微小孔非傍轴衍射光束传输特性的研究

基于非傍轴标量光束横截面上光强的精确定义,对桶中功率和二阶矩定义的光束束宽、远场发散角和M2因子等进行了比较研究.以平面波的微小圆孔衍射光束为例,进行了详细的数值计算.结果表明,桶中功率定义的光束束宽、远场发散角和M2因子较二阶矩定义的小,两种定义的M2因子都随束腰趋于零而趋向于零,桶中功率定义的光束束宽的传播规律与双曲线存在一定偏差.     

Optically trapping Rayleigh particles by using focused partially coherent multi-rotating elliptical Gaussian beams

By investigating the cross-spectral density of partially coherent multi-rotating elliptical Gaussian beams(REGBs) that propagate through a focusing optical system, we obtain the radiation force on a Rayleigh particle.The radiation force distribution is studied under different beam indexes, coherence widths, and elliptical ratios of the partially coherent multi REGBs. The transverse and the longitudinal trapping ranges can increase at the focal plane by increasing the beam index or decreasing the coherence width. The range of the trapped particle radii increases as the elliptical ratio increases. Furthermore, we analyze the trapping stability.     

Spatial structure of the degree of coherence for broadband dispersed laser beams

1D and 2D spatial distributions of a complex degree of coherence are obtained using numerical simulation for the broadband dispersed laser beams. The configuration of the spatial inhomogeneities (speck-les) of the degree of coherence is studied at various spectral widths and dispersions of the beam. It is shown that the speckle size increases with increasing distance from the initial plane where the light field is assumed to be spatially coherent. The small- and large-scale structures of the degree of coherence are shown for the transverse cross section of the beam. The results are compared with the analytically calculated correlation functions of the spatial coherence. The features of the speckle structure are interpreted using the concept of partial speckles.     

A Rotationally Symmetric Multifocal Pearcey Beam with a Polygonal Profile

The polygonal multifocal propagation properties and particle trapping experiments of a new kind of power-exponential-phase vortex chirped circular Pearcey Gaussian beam are investigated. Theoretical and experimental results show that it is possible to adjust the position and transverse spatial distribution of the beam in focal plane, as well as to make the transverse intensity distribution emerge an arbitrary polygonal profile by changing the distribution factor and vortex topological charge. Stable trapping of multiple particles is achieved in the experiment with transverse intensity fields in the polygon plane with different topological charges.     

分数傅里叶变换面上余弦-高斯光束的变换特性

 利用Wigner分布函数的方法，研究了余弦-高斯光束的分数傅里叶变换特性。导出了余弦-高斯光束在分数傅里叶变换面上光强分布和束宽的解析计算公式，并对此进行了数值模拟计算。研究表明：分数傅里叶变换阶数对余弦-高斯光束的光强分布有明显影响，余弦-高斯光束的轴上光强随分数傅里叶变换阶数呈周期性变化，束宽随分数傅里叶变换阶数也呈周期性变化，周期为2；对给定调制参数的余弦-高斯光束，通过适当选取分数傅里叶变化阶数可以获得平顶的光强分布。     

Miniaturized bending-free solitons by restoring symmetry in periodically biased photorefractives

We consider optical propagation through a centrosymmetric photorefractive crystal with the externally applied bias voltage modulated along the optical propagation direction. We analytically prove that, if the modulation scale is smaller than the optical diffraction length, the resulting effective nonlinearity has an even parity in the transverse plane for an even-symmetric intensity profile and supports bending-free solitons down to few-micrometer beam widths. Numerical integration of the full photorefractive model for light-matter interaction allows us to confirm the feasibility of these miniaturized solitons and, for longer modulation periods, to investigate the excitation of self-trapped wiggling optical beams.     

30 MeV电子束轰击旋转钽靶产生轫致辐射分析

提出了一种基于射频直线加速器的多脉冲X光照相系统,有望用于材料动态性能诊断等流体物理动力学研究。基于射频加速器的特点,该套照相系统能够产生时间跨度10 s以上、数个脉冲间隔可调、脉宽为几十至一百ns的脉冲电子束,产生电子束束斑半高宽尺寸小于1 mm。通过蒙特卡罗模拟程序Geant4,分析计算了特定的几何布局以及不同厚度及电子束束斑条件下,电子束打靶后在靶中的能量沉积,靶中的电子束散射对X光焦斑的影响,以及1 m处的照射量,探讨了这套X光照相系统的应用可行性。结果表明,在30 MeV,400 nC电子束轰击厚度为1 mm的靶条件下,1 m处照射量约为9.1 R,靶厚在1~2 mm范围内并未引起X光焦斑的明显增大。较小横向尺寸的电子束会引起靶体局部升温严重,将会制约脉冲数量;采用旋转靶能够提升脉冲数量,通过分析二维旋转靶的应力,分析了靶材升温以及钽/钽合金屈服强度对脉冲间隔的限制作用。     

Fractional Fourier transform of Airy beams

An analytical expression of an Airy beam passing through a fractional Fourier transform (FRFT) system is presented. The effective beam size of the Airy beam in the FRFT plane is also derived. The influences of the order of FRFT, the modulation parameter, and the transverse scale on the normalized intensity distribution and the effective beam size of an Airy beam in the FRFT plane are examined. The order of FRFT controls the effective beam size and the orientation of the beam spot. The modulation parameter affects the effective beam size and the number of lateral side lobes. The normalized intensity distribution and the effective beam size are both proportional to the transverse scale.     

任意偏振态光束全反射时的侧向和横向位移

光束在电介质界面发生全反射时,实际反射光束会在入射面内相对于几何反射光束产生一侧向位移,在垂直于入射面的方向产生一横向位移。利用改进的能流法研究了任意偏振态光束发生全反射时的侧向和横向位移特性。研究表明,侧向位移的大小与入射光束的两组成部分———TE和TM偏振光的相位差无关,而与两组分的光强比密切相关,且该位移可以表示为TE和TM偏振光束各自的位移按光强的加权平均。横向位移的大小不仅与入射光束两组分的光强比相关,还与组分的相位差密切相关。另外,反射光束不仅在椭圆偏振态入射的情况下会产生横向位移,而且在TE和TM偏振态之外的其他线偏振态入射时,也会产生横向位移。     

Optical resonators with inhomogeneous mirrors

It is shown that the possibility of changing a laser light beam intensity distribution exists when the output mirror transmittance depends on the transverse plane coordinates. Methods of manufacture of inhomogeneously transmitting laser mirrors with constant phase of the reflected beam over the whole surface are described. Two examples of application of such systems are considered: one for obtaining a laser beam with constant intensity distribution in the transverse plane, the other for single-segment energy extraction from high-order transverse modes. The total laser output power is shown to be almost the same as for use of a homogeneous output mirror.     

有横向分布为TEM10模的信号注入的环形腔激光器的双稳性

本文假设注入环形腔激光器的信号的横向场分布为TEM_(10)模.利用平均场近似方法得到了单横模共振时的定态方程,并进行了线性稳定性分析.结果表明,对优质腔来说,与Lugiato等讨论的注入信号为平面波光束及高斯光束(TEM_(00)模)的情况不同,注入光场横向分布具有TEM_(10)模的横式时,环形激光器的输出呈现双稳性.     

环形障碍物-轴棱锥产生局域空心光束

提出一种产生局域空心光束(bottle beam)的新方法,它是在传统的轴棱锥底面上放置一个环形障碍物.平面波入射到带有环形障碍物的轴棱锥底面时,将在轴棱锥产生的最大无衍射距离内形成bottle beam,而在bottle beam的前后仍然保持无衍射贝塞尔光.通过变化障碍物大小及轴棱锥底角实现bottle beam尺寸的控制.利用衍射理论描述了平面波经带有环形障碍物的轴棱锥产生bottle beam的原理,分析和模拟了传输过程中不同位置的截面光强分布及整个过程的三维光强分布,并给出相关的实验结果.研究结果对bottlebeam的实际应用具有积极的意义.     

Generation of a two-photon singlet beam

Controlling the pump beam transverse profile in multimode Hong-Ou-Mandel interference, we generate a "localized" two-photon singlet state, in which both photons propagate in the same beam. This type of multiphoton singlet beam may be useful in quantum communication to avoid decoherence. We show that although the photons are part of the same beam, they are never in the same plane-wave mode, which is characterized by spatial antibunching behavior in the plane normal to the propagation direction.     

光束控制系统热效应与球差对激光光束质量的影响

采用Strehl比、桶中功率、β参数和束宽描述远场光束质量，研究了光束控制系统热效应和球差对远场激光光束质量的影响.用四维仿真程序作了详细计算，给出了数值计算结果并进行了物理分析.研究表明，传输通道的热效应和正球差会使远场光强分布扩展，可聚焦能力下降，光束质量变差.有意义的是，当激光发射功率达到一定值时，合适选取负球差会在几何焦面得到比无球差时更高的峰值光强和更好的可聚焦能力.但在以前的工作中，由于焦移，类似的结论是在实际焦面上得出的.非线性热晕的存在是其物理原因，对此用数值计算例子作了说明. 关键词： 高功率激光 热效应 光束控制系统 球差     

A planar relativistic electron beam in a plasma channel bounded by conducting walls

The force interaction of a relativistic electron beam with a plasma in a channel bounded by planegeometry highly conducting walls is studied. The steady-state interaction regime, ω=ku, is analyzed using the model of a cold collisional electron plasma. The formulas for the transverse component of the force acting on the beam electrons are derived for an arbitrary deviation of the beam from the symmetry plane of the channel.     

Diagnostics of a hollow relativistic electron beam from the transfer function of a converting target

Methods for measuring angular, geometrical, and energy characteristics of a hollow relativistic electron beam using the transfer function of a converting target are developed. A relationship between the anisotropy parameters of bremsstrahlung behind the target, angular characteristics of electrons, and geometrical parameters of the electron beam in the target plane is established. The energy balance equation that describes the interaction between the electron beam and converting target is studied. The transfer function of the target and its behavior versus electron energy are found. The time and integral characteristics of the electron beam are examined.     

Reconstruction of the Wigner transform of a rotationally symmetric two-dimensional beam from the Wigner transform of the beam's one-dimensional sample

A coherent rotationally symmetric two-dimensional beam is essentially one-dimensional in content: It is fully determined by the one-dimensional sample along a diagonal of the circularly symmetric field distribution in a transverse plane. The linear transform that reconstructs the four-dimensional Wigner distribution of the full two-dimensional beam from the two-dimensional Wigner distribution of the one-dimensional sample is presented.     

Doppler shift of a laser pulse beam scattered by a rotating cone and cylinder

Based on laser radar equations, a Doppler shift model of a laser pulse beam scattered by a rotating arbitrary convex target is reported in this paper. The boundary relations between an incident pulse beam and the detected area elements are analyzed by geometric methods. The Doppler shift characteristics of the rotating cone and cylinder are discussed and the difference between the laser pulse beam and the plane wave scattered from the same rotating target is compared accordingly. Numerical simulations show that the Doppler shift is tightly relevant to their dimensions, speeds, and so on. In the same incidence conditions, the pulse beam and plane wave have difference peak values and the same Doppler shift bandwidth. If the waist radius of the pulse beam is larger, the peak value is higher, and the Doppler shifts are proportional to the speed of the rotating target. By virtue of our theoretical model, we probe into the scattered characteristics of the Doppler shifts of a laser pulse beam, which would benefit target identification in national defense.     

Nuclear shadowing effect in relativistic heavy-ion collisions

Two complementary approaches are used in studying the nuclear shadowing effect in heavy-ion collisions at SIS/GSI beam energies within a hadronic transport model. By analysing simultaneously the average transverse momentum of nucleons and pions in the reaction plane as a function of rapidity, the shadowing effect is revealed as an anticorrelation of the average transverse momentum distributions of nucleons and pions in semicentral and peripheral collisions. While by studying the azimuthal angle distribution of pions with respect to the reaction plane, the shadowing effect appears as an azimuthal anisotropy with a preferential emission of pions perpendicular to the reaction plane. The dependence of the nuclear shadowing effect on the impact parameter and the beam energy is also studied.     

Scattering of a general partially coherent beam from a diffuse target in atmospheric turbulence

The second-order and fourth-order statistical moments of the speckle field from a diffuse target in atmospheric turbulence are studied which have great influence on the performance of lidar systems. By expanding a general rotationally symmetric beam as a sum of Gaussian–Schell model(GSM) beams, the mean intensity of the general beam propagating over a distance in an atmospheric turbulence is formulated. Expressions for the degree of coherence(DOC) and the normalized intensity variance of the scattered field of a general beam from a rough surface in turbulence are derived based on the extended Huygens–Fresnel principle. The general expressions reduce to the well-known forms for a GSM beam. Another example for the general beam used in this paper is the collimated flat-topped beam. The results of both kinds of beams show that the intensity profile on the target plane is a key factor affecting the statistical characteristics of the speckle field.A larger beam spot on the target plane induces a smaller coherence length and a smaller normalized intensity variance of the received field. As turbulence gets stronger, the coherence length becomes smaller, and the normalized intensity variance firstly increases and declines to unity finally.