Cost-effective solution for phase-OTDR distributed acoustic/vibration sensing

Self-injection locking - an efficient method to improve the spectral performance of semiconductor lasers without active stabilization - has already demonstrated its high potential for operation with single-longitude-mode fiber lasers. Recently, we demonstrated that self-injection locking of a conventional DFB laser through an external fiber optic ring cavity causes a drastic decrease of the laser linewidth and makes possible its direct application in a phase-sensitive optical time domain reflectometry (φ-OTDR) acoustic sensor system. Detection and localization of dynamic perturbations in the optical fiber were successfully demonstrated at the distance of 9270 m. However, the ability of the system to restore the perturbating frequency spectrum was not quantified. Here, we have evaluated the performance of a φ-OTDR system for acoustic/vibration measurements utilizing a conventional telecom DFB laser self-stabilized through an external PM optical fiber ring resonator. The use of PM fiber components prevents the polarization mode-hopping that is proved to be a major source of the laser instability, resulting in single frequency laser operation with 6 kHz linewidth. The laser diode current and the laser fiber configuration temperature both have been stabilized with accuracies better than 0.3%. All laser components have been placed into a special insulating box to protect the laser from external perturbations. Under these conditions, the typical duration of laser operation in self-maintaining stabilization regime is ~30 minutes. The laser long-term frequency drift is estimated to be less than ~30 MHz/min. This low-cost solution is directly compared with the use of a commercial, ultra-narrow linewidth (~ 100 Hz) fiber laser implemented into the same setup. Both systems are tested for measurement of the frequency of vibration applied to a fiber at a distance of 3500 m. The obtained SNR value higher than 6 dB demonstrates the ability of the DFB laser to be used in distributed measurements of vibrations with frequencies up to 5600 Hz with a spatial resolution of 10 meters.