Noncontact pulsed laser-scanning laser Doppler vibrometer (PL-SLDV) phased array imaging for damage detection in composites.

Abstract Threaded pipe structures are critical components in equipment used in various fields. The threaded sections are prone to induce defects due to stress concentration, threatening the safe operation of the equipment. Consequently, the inspection of these structures is essential. While the ultrasonic guided wave (UGW) method has been applied to inspect threaded pipes, the influence of thread parameters on UGW propagation characteristics is yet to be examined. This study analyzed the dispersion characteristics of UGWs. It reveals that the group velocity dispersion curve of the L(0,2) mode initially increases and then decreases over frequency range of 60–140 kHz. The group velocity is higher in trapezoidal threads than in rectangular threads. Dispersion curves for different thread heights exhibit a crossover near 85 kHz. Dispersion curves for different pitches intersect around 83 kHz. The effect of thread parameter variations on the reflection characteristics was investigated. It was found that the trapezoidal thread exhibits a higher reflection coefficient than the rectangular thread. The reflection coefficient increases with thread height and decreases with pitch. The influence of thread parameters on defect detection sensitivity was examined. Results demonstrate that the L(0,2) mode offers high sensitivity for defect detection in threaded pipes featuring trapezoidal threads, a thread height of 1 mm, and a pitch of 4.5 mm. Finally, the effectiveness of the L(0,2) mode in detecting defects of varying depths within threaded pipes was validated. This research provides a novel method for the inspection of threaded pipe structures.

During the active experiment FENICS-2024 on the Kola Peninsula using a decommissioned power transmission line as a horizontal radiating antenna, ultra-low-frequency signals of the 1–6 Hz range were recorded at magnetic stations located from ~1600 km to ~2100 km from the transmission line with normalized amplitudes from ~0.3 fT/A to ~0.8 fT/A. Observational results are compared with approximate analytical estimates of the magnetic field excited by the magnetic dipole. The calculations turned out to be in qualitative agreement with the observational results. To assess the possible response in the upper ionosphere, a numerical model of the ULF field in the atmosphere and ionosphere generated by the horizontal surface current was employed. The model is based on solving the system of Maxwell equations in the vertically inhomogeneous atmosphere and ionosphere. The fundamental feature of this model is that it correctly takes into account the contribution of ionospheric waveguide propagation. The observational results supported by numerical simulation have shown the potential of active experiments of the new type for signal generation for large-area magnetotelluric sounding and for modification of near-Earth plasma with artificial signals.

During the active experiment FENICS-2024 on the Kola Peninsula using a decommissioned power transmission line as a horizontal radiating antenna, ultra-low-frequency signals of the 1–6 Hz range were recorded at magnetic stations located from ~1600 km to ~2100 km from the transmission line with normalized amplitudes from ~0.3 fT/A to ~0.8 fT/A. Observational results are compared with approximate analytical estimates of the magnetic field excited by the magnetic dipole. The calculations turned out to be in qualitative agreement with the observational results. To assess the possible response in the upper ionosphere, a numerical model of the ULF field in the atmosphere and ionosphere generated by the horizontal surface current was employed. The model is based on solving the system of Maxwell equations in the vertically inhomogeneous atmosphere and ionosphere. The fundamental feature of this model is that it correctly takes into account the contribution of ionospheric waveguide propagation. The observational results supported by numerical simulation have shown the potential of active experiments of the new type for signal generation for large-area magnetotelluric sounding and for modification of near-Earth plasma with artificial signals.

Experimental evidence of a low-frequency longitudinal core wire guided wave mode in a seven-wire strand under tensile loading.

A space-time discontinuous Petrov-Galerkin finite element formulation for a modified Schrödinger equation for laser pulse propagation in waveguides

In this study, we investigate the coupled nonlinear integrable system known as the Akbota–Gudekli–Kairat–Zhaidary (AGKZ) equation. By employing a generalized extended simple equation method, we examine soliton and various solitary wave solutions with diverse physical structures. The nonlinear complex AGKZ equation is a newly introduced integrable model arising in the study of space curves and surfaces; therefore, its analytical exploration is essential for understanding its physical applications. The investigated solutions display distinct physical structures, including bright solitons, kink wave structures, dark solitons, peakon-type bright and dark waves, anti-kink wave structures, periodic waves with varying profiles, solitary waves through contour plots, two-dimensional plots, and three-dimensional visualizations using Mathematica tool. The novelty of this work lies in establishing enriched and distinct soliton solutions to the AGKZ equation and performing a comparative analysis of the proposed method, which has not been previously addressed in the literature. The derived solutions of the AGKZ equation may be applied to model ultrashort pulse propagation in nonlinear optical fibers, photonic crystals, waveguides, and solitary waves in shallow water. The results demonstrate that the proposed approach is practical, straightforward, and effective for generating a wide variety of soliton solutions applicable to other nonlinear equations.

Conventional directional dipoles (CDDs), which consist of electric and/or magnetic dipoles that satisfy specific amplitude and phase conditions, can give rise to directional coupling and radiation in both the near and far fields. Theoretically, a toroidal dipole can replace an electric dipole of the CDDs and form pseudo-directional dipoles (PDDs), providing more degrees of freedom for directional manipulation of light. However, the realization of PDDs in practical structures has yet to be reported due to the difficulty of achieving desirable toroidal dipoles. Here, we employ coupled helices to achieve PDDs and demonstrate flexible manipulation of light propagation in a silicon waveguide. By tuning the incidence and helix geometry, we can control the relative amplitude and phase between the toroidal dipole, electric dipole, and magnetic dipole. This enables the realization of pseudo-circular dipole, pseudo-Huygens dipole, and pseudo-Janus dipole, providing three types of directional sources with distinct near-field properties. The near-field directionality of these sources can be tuned by changing the separation distance between the helices and the waveguide, leading to tunable asymmetric excitation of guided light. The proposed realization may facilitate the design of directional sources and switches in photonic integrated circuits, with broad applications in on-chip information processing and optical communication.

Aluminum nitride (AlN) is a wide-bandgap semiconductor (6.2 eV) with a broad transparency window spanning from the ultraviolet (UV) to the mid-infrared (MIR) wavelength region, making it a promising material for integrated photonics. In this work, AlN thin films using reactive RF sputtering are deposited, followed by annealing at 600 °C in a nitrogen atmosphere to reduce slab waveguide propagation losses. After annealing, the measured loss is 0.84 dB/cm at 978 nm, determined using the prism coupling method. A complete microfabrication process flow is then developed for the realization of optical channel waveguides. A key challenge in the processing of AlN is its susceptibility to oxidation when exposed to water or oxygen plasma, which significantly impacts device performance. The process is validated through the fabrication of microring resonators (MRRs), used to characterize the propagation losses of the AlN channel waveguides. The fabricated MRRs exhibit a quality factor of 12,000, corresponding to a propagation loss of 4.4 dB/cm at 1510–1515 nm. The dominant loss mechanisms are identified, and strategies for further process optimization are proposed.

High-harmonic upconversion driven by a mid-infrared femtosecond laser can generate coherent soft x-ray beams in a tabletop-scale setup. Here, we report on a compact ytterbium-pumped optical parametric chirped pulse amplifier (OPCPA) laser system seeded by an all-fiber front-end and employing periodically poled lithium niobate (PPLN) nonlinear media operated near the pulse fluence limits of current commercially available PPLN crystals. The OPCPA delivers 3 µm wavelength pulses with 775 µJ energy at 1 kHz repetition rate, with transform-limited 120 fs pulse duration, diffraction-limited beam quality, and ultrahigh 0.33% rms energy stability over >18 h. Using this laser, we generate soft x-ray high harmonics (HHG) in argon gas by focusing into a low-loss, high-pressure gas-filled anti-resonant hollow core fiber (ARHCF), generating coherent light at photon energies up to the argon L-edge (250 eV) and carbon K-edge (284 eV), with high beam quality and ∼1% rms energy stability. This work demonstrates soft x-ray HHG in a high-efficiency guided-wave phase matched geometry, overcoming the high losses inherent to mid-IR propagation in unstructured waveguides, or the short interaction lengths of gas cells or jets. The ARHCF can operate in the long term without damage and with the repetition rate, stability, and robustness required for demanding applications in spectromicroscopy and imaging. Finally, we discuss routes for further optimizing the soft x-ray HHG flux by driving He at higher laser intensities using either the signal (1.5 μm) or idler wavelengths (3 μm).

Analysis of guided wave propagation in highly damped viscoelastic multilayered composite structures using the biot model.

Abstract The propagation characteristics of Lamb waves in structural waveguides provide valuable insights into the health of a structure. As these waves travel through the medium, they are reflected or scattered by structural boundaries, discontinuities, and defects. This study introduces a fast and efficient modeling approach for Lamb wave propagation in large, built-up, plate-like structures, aimed at supporting damage localization in realistic scenarios. The proposed methodology combines a semi-analytical Modal Pencil approach with the Wave Finite Element and \textcolor{blue}{Hybrid Finite Element/Wave Finite Element methods}. This framework enables accurate simulation of transducer-excited ultrasonic fields, transient guided wave propagation, and wave interactions such as scattering and reflection at defects, interfaces, and boundaries. Importantly, the computational cost of this method is independent of the propagation path length, making it particularly suitable for long-range inspection tasks. Case studies include isotropic plates joined by both flat and bent connectors. The model’s accuracy is validated through comparison with time-domain 3D finite element simulations. Furthermore, the practical capability of the proposed approach is demonstrated via experimental damage localization in a real bent aluminum plate. The results highlight the potential of this digital twin framework to enhance the damage detection capabilities of ultrasound-based structural health monitoring systems, while requiring only a minimal number of actuator–sensor pairs.

Pipeline Unidirectional Guided Wave Generation Based on a Hybrid Piezoelectric Transducer Ring and Wave-Packet Modification

Abstract Eigenmode expansion (EME) is a widely used method for modeling the electromagnetic wave propagation in multimode waveguides, where it breaks down signals into local eigenmodes and calculates them independently. Nevertheless, this methodology may challenge the causality mandated by the theory of special relativity, thus potentially disrupting the cause-and-effect relationship. This study experimentally explored light transmission in the multimode coreless fiber and found discrepancies between the EME method and measurement. To reconcile these inconsistencies, we introduced a light cone model, providing an alternative interpretation guided by the principles of special relativity. Remarkably, this innovative model did not merely resolve the observed discrepancies between the theory and experiments, but also presented a pioneering technique for designing microbend sensors. Through experimentation, we achieved the remarkable sensitivity of 500 dB/m −1 at a bending curvature of 0 m −1 . Our research advances the understanding of multimode systems and paves the way for innovative sensing and communications applications in compact devices.

In order to understand nonlinear wave propagation in applications such as exhaust pipes, jet engines, pipe flows, and blood flow in veins one can model them as nonlinear waveguides. There is a reasonable amount of numerical or analytical work in the literature on nonlinear rigid waveguides and much less on nonlinear flexible waveguides. However, most of these studies ignore the mean flow of the fluid. These waveguides under harmonic excitation, generate multiple harmonic waves and modes at frequencies dictated by the dispersion relation. Due to weak nonlinearity, these modes interact, leading to the formation of higher harmonics that either grow spatially or oscillate in amplitude. The current study examines nonlinear acoustic wave propagation in an infinite rigid cylindrical waveguide containing an acoustic fluid with uniform mean flow. The system is modelled using nonlinear acoustic equations, separated into first and second order components using the regular perturbation method. At first order, the dispersion equation determines the primary waves, revealing that an increase in Mach number reduces the primary wavenumber's magnitude. A closed form solution for pressure at both linear and nonlinear orders is derived, and wave interactions are explored. Resonance conditions and Mach number effects on the wave behaviour are investigated.

This study explores the use of elastic waveguide propagation and signal analysis for monitoring structural evolution in heterogeneous media, with bone analogues employed as a case example. Synthetic models representing low, intermediate, and high stiffness states were examined using piezoelectric sensors to capture transmitted waveforms. Four parameters velocity, attenuation, dispersion index, and spectral entropy were extracted according to defined procedures. Results showed consistent trends: velocity increased, attenuation decreased, dispersion diminished, and entropy reduced as stiffness increased, confirming the sensitivity of wave-derived features to structural transitions. A Random Forest classifier was applied to these features, demonstrating highly accurate discrimination among the three states under controlled conditions. The integration of elastic wave descriptors with supervised learning highlights the potential of vibration-based diagnostics for tracking stiffness evolution in heterogeneous composites. While bone consolidation provides a compelling case study, the framework is generalisable to other composite systems, thereby reinforcing the contribution of elastic wave analysis to the broader field of sound and vibration.

We experimentally investigate frequency-selective spin wave (SW) transmission in a micrometer-scale, ring-shaped magnonic resonator integrated with a linear yttrium iron garnet stripe. Using super-Nyquist-sampling magneto-optical Kerr effect (SNS-MOKE) microscopyand micro-focused Brillouin light scattering (μ-BLS), we probe SW dynamics in the dipolar regime under in-plane magnetization. Spatially resolved measurements reveal a sharp transmission peak at 3.92 GHz for an external field of 74 mT, demonstrating strong frequency selectivity. This behavior arises from interference and scattering among multiple SW modes in the ring, shaped by anisotropic dispersion, geometric confinement, and local magnetic field inhomogeneities. Caustic-like propagation further limits outcoupling due to fixed group velocity directions. Fourier analysis reveals discrete wavevector components consistent with quantized ring eigenmodes. μ-BLS measurements at 70 mT show a shift of the transmission peak, demonstrating tunability via the external magnetic field.

This study presents a novel statistical structural damage diagnosis framework using ultrasonic guided wave signals. The approach employs functional series time-dependent autoregressive (FS-TAR) models to capture the non-stationary dynamics of guided wave propagation. Unlike traditional methods that analyze only initial wave packets, this framework utilizes complete guided wave signals, including reflected waves, providing a comprehensive assessment of structural state. The FS-TAR model represents time-varying parameters through deterministic evolution using orthogonal basis functions. Three basis function families, namely, wavelet, Chebyshev, and trigonometric, have been evaluated to determine optimal signal representation. The covariance structure of the estimated time-invariant coefficients of projection vector and time-varying model parameters is extensively investigated, and their role in damage diagnosis is assessed. Two complementary damage diagnosis approaches are developed: one based on time-invariant projection coefficients and another using time-dependent model parameters. Both approaches employ statistical hypothesis testing with established confidence bounds derived from the asymptotic properties of the parameter estimators. Experimental validation is conducted on an aluminum plate under various damage scenarios, including both damage-intersecting and non-intersecting wave propagation paths. Results demonstrate accurate and robust damage detection and classification across all tested states. The wavelet basis functions show superior performance, providing the clearest parameter separation between healthy and damaged states. Key advantages include (i) utilization of complete wave signals rather than isolated wave packets, (ii) response-only operation without requiring input measurements, (iii) established statistical framework with quantified uncertainties, and (iv) real-time applicability with minimal computational requirements.

Interpreting material anisotropy through the fractional wave equation.

Minimizing background optical emission originating from waveguides is critical for achieving high-sensitivity performance in integrated photonic platforms, particularly for applications in biosensing, chemical detection, environmental monitoring, and quantum sensors. These background signals, often stemming from intrinsic fluorescence (also known as autofluorescence or photoluminescence) or Raman scattering in the waveguide core or cladding, can obscure weak optical signals and limit the accuracy of integrated photonic sensors. In this work, we introduce a novel parameter-extraction technique that quantitatively separates waveguide losses and background-generation efficiency, and identifies the individual contributions of the waveguide core and cladding to the total background signal. Our method utilizes only standard photonic waveguides and requires no specialized sample preparation or auxiliary test wafers. We validate our technique experimentally by characterizing silicon-nitride waveguides across nine wafers fabricated in a 300-mm-wafer platform developed at AIM Photonics. Applying our parameter-extraction method, we demonstrate its utility in showing a clear correlation between waveguide propagation loss and fluorescence, and its sensitivity in resolving material-specific differences in Raman spectra between wafers fabricated using different processes, including those arising from hydrogen-related impurities in non-annealed cores. This work provides a practical and comprehensive tool for quantifying, diagnosing, and reducing material-specific propagation loss and background signals in integrated photonic waveguides, ultimately enabling more effective development of low-noise and low-loss photonics platforms.