Modes In Frequency Domain Research Articles

Tunable temporal dynamics of dipole response in graphene-wrapped core–shell nanoparticles

The investigation on the temporal dynamics of graphene-wrapped core–shell nanoparticles under the illumination of a Gaussian impulse have been carried out. By altering the graphene layers and the aspect ratio of the core–shell structure, we can adjust the resonant modes into typical cases in regime of terahertz. Accordingly, different scenarios for the temporal evolution are detected, which include two kinds of ultrafast oscillation with exponential decay tendency, pure exponential decay, and Gaussian shape, when the pulse duration of the incident pulse is much shorter than, similar to, and much longer than the localized surface plasmon lifetime. To one's interest, when the coupling between two resonant modes exists, one predicts the long-periodic oscillation, whose period is just the difference between the frequencies of the resonant modes. Hence, the intrinsic properties of the ultrafast oscillation can be hardly influenced by the input signals. Further quantitative calculation demonstrate that the periods of the ultrafast oscillations can be tuned by different physical mechanisms, which are, respectively, based on the self-interacting correction of a single resonance and the strong coupling between the resonant modes in frequency domain. Our results may be applicable in the fields of optical sensors, optical information processing, and other nanophotonic devices.

Data-Driven Interarea Oscillation Analysis for a 100% IBR-Penetrated Power Grid

In this research, a 100% inverter-based resource (IBR)-penetrated bulk power system (BPS) is examined for possible interarea oscillations. A testbed of a BPS constructed in PSCAD, with both grid-forming inverter (GFM)-based IBRs and grid-following inverter (GFL)-based IBRs, is utilized for simulation. For linear analysis, a data-driven method is used to obtain the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> admittance models of the IBRs. This information, combined with our knowledge of the network admittance, leads to eigenvalue analysis of the power grid. Furthermore, for the dominant 3-Hz mode, extended frequency-domain mode shape analysis, along with subgroup effect analysis, and network decomposition analysis are conducted. The analysis results can precisely tell the influencing factors of the oscillation mode. This mode is identified as an interarea oscillation mode. It has a different characteristics compared to the traditional electromechanical interarea mode. Specifically, the network decomposition analysis provides new insights on interarea oscillation modes. The analysis results are all confirmed by the electromagnetic transient (EMT) simulation results.

Implementation of a finite difference frequency domain mode solver incorporating subpixel smoothing.

Finite difference frequency domain (FDFD) mode solvers are straightforward to implement but can suffer from slow convergence when applied to high-contrast refractive index structures. In this work, we show how subpixel smoothing can improve the convergence properties of a full-vectorial FDFD mode solver. Based on a standard Yee grid, we formulate a generalized eigenbproblem whose solutions provide the modes of the waveguides taking into account the tensor nature of the effective dielectric constant. We investigate the convergence of the proposed FDFD mode solver in several cases including a step index fiber, a microsctuctured fiber, and a cylindrical plasmonic waveguide. The results show that tensor smoothing can significantly improve the convergence of the solver, thus allowing the use of less dense grids in the calculations. Our implementation is freely available on the web under an open-source licence.

Cellular Features Revealed by Transverse Laser Modes in Frequency Domain (Adv. Sci. 1/2022)

Transverse Laser Modes In article number 2103550 by Changjin Huang, Yu-Cheng Chen, and co-workers, the key mechanism governing the frequency distributions of transverse modes in single cell biological lasers are revealed. The cover art illustrates the concept of distinctive laser fingerprints generated by cells, which could be employed to provide cellular information. Morphological information of cells could be interpreted through hyperspectral imaging of laser modes. Applications including cell classification and monitoring of cell adhesion are explored and demonstrated.

A Longitude-Purification Mechanism for Tunable Fiber Laser Based on Distributed Feedback

Advanced applications in optical frequency metrology demand improved tunable lasers with high coherence. Herein, a tunable single-frequency fiber laser based on the longitude-purification induced by the distributed feedback has been demonstrated. The key device in the proposed laser structure is a distributed self-injection feedback structure (DSIFS) which function as a mode selector to have a robust frequency-domain mode suppression and wavelength adaptability. Our work is distinctly focused on the theoretical analysis of the formation process of single-longitudinal-mode (SLM) laser output and the wavelength adaptive characteristics in the DSIFS. In this experiment, the side mode suppression of the resonant longitudinal modes can be achieved in a fiber ring laser without any accurate control of the main cavity. The fiber laser obtained a compressed laser linewidth of 855 Hz, enhanced side mode suppression ratio (SMSR) of ∼67 dB, and frequency noise suppression of 40 dB. Furthermore, the fiber laser can be tuned over the entire flat gain region. The measured output power, wavelength and SMSR variations of the proposed laser over a long-term observation are less than 0.034 mW, 0.028 nm and 2.53 dB, respectively. In addition, the SLM operation of the laser can also be obtained at different pump power for every wavelength channel. Moreover, this method is applicable to any other gain-type lasers, indicating that the longitude-purification mechanism has a broad application prospect in other highly coherent tunable lasers.

Cellular Features Revealed by Transverse Laser Modes in Frequency Domain.

Biological lasers which utilize Fabry–Pérot (FP) cavities have attracted tremendous interest due to their potential in amplifying subtle biological changes. Transverse laser modes generated from cells serve as distinct fingerprints of individual cells; however, most lasing signals lack the ability to provide key information about the cell due to high complexity of transverse modes. The missing key, therefore, hinders it from practical applications in biomedicine. This study reveals the key mechanism governing the frequency distributions of transverse modes in cellular lasers. Spatial information of cells including curvature can be interpreted through spectral information of transverse modes by means of hyperspectral imaging. Theoretical studies are conducted to explore the correlation between the cross‐sectional morphology of a cell and lasing frequencies of transverse modes. Experimentally, the spectral characteristics of transverse modes are investigated in live and fixed cells with different morphological features. By extracting laser modes in frequency domain, the proposed concept is applied for studying cell adhesion process and cell classification from rat cortices. This study expands a new analytical dimension of cell lasers, opening an avenue for subcellular analysis in biophotonic applications.

Seismic Protection of RC Buildings by Polymeric Infill Wall-Frame Interface.

This paper is aimed at investigating the usage of flexible joints in masonry infilled walls surrounded by reinforced concrete (RC) frames. For this purpose, a real-size specimen was numerically created and exposed to the seismic loads. In order to evaluate both in-plane and out-of-plane performances of the infill walls, the system was chosen as a box shaped three-dimensional structure. In total, three different one-story constructions, which have single bays in two perpendicular directions, were modeled. The first type is the bare-frame without the infill walls, which was determined as a reference system. The second and third types of buildings are conventional mortar joint and PolyUrethane Flexible Joint (PUFJ) implemented ones, respectively. The influence of these joints on the material level are investigated in detail. Furthermore, general building dynamic characteristics were extracted by means of acceleration and displacement results as well as frequency domain mode shapes. Analyses revealed that PUFJ implementation on such buildings has promising outcomes and helps to sustain structural stability against the detrimental effects of earthquakes.

EMC Chamber Quiet Zone Qualification for Applications Above 1 GHz Using Frequency Domain Mode Filtering

Anechoic chambers used for electromagnetic compatibility (EMC) measurements above 1 GHz are qualified based on the site voltage standing wave ratio (SVSWR) method as per the international standard CISPR 16-1-4. With one antenna at the fixed position, some distance away from the quiet zone (QZ), the SVSWR is acquired by moving a dipole-like antenna along several linear paths that are located at the edge of the QZ. To reduce test burden, the SVSWR method under-samples the measurement by design, in that only six discrete points along each 40 cm linear travel path are measured. As a result, the test results are generally overly optimistic. In this article, we propose to use a novel cylindrical mode coefficient (CMC)-based frequency domain mode filtering techniques to obtain the VSWR. Here, we measure the vector pattern cut of the dipole-like test antenna with an offset placement at the outer edge of QZ. The antenna is then mathematically translated to the rotation center, whereupon a bandpass filter that tightly encloses the test antenna mode spectrum is applied. Two approaches are studied herein for translating the rotation center. One is by applying a path length correction to both magnitude and phase, and the other is by performing a cylindrical far-field transformation on the quasi-far-field data. The difference between the mode filtered antenna pattern and the original perturbed pattern is used to derive the chamber SVSWR. In contrast to the conventional technique, the proposed, novel method does not suffer from positional under-sampling, so it is well-placed to be applied at microwave frequencies and above.

INFLUENCE OF WINDING ENDS ON THE PARAMETERS OF PULSE INDUCTOR WITH U-SHAPED CORE

It is known from the scientific literature that magnetic pulse processing of electrically conductive non-magnetic sheet materials helps to reduce residual stresses, especially in welded joints. This is due to magnetoplastic and electroplastic effects. To create such effects in non-magnetic electrically conductive materials with welded joints, an inductor with pulsed magnetic field, U-shaped magnetic circuit and hollow conductor for possibility of active cooling of the winding is proposed. Such inductor allows inducing high-density pulsed currents in electrically conductive non-magnetic sheet materials with welded joints. It studies the parameters of the inductor - active resistance and inductance in the frequency-domain mode. The parameters calculated in two-dimensional and three-dimensional models are compared. The electromagnetic field is calculated using Maxwell equations and finite element method. Parameters of an ends of winding are determined by the difference in the parameters of the three-dimensional and two-dimensional models of the induction system. Resistance is calculated separately in the groove`s part of the winding, the outer part and on the frontal parts. The parameters of the induction system with a ferromagnetic core and non-magnetic thin-sheet alloy AMg6 are calculated for various values of complex amplitude of current in winding. Additionally, the parameters are calculated both without the magnetic core and without the non-magnetic metal. The quantitative comparison of the parameters of the three-dimensional model with the two-dimensional one is performed. The active resistance and inductance of end parts of the inductor are investigated by well-known analytical expressions from handbooks of electric machines. References 11, figures 3, tables 6.

Hybrid Fourier-domain mode-locked laser for ultra-wideband linearly chirped microwave waveform generation

We show the generation of a tunable linearly chirped microwave waveform (LCMW) with an ultra-large time-bandwidth product (TBWP) based on a hybrid Fourier-domain mode-locked (FDML) laser. The key device in the hybrid FDML laser is a silicon photonic integrated micro-disk resonator (MDR) which functions as an optical bandpass filter, to have strong wavelength selectivity and fast frequency tunability. By incorporating the integrated MDR in the fiber-based ring cavity to perform frequency-domain mode locking, an FDML laser is realized and a broadband frequency-chirped optical pulse is generated. By beating the frequency-chirped optical pulse with an optical carrier from a laser diode (LD) at a photodetector (PD), an LCMW is generated. The bandwidth of the LCMW is over 50 GHz and the temporal duration is over 30 µs, with an ultra-large TBWP of 1.5 × 106. Thanks to the strong tunability of the MDR in the FDML laser, the generated LCMW is fully tunable in terms of bandwidth, temporal duration, chirp rate, and center frequency.

Longitudinal and transverse mode coupling instability: Vlasov solvers and tracking codes

The study of collective effects in circular accelerators can be tackled by solving numerically the Vlasov equation or by using tracking codes. The two methods are obtained with different approaches: Vlasov solvers consider a continuous distribution function and describe the beam with coherent oscillation modes in frequency domain (ending up usually with an eigenvalue system to solve), while tracking codes use macroparticles and wakefields in time domain. In this paper we present two Vlasov solvers for the study of collective effects (from impedances/wakefields only) which evaluate the frequency shift of coherent oscillation modes and possible mode coupling instability in the single-bunch case for both longitudinal and transverse planes. In the longitudinal plane the Vlasov solver also takes into account the potential well distortion due to the wakefields under some conditions. In parallel to this theoretical approach, tracking codes, which include collective effects, have been used as benchmark. In particular, starting from their results, we also propose a new method to study the frequency shift of coherent modes and compare it with the output of the Vlasov solvers.

Optical Angle Sensor Technology Based on the Optical Frequency Comb Laser

A mode-locked femtosecond laser, which is often referred to as the optical frequency comb, has increasing applications in various industrial fields, including production engineering, in the last two decades. Many efforts have been made so far to apply the mode-locked femtosecond laser to the absolute distance measurement. In recent years, a mode-locked femtosecond laser has increasing application in angle measurement, where the unique characteristics of the mode-locked femtosecond laser such as the stable optical frequencies, equally-spaced modes in frequency domain, and the ultra-short pulse trains with a high peak power are utilized to achieve precision and stable angle measurement. In this review article, some of the optical angle sensor techniques based on the mode-locked femtosecond laser are introduced. First, the angle scale comb, which can be generated by combining the dispersive characteristic of a scale grating and the discretized modes in a mode-locked femtosecond laser, is introduced. Some of the mode-locked femtosecond laser autocollimators, which have been realized by combining the concept of the angle scale comb with the laser autocollimation, are also explained. Angle measurement techniques based on the absolute distance measurements, lateral chromatic aberration, and second harmonic generation (SHG) are also introduced.

Comparison of analytical and difference sensitivity methods for structural damage identification

The sensitivity-based damage identification method receives much attention due to its ability to identify structural damages at element level. This paper aims to discuss the influence of two different deduction algorithms of sensitivity matrix, the analytical method and the numerical difference method, on the parameter identification results of structures with the same identification index. In order to compare the difference between the two sensitivity-based damage identification methods, the frequency-domain mode shape and the time-domain acceleration response information are used as the updating objectives, respectively. And then, the sensitivity matrices of the updating objectives to the same identification index, structural element stiffness parameters, are respectively derived based on the analytical method and the numerical difference method. Finally, a simply supported box girder bridge is used to illustrate the difference between the two sensitivity-based methods. Numerical simulations with initial model errors and measurement noise show the two types of methods both can accurately detect local damages and identify unknown initial model errors. The presence of random noise has some bad effect on the identified results of structural parameters, but the identified accuracy is still acceptable for normal noise level.

A quadratic penalty item optimal variational mode decomposition method based on single-objective salp swarm algorithm

A quadratic penalty item optimal variational mode decomposition (VMD) method based on single-objective salp swarm algorithm (SSA) is proposed to alleviate the mode mixing of complex vibration signals. A novel frequency domain mode mixing density is proposed to quantitatively measure the mode mixing severity between intrinsic mode functions (IMFs). Then, an objective function is constructed by integrating correlation coefficient into frequency domain mode mixing density. Based on the SSA, the optimal quadratic penalty item is obtained through the objective function. The merits of this method are the alleviation of the mode mixing and maintenance of the signal fidelity for complex vibration signal without prior knowledge. Experimental results demonstrate the effectiveness of the proposed method.

Measurement and Visualization System of Eigenmode Shapes Considering Orthogonal Modes on Circular Membrane

This work proposes a visualization system of eigenmode shapes of a tapped circular membrane of membranophones. Under tapped membrane vibration, it is not enough to divide modes in frequency domain, because of the existence of eignmodes whose frequencies are close to each other including orthogonal modes. Therefore, we structured this visualization system of eigenmode shapes, which could divide the orthogonal modes appearing at slightly different frequencies. This system firstly measures the sound pressures emitted from the vibration on tapped circular membrane of membranophone with a circular microphone array placed in proximity to the membrane. Secondly, it extracts the circumferential shape of each eigenfrequency. In this system, dominant frequencies of estimated power spectrum density obtained by Yule-Walker method are considered as approximate eigenfrequences of this vibration. Multiple eigenmodes whose azimuthal mode order is different, are divided by Fourier series expansion of the circumferential shapes of each eigenfrequency. In addition, orthogonal modes, which have the same mode order and slightly different frequencies are divided by considering the orthogonality of mode shapes, sine and cosine functions. Lastly, the system estimates the eigenmode shape of the whole membrane using the Bessel function of the first kind correspond to each mode order as radial shape, and shows them to users. It is expected that user could recognize the unbalance of tension distribution intuitively using this system. It is also expected the system to be applied to the tuning assistive system of the tuning of circular membranophone.

Free-edge sensor placement for identifying vibration modes of structures subjected to impact loadings using fiber Bragg gratings

For modal testing, active vibration control, or structural health monitoring, determination of sensing locations becomes a crucial issue once the interesting mode has nodal lines on which the sensors are bonded. Theoretically, only nodal points but not nodal lines would exist along free edges of the structures. Thus, measurements of vibrations along free edges of structures would maximize the number of detecting (i.e., for monitoring application) or observing (i.e., for active control application) modes. However, conventional sensors such as strain gauges or accelerometers can only be bonded on flat or smooth surfaces. In this work, fiber Bragg gratings, which possess linear geometric shape, are employed to investigate the feasibility of detecting more vibration modes along free edges of structures. A traction-free aluminum solid with symmetric geometry subjected to various impact loadings is considered. To verify the capability of the free-edge bonded FBG sensors for detecting vibration modes from impact-induced responses, relationships between sensing locations, impact locations, and intensities of modes, are discussed with the aid of mode shapes obtained by the finite element me\thod. Within frequencies up to 30 kHz, 14 out-of-plane dominated and 25 in-plane dominated modes in frequency domain are examined. Multiple modes, that is, repeated roots, two or more modes with identical frequencies due to symmetric geometry, are also analyzed. Our experimental results show that placing the FBGs along free edges of structures to acquire and analyze more vibration modes is feasible. Copyright © 2016 John Wiley & Sons, Ltd.

Modal and Dynamic Analysis of a Vehicle with Kinetic Dynamic Suspension System

A novel kinetic dynamic suspension (KDS) system is presented for the cooperative control of the roll and warp motion modes of off-road vehicles. The proposed KDS system consists of two hydraulic cylinders acting on the antiroll bars. Hence, the antiroll bars are not completely replaced by the hydraulic system, but both systems are installed. In this paper, the vibration analysis in terms of natural frequencies of different motion modes in frequency domain for an off-road vehicle equipped with different configurable suspension systems is studied by using the modal analysis method. The dynamic responses of the vehicle with different configurable suspension systems are investigated under different road excitations and maneuvers. The results of the modal and dynamic analysis prove that the KDS system can reduce the roll and articulation motions of the off-road vehicle without adding extra bounce stiffness and deteriorating the ride comfort. Furthermore, the roll stiffness is increased and the warp stiffness is decreased by the KDS system, which could significantly enhance handing performance and off-road capability.

Canonical information flow decomposition among neural structure subsets.

Partial directed coherence (PDC) and directed coherence (DC) which describe complementary aspects of the directed information flow between pairs of univariate components that belong to a vector of simultaneously observed time series have recently been generalized as bPDC/bDC, respectively, to portray the relationship between subsets of component vectors (Takahashi, 2009; Faes and Nollo, 2013). This generalization is specially important for neuroscience applications as one often wishes to address the link between the set of time series from an observed ROI (region of interest) with respect to series from some other physiologically relevant ROI. bPDC/bDC are limited, however, in that several time series within a given subset may be irrelevant or may even interact opposingly with respect to one another leading to interpretation difficulties. To address this, we propose an alternative measure, termed cPDC/cDC, employing canonical decomposition to reveal the main frequency domain modes of interaction between the vector subsets. We also show bPDC/bDC and cPDC/cDC are related and possess mutual information rate interpretations. Numerical examples and a real data set illustrate the concepts. The present contribution provides what is seemingly the first canonical decomposition of information flow in the frequency domain.

Percolation and Electrical Conductivity Modeling of Novel Microstructured Insulator-Conductor Nanocomposites Fabricated from PMMA and ATO

ABSTRACTThe electrical conductivity of insulating polymer matrix composites undergoes radical increase at a certain concentration of conductive filler, which is known as the percolation threshold. Polymer matrix conductive nanocomposites were fabricated by compression molding the mechanically mixed poly (methyl methacrylate) (PMMA) and antimony tin oxide (ATO) nanoparticles, as has been done with other polymer composites before. The electrical conductivity of PMMA/ATO nanocomposites increased by several orders of magnitude at a small concentration of ATO (∼ 0.27 vol %). The continuous 3D network like distribution of ATO nanoparticles contributed to this percolation at subcritical filler concentrations. The effects of processing parameters on these unique microstructures and electrical properties were investigated. The tetrakaidecahedron-like microstructure was observed by scanning electron microscopy (SEM) and was found to be affected by the molding pressure, temperature and amount of nanoparticles. The viscoelastic flow of matrix under the optimum processing conditions allowed the shape transformation of PMMA into space filling polyhedra and an ordered distribution of ATO nanoparticles along the sharp edges of the PMMA. Parametric finite element analysis was performed to model this unique microstructure-driven percolation. The 2D simplified model was generated in AC/DC frequency domain mode in COMSOL Multiphysics®to solve the effects of ordered distribution of conductive nanoparticles on the electrical properties of the composite. There was excellent agreement between experimental and simulated values of electrical conductivity and percolation concentration. This model can be used to predict percolation threshold and electrical properties for any types of composite systems containing insulating matrix and conductive fillers that can form this unique microstructure.

MODE TUNING IN PHOTONIC CRYSTAL CAVITIES USING GEOMETRY OF FIRST NEIGHBORING LAYER

The relation between position of optical source and excitation of photonic crystal cavity modes are investigated. For multimode cavities, the source position considerably affects the modes' excitation. The frequency distributions of local density of photonic states in a multimode cavity differ for source positions at structural symmetric locations and nonsymmetrical ones. It was also found that the geometry of cavity's neighboring layers has strong influence on the spatial and spectral distributions of modes. The results also show that the shifts of various cavity modes in frequency domain have different sensitivity with respect to geometrical tolerance. This can be used in various applications which need mode tuning.