Formation of a weakly diverging caustic beam in an underwater sound channel open to the bottom

The paper studies the patterns manifesting themselves in the formation and propagation of caustic and weakly diverging beams using the geometric acoustic approximation and mode theory applied to the simplest model of an underwater sound channel open to the bottom in the form of the bilinear dependence of the square of the refractive index on depth. It is established that when a vertical array emitting a tonal sound signal is located at a certain critical depth, the multimode caustic beam which forms near the reference ray horizontally departing from its center and which predominates in intensity is simultaneously the most weakly diverging beam. It is shown that the reference ray of such a weakly diverging caustic beam corresponds to the smoothest minimum for the dependence of the length of the ray cycle on their angle of departure from the center of the array and to the depth level of revolution at the critical depth characteristic of the given oceanic waveguide.     

Power emitted by a vertical compensated linear array in a Pekeris waveguide

An expression is derived for the acoustic power emitted by a vertical compensated discrete linear array in a Pekeris waveguide. The sound field is represented by the sum of a discrete spectrum and a continuous one. We consider the dependence of the power emitted by the array on both the number of array elements and the array compensation angle.     

Formation,propagation, and focusing of acoustic beams in oceanic waveguides. The quasi-optics approximation

Using the WKB approximation and assuming that the spatial spectrum of modes excited by a vertical array is sufficiently narrow, a quasi-optical theory is developed to describe the fundamental regularities manifesting themselves in the formation, propagation, and focusing of multimode acoustic beams in oceanic waveguides. Functional dependences on the parameters of both the transmitting array and the oceanic waveguide are obtained for horizontal distances at which ordinary beams form refraction focusing zones. Conditions ensuring the formation of a beam with the minimum wave front divergence are formulated for the distribution of the source excitation factor over the array aperture.     

Optimal distance from high power laser diode to cylindrical microlens in a coupling system

In a practical coupling system, a cylindrical microlens is used to collimate the emission of a high power laser diode (LD) in the dimension perpendicular to the junction plane. Using passive alignment, the LD is placed in the focus of the cylindrical microlens generally, regardless of the performance of the multimode optical fiber and the LD. In this paper, a more complete analysis is arrived at by ray-tracing technique, by which the angle 9 of the ray after refraction is computed as a function of the angle θo of the ray before refraction. The focus of the cylindrical microlens is not always the optimal position of the LD. In fact, in order to achieve a higher coupling efficiency, the optimal distance from the LD to the cylindrical microlens is dependent on not only the radius R and the index of refraction n of the cylindrical microlens, but also the divergence angle of the LD in the dimension perpendicular to the junction plane and the numerical aperture (NA) of the multimode optical fiber. The result     

Sub-diffraction-Limit Imaging in Optical Hyperlens

Sub-diffraction-limit imaging in the optical hyperlens based on cylindrical metamaterials is studied. Some parameters of hyperlens, such as the dispersive relation and the divergence angle of imaging, are numerically analysed with the ray trajectory method and effective medium theory. The dependence of imaging properties on dielectric constant is discussed. As a result, a 0° divergence angle is obtained for the best imaging effect. This work will be helpful for the design, structure fabrication and resolution improvement of the optical hyperlens.     

Determination of the path of a sound source moving in an inhomogeneous medium

The possibility of determining the actual paths of moving sound sources from the signals received by a linear horizontal array whose size is large compared to the wavelength is investigated with the use of procedures suggested earlier for the imaging of dynamic objects moving below an inhomogeneous layer. Two cases of signal reception are considered: when the signals propagate in the oceanic waveguide and when the signals propagate through an inhomogeneous layer located near the array. It is shown that, unlike the standard spatial processing procedures, the proposed methods allow one to measure the absolute angular displacements to within the diffraction resolution of the array and to eliminate the ambiguity in angular measurements. An important point is that the proposed methods require no prior data on the parameters of the inhomogeneous layer or the multimode waveguide.     

多模光纤产生用于大气光通信的部分相干光源

 克服大气湍流对近地大气光通信质量影响,可归结为在接收端的照明均匀化,而采用部分相干光照明则是提高照明均匀性的一种有效途径。提出利用多模光纤的模间色散产生一个具有部分相干光特性的合成光源的方法,分析了该光源的产生原理,表明光束发散角由光纤芯径和发射透镜的焦距决定,由模间色散产生的光程差越大,该合成光源的空间相干性越差。仿真结果表明形成1 mrad远场发散角的合成光源是可行的,因而适于在空间光通信中应用。     

Generation of a hollow laser beam by a multimode fiber

A simple method to generate a hollow laser beam by multimode fiber is reported. A dark hollow laser beam is generated from a multimode fiber and the dependence of the output beam profile on the incident angle of laser beam is analyzed. The results show that this hollow laser beam can be used to trap and guide cold atoms.     

Radiation passage through a twisted multimode light guide

In the geometrical optics approximation, it is shown that the meridional ray plane in a twisted multimode light guide is rotated through an angle given by Berry's geometrical phase. The results obtained allow the experimentally established effect of rotation of a speckle pattern for opposite helicity of the multimode lightguide to be described. All-Union Scientific Research Institute of Engineering Physics at the Russian Federal Nuclear Center. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 58–60, October, 1999.     

Analysis of the beam divergence for one-rod core microstructured optical fibres

The evolution of microstructured optical fibers with hexagonal array (H-MOFs) of air-holes rooted in the background of undoped silica has led to the realization of an ideal host for encouraging and technologically entitled optical properties. We focus to explore the divergence of radiation into free space from the end-facet of solid-core H-MOFs by using the improved theoretical model. Also, we investigated the wavelength dependence of beam divergence angle for principal core mode of H-MOFs under step-index fiber approximation (SIFA). Experimental results have been included for comparison.     

用于Z箍缩X辐射谱诊断的柱面凸晶摄谱仪的性能分析

 考虑X光源的空间尺寸和晶体的摇摆曲线,用光线追迹方法计算了诊断丝阵Z箍缩等离子体X辐射谱的柱面凸晶摄谱仪的色散、能谱分辨和空间分辨以及系统参数对它们的影响。结果表明：决定柱面凸晶摄谱仪能谱分辨的主要因素是光源尺寸以及光源-晶体之间距离;增大光源-晶体之间距离能够改善能谱分辨,但导致径向空间分辨能力下降。并依此建立了云母晶体摄谱仪,在“强光一号”装置上对系统进行了测试,获得时间积分铝丝阵负载Z箍缩等离子体X辐射谱。     

利用含二面角误差的角锥棱镜阵列实现反射光束均匀发散的方法

角锥棱镜常应用于光电跟踪、卫星通信、干涉仪等领域.在一些特殊应用场合中,要求经角锥棱镜反射的光束具有一定的发散角,以实现对距离激光器较远位置处探测器的覆盖.由于标准角锥棱镜不具备对光束发散的功能,本文利用含二面角误差的角锥棱镜对反射光束的不均匀发散特性,提出利用角锥棱镜阵列实现对反射光束均匀发散的方法和设计原则.采用衍射光学理论分析了所提方法及其设计原则的可行性,并依此设计了一个发散半角为0.5 mrad的角锥棱镜阵列.分析了光束参数、结构参数对反射光束远场衍射特性的影响,结果表明,入射光斑强度分布对反射光束发散半角没有影响,当角锥阵列满足点光源条件时,传输距离对反射光斑的角向均匀性没有影响;当阵元数超过一定值时,均匀性不再显著变化,但反射光斑的强度将进一步增加.在工程应用中,角锥棱镜阵列安装方位角误差对反射特性影响不显著,但角锥棱镜二面角的加工精度对反射特性影响较大,可通过进一步增加阵元数加以解决.     

The light acceptance angle of a graded index fibre

The acceptance angle of a graded index fibre, which corresponds to the numerical aperture, is presented. A geometrical technique is used to obtain the exact ray trajectory of the beam propagating along the fibre. The acceptance angle in a conventional multimode fibre is not influenced by the position in the fibre cross-sectional area, especially for the meridional rays. However in a graded index fibre, it is a function of position on the cross section of the fibre and of the incident angle of the beam. As the position moves into the fibre periphery, the acceptance angle decreases monotonically and approaches zero.     

Combining directional light output and ultralow loss in deformed microdisks

A drawback of optical modes in microdisk cavities is their isotropic light emission. Here we report a novel, robust, and general mechanism that results in highly directional light emission from high-quality modes. This surprising finding is explained by a combination of wave phenomena (wave localization along unstable periodic ray trajectories) and chaotic ray dynamics in open systems (escape along unstable manifolds) and applies even to microlasers operating in the common multimode regime. We demonstrate our novel mechanism for the lima?on cavity and find directional emission with narrow angular divergence for a significant range of geometries and material parameters.     

Single-mode operation of a laser-diode array with frequency-selective phase-conjugate feedback

A new technique for single-mode operation of laser-diode arrays is presented. A gain-guided GaAlAs laser diode array is coupled to an external frequency-selective phase-conjugate feedback system that contains a photorefractive barium titanate crystal, a Fabry-Perot etalon, and a spatial filter. The etalon is the key component, and it forces the array, which has low spatial and spectral coherence when it runs freely, to oscillate in a single spatial and a single longitudinal mode. At a drive current of two times the threshold current, the far-field pattern is reduced to only 1.4 times the diffraction limit, the spectral bandwidth is less than 0.02 nm, and the coherence length is increased by a factor of 70. The technique has general validity and can be applied to various other multimode laser systems.     

用位相共轭镜实现多模光纤束光束发散角压缩及光学不均匀性补偿

数值孔径为０．０７的多模光纤束与受激布里渊散位相共轭镜的组成的双光程装置中，输出光束的远场分布呈典型的二维列阵孔的径的衍射花样，输出光能分布在０．０２６ｒａｄ内，即由单根光纤芯径的衍射极限所决定的范围内，远小于由数值孔径所决定的高阶模相应的发散范围，并且偿了由纤束不均匀性带来的缺陷。     

Spatial diffusion in a periodic optical lattice: revisiting the Sisyphus effect

We numerically study the spatial diffusion of an atomic cloud experiencing Sisyphus cooling in a three-dimensional linlin optical lattice in a broad range of lattice parameters. In particular, we investigate the dependence on the size of the lattice sites which changes with the angle between the laser beams. We show that the steady-state temperature is largely independent of the lattice angle, but that the spatial diffusion changes significantly. It is shown that the numerical results fulfill the Einstein relations of Brownian motion in the jumping regime as well as in the oscillating regime. We finally derive an effective Brownian motion model from first principles which gives good agreement with the simulations. Received 8 August 2001 and Received in final form 6 November 2001     

高斯-谢尔模型列阵光束的远场发散角和远场辐射强度

推导出了高斯-谢尔模型（GSM）列阵光束通过自由空间传输其二阶矩束宽、远场发散角和远场辐射强度的解析表达式,并给出了GSM列阵光束与高斯光束具有相同远场发散角的条件.研究表明,具有相同远场发散角的GSM列阵光束与对应的高斯光束并不具有相同的远场辐射强度分布,这一特性与单束GSM光束的差异很大. 关键词： 高斯-谢尔模型列阵光束 远场发散角 远场辐射强度     

Multi-focus optical fiber lens based on all-dielectric metasurface

A multi-focus optical fiber lens is numerically demonstrated based on an all-dielectric metasurface structure. The metasurface consists of an array of rectangular silicon resonators with varying widths in order to obtain the required phase distribution. The core diameter of the multimode fiber is large enough to contain sufficient resonance units. The spatial distribution of the dielectric resonators is dictated by spatial multiplexing, including interleaving meta-atoms and lens aperture division, to achieve multi-focus properties. The proposed optical fiber metalens can produce two or three focal points along the longitudinal direction with high focusing efficiency. The size of every focal point is close to the diffraction limit, and the relative intensity on each focus can be controlled by adjusting the number of the respective resonators. The proposed optical fiber lens will have a great potential in the fields of integrated optics and multifunctional micro/nano devices.     

LED微阵列投影系统设计

为满足便携式投影仪的市场需求,设计了一种基于LED微型阵列的投影系统。该系统由显示单元和投影物镜构成。采用尺寸为12 mm×9 mm的自发光LED微型阵列作为系统的显示单元,利用光学设计软件设计了投影物镜。投影物镜采用反远距光学结构,全视场角为80°,焦距为8 mm,属于强光、广角镜头。在空间频率20 lp/mm处,该物镜的调制传递函数大于0. 85,畸变小于2%,符合投影系统的设计要求。该投影系统具有体积小,结构简单,投影效果好,易加工等诸多优势,可为第三代投影技术的发展提供参考。