier Modulation

Traditional hybrid reflectometry-interferometer vibration sensing systems primarily target high-frequency vibration detection. In this work, we propose a vibration sensing scheme based on residual carrier modulation, which integrates pulse-compression$\varphi$-OTDR with a forward single-fiber interferometer. This configuration enables simultaneous long-distance interrogation and high-frequency vibration detection. By utilizing a shared transceiver and single-fiber architecture, both high-quality Rayleigh backscattering (RBS) and interferometric signals are acquired. RBS-based positive-negative frequency demodulation provides fully distributed vibration sensing in the near section of the fiber under test (FUT), while interferometric demodulation enables effective detection in the far section, where the RBS signal becomes too weak. Furthermore, the joint analysis of RBS and interferometric signals enhances the detection of high-frequency vibration. Experimental results demonstrate the system's capability for robust vibration demodulation over a 202 km FUT with a frequency response of up to 10 kHz. Compared to traditional hybrids that are typically limited to the kilometers scale, the proposed scheme enables long-reach while maintaining a shared-transceiver, single-fiber architecture, thereby substantially reducing system complexity.