Waveguide Research Articles

One-way multiple beam splitter designed by quantum-like shortcut-to-adiabatic passage

Abstract In this work, we introduce a quantum-mechanical shortcut-to-adiabatic passage (STAP) into the design of multiple beam splitter. The device consists of one input and N output waveguide (WG) channels, which are connected via a mediator WG. After reducing such (N + 2)-WG structure into a controllable 3-WG counterpart by Morris-Shore transformation, we point out that the structure is available for all possible three-level STAP methods. By implementing one of them which does not require additional couplings, we can achieve the multiple beam splitting with arbitrary ratios among the N outputs. What is more, the device length is significantly shortened. Except for this, it is quite unique that the design exhibits a one-way energy transport. The underlying physics is presented. These features may have profound impacts on exploring quantum technologies for promoting advanced optical devices.

Deep guided wave convolution neural network committee-based multi-path fusion diagnosis method for fatigue corner crack

Accurate diagnosis of crack size is a critical task for guided wave (GW)-based structural health monitoring (SHM). However, fatigue cracks would have complex morphology due to complex structural geometries and loading conditions, in which multiple dimension characteristics, like crack length, depth, and angle are involved. It is challenging to quantitatively evaluate these characteristics with GW signals from a single excitation-sensing path. This paper proposes a novel deep guided wave convolution neural network (CNN) committee-based multi-path GW fusion diagnosis method, aiming at quantitative evaluation of dimension characteristics of the complex fatigue damage. GW signals from multiple excitation-sensing paths are synthesized as a high-dimension input image to enhance the effects of the fatigue crack. Besides, the deep GW-CNN committee is developed for damage quantification, in which each GW-CNN is trained with a portion of the training dataset to reduce the impact of small sample size. The proposed method is validated on fatigue tests of landing gear beam specimens under variable amplitude loading, which is designed referring to the critical region of a real aircraft and its fatigue crack presents as a corner crack. The leave-one-out validation results show the effectiveness of the proposed method, especially improvements in the diagnosis of small cracks.

Guided Waves in Composite Overwrapped Pressure Vessels and Considerations for Sensor Placement Towards Structural Health Monitoring – An Experimental Study

Abstract The utilization of composite overwrapped pressure vessels (COPVs) to store hydrogen, especially at high pressures, is gaining more popularity due to their lightweight design and high storage density, offering significant economic advantages. However, the presence of material defects or fatigue can lead to critical failures, requiring an innovative and robust approach to ensure safe operation and system integrity. Developing a continuous structural health monitoring (SHM) system for COPVs can provide comprehensive real-time information about their condition, facilitating a shift away from periodic inspections. This study scrutinizes the behavior of guided waves (GWs) within COPVs to design a sensor network for damage detection and localization. First, the dispersive and multimodal propagation behavior of GWs is experimentally investigated. Subsequently, important parameters for the network design are derived and finally a sensor network consisting of fifteen piezoelectric transducers is designed to cover the entire cylindrical area. The effectiveness is then evaluated experimentally by placing artificial defects on the surface of the COPV. The multi-layered dataset of GW signals was analyzed using both commonly used ultrasonic features (e.g., amplitude, frequency, time of flight) as well as statistical features (kurtosis, skewness, variance, etc.). These features were utilized to compute a damage index, and the effectiveness of the detection performance was assessed using receiver operating characteristic curves. It can be seen that some features are more sensitive and robust under varying experimental conditions. The results show that ultrasonic GW SHM system is a promising solution for damage detection and localization in COPVs.

A piezoelectric coded-excitation scanning acoustic transducer for Lamb wave inspections

Abstract Several techniques for Guided Wave (GW) inspections have already been developed. Most of them rely on extensive sensor deployment and damage localization algorithms characterized by significant computational costs. However, there is a growing demand for simpler, more autonomous, and more affordable systems across various fields. In particular, the implementation of wireless, battery-powered systems with a reduced number of sensor nodes and simpler processing would greatly facilitate the transition of this inspection technology on the field. Following this direction, this work presents the design of a novel piezoelectric transducer composed of four different patches, i.e. with only four input/output channels, to scan a given area. The peculiar piezo-load distributions allow the association of different spectral binary sequences for each 6∘ discrete angular step. By evaluating the distance-of-flights (DoFs) of detected peaks, the range coordinates for multiple defects are identified. Meanwhile, the angular information is extracted by demodulating binary sequences of peaks with comparable DoFs across several frequency bands. Since the transducer is designed as an encoder, it is referred to as coded-excitation scanning acoustic transducer (CESAT). More specifically, the Gray code is used to generate spectral patterns to reduce the uncertainty between two adjacent angular steps. A new quantization procedure for the optimal generation of the piezo distributions is also proposed. Ad hoc signal processing algorithms, suitable for embedded applications, were developed to extract multi-target range and angle information. The processing is based on the frequency decomposition of the recorded signal using an FIR filter bank and on dispersion compensation procedures for pulse ‘re-compression’. The transducer encoder behavior is validated through a finite element analysis. Finally, numerical simulations were performed to assess the effectiveness of the CESAT and associated signal processing in multi-defect detection and localization tasks.

Development of wireless optically stimulable upconversion nanoparticle devices for non-invasive photodynamic therapy

Abstract Photodynamic therapy (PDT) is one of the treatment methods used for brain tumors. In PDT, red laser light is irradiated onto photosensitizers accumulated in tumor tissues. However, the red light used in PDT has low tissue penetration, requiring the opening of the head for surgery, which imposes a significant burden on the patient. Therefore, in this study, we proposed wireless optically stimulable implant devices using upconversion nanoparticles (UCNPs), including the UCNP fiber and the waveguiding (WG)-UCNP mesh sheet. The proposed devices enable wireless red light emission by external NIR irradiation, resulting in non-invasive PDT. Furthermore, the UCNP fiber and WG-UCNP mesh sheet have separate NIR light receiving and red light emitting areas, which reduces the attenuation of the entering NIR light in bone, tissue, and so on. In this study, the UCNP fiber and WG-UCNP mesh sheet were successfully fabricated, and the light strength characteristics were evaluated using emission tests.

Unidirectional Alleviation of the Diffraction Limit in Geometrically Anisotropic Photonics

Rationally constructed materials have enabled access to optical capabilities beyond nature’s limitations, thanks to advances made in both theory and experiment. These synthetic composites allow subwavelength confinement of electromagnetic energy and facilitate unparalleled control over different aspects of electromagnetic waves (polarization, amplitude, frequency, etc.). However, the diffraction phenomenon is severely hindering the efficacy and performance of dielectric photonic components. Diffraction causes the electromagnetic wave to spread and deviate from its intended path, thereby, making the collimated light beam scatter, leading to lower power density and inaccurate targeting. This is particularly detrimental for applications requiring precise control of high-frequency with shorter wavelengths. Herein, we report on the effect of anisotropic geometrical scaling of dielectric photonic crystals to alleviate the diffraction barrier along the Γ → X path of the irreducible Brillouin region. Thus, achieving the long-sought goal of high-frequency electromagnetic wave steering. We harness the full weight of modal and harmonic analysis based on the Finite Element Method to demonstrate that scaling the direction perpendicular to the wave’s propagation reduced by fourfold the diffraction limit from 100 THz to 400 THz.

Towards predictive maintenance of hydrogen pressure vessels based on multi-sensor data

In this paper, we report on a sensor network for structural health monitoring (SHM) of Type IV composite overwrapped pressure vessels (COPVs) designed for hydrogen storage. The sensor network consists of three different SHM sensing technologies: ultrasonic guided waves (GW), acoustic emission (AE) testing, and distributed fiber optic sensors (DFOS). We present an experimental setup for a lifetime test, where a COPV is subjected to cyclic loading. Data from all sensors are collected and centrally evaluated. The COPV failed after approximately 60,000 load cycles, and the sensor network proved capable of detecting and localizing the damage even before the failure of the COPV. This multi-sensor approach offers significantly more channels of information and could therefore enable a transition from costly and time-consuming periodic inspections to more efficient and modern predictive maintenance strategies, including artificial intelligence (AI)-based evaluation. This not only has a positive effect on operational costs but enhances safety through the early identification of critical conditions in the overall system in real-time. In the future, we aim to integrate the measurement setup into a hydrogen refueling station with the data stream implemented into a digital signal processing chain and a digital twin.

Design and implementation of a digital smart layer for ultrasonic-guided wave-based structural health monitoring

Ultrasonic-guided wave (UGW) is an important technology for realizing structural health monitoring (SHM), and the piezoelectric transducer (PZT) is a commonly used actuator and sensor. The PZT layer is fabricated using a flexible printed circuit (FPC), and the interface of the PZTs is connected to the host of the monitoring system via cables. The flexible PZT layer (FPL) is easy to install, highly reliable, and widely used in industrial sites. However, as the distance between the host and the FPL gradually increases, the problem of guided wave (GW) signal attenuation caused by cables cannot be ignored. In addition, the use of shielded cables has the problem of heavy quality and high cost, and the use of unshielded cables has the problem of signal interference. Therefore, this paper proposes a digital smart layer (DSL) composed of a flexible signal acquisition system and a PZT array. The DSL acquires excitation and GW signals acquisition and performs PZT diagnostics. It connects to the host via a fixed six-wire cable and uses digital signals for communication. Multiple DSLs can be interconnected to form a network and perform damage detection simultaneously, improving the monitoring area and damage detection efficiency of the system. An experimental system was constructed on an aluminum plate to verify the functionality of the DSL. The results prove that the DSL solves the problems of signal attenuation and interference susceptibility of GW signal in long-distance cable transmission and realizes the diagnosis of PZT faults such as debonding and breakage. This paper provides a highly reliable and practical means of monitoring the condition of equipment structures such as airplanes, trains, and bridges.

Chirality-induced phonon spin selectivity by elastic spin–orbit interaction

Spin and orbital degrees of freedom are crucial in not only fundamental particles but also classical waves such as optical systems, wherein the spin-orbit interaction (SOI) of light provides new perspectives for manipulating electromagnetic waves. Elastic waves possess similar spin angular momentum (SAM) and orbital angular momentum (OAM). However, the elastic counterpart of SOI remains unexplored, even for ubiquitous elastic waveguides (WG). Here, we demonstrate the existence of elastic SOI in helical WG. We prove that the torsion and curvature of helical WG induces synthetic gauge potentials in describing the elastic vibrations. Through analytical theory and simulations, we unveil the interplay among elastic SAM, intrinsic OAM, and extrinsic OAM, impacted by the elastic SOI. Importantly, results show that elastic SOI can introduce the Chirality-Induced Phonon Spin Selectivity. These findings advance our understanding of angular momentum physics in elastic waves and enable practical strategies for wave manipulation.

Sub-picosecond mode-locked laser operation in a femtosecond-laser-inscribed Yb:CaF2 channel waveguide

We present the successful demonstration of both Q-switched mode-locked (QSML) and continuous-wave mode-locked (CWML) operation in a femtosecond-laser-inscribed Yb:CaF2 double-track waveguide (WG) structure. A semiconductor saturable absorber output coupler (SOC) was used as the mode locker with fine tuning of the intracavity group delay dispersion (GDD) achieved through the Gires-Tournois interference (GTI) effect induced by an air gap. To ensure sufficient fluence on the saturable absorber, the cavity was extended to 50 cm by inserting two lenses between the SOC and WG, resulting in a repetition frequency of ∼300 MHz. In the QSML regime, the laser exhibited an amplitude modulation period of 65 kHz within Q-switched pulses of 3-µs duration. Notably, in the purely CWML regime, the laser generated a maximum output power of 51 mW near 1036 nm with a pulse width of 979 fs.

Investigating the suitability of the matched fiber Bragg grating approach for guided wave based structural health monitoring

Fiber Bragg grating (FBG) sensors are thought to be ideal sensors for structural health monitoring (SHM). Amplitude based techniques such as matched filters and the edge-filtering have been proposed to fulfill the high sampling rates necessary for guided waves (GW) sensing. The current research for the first time shows the inherent robustness the matched filter technique provides to the sensing system. The matched system is realized through a FBG deployed on a low cost micrometer screw gauge which allows flexibility in matching the reflected wavelengths of the FBGs. Based on the results it is shown that, in the remote configuration the matched FBG approach is robust under changing temperature conditions. This concept has been shown analytically and experimentally. Furthermore, the approach is then extended for multiplexing of the sensors and the deployed system is used for detecting and localizing damage in the plate.

Tailored Growth of Transition Metal Dichalcogenides' Monolayers by Chemical Vapor Deposition.

Here, results on the tailored growth of monolayers (MLs) of transition metal dichalcogenides (TMDs) are presented using chemical vapor deposition (CVD) techniques. To enable reproducible growth, the flow of chalcogen precursors is controlled by Knudsen cells providing an advantage in comparison to the commonly used open crucible techniques. It is demonstrated that TMD MLs can be grown by CVD on large scale with structural, and therefore electronic, photonic and optoelectronic properties similar to TMD MLs are obtained by exfoliating bulk crystals. It is shown that besides the growth of the "standard" TMD MLs also the growth of MLs that are not available by the exfoliation is possible including examples like lateral TMD1-TMD2 ML heterostructures and Janus TMDs. Moreover, the CVD technique enables the growth of TMD MLs on various 3D substrates on large scale and with high quality. The intrinsic properties of the grown MLs are analyzed by complementary microscopy and spectroscopy techniques down to the nanoscale with a particular focus on the influence of structural defects. Their functional properties are studied in devices including field-effect transistors, photodetectors, wave guides and excitonic diodes. Finally, an outlook of the developed methodology in both applied and fundamental research is given.

Probabilistic diagnostic imaging method based on the difference between the arrival moments of ultrasonic guided wave signals

Ultrasonic guided wave (UGW), using the probabilistic diagnostic imaging (PDI) method is widely used in structural health monitoring. The PDI method, which incorporates signal time-of-flight (TOF), has significant advantages in high-precision localization and multi-damage recognition. However, the TOF acquisition process introduces errors. This paper proposes a modified PDI method based on the difference between the arrival moments of guided wave (GW) signals. This method eliminates the need to acquire excitation to obtain TOF, reducing the requirements for GW detector, and minimizing the impact of TOF errors on damage localization. An experimental system for composite laminates was constructed, and the experimental results showed that the method had an average localization error of 4.73 mm for single damages. The average localization error for both multiple damages and damage at edge locations was less than 20.0 mm. The localization error is satisfactory with reference to the size of the monitored area and damages.

A multi-scale deep residual network-based guided wave imaging evaluation method for fatigue crack quantification

Abstract As a promising structural health monitoring technology, guided wave (GW) imaging is gaining increasing attention for crack monitoring of aircraft structures. However, actual fatigue crack propagation is a complex dynamically evolving process affected by various variabilities. It is still challenging to accurately track and quantify the dynamic fatigue crack propagation with GW imaging methods. Therefore, in order to achieve more accurate fatigue crack quantification, this paper proposes a multi-scale deep residual network-based GW imaging evaluation method. A convolutional neural network (CNN) is utilized to evaluate the entire pixel distribution of GW imaging maps to fuse damage-related information from multiple GW monitoring paths. By designing multi-scale convolutional kernels and deep residual learning, a robust quantitative image feature extraction is ensured with the dynamic evolution process of fatigue crack growth and the performance degradation is avoided as the CNN goes deeper, thereby improving the quantification accuracy. The method is validated on a fatigue test of landing gear beams, which are important load-carrying aircraft structural components. The results demonstrate that the proposed method can extract multi-scale crack length-related features and accurately track fatigue crack propagations. For batch specimens, the maximum quantification error is reduced from the original 6.1 mm to 1.6 mm, marking a significant improvement.

Attenuation of Lamb waves in coupled-resonator viscoelastic waveguide

Guidance of elastic waves is one of the main applications of artificial crystal structures. The attenuation of the guided waves is, however, often overlooked, as most of the proposed waveguides only comprise ideal elastic materials. In this work, we study the propagation of evanescent Lamb waves guided in coupled-resonator viscoelastic waveguide (CRVW), with special attention to attenuation. CRVW is defined by considering a linear chain of coupled defect cavities in a phononic plate made of epoxy. The viscoelastic behavior of epoxy is characterized numerically by the Kelvin–Voigt (K–V) model. Based on finite element analysis, the complex band structure and the spectrum of frequency response function (FRF) are obtained. Due to viscosity, guided Lamb waves are spatially damped. Two theoretical models are devised to predict the displacement distributions inside and outside a bandgap for guided waves, respectively, considering either the first or the first two least evanescent Bloch waves identified in the complex band structure. A CRVW sample is fabricated and characterized experimentally by laser vibrometry. Evanescent Lamb waves are observed to be strongly confined along the waveguide and at the same time to decay rapidly along the waveguide axis. Experiments and numerical simulations are found to be in fair agreement. The present work is expected to inspire practical applications of highly confined viscoelastic phononic waveguides.

Smart structural health monitoring (SHM) system for on-board localization of defects in pipes using torsional ultrasonic guided waves

Most reported research for monitoring health of pipelines using ultrasonic guided waves (GW) typically utilize bulky piezoelectric transducer rings and laboratory-grade ultrasonic non-destructive testing (NDT) equipment. Consequently, the translation of these approaches from laboratory settings to field-deployable systems for real-time structural health monitoring (SHM) becomes challenging. In this work, we present an innovative algorithm for damage identification and localization in pipes, implemented on a compact FPGA-based smart GW-SHM system. The custom-designed board, featuring a Xilinx Artix-7 FPGA and front-end electronics, is capable of actuating the PZT thickness shear mode transducers, data acquisition and recording from PZT sensors and generating a damage index (DI) map for localizing the damage on the structure. The algorithm is a variation of the common source method adapted for cylindrical geometry. The utility of the algorithm is demonstrated for detection and localization of defects such as notch and mass loading on a steel pipe, through extensive finite element (FE) method simulations. Experimental results obtained using a C-clamp for applying mass loading on the pipe show good agreement with the FE simulations. The localization error values for experimental data analysed using C code on a processor implemented on the FPGA are consistent with algorithm results generated on a computer running Python code. The system presented in this study is suitable for a wide range of GW-SHM applications, especially in cost-sensitive scenarios that benefit from on-node signal processing over cloud-based solutions.

Si-waveguide-based optical power monitoring of a 2 × 2 Mach-Zehnder interferometer based on a InGaAsP/Si hybrid MOS optical phase shifter.

Transparent in-line optical power monitoring in Si programmable photonic integrated circuits (PICs) is indispensable for calibrating integrated optical devices such as optical switches and resonators. A Si waveguide (WG) photodetector (PD) based on defect-mediated photodetection is a promising candidate for a transparent in-line optical power monitor, owing to its simplicity and ease of integration with a fully complementary metal-oxide-semiconductor (CMOS)-compatible process. Here, we propose a simple optical power monitoring scheme for a 2 × 2 Mach-Zehnder interferometer (MZI) optical switch based on InGaAsP/Si hybrid MOS optical phase shifters. In the proposed scheme, a low-doped p-type Si WG PD with a response time of microseconds is utilized as a transparent in-line optical power monitor, and the ground terminal of the MOS optical phase shifter is shared with that of the Si WG PD to enable the simple monitoring of the output optical power of the MZI. Based on this scheme, we experimentally demonstrate that the output optical power of a 2 × 2 MZI can be simply monitored by applying a bias voltage to the Si slabs formed at the output WGs of the MZI without excess optical insertion loss.

Simulating plane wave imaging with the fast nearfield method

Conventional B-mode imaging suffers from limited frame rates due to compounded travel times. In contrast, plane wave imaging has emerged as a high-speed solution, offering superior frame rates while preserving resolution and contrast. Simulation plays a pivotal role in testing and refining new imaging algorithms, which is crucial for further development of plane wave imaging. However, limited support exists in current ultrasound simulation packages for plane wave imaging simulations. To address this deficiency, plane wave imaging support is incorporated into FOCUS, the “Fast Object-oriented C++ Simulator” (https://www.egr.msu.edu/∼fultras-web/). This is achieved by adopting a pulse-echo imaging model that combines received signals from individual scatterers while assuming linear propagation of acoustic pressure fields within an isotropic, homogeneous, and non-dissipative medium. The fast nearfield method is deployed for the acoustic pressure field calculations, providing superior performance in terms of speed, accuracy, and memory-efficiency compared to other methods. The pulse-echo imaging model is extended to standard plane wave imaging techniques, such as electronic steering of plane waves and beamforming of the received RF data. Simulation results with a scatterer population that mimics a soft-tissue phantom featuring 5-point targets, 5 hyperechoic regions, and 5 anechoic regions of varying sizes will be demonstrated.

Optimal Design of a Sensor Network for Guided Wave-Based Structural Health Monitoring Using Acoustically Coupled Optical Fibers.

Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use has been limited for the GW sensing due to its limited sensitivity. FBG sensors in the edge-filtering configuration have overcome this issue with sensitivity and there is a renewed interest in their use. Unfortunately, the FBG sensors and the equipment needed for interrogation is quite expensive, and hence their number is restricted. In the previous work by the authors, the number and location of the actuators was optimized for developing a SHM system with a single sensor and multiple actuators. But through the use of the phenomenon of acoustic coupling, multiple locations on the structure may be interrogated with a single FBG sensor. As a result, a sensor network with multiple sensing locations and a few actuators is feasible and cost effective. This paper develops a two-step methodology for the optimization of an actuator-sensor network harnessing the acoustic coupling ability of FBG sensors. In the first stage, the actuator-sensor network is optimized based on the application demands (coverage with at least three actuator-sensor pairs) and the cost of the instrumentation. In the second stage, an acoustic coupler network is designed to ensure high-fidelity measurements with minimal interference from other bond locations (overlap of measurements) as well as interference from features in the acoustically coupled circuit (fiber end, coupler, etc.). The non-sorting genetic algorithm (NSGA-II) is implemented for finding the optimal solution for both problems. The analytical implementation of the cost function is validated experimentally. The results show that the optimization does indeed have the potential to improve the quality of SHM while reducing the instrumentation costs significantly.

Waveguide-Enhanced Nanoplasmonic Biosensor for Ultrasensitive and Rapid DNA Detection.

DNA is fundamental for storing and transmitting genetic information. Analyzing DNA or RNA base sequences enables the identification of genetic disorders, monitoring gene expression, and detecting pathogens. Traditional detection techniques like polymerase chain reaction (PCR) and next-generation sequencing (NGS) have limitations, including complexity, high cost, and the need for advanced computational skills. Therefore, there is a significant demand for enzyme-free and amplification-free strategies for rapid, low-cost, and sensitive DNA detection. DNA biosensors, especially those utilizing plasmonic nanomaterials, offer a promising solution. This study introduces a novel DNA-functionalized waveguide-enhanced nanoplasmonic optofluidic biosensor using a nanogold-linked sorbent assay for enzyme-free and amplification-free DNA detection. Integrating plasmonic gold nanoparticles (AuNPs) with a glass planar waveguide (WG) and a microfluidic channel, fabricated through cost-effective, vacuum-free methods, the biosensor achieves specific detection of complementary target DNA sequences. Utilizing a sandwich architecture, AuNPs labeled with detection DNA probes enhance sensitivity by altering evanescent wave distribution and inducing plasmon resonance modes. The biosensor demonstrated exceptional performance in DNA detection, achieving a limit of detection (LOD) of 33.1 fg/mL (4.36 fM) with a rapid response time of approximately 8 min. This ultrasensitive, rapid, and cost-effective biosensor exhibits minimal background nonspecific adsorption, making it highly suitable for clinical applications and early disease diagnosis. The innovative design and fabrication processes offer significant advantages for mass production, presenting a viable tool for precise disease diagnostics and improved clinical outcomes.