Direct Laser Sweep Synchronization Enabled Dual-Wavelength φ-OFDR With an Enhanced Measurement Range

Measurement range, as an important performance metric in distributed fiber-optic sensing, is inherently limited due to the ±π range in most of the phase demodulation based sensing techniques. In this work, we report a dual-wavelength phase-sensitive optical frequency-domain reflectometry (φ-OFDR) that permits an enhanced measurement range based on direct laser sweep synchronization. By exploiting the broadband frequency sensitivity, a common unbalanced Mach-Zehnder interferometer is shared by two individual lasers to extract the frequency errors during their respective frequency sweeps regardless of their frequency interval. Based on this, common-referenced optical phase-locked loops for each of the lasers are established, allowing synchronizing the two sweeps at arbitrary frequency interval. By constructing virtually a synthetic probe of a longer wavelength, equivalently an enhanced phase demodulation range is achieved for the phase difference of the two lasers. This thus enables an enhancement in the measurement range using direct frequency-swept lasers with a significant reduction of system complexity. Demonstrations based on direct modulation fiber lasers testified the permitted enhancement in strain measurement range as large as ∼190 times. The performance such as the precision has been assessed at different intervals along the distance. This method provides an efficient, cost-effective, and compact implementation of φ-OFDR based distributed sensing with an enhanced measurement range from a practical point of view.