Phase-locked and narrow-linewidth OFC generation using cascaded optical injection of semiconductor lasers for phi-OTDR systems.

High laser coherence is essential for stable phase demodulation in phase-sensitive optical time-domain reflectometry (phi-OTDR). For future wavelength-division-multiplexed (WDM) distributed acoustic sensing (DAS) architectures, multiple stable and mutually coherent optical probe carriers are preferred. Optical frequency combs (OFCs), which generate multiple equally spaced carriers from a common optical source, provide an attractive light-source platform for this purpose. In this study, we generate phase-locked narrow-linewidth OFCs (PNOFCs) through cascaded optical injection of semiconductor lasers. Using a kHz-linewidth master laser and a 20-GHz microwave reference, we obtain 20-GHz-spaced comb lines with kHz-level linewidths. Furthermore, long-term stability measurements confirm that the PNOFC lines maintain stable linewidth and power characteristics, demonstrating mode-hopping-free capability during continuous operation. Each comb line is individually selected and used as a phi-OTDR probe for 600-Hz vibration sensing over 1-km fiber, achieving signal-to-noise ratios above 24 dB.