- 标题
- 摘要
- 关键词
- 实验方案
- 产品
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A Domain Decomposition Finite Element Method for Modeling Electromagnetic Scattering from Rough Sea Surfaces with Emphasis on Near-Forward Scattering
摘要： A high fidelity full-wave simulator is presented to perform numerical experiments for rough sea scattering problem by considering different polarizations, frequencies, grazing angles, wind speeds and sea surface spectra. The simulator is based on a novel Finite Element Domain Decomposition (FEDD) method for solving the problem of two dimensional electromagnetic scattering over one-dimensional sea surface. This non-iterative method partitions the computational domain into a number of overlapping subdomains and solves each domain individually by employing the Locally-Conformal Perfectly Matched Layer (LC-PML) at the truncation boundaries. LC-PML has a unique feature such that it can be applied to irregular domains on the contrary to standard PML methods, and hence inspired the birth of FEDD. The FEDD method is used at each Monte Carlo realization corresponding to a sample from random rough surfaces, and decreases the computational load especially for electrically-large problems. The accuracy and computational efficiency of the method is investigated through several simulations. Using the FEDD method, the statistical behavior of the bistatic Radar Cross Section (RCS) is obtained for both horizontal and vertical polarizations. A special emphasis is given to forward-scattered RCS and the mean reflection coefficient for sea surface especially at low grazing angles, and it is shown that the simulator produces results in agreement with the Ament and Miller-Brown approximations, and experimental data, proving the reliability of the simulation approach. The results are also compared with the standard finite element method and method of moments. Rough sea surfaces are created by using both Pierson-Moskowitz and Elfouhaily spectra.
关键词： Locally-Conformal Perfectly Matched Layer (PML),forward scattering,sea surface,Domain decomposition,Monte Carlo,rough surface scattering,Finite Element Method (FEM),propagation,reflection coefficient,bistatic Radar Cross Section (RCS),multipath
更新于2025-09-23 15:21:21
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Analytic Mode Normalization for the Kerr Nonlinearity Parameter: Prediction of Nonlinear Gain for Leaky Modes
摘要： Based on the resonant-state expansion with analytic mode normalization, we derive a general master equation for the nonlinear pulse propagation in waveguide geometries that is valid for bound and leaky modes. In the single-mode approximation, this equation transforms into the well-known nonlinear Schr?dinger equation with a closed expression for the Kerr nonlinearity parameter. The expression for the Kerr nonlinearity parameter can be calculated on the minimal spatial domain that spans only across the regions of spatial inhomogeneities. It agrees with previous vectorial formulations for bound modes, while for leaky modes the Kerr nonlinearity parameter turns out to be a complex number with the imaginary part providing either nonlinear loss or even gain for the overall attenuating pulses. This nonlinear gain results in more intense pulse compression and stronger spectral broadening, which is demonstrated here on the example of liquid-filled capillary-type fibers.
关键词： resonant-state expansion,Kerr nonlinearity,leaky modes,nonlinear pulse propagation,waveguide geometries
更新于2025-09-23 15:21:21
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[IEEE 2018 IEEE International Conference on Computational Electromagnetics (ICCEM) - Chengdu, China (2018.3.26-2018.3.28)] 2018 IEEE International Conference on Computational Electromagnetics (ICCEM) - A Vector Parabolic Equation Method for Propagation Predictions Over 3-D Irregular Terrains
摘要： In this paper, a vector parabolic equation (PE) method based on perfectly absorbing thin screen is applied to predicting the propagation of electromagnetic waves over three-dimensional (3-D) irregular terrains. Under the assumption of forward propagation, the 3-D PE is obtained and the split-step Fourier transform algorithm is adopted to march the potentials from one aperture plane to the next. Terrains are equivalent to a series of perfectly absorbing thin screens arranged along the direction of propagation and the Tukey window is used to attenuate the fields smoothly at the upper boundary without reflections. Finally, in order to validate the proposed method, several numerical simulations are made and the results are compared with the two-dimensional PE method. As a result, good agreements are observed and the proposed method was confirmed to take the effect of lateral terrains into account.
关键词： split-step Fourier transform,vector parabolic equation,propagation prediction,irregular terrain
更新于2025-09-23 15:21:21
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Locality estimates for Fresnel-wave-propagation and stability of x-ray phase contrast imaging with finite detectors
摘要： Coherent wave-propagation in the near-field Fresnel-regime is the underlying contrast-mechanism to (propagation-based) x-ray phase contrast imaging (XPCI), an emerging lensless technique that enables 2D- and 3D-imaging of biological soft tissues and other light-element samples down to nanometer-resolutions. Mathematically, propagation is described by the Fresnel-propagator, a convolution with an arbitrarily non-local kernel. As real-world detectors may only capture a finite field-of-view, this non-locality implies that the recorded diffraction-patterns are necessarily incomplete. This raises the question of stability of image-reconstruction from the truncated data—even if the complex-valued wave-field, and not just its modulus, could be measured. Contrary to the latter restriction of the acquisition, known as the phase-problem, the finite-detector-problem has not received much attention in literature. The present work therefore analyzes locality of Fresnel-propagation in order to establish stability of XPCI with finite detectors. Image-reconstruction is shown to be severely ill-posed in this setting—even without a phase-problem. However, quantitative estimates of the leaked wave-field reveal that Lipschitz-stability holds down to a sharp resolution limit that depends on the detector-size and varies within the field-of-view. The smallest resolvable lengthscale is found to be ≈1/–f times the detector’s aspect length, where –f is the Fresnel number associated with the latter scale. The stability results are extended to phaseless imaging in the linear contrast-transfer-function regime.
关键词： resolution,phase contrast,image reconstruction,stability,x-ray imaging,Fresnel propagation
更新于2025-09-23 15:21:21
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Performance analysis of slot waveguide using aluminum nitride in slot region
摘要： The major obstacle to accomplish an efficient photonic slot waveguide is its comparatively high propagation loss. In the current work, the silicon-on-insulator based slot waveguide with ultra-low propagation loss has been presented using aluminum nitride (AlN) in the slot region. The performance analysis of the slot waveguide has been done by varying the slot gaps from 20 to 240 nm and by varying the arm width from 160 to 340 nm. As compared with the recently reported works based on different slot waveguides, relatively smaller propagation loss of ~ 0.7 dB/cm has been predicted using the AlN based slot waveguide having a slot gap of 120 nm and arm width of 240 nm. Simulation results have demonstrated that the presence of AlN in the slot gap/low-index region, sandwiched between two high-index (silicon) regions, causes the discontinuity of the electric field of quasi-TE mode between the high-index regions. This results in high confinement of light with the smaller propagation loss in the slot region. Along with this, it has also been observed that the presented slot waveguide has the appreciably good propagation length, with low dispersion characteristics. Hence, this kind of analysis can be extended to realize the various optical devices/applications, such as for coupling, sensing, switching, etc.
关键词： Propagation loss,Aluminum nitride,Slot waveguide,Effective refractive index (ERI),Dispersion
更新于2025-09-23 15:21:01
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A perturbative approach to self-phase modulation and self-steepening of short laser pulses propagating in nonlinear media
摘要： The solution of the wave equation in the envelope approximation with temporal corrections for a laser pulse propagating in a medium where the Kerr effect, field ionization, and associated absorption take place, is obtained through a first order perturbative approach. Dispersion is, however, neglected to decouple the equations for the amplitude and the phase. The closed-form expressions so obtained clarify the influence of the various terms of the equation on the laser amplitude and on the frequency generation as a function of the retarded time. Furthermore, they allow extracting scaling parameters which size the nonlinear effects. The results are illustrated quantitatively in the case of a femtosecond pulse focused in the air with typical parameters.
关键词： perturbative approach,self-phase modulation,self-steepening,short laser pulse propagation
更新于2025-09-23 15:21:01
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A comprehensive approach for characterisation of the deposited energy density during laser-matter interaction in liquids and solids
摘要： We present a novel approach for characterisation of ultrafast laser-matter interaction processes both in solids and liquids under extreme conditions of microplasma generation. By combination of three-dimensional propagation imaging, absorption measurements, shadowgraphy and photoacoustic imaging we can restore plasma electron density distribution, laser pulse fluence profile and the value of deposited energy density inside the bulk of the material and characterise the regime of the laser pulse propagation. The developped concept is important to understand the physics of ultrafast laser-matter interaction with strong implications for precision control of laser micromachining, bioprocessing and biotreatment.
关键词： three-dimensional propagation imaging,photoacoustics,femtosecond filamentation,laser-matter interaction,deposited energy density,shadowgraphy
更新于2025-09-23 15:21:01
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[IEEE 2019 International Conference on Communication and Electronics Systems (ICCES) - Coimbatore, India (2019.7.17-2019.7.19)] 2019 International Conference on Communication and Electronics Systems (ICCES) - Synchronization of Distributed Photovoltaic Generation with an Active network using Phase Locked Loop technique
摘要： Optical tomographic imaging requires an accurate forward model as well as regularization to mitigate missing-data artifacts and to suppress noise. Nonlinear forward models can provide more accurate interpretation of the measured data than their linear counterparts, but they generally result in computationally prohibitive reconstruction algorithms. Although sparsity-driven regularizers significantly improve the quality of reconstructed image, they further increase the computational burden of imaging. In this paper, we present a novel iterative imaging method for optical tomography that combines a nonlinear forward model based on the beam propagation method (BPM) with an edge-preserving three-dimensional (3-D) total variation (TV) regularizer. The central element of our approach is a time-reversal scheme, which allows for an efficient computation of the derivative of the transmitted wave-field with respect to the distribution of the refractive index. This time-reversal scheme together with our stochastic proximal-gradient algorithm makes it possible to optimize under a nonlinear forward model in a computationally tractable way, thus enabling a high-quality imaging of the refractive index throughout the object. We demonstrate the effectiveness of our method through several experiments on simulated and experimentally measured data.
关键词： Optical phase tomography,sparse reconstruction,beam propagation method,total variation regularization,compressive sensing,stochastic proximal-gradient
更新于2025-09-23 15:21:01
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Updated Fast Solution for Quasi-Optical Beam Waveguides With Tilted FFT Beam Propagator
摘要： An updated fast solution in modeling quasi-optical beam waveguides is reported, where several mirror reflectors are utilized for the beam transformation. The fast Fourier transform Poynting vector ray tracing between virtual apertures (FFT-PVVAs) algorithm has been introduced and applied in modeling the reflected beam, based on the FFT-accelerated beam propagator between parallel apertures, and the Poynting vector ray tracing for the mirror reflection. However, for designing and optimizing a beam path with multireflectors, it is desired that the accumulated precision loss due to the Poynting vector ray tracing can be further reduced. As a modification, angular spectrum transform-based beam propagator between oblique apertures is introduced, so as to reduce the ray tracing path length. The improved method is named as FFT-PVVA-tilted propagation (FFT-PVVA-TP). As validation, beam propagation in a four-mirror beam waveguide is simulated by FFT-PVVA, FFT-PVVA-TP, and physical optics integration. Numerical results suggest that FFT-PVVA-TP provides clearly better accuracy after four times of beam reflection.
关键词： Angular spectrum (AS),Poynting vector,beam propagation,quasi-optical (QO) beam waveguides
更新于2025-09-23 15:21:01
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Atomic picture of crack propagation in Li <sub/>2</sub> Oa??2SiO <sub/>2</sub> glassa??ceramics revealed by molecular dynamics simulations
摘要： Fundamental understanding of the interaction between glass and ceramic phases with a running crack and the mechanisms by which propagation is hindered or altered as compared to the precursor parent glasses remains elusive for the lithium disilicate glass-ceramics. We herein conduct extensive molecular dynamics simulations to reveal some atomic details that are otherwise extremely challenging to be probed by experiments. The results show that the crack propagation pathway in glass-ceramics is dramatically different as compared to the precursor parent glass. In glassy sample, clean crack branching seems to be triggered by multiple cavitation events ahead of crack tip. In glass-ceramic samples, however, branching is characterized by microcracks at multiple glass-nanocrystal interface sites, clean nanocrystal cleavage along certain crystalline plane, and even secondary cracks due to the percolation of multiple microcracks. Additionally, the nanocrystal distribution, total volume fraction, and aspect ratio also have pronounced effects on the propagation of a primary crack and can lead to very diverse crack patterns.
关键词： lithium disilicate glass-ceramics,crack branching,microcracks,crack propagation
更新于2025-09-23 15:21:01