Direct Solution of the Inverse Problem for Rough Surface Scattering

We consider the inverse scattering problem for a scalar wave field incident on a perfectly conducting one-dimensional rough surface. The Dirichlet Green function for the upper half-plane is introduced, in place of the free-space Green function, as the fundamental solution to the Helmholtz equation. Based on this half-plane Green function, two reasonable approximate operations are performed, and an integral equation is formulated to approximate the total field in the two-dimensional space, then to determine the profile of the rough surface as a minimum of the total field. Reconstructions of sinusoidal, non-sinusoidal and random rough surface are performed using numerical techniques. Good agreement of these results demonstrates that the inverse scattering method is reliable.     

光束在自由空间中传输的偏振特性变化研究

光束在自由空间中传输时,其偏振特性随传输距离和光源参数的改变显示出不同的变化规律。为了研究其变化规律,以偏振相干统一理论为基础,仿真分析了各向同性和异性光束在自由空间传输时的偏振度变化规律。结果表明,光束在自由空间中传输时,光谱总密度随着传输距离的增加而逐渐降低,偏振度在经过长距离传输后会达到稳定;波长及光斑影响偏振度达到稳定状态距离,相干长度影响其到达稳定值的大小。对于非偏振和全偏振光束在自由空间传输时,只在特定条件下光谱的偏振度会保持不变。     

如何使光线弯曲

5年前,物理学家们演示了某类激光束可在自由空间中沿着曲线路径传播。这种违反常识的现象可以有许多方面的应用,如操控纳米颗粒,杀死药物难以达到的肿瘤细胞等。但是在这种奇异的效应付诸应用之前,研究人员必须解决如何使光线弯曲的角度足够大才行。最近有两个独立的研究小组解决了这一问题,并且宣称他们将研究如何使声波及其他种类的波沿弯曲路径传播的问题。     

The Wigner distribution function of a super Lorentz–Gauss SLG_(11) beam through a paraxial ABCD optical system

An orthonormal beam family of super Lorentz-Gauss （SLG） beam model is proposed to describe the higher-order mode beams with high divergence, which are generated by a high power diode laser. Here we consider the simplest case of the SLG beams, where there are four mutually orthogonal SLG beams, namely SLG00, SLG01, SLG10, and SLGll beams. The SLG00 beam is just the Lorentz-Gauss beam. Based on the Collins integral formula and the Hermite-Gaussian expansion of a Lorentz function, an analytical expression for the Wigner distribution function （WDF） of an SLG11 beam through a paraxial ABCD optical system is derived. The properties of the WDF of an SLG11 beam propagating in free space are demonstrated. The normalized WDFs of an SLG11 beam at the different spatial points are depicted in several observation planes. The influence of the beam parameter on the WDF of an SLGI 1 beam in free space is analyzed at different propagation distances. The second-order moments of the WDF of an SLG11 beam in free space are also examined. This research reveals the propagation properties of an SLGll beam from another perspective. The WDFs of SLG01 and SLG10 beams can be easily obtained by using the WDFs of Lorentz-Gauss beam and the SLG11 beam.     

径向洛伦兹阵列光束在自由空间的传输特性

推导出有初始相位分布的径向洛伦兹阵列光束在自由空间传输的解析公式,用以研究相干合成光束在自由空间的传输特性。结果表明:在远场,具有不同相位分布的光束光强剖面成为空心形状,具有相同相位分布的光束光强剖面中心为亮斑。合成光束束宽和桶中功率与子光束束腰宽度、径向阵列的初始相位分布和半径有关。对所得结果用数值计算例做了说明。当阵列半径较大和束腰宽度较小时,具有不同相位分布和相同相位的光束束宽随传输距离的变化曲线和桶中功率都趋于一致。     

盲优化波前校正提高自由空间光通信光纤耦合效率

为了提高自由空间光通信系统的光纤耦合效率,介绍了一种新颖的基于盲优化波前校正的自适应光学系统。该系统不使用波前传感器,而是在信号进入接收端前,将畸变光信号分束后送入光电探测器,进行耦合效率计算;采用耦合效率的估计公式斯特列尔比为目标函数,用随机并行梯度优化算法最大化目标函数,以实时控制变形镜,从而使耦合进入单模光纤的光达到最优。数值结果表明,此系统能使光纤耦合效率从6%提高到约60%。     

Entanglement Properties Between Two Spatially Separated Atoms with Cascade Configuration in Free Space

We investigate the entanglement properties between two identical atoms with cascade configuration through the retarded dipole-dipole interaction in free space when their spatial separation is on the order of radiation wavelength or less. We analyze the function of Hamiltonian induced by dipole-dipole interaction. By solving master equation, we show that the spontaneous emission induce entanglement and destroy entanglement too. We also show the long life time of entanglement within cascade configuration.     

Free-space 1×2 wavelength-selective switches for wavelength-division multiplexing optical networks

A free-space 1 ×2 wavelength-selective switch （WSS） based on thin-film filter technology is proposed. The 1 × 2 WSS is fabricated with an electromagnetic actuator, a reflecton prism, a narrow-band thin-film filter, and three fiber collimators. The working principle and the configuration of WSS are illuminated. The experimental results indicate a fiber-to-fiber insertion loss ranging from 1.109 to 1.249 dB with 2-V voltage input, which satisfies the application of optical fiber communication.     

Multiband terahertz metamaterial absorber

This paper reports the design of a multiband metamaterial (MM) absorber in the terahertz region. Theoretical and simulated results show that the absorber has four distinct and strong absorption points at 1.69, 2.76, 3.41 and 5.06 THz, which are consistent with `fingerprints' of some explosive materials. The retrieved material parameters show that the impedance of MM could be tuned to match approximately the impedance of the free space to minimise the reflectance at absorption frequencies and large power loss exists at absorption frequencies. The distribution of the power loss indicates that the absorber is an excellent electromagnetic wave collector: the wave is first trapped and reinforced in certain specific locations and then consumed. This multiband absorber has applications in the detection of explosives and materials characterisation.