Shaping the beam profile of a partially coherent beam by a phase aperture

By use of a tensor method, an analytical formula for a partially coherent Gaussian Schell-model (GSM) beam truncated by a circular phase aperture propagating through a paraxial ABCD optical system is derived. The propagation properties of a GSM beam truncated by a circular phase aperture in free space are studied numerically. It is found that the circular phase aperture can be used to shape the beam profile of a GSM beam and generate partially coherent dark hollow or flat-topped beam, which is useful in many applications, e.g., optical trapping, free-space optical communication, and material thermal processing. The propagation factor of a GSM beam truncated by a circular phase aperture is also analyzed.     

Nonlinear beam shaper for femtosecond laser pulses, from Gaussian to flat-top profile

We present a straightforward method to transform a spatially Gaussian femtosecond laser beam into a flat-top shaped beam. The proposed technique takes advantage of a nonlinear phase induced in positive Kerr medium followed by a simple optical system. The variation of the refractive index with the laser intensity creates a phase plate which induces changes in the beam profile after propagation; flat-top and doughnut profiles are observed. The shaping conditions are computed numerically and confirmed experimentally. The method does not introduce energy losses. The device is very simple, self-regulated, flexible and does not need a manufactured phase plate or precise alignment. This method can be useful for light-matter interaction and laser machining.     

Flat-top profile of an excimer-laser beam generated using beam-splitter gratings

We present a simple solution to the beam-shaping problem in which a wide, typically rectangular spatial distribution of an excimer-laser beam should be transformed into a flat-top distribution, a few millimeters wide in both directions. The set-up can be made practically lossless and it allows the energy density of the pulse to be increased by a factor of 50. The method finds use, e.g., in pulsed laser deposition, UV lithography and micro machining.     

Generating of quadratic gray scale beam profile using binary gray scale masks

Generating beams with a desired quadratic lateral intensity distribution using 1D binary masks was analyzed, in detail. Effect of width of the first bar as well as the gray scale increment rate on the generated shaped beam was examined. It is shown that increasing the gray scale rate produces smoother and broaden profile. Besides, it was demonstrated that width of the first bar has great impact on the generated profile, so by increasing the width, the generated profile becomes sharper. Theoretical results are confirmed by experiment, as well.     

Design guidelines and characteristics of beam-shaping microstructure optical fibers

Microstructure optical fibers with flat-top fundamental mode are first proposed by introducing a low-index inner core into the core of index-guiding microstructure optical fibers. The design guidelines and characteristics of beam-shaping microstructure optical fibers are demonstrated. The interrelationships of inner-core index with laser wavelength, air hole diameter and size of inner core are investigated. The influence of the relative size of inner core on the spatial profile of the fundamental mode is demonstrated. Moreover, sensitivity of the flat-top fundamental mode profile from the slight change of the optimum inner-core index value is studied. Starting from these results we deduce that it is possible to fabricate beam-shaping microstructure fibers with nowadays technique.     

Estimation of the laser beam enabling rectangular processing by the inverse diffraction theory

A small rectangular laser spot has been widely studied for laser processing and laser repair technology. It is usually made by converging the input beam with the lens. Faithful reproduction of the laser spot is dependent on NA of the imaging lens. The small rectangular spot can be obtained by high NA lens, which is limited by many factors, such as high energy loss due to the reflection on the surface, large mass and volume, and strong sensitivity to aberrations and misalignment. On the other hand, the beam cannot be faithfully reproduced because of the diffraction with the low NA lens which has no such the limitations. One of the alternative ways to produce small rectangular profiles by using the low NA lens system is to estimate the input beam profile leading to the output profile of the sharp rectangular shape.     

半导体激光器线阵的棱镜组光束整形器和光纤耦合输出

针对半导体激光器线阵输出光束快慢轴方向光参数积不对称问题,提出并制作了基于直角棱镜片的光束整形器,其具有结构简单紧凑,制作安装容易的特点。通过数值仿真和实验对光束整形器进行了分析,研究表明整形器实现了半导体激光器线阵输出光束的对称化,并且光束经过透镜聚焦后对数值孔径为0.46,直径为200μm的光纤进行耦合,效率为53%。     

Numerical simulation of a Gaussian beam profile passing through a Kerr medium

The circular Gaussian beam shape has been simulated by numerical evaluation on beam shape based on z-scan method through beam radius. This method is useful in many studies such as z-scan experiments. The validity of the relation between peak and valley separation in close aperture data and the diffraction length of the beam has been investigated by this method for some real cases. The results are in good agreement with the experimental studies.     

1D Bessel-like beam generation by highly directive transmission through a sub-wavelength slit embedded in periodic metallic grooves

A new method to generate a quasi-one dimensional (1D) Bessel-like beam whose non-diffracting length extends to macroscopic scale is proposed by utilizing highly directive transmission through a sub-wavelength single slit embedded in periodic metallic grooves. We employed finite-difference time-domain methods for full-vectorial diffraction and spectral analysis. In optimal conditions, unique quasi-1D Bessel-like beams were generated in transmission for the incident p-polarized plane wave such that highly asymmetric far-field distribution was achieved; Bessel-like beam along the slit axis and a flat-top super-Gaussian beam along the other perpendicular axis. Detailed parametric studies for the proposed structure are reported in terms of the operating spectral range and a general window of optimal conditions.     

光纤激光器中基于偏振控制器的脉宽可调平顶光脉冲生成技术

提出了主动锁模联合非线性偏振旋转光纤激光器中基于偏振控制器的脉宽可调平顶光脉冲生成技术.光纤激光器由主动锁模部分和一根掺Bi的高非线性光纤联合两个偏振控制器及光纤检偏器构成的非线性偏振旋转结构组成.分析了该实验装置在不同状态下的工作机制,通过调节激光器中的两个偏振控制器,实验获得了10GHz的脉宽可调平顶光脉冲,脉宽调节范围在12~19ps之间,调节激光腔中的宽带滤波器,可在不同波长处获得平顶光脉冲.实验表明:输出的平顶光脉冲性能稳定,边模抑制比为65dB,定时抖动145fs.     

Direct ultrafast laser micro-structuring of materials using programmable beam shaping

Using active and programmable focal-spot shaping, we demonstrate direct femtosecond laser micro-structuring of materials. Surface marking as well as drilling with user-defined geometrical shapes of small dimensions (20 μm) are performed, without moving the laser beam nor the material target.     

Laser annealing process of ITO thin films using beam shaping technology

In this study, we develop a laser annealing system for In₂O₃Sn (ITO) to carry out heat treatment on oxides with high melting temperature on substrates with low melting temperature. It is known that the working temperature of traditional heat treatments is usually limited by the melting point of the substrate materials. To overcome this problem, we apply a laser annealing technique to modify the film properties, and to measure the electrical and surface properties, we use Hall measurement, a four-point probe, and an atomic force microscope in our experiment. We will discuss how the annealing is affected by the laser machining parameters, including the beam profile, intensity distribution, laser spot overlap, and laser operation mode. We will further show through experimental results that the beam profile greatly affects the surface roughness of the ITO films. With the use of a uniform beam profile with proper laser intensity, the surface roughness and the sheet resistance of the ITO films can be reduced from 23 nm to 4.2 nm and from 417 Ω/sq to 400.4 Ω/sq, respectively.     

梯度轴棱锥产生单个Bottle beam

首次提出一种新的梯度轴棱锥模型,这种梯度轴棱锥在传统的轴棱锥基础上,将轴棱锥顶点的锥角设计成具有一定的梯度.平面波入射到这种梯度轴棱锥,其衍射场会形成两个区域的Bessel光,并且在这两个Bessel光之间形成一个Bottle beam（局域空心光束）.由于Bottle beam具有三维封闭的空心区域和极高的强度梯度,可用作激光导管、光镊和光学扳手等,在生命科学和纳米科技中有重要应用.从几何光学角度分析了梯度轴棱锥形成单个Bottle beam的原理,由衍射及相干理论,分析和模拟了纵向剖面光强分布和横截面光强分布.研究结果对梯度轴棱锥的设计和应用具有指导意义. 关键词： 梯度轴棱锥 Bottle beam 轴棱锥 Bessel光     

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.     

KB镜束流截面测量系统研制

为了对衍射极限储存环的束流横向截面尺寸及发射度进行测量,设计了一套Kirkpatrick-Baez(KB)反射镜聚焦成像系统,并在上海光源(SSRF)储存环进行预制研究。该系统主体由两面垂直放置的KB反射镜组成,分别在水平及垂直方向对弯转磁铁光源点进行成像,系统工作在硬X射线波段,聚焦光斑被闪烁体X射线相机采集。对影响系统成像质量的像差和点扩散函数进行了计算。目前,实现了对束流的实时成像,可精确测量束流横向截面尺寸为75.9 μm(水平方向)和20.2 μm(垂直方向),系统稳定性(RMS)小于0.1 μm。     

Longitudinal phase-space coating of beam in a storage ring

In this Letter, I report on a novel scheme for beam stacking without any beam emittance dilution using a barrier rf system in synchrotrons. The general principle of the scheme called longitudinal phase-space coating, validation of the concept via multi-particle beam dynamics simulations applied to the Fermilab Recycler, and its experimental demonstration are presented. In addition, it has been shown and illustrated that the rf gymnastics involved in this scheme can be used in measuring the incoherent synchrotron tune spectrum of the beam in barrier buckets and in producing a clean hollow beam in longitudinal phase space. The method of beam stacking in synchrotrons presented here is the first of its kind.     

Beam propagation of a higher-order cosh-Gaussian beam

On the basis of the second-order moment, the beam propagation of a higher-order cosh-Gaussian beam has been investigated. Two analytical expressions of the M² factor have been derived, which corresponds to the parity of the beam order n. The M² factor is determined by the beam order n and the parameter δ. The influences of the beam order n and the parameter δ on the M² factor are illustrated with numerical examples. The analytical kurtosis parameter has also been presented. The analytical formula is further simplified in the source plane and the far field, respectively. With given appropriate values of n and δ, the kurtosis parameter in the far field is smaller than 3. This research is helpful to the practical applications of a higher-order cosh-Gaussian beam.     

束晕-混沌的神经网络自适应控制

 理论分析了强流离子束在周期磁场聚焦通道中传输时产生的束晕 混沌动力学行为,给出了近似反映实际聚焦磁场的余弦函数形式。然后利用神经网络方法对非线性复杂系统控制的优越性,提出前馈反传神经网络方法对强流离子束中束晕 混沌进行自适应控制。通过适当选择的神经网络控制结构和线性反馈系数以及自适应调整神经网络的权系数,可将强流离子束的包络半径达到束匹配半径的控制目标,且束包络的抖动大小明显减少,束晕 混沌现象得到了明显的抑制。     

Beam combination of a radial laser array: Flat-topped beam

An analytical expression of a radial laser array for flat-topped beam is derived based on the generalized Collins formula. The intensity distribution of the resulting beam focused by a lens at the focus plane, for phase-locked and nonphase-locked cases, is studied numerically. The effect of the Fresnel number and normalized radius on intensity distribution for phase-locked and nonphase-locked cases is also presented. It is found that intensity distribution for nonphase-locked case is much less sensible to the Fresnel number and normalized radius than that of phase-locked case.