High-cycle fatigue crack initiation and propagation in laser melting deposited TC18 titanium alloy

This article examines fatigue crack nucleation and propagation in laser deposited TC18 titanium alloy. The Widmanstatten structure was obtained by double-annealing treatment,. High-cycle fatigue （HCF） tests were conducted at room temperature with the stress ratio of 0.1 and the notch concentration factor Kt = 1. Fatigue cracks initiated preferentially at micropores, which had great effect on the HCF properties. The effect decreased with the decrease of pore size and the increase of distance from the pore location to the specimen surface. The crack initiation region was characterized by the cleavage facets of a lamella and the tearing of β matrix. The soft a precipitated-free zone formed along grain boundaries accelerated the crack propagation. Subsurface observation indicated that the crack preferred to propagate along the grain boundary α or border of a lamella or vertical to a lamella.

Adaptive split-step Fourier method for simulating ultrashort laser pulse propagation in photonic crystal fibres

In this paper, the generalized nonlinear Schrodinger equation （GNLSE） is solved by an adaptive split-step Fourier method （ASSFM）. It is found that ASSFM must be used to solve GNLSE to ensure precision when the soliton selffrequency shift is remarkable and the photonic crystal fibre （PCF） parameters vary with the frequency considerably. The precision of numerical simulation by using ASSFM is higher than that by using split-step Fourier method in the process of laser pulse propagation in PCFs due to the fact that the variation of fibre parameters with the peak frequency in the pulse spectrum can be taken into account fully.

First demonstration of improving laser propagation inside the spherical hohlraums by using the cylindrical laser entrance hole

The octahedral spherical hohlraums have natural superiority in maintaining high radiation symmetry during the entire capsule implosion process in indirect drive inertial confinement fusion.While,in contrast to the cylindrical hohlraums,the narrow space between the laser beams and the spherical hohlraum wall is usually commented.In this Letter,we address this crucial issue and report our experimental work conducted on the SGIII-prototype laser facility which unambiguously demonstrates that a simple design of cylindrical laser entrance hole(LEH)can dramatically improve the laser propagation inside the spherical hohlraums.In addition,the laser beam deflection in the hohlraum is observed for the first time in the experiments.Our 2-dimensional simulation results also verify qualitatively the advantages of the spherical hohlraums with cylindrical LEHs.Our results imply the prospect of adopting the cylindrical LEHs in future spherical ignition hohlraum design.

An exact solution to paraxial propagation of laser beams in longitudinal inhomogeneous plasmas

An exact, general solution for laser beams propagating in longitudinally inhomogeneous plasmas is obtained in the form of the diffraction integral. The Gaussian beam and the Hermite-Gaussian beam are taken for example. In the case of an increasing plasma density along the propagation distance, natural diffraction of the Gaussian beam is retarded. This retardance has a less effect on the central part of the Hermite-Gaussian beam while a considerable rise of the power in bucket （PIB） occurs in the surrounding part of the beam.

Propagation of high-power partially coherent fibre laser beams in a real environment

The propagation performance of high-power partially coherent fibre laser beams in a real environment is inves- tigated and the theoretical model of a high-power fibre laser propagating in a real environment is established. The influence of a collimating system and thermal blooming is considered together with atmospheric turbulence and me- chanical jitter. The laser energy concentration of partially coherent beams in the far field is calculated and analysed based on the theoretical model. It is shown that the propagation performance of partially coherent beams depends on the collimating system, atmospheric turbulence, mechanical jitter and thermal blooming. The propagation performance of partially coherent beams and fully coherent beams is studied and the results show that partially coherent beams are less sensitive to the influence of thermal blooming, which results in that the energy degeneration for partially coherent beams is only 50% of that for fully coherent beams. Both partially coherent beams and fully coherent beams become less sensitive to thermal blooming when the average structural constant of the refraction index fluctuations increases to 1.7×10^-14 m^-2/3. The investigation presents a reference for applications of a high-power fibre laser system.

Numerical simulation of super-continuum laser propagation in turbulent atmosphere

Considering the atmospheric extinction and turbulence effects,we investigate the propagation performances of supercontinuum laser sources in atmospheric turbulence statistically by using the numerical simulation method,and the differences in propagation properties between the super-continuum(SC)laser and its pump laser are also analyzed.It is found that the propagation characteristics of super-continuum laser are almost similar to those of the pump laser.The degradation of source coherence degree may cause the relative beam spreading and scintillation indexes to decrease at different propagation distances or different turbulence strengths.The root-mean-square value of beam wandering is insensitive to the variation of source correlation length,and less aperture averaging occurs when the laser source becomes less coherent.Additionally,from the point of view of beam wandering,the SC laser has no advantage over the pump laser.Although the pump laser can bring about a bigger aperture average,the SC laser has a lower scintillation which may be due to the multiple wavelength homogenization effects on intensity fluctuations.This would be the most important virtue of the SC laser that can be utilized to improve the performance of laser engineering.

The propagation dynamics and stability of an intense laser beam in a radial power-law plasma channel

By containing ponderomotive self-channeling,the propagation behavior of an intense laser beam and the physical conditions are obtained theoretically in a radial power-law plasma channel.It is found that ponderomotive self-channeling results in the emergence of a solitary wave and catastrophic focusing,which apparently decreases the region for stable propagation in a parameter space of laser power and the ratio of the initial laser spot radius to the channel radius(RLC).Direct numerical simulation confirms the theory of constant propagation,periodic defocusing and focusing oscillations in the parameter space,and reveals a radial instability which prevents the formation of bright and dark solitary waves.The corresponding unstable critical curve is added in the parameter space numerically and the induced unstable region above the unstable critical curve covers that of catastrophic focusing,which shrinks the stable region for laser beams.For the expected constant propagation,the results reveal the need for a low RLC.Further study illustrates that the channel power-law exponent has an obvious effect on the final stable region and laser propagation,for example increasing this exponent can enlarge the stable region significantly,which is beneficial for guiding of the laser and increases the lowest RLC for constant propagation.Our results also show that the initial laser amplitude has an apparent influence on the propagation behavior.

Nonlinear propagation of an intense Laguerre-Gaussian laser pulse in a plasma channel

The nonlinear propagation of an intense Laguerre-Gaussian(LG)laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method.The evolution equation of the spot size is derived including the effects of relativistic self-focusing,preformed channel focusing,and ponderomotive self-channeling.The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size,periodically focusing and defocusing oscillation,catastrophic focusing,and solitary waves are obtained.Compared with the laser pulse with fundamental Gaussian(FG)mode,it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse,while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse.Thus,the matched condition for the intense LG laser pulse with constant spot size is released obviously,while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.

4-8 Test Experiments of Laser Propagation and Control for Laser Cooling Experiment at the CSRe

Laser cooling of heavy ion beams is the most promising method to realize crystalline beams at storage rings[1].In order to perform the laser cooling experiment of C3+ at the CSRe[2], we tested the laser propagation and controlby using the He-Ne laser at the straight part (about 25 m long) before the experiment. The experimental resultsindicated that the laser was collimated and stable, and the laser intensity was strong enough after a long distancepropagation at the CSRe which satis ed the requirement of laser cooling of heavy ion beams.

Propagation of phase-locked truncated Gaussian beam array in turbulent atmosphere

Truncation manipulation is a simple but effective way to improve the intensity distribution properties of the phase-locked Gaussian beam array at the receiving plane. In this paper, the analytical expression for the propagation of the phase-locked truncated Gaussian beam array in a turbulent atmosphere is obtained based on the extended Huygens--Fresnel principle. Power in the diffraction-limited bucket is introduced as the beam quality factor to evaluate the influence of different truncation parameters. The dependence of optimal truncation ratio on the number of beamlets, the intensity of turbulence, propagation distance and laser wavelength is calculated and discussed. It is revealed that the optimal truncation ratio is larger for the laser array that contains more lasers, and the optimal truncation ratio will shift to a larger value with an increase in propagation distance and decrease in intensity of atmosphere turbulence. The optimal truncation ratio is independent of laser wavelength.

Propagation properties of off-axis Hermite-cosh-Gaussian beam combinations through a first-order optical system

Based on the Collins integral formula, the analytic expressions of propagation of the coherent and the incoherent off-axis Hermite-cosh-Gaussian （HChG） beam combinations with rectangular symmetry passing through a paraxial first-order optical system are derived, and corresponding numerical examples are given and analysed. The resulting beam quality is discussed in terms of power in the bucket （PIB）. The study suggests that the resulting beam cannot keep the initial intensity shape during the propagation and the beam quality for coherent mode is not always better than that for incoherent mode. Reviewing the numerical simulations of Gaussian, Hermite-Gaussian （HG） and cosh- Gaussian （ChG） beam combinations indicates that the Hermite polynomial exerts a chief influence on the irradiance profile of composite beam and far field power concentration.

INFLUENCE OF DIFFRACTION ON LASER-GENERATED ACOUSTIC WAVES PROPAGATING IN LIQUIDS

The effect of diffraction on the geometry of initially plane acoustic transients propagating in liquids is studied.Theoretical considerations concern the transition from plane to spherical waves upon passing the border between near and far wave zone.Experiments with CO_(2)-laser-generated acoustic waves in absorbing liquids are discussed.The wavefront curvatures and lateral extensions at different propagating distances in n-heptane are analyzed by high-speed shadowgraphy and measurements with a piezoelectric transducer.Good agreement between theory and experiment is obtained.

Dynamic Characteristics of Growing Modes of Raman Instability from Intense Laser Beam Propagating Through Plasma

An essential dispersion relation,which can describe the dynamic properties of stimulated Raman scattering instability as a laser beam propagates through plasmas,is derived analytically.The development of growth mode,angle distribution,and temperature dependence of the instabilities are presented by solving this dispersion relation numerically.A significant dynamic characteristic has been revealed that the temperature increasing of the electron would result in redshift of scattered spectrum at high laser intensities.Furthermore,a novel modulational instability with double-peak temporal structure appears in a limited density region because of the coupling of scattered upshift and downshift waves.

Effect of laser focus in two-color synthesized waveform on generation of soft x-ray high harmonics

Synthesis of multi-color laser pulses has been developed as a promising way to improve low conversion efficiency of high-order harmonic generation(HHG). Here we systematically study the effect of laser focus in a two-color waveform on generation of macroscopic HHG in soft x-rays. We find that the dependence of HHG yields on laser focus at low or high gas pressure is sensitive to the characteristics of single-atom harmonic response, in which “short”-or “long”-trajectory emissions can be selectively controlled by changing the waveform of two-color synthesized laser pulse. We uncover the phase-matching mechanism of HHG in the gas medium by examining the propagation of the two-color waveform and the evolution of time-frequency emissions of high-harmonic field. We further reveal that the nonlinear effects, such as geometric phase, atomic dispersion, and plasma defocusing, are responsible for modification of two-color waveform upon propagation. This work can be used to find better macroscopic conditions for generating soft x-ray HHG by employing two-color optimized waveforms.

弱相对论涡旋光在等离子体中传播的波前畸变及补偿

涡旋光与等离子体相互作用近年来在激光等离子体领域引起了广泛的关注.深入研究涡旋光在等离子体中的传播对粒子加速和辐射源产生等工作具有重要意义.本文着重探讨了弱相对论涡旋光在等离子中传播时,传播过程对电磁波结构的影响.基于三维粒子模拟,发现弱相对论涡旋光在等离子体中传播时会产生波前畸变.在给定等离子体密度时,畸变程度与电磁波强度及传播距离密切相关.基于相位修正模型,通过考虑电子相对论质量修正,在理论上对该现象进行了解释.此外,研究还发现可以通过设置适当的初始密度调制对波前畸变进行补偿抑制,并通过三维粒子模拟进行了验证.本工作加强了对涡旋光在等离子体中传播过程的理解,并为设计应用于相对论涡旋光的等离子体器件提供了参考.

激光增材制造体育器材用TC4钛合金疲劳裂纹扩展行为研究

目的揭示应力比对增材制造TC4钛合金疲劳裂纹扩展行为的影响规律。方法采用紧凑型拉伸试样,在恒载荷幅条件下对激光增材制造TC4钛合金进行了应力比为0.1、0.3和0.5的疲劳裂纹扩展实验,定量评价了不同应力比下合金的疲劳裂纹扩展速率和变化规律。基于Paris公式对裂纹扩展速率进行了拟合,分析了应力比对各参数的影响规律。最后通过扫描电镜对断口表面形貌进行了观察,分析了应力比对断裂模型的影响。结果在相同的∆K条件下,疲劳裂纹扩展速率随着应力比的增大而增大。在Paris公式中,参数C随应力比的增大而减小,参数m随应力比的增大而增大,并且m和lgC呈现线性关系。随应力比的增大,断口表面的河流花样增多、疲劳辉纹变浅、二次裂纹数量增加。结论应力比引起的裂纹尖端闭合效应和平面应力比变化是导致裂纹扩展速率发生改变的主要原因。

激光系统组件化仿真软件EasyLaser

针对国内一直以来缺乏一套能够对复杂激光系统进行全面仿真模拟工具的问题,在成熟仿真模块的基础上,基于组件化建模仿真思想研发了通用易用的激光系统仿真软件EasyLaser。它由通用的组件化建模仿真支撑平台SciSimu和8个仿真组件库构成,具备对光束的光学元件间传输、大气传输、电视成像、自适应光学校正、识别提取跟踪等复杂过程的建模与仿真能力。详细介绍了EasyLaser的物理仿真能力和软件特色,并利用EasyLaser软件搭建仿真光路模型,进行自适应光学系统对大气湍流效应的校正效果的分析和研究。

激光选区熔化生物医用Ti-6Al-4V合金的弯曲疲劳行为

分析了SLM Ti-6Al-4V合金的弯曲疲劳行为及相应的微观组织和断口形貌,设置与SLM试样化学成分相同的轧制态Ti-6Al-4V合金进行对比研究.采用X射线衍射(XRD)、结合电子背散射衍射(EBSD)的扫描电子显微镜(SEM)对合金的微观组织进行分析.结果表明,三点弯曲疲劳裂纹起始于准解理断裂表面附近的应力集中,随后向内扩展.SLM成形Ti-6Al-4V合金的弯曲疲劳寿命高于轧制成形Ti-6Al-4V合金,SLM Ti-6Al-4V合金内部孔洞缺陷导致表面附近应力集中,促进疲劳裂纹形核;而SLM Ti-6Al-4V合金中随机取向的α+β晶粒、二次裂纹和孔洞延缓了裂纹扩展,提高了疲劳寿命.对于轧制态Ti-6Al-4V合金,由大量近似取向α晶粒组成的宏观区引起应力集中,形成微裂纹,导致疲劳裂纹形核,而且宏观区对裂纹扩展的阻碍作用较小,不利于材料的疲劳寿命.

基于激光诱导击穿光谱的瞬态温度测量方法

温度是影响材料力学性能的重要因素之一,准确测量器件温度是认识材料在应力作用下其力学性能演变以及评估设备健康状态和寿命的重要方式。面向功率器件开关过程中焊接界面快速温变测量的需求,传统方法存在时间分辨能力不足、难以测量瞬态温度的问题。文中基于激光诱导元素特征谱线强度与温度的密切相关性,提出了一种微秒量级时间分辨能力的表面温度测量方法,并建立了样品表面温度与光谱特性之间的定量关系。研究结果表明,物质表面温度提升导致激光诱导等离子体光谱强度和信噪比增强,且增强效果受到光谱采集延时和门宽影响。采用反向传播-人工神经网络(back propagation-artificial neural network,BP-ANN)和偏最小二乘(partial least squares,PLS)法对表面温度与光谱特性关系定量拟合并校准,拟合模型线性相关性拟合度指标均大于0.99。BP-ANN拟合模型的拟合偏差更小,其均方根误差(root mean squared error,RMSE)为2.582,正确率为98.3%。该方法为物体瞬态温度测量提供了一种有效手段,对功率器件焊接界面健康状态的评估给予了有力支撑。

基于激光干涉光刻技术的产品防伪包装设计研究

为了提高产品防伪包装设计效果,提出基于激光干涉光刻技术的产品防伪包装设计方法。利用光纤耦合激光传输控制多光束干涉信息,基于光纤波导分束形成技术,集成处理光束传播路径,基于光束振幅、相位和偏振联合调制方法,提取产品防伪包装的防伪状态特征量,基于振幅型或相位型的微结构衍射单元阵列传输方法,构建激光干涉光刻模型,实现四光束、五光束和六光束的多光束激光防伪。测试结果表明,所提方法的被篡改率最低为0.05%,解密时间较短,均在30 ms左右,MSE为0.673%,PSNR为47.40 dB,SSIM为0.975,采用该方法进行产品防伪包装设计,提高了产品的激光防伪能力,降低被篡改率。