Interval-locked dual-frequency φ-OFDR with an enhanced strain dynamic range and a long-term stability.

We report on an interval-locked dual-frequency phase-sensitive optical frequency-domain reflectometry relying on a common-reference optical phase-locked loop. With a shared unbalanced interferometry, this design allows for synchronizing the frequency drift of two lasers, leading to a steadily stabilized dual frequency with an arbitrary interval. Equivalently to a longer synthetic wavelength, their phase difference is utilized to demodulate the ambient changes of interest with an enhanced dynamic range and long-term stability.