Coupling Scheme Research Articles

Mitigation and suppression of rare events in weakly coupled lasers

We consider an asymmetric laser system which displays mixed-mode oscillations and hyperchaos. This system consists of two laser oscillators which are weakly and bidirectionally coupled, where the asymmetry is induced by a mismatch between the laser pump parameters. The coupling scheme via saturable absorbers, however, is built up with a beam combining process which incorporates the electric fields of both laser oscillators. This procedure, which is relevant in semiconductor and solid-state lasers, is implemented in the present molecular laser model.In our numerical simulations and time series analysis we show that for small enough coupling strength, below the chaotic phase synchronization threshold, the asymmetric laser system displays extreme rare events in one of the lasers whose occurrence can be mitigated and suppressed by optical feedback. These episodes are outliers in the time series from the laser intensity and the return time intervals, and are reminiscent of Dragon-Kings. We estimate the correlation dimensions and Lyapunov exponents from the time series, which suggest the ubiquity of hyperchaotic dynamics irrespective of the occurrence or lack of extreme rare events in these time series.

A nodal position finite element model for fluid-structure interaction analysis of floating bodies with mooring

A numerical model based on the Nodal Position Finite Element Method (NPFEM) is proposed in this work for fluid-structure interaction (FSI) analysis of moored bodies subject to multiphase free surface flows. The semi-implicit CBS (Characteristic-Based Split) method is utilized here for discretization of the flow fundamental equations in the context of the Finite Element Method, where linear tetrahedral elements are adopted. Turbulence is analyzed using Large Eddy Simulation (LES) with the dynamic sub-grid scale model and the Level Set method is utilized for simulation of multiphase free surface flows. The structural system is modeled using a three-dimensional rigid body approach for the floating object, while the mooring is discretized using a geometrically nonlinear cable formulation based on the NPFEM. A partitioned coupling scheme is adopted for fluid-structure interaction analysis taking into account the fluid-structure and cable-structure couplings, where the flow equations are kinematically described employing an Arbitrary Lagrangean-Eulerian (ALE) formulation with a numerical scheme for mesh motion. The algorithms constituting the numerical model are verified first considering classical applications on fluid dynamics and FSI, in addition to applications involving the dynamic analysis of cables. Finally, problems involving floating bodies with and without mooring are simulated to demonstrate the applicability and accuracy of the proposed numerical model.

Metasurface Deflector Enhanced Grating Coupler for Perfectly Vertical Coupling

We propose a perfectly vertical coupling scheme based on metasurface deflectors (meta-deflectors) and grating couplers (GCs). An approach for optimizing the GCs based on the Gaussian-fitting using the genetic algorithm is proposed. An meta-deflector based on amorphous silicon (a-Si) pillars is designed to the optimal coupling angle of the GC to ensure good coupling efficiency (CE). Simulations predict peak vertical CE to be 78% at the wavelength of 2 μm, with 1 dB bandwidth ≥35 nm. The design process of GC and meta-deflector is provided in detail, and the influence of fabrication error on the CE is analyzed.

BiMn7O12 : Polar antiferromagnetism by inverse exchange striction

Despite extensive research on magnetically induced ferroelectricity there exist relatively few studies on how a preexisting electric polarization affects magnetic order. Given that well-established magnetoelectric coupling schemes can in principle work in reverse, one might anticipate that primary, polar magnetic structures could be uniquely stabilized in ferroelectric crystals, however, this scenario is apparently rare. Here, we show that in ferroelectric $\mathrm{Bi}{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$, a pure, polar $E$-type antiferromagnetic order emerges below ${T}_{1}=59$ K, and we present a phenomenological model of trilinear magnetoelectric coupling consistent with ${\mathrm{Bi}}^{3+}$ lone-pair driven polar distortions uniquely stabilizing the polar antiferromagnetism via modulation of Heisenberg exchange pathways, i.e., inverse exchange striction. In addition, below ${T}_{2}=55$ K there occurs large commensurate canting of the $E$-type structure due to the onset of ferrimagnetic order on a separate crystallographic sublattice that may be exploited for additional magnetoelectric functionality.

Multi-feed, loop-dipole combined dielectric resonator antenna arrays for human brain MRI at 7 T

ObjectiveTo determine whether a multi-feed, loop-dipole combined approach can be used to improve performance of rectangular dielectric resonator antenna (DRA) arrays human brain for MRI at 7 T.Materials and methodsElectromagnetic field simulations in a spherical phantom and human voxel model “Duke” were conducted for different rectangular DRA geometries and dielectric constants εr. Three types of RF feed were investigated: loop-only, dipole-only and loop-dipole. Additionally, multi-channel array configurations up to 24-channels were simulated.ResultsThe loop-only coupling scheme provided the highest B1+ and SAR efficiency, while the loop-dipole showed the highest SNR in the center of a spherical phantom for both single- and multi-channel configurations. For Duke, 16-channel arrays outperformed an 8-channel bow-tie array with greater B1+ efficiency (1.48- to 1.54-fold), SAR efficiency (1.03- to 1.23-fold) and SNR (1.63- to 1.78). The multi-feed, loop-dipole combined approach enabled the number of channels increase to 24 with 3 channels per block.DiscussionThis work provides novel insights into the rectangular DRA design for high field MRI and shows that the loop-only feed should be used instead of the dipole-only in transmit mode to achieve the highest B1+ and SAR efficiency, while the loop-dipole should be the best suited in receive mode to obtain the highest SNR in spherical samples of similar size and electrical properties as the human head.

Synchronization dynamics of phase oscillators with generic adaptive coupling

Adaptive coupling schemes among interacting elements are ubiquitous in real systems ranging from physics and chemistry to neuroscience and have attracted much attention in recent years. Here, we extend the Kuramoto model by considering a particular adaptive scheme in a system of globally coupled oscillators. The homogeneous coupling is correlated with the global coherence of the population that is weighted by the generic nonlinear feedback function of the amplitude of the order parameter. The studied model is analytically tractable that generalizes the theory of Kuramoto for synchronization transition. We develop a mean-field theory by establishing the self-consistent equation describing the stationary dynamics in the thermodynamic limit. Importantly, the Landau damping effect, which turns out to be far more generic, is revealed in the framework of the linear stability analysis of the resonant pole theory. Furthermore, the relaxation rate of the order parameter in the subcritical region is obtained from a universal formula. Our study can deepen the understanding of synchronization transitions and other related collective dynamics in networked oscillators with adaptive interaction schemes.

Collective behavior of identical Stuart-Landau oscillators in a star network with coupling asymmetry effects.

In this study, we investigated the impact of the asymmetry of a coupling scheme on oscillator dynamics in a star network. We obtained stability conditions for the collective behavior of the systems, ranging from an equilibrium point over complete synchronization (CS) and quenched hub incoherence to remote synchronization states using both numerical and analytical methods. The coupling asymmetry factor α significantly influences and determines the stable parameter region of each state. For α ≠ 1, the equilibrium point can emerge when the Hopf bifurcation parameter a is positive, which is impossible for diffusive coupling. However, CS can occur even if a is negative under α < 1. Unlike diffusive coupling, we observe more behavior when α ≠ 1, including additional in-phase remote synchronization. These results are supported by theoretical analysis and validated through numerical simulations and independent of network size. The findings may offer practical methods for controlling, restoring, or obstructing specific collective behavior.

Bidirectional high sidelobe suppression silicon optical phased array

An optical phased array (OPA), the most promising non-mechanical beam steering technique, has great potential for solid-state light detection and ranging systems, holographic imaging, and free-space optical communications. A high quality beam with low sidelobes is crucial for long-distance free-space transmission and detection. However, most previously reported OPAs suffer from high sidelobe levels, and few efforts are devoted to reducing sidelobe levels in both azimuthal ( φ ) and polar ( θ ) directions. To solve this issue, we propose a Y-splitter-assisted cascaded coupling scheme to realize Gaussian power distribution in the azimuthal direction, which overcomes the bottleneck in the conventional cascaded coupling scheme and significantly increases the sidelobe suppression ratio (SLSR) in the φ direction from 20 to 66 dB in theory for a 120-channel OPA. Moreover, we designed an apodized grating emitter to realize Gaussian power distribution in the polar direction to increase the SLSR. Based on both designs, we experimentally demonstrated a 120-channel OPA with dual-Gaussian power distribution in both φ and θ directions. The SLSRs in φ and θ directions are measured to be 15.1 dB and 25 dB , respectively. Furthermore, we steer the beam to the maximum field of view of 25°×13.2° with a periodic 2λ pitch (3.1 μm). The maximum total power consumption is only 0.332 W with a thermo-optic efficiency of 2.7 mW/π .

Open-source simulation of strongly-coupled fluid-structure interaction between non-conformal interfaces

Design code-based “life-safety” requirements for structural earthquake and tsunami design offer reasonable guidelines to construct buildings that will remain standing during a tsunami or seismic event. Much less consideration has been given to assessing structural resilience during sequential earthquake and tsunami multi-hazard events. Such events present a series of extreme loading scenarios, where damage sustained during the earthquake influences structural performance during the subsequent inundation. Similar difficulties exist with respect to damage sustained during tropical events, as wind and fluid loading may vary with structural response or accumulated damage. To help ensure critical structures meet a “life-safety” level of performance during such multi-hazard events, analysis software capable of simulating simultaneous structural and fluid dynamics must be developed. To address this gap in understanding of non-linear fluid-structure-interaction (FSI), an open-source tool (FOAMySees) was developed for simulation of tsunami and wave impact analysis of post-earthquake non-linear structural response of buildings. The tool is comprised of the Open-source Field Operation And Manipulation software package and OpenSeesPy, a Python 3 interpreter of OpenSees. The programs are coupled via preCICE, a coupling library for partitioned multi-physics simulation. FOAMySees has been written to work in a Linux OS environment with HPC clusters in mind. The FOAMySees program offers a partitioned conventional-serial-staggered coupling scheme, with optional implicit iteration techniques to ensure a strongly-coupled two-way FSI solution. While FOAMySees was developed specifically for tsunami-resilience analysis, it may be utilized for other FSI applications with ease. With this coupled Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) program, tsunami and earthquake simulations may be run sequentially or simultaneously, allowing for the evaluation of non-linear structural response to multi-hazard excitation.

Evaluating the Combined Effects of Water and Fertilizer Coupling Schemes on Pear Vegetative Growth and Quality in North China

Unreasonable fertilizer and irrigation applications and dosages in orchards in northern China result in poor vegetative growth and fruit quality. To reveal the combined effect of water and fertilizer coupling on vegetative growth and fruit quality, this study used pear as a field experiment material, considering: (1) irrigation lower limits (55%, 65%, 75%θf, θf is field capacity) and (2) nitrogen fertilizer application (162, 324, 486 kg·ha−1). Nine coupling schemes and a control treatment (C) were set up in the orthogonal combination. The results showed that, under the higher irrigation rate and nitrogen dose, the spring shoot length, base diameter, and leaves relative chlorophyll content values were increased by 36.77%, 31.86% and 12.91%, respectively. The response of each coupling scheme was different. However, selected water and nitrogen coupling schemes improved the fruit quality. The evaluation results indicated that medium irrigation and high fertilizer scheme were optimal. In conclusion, integrating the vegetative growth and fruit quality, it is recommended that the water and fertilizer coupling scheme for pear in the northern China is as follows: a lower irrigation limit of 65%θf and a nitrogen fertilizer amount of 486.00 kg·ha−1.

Free‐Form Micro‐Optics Enabling Ultra‐Broadband Low‐Loss Off‐Chip Coupling

AbstractEfficient fiber‐to‐chip coupling has been a major hurdle to cost‐effective packaging and scalable interconnections of photonic integrated circuits. Conventional photonic packaging methods relying on edge or grating coupling are constrained by high insertion losses, limited bandwidth density, narrow band operation, and sensitivity to misalignment. This work presents a new fiber‐to‐chip coupling scheme based on free‐form reflective micro‐optics. A design approach which simplifies the high‐dimensional free‐form optimization problem to as few as two full‐wave simulations is implemented to empower computationally efficient design of high‐performance free‐form reflectors while capitalizing on the expanded geometric degrees of freedom. This work demonstrates fiber array coupling to waveguides taped out through a standard foundry shuttle run and backend integrated with 3‐D printed micro‐optics. A low coupling loss down to 0.5 dB is experimentally measured at 1550 nm wavelength with a record 1‐dB bandwidth of 300 nm spanning O to U bands. The coupling scheme further affords large alignment tolerance, high bandwidth density, and solder reflow compatibility, qualifying it as a promising optical packaging solution for applications such as wavelength division multiplexing communications, broadband spectroscopic sensing, and nonlinear optical signal processing.

Explosive synchronization in phase oscillator populations with attractive and repulsive adaptive interactions

The attractive and repulsive adaptive coupling schemes among dynamical agents are ubiquitous in systems ranging from physics, biology to neuroscience, which have attracted increasing interest and ample attention during recent years. Here, we extend the classical Kuramoto model to the coupling with mixed signs by considering a particular adaptive scheme in a system of globally coupled phase oscillators. The time-varying coupling weight between each pair of oscillators is correlated with the local coherence that involves according to a nonlinear differential equation. The studied model is capable of capturing the essential properties of the explosive synchronization induced by the adaptive interactions. We carry out extensive simulations to explore the collective dynamics in different perspectives. Remarkably, we reveal that the occurrence of the abrupt transitions of the macroscopic overall weights, as well as the emergence of the correlations between the microscopic structure features (including degrees and frequency dissasortativity) and local dynamics trigger the explosive synchronization, which manifests the suppression rule for the formation of small synchronized clusters. Furthermore, we provide an analytical treatment for the reduced system that allows us to grasp the underlying mechanism of the observed phenomena. Our study can deepen the understanding of the explosive synchronization transitions and other related abrupt dynamic phenomena occurring in networked oscillators with generic adaptive schemes.

An Aero-Structural Model for Ram-Air Kite Simulations

Similar to parafoils, ram-air kites are flexible membrane wings inflated by the apparent wind and supported by a bridle line system. A major challenge in estimating the performance of these wings using a computer model is the strong coupling between the airflow around the wing and the deformation of the membrane structure. In this paper, we introduce a staggered coupling scheme combining a structural finite element solver using a dynamic relaxation technique with a potential flow solver. The developed method proved numerically stable for determining the equilibrium shape of the wing under aerodynamic load and is thus suitable for performance measurement and load estimation. The method was validated with flight data provided by SkySails Power. Measured forces on the tether and steering belt of the robotic kite control pod showed good resemblance with the simulation results. As expected for a potential flow solver, the kite’s glide ratio was overestimated by 10–15%, and the measured tether elevation angle in a neutral flight scenario matched the simulations within 2 degrees. Based on the obtained results, it can be concluded that the proposed aero-structural model can be used for initial designs of ram-air kites with application to airborne wind energy.

Tunable terahertz slow light with hybrid coupling of a magnetic toroidal and electric dipole metasurface

We present a hybrid coupling scheme of a magnetic toroidal and electric dipole metasurface with suppressed radiation loss, which can produce the tunable plasmon-induced transparency (PIT) with an enhanced slow-light effect in the terahertz regime. The terahertz metasurface is constructed by nesting a dual-split ring resonator (DSRR) inside a ring resonator (RR) to exploit the destructive coherence of hybrid electromagnetic mode coupling at the PIT resonance. The polarization-dependence excitation performs the active tunability of a PIT-induced group slowing down by rotating the polarization angle, experimentally achieving a maximum group delay of 3.5 ps. Furthermore, the modified terahertz metasurface with a four-split ring resonator (FSRR) nested in an RR is prepared on photoconductive silicon, demonstrating the pump-controllable group delay effect at the PIT resonance. The large group delay from 2.2 to 0.9 ps is dynamically tunable by adjusting the pump power. The experimental results are in good accord with the theoretical simulations.

Bandpass Filter and Diplexer Based on Short-Circuited Stub-Embedded Resonators and Novel Coupling Scheme

A novel coupling scheme is proposed based on short-circuited stub-embedded resonator (SCSER) for bandpass filters (BPFs). The BPF utilizing SCSER owns three transmission zeros (TZs) that are controlled by short-circuited and open-circuited stubs. Based on the proposed SCSER and coupling structure, a novel high-isolation diplexer is presented. When the TZ is allocated in the opposite channel and impedance transformation is carried out in the common port, the proposed diplexer can be realized by directly combining two different BPFs without an additional matching network. For the demonstration, a BPF and a diplexer based on SCSER are fabricated and measured. The measured results agree well with the simulated ones.

First steps towards modeling the interaction of cardiovascular agents and smooth muscle activation in arterial walls

AbstractNumerical simulation of the response of healthy and pathological arteries to cardiovascular agents can provide valuable information to the physician in the treatment of diseases such as hypertension, atherosclerosis, and the Marfan syndrome. Here, we provide a first step towards a computational framework to model the effects of antihypertensive agents on the mechanical response of arterial walls. A material model is developed by extending an existing formulation for wall tissue to incorporate the effects of calcium‐ion channel blockers. The resulting coupled deformation‐diffusion problem is then solved using the finite element method. Simulation results with drug activity show that, indeed, an increased lumen diameter due to reduced contraction is obtained. Additionally, a decrease in the rate of arterial contraction is observed, which is also consistent with expected behavior. Finally, we compare results for an implicit or explicit treatment of the the deformation‐diffusion coupling, and we observe that both coupling schemes yield comparable results for a wide range of time step sizes.

A Nonlinear Fractional BEM Model for Magneto-Thermo-Visco-Elastic Ultrasound Waves in Temperature-Dependent FGA Rotating Granular Plates

The primary goal of this study is to create a nonlinear fractional boundary element method (BEM) model for magneto-thermo-visco-elastic ultrasound wave problems in temperature-dependent functionally graded anisotropic (FGA) rotating granular plates in a constant primary magnetic field. Classical analytical methods are frequently insufficient to solve the governing equation system of such problems due to nonlinearity, fractional order heat conduction, and strong anisotropy of mechanical properties. To address this challenge, a BEM-based coupling scheme that is both reliable and efficient was proposed, with the Cartesian transformation method (CTM) used to compute domain integrals and the generalized modified shift-splitting (GMSS) method was used to solve the BEM-derived linear systems. The calculation results are graphed to show the effects of temperature dependence, anisotropy, graded parameter, and fractional parameter on nonlinear thermal stress in the investigated plates. The numerical results validate the consistency and effectiveness of the developed modeling methodology.

Asymmetric spiral chimeras on a spheric surface of nonlocally coupled phase oscillators.

The spiral chimera state shows a remarkable spatiotemporal pattern in a two-dimensional array of oscillators for which the coherent spiral arms coexist with incoherent cores. In this work, we report on an asymmetric spiral chimera having incoherent cores of different sizes on the spherical surface of identical phase oscillators with nonlocal coupling. This asymmetric spiral chimera exhibits a strongly symmetry-broken state in the sense that not only the coherent and incoherent domains coexist, but also their incoherent cores are nonidentical, although the underlying coupling structure is symmetric. On the basis of analyses along the Ott-Antonsen invariant manifold, the bifurcation conditions of asymmetric spiral chimeras are derived, which reveals that the asymmetric spiral chimera state emerges via a supercritical symmetry-breaking bifurcation from the symmetric spiral chimera. For the coupling function composed of two Legendre modes, rigorous stability analyses are carried out to present a complete stability diagram for different types of spiral chimeras, including the stationary symmetric and asymmetric spiral chimeras as well as breathing asymmetric spiral chimera. For the general coupling scheme the asymmetric spiral chimera occurs as a result of competition between the odd and even Legendre modes of the coupling function. Our theoretical findings are verified by using extensive numerical simulations of the model system.

Lax-Wendroff solvers-based Hermite reconstruction for hyperbolic problems

This paper develops a new family of 2k-th order accurate Lax-Wendroff solvers-based Hermite reconstructions for hyperbolic problems systematically and implements the resulting schemes in the two-stage fourth order time-stepping framework. In order to cope with numerical difficulties around discontinuities, the WENO technology is applied and in practice a hybrid choice is made between a second order (k=1) and any other higher order Hermite reconstructions (k≥2) for efficiency. This approach unifies the Hermite-type reconstructions and the Lax-Wendroff type time discretization to form compact spacetime coupling schemes. Numerical experiments demonstrate the performance of the resulting schemes.

Numerical Study of the Effects of Asymmetric Velocity Profiles in a Curvilinear Channel on Migration of Neutral Buoyant Particle

The microstructure and suspended particle behavior should be considered when studying the flow properties exhibited by particle suspension. In addition, particle migration, also known as Segré–Silberberg effects, alters the microstructure of the suspension and significantly affects the viscosity properties of the suspension. Therefore, particle behavior with respect to the changes in mechanical factors should be considered to better understand suspension. In this study, we investigated the particle behavior in asymmetric velocity profiles with respect to the channel center numerically using the lattice Boltzmann method and a two-way coupling scheme. Our findings confirmed that the final equilibrium position of particles in asymmetric velocity profiles converged differently between the outer and inner wall sides with respect to the channel center. This indicates that the mechanical equilibrium position of particles can be changed by asymmetric velocity profiles. In addition, centrifugal force acting on the particles is also important in the study of equilibrium position. These results suggest that the microstructure and viscosity characteristics of a suspension in a pipe could be handled by changes in velocity profiles.