Plane Wavefront Research Articles

Quantification of Site City Interaction Effects on Responses of Buildings and Basin Under Earthquake Loading

ABSTRACT The paper presents quantification of site-city-interaction (SCI) effects on buildings and free-field motion under earthquake loading. The obtained reduction in the response of buildings in wide bandwidth conflicts with the reported splitting of the bandwidth of fundamental frequency in past SCI studies, which used a simple plane wave-front. However, the achieved reduction in fundamental-frequency and free-field motion corroborates with past studies. Obtained largest SCI effect was highly dependent on building type, city, and basin-heterogeneity in contrast to general perception. It is recommended that city should be homogeneous and fundamental-frequency of buildings should be less than 1.4 times to that of basin and buildings should preferably be of steel.

Gain enhancement of mm-wave antenna using graded index lens integrated frequency selective surface for 5G NR FR2 applications

In this study, the gain of a printed antenna is boosted for utilization in 5G systems by employing a phase-oriented graded index-type lens system. In the prescribed design, various metamaterial unit cells are arranged as arrays with radial phases for the graded meta lens to optimize transmission characteristics. The suggested arrangement creates a meta lens with spatially varying surface impedance and locally varying refractive index, allowing the conversion of spherical wavefronts into planar wavefronts for beam collimation in the 5G band. The focal length-to-diameter ratio (f/d) is fixed at 0.38 in this prescribed design. The integration of the FSS structure with the patch antenna enhances the efficiency above 98% and increases gain by 2.635dBi owing to the focusing effect of the properly placed lens. The prescribed planar lens is easy to realize during manufacturing compared to 3D lenses. The meta lens integrated patch yields a gain of around 8.936dBi at 29 GHz. The performance of the lens antenna is evaluated through simulation studies and then validation of results is performed with experimental arrangement to ensure accuracy and reliability of the suggested methodology for gain enhancement of the functioning antenna at FR2 5G spectrum.

Experimental validation of an inverse method for defect reconstruction in a two-dimensional waveguide model.

Defect reconstruction is essential in non-destructive testing and structural health monitoring with guided ultrasonic waves. This paper presents an algorithm for reconstructing notches in steel plates, which can be seen as artificial defects representing cracks by comparing measured results with those from a simulation model. The model contains a parameterized notch, and its geometrical parameters are to be reconstructed. While the algorithm is formulated and presented in a general notation, a special case of guided wave propagation is used to investigate one of the simplest possible simulation models that discretizes only the cross section of the steel plate. An efficient simulation model of the plate cross section is obtained by the semi-analytical scaled boundary finite element method. The reconstruction algorithm applied is gradient-based, and algorithmic differentiation calculates the gradient. The dedicated experimental setup excites nearly plane wave fronts propagating orthogonal to the notch. A scanning laser Doppler vibrometer records the velocity field at certain points on the plate surface as input to the reconstruction algorithm. Using two plates with notches of different depths, it is demonstrated that accurate geometry reconstruction is possible.

Omnipolar conduction velocity mapping for ventricular substrate characterization: Impact of CV estimation method and EGM type on in vivo conduction velocity measurements

BackgroundAreas of abnormal or heterogeneous conduction velocity (CV) are important ablation targets for ventricular tachycardias, yet precise assessment of CV in clinical contact mapping remains challenging. Numerous different CV estimation methods have been proposed. ObjectiveThis study aimed to compare the automated local activation time (LAT)–independent omnipolar-based CV estimation method termed wave speed (WS) with 4 established LAT-based methods to formally establish the quantitative differences between them. MethodsHigh-density contact maps in patients with structurally normal hearts during sinus rhythm (SR) and ventricular ectopy (VE) were retrospectively analyzed. CV was assessed and compared by 5 methods: omnipolar WS, gradient method, planar wavefront fitting, circular wavefront fitting, and radial basis function. CV variations based on electrogram (EGM) type (unipolar, bipolar, and omnipolar), catheter movement, and surrogate markers for catheter contact were analyzed. ResultsThe study included 23 patients (47.8% male; 45.7 ± 17.3 years) with 22 SR maps (11 left ventricle, 11 right ventricle) and 16 VE maps (9 left ventricle, 7 right ventricle). The WS algorithm yielded statistically significant higher CV estimates in SR (mean, 1.41 ± 0.18 m/s) and VE (mean, 1.23 ± 0.18 m/s) maps compared with all LAT-based estimation methods, with absolute differences ranging from 0.1 m/s to 0.81 m/s. Median pointwise differences in SR and VE between WS and LAT-based methods were high, ranging from 0.55 ± 0.15 m/s (WS vs planar wavefront fitting) to 0.67 ± 0.16 m/s (WS vs radial basis function). For LAT-based methods, use of unipolar EGMs yielded significantly higher CV estimates than bipolar or omnipolar EGMs in SR. ConclusionThe CV estimation method has an important, statistically significant impact on ventricular CV measurements. Future work will focus on how these differences affect identification of pathologic conduction slowing in scar-related substrate.

Design Formulas for Flat Gradient Index Lenses With Planar or Spherical Output Wavefront

Design Formulas for Flat Gradient Index Lenses With Planar or Spherical Output Wavefront

Method of characteristics for transient, planar, cylindrical flows

Cylindrical radiation of individual pulses is studied and strong differences from spherical radiation are highlighted. Prior to this study, the class of available analytical solutions for cylindrical radiation was rather limited. It included (i) a well-known solution for a steady-state, time-harmonic line source and (ii) the solution for a delta-pulse line source. The first of these has no analytical counterpart for individual pulses. By means of the second (actually the Green's function) a more general class of line source solutions can be formulated, but each one is in the form of an integral that has to be evaluated numerically and requires tedious preparation of a singular integrand, making it difficult to achieve perfect accuracy. In the present paper, use is made instead of a particular family of analytically-exact (linear) solutions that has been developed recently. It is used to assess the accuracy of solutions obtained numerically by the method of characteristics (MoC), which is then applied to more general sources than those amenable to analytical treatment. The MoC analysis is specially formulated to reduce the influence of terms involving the reciprocal of the radius (such terms are numerically unmanageable close to the origin). It is found that the qualitative behaviour of waves radiating from the surface of a cylinder of finite radius can depend strongly on the duration of the initiating disturbance and that this can be interpreted in a convolution-like manner. After validation, the numerical method is coupled with a conventional MoC analysis of planar wave propagation and is used to simulate the reflection and radiation of an initially planar wavefront arriving at a flanged duct exit. The numerical representation is one-dimensional in both domains – (x,t) inside the duct and (r,t) outside it. Although its use implies identical behaviour in all radial directions in the external domain, the solution is found to be in close agreement with circumferential-average values from a CFD solution that simulates azimuthal variations as well as radial variations. Whilst being less comprehensive than the CFD simulation, the MoC analysis has the important practical benefit of making highly efficient use of human and computational resources. By comparing the MoC and CFD solutions, it is found that the nominally-cylindrical radiation from the duct approximates closely to radiation from a line source a small distance inside the duct (not at the exit plane). An approximate location of the effective source is quantified.

A model about regulation on three division modes of stem cell

We construct a multi-stage cell lineage model for cell division, apoptosis and movement. Cells are assumed to secrete and respond to negative feedback molecules which act as a control on the stem cell divisions (including self-renewal, asymmetrical cell division (ACD) and differentiation). The densities of cells and molecules are described by coupled reaction–diffusion partial differential equations, and the plane wavefront propagation speeds can be obtained analytically and verified numerically. It is found that with ACD the population and propagation of stem cells can be promoted but the negative regulation on self-renewal and differentiation will work slowly. Regulatory inhibition on differentiation will inversely increase stem cells but not affect the population and wave propagation of the cell lineage. While negative regulation on self-renewal and ACD will decrease the population of stem cells and slow down the propagation, and even drive stem cells to extinction. Moreover we find that inhibition on self-renewal has a strength advantage while inhibition on ACD has a range advantage to kill stem cells. Possible relations to model cancer development and therapy are also discussed.

Non-coincidence detection of fiber optic circulators based on Hertz-level frequency-shifting heterodyne interferometry

The fiber optic circulator in the form of point diffraction is a key component for coupling fiber optical path and spatial optical path in a laser Doppler vibration measurement system. The coupling efficiency and other performance parameters of fiber optic circulator are great significant for improving measurement accuracy and working distance of vibration measurement system. The conventional circulator coincidence detection methods include energy monitoring method and far-field coincidence monitoring method, which cannot be used to quantitatively analyze the fiber mismatch factors. Therefore, the consistency of the circulator coupling efficiency cannot be guaranteed. To solve these problems, a phase detection technology based on Hertz-level frequency-shifting heterodyne interferometry is proposed. The interferometry phase information is used to calculate the relative spatial positions of optical fibers, and this technology performs quantitative detection in the fiber alignment process. The interference wavefront formed by relative spatial positions of optical fibers is simulated and validated experimentally. The curves of coupling efficiency and wavefront PV value versus different kinds of alignment errors are simulated and analyzed. By fitting the interference wavefront with the Zernike polynomials, the correspondence between different kinds of alignment errors and Zernike coefficients is obtained. The value of Z2 (Zernike coefficient) can be used as the basis for judging whether there is transverse displacement in the Y direction. Similarly, Z3 corresponds to the transverse displacement in the X direction, Z4 corresponds to the longitudinal displacement in the Z direction and Z5 corresponds to the optical axis angle. Through this correspondence relationship, the quantitative separation and analysis of fiber mismatch factors are realized. The experimental results show that the accuracy of this method for measuring lateral displacement is better than 1μm. According to the phase diagram obtained from the experiment, Zernike coefficient fitting is performed. The lateral displacement deviation, longitudinal displacement deviation, and angular deviation are calculated by the coefficients of Z2 to Z5. The fiber adjustment mechanism corrects the transverse displacement deviation. It provides a new detection method for realizing fiber alignment and mismatch correction. Compared with the existing detection methods, the phase detection method based on Hertz-level frequency-shifting heterodyne interferometry solves the quantification problem of fiber coincidence adjustment. This method has the advantages of high measurement accuracy, compact detection structure and composition, and low detection cost. This method has great potential applications in the fields of optical fiber and spatial optical device alignment, optical system aberration detection, and planar wavefront detection.

Plane Wavefront Distortion and Its Effect on Laser Coherent Synthesis Efficiency (Invited)

Plane Wavefront Distortion and Its Effect on Laser Coherent Synthesis Efficiency (Invited)

Cherenkov-Type Terahertz Generation by Long-Wavelength Ultrafast Laser Excitation of a GaP Crystal

We explore, both theoretically and experimentally, the potential of semiconductor materials for the Cherenkov scheme of terahertz generation on the example of GaP crystal pumped by femtosecond laser pulses of 1.54 μm wavelength. We use a convenient scheme with a focused-to-a-line laser beam which is introduced into the crystal by oblique incidence at the Brewster angle on the crystal front face and propagates in the crystal at the Cherenkov angle to its normal. A half of the generated Cherenkov wedge impinges normally the rear face of the crystal and generates an output terahertz beam with a plane wavefront. The whole generation process is simulated by FDTD method. In experiments, the laser pulses of 140 fs duration and 10 μJ energy were converted to wideband (~2.5 THz bandwidth) terahertz radiation with the efficiency of ~3 × 10−5, which exceeds the efficiency of the standard collinear scheme by at least an order of magnitude.

Фокусировка монополярного электромагнитного импульса двумерным параболическим зеркалом

Using the methods of a computational experiment, a study was made of the process of focusing a monopolar electromagnetic pulse by a cylindrical paraboloid. It is shown that the field of a monopolar pulse with a plane wavefront, reflected and transmitted through the focus of a cylindrical paraboloid, becomes bipolar, and the time dependence of the electric field is proportional to the time derivative of the incident pulse field.

Hybrid Far- and Near-Field Modeling for Reconfigurable Intelligent Surface Assisted V2V Channels: A Sub-Array Partition Based Approach

Reconfigurable intelligent surface (RIS)-assisted communications has been a hot topic due to its promising advantages for future wireless networks. Existing works on RIS-assisted channel modeling have mainly focused on far-field propagation condition with planar wavefront assumption. In essence, the far-field condition does not always hold because the RIS array dimension may be comparable to the terminal distance, especially in RIS-assisted mobile networks. To this end, we propose a hybrid far- and near-field stochastic channel model for characterizing a RIS-assisted vehicle-to-vehicle (V2V) propagation environment, which takes into account both far-field and near-field propagation conditions. To achieve the balance between the modeling accuracy and complexity for the investigation of the RIS-assisted V2V propagation characteristics, we develop a sub-array partitioning scheme to dynamically divide the entire RIS array into several smaller sub-arrays, which makes planar wavefront assumption applicable for the sub-arrays. Important channel statistical properties, including spatial cross-correlation functions (CCFs), temporal auto-correlation functions (ACFs), and frequency correlation functions (FCFs), are derived and investigated. Simulation results are provided to show the performance of the proposed sub-array partition based hybrid far- and near-field modeling solution for RIS-assisted V2V channels.

Method for testing freeform surfaces based on a Shack-Hartmann sensor with plane wavefront scanning and stitching.

Currently, the surface error measurement technology for freeform faces a significant contradiction between measurement accuracy and dynamic range. The study proposes a non-null testing method for measuring freeform surfaces by utilizing a Shack-Hartmann wavefront sensor to emit a small aperture parallel beam and scan along the normal direction at the center of subapertures for stitching (SHPSS). A mathematical model based on ray tracing and the reflection theorem is established to calculate the sampling points on an ideal freeform surface, the reference spot array on CCD, and the corresponding relationship between microlens array and spots. An algorithm is proposed to iteratively calculate the wavefront aberration and gradually approach the actual sampling points using the established model. Theoretical analysis and numerical simulation results indicate that SHPSS can increase the dynamic range and improve the accuracy of wavefront reconstruction. The error analysis of the SHPSS method is carried out, the measurement accuracy of full aperture freeform surface is 11.45 nm. A testing system is set up and experiments are conducted on a 100 mm aperture freeform reflective mirror. The RMS of the SHPSS test results is less than λ/30 (λ=635 nm) compared to the interferometric test results. By analyzing five groups of repeated measurement experiments, the repeatability accuracy of SHPSS method is less than 1/80 λ (RMS). This demonstrates the feasibility and measurement capabilities of the method for freeform surface testing.

New substituted pentazadienes as initiators of free-radical polymerization: synthesis, photochemical properties and perspectives for holographic media

The series of 4,4′-substituted 3-hydroxyethyl-pentaza-1,4-dienes were synthesized in the reaction of aromatic diazonium salts with an ethanol amine. Photolysis of pentaza-1,4-dienes was investigated by means of UV/Vis spectroscopy. Electron donating methyl and methoxy substituents were found to increase photosensitivity whereas electron withdrawing chloro-substituents cause higher photolytic stability. 3-hydroxyethyl-pentaza-1,4-dienes as a radical polymerization initiator has been successfully investigated in 10% methyl methacrylate solution in DMF. Gel permeation chromatography (GPC) and differential scanning calorimetry (DSC) were used for obtained polymers characteristics evaluation. The highest values of PMMA Mn and Đ can be obtained in the case of chloro-substituted derivative. The holograms of a plane wavefront were recorded in media based on obtained PMMA doped by azo-dye by a 532 nm laser in a ∼1-μm-thick sample. The diffraction efficiency values for parallel polarized acting light increased in Cl→Me→OMe series.

Fast modal reconstruction of large plane wavefronts from sparse measurements using Shack-Hartmann sensors.

A concept for the fast measurement and reconstruction of optical wavefronts using Shack-Hartmann sensors (SHSs) is presented. For wavefronts with a diameter at the scale of several tens of millimeters, hundreds of measurements with an SHS may be necessary to cover the wavefront. In the proposed concept, a few SHSs are used to measure about 2% of the entire wavefront, providing sufficient measurement data for its successful reconstruction. The small number of SHSs mounted in parallel makes the concept suitable for time-critical applications. A simulation analysis is performed, and an experimental validation of the concept is presented, demonstrating that the wavefront can be reconstructed with an RMS error of about 100nm.

Parametric Sparse Channel Estimation for RIS-Assisted Terahertz Systems

To support extremely high data rates in 6G wireless networks, reconfigurable intelligent surface (RIS)-assisted terahertz (THz) communications have gained much attention in recent years. By manipulating the phase shifts of reflecting elements, the RIS can proactively adjust the wireless propagation environment of THz systems, thereby enhancing the overall throughput significantly. To realize the full potential of RIS-assisted THz systems, an acquisition of accurate channel information is of great importance. However, since the wavefront of the THz electromagnetic signal is spherical, the conventional channel estimation techniques using the planar wavefront assumption suffer from severe performance degradation in the near-field RIS-assisted THz systems. An aim of this work is to propose an efficient channel estimation technique for near-field RIS-assisted wideband THz systems. Key idea of the proposed polar-domain frequency-dependent RIS-assisted channel estimation (PF-RCE) scheme is to estimate the sparse multipath components (i.e., angles, distances, and path gains) of the near-field THz channel by exploiting the polar-domain sparsity and common support properties. We demonstrate from the numerical evaluations that PF-RCE achieves a significant performance gain over the conventional THz channel estimation schemes in terms of the normalized mean square error (NMSE).

Visibility region and correlation in ELAA-MIMO LoS scenarios under planar and spherical wavefront propagation

Channel is one of the critical aspects of designing reliable wireless communication systems by accurately modeling the signal propagation along different physical scenarios where the wireless data transmission occurs. Although several channel models have already been developed for Massive MIMO (M-MIMO) systems, the novel extra-large array aperture multiple-input-multiple-output (ELAA-MIMO) systems impose a new challenge in the development of wireless channel models due to the proximity between users, scatterers, and the base station (BS) array, by establishing the spherical wavefront hypothesis. Spherical wavefront propagation and visibility regions (VR) are known as phenomenons that physically affects the ELAA-MIMO system performance and channel modeling, being essential for practical deployments of the next-generation wireless communications (B5G). In this work, we model and discuss the critical parameters of Line-of-Sight (LoS) M-MIMO and recent ELAA-MIMO channel models, such as physical aspects and generation of these wireless channels. We aim to provide insights on the parameters setting and VR determination based on a power threshold criterion to be considered in specific LoS ELAA-MIMO channel modeling.

Spherical Wavefronts Improve MU-MIMO Spectral Efficiency When Using Electrically Large Arrays

Modern MIMO communication systems are almost exclusively designed assuming locally plane wavefronts over antenna arrays. This is known as the far-field approximation and is soundly justified at sub-6-GHz frequencies at most relevant transmission ranges. However, when higher frequencies and shorter transmission ranges are used, the wave curvature over the array is no longer negligible, and arrays operate in the so-called radiative near-field region. This letter aims to show that the classical far-field approximation may significantly underestimate the achievable spectral efficiency of multi-user MIMO communications operating in the 30-GHz bands and above, even at ranges beyond the Fraunhofer distance. For planar arrays with typical sizes, we show that computing combining schemes based on the far-field model significantly reduces channel gain and spatial multiplexing capability. When the radiative near-field model is used, interference rejection schemes, such as the optimal minimum mean-square-error combiner, appear to be very promising, when combined with electrically large arrays, to meet the requirements of next-generation networks.

On the critical initiation of planar detonation in Noble-Abel and van der Waals gas

The critical decay rate theory (C.A. Eckett, J.J. Quirk, J.E. Shepherd, Journal of Fluid Mechanics 421 (2000) 147–183) on detonation initiation in unsteady flow was extended from ideal to Noble-Abel and van der Waals gas, in which the finite molecular volume effect (or repulsion force) and the inter-molecular attraction effect (or attraction force) were included. Considering the reactive Euler equations, the reaction zone temperature-gradient equation was established independently of the equation of state and wave geometry, and then solved asymptotically by assuming large activation energy, planar wave front and a one-step irreversible reaction model. A critical decay time was derived as the critical condition for detonation initiation. It was found that the inter-molecular attraction effect makes initiation more difficult by increasing the critical decay time, whereas the finite molecular volume effect promotes initiation. The real gas effects are enhanced when increasing the reduced activation energy and/or decreasing the heat capacity ratio of perfect gas. The applicability and generality of the asymptotic solutions were evaluated by comparison with quasi-unsteady simulation employing detailed reaction models for hydrogen-air and ethylene-air mixtures, and with the further simplified, fully analytical solutions based on strong-shock assumption. The critical conditions of the asymptotic solutions are qualitatively consistent with those obtained with the detailed mechanisms. The asymptotic solutions can also be well reproduced by the simple analytical solutions in most conditions. This study thus establishes a simple, yet reliable, method to evaluate the real gas effect on detonation initiation in high-pressure conditions.

Image plane wavefront sensing and Kalman filtering for automated alignment of off-axis aspherics.

Automated alignment can significantly increase optical system precision and flexibility, and reduce time and labor costs in system setup and maintenance. We present an automated alignment technique on a double off-axis parabolic mirror system, which poses challenging alignment problems due to the mirrors' high sensitivity to aberrations, rotational asymmetry, and non-orthogonality in stage adjustments. In our methodology, we use focal plane wavefront sensing to eliminate the non-common path error, increase optical throughput, and reduce cost and system complexity. We incorporate model-based optimal estimation and control to better handle the nonlinearity, model uncertainty, and noise. Using either an iterated extended Kalman filter or a square-root unscented Kalman filter as the optimal nonlinear misalignment state estimator, we are able to consistently reduce the linear misalignment from around 1mm to <5µm and the angular misalignment from around 500 to <6a r c s e c in simulation, achieving a final wavefront error of <5∗10-5 waves in the field of view when tested at wavelength 635nm. We discover a multi-state coupling effect, which implies that different misalignment states have compensating effects on system measurements, thus interfering with the estimator's observation of misalignment state changes. We further investigate the coupling's effects on alignment quality through observability analysis.