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 +/-pi range in most of the phase demodulation based sensing techniques. In this work, we report a dual-wavelength phase-sensitive optical frequency-domain reflectometry (phi-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 similar to 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 phi-OFDR based distributed sensing with an enhanced measurement range from a practical point of view.