Large Number Of Modes Research Articles

Investigation of the Interaction of Water Molecules with the Surface of a Quartz Tube Using Diode Laser Spectroscopy

The behavior of water molecules at the density of 1013 to 1016 cm−3 in a fused silica tube at room temperature has been studied. The number of molecules in the gas phase initially injected in the tube is comparable to the number of molecules adsorbed on the walls of the pre-evacuated tube. The concentrations of molecules in the gas phase were measured by diode laser spectroscopy with an external optical cavity. An off-axis alignment of the cavity with a large set number of transverse modes was used, which made it possible to measure the concentrations of molecules with narrow absorption profiles, temporal resolution of 5 s, and the accuracy better than ±5%. A strong interaction of molecules with the walls was observed. The time behavior of molecules in the gas phase both after the injection of gas into the vacuum volume and after its rapid evacuation under dynamic equilibrium between capture and desorption processes is non-exponential. The characteristic time of these processes 10−11 to 102 s depends on the redistribution of molecules between the gas and the near-wall layers, which is governed by the physical and chemical adsorption mechanisms. The theoretical results on the kinetics of the processes are in good agreement with the experimental data.

Nonlinear dynamic response and transmissibility of a flexible rotor system mounted on viscoelastic elements

In this paper, the three-dimensional nonlinear dynamics and force transmissibility characteristics of a flexible shaft-disk rotor system are investigated numerically. The rotor system is comprised of a flexible shaft supporting a rigid disk, and the shaft ends are mounted on viscoelastic elements (bearing/support). The dynamic model of the system is developed considering the coupled axial and bending–bending elastic motions of the shaft while the disk is modeled as a rigid body. The equations of motion (EOM) obtained using Lagrange’s equation are discretized using the assumed mode method utilizing large number of modes to examine the system dynamics accurately for multiple resonance regions. The EOM are then linearized to obtain the natural frequencies to construct the Campbell diagrams and investigate the influence of the bearing stiffness and disk location on critical speeds and whirling modes. The harmonic balance method combined with the pseudo-arclength continuation method is used to construct the nonlinear frequency response and force transmissibility curves due to disk unbalance force. The effects of the bearing stiffness and damping, unbalance force magnitude and disk location on the nonlinear multi-mode primary frequency response and force transmissibility are then thoroughly investigated.

Amplitude-multiplexed readout of single photon detectors based on superconducting nanowires

The realization of large-scale photonic circuit for quantum optics experiments at telecom wavelengths requires an increasing number of integrated detectors. Superconductive nanowire single photon detectors (SNSPDs) can be easily integrated on chip and they can efficiently detect the light propagating inside waveguides. The thermal budget of cryostats poses a limit on the maximum number of elements that can be integrated on the same chip due to the thermal impact of the readout electronics. In this paper, we propose and implement a novel scheme able for an efficient reading of several SNSPDs with only one readout port, enabling the realization of photonic circuits with a large number of modes.

High-efficiency sorting and measurement of orbital angular momentum modes based on the March–Zehnder interferometer and complex phase gratings

With the recent exponential developments of high-speed optical communications, traditional optical communications systems are showing their limitations. Over the past decade, the orbital angular momentum (OAM) is attracting more and more research attention, due to its orthogonality between two OAM states, which supplies new degrees of freedom and can be used in fiber or free-space optical communication systems for increasing the communication capacity. Here, we propose a novel OAM de/multiplexing method based on the combination of the March–Zehnder interferometer (MZI) implanted pair of dove prisms (DP) (MZI-DP) and a complex phase gratings (CPG), which can be used to sort and detect multiple OAM modes simultaneously. The fundamental Gaussian beam illuminates the spatial light modulator (SLM) to produce the multiplexed Laguerre–Gaussian modes with different topological charges. The MZI-DP system can sort different OAM modes into odd and even values of topological charges. And then, the designed CPGs perform the concrete measurements of different sorted OAM modes. The propagating directions of different modes are arbitrarily steered by two CPGs in two MZI-DP outputting planes. With this designed system, we can simultaneously sort the OAM modes into odd and even values of topological charges, as well as performing the measurement of a large number of OAM modes without any overlapping between adjacent modes. Furthermore, we have simultaneously demultiplexed 50 different OAM modes and controlled their propagation directions successfully, which shows high-recognition rates and superior performances.

Computing the Double-Gyroaverage Term Incorporating Short-Scale Perturbation and Steep Equilibrium Profile by the Interpolation Algorithm

In the gyrokinetic model and simulations, when the double-gyroaverage term incorporates the combining effect contributed by the finite Larmor radius, short scales of the perturbation, and steep gradient of the equilibrium profile, the low-order approximation of this term could generate unignorable error. This paper implements an interpolation algorithm to compute the double-gyroaverage term without low-order approximation to avoid this error. For a steep equilibrium density, the obvious difference between the density on the gyrocenter coordinate frame and the one on the particle coordinate frame should be accounted for in the quasi-neutrality equation. A Euler–Maclaurin-based quadrature integrating algorithm is developed to compute the quadrature integral for the distribution of the magnetic moment. The application of the interpolation algorithm to computing the double-gyroaverage term and to solving the quasi-neutrality equation is benchmarked by comparing the numerical results with the known analytical solutions. Finally, to take advantage of the interpolation solver clearer, the numerical comparison between the interpolation solver and a classical second order solver is carried out in a constant theta-pinch magnetic field configuration using SELALIB code. When the equilibrium profile is not steep and the perturbation only has the non-zero mode number along the parallel spatial dimension, the results computed by the two solvers match each other well. When the gradient of the equilibrium profile is steep, the interpolation solver provides a bigger driving effect for the ion-temperature-gradient modes, which possess large polar mode numbers.

Simultaneous Measurement of MDL and DMGD in FMFs by Analyzing the Rayleigh Backscattering Amplitudes

In this paper, we propose and experimentally demonstrate a novel method for simultaneously measuring mode-dependent loss (MDL) and differential modal group delay (DMGD) in few mode fibers (FMFs). This method is based on analyzing the Rayleigh backscattering amplitudes obtained with a special nondestructive measurement system. With this method, the MDL and DMGD in about 9.8 km 3-mode fiber are successfully measured, which are 0.186 dB and 3.581 ps/m between LP 01 and LP 11a and 0.157 dB and 3.604 ps/m between LP 01 and LP 11b , respectively. Then, the experimental results of DMGD and MDL with different methods are compared, which show that the values of DMGD and MDL obtained by the proposed method agree well with that using the time-of-flight (TOF) and conventional transmission methods, respectively. In addition, it is also demonstrated that the proposed scheme can be scalable to measure FMFs with a large number of modes.

Hydraulic drive of vibration stand for testing the robotic systems units by random vibration method

This paper reviews the development of hydraulic drive of vibration stand for testing the robotic systems units (technical products) for vibration resistance by random vibration method. The physical laboratory model was developed for studying the dynamic properties of a test stand which allows to realize smooth sweep excitation frequency f1 within the range of 8…56 Hz and detect resonant modes. The new design allows adequate simulation of vibration during transportation and using gapless narrow-band spectrum of random vibrations from power units. The stand allows to implement a large number of test modes that demonstrate the prospects of its application for testing technical products by random vibration method.

Cosmological inference from galaxy-clustering power spectrum: Gaussianization and covariance decomposition

Likelihood fitting to two-point clustering statistics made from galaxy surveys usually assumes a multivariate normal distribution for the measurements, with justification based on the central limit theorem given the large number of overdensity modes. However, this assumption cannot hold on the largest scales where the number of modes is low. Whilst more accurate distributions have previously been developed in idealized cases, we derive a procedure suitable for analysing measured monopole power spectra with window effects, stochastic shot noise, and the dependence of the covariance matrix on the model being fitted all taken into account. A data transformation is proposed to give an approximately Gaussian likelihood, with a variance--correlation decomposition of the covariance matrix to account for its cosmological dependence. By comparing with the modified-$t$ likelihood derived under the usual normality assumption, we find in numerical tests that our new procedure gives more accurate constraints on the local non-Gaussianity parameter $f_\mathrm{NL}$, which is sensitive to the large-scale power. A simple data analysis pipeline is provided for straightforward application of this new approach in preparation for forthcoming large galaxy surveys such as DESI and Euclid.

Low-Complexity Detection for Multiple-Mode OFDM with Index Modulation

Multiple-mode orthogonal frequency division multiplexing with index modulation (MM-OFDM-IM) is a recently proposed multicarrier transmission technique, which transmits distinguishable constellations (modes) according to their full permutations. For an uncoded system, the maximum-likelihood (ML) detector achieves the optimal performance, but its computational complexity is extremely high due to a large number of symbol modes. Existing low-complexity detectors may output an illegal mode permutation, which leads to catastrophic errors in the demodulation. redIt is well known that sphere decoding algorithms can reduce the computational complexity effectively and achieve near-ML bit error rate (BER) performance. In this paper, employing a sphere-decoding-like algorithm, we propose a novel detector, which can achieve near-ML performance with considerably low computational complexity. For coded MM-OFDM-IM systems, we propose a simplified log-likelihood ratio (LLR) calculation algorithm to achieve near-optimal coded BER performance with considerably low computational complexity. Computer simulation and numerical results demonstrate that the proposed detectors can achieve outstanding performance at the cost of low computational complexity.

Sequential clustering of synchrosqueezed wavelet transform coefficients for efficient modal identification

Time–frequency-based signal processing for efficient modal parameter identification is the theme of this paper. For this purpose, recently developed synchrosqueezed transformation is adopted along with sequential clustering to bypass heuristic intermittency required in traditional wavelet transform-based modal identification. Here, k-means algorithm is used to locate the energy content of the recorded response in different frequency scales, where synchrosqueezing offers better resolution. The rationale behind the use of the unsupervised learning lies in its ability to segregate the energy content of the signals without any requirement of prior training. Two validation exercises are presented to establish the performance of the proposed identification strategy. Finally, the response of a thin beam tested in the laboratory is used that has large number of active modes with many of them are closely spaced. Results presented in this paper clearly proves the efficiency of the proposed algorithm that can be adopted for modal identification for structural health monitoring and control.

Axisymmetric density waves in Saturn’s rings

Density waves in Saturn's rings are usually tightly wrapped spiral patterns generated by resonances with either Saturn's moons or structures inside the planet. However, between the Barnard and Bessel Gaps in the Cassini Division (i.e. between 120,240 and 120,300 km from Saturn's spin axis), there are density variations that appear to form an axisymmetric density wave consisting of concentric zones of varying densities that propagate radially through the rings. Axisymmetric waves cannot be generated directly by a satellite resonance, but instead appear to be excited by interference between a nearby satellite resonance and normal mode oscillations on the inner edge of the Barnard Gap. Similar axisymmetric waves may exist just interior to other resonantly confined edges that exhibit a large number of normal modes, including the Dawes ringlet in the outer C ring and the outermost part of the B ring.

The finite-element time-domain method for elastic band-structure calculations

The finite-element time-domain method for elastic band-structure calculations is presented in this paper. The method is based on discretizing the appropriate equations of motion by finite elements, applying Bloch boundary conditions to reduce the analysis to a single unit cell, and conducting a simulation using a standard time-integration scheme. The unit cell is excited by a wide-band frequency signal designed to enable a large number of modes to be identified from the time-history response. By spanning the desired wave-vector space within the Brillouin zone, the band structure is then robustly generated. Bloch mode shapes are computed using the well-known concept of modal analysis, especially as implemented in an experimental setting. The performance of the method is analyzed in terms of accuracy, convergence, and computation time, and is compared to the finite-difference time-domain method as well as to a direct finite-element (FE) solution of the corresponding eigenvalue problem. The proposed method is advantageous over FD-based methods for unit cells with complex geometries, and over direct FE in situations where the formulation of an eigenvalue problem is not straightforward. For example, the new method makes it possible to accurately solve a time-dependent Bloch problem, such as the case of a complex unit cell model of a topological insulator where an internal fluid flow or other externally controlled physical fields are present.

High Resolution 2-D Electromagnetic Vortex Imaging Using Uniform Circular Arrays

The vortex electromagnetic (EM) wave carrying orbital angular momentum (OAM) has been introduced in radar imaging to take full advantage of its special phase front. It has been demonstrated that the azimuthal and range resolution are decided by the number of OAM mode and frequency bandwidth respectively. In this paper, theelevation resolution of the vortex wave transmitted by a uniform circular array is derived for the first time through the calculation of correlation function of two closely adjacent points. Since large number of OAM modes should be applied to achieve high-resolution image, which is difficult to realize in practice, the sparse Bayesian learning (SBL) is introduced to reconstruct the targets for 2-D EM vortex imaging. Simulation results show that the targets located at different elevation angle can be reconstructed correctly and higher resolution can be achieved based on the SBL reconstruction algorithm.

重点脆弱生态区生态恢复模式现状与发展方向

PDF HTML阅读 XML下载 导出引用 引用提醒 重点脆弱生态区生态恢复模式现状与发展方向 DOI: 10.5846/stxb201909051848 作者: 作者单位: 中国科学院生态环境研究中心,中国科学院生态环境研究中心,中国科学院生态环境研究中心 作者简介: 通讯作者: 中图分类号: 基金项目: 中国科学院科技服务网络计划项目（KFJ-STS-ZDTP-036-7）；国家重点研发计划课题（2016YFC0501603，2016YFC0502102）；国家自然科学基金项目（41807153）；中国科协青年人才托举工程（2017QNRC001）；中国科学院青年创新促进会（2018056） Ecological restoration in the key ecologically vulnerable regions: Current situation and development direction Author: Affiliation: State Key Laboratory of Urban and Regional Ecology,Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences,State Key Laboratory of Urban and Regional Ecology,Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences,State Key Laboratory of Urban and Regional Ecology,Research Center for Eco-Environmental Sciences,Chinese Academy of Sciences Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:生态系统退化严重影响区域可持续发展，控制生态系统退化和生态恢复已成为重要议题。我国脆弱生态区分布广泛，生态系统退化显著，荒漠化、水土流失、石漠化等问题突出，针对这一问题，我国已开展一系列生态恢复工程，积累了数量众多的生态恢复模式。在梳理重点脆弱生态区（北方风沙区、青藏高寒区、干旱荒漠区、黄土高原区和喀斯特地区）生态环境问题现状基础上，整理了针对不同脆弱生态区及亚区的生态恢复模式，构建了一套全面评估生态恢复模式的综合效益的指标体系，结合实地调查和数据收集，评估了脆弱生态区的生态恢复模式的综合效益，基于此提出生态恢复实践中应将生物多样性和生态系统服务提升作为重要目标，同时加强针对生态恢复模式的成本效益分析，综合设计生态恢复管理制度，搭建重点脆弱生态区生态恢复信息平台可为生态恢复模式筛选和优化提供决策支持。 Abstract:Ecosystem degradation seriously affects regional sustainable development. Therefore, ecosystem restoration has become a critical issue. China's ecologically vulnerable regions are widely distributed and the degradation of ecosystems is remarkable. Desertification, soil erosion, and rocky desertification have become prominent problems. To address these problems, China has launched a series of ecological restoration projects and accumulated a large number of ecological restoration modes. This study clarified the current situation and eco-environmental problems in the key ecologically vulnerable regions (northern wind-sand, Tibetan Plateau, arid desert, Loess Plateau, and karst regions), listed the ecological restoration modes according to the ecologically vulnerable regions and their sub-regions, and developed a comprehensive evaluation index system for the restoration modes. The comprehensive benefits of ecological restoration were assessed based on field surveys and data collection. This study suggested that biodiversity and ecosystem services should be promoted as an important goal for restoration practices, the cost-benefit analysis of ecological restoration modes should be strengthened, and a comprehensive design of an ecological restoration management system should be developed. The establishment of an ecological restoration information platform for ecologically vulnerable regions can provide decision support for the selection and optimization of ecological restoration modes. 参考文献 相似文献 引证文献

Measurement of gravity acceleration by cold atoms in a harmonic trap using Kapitza-Dirac pulses

The interferometry of two Kapitza-Dirac (KD) pulses acting on cold atoms in a harmonic oscillator potential well is investigated by Feynman path integral method. The wave function and density distribution function of the system at a given time are calculated analytically by using the propagator under the action of an external field. The first KD pulse acts on cold atoms to produce a large number of modes in the harmonic oscillator potential well. The maximum value of wave packet of mode 0 is larger than those of other modes. These modes evolve along different paths. The external field changes the phase of each mode and makes the evolution path of the mode deviate from that without the external field. When the second KD pulse is added, it splits the mode of the first KD pulse, and thus generates more modes. These modes will evolve along different paths under the action of external field and harmonic potential well, and interfere with each other. At the moment of measurement, all the wave packets are separated without overlapping. The effect of the external field does not change the magnitude of the density distribution at the time of measurement, but makes the wave packet of each mode shift. The phase difference between adjacent modes decreases linearly with the increase of field intensity. When the external field is a gravity field, we calculate the Fisher information and the Cramer-Rao lowér bound. The Fisher information is proportional to the mass of atoms and inversely to the third power of harmonic potential well frequency. We can improve the measurement accuracy of interferometer by reducing the frequency of harmonic potential well and increasing atomic mass. When the initial state is the ground state of the harmonic potential well, the accuracy of the gravity acceleration measured by the interference device can be obtained to be 10<sup>–9</sup> by using the experimental parameters. The initial state is the ground state of the harmonic potential well and the external field, and the calculation result indicates that the measurement accuracy will decrease. At the same time, the enhancement of inter-atomic repulsion and attraction interaction will also lead the measurement accuracy to increase.

Mode Shift Keying for Reconfigurable MIMO Antennas: Performance Analysis and Antenna Design

Space-shift-keying (SSK) and spatial modulation (SM) enable multiple antenna transmission systems to convey information on antenna indices. While SSK/SM helps reduce the number of radio frequency (RF) chains, large numbers of antennas and low spatial correlations are required to achieve high data rates. This work investigates the use and design of multifunctional reconfigurable antennas (MRAs) for SSK/SM based transmission where a single-element MRA generates large numbers of modes. To enhance legacy SSK/SM performance while reducing RF hardware complexity, we propose single- and multi-carrier antenna mode-shift keying (MoSK) and mode modulation (MoM) schemes facilitated by MRAs. Based on an error probability analysis, we determine criteria for MRA design and mode set selection suitable for MoSK/MoM. We also develop two MRA designs and investigate their performances over Rayleigh fading channels. We argue that by creating MRA modes with low pattern correlations, channel correlations can be reduced to improve the detection performance. Extensive simulations demonstrate that MoSK/MoM performance exceeds that of SSK/SM along with significant complexity reduction. For instance, a single-carrier MoSK/MoM using a single MRA with 8 modes achieves about 2 dB gain compared to legacy SSK/SM requiring 8 antennas, and by multi-carrier MoSK/MoM using 4 subcarriers, an MRA with 32 modes can attain an error rate performance comparable to this single-carrier system.

Hermite–Gaussian mode sorter

The Hermite-Gaussian (HG) modes, sometimes referred to as transverse electromagnetic modes in free space, form a complete and orthonormal basis that have been extensively used to describe optical fields. In addition, these modes have been shown to be helpful in enhancing information capacity of optical communications as well as achieving super-resolution imaging in microscopy. Here we propose and present the realization of an efficient, robust mode sorter that can sort a large number of HG modes based on the relation between HG modes and Laguerre-Gaussian (LG) modes. We experimentally demonstrate the sorting of 16 HG modes, and our method can be readily extended to a higher-dimensional state space in a straightforward manner. We expect that our demonstration will have direct applications in a variety of fields including fiber optics, classical and quantum communications, as well as super-resolution imaging.

Multipolar gravitational waveforms and ringdowns generated during the plunge from the innermost stable circular orbit into a Schwarzschild black hole

We study the gravitational radiation emitted by a massive point particle plunging from slightly below the innermost stable circular orbit into a Schwarzschild black hole. We consider both even- and odd-parity perturbations and describe them using the two gauge-invariant master functions of Cunningham, Price, and Moncrief. We obtain, for arbitrary directions of observation and, in particular, outside the orbital plane of the plunging particle, the regularized multipolar waveforms, i.e., the waveforms constructed by summing over of a large number of modes, and their unregularized counterparts constructed from the quasinormal-mode spectrum. They are in excellent agreement and our results permit us to especially emphasize the impact on the distortion of the waveforms of (i) the harmonics beyond the dominant $(\ell=2,m=\pm 2)$ modes and (ii) the direction of observation, and therefore the necessity to take them into account in the analysis of the last phase of binary black hole coalescence.

Linear analysis on the interfacial instability of a spherical liquid droplet subject to a radial vibration

Precursory surface standing waves for liquid atomization occur on a spherical droplet subjected to a radial time-periodic force. In this paper, we carried out a linear stability analysis on the spherical Faraday instability. With the Floquet analysis, a derived difference equation gives the dispersion relation between the Floquet exponent and the spherical modes. For inviscid instability, the problem can also be reduced to the standard Mathieu equation as the same as its planar counterpart, but the parameters in the equation correspond to different quantities due to the spherical configuration. The analysis shows that increasing the density ratio of the ambient fluid to the droplet narrows the range of possibly excited spherical modes under the same forcing condition. For viscous instability, an additional parameter corresponding to the viscous effects was introduced into the difference equation. With increasing the droplet viscosity, the surface waves with large mode numbers are stabilized and hence a larger forcing amplitude is required to cause instability. Furthermore, the most-unstable spherical mode of the largest growth rate excited in the experimental condition is determined and discussed for its physical interpretation for droplet atomization caused by Faraday instability.

Alternative description of gravitational radiation from black holes based on the Regge poles of the S -matrix and the associated residues

We advocate for an alternative description of gravitational radiation from black holes based on complex angular momentum techniques (analytic continuation of partial wave expansions, duality of the ${\cal S}$-matrix and effective resummations involving its Regge poles and the associated residues, Regge trajectories, semiclassical interpretations, etc.). Such techniques, which proved to be very helpful in various areas of physics to describe and analyze resonant scattering, were only marginally used in the context of black hole physics. Here, by considering the multipolar waveform generated by a massive particle falling radially from infinity into a Schwarzschild black hole, we show that they could play a fundamental role in gravitational-wave physics. More precisely, from the multipole expansion defining the Weyl scalar $\Psi_4$, we extract the Fourier transform of a sum over Regge poles and their residues which can be evaluated numerically from the associated Regge trajectories. This Regge pole approximation permits us to reconstruct, for an arbitrary direction of observation, a large part of the multipolar waveform $\Psi_4$. In particular, it can reproduce with very good agreement the quasinormal ringdown as well as with rather good agreement the tail of the signal. This is achieved even if we take into account only one Regge pole and, if a large number of modes are excited, the result can be improved by considering additional poles. Moreover, while quasinormal-mode contributions do not provide physically relevant results at "early times" due to their exponentially divergent behavior as time decreases, it is not necessary to determine from physical considerations a starting time for the Regge ringdown.