Fading-suppressed Φ-OTDR with high spatial-resolution based on a chirped electro-optic frequency comb

We propose a fading-suppressed phase-sensitive optical time-domain reflectometer (φ-OTDR) sensing system based on a chirped electro-optic frequency comb (CEOFC), which achieves high spatial resolution and simultaneously enhances the potential for long-distance sensing. A dual-parallel Mach-Zehnder modulator and a phase modulator are employed to generate multiple parallel chirped probe channels from a single carrier electrical waveform, enabling simultaneous wideband interrogation. After matched filtering, each comb line preserves the full spatial resolution determined solely by the electrical chirp bandwidth, while the combination of multi-channel Rayleigh traces effectively suppresses the fading. In a 10.84-km sensing experiment, the proposed system achieves a 50-cm spatial resolution, improving the average normalized intensity signal-to-noise ratio (SNR) by 12.92 dB, reducing the fading rate by 54%, and enhancing the phase SNR by 22.36 dB compared with the conventional chirped-pulse φ-OTDR. Two remote vibration events are accurately recovered with high fidelity. The results prove that the CEOFC-based φ-OTDR offers high spatial resolution, strong fading robustness, and a great potential for long-distance distributed acoustic sensing applications.