Angular spectrum characters of high gain non-critical phase match optical parametric oscillators

The angular spectrum gain characters and the power magnification characters of high gain non-walk-off colinear optical parametric oscillators have been studied using the non-colinear phase match method for the first time. The experimental results of the KTiOAs04 and the KTiOP04 crystals are discussed in detail. At the high energy single resonant condition, low reflective ratio of the output mirror for the signal and long non-linear crystal are beneficial for small divergence angles. This method can also be used for other high gain non-walk-off phase match optical parametric orocesses.

Modified scaling angular spectrum method for numerical simulation in long-distance propagation

The angular method(AS)cannot be used in long-distance propagation because it produces severe numerical errors due to the sampling problem in the transfer function.Two ways can solve this problem in AS for long-distance propagation.One is zero-padding to make sure that the calculation window is wide enough,but it leads to a huge calculation burden.The other is a method called band-limited angular spectrum(BLAS),in which the transfer function is truncated and results in that the calculation accuracy decreases as the propagation distance increases.In this paper,a new method called modified scaling angular spectrum(MSAS)to solve the problem for long-distance propagation is proposed.A scaling factor is introduced in MSAS so that the sampling interval of the input plane can be adjusted arbitrarily unlike AS whose sampling interval is restricted by the detector’s pixel size.The sampling interval of the input plane is larger than the detector’s pixel size so the size of calculation window suitable for long-distance field propagation in the input plane is smaller than the size of the calculation window required by the zero-padding.Therefore,the method reduces the calculation redundancy and improves the calculation speed.The results from simulations and experiments show that MSAS has a good signal-to-noise ratio(SNR),and the calculation accuracy of MSAS is better than BLAS.

Information Entropy of Angular Spectrum for Quantitatively Evaluating Eddy Current Distribution Varying in Time Domain

Eddy current (EC) distribution induced by EC sensors determines the interaction between the defectin the testing specimen and the EC, so quantitatively evaluating EC distribution is crucial to the design of ECsensors. In this study, two indices based on the information entropy are proposed to evaluate the EC energyallocated in different directions. The EC vectors induced by a rotational field EC sensor varying in the timedomain are evaluated by the proposed methods. Then, the evaluating results are analyzed by the principle ofEC testing. It can be concluded that the two indices can effectively quantitatively evaluate the EC distributionsvarying in the time domain and are used to optimize the parameters of the rotational EC sensors.

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.

基于空间光调制器的艾里光束能量调制方法

艾里光束由于具有无衍射、自加速和自愈合等特性,具有重要的应用前景。本文基于空间光调制器可编程特性,提出了一种可调谐的艾里光束能量调制新方法。从理论上深入分析通过在相位图中引入一个可控的调制相位而实现了艾里光束能量调控的方法,并且量化了调制参数与能量调制范围的关系。仿真和实验结果表明通过改变调制相位可以灵活地调节光束能量分布,本文的研究有助于进一步推动艾里光束的应用。

基于单一元件产生太赫兹无衍射波束技术研究

提出了一种新型的光学元件,可以产生与元件具有一定距离的无衍射波束。普通轴棱锥的横截面为等腰三角形,通过在两个腰上限定抛物线进行元件设计,采用3D打印技术制作了不同结构参数的光学元件。角谱理论的仿真计算结果以及在0.1 THz频率下进行的实验结果,都清晰地表明了产生的无衍射波束与元件之间具有一定距离。利用空间频谱对元件的波束行为进行分析,结果表明,该元件产生了一种新型的无衍射波束,根据元件的空频形状,将元件产生的无衍射波束命名为“梳状空频无衍射波束”。

Three-Dimensional Scene Encryption Algorithm Based on Phase Iteration Algorithm of the Angular-Spectral Domain

In order to increase the capacity of encrypted information and reduce the loss of information transmission, a three-dimensional(3 D) scene encryption algorithm based on the phase iteration of the angular spectrum domain is proposed in this paper. The algorithm, which adopts the layer-oriented method, generates the computer generated hologram by encoding the three-dimensional scene. Then the computer generated hologram is encoded into three pure phase functions by adopting the phase iterative algorithm based on angular spectrum domain,and the encryption process is completed. The three-dimensional scene encryption can improve the capacity of the information,and the three-phase iterative algorithm can guarantee the security of the encryption information. The numerical simulation results show that the algorithm proposed in this paper realized the encryption and decryption of three-dimensional scenes. At the same time, it can ensure the safety of the encrypted information and increase the capacity of the encrypted information.

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.

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.

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.

Angular Distribution and Cross Section Measurements of ^(64)Zn(n,α)^(61)Ni Reaction for Neutron Energy 5 MeV

A twin gridded ionization chamber with dual parameter data acquisition sys-tem is used to study neutron induced charged particle emission reaction.The angulardistribution and cross section of n-particles from the 64Zn（n,α）61Ni reaction are meas-ured at neutron energy 5 MeV.

基于角谱传播理论的衰荡腔光场传输模型及调腔评价准则研究

在光腔衰荡技术中,腔失调与腔损耗观测值之间存在复杂的非线性映射关系,导致调腔过程易陷入局部寻优,对测量准确性造成影响.本文基于角谱传播理论,建立了一种衰荡腔高斯光场传输模型,对典型调腔评价准则进行对比,并以模型仿真与实验研究相结合的方式,验证模型合理性.在仿真模型和实验系统中对特定腔镜施加二维倾斜角度扫描,获取两种典型调腔评价准则(即光强峰值和衰荡时间)的二维扫描分布.对比光强最大峰值和最长衰荡时间所对应的腔损耗观测状态.仿真结果表明光强最大峰值对应的腔损耗观测误差更小,观测重复性更高.实验结果同样表明光强最大峰值评价准则具有更好效果.模型仿真和实验研究的结果验证光强最大峰值评价准则在仿真和实验中具有更好的调腔重复性精度.同时,仿真与实验结果基本吻合,初步验证了本衰荡腔光场传输模型的合理性.本光场传输仿真模型对光腔衰荡技术在测量应用、光场响应及自动化调腔技术等方面的研究具有一定参考意义.

Directional Spectrum of Wind Waves: Part Ⅱ- Comparison and Confirmation

A model on the directional frequency spectrum of wind waves for deep water is introduced. The comparisons of the proposed model with other existing models show that the proposed model is very close to the JONSWAP model and DHH model for describing the developing waves under the normal spectral bandwidth, and has a better description for the transition of the unidirectional spectrum from ω -4 to ω -5 at a position around 3ω p, i.e., three time the peak frequency. Comparisons also show that the proposed model describes closely both field data measured by a four-frequency radar and a laser-optical sensor, and laboratory data measured by a laser slope gauge and an imaging optical method. The comparisons further demonstrate that the inverse spectral bandwidth as a new wave parameter is robust for describing the spectral steepness. Finally, the formula on the local spectral-peak angular frequency is confirmed using the observed two-dimensional spectra.

Directional Spectrum of Wind Waves: Part Ⅰ- Model and Derivation

A new model on the directional spectrum of wind waves for deep water is proposed based on the statistics of wind waves. This model contains three parameters: the wave age, the inverse spectral bandwidth and the local spectral-peak angular frequency. The inverse spectral bandwidth is a robust parameter for describing the spectral steepness of wind waves. Using the inverse spectral bandwidth parameter, the proposed model can well describe various observations obtained from the open ocean and laboratory tank.

基于点源成像的微球成像分辨能力仿真分析

基于微球透镜的显微成像技术具有方法简单、无需标记等优点,近年来受到了国内外众多研究者的关注。微球透镜成像系统参数变化将直接影响分辨能力,然而,目前缺乏此方面的仿真分析工作。因此,基于点源成像,提出了一种分析微球透镜成像分辨能力的方法,仿真分析了微球尺寸以及微球和点光源距离变化对成像分辨能力的影响。结果表明,成像分辨能力随微球尺寸的增大而下降,微球和点光源距离小于800 nm时,分辨能力随距离的增加呈下降趋势,当距离大于800 nm时,分辨能力不再发生明显变化。仿真结果对微球透镜显微成像实验具有指导意义,提出的方法为仿真分析微球成像分辨能力的研究工作提供了理论基础。

菲涅尔衍射积分的角谱算法

菲涅尔衍射可看作一个线性空间平移不变系统,因此可以在频谱域中,利用角谱算法来数值计算菲涅尔衍射积分.首先详细阐明了角谱算法的具体实施过程,澄清了理想采样条件、卷绕误差产生的原因和影响范围以及计算窗口尺寸的选取依据等相关问题.然后采用角谱算法数值计算了高斯光束照明余弦光栅的菲涅尔衍射场.数值解与解析解的对比表明,当满足理想采样条件并正确选取计算窗口尺寸时,角谱算法可以达到非常高的计算精度.最后数值仿真了单透镜相干成像实验,进一步展示了角谱算法的应用过程.

基于角谱域和时间反演的任意均匀场的生成方法

现有均匀场往往基于阵列天线的特殊排布,通过平顶波束赋形在角远场区域或者通过点聚焦在近场区域生成的,生成的均匀场直接受制于阵列排布形态且无法灵活调控.提出了一种基于角谱域和改进时间反演方法相结合的均匀场生成方法,该方法不受阵列排布的限制,能够以同一阵列排布形态,在包括近场区域在内的任意位置,生成指定大小、形状以及偏转角度的多种均匀场.首先理论解析了本方法不受阵列排布限制的原因;其次数值验证了固定阵列排布形态灵活生成多种均匀场的能力;最后引入时间反演方法,并做出反演信号幅度倒数加权的改进,解决了上述均匀场在生成过程中由幅度衰减和相位延迟带来均匀场平坦度恶化等问题.研究结果表明,合成场质量与其对应角谱域包络的主瓣和副瓣信息有关,且生成任意均匀场必须包含至少1/2的角谱域主瓣信息和1/2的副瓣信息.本方法能够灵活调控一维和二维均匀场的位置、大小、形状以及偏转角度,为灵活生成均匀场提供了一条新思路.

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.