发射系统遮拦比对均强聚焦光束光斑扩展的影响

 利用数值模拟方法讨论了发射系统遮拦比对均强聚焦光束湍流大气传输光斑扩展的影响。结果表明，对于均强聚焦光束湍流大气传输，在弱湍流效应条件下，光斑扩展受遮拦比影响较大；在较强湍流效应条件下，不同遮拦比光斑扩展变化趋势相同，此时远场光斑扩展主要还是受湍流效应强弱的影响。获得了常用遮拦比条件下均强聚焦光束光斑扩展和湍流效应特征参数的定标关系。     

基于迈克耳孙干涉仪的微机械光纤信号调制器

基于迈克耳孙干涉仪原理,设计了利用双光束干涉效应的微机械光纤信号调制器.实验中从输入端输入的光纤信号,经分光板后分为光强相等的透射光束与反射光束;调制信号以电压形式加载于压电陶瓷,使其伸缩振荡,以调制透射光束与反射光束之间的光程差;透射光束与反射光束经透镜聚焦后在输出端面发生双光束干涉,在输出端输出被调制的光信号,再耦...     

激光反射法测量弯曲液面特性

发现了弯曲液面上边界反射光场的带状暗场扩张效应.当平行激光束垂直入射到由平板润湿效应引起的弯曲液面上时，边界反射产生特殊的反射图样，图样的中间为带状暗场，两侧有清晰的衍射条纹，并且带状暗场宽度随入射光束宽度的缩小而迅速变大.分析了反射图样的带状暗场宽度与入射光束宽度的解析关系，得出了弯曲液面的斜率曲线和高度曲线.建立了一种利用弯曲液面边界反射的暗场扩张效应来实时测量液面特性的新方法.     

跟踪抖动对激光湍流大气传输光束扩展的影响

对发射系统的跟踪抖动对激光湍流大气传输光束扩展的影响进行了数值分析,并且得到了跟踪抖动对光束扩展影响的定标关系。由此定标关系可知,当系统衍射角σ0较大时,跟踪抖动对光束扩展的影响比较小;但是当系统衍射角σ0较小时,特别是在湍流效应较弱的情况下,抖动的影响非常显著,此时光束扩展主要是由系统抖动项所决定的。通常在湍流效应较弱时,系统跟踪抖动对光束扩展的影响相对较大,随湍流效应的不断增强,湍流扩展逐渐起主导作用,而系统跟踪抖动对光束扩展的影响相对逐渐减小。因此,由此定标关系可以定量了解不同条件下系统衍射、系统跟踪抖动以及湍流效应对光束扩展影响的大小,并为系统优化设计提供定量的科学依据。     

手征和运动介质界面异常光自旋霍尔效应

文章讨论了偏振高斯光束在空气-手征介质界面的反射和折射问题,发现了反射和折射光束的异常自旋霍尔效应.在某些特定的角度下,反射高斯光束重心的移动可以达到几十个波长的量级,通过调节手征参数,甚至会出现折射光重心移动方向由正到负的转变.文章还讨论了任意偏振高斯光束在空气-运动介质界面的反射和透射问题,当介质垂直于入射平面运动时,反射和透射光束的横向移动可以达到几个波长的量级,甚至在某些运动速度下可以实现移动方向正负的改变.这些研究对控制光的自旋霍尔效应提供了可能的途径.     

双棱镜结构中透射光束的古斯-汉欣位移

当入射角大于全反射临界角时,双棱镜结构中透射和反射光束的古斯-汉欣(Goos-Haenchen)位移具有饱和效应,并且只有波长数量级。利用稳态相位法研究了当入射角小于全反射临界角时双棱镜结构中透射光束的古斯-汉欣位移。研究表明,传播模式下透射光束的古斯-汉欣位移是空气层厚度、入射角和双棱镜折射率的周期性函数。当透射共振时,透射光束的古斯-汉欣位移可达入射波长的几十倍,与入射角大于全反射临界角的情况相比,透射光束的位移通过边界的相互作用具有共振增强效应;在非共振点处,对称结构中的反射光束具有与透射光束相同的古斯-汉欣位移。共振增强的透射光束的位移在光开关及光耦合器中具有潜在的应用。     

利用平凹透镜和标准角锥棱镜实现反射光束发散的方法

角锥棱镜常被用作光电测距和光电跟踪的合作信标。在一些特殊应用场合中,要求被角锥棱镜反射的光束具有一定的发散角,以实现对远场不同位置处激光器和探测器的全覆盖。标准角锥棱镜不具备对光束发散的功能,但是可以利用角锥棱镜中光束出射点与入射点不同的特性,在标准角锥棱镜前加装平凹透镜来实现对反射光束的发散。采用理论分析方法和几何光学追迹模拟方法分析了利用平凹透镜和标准角锥棱镜实现反射光束发散的可行性,同时研究了反射光束发散半角与入射光束参数和平凹透镜几何参数的定量关系。理论和模拟结果都表明,当入射光束半径小于平凹透镜半径时,反射光束的发散半角随入射光束半径准线性增加;平凹透镜曲率半径越小,反射光束发散半角越大。     

球差光束在大气湍流中传输特性的实验研究

采用空间光调制器产生球差光束,并利用旋转随机相位板模拟大气湍流,实验上研究了球差光束在大气湍流中的传输特性.研究表明:在自由空间传输时,正、负球差光束光强分布均为环形多层分布,但经过大气湍流传输后光强均会变为类高斯分布.正球差导致光束扩展,负球差会导致光束聚焦.正球差越大光束能量集中度越差.负球差对光束能量集中度的影响是非单调的.特别地,大气湍流会削弱球差效应对光束扩展的影响.     

矢量光束界面反射透射的自旋霍耳效应

矢量光束的一般表示方法是利用投影矩阵和广义琼斯矢量的乘积描述.投影矩阵存在一个自由度,该自由度与有限光束的场矢量偏振状态有关,由特定的单位矢量与波矢量间的方位角决定,可以定量地描述矢量光束的偏振状态.本文在矢量光束描述的理论基础上,通过对投影矩阵进行与反射光束与透射光束传播方向相应的坐标旋转,根据麦克斯韦方程组及其边界条件,计算讨论在各向同性介质界面上反射、透射矢量光束的表示形式以及其自旋霍尔效应表现出的横向位移.线偏振光(光子自旋量为σ＝0)横向位移为零,圆偏振光束(光子自旋量为σ＝±1)位移量最大且左圆偏振与右圆偏振光束的位移大小相等方向相反,进一步分析了左圆偏振光束在界面上的反射、透射光束的横向位移与入射角的关系.     

应用变换光学理论调控电磁场光束束宽

 根据变换光学理论,提出了利用超常材料调控光束束宽及将有限束宽光束变成线光束的理论方案,得到了所需超常材料的介电常数和磁导率的解析结果。应用这些结果,可以实现对光束束宽的调控或者将有限束宽光束压缩成线光束。利用有限元分析方法进行全波仿真,仿真结果与理论相符合,可应用于压缩或扩展电磁波束的能量密度。线光束中心处的场能量密度极大,可应用于增强非线性光学效应等方面。     

Chaos mirror for free-space links

We propose a novel reflector, called a chaos mirror, designed by use of the principles of chaotic reflection. A feature of the reflector is that it converts a one-dimensional spread of incident rays into a two-dimensional spread of reflected rays. This can be useful for making free-space optical beam links by sweeping a transmission beam in only one dimension.     

Reflection and transmission of an Airy beam in a dielectric slab

The reflection and transmission of a finite-power Airy beam incident on a dielectric slab are investigated by an analytical method. Based on the plane-wave angular spectrum expansion and Fresnel approximation, the analytical expressions of the reflected field, internal field as well as transmitted field in each region are obtained. Through numerical simulations, the intensity distributions of the incident beam, reflected beam, internal beam as well as transmitted beam are presented at oblique incidence. Besides, we also compare the intensity distributions of the geometrical-optics beam field, the first order beam mode field and the actual beam field, which indicates that the contribution of each order beam mode field to the actual beam field is related to the refractive index of the dielectric slab. Meanwhile, the reflection characteristics of the Airy beams in the special cases of Brewster incidence and total reflection are investigated. Finally, the effects of the optical thickness and refractive index of the dielectric slab on the peak intensity distributions and beam shifts of the reflected and transmitted beams are also discussed in detail. The analytical and numerical results will be useful to analyze the propagation dynamics of Airy beam in the dielectric slab and provide some theoretical supports to the design of optical film.     

Goos-Hänchen and Imbert-Fedorov shifts of a nondiffracting Bessel beam

Goos-H?nchen (GH) and Imbert-Fedorov (IF) shifts are diffractive corrections to geometric optics that have been extensively studied for a Gaussian beam that is reflected or transmitted by a dielectric interface. Propagating in free space before and after reflection or transmission, such a Gaussian beam spreads due to diffraction. We address here the question of how the GH and IF shifts behave for a "nondiffracting" Bessel beam.     

Resonant excitation of intense acoustic waves in crystals

The resonant excitation of an intense elastic wave through nonspecular reflection of a special pump wave in a crystal is described. The choice of the plane and angle of incidence is dictated by the requirement that the excited reflected wave be close to the bulk eigenmode with its energy flow along a free boundary. The resonance parameters have been found for a medium with an arbitrary anisotropy. General relations are concretized for monoclinic, rhombic, and hexagonal systems. A criterion is formulated for an optimal selection of crystals in which the resonant reflection is close to the conversion one, when almost all of the energy from the incident beam of the pump wave falls into the near-surface narrow high-intensity reflected beam. Estimates and illustrations are given for such crystals as an example. The intensity of the reflected beam increases with its narrowing, but its diffraction divergence also increases with this narrowing. Nevertheless, the intensity of the beam can be increased by a factor of 5–10 at sufficiently high frequencies while keeping its divergence at an acceptable level. Amplification by two orders of magnitude can be achieved by compressing the beam in two dimensions through its double reflection.     

Frustrated Total Internal Reflection: Resonant and Negative Goos–Hänchen Shifts in Microwave Regime

It is well-known that the variations of Goos–Hänchen shifts (GHSs) are closely associated with the energy flux provided by evanescent states in the case of total internal reflection. However, when the frustrated total internal reflection (FTIR) is realized with double-prism system operated in the microwave frequency, we observe that the GHSs for the reflected beam show periodic, resembling the phenomenon for transmitted beams reported in the literatures, versus either the operating frequency or the air layer thickness, which is different from the variation of the corresponding reflected energy. Moreover, in another FTIR based system fabricated by a composite absorptive material slab with a two-dimensional top layer of frequency selective surface (FSS), the GHSs for reflected beam are discovered as not only resonant but also negative with the incidence of transverse electric that is TE polarized, just as predicted theoretically in the literatures.     

Reflection and transmission of Laguerre Gaussian beam from uniaxial anisotropic multilayered media

Based on angular spectrum expansion and 4 × 4 matrix theory, the reflection and transmission characteristics of a Laguerre Gaussian(LG) beam from uniaxial anisotropic multilayered media are studied. The reflected and transmitted beam fields of an LG beam are derived. In the case where the principal coordinates of the uniaxial anisotropic media coincide with the global coordinates, the reflected and transmitted beam intensities from a uniaxial anisotropic slab and three-layered media are numerically simulated. It is shown that the reflected intensity components of the incident beam, especially the TM polarized incident beam, are smaller than the transmitted intensity components. The distortion of the reflected intensity component is more evident than that of the transmitted intensity component. The distortion of intensity distribution is greatly affected by the dielectric tensor and the thickness of anisotropic media. We finally extend the application of the method to general anisotropic multilayered media.     

Retrieval of depth profile of nano-scale thin films by one directional polarization analysis in neutron specular reflectometry

Recently it has been shown that the modulus and phase of complex reflection coefficient can be determined using a magnetic substrate and polarized neutrons. Several other methods have also been worked out based on the measurement of polarizations of reflected neutrons from magnetic reference layers and magnetic substrate. However, due to the fact that available reflectometers are limited in the choice of polarization of reflected beam in the same direction as the polarization of the incident beam, neither of the methods, which are based on polarization analysis, have been proven to be experimentally practical. In this paper, we have proposed a new method for determining the phase of reflection coefficient that is based on two measurements of polarization, which correspond to two magnetic fields with the same magnitude and different orientations. The polarization analysis is performed in the same direction as the polarization of the incident beam and is well suited for available reflectometers.The problems envisaged in implementation of the method are also discussed.     

计算海面激光反射特性的几何光学方法研究

研究了计算海面激光反射的几何光学方法。首先根据JONSWAP非稳态海谱模型数值模拟出二维随机粗糙海面,再采用几何光学方法对入射激光光束在海面上的反射光进行建模,最后计算出海面激光光斑反射光强的空间分布。编制了相关的计算机程序,分析了在不同入射角度和不同风速下的激光光束反射特性。计算结果表明该方法对研究二维随机粗糙海表面激光反射特性有一定参考价值。     

Multilayer neutron monochromator-polarizer based on iron

In this paper, a multilayer neutron monochromator-polarizer, consisting of 490 alternating layers of iron and niobium, is proposed. Each layer of iron or niobium has a thickness of 25 Å. The use of thin layers in the structure period improves the quality of the monochromator-polarizer and yields a neutron beam reflected from this structure with high wavelength resolution and a small fraction of minor nonmonochromatic contributions to the intensity of the reflected beam. The fact that the layer thicknesses are the same in the period suppresses even-order Bragg peaks. The parameters of the first-order Bragg peak for eight model neutron monochromators-polarizers based on iron and cobalt are given. It follows from the calculated curve of the coefficient of neutron reflection from the Fe/Nb structure that the first-order Bragg peak has a half width of 1.1%, a relative reflection coefficient of 0.997, and a maximum polarizing efficiency of unity. We also present the first experimental curves of the coefficient of neutron reflection from this structure, depending on the momentum transfer for both spin components of the beam.