System degradation due to multipath coherent crosstalk in WDMnetwork nodes

This paper shows-both experimentally and through Monte Carlo simulation-that in wavelength division multiplexing (WDM) lightwave networks coherent crosstalk in network nodes causes multipath fading on a seconds to minutes time scale. The result of such fading is that the bit error rate (BER) fluctuates significantly as a function of time. We find that for eight crosstalk paths a signal-to-crosstalk ratio (SXR) greater than 40 dB is required to make this fading negligible. The simulation also shows that for 32 crosstalk paths an SXR as much as 55 dB is required to combat the crosstalk penalty     

Tunable coherent radiation in the soft X-ray and extremeultraviolet spectral regions

Undulator radiation, generated by relativistic electrons traversing a periodic magnet structure, can provide a continuously tunable source of very bright and partially coherent radiation in the extreme ultraviolet (EUV), soft X-ray (SXR), and X-ray regions of the electromagnetic spectrum. Typically, 1-10 W are radiated within a 1/N relative spectral bandwidth, where N is of order 100. Monochromators are frequently used to narrow the spectral bandwidth and increase the longitudinal coherence length, albeit with a more than proportionate loss of power. Pinhole spatial filtering is employed to provide spatially coherent radiation at a power level determined by the wavelength, electron beam, and undulator parameters. In this paper, experiments are described in which broadly tunable, spatially coherent power is generated at EUV and soft X-ray wavelengths extending from about 3 to 16 nm (80-430-eV photon energies). Spatially coherent power of order 10 μW is achieved in a relative spectral bandwidth of 9×10^-4, with 1.90-GeV electrons traversing an 8-cm period undulator of 55 periods. This radiation has been used in 13.4-nm interferometric tests that achieve an rms wavefront error (departure from sphericity) of λ_euv/330. These techniques scale in a straightforward manner to shorter soft X-ray wavelengths using 4-5-cm period undulators at 1.90 GeV and to X-ray wavelengths of order 0.1 nm using higher energy (6-8 GeV) electron beams at other facilities     

On the CNET: a rearrangeable nonblocking optical interconnection network

A planar, circular, rearrangeable nonblocking optical switching architecture using N(N-1) Ti:LiNbO/sub 3/ directional coupler switches is suggested. Various figures of merit such as the signal-to-crosstalk ratio (SXR), insertion loss and fault tolerance are presented. The outline for a distributed routing algorithm in O(N) time is given.<>     

部分相干cosh-Gaussian光束通过大气湍流后的聚焦特性

为了研究部分相干双曲余弦高斯光束通过大气湍流中传输后的聚焦特性,采用了ABCD矩阵光学的方法进行分析研究,取得了光强分布、束腰半径以及桶中功率的解析式,并用于描述部分相干双曲余弦高斯光束通过大气湍流的聚焦特性。结果表明,在不同湍流强度下,部分相干双曲余弦高斯光束和双曲余弦高斯光束在经过大气湍流后有不同的聚焦特性;使用桶中功率和束腰半径作为评价参量,大气湍流对完全相干双曲余弦高斯的影响更严重;大气湍流对聚焦部分相干双曲余弦高斯光束的影响要大于准直光束。该结果对部分相干光束聚焦特性的研究有一定的理论指导意义,对合成光束傍轴聚焦系统的设计有工程指导意义。     

洛伦兹光束通过一阶轴对称光学系统的传输变换特性

从广义惠更斯-菲涅耳衍射积分出发,导出了洛伦兹光束通过一阶轴对称光学系统传输变换的解析公式.给出了洛伦兹光束的束宽、瑞利长度和发散角的表达式,讨论了这些参量通过自由空间的变换规律.结果表明,洛伦兹光束的瑞利长度大于高斯光束的瑞利长度,洛伦兹光束光斑半径的扩展速度慢于高斯光束,洛伦兹光束的远场发散角小于高斯光束的远场发散角.结果还表明,自由空间中洛伦兹光束的束宽、瑞利长度和发散角的变化规律与高斯光束相应参量的变化规律相同.     

计算机控制下的多路激光束合成系统

在激光应用领域中,需要将多束不同波长的激光束同时作用于研究对象,多路激光束的合成多是采用手动操作反射镜,合成度靠人眼观测确定,可靠性难以保证。提出的计算机控制的多路激光束合成系统是分别将每束光路位置信号采集到计算机,并控制步进电机带动反射镜调整具体光路。该系统优点是结构简单、效果好,在实际使用中可以实现光束的有效合成。     

无衍射分立光子晶格

在加压光折变掺铈铌酸锶钡(SBN)晶体中观察了由无衍射分立光束所产生的光子晶格。重点实验观察了三束光干涉和六束光干涉两种情况下所产生的光子晶格,并利用远场衍射图、布里渊区光谱图和傅里叶空间频谱图比较了这两种光子晶格的相同点和区别,并且进行了理论分析。结果表明由三束光干涉产生的光子晶格更均匀稳定。另外,又分别模拟了由四光束干涉、五光束干涉、七光束干涉、八光束干涉、九光束干涉和十光束干涉这几种情况下所获得的光子晶格的传输图。     

厄米-拉盖尔-高斯光束的传输特性

利用Collins公式,推导出厄米-拉盖尔-高斯(HLG)光束通过近轴ABCD光学系统的解析传输公式,并用来研究通过自由空间和薄透镜的传输特性.结果表明,除模指数m,n之外,对HLG光束新引入的α参数,它影响光强分布和对称性;HLG光束在传输中保持形状不变;HLG光束的焦移与模指数m,n和α参数无关,这意味着m,n和α不同的HLG光束聚焦在同一位置.并给出了HLG光束的M2因子.     

直接输出高阶横模激光器的实验研究

从理论和实验上研究了从激光器直接输出高阶拉盖尔-高斯（LG）光束和高阶厄密-高斯（HG）光束。首先从理论上研究了高阶LG光束和高阶HG光束的光强分布特性,并进行数值仿真。在实验研究中,利用445 nm的蓝光半导体激光器端面泵浦Pr:YLF晶体,在一定的条件下,能从平凹腔直接输出640 nm波长高阶LG光束和高阶HG光束。实验结果表明:从激光腔内输出的高阶LG光束和高阶HG光束与理论仿真基本一致。文中所报道的获得高阶模的实验装置简单,对产生高阶光束及其应用具有较重要价值。     

分数阶无衍射光束的实验研究

实验研究了轴棱锥聚焦分数阶涡旋光束获得分数阶贝塞尔光束。研究结果表明：当分数阶涡旋光束通过轴棱锥聚焦后,获得的分数阶贝塞尔光束其横截面上的光强分布具有高阶无衍射贝塞尔光束的特点;在轴棱锥后的最大准直距离内,光斑的形状基本保持不变,即呈现出很好的无衍射传输特性;另外还研究了轴棱锥角度改变对分数阶贝塞尔光束的影响,实验结果表明,轴棱锥的角度越大时,横截面光强图中的中心空心越小。     

Numerical simulation and experimental study of magnetron-injection guns for powerful short-wave gyrotrons

The results of a numerical simulation and experimental study of magnetron-injection guns forming intense helical electron beams for powerful gyrotrons are compared. Beams of three types—laminar beams, beams with regularly intersecting trajectories and beams with irregularly intersecting (mixed) trajectories are studied. The numerical simulation is performed taking into account the influence of the space charge field. The experimental beam characteristics are measured by the retarding field method. The experimental data agree satisfactorily with the results of the theory if the temperature deformation of the electrode systems is taken into account. It is shown that the best velocity distribution characteristics are realized for guns forming laminar beams and boundary beams (intermediate topology between beams with regularly intersecting trajectories and laminar beams).     

Relationships between squinted, sum and difference radiation patterns of amplitude monopulse antennas with mutual coupling between feeds

The relationships between the squinted beams and sum and difference beams of amplitude monopulse antennas are usually given on the basis that mutual coupling between feeds is negligible. These relationships can lead to gross conceptual errors regarding the basic capabilities of amplitude monopulse antennas. This paper develops similar relationships which include the effects of mutual coupling. It is found that the sum and difference beams are still proportional to the sum and to the difference of the squinted beams but the constants of proportionality, which are related to the VSWR's at the sum and difference ports before matching, are generally different. These constants of proportionality, or VSWR's, can be evaluated in terms of the squinted beams in the case of lossless antennas by applying the principle of conservation of energy. The matched sum pattern gain on boresight and the matched difference pattern slope on boresight are computed for a lossless antenna with squinted beams of the formsin ktheta/kthetaversus squint angle. The results obtained are drastically different from previously published results. Both of these antenna parameters are maximum in the limit of zero squint angle. However, for these squinted beam shapes, a small squint angle implies that a very high VSWR is to be matched at the difference port. It is noted that a given pair of matched sum and difference patterns can be produced by different pairs of squinted beams. These pairs of squinted beams can differ in their amplitudes and shapes. However, a single pair of squinted beams corresponds to inherently matched sum and difference ports.     

In vitro and in vivo real-time imaging with ultrasonic limited diffraction beams

Recently, there has been great interest in a new class of solutions to the isotropic/homogeneous scaler wave equation which represents localized waves or limited diffraction beams in electromagnetics, optics, and acoustics. Applications of these solutions to ultrasonic medical imaging, tissue characterization, and nondestructive evaluation of materials have also been reported. The authors report a real-time medical imager which uses limited diffraction Bessel beams, X-waves, Axicons, or conventional beams. Results (in vitro and in vivo) show that the images obtained with limited diffraction beams have higher resolution and good contrast over larger depth of field compared to images obtained with conventional focused beams. These results suggest the potential clinical usefulness of limited diffraction beams.     

Design of a spread-spectrum modulation signal bearing directional information

In this article, we propose a multiple beams spread-spectrum modulation signal transmitted by a Cassegrain antenna for communication and direction-finding applications. The main idea is that a sum beam and two difference beams are transmitted by the Cassegrain antenna similar to the Monopulse technique. According to the beams’ radiation pattern, a multiple beams spread-spectrum modulation scheme is designed to modulate the beams’ spatial parameters into the transmit signal constellation. In this way, a receiver with a single antenna demodulates the data information and the directional information from the receive signal constellation, simultaneously. Simulation results show that the proposed signal can be applied to the integrated system of communication and direction-finding in a low signal-to-noise ratio environment.     

厄米-高斯光束在饱和非线性介质中的传输特性

为了研究厄米-高斯光束在光折变饱和非线性介质中的传输特性,采用有限差分方法数值求解了光波演化方程,理论分析了厄米-高斯光束的传输特性。结果表明,1维1阶、2阶和3阶厄米-高斯光束在光折变非线性介质中传输时,在合适的非线性条件下,均可以形成呼吸模式的孤子;随着非线性的加大,厄米-高斯光束的光场分量之间的相互分离趋势将逐渐变弱,同时,每个光场分量的振幅起伏效应会更加明显;改变厄米-高斯光束的入射位置、入射角度对其传输特性没有影响;2维厄米-高斯光束的传输特性和1维情况是类似的。厄米-高斯光束的这些特性在光开关领域有一定的应用前景。     

Numerical calculations of two-beam energy coupling gains inphotorefractive BaTiO3 crystal

Steady-state two-beam energy coupling gains are numerically obtained in photorefractive BaTiO₃ crystal. The energy coupling gains are calculated by changing the angle subtended by two incident beams and by rotating the crystal relative to the grating wave vector. Polarizations of the two incident beams are assumed to be (ordinary, ordinary), (extraordinary, extraordinary), (ordinary, extraordinary), or (extraordinary, ordinary). The wavelength of the beams is assumed to be 515 nm. The maximum coupling gain is found to be 4.8/cm when both beams are ordinary and 74.7/cm when the beams are extraordinary. It is also shown that energy can transfer between ordinary (extraordinary) and extraordinary (ordinary) beams. In this case the maximum energy coupling gain is found to be 9.2/cm     

部分相干Airy涡旋光束在非Kolmogorov谱中的模态强度

为了探究部分相干Airy涡旋光束在非Kolmogorov谱中模态强度的演化规律,基于广义的Huygens-Fresnel原理和Rytov近似理论,推导了部分相干Airy涡旋光束的轨道角动量模态概率的解析式.结合MATLAB的数值模拟,研究了部分相干Airy涡旋光束在非Kolmogorov谱湍流大气中传输时湍流参量和波束...     

激光空间整形实现直接驱动惯性约束聚变球靶均匀辐照

在保持间接驱动光束排布结构及束数不变的条件下,通过对各路入射光横截面强度分布进行调整,消除多光束叠加后球靶表面光强大尺度的不均匀性。建立了描述球靶表面多光束叠加分布的计算机模拟平台。根据光束入射条件,计算出多束光在靶面叠加后的光强分布及其不均匀度。利用加权平均的思想,得出实现均匀强度叠加所需要的各入射光焦面强度分布状态。根据激光传输理论,计算出可实现球靶均匀辐照的进入靶室聚焦透镜之前的光电场分布。     

空心高斯光束通过光阑透镜系统的传输特性

为了研究空心光束通过硬边光阑透镜傍轴ABCD光学系统后的传输特性,利用柯林斯衍射积分公式,推导出了空心高斯光束通过受圆孔硬边光阑限制的傍轴ABCD光学系统的传输公式,所得公式可用来描述空心高斯光束通过任意傍轴ABCD光学系统的传输.研究了光阑、透镜以及光阑透镜系统对空心高斯光束传输特性的影响,并用数值例做了详细说明.结果表明,光阑和透镜均会使光束的光场分布向源平面前移,光阑会破坏空心高斯光束的空心性,而单纯的透镜只改变光场分布,不会破坏光束的空心性.这一结果对于空心光束的产生和应用有理论指导意义.