Search Results (406)

Interferometric Synthetic Aperture Radar (InSAR) enables the measurement of ground deformation with high spatial coverage and frequent temporal sampling, making it a valuable tool for landslide monitoring in remote mountainous regions. However, in tropical mountainous environments, dense vegetation, steep topography, and rapid surface changes often reduce interferometric coherence, limiting the reliability of C-band observations. This study evaluates the applicability of C-band and L-band spaceborne InSAR data for detecting slow-moving landslides in the northern tropical Andes. The analysis focuses on two case studies in the northern Colombian Andes, where slow-moving landslides have affected two municipalities. Time-series deformation and velocity analysis were derived using a Small Baseline Subset (SBAS) approach applied to Sentinel-1 (C-band) and SAOCOM-1 (L-band) SAR datasets. The detected deformation patterns were compared with independent field evidence reported in previous geotechnical investigations of the study area. Results show that both sensors successfully identified the spatial extent of the slow-moving landslide. However, L-band data exhibited higher coherence and reduced noise levels in densely vegetated zones. These differences highlight the improved performance of L-band observations under tropical vegetation conditions, while C-band data remain effective for detecting moderate surface displacements where coherence is preserved. The findings provide guidance for selecting appropriate SAR wavelengths for landslide detection in tropical mountainous regions and support the integration of multi-frequency InSAR for operational hazard assessment. Full article

When a material is subjected to external forces, it undergoes internal mechanical deformations. To measure this type of deformation, mechanical sensors such as load cells, strain gauges, etc., are typically used. These sensors convert mechanical energy into electrical energy, which can then be displayed on electronic displays. Noninvasive methods can be used to measure deformations in materials. These methods use light as a measurement medium. These include non-destructive optical testing (Ronchi test, Foucault test, etc.) and interferometric testing. Among the most commonly used interferometers for strain measurement are shearography configurations, as they are suitable for measuring in-plane or out-of-plane deformations. In the present work, the deformations of four rectangular pieces made of materials used in the automotive and aerospace industries will be analyzed: 6061 aluminum, as well as 304, 1045, and 4041 steels. The analysis will be performed using optical interferometry, using a shearing interferometer, with shear in the direction of the applied compression force, in a Y-shape. In addition, the results will be compared with a Wheatstone bridge arrangement made with four strain gauges, as well as with finite element analysis. Compressive forces of 491 N to 2453 N were applied to each sample, yielding different fringe patterns in the obtained interferograms. When processed to obtain their respective phases, the proposed materials exhibited different elasticity. The interferometric results were compared with those obtained with strain gauges, which were placed on the opposite side of the test objects. Furthermore, the interferometric results reliably reflect the results obtained with the finite element analysis, showing the deformation of the material when the compressive force is applied vertically. Full article

Conventional dual-beam interferometers typically provide a single interferometric phase observable, while simultaneous measurement of displacement and angular motion may require an additional optical path or auxiliary sensor. In this work, we develop a shared-optics dual-port interferometer for simultaneous displacement and angular-motion measurement without a substantial increase in optical complexity. The proposed configuration integrates two Michelson-like interferometers in a nearly common-path shared-optics architecture, using polarization-based routing to share key optical components while providing two independent phase readouts. Under a $120\mathrm{nm}$ axial displacement at $70\mathrm{Hz}$, the two phase readouts exhibited closely matched responses and broadband phase levels at the ${10}^{-4}\mathrm{rad}/\sqrt{\mathrm{Hz}}$ level, while their difference reduced the shared displacement component to below 0.3% of the individual-channel response. In the simultaneous pitch and yaw excitation at 100 and $70\mathrm{Hz}$, the pitch- and yaw-driven responses were separated in the common and differential phase signals, while weak sum- and difference-frequency components associated with nonlinear and cross-axis actuator coupling were resolved at the ${10}^{-3}\mathrm{rad}/\sqrt{\mathrm{Hz}}$ level. These results demonstrate that the shared-optics architecture can extend a compact dual-beam interferometer from differential phase sensing to simultaneous displacement and angular-motion measurement, while retaining sufficient phase resolution to identify weak cross-axis actuator responses. Full article

Continuous measurement of small deformation in cementitious materials requires both sensitive displacement readout and control of environmental effects. This study develops a long-gauge strain sensor based on a low-finesse extrinsic Fabry–Pérot interferometer (EFPI). Axial displacement of a 250 mm mortar prism is transferred to an external reflector, and the air cavity length is recovered from swept-wavelength reflection spectra. Loading–unloading measurements cover a nominal strain range of 0–10,000 µε. A separate incremental test resolves a nominal 2 nm displacement step, equivalent to 8 nε, with a local strain-response slope of 0.960 and R² = 0.99550. For 100 consecutive readings at a chamber setting of 20 °C, the sample standard deviation is 1.162 nε; division by the local response slope gives an estimated input-referred noise-equivalent strain of 1.21 nε (1σ). These short-term metrics do not establish long-term accuracy. During 14-day monitoring, individual mortar prisms with water-to-cement ratios of 0.4, 0.5, and 0.6 reach apparent compressive strains of 452.0, 532.5, and 599.7 µε, respectively. The results demonstrate the feasibility of continuous long-gauge optical monitoring. Full article

Wildfire monitoring using synthetic aperture radar (SAR) provides critical capabilities under challenging atmospheric conditions where optical sensors are limited by smoke and cloud cover. We evaluated Sentinel-1 C-band SAR interferometric coherence for Burned-area detection of the 2017 “Las Máquinas” megafire (Maule, Chile), comparing Ascending (Asc) and Descending (Dsc) orbital geometries processed with the AMSTer InSAR software. Multi-temporal RGB Coherent Change Detection composites were constructed using two interferometric pairs per orbit: the Normalised Differential Activity Index (NDAI, R channel), pre-fire coherence (G channel), and co-event coherence (B channel), clearly delineating the fire scar through red and orange signatures reflecting fire-induced vegetation loss and soil exposure. Seven machine-learning classifiers (Random Forest (RF), Support Vector Machine (SVM), Decision Tree (DT), Logistic Regression (LR), K-Nearest Neighbours (KNN), Gradient Boosting Classifier (GBC), and XGBoost) were trained on the three-band coherence feature space. For the Ascending orbit, XGBoost achieved the highest performance (OA = 0.9328; F1 = 0.9195) and mapped 143,950 ha (76.2%) as Burned. For the Descending orbit, XGBoost also performed best (OA = 0.9221; F1 = 0.9055) and mapped 144,475 ha (76.5%) as Burned. In this case study, the Ascending geometry performed marginally better than the Descending one; however, the leading classifiers were statistically indistinguishable, indicating that the Burned and Unburned classes are close to linearly separable in the coherence feature space. These results confirm the effectiveness of coherence-based SAR analysis for large-scale wildfire mapping under adverse atmospheric conditions. Full article

Human presence detection is critical when building smart cities with use cases in sectors like smart homes, emergency evacuation, health-care monitoring and others. Existing human detection systems predominantly rely on camera-based imaging, raising privacy concerns. Moreover, conventional FMCW radar approaches are primarily motion-based, thus often failing to detect the presence of unconscious individuals, as in the case of search and rescue (SAR) operations. Some radar approaches use Doppler or spectral peak analysis to estimate respiration but fail to exploit phase coherence to resolve sub-millimeter chest displacement and higher-order physiological harmonics. This paper presents an interferometric radar framework that models multi-feature vital-sign signatures for human detection under controlled clinical settings using respiratory harmonic relationships, inter-harmonic consistency, chest-displacement spectral characteristics, and radar-derived cardiac mechanical signatures. Physiological relationships are used to establish the expected structure of the extracted features, while subject-to-subject variability and measurement uncertainty are used to determine practical acceptance regions from the training cohort. Experimental data from 30 healthy subjects were analyzed using a single interferometric radar sensor under controlled clinical conditions. The resulting signatures were subsequently evaluated using a machine-learning validation pipeline. With 243 test cases, the proposed framework achieved 89.71% accuracy, 95.26% precision, 94.15% F1-score, and 93.06% sensitivity. The study demonstrates that interferometric chest-displacement sensing can provide a privacy-preserving physiological feature space for human presence detection, while also identifying the limitations associated with unresolved multi-person signal superposition and hardware-induced phase uncertainty. Moreover, interferometric sensing by principle will work better than conventional radar approaches for SAR operations. Although validated in a controlled clinical environment, the framework establishes a foundational pathway towards future research for eventual deployment in next-generation smart systems. Full article

Geohazards are present at every stage of a mine’s life cycle (initial exploration, site investigations, active operations, closure, reclamation, legacy management). This study has reviewed the literature covering mining and geohazards gathered from the Scopus bibliographic database. Quantitative metadata analysis was conducted on keyword co-occurrence within the 150 selected publications, and thematic clustering was mapped. In addition, five case studies were analysed to add further in-depth analysis. The publication volume shows a sharp uptrend starting around 2015, and 45% of the articles indicate a corresponding author from China. Publications more often cover geohazards in underground mines than open-pit/surface mining. AI methods are a rapidly evolving subject. Overall, the review reveals an imbalance in research attention across different stages of the mine life cycle. While only approximately 6% of the literature addresses exploration-stage geohazards, the case studies demonstrate that early identification is critical. In conclusion, technical capabilities for geohazard monitoring have advanced dramatically, especially with interferometric synthetic aperture radar (InSAR) deformation analysis, machine learning classification, and multi-sensor data fusion; however, the field suffers from systematic integration of geohazards across the mine life cycle. Full article

Interferometric fiber-optic hydrophones are widely deployed in passive underwater acoustic detection systems due to their high sensitivity, broad dynamic range, and immunity to electromagnetic interference. However, their adoption in active acoustic systems has been largely constrained by the limited operational bandwidth of conventional architectures. Through coupled acoustic–structural simulations, this work identifies the operational bandwidth bottleneck as arising from low-order mechanical resonance of high-modulus mandrel structures combined with acoustic cavity resonance and near-field scattering within the enclosed cylindrical geometry. To address these limitations, an optimized push–pull mandrel structure, featuring a miniaturized profile of φ16 mm × 16 mm and a low-acoustic-impedance material, is implemented to eliminate mechanical resonance within the operational bandwidth. Additionally, an integrated acoustic metamaterial liner is incorporated into the inner tube to mitigate scattering and reverberation effects. Experimental characterization of the prototype demonstrates an average phase sensitivity of −132 dB re 1 rad/μPa, with fluctuations below ±1.5 dB across 20 Hz to 31.5 kHz. Utilizing a custom-built demodulation system, the hydrophone achieves a minimum detectable pressure of 42 dB re 1 µPa/√Hz (126 µPa/√Hz) at 20 Hz and 27 dB re 1 µPa/√Hz (22 µPa/√Hz) above 2 kHz, remaining over 28 dB below the deep-sea state zero ambient noise floor at 20 Hz, and hence confirms its passive acoustic monitoring capability. In the high-frequency regime above 2 kHz, the hydrophone maintains ±1.5 dB sensitivity flatness with a horizontal directivity variation of only ±1.51 dB at 30 kHz, providing a robust platform for broadband active acoustic applications, including active sonar, underwater acoustic imaging and acoustic communications. These results establish a viable pathway toward next-generation dual-mode underwater acoustic systems requiring both high-fidelity passive listening and broadband active detection. Full article

This study focuses on the Significant Wave Height (SWH) retrieval from the Ka-band radar interferometer (KaRIn) on the Surface Water and Ocean Topography (SWOT) satellite by combining backscattering and interference characteristics. To this end, the backscattering-related and interference-related parameters were jointly used as inputs to develop a machine learning model. Here, the backscattering-related data include normalized radar cross-section (NRCS), incidence angle, and the image spectra parameters extracted from KaRIn Level 1B (L1B) data, while the interference-related data correspond to the Level 2 (L2) volumetric correlation, which characterizes the influence of ocean wave scattering on interferometric coherence. The machine learning model is built upon a Multi-Layer Perceptron (MLP), which serves as a nonlinear fitting tool. SWH retrievals from the proposed method and the existing L2 SWH product as a reference were validated by the collocated European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis data, Haiyang2C (HY2C) and Haiyang2D (HY2D) altimeter data, and National Data Buoy Center (NDBC) buoy data. Validations show that both KaRIn SWH have a good agreement with collocations in terms of correlation coefficient (COR), BIAS and root mean square error (RMSE). Moreover, the retrieval accuracy from the proposed method (with an RMSE of about 0.29 m) is better than that of the L2 product (with an RMSE of about 0.46 m) when validated against the collocated ECMWF datasets. Ablation analysis further confirms that image spectra parameters and volumetric correlation are the dominant factors driving the retrieval accuracy improvement, with notable contribution differences among the sub-parameters of spectral features. This performance gain arises from the complementary physical mechanisms of backscattering and interferometric observables, which describe sea state information from independent dimensions. These accurate SWH retrievals can help correct sea state biases for collocated KaRIn sea surface height products and complement wave products from other satellite sensors. Full article

Microwave dielectric characterization is essential for material evaluation, process monitoring, biomedical sensing, and nondestructive testing. This review critically evaluates planar microwave sensors for dielectric characterization, with the core scope restricted to printed microstrip and coplanar-waveguide structures, SRR/CSRR and DGS configurations, substrate-integrated waveguides, interferometric sensors, and microfluidic platforms. Adjacent non-planar or system-level techniques are included only when they provide transferable lessons in calibration, inversion, or deployment. Unlike earlier surveys that primarily catalog devices or extraction methods, the literature is organized here through a design-decision hierarchy linking architecture, operating principle, readout mechanism, sample interface, and application. Representative approaches are compared not only by frequency, sensitivity, Q-factor, and sample volume, but also by calibration burden, fabrication tolerance, environmental robustness, cost, and scalability. Particular attention is given to uncertainty sources in practical measurement chains, FR-4 and PCB manufacturing variability, long-term drift and sensor aging, and the application-specific limitations of machine-learning-assisted inversion. The resulting synthesis provides design-oriented guidance for selecting and translating planar microwave sensors into reliable industrial, biomedical, and microwave-processing measurement systems. Full article

A cascaded dual-parameter fiber-optic sensor is presented, in which a single-mode–no-core–single-mode (SNS) multimode-interference (MMI) interferometer is integrated with a fiber Bragg grating (FBG) to achieve simultaneous refractive index (RI) and temperature sensing. The SNS segment, fabricated by fusion splicing a no-core fiber (NCF) between two single-mode fibers, exploits MMI to generate spectral features that are highly responsive to ambient RI changes. Meanwhile, the FBG serves as an independent temperature reference, owing to its negligible RI sensitivity. Based on the beam propagation method, the MMI characteristics in the NCF are analyzed. By combining the simulation results with the experimental spectra, the NCF length is optimized by comprehensively considering the interference-fringe visibility, free spectral range, and spectral separation from the FBG wavelength. Experimental results show that the maximum RI sensitivity of the SNS interferometric structure reaches 136.29 nm/RIU, with a corresponding temperature sensitivity of 9.14 pm/°C. The FBG exhibits a temperature sensitivity of 9.83 pm/°C while remaining virtually unresponsive to surrounding RI variations. By establishing a dual-parameter sensitivity matrix, RI and temperature variations can be simultaneously demodulated, enabling effective temperature compensation for RI sensing. The sensor requires no tapering, etching, or surface modification and can be fabricated using only conventional fiber-cleaving and fusion-splicing processes. With its simple fabrication, compact structure, low cost, and good mechanical stability, the sensor shows promising potential for temperature-compensated RI sensing, biochemical detection, liquid-concentration monitoring, and environmental sensing. Full article

Plasmonic biosensing technologies have emerged as powerful analytical tools for sensitive and label-free characterisation of biomolecular interactions and complex samples. The increasing demand for comprehensive molecular profiling has accelerated the development of multiplexing strategies that enable simultaneous analysis of multiple analytes and molecular interactions. This Feature Paper examines multiplexing through the complementary spatial, spectral, and temporal dimensions of multiplexing, together with their hybrid combinations and associated analytical trade-offs. Compared with other optical biosensing approaches, including interferometric, photonic, and fluorescence-based sensing platforms, plasmonic biosensors remain attractive owing to their combination of label-free detection, real-time interaction monitoring, sensitive interfacial analysis, and compatibility with multiplexed assay formats. This Feature Paper critically discusses current multiplexing strategies, focusing primarily on surface plasmon resonance (SPR), imaging SPR (SPRi), localised SPR (LSPR), surface-enhanced Raman scattering (SERS), and related nanoplasmonic biosensing approaches, together with recent advances in surface biofunctionalisation, antifouling interfaces, and molecular recognition strategies. Representative applications in biomedical diagnostics and non-clinical settings are highlighted, with examples such as liquid biopsy, glycoprofiling, extracellular vesicle profiling, and food and environmental analysis, alongside key challenges in reproducibility, standardisation, data interpretation, and clinical translation. In addition, selected non-plasmonic optical biosensing technologies are briefly discussed to position plasmonic biosensing within the broader landscape of multiplexed optical biosensing. This Feature Paper argues that the future of multiplexed plasmonic biosensing will depend less on further improvements in sensor performance than on robust, standardised analytical systems. Full article

In the field of long-distance partial discharge (PD) detection for submarine cables, there is an urgent need for a remote demodulation distributed detection technology deployable at multiple critical locations to overcome the limitations of single-point measurement. Factory joints of high-voltage submarine cables are high-risk components for PD, and long-distance fiber optic acoustic sensing technology holds the greatest potential for online monitoring. However, due to the viscoelasticity of the joint’s polymer insulation structure, sound propagation distance is severely limited, and non-multi-point measurement cannot achieve effective coverage of the measurement area. This paper proposes a quasi-distributed remote demodulation sensing system, in which both the fiber optic interferometer and the distributed feedback fiber laser (DFB-FL) serve as sensors, enabling highly sensitive quasi-distributed PD detection. The DFB-FL itself is highly sensitive to strain, and the multiple fiber coils formed by the interferometer arms are also highly sensitive to strain. Both can be modulated by the micro-strain induced by the acoustic field generated from PD in the polymer solid, producing phase shifts of the optical waves, which are then intrinsically demodulated by the interferometer system to extract the vibration signals caused by the discharge. Theoretical analysis shows that the sensitivity increases with the length of the unbalanced arm, with the upper limit constrained by laser coherence and optical attenuation; for the PD frequency band, the optimal unbalanced length is below 200 m—this design rule is applicable to on-chip interferometric sensors. The interferometer coils employ bend-insensitive fibers to suppress optical loss and improve fringe visibility. Meanwhile, three fiber coil configuration schemes are constructed to enhance the detection sensitivity to acoustic signals. Simulation results generate frequency response contour maps based on Young’s modulus, indicating that the solid-wound coil achieves the highest amplitude and the broadest bandwidth, with an optimal response frequency of approximately 50 kHz. Experimental results demonstrate that among the three structure types, the solid-wound coil also achieves the largest response ratio. Finally, in tests performed on a 220 kV submarine cable intermediate joint (with the system installed inside the metallic sheath), the minimum detectable discharge level in the DFB-FL region reached 6.75 pC, while that for the fiber coil reached 12.66 pC; when installed outside the metallic sheath, the minimum detectable discharge levels were 53.2 pC for the grating region and 89.6 pC for the fiber coil. Full article

We present a rank-based method for detecting short-duration gravitational-wave transients in 46 days of coincident data from the first Advanced LIGO observing run (O1). The method applies a moving-window implementation of Chatterjee’s rank correlation coefficient to whitened interferometric sensor strain data. This produces a computationally efficient statistic sensitive to temporally ordered signal structure without relying on waveform templates. Compared with traditional excess power and coherent burst searches, the rank-based formulation is potentially less sensitive to certain non-Gaussian noise transients. Furthermore, it processes dual-detector data faster than real time on a single CPU core. We evaluate the method using 60 hardware injections from the O1 dataset, recovering 28 compact binary coalescence injections, primarily for events with a single-detector signal-to-noise ratio above approximately 13. The pipeline identifies 31 transient candidates, including the astrophysical event GW150914 and two instrumental glitches. Although the present implementation is less sensitive than established search pipelines, these results demonstrate the feasibility of the new method. Rank-based detection statistics provide a computationally efficient and complementary method for low-latency transient detection in interferometric sensor networks. Full article

Vibration measurements are essential for the early detection of faults in rotating machinery and are particularly important for hydrogenerators in hydro power plants. Industrial applications of vibration measurements typically rely on displacement, velocity, and acceleration sensors, each offering distinct advantages and limitations. This paper discusses and proposes a cost-effective, contactless optical vibration sensing system based on standard fiber-optic telecommunication components, enabling its integration into existing fiber-optic networks. The proposed system utilizes interferometric sensing principles, providing inherent immunity to electromagnetic interference and galvanic effects while achieving micrometer-scale resolution. The key advancement of the proposed sensor lies in the relatively simple and cost-effective configuration—the realization of the Michelson interferometer. It facilitates a combination of standard optical communication hardware, including a 3 × 3 fiber-optic coupler and two photodetectors for reliable discrimination of vibration displacement directions. The associated signal processing platform is based on a cross-correlation algorithm by which both the direction and the magnitude of displacement are determined. The optical sensor was experimentally validated using a realistic-scenario laboratory setup, which demonstrates the feasibility and performance of the proposed approach. Full article