Multichannel singular spectrum analysis of the axial atmospheric angular momentum

Earth＇s variable rotation is mainly produced by the variability of the AAM（atmospheric angular momentum）. In particular, the axial AAM component X_3, which undergoes especially strong variations,induces changes in the Earth＇s rotation rate. In this study we analysed maps of regional input into the effective axial AAM from 1948 through 2011 from NCEP/NCAR reanalysis. Global zonal circulation patterns related to the LOD（length of day） were described. We applied MSSA（Multichannel Singular Spectrum Analysis） jointly to the mass and motion components of AAM, which allowed us to extract annual, semiannual, 4-mo nth, quasi-biennial, 5-year, and low-frequency oscillations. PCs（Principal components） strongly related to ENSO（El Nino southern oscillation） were released. They can be used to study ENSO-induced changes in pressure and wind fields and their coupling to LOD. The PCs describing the trends have captured slow atmospheric circulation changes possibly related to climate variability.

Beam alignments based on the spectrum decomposition of orbital angular momentums for acoustic-vortex communications

Given the enhanced channel capacity of wave chirality,acoustic communications based on the orbital angular momentum(OAM)of acoustic-vortex(AV)beams are of significant interest for underwater data transmissions.However,the stringent beam alignment is required for the coaxial arrangement of transceiver arrays to ensure the accuracy and reliability of OAM decoding.To avoid the required multiple measurements of the traditional orthogonality based algorithm,the beam alignment algorithm based on the OAM spectrum decomposition is proposed for AV communications by using simplified ring-arrays.Numerical studies of the single-OAM and OAM-multiplexed AV beams show that the error of the OAM spectrum increases with the translation distance and the deflection angle of the transceiver arrays.To achieve an ideal arrangement,two methods of the single-array translation alignment and the dual-array deflection alignment are developed based on the least standard deviation of the OAM spectrum(SD-OAM).By decreasing the SD-OAM towards zero using transceiver arrays of 16 transmitters and 16 receivers,accurate beam alignments are accomplished by multiple adjustments in three dimensions.The proposed method is also demonstrated by experimental measurements of the OAM dispersion and the SD-OAM for misaligned beams.The results demonstrate the feasibility of the rapid beam alignment based on the OAM spectrum decomposition by using simplified transceiver ring-arrays,and suggest more application potentials for acoustic communications.

Tailoring OAM spectrum of high-order harmonic generation driven by two mixed Laguerre–Gaussian beams with nonzero radial nodes

The extreme ultraviolet(XUV)light beam carrying orbital angular momentum(OAM)can be produced via high-order harmonic generation(HHG)due to the interaction of an intense vortex infrared laser and a gas medium.Here we show that the OAM spectrum of vortex HHG can be readily tailored by varying the radial node(from 0 to 2)in the driving laser consisting of two mixed Laguerre-Gaussian(LG)beams.We find that due to the change in spatial profile of HHG,the distribution range of the OAM spectrum can be broadened and its shape can be modified by increasing the radial node.We also show that the OAM mode range becomes much wider and its distribution shape becomes more symmetric when the harmonic order is increased from the plateau to the cutoff when the driving laser has the nonzero radial nodes.Through the map of coherence length and the evolution of harmonic field in the medium,we reveal that the favorable off-axis phase-matching conditions are greatly modified due to the change of intensity and phase distributions of driving laser with the radial node.We anticipate this work to stimulate some interests in generating the XUV vortex beam with tunable OAM spectrum through the gaseous HHG process achieved by manipulating the mode properties of the driving laser beam.

质心系能量200 GeV下Au-Au碰撞中的轻核产生

STAR实验组最新发布的氘、氚等一系列实验数据,为轻核产生机制的理论研究提供了新机遇.利用核子重组合模型,系统研究质心系能量200 GeV下Au-Au碰撞中氘、氚、^(3)He的横动量谱、产额快度密度以及平均横动量.氘、氚的理论计算结果能够自洽描述其最新实验数据,^(3)He的理论预言为相对论重离子对撞机(RHIC)实验进一步测量提供理论依据.研究结果表明,核子重组合是RHIC高能区Au-Au碰撞中氘、氚、^(3)He等轻核的主要产生来源.

基于轨道角动量谱分解的多通道水声通信及系统实现

水声通信能够实现信息的长距离传输,在深海探索和军事领域发挥重要作用.然而,由于换能器带宽和声波波长的影响,水声通信的发展与应用受到传输衰减大和信道容量低以及频谱利用率不足的限制.本文结合复用涡旋声束(AV)和轨道角动量(OAM)谱分解技术,研究了基于环形收发阵列的多通道水声通信方法,并系统实现.基于发送数据的ASCII编码,利用全息信号的幅度和相位调制,驱动环形换能器阵列发射沿轴传输的多通道OAM复用涡旋声束,并通过环形换能器阵列的信号采样和OAM谱分解完成复用涡旋声束的OAM模态解码,实现了高效的并行数据传输.构建了8声源环形收发阵列和全息相控系统,开展了6通道水声通信研究,成功实现了复用涡旋声场的实验测量和OAM解码,并与传统单通道二进制串行数据通信进行性能比较.理论和实验结果证明,基于OAM谱分解的通信通道数由发射阵列的阵元数N决定,其解码精度由发射信号的相位精度和接收阵列的阵元数决定;阵元数N≥1的大阵列系统在满OAM模式复用条件下的传输速率约为传统水声通信的N倍.本研究表明基于OAM谱分解的多通道通信可大幅提高信道容量和频谱利用率,为其在水声通信中的实际应用提供了一种实用方案.

A new method of calculating the orbital angular momentum spectra of Laguerre-Gaussian beams in channels with atmospheric turbulence

Studying orbital angular momentum（OAM） spectra is important for analyzing crosstalk in free-space optical（FSO）communication systems. This work offers a new method of simplifying the expressions for the OAM spectra of Laguerre-Gaussian（LG） beams under both weak/medium and strong atmospheric turbulences. We propose fixing the radius to the extreme point of the intensity distribution, review the expression for the OAM spectrum under weak/medium turbulence,derive the OAM spectrum expression for an LG beam under strong turbulence, and simplify both of them to concise forms.Then, we investigate the accuracy of the simplified expressions through simulations. We find that the simplified expressions permit accurate calculation of the OAM spectrum for large transmitted OAM numbers under any type of turbulence. Finally,we use the simplified expressions to analytically address the broadening of the OAM spectrum caused by atmospheric turbulence. This work should contribute to the concise theoretical derivation of analytical expressions for OAM channel matrices for FSO-OAM communications and the analytical study of the laws governing OAM spectra.

Wave-Mean Flow Interaction: the Role of Continuous-Spectrum Disturbances

Traditionally, “ eddy feeds zonal flow” in the atmosphere is considered as a result of decaying unstable waves. We show that disturbances made of non-modal solutions -the continuous-spec-trum disturbances-can also effectively transport zonal angular momentum and interact with the zonal basic flow. These disturbances, though stable, eventually decay, losing their energy to strengthen the westerly jets in the atmosphere. Calculations with observational data illustrate that the atmospheric zonal flow is maintained primarily by continuous-spectrum disturbances rather than by unstable waves. Angular momen-tum transport by continuous-spectrum disturbances is one order of magnitude larger than that by all kinds of normal modes (referred as discrete-spectrum disturbances) including unstable waves. Key words Maintenance of zonal flow - Continuous-spectrum - Angular momentum - Transport This research was partly supported by the Chinese Program on Basic Research and by the Institute of Atmospheric Physics, Chinese Academy of Sciences. It was also supported by NASA Grant NAGW 3517, and DOE Grant DEFG0285-ER60314 to the State University of New York at Stony Brook.

Statistical Distribution of Depth-Integrated Local Horizontal Momentum for Second-Order Random Ocean Waves in Finite Water Depth

Based on the second order random wave solutions of water wave equations in finite water depth, statistical distributions of the depth integrated local horizontal momentum components are derived by use of the characteristic function expansion method. The parameters involved in the distributions can be all determined by the water depth and the wave number spectrum of ocean waves. As an illustrative example, a fully developed wind generated sea is considered and the parameters are calculated for typical wind speeds and water depths by means of the Donelan and Pierson spectrum. The effects of nonlinearity and water depth on the distributions are also investigated.

低横动量Υ(1S)介子在强子气体中的分布

Υ(1S)是高能重离子碰撞中的干净探针,其研究具有重要意义.强子气体中Υ(1S)的分布受到集体流、量子效应和强相互作用三种效应的影响.此前的模型均只分析了其中一到两种效应而忽略了其他效应,将忽略的效应放入到了拟合参数中.本文从新的视角提出了两体分形模型,从物理计算而非参数拟合的方法开展研究.本文认为Υ(1S)与最相近的介子形成两体结构,与介子内部的两夸克结构具有自相似性.引入环境影响因子qfqs来描述上述三种效应影响;引入伴随因子q2来描述b,b之间的相互作用和上述三种效应的影响.通过求解概率与熵方程,得到不同碰撞条件下qfqs与q2的值.将qfqs代入到分布函数中,得到低横动量区的横动量谱并与实验对比,结果表明理论与实验符合良好.还分析了qfqs随温度的变化关系,并发现qfqs>1,这是由环境效应减少了Υ(1S)的微观状态数所致.另外,计算显示qfqs随着系统温度的降低而减小,这与随着系统膨胀强子气体影响减弱的现象相一致.

斜程湍流大气中矢量涡旋光束的OAM特性研究

基于螺旋谱理论推导出矢量涡旋光束在斜程大气湍流传输中的轨道角动量(orbital angular momentum,OAM)谱,讨论了修正Kolmogorov大气湍流对不同阶矢量涡旋光束光场分布及光束OAM谱的影响。利用旋转毛玻璃作为随机相位屏,实验采集了矢量涡旋光束通过旋转毛玻璃后的光强图像与归一化强度。结果表明:经大气湍流斜程传输后,矢量涡旋光束的光强分布产生畸变且OAM谱发生弥散,OAM谱弥散程度随传输距离的增大而增大。传输距离不变时,天顶角越大,主OAM模相对功率越小,湍流内尺度减小与折射率结构常数增大均会导致主OAM模相对功率减小。同一传输路径下,随着湍流强度的增大,矢量涡旋光束中心相位奇点的强度逐渐增大,并且拓扑荷不变时,偏振阶数高的矢量涡旋光束受到的湍流影响较小。

杂化偏振涡旋合成光束阵列的轨道角动量谱

轨道角动量(OAM)是高容量光通信和超分辨成像技术的重要参数。利用惠更斯-菲涅尔原理和相干合成理论,提出了杂化偏振涡旋合成光束阵列。详细研究了涡旋、偏振、附加拓扑电荷及子光束数对输入和输出平面光束的OAM谱的影响。结果表明:子光束的数量和杂化偏振共同影响了OAM模式的最大权重,子光束数量增加会显著提升OAM谱的最大权重,但杂化偏振却不能显著提升OAM谱的最大权重。OAM谱的最大模式位置总是等于光束中心光涡旋的总拓扑数,与子光束数无关。OAM谱所有非零权重模式的位置由涡旋、偏振、附加拓扑电荷和子光束数目共同决定。本文结果对光通信与偏振成像技术有着潜在的应用价值。

金属目标对贝塞尔涡旋波束的近场电磁散射特性

相较于平面波,贝塞尔涡旋波束作为典型的涡旋波束,以其携带轨道角动量(orbital angular momentum,OAM)、无衍射、自重建的特性,使其在雷达检测、目标识别与成像等领域具有更加显著的竞争力.本文根据物理光学法和三角形面片建模,并结合贝塞尔涡旋波束的角谱展开法,导出了贝塞尔涡旋波束入射到任意金属目标的散射近场表达式,通过与FEKO软件仿真结果对比验证了本文方法的正确性.数值计算了简单和组合目标的散射近场的幅相和OAM分布以及雷达散射截面(radar cross section,RCS),分析了波束参数、接收面的距离和目标偏移波束距离等因素的影响.结果表明贝塞尔涡旋波束照射下的目标的近场幅相和OAM扰动与多种因素相关,这为进一步利用近场散射特征获得更多的目标信息奠定了基础.

湖北省1954/2008年历史罕见持续低温冰雪过程对比分析

1954年12月26日-1955年1月18日湖北省遭遇了120年一遇的历史罕见持续低温冰雪灾害。2008年1月13日-2月3日,湖北省再次出现了一次持续低温冰雪过程,严重程度仅次于1954年,为70年一遇。利用常规天气图和数值预报产品资料,应用天气学和波谱分析方法,对比分析了两次过程的环流形势特征及能谱特征。结果表明:两次过程具有基本一致的长波系统背景场、水汽条件、地面冷空气条件;在低温冰雪过程中,850～700hPa存在明显逆温层;中高纬度、中低纬度能量谱强度、输送和转换有明显的变化,并且与天气系统的变化相对应。但是,两次天气过程的表现形式有明显差异,而且前者的主要影响系统,如阻塞高压、低涡、副高和地面冷高压等的强度均明显比后者强。

轨道角动量在无线通信中的研究新进展综述

随着无线通信技术的蓬勃发展,频谱利用率和系统容量已经趋近香农极限.轨道角动量作为一项新型技术,拥有高效频谱利用率和抗干扰能力,引起了国内外学术界的关注.本文首先介绍了轨道角动量的应用与无线通信系统的基本原理;其次综述了轨道角动量在相关领域的研究进展,并对轨道角动量产生的关键技术进行深入分析,以及对现有的轨道角动量接收方法进行梳理总结;最后在目前已有研究的基础上,展望并提出了未来轨道角动量在无线通信研究及应用中需要重点解决和关注的一些突出问题,包括携带轨道角动量涡旋电磁波的产生,不同模态轨道角动量电磁波相互干扰的抑制,轨道角动量模态的编码方式以及不同模态涡旋电磁波的分离与检测等方面.

大气斜程传输中高阶贝塞尔高斯光束轨道角动量的研究

大气湍流引起大气折射率随机变化,导致空间不均匀性。高阶贝塞尔光束在大气湍流中传输时,空间不均匀性会使光子波函数改变,形成不同的光子态引起轨道角动量的弥散。在Rytov近似下,计算了高阶贝塞尔光束在大气斜程传输中各分量所占光束总能量的权重。讨论并对比折射率结构常数,光束波长,天顶角,轨道角动量数,接收孔径和光斑大小等参数对螺旋谱的影响,并给予相应的物理解释。结果表明:随着折射率结构常数,天顶角和传输距离的增加以及光束波长的减小,螺旋谐波主分量对应的谱减小,轨道角动量弥散越大,而且望远镜接收孔径和光斑大小对轨道角动量弥散的影响非常小。

2009年初湖北省持续雨雪过程分析

利用常规天气图和数值预报产品资料,采取天气学和波谱方法,分析了2009年2月13日至3月4日湖北持续雨雪过程的环流形势及能量谱特征,并与2008年1月10日至2月1日湖北罕见持续低温雨雪冰冻过程进行对比。结果表明:两次持续雨雪过程具有基本相同的长波系统背景场条件和地面冷空气影响路径;对流层850—700 hPa存在逆温层,后一过程的逆温层维持时间更长、强度更大且无中断;两次雨雪过程的温度分布和强度差异明显,后者气温异常偏低,冰冻现象严重;两次雨雪天气期间,中高纬、中低纬地区的能量谱强度、输送和转换变化明显,且与天气系统的变化相对应。

动量轮微振动扰振频谱对三反同轴相机的影响

随着遥感卫星分辨率的不断提高,动量轮扰振对遥感成像的影响逐渐成为卫星总体设计时需要关注的问题。目前对动量轮微振动的研究多侧重于时域分析,经常把卫星平台和载荷分立开来单独研究。鉴于平台和相机之间具有耦合作用,分立研究会带来较大误差。动量轮的输出响应并不是一个单一频率的时域信号,不同频谱对相机的影响不同,频谱分析往往更能反映问题。文章联合卫星平台和相机进行整星建模,分析了微振动扰振从动量轮处传递到相机光学元件的频谱特征,得到动量轮扰振不同频谱对相机造成的具体影响。分析结果显示,低于相机基频的动量轮扰振频谱造成相机光轴晃动,相机特征频率区间的扰振频谱不仅会放大光轴晃动,还会造成相机内部的光学元件光轴不一致。最后文章定量计算比较了这两种影响带来的MTF下降。

植物冠层动量交换特征的实验研究

使用湍流梯度测试资料，对植物冠层动量交换特征进行了详细研究，结果表明；森林冠层内惯性副区能谱曲线仍可用幂指数描述，但斜率比－２／３更负；森林冠层内湍流尺度有变小的趋势；森林上层的耗散系数比下层大；由植被吸收引起动量及动量通量随冠层深度增加而明显减小；冠层下层的动量通量和耗散系数分别与上层的量有好的正相关；森林冠层内耗散系数和动量通量随大气稳定度有明显变化。

大气湍流下轨道角动量复用态串扰分析

以加载QPSK调制信号的轨道角动量(OAM)光束为传输载波,以多个相位屏模拟大气湍流,研究不同大气湍流强度下OAM复用态的串扰情况。通过对复用光束光强和相位研究,得出在OAM复用态光强受到湍流影响时会发生明显的闪烁现象,光功率分散,相位发生旋转弯曲,且湍流强度越大,受到的影响越大。选用螺旋谱分析不同湍流强度下各OAM复用态之间的弥散程度,当大气湍流强度增加时,OAM态之间的弥散程度增加,且较强湍流会导致OAM复用态失真。同时,考虑OAM复用态之间的模式串扰以及每路携带信息的OAM态因大气信道引起的混合噪声而造成的码间干扰,对比研究了不同大气湍流强度下系统误码率随传输距离的变化,结果表明:系统误码率随传输距离的增长而增大,强湍流之下光束误码率会随着传输距离增长到一定程度后趋于平稳,弱湍流之下光束误码率会随着传输距离的增长而增大。

基于μ子动量谱的多群模型测量平板材料的厚度

在基于μ子多重库仑散射的断层成像技术中,天然宇宙射线中不同动量的μ子在同一目标对象中的散射特性存在明显的差异,这在一定程度上影响了成像分辨率。为了减少或消除这种不利影响,建立了一套基于天然宇宙射线μ子动量谱的多群模型,并检验其用于铅板厚度测量及平板材料识别中的性能和特征。结果表明,采用基于μ子动量谱的多群模型能够实现1~16cm范围内铅板厚度的准确测量,相对误差小于5%。此外,该多群模型还可以通过计算辐射长度来识别未知平板的材料组成,尤其是对于高原子序数材料组成的平板,其辐射长度相对误差小于4%。