Cost-effective Φ-OTDR with laser phase noise mitigation using self-mixing interferometry

The performances and cost of the phase-sensitive optical time-domain reflectometry (Φ-OTDR) systems are heavily influenced by the lasers used. Traditionally, Φ-OTDR systems rely on highly coherent ultra-narrow linewidth lasers (NLL). This paper proposes a Φ-OTDR system that utilizes self-mixing interferometry to mitigate the impact of laser phase noise and a triple-frequency scheme to achieve fading-free detection over 40 km. The proposed scheme employs an inexpensive fixed wavelength distributed feedback semiconductor laser (DFB-SL) with a 93 kHz linewidth as the light source and successfully mitigates the noise floor by 8 to 22 dB within a range of 35 km compared to the performance of conventional systems. Leveraging the high-power output of the DFB-SL, the proposed scheme eliminates the need for an online erbium-doped optical fiber amplifier (EDFA) and achieves fading-free detection over 10 km. The results in this study offer a practical solution to address the bottleneck issue of laser phase noise in Φ-OTDR systems and contribute to the development of cost-effective systems and on-chip integration, eliminating the requirement for NLL and online amplifiers.