Large-Scale Strain Measurement for OFDR via Permanent-Scatterer-Based Distance Repositioning

Large-scale strain (i.e., extensive deformation over long gauge lengths in optical fibers) introduces significant distortions in distance referencing for spectrum-based optical frequency-domain reflectometry (OFDR), leading to severe positional segment mismatches and degraded measurement accuracy. To address this issue, this letter proposes a distance reposition method (DRM) that utilizes a femtosecond-laser-inscribed permanent scatterer (PS) array as intrinsic reference markers within the fiber. By realigning adjacent fiber segments to the repositioned PS locations, the DRM achieves effective self-correction of strain-induced spatial mismatches during each local demodulation process. This enhances both the correlation and accuracy of the Rayleigh backscattering (RBS) spectra derived from reference and measurement signals. The PS array was fabricated at 12 cm intervals over a 2.29 m region, exhibiting an approximate 25 dB signal enhancement with negligible insertion loss. Experimental results confirm that the DRM substantially improves large-scale strain sensing performance, enabling distributed strain measurements of up to 5000 µε over a 1 m segment with a spatial resolution of 6.4 mm. The proposed technique offers a simple and practical solution for high-accuracy distributed large-scale strain sensing in OFDR systems.