Distance Repositioning Using Permanent Scatterers for Large-Scale Strain Measurement in OFDR

Large-scale strain, characterized by extensive deformation over long fiber segments, which induces significant distortions in distance referencing in spectrum based optical frequency-domain reflectometry (OFDR), resulting in severe positional mismatch and degraded measurement accuracy. To overcome this limitation, this letter presents a distance reposition method (DRM) based on a femtosecond-laser-inscribed permanent scatterer (PS) array embedded in the fiber as intrinsic reference markers. By realigning adjacent fiber segments to the repositioned PS locations, the DRM enables effective self-correction of strain-induced spatial mismatches during local demodulation. This process enhances both correlation and accuracy of the Rayleigh backscattering (RBS) spectra obtained from reference and measurement signals. A PS array fabricated at 12 cm intervals across a 2.29 m section exhibits approximately 25 dB signal enhancement with negligible insertion loss. Experimental validation shows that the DRM significantly improves large-scale strain sensing, achieving distributed strain measurements up to 5000 µε over a 1 m segment with a spatial resolution of 6.4 mm. The proposed technique provides a simple and practical approach for high-accuracy distributed large-scale strain sensing in OFDR systems.