Fast-swept OFDR interrogation of inhomogeneous WFBG arrays using a Cleaved-end Fresnel reference and adaptive spectral reconstruction

Abstract

Fast-swept optical frequency domain reflectometry (OFDR) enables high-rate interrogation of weak-reflection fiber Bragg grating (WFBG) arrays, but its performance is limited by laser sweep nonlinearity and spectral variations among individual gratings. An in-line self-calibrated fast-swept OFDR demodulation framework is presented for inhomogeneous WFBG arrays. The intrinsic Fresnel reflection from a standard cleaved fiber end facet is exploited as an in-line phase reference, enabling practical sweep nonlinearity compensation without an auxiliary interferometer or specialized end-facet fabrication. At a sweep speed of approximately 1000 nm/s, the reconstructed optical frequency shows a relative error of 0.0023% with respect to an auxiliary Mach–Zehnder interferometer. A multi-scale Hanning-window correlation method combined with adaptive spectral boundary searching is further developed to accommodate variations in reflectivity, grating length, and spectral profile. A nine-element WFBG array with reflectivities of 0.01%–0.5% and grating lengths of 2–8 mm was experimentally evaluated. The system achieved a 200 Hz optical acquisition rate, wavelength repeatability better than 1.35 pm, and mean spectral cross-correlation coefficients above 0.996. Compared with conventional demodulation approaches, the proposed method reduces wavelength extraction errors for WFBGs with distorted spectra or low reflectivity. The end-facet Fresnel reference introduces only limited SNR degradation and does not cause significant crosstalk to the WFBG channels. Static temperature sensitivities ranged from 12.60 to 13.25 pm/°C, and dynamic cooling measurements gave Pearson correlation coefficients of 0.9919–0.9941 with RMSEs of 1.08–1.20°C. These results demonstrate simplified sweep calibration and stable demodulation of spectrally inhomogeneous WFBG arrays under fast-swept conditions.