Unwanted Modes Research Articles

Suppression of background emission for efficient single-photon generation in micropillar cavities

We optimize the efficiency of a quantum-dot-based micropillar single-photon source by minimizing the spontaneous emission into unwanted background modes. We perform a numerical investigation of the background emission, where we identify a semi-periodic enhancement with the diameter. At these peaks of the background emission, the efficiency is reduced by ∼20%, and we show how this reduction can be avoided simply by choosing a diameter away from the peaks. Finally, we analyze the discrepancy between the Purcell factor estimated from experimental lifetime measurements and the true Purcell factor.

The Novel Successive Variational Mode Decomposition and Weighted Regularized Extreme Learning Machine for Fault Diagnosis of Automobile Gearbox

In order to improve the diagnosis accuracies of the current diagnosis methods, a novel fault diagnosis method of automobile gearbox based on novel successive variational mode decomposition and weighted regularized extreme learning machine is presented for fault diagnosis of gearbox in this paper. The novel successive variational mode decomposition (SVMD) is presented to improve the traditional variational mode decomposition, which finds modes one after the other, and this succession helps increase convergence rate and also not extract the unwanted modes; weighted regularized extreme learning machine (WRELM) is presented to improve the traditional extreme learning machine, which uses the weight of each sample with the nonparametric kernel density estimation and can find the optimal weight for each sample. The test results indicate that the diagnosis accuracy of SVMD-WRELM for gearbox is better than that of VMD-WRELM, VMD-ELM.

Mitigating Unwanted Mode in a Microstrip Patch by a Simpler Technique to Reduce Cross-Polarized Fields Over the Orthogonal Plane

This letter explores a new concept for the first time to control a specific unwanted higher mode in a microstrip patch with a target to reduce its cross-polarized (XP) radiation over the orthogonal plane. A strategic metal pole has been introduced to control the modal current of an orthogonal higher mode (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> ) but leaving the fundamental radiating mode (TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> ) untouched. The conjecture has been successfully verified in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> -band. The approach is the simplest of all explored earlier, requiring no multiparameter optimization or fabrication complexity except requiring an additional vertical space. Antenna impedance and primary radiations are insensitive to this technique but it ensures 11dB XP suppression over the orthogonal plane. The pole height can be reduced by 40%–63% by accepting a tradeoff in XP suppression by 3 dB.

Physics-Driven Machine-Learning-Based Borehole Sonic Interpretation in the Presence of Casing and Drillpipe

SummaryInterpretation of sonic data acquired by a logging-while-drilling (LWD) tool or wireline tool in cased holes is complicated by the presence of drillpipe or casing because those steel pipes can act as a strong waveguide. Traditional solutions, which rely on using a frequency bandpass filter or waveform arrival-time separation to filter out the unwanted pipe mode, often fail when formation and pipe signals coexist in the same frequency band or arrival-time range. We hence developed a physics-driven machine-learning-based method to overcome the challenge. In this method, two synthetic databases are generated from a general root-finding mode-search routine on the basis of two assumed models: One is defined as a cemented cased hole for a wireline scenario, and the other is defined as a steel pipe immersed in a fluid-filled borehole for the logging-while-drilling scenario. The synthetic databases are used to train neural network models, which are first used to perform global sensitivity analysis on all relevant model parameters so that the influence of each parameter on the dipole dispersion data can be well understood. A least-squares inversion scheme using the trained model was developed and tested on synthetic cases. The scheme showed good results, and a reasonable uncertainty estimate was made for each parameter. We then extended the application of the trained model to develop a method for automated labeling and extraction of the dipole flexural dispersion mode from other disturbances. The method combines the clustering technique with the neural-network-model-based inversion and an adaptive filter. Testing on field data demonstrates that the new method is superior to traditional methods because it introduces a mechanism from which unwanted pipe mode can be physically filtered out.This novel physics-driven machine-learning-based method improved the interpretation of sonic dipole dispersion data to cope with the challenge brought by the existence of steel pipes. Unlike data-driven machine learning methods, it can provide global service with just one-time offline training. Compared with traditional methods, the new method is more accurate and reliable because the processing is confined by physical laws. This method is less dependent on input parameters; hence, a fully automated solution could be achieved.

Broadband polarization filter based on hybrid silver-graphene coated pentagonal photonic crystal fiber

In this paper, a polarization filter based on hybrid silver-graphene coated pentagonal Photonic Crystal Fiber (PCF) is proposed. The finite element method is used to analyse the confinement loss characteristics of the proposed PCF. The proposed PCF based filter selects the desired X-polarization mode with a low confinement loss of 0.12 dB/cm and 0.59 dB/cm and rejects the unwanted Y-polarization mode with a very high confinement loss of 361.26 dB/cm and 1508.37 dB/cm at the mostly used wavelengths of 1.31 μm and 1.55 μm for optical communication. The proposed PCF with a length of 300 μm achieves the cross-talk of 216.68 dB and 904.67 dB at 1.31 μm and 1.55 μm. It also accomplishes a broad spectral width of 816 nm with the cross-talk value greater than 20 dB. The influence of the structural parameters of the proposed PCF is studied. The fabrication feasibility of the proposed PCF is also investigated. The proposed PCF based polarization filter will be an inevitable candidate for the integrated optical system.

Characteristic mode analysis of wideband high-gain and low-profile metasurface antenna* *Project supported by the National Natural Science Foundation of China (Grant Nos. 61671464 and 61801508), the Postdoctoral Innovative Talents Support Program of China (Grant No. BX20180375), and the Natural Science Research Fund of Shaanxi Province, China (Grant No. 2019JQ-103).

A wideband high-gain and low-profile metasurface antenna is proposed by analyzing characteristic quantities and parameters in the characteristic modes (CMs). The detailed modal current and modal weighting coefficient are analyzed to explain the broadband operation and high gain. A dominant characteristic mode is well excited, leading to a broadband operation. The mode behaviors of the excitation are changed to suppress the unwanted higher-order modes and improve the radiation performance by changing the widths of two patches. The measured impedance bandwidth for –10 dB is 39.8% (5.3 GHz–7.94 GHz) with a gain of 7.8 dBi–10.04 dBi over the operating bandwidth.

Numerical studies on RF tuning of an RFQ in a simulation environment using a tuning program

A four-vane type Radio Frequency Quadrupole (RFQ) is very sensitive to geometrical errors since these may result into mixing of unwanted modes with the desired operating mode. This perturbs the voltage profile of the RFQ, which may deteriorate the beam dynamics, and increase the emittance growth and beam loss. In order to tune the RFQ for a desired voltage profile, a computer program based on transmission line model of RFQ is typically used. Although the theory of tuning program and its experimental validation has been discussed in earlier publications (Palmieri et al., 2010; Sanyasirao, 2014), here we discuss a methodology to perform the tuning of an RFQ in a simulation environment. This approach enables us to do an in-depth analysis of the tuning process, which is illustrated by performing the studies for an example case of the RFQ for the proposed Indian Facility for Spallation Research (IFSR), using the 3D Electro-Magnetic (EM) code CST-MWS. In these simulations, modelling of the vane-tip modulations of the RFQ has been performed in the code CST-MWS, for which a new procedure, using a macro, has been developed, which is presented here. We study perturbation in the voltage profile for two specific situations — first, due to vane-tip modulations, and second, due to misalignment errors. We correct the effect due to each of these perturbations in the simulation environment, using the tuning algorithm, and verify it using the 3D EM code. It has been shown in the paper that the same tuning algorithm with suitable modification can be used also for tuning of the end-cells of RFQ.

Correction of coupled higher-order modes in the S-parameter characterization of wideband cavity beam position monitors.

Wideband RF cavity beam position monitors (CBPM) have been increasingly employed for short bunch interval operations in several linear accelerators. At a few nanoseconds of bunch spacing, the loaded quality factor of the CBPM TM110 dipole eigenmode is required to be sufficiently low to minimize the signal interference between consecutive bunches. Moreover, the bunched beam also couples to several unwanted higher-order modes (HOM), such as the TM210 and TE111 eigenmodes, which also may result in an error of the bunch position measurement if no precautions are taken. In the fabrication phase of CBPMs, the mode coupling can alter the TM110 mode frequency and therefore causes an error in its tuning, e.g., 0.3% for a 4.875GHz CBPM with QL = 22.5. This error needs to be identified so that the precise tuning is enabled since the dipole mode frequency becomes critical for the position evaluations as the signal processing is relatively phase-sensitive. This paper presents an approach to extract the pure unperturbed frequency of the dipole mode, not altered by the response to coupled HOMs. An analytic model based on the superposition law applied to electromagnetic fields has been developed to characterize the response of coupled HOMs in the S-parameter measurement. The model has been further verified with numerical simulation in the CST-Studio software by analyzing an approximate single-mode response CBPM. The correction method was applied on a wideband CBPM prototype for the pre-research plan of future high repetition rate hard XFEL at the Chinese Academy of Engineering Physics.

Improved Design of Uniform SIW Leaky Wave Antenna With Suppression of Unwanted Mode

Improved Design of Uniform SIW Leaky Wave Antenna With Suppression of Unwanted Mode

Variant Characteristic Mode Equations Using Different Power Operators for Material Bodies

This paper is concerned with the electromagnetic theory of characteristic modes for general homogeneous material bodies using surface integral equations (SIEs), which are computationally more efficient than using volume integral equations (VIEs). However, the generalized eigenvalue equations (GEEs) based on SIEs are prone to generate spurious or extra modes, unlike their counterparts based on VIEs. In this work, several variant SIE-based GEEs are proposed to eliminate the spurious/extra modes and improve the numerical performances. These variant GEEs share a common point that the reactive power operator related to the material domain should be removed from the left-hand sides of GEEs by explicitly enforcing the boundary condition, because it seems sensitive to incur unwanted modes. These variant GEEs are diverse in choosing the active power operator related to the material domain on the right-hand sides to suit different medium parameters, which even can be dropped directly for lossless cases. Numerical results show that these variant GEEs do not generate spurious/extra resonant modes in observed frequency range, and at least one of the variants would have acceptable numerical accuracy.

Scientific Visualization of the Results of a Numerical Experiment of the Nonlinear Dynamics of a Nanoscale Beam Structure

The paper presents the results of scientific visualization of the nonlinear dynamics of contact interaction of a nanoscale beam structure under the action of an external harmonic load. The beam structure consists of two beams obeying the kinematic hypotheses of Euler-Bernoulli and S.P. Timoshenko. The constructed mathematical model takes into account geometric and constructive nonlinearities. The size-dependent behavior of the structure is implemented on the basis of the modified moment theory of elasticity. The resulting system of partial differential equations is reduced to a system of ordinary differential equations by the second order finite difference method. The Cauchy problem is solved by the fourth order Runge-Kutta method. In this work, using the methods of scientific visualization of the results of applying the methods of nonlinear dynamics, the influence of the size-dependent parameter and external load on the vibrations of the beam structure is investigated. As methods for studying nonlinear dynamics, the work uses wavelet spectra based on the mother Morlet, Fourier power spectra, signals. The use of scientific visualization methods makes it possible to develop specific recommendations for the operating conditions of the beam structure. This, in turn, makes it possible to avoid unwanted vibration modes of beam nanostructures, which are widely used as sensitive elements of sensors of micro and nano electromechanical systems.

A Compact Omnidirectional Patch Antenna With Ultrawideband Harmonic Suppression

This communication presents an effective approach to design a novel patch antenna with the simultaneous realization of ultrawideband harmonic suppression, omnidirectional radiation, and compact configuration. Initially, the resonant frequency and the input impedance of a short-circuited ring-patch (SCRP) antenna are deeply analyzed. It indicates that the resonant frequency of the desired TM01 mode could be effectively lowered by properly adjusting the radius of the ring patch. Thus, a compact dimension could be achieved. Meanwhile, an attractive property of a large input resistance ratio between TM01 and high-order modes is obtained. Then, different from the previous works with the aid of additional harmonic rejected elements, a compact $50~\Omega $ coplanar waveguide (CPW) with the function of an impedance transformer is employed to feed the antenna. After the proper design is executed, the TM01 mode could be well matched and those unwanted high-order modes are totally mismatched at impedance. The ultrawideband harmonic suppression is thus obtained and additional elements are not required at all. The equivalent circuit model is presented to give a clear insight into its working principle and a unified design procedure is generalized. Finally, two prototypes are designed, fabricated, and tested. The simulated and measured results are in good agreement with each other, certifying the validity of the proposed method. All these results show that an ultrawideband harmonic suppression (7 ${f}_{0}$ , ${f}_{0}$ is the operating frequency) has been successfully achieved while keeping the omnidirectional radiation, low profile ( $0.037~\lambda _{0}$ ), and compact patch size ( $0.096\,\,\lambda _{0} \times 0.096\,\,\lambda _{0}$ , $\lambda _{0}$ is the free-space wavelength at ${f}_{0}$ ).

Optimal Design of TSM Langasite Resonator for High-Temperature Applications: A Review.

In this review article, we address two vital design considerations that govern the high-temperature operation of a thickness-shear mode langasite resonator: 1) electrode design and 2) electrode material. Optimal electrode designs to mitigate unwanted spurious modes and achieve a high Q-factor for fundamental and higher overtone modes have been discussed in great detail. Governing equations that determine the size, shape, and orientation of these electrodes have also been presented. In addition, the suitability of six platinum-group metals as electrode materials for high-temperature resonators have been assessed and summarized. Furthermore, the adhesion to the substrate, electrical conductivity, thermal stability, and various other temperature-dependent properties of these metals have been discussed. Finally, several combinations and operating ranges of these electrode materials have been thoroughly evaluated.

Scalable selective high order mode pass filter architecture with asymmetric directional couplers.

We propose a novel design architecture to realize scalable selective mode filter based on the asymmetric directional coupler structure. In this structure, any arbitrary high-order mode can pass, whereas other unwanted modes are blocked. Furthermore, multiple optical modes can be blocked by only adjusting the structural parameters. As a proof of concept, we experimentally demonstrated a three-mode device and the scalability of the proposed structure is demonstrated by another design of four-mode filter. The proposed architecture offers scalability and high-design flexibility, and it has excellent potential to be used in advanced mode division multiplexing optical networks.

An Epsilon-Near-Zero (ENZ) Based, Ultra-Wide Bandwidth Terahertz Single-Polarization Single-Mode Photonic Crystal Fiber

A novel terahertz (THz) photonic crystal fiber (PCF) that yields single-polarization single-mode (SPSM) propagation over an ultra-wide bandwidth is designed and analyzed. The PCF is based upon a triangle-based lattice of air holes in a high resistivity silicon substrate with three selectively-filled rectangular slots introduced into the core area. Four air holes surrounding the core region are chosen to be loaded with an epsilon-near-zero (ENZ) material. The configuration, and the large loss of the ENZ material establish a large loss difference (LD) between the two fundamental propagating polarization modes, and any higher order modes. When the central slot of the three in the core is filled with a gain material, and the adjacent two slots are air-filled, the LD values between the one desired propagating mode, and all other modes are significantly enhanced. Consequently, essentially only the desired mode will exist in the PCF after a short propagation distance resulting in the SPSM behavior. The optimized design provides large LD values, greater than 9.4 dB/cm, over a SPSM spectrum of 0.64 THz (from 1.10 to 1.74 THz), which, to the best of our knowledge, is the widest SPSM bandwidth achieved to date in the THz regime. The unwanted modes are 30 dB smaller than the wanted mode after a 3.2 cm length of the PCF. This outcome is highly desired for polarization sensitive THz communications, and sensor systems that rely on waveguiding structures.

Low-Profile High-Directivity Circularly-Polarized Differential-Fed Patch Antenna With Characteristic Modes Analysis

A low-profile high-directivity circularly polarized patch antenna is proposed in this article. Two orthogonal slots are etched out in the middle of a square patch. As a result, radiation patterns of original TM12/21 modes are reshaped to the desired ones that are similar to those of TM10/01 modes, leading to significant enhancement of broadside radiation. To accommodate the modified current distributions of the two operating modes, a pair of perturbed stubs is proposed and then loaded on the slot-loaded patch, which leads to the quadrature-phase difference. In order to suppress unwanted modes that tilt the radiation pattern away from broadside direction, differential-fed configuration instead of single-fed one is adopted to excite modified TM12/21 modes. Characteristic modes theory (CMT) is used to analyze the input impedance of differential-fed patch antenna and determine the two feeding positions. Next, the proposed antenna is compared with three conventional ones, which reveals its superiority in the highest directivity. Besides, it is also discussed how to determine the size of the finite ground plane. We find that the size of 1.5λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 1.5λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> can increase directivity slightly and effectively prevent radiation pattern from tilting tendency. Finally, a prototype antenna is fabricated and measured for validation.

Improved Design of Uniform SIW Leaky Wave Antenna by Considering the Unwanted Mode

An improved design for the uniform substrate integrated waveguide (SIW) long slot leaky wave antenna (LWA) is presented in this communication. To achieve the desired sidelobe level (SLL), beamwidth and beam direction, proper distributions for leakage constant and phase constant should be created along the slot. An accurate calculation of leakage and phase constants is performed by considering an unwanted mode, in addition to the leaky mode, along the uniform LWA structure. Considering this unwanted mode in the design procedure results in good consistency between the desired and designed radiation patterns. The presented LWA realizes a Taylor distribution with predefined SLL of -25 dB at 17 GHz. The radiation pattern, amplitude distribution, and phase distribution of the designed antenna are compared with the desired ones. This comparison shows the high accuracy of the proposed design procedure. Measurement results are also presented at 16.5, 17, and 17.5 GHz for validation of the design procedure.

Enhancing the modal purity of orbital angular momentum photons

Orbital angular momentum (OAM) beams with topological charge l are commonly generated and detected by modulating an incoming field with an azimuthal phase profile of the form exp(ilϕ) by a variety of approaches. This results in unwanted radial modes and reduced power in the desired OAM mode. Here, we show how to enhance the modal purity in the creation and detection of classical OAM beams and in the quantum detection of OAM photons. Classically, we combine holographic and metasurface control to produce high purity OAM modes and show how to detect them with high efficiency, extending the demonstration to the quantum realm with spatial light modulators. We demonstrate ultra-high purity OAM modes in orders as high as l = 100 and a doubling of dimensionality in the quantum OAM spectrum from a spontaneous parametric downconversion source. Our work offers a simple route to increase the channel capacity in classical and quantum communication using OAM modes as a basis.

Compact SIW Diplexers and Dual-Band Bandpass Filter With Wide-Stopband Performances

A realization method of wide-stopband substrate-integrated waveguide (SIW) diplexers and dual-band bandpass filters (BPFs) with widely realizable frequency ratios is presented using modified intrinsic modes suppression technique. Design procedures of dual-band and wide-stopband characteristics for different frequency ratios are systematically summarized by sufficiently using the limited design degrees of freedom. The mixed dual- and single-mode cavities and the offset external coupling slots could help to flexibly control the dual-band frequencies and bandwidths. Meanwhile, the internal coupling windows could produce predictable wide-stopband performance when properly arranged at the positions of weakest electric fields of specific unwanted spurious modes determined by the frequency distributions for the required frequency ratio. Two SIW diplexers and an SIW dual-band BPF with frequency ratios of 1.225, 1.5, and 1.425 are synthesized and designed for demonstration, extending the stopband up to $1.79{f} _{0}$ with better than 24 dB rejection level, $2.05{f} _{0}$ and $2.34{f} _{0}$ with better than 20 dB rejection level, respectively.

An Eigenmode Correlation‐Based Algorithm for Approaching Antenna Optimal Currents With Multiple Feeds

AbstractA feeding algorithm has been proposed for the excitation of the optimal electric current of Rayleigh quotient form optimization problems. Modal weighting coefficients of the corresponding generalized eigenvalue problem are calculated to form the correlation information between the desired and all other unwanted modes. Based on this information, multiple feeds are arranged to approximate the desired current distributions. The eigencurrent distribution is also utilized for some shape modification to the radiator for a possible feed number reduction. Three canonical optimization problems for electrically small antennas, minimum Q, maximum G/Q, and maximum G are studied and discussed. Several numerical examples with a certain arbitrary shape and electrical size are given. With the proposed feeding algorithm, calculated and simulated current distributions and performance indices are close to the theoretical optimum solution. Finally, a practical feeding network design for a classic characteristic mode problem is shown as a validation.