Large-scale strain sensor based on a distance reposition method by permanent scatters in an OFDR system

Large-scale strain (i.e., large strain in wide-range optical fiber) distorts distance referencing in spectrum-based optical frequency-domain reflectometry (OFDR), resulting in severe position segment mismatches and degraded accuracy. In this Letter, a distance reposition method (DRM) is proposed, which employs a femtosecond-laser-inscribed permanent scatter (PS) array as an in-fiber reference marker. By realigning adjacent fiber segments to repositioned PS positions, the DRM enables sufficient self-correction against strain-induced spatial mismatches for fiber sections during each local demodulation process, thereby enhancing the correlation and accuracy of Rayleigh backscattering spectra obtained by reference and measurement signals. The PSs are inscribed with an interval of 12 cm in the 2.29-m-long region, with the enhancement being approximately 25 dB and having negligible insertion loss. Experimental results demonstrate that the DRM significantly improves the performance of large-scale strain sensing, enabling distributed strain sensing up to 5000 µε over a 1-m segment with 6.4 mm spatial resolution. This technique provides a simple and practical approach to distributed large-scale strain real-time sensing for OFDR systems.