We report a noble coherent optical frequency domain reflectometry (OFDR) system that simply enables measurement range enhancement up to full coherence length of a laser source. The proposed technique is based on bidirectional determination of Rayleigh backscattered signal along a fiber. To do this, complex OFDR signals are acquired with the help of $\pi / 2$ phase-shifting interferometry in a detection part and an additional delay fiber in a reference arm. Through a bidirectional determination process using the complex OFDR signals, mirrored signals appearing at both sides of the spatial domain were completely discriminated, so that ambiguity arising due to a positional origin can be removed. Bidirectional distributed strain and temperature sensing is successfully performed for the first time. Spectral shifts by applying strain to and heating of the test fiber were found to be independent on both sides of the reference position, indicating that twofold enhancement of the measurement range can be obtained compared to previous systems.

This paper presents a compact dynamic optical frequency domain reflectometry (D-OFDR) platform enabling millimeter-scale, distributed strain sensing for real-time structural health monitoring (SHM) of reinforced concrete subjected to seismic loading. The proposed D-OFDR interrogator employs a dual-interferometer architecture: a main interferometer for strain sensing and an auxiliary interferometer for nonlinear frequency sweep compensation. Both signals are detected by photodetectors and digitized via a dual-channel FPGA-based DAQ board, enabling high-speed embedded signal processing. A dual-edge triggering scheme exploits both the up-chirp and down-chirp of a 50 Hz bidirectional sweep to achieve a 100 Hz interrogation rate without increasing the sweep speed. Laboratory validation tests on stainless steel cantilever beams showed sub-hertz frequency fidelity (an error of 0.09 Hz) relative to conventional strain gauges. Shake-table tests on a 2 m RC column under incremental seismic excitations (scaled 10–130%, peak acceleration 0.864 g) revealed distinct damage regimes. Distributed strain data and frequency-domain analysis revealed a clear frequency reduction from approximately 3.82 Hz to 1.48 Hz, signifying progressive stiffness degradation and structural yielding prior to visible cracking. These findings demonstrate that the bidirectional sweep-triggered D-OFDR method offers enhanced real-time monitoring capabilities, substantially outperforming traditional point sensors in the early and precise detection of seismic-induced structural damage.

Distributed acoustic sensing (DAS), which can provide dense spatial and temporal measurements using optical fibers, is quickly becoming critical for large-scale infrastructure monitoring. However, anomaly detection in DAS data is still challenging owing to the spatial correlations between sensing channels and nonlinear temporal dynamics. Recent approaches often disregard the explicit sensor layout and instead handle DAS data as two-dimensional images or flattened sequences, eliminating the spatial topology. This work proposes GraphDiffusion, a novel generative anomaly-detection model that combines a conditional denoising diffusion probabilistic model (DDPM) and a graph neural network (GNN) to overcome these limitations. By treating each channel as a graph node and building edges based on Euclidean proximity, the GNN explicitly models the spatial arrangement of DAS sensors, allowing the network to capture local interchannel dependencies. The conditional DDPM uses iterative denoising to model the temporal dynamics of standard signals, enabling the system to detect deviations without the need for anomalies. The performance evaluations based on real-world DAS datasets reveal that GraphDiffusion achieves 98.2% and 98.0% based on the area under the curve (AUC) of the F1-score at K different levels (F1_K-AUC), an AUC of receiver operating characteristic (ROC) at K different levels (ROC_K-AUC), outperforming other comparative models.

We investigated the effect of externally introduced 980 nm photobleaching on radiation-induced attenuation (RIA) in pure silica core (PSC) single-mode fibers and demonstrated that it enhanced the radiation-hard characteristics of PSC fiber based Raman-distributed temperature sensors (Raman-DTS) operating at 1550 nm. Gamma-ray irradiation experiments confirmed that 980 nm photobleaching effectively reduces the RIA of PSC fibers at 1550 nm. RIA spectra were measured in the wavelength range of 1400–1650 nm during gamma-ray irradiation to evaluate the influence of 980 nm photo-bleaching on the wavelength dependence of RIA. We also analyzed the spectral changes in RIA of the PSC fibers with different photobleaching power and showed that the wavelength dependence of RIA can be tuned by adjusting the bleaching power. Finally, we demonstrated that applying the external photo-bleaching strategy effectively mitigates radiation-induced temperature measurement distortion in Raman-DTS by reducing the RIA imbalance between the Raman Stokes and anti-Stokes scattering bands.

We present a dual-frequency Terahertz (THz) heterodyne interferometry system for nondestructive 3D profiling of large step heights without phase unwrapping. The system employs photonic generation of two THz frequencies around a center frequency of 306 GHz with tunable frequency spacing from 8 to 11.5 GHz, corresponding to synthetic wavelengths of 37.5 to 26.1 mm. Through phase-sensitive heterodyne detection, minimum detectable height differences of 6.7 μm at 10 GHz and 19.7 μm at 8 GHz were achieved. This allows for direct measurement of millimeter-scale discontinuities within a single 2π phase cycle. Furthermore, experimental validation in 3D-printed polylactic acid (PLA) samples demonstrates accurate measurements up to 27 mm with the measured refractive index of 1.638. Taking into account a frequency modulation of 8 GHz and the refractive index of PLA, step heights up to 29.3 mm are measurable within a single 2π phase cycle. The system also enables non-destructive profiling, passing through a 200-μm paper envelope that is an optically opaque material, and images embossed structures. This technique facilitates non-contact metrology for composite material inspection, automotive components, and multilayer coatings where optical methods fail due to material opacity.