光谱色散后的相位调制光束衍射特性研究

用数值方法研究了激光驱动器系统中使用光谱色散平滑单元后光束的衍射特性.模拟结果表明，光谱色散会使光束衍射光斑变大，近场空间强度均匀性改善，而远场光斑内部存在光强接近于均匀分布的区域.进一步分析了光谱色散平滑单元中相位调制器的调制深度、调制频率及光栅色散系数等主要元件参数对光束传输特性的影响，发现在一定情况下光斑内部会出现较强的强度调制. 关键词： 光谱色散平滑 光束衍射 近场 远场     

划痕型缺陷对光场质量的影响

为了研究高功率激光系统中划痕型缺陷对光场质量的影响,采用分步傅里叶算法对非线性近轴波动方程进行求解,模拟分析了不同划痕参数(长度、宽度、深度)下,元件内部、光束近场、元件后传输光场以及光束远场的光强分布情况。数值模拟结果表明:随着划痕长度、宽度或深度的增加,元件内部以及元件后传输光场的峰值强度和对比度均会相应增强;光束近场的光强对比度也会略微增大;对于光束远场的强度分布,与划痕宽度方向所对应的频谱能量会不断增强。该项研究工作可以为划痕检验标准的制定提供一定的定量分析依据。     

用于光束匀滑的连续相位板近场均匀性

 为降低高功率激光系统中连续相位板(CPP)后续元件的强激光损伤风险,综合考虑入射光强调制、干涉及衍射作用等多种影响因素,建立了CPP近场计算分析模型,模拟和分析了这些因素对CPP后的近场均匀性的影响。理论分析结果表明：CPP后的光束近场均匀性主要受入射光调制、CPP表面剩余反射率和衍射传输距离的影响;当入射光束质量较差时,CPP后的近场均匀性主要由入射光束质量决定,CPP剩余反射率和衍射传输距离对近场均匀性影响相对较小;但当光束质量比较理想时,干涉和衍射作用会破坏CPP的近场均匀性,衍射传输距离影响尤为突出。     

高功率固体激光装置光学元件"缺陷"分布与光束近场质量的定量关系研究

基于光传输理论,获得了弱调制情况下光学元件"缺陷"分布功率谱密度 (power spectral density, PSD)与光束近场强度分布PSD之间的定量关系;通过数值模拟的方法,针对高功率固体激光装置的基本单元(线性介质、非线性介质以及空间滤波器)对获得的理论关系进行了具体的验证和讨论.研究结果表明,弱调制下,只存在振幅型或位相型"缺陷"分布时,光学元件"缺陷"分布PSD与光束近场强度分布PSD通过近场强度分布PSD的系统传输因子联系,传输因子与系统的构型和运行状态有关.研究结果为光学元件"缺陷"分布指标的获得提供了理论基础,对高功率固体激光装置负载能力的提升起到了一定的指导作用. 关键词： 缺陷分布 功率谱密度 光学元件 光束质量     

高功率激光系统中近场滤波设计与理论模拟

基于经典耦合波理论,分析了透射型体Bragg光栅的衍射特性,建立了透射型体Bragg光栅的设计体系。采用带有调制的超高斯光束,对高功率激光系统的近场滤波进行了理论模拟,采用软化因子、近场调制度、近场对比度分析了输出光束的近场质量,采用功率谱密度函数分析了近场滤波对不同空间频率的滤除能力。研究结果表明,近场滤波对0.3～2.0mm-1的中、高频调制具有显著的滤除作用。     

三倍频光学系统中近场调制增长规律研究

高功率固体激光装置的光束质量控制既要满足精密物理实验的苛刻要求,又要保证激光装置在高负载条件下的安全运行,是建造激光器的核心环节。三倍频光学系统承担着将1.053 μm的基频光转换成0.351 μm的三倍频光并聚焦到靶上的任务,是影响激光装置光束质量的重要环节。基于B-T理论研究了非线性传输条件下三倍频光学系统中光束调制的增长规律,基于典型的KDP晶体面形数据建立的中高频相位屏模型,通过数值计算获得了三倍频光学系统不同B积分下光束近场的变化规律。研究表明,三倍频光学系统中最快增长频率和相应的最快增长因子都是为基频光系统的三倍,空间调制周期在0.1～1 mm之间的空间频率成分增长最快。当三倍频光学系统B积分的设计值为1.5,要求三倍频光近场调制度小于1.15时,中高频噪声的RMS值必须控制在1.6 nm以下。     

用高阶对比度研究光束的小尺度自聚焦

提出了高阶对比度这一新参数来描述光束的近场均匀性，并用之研究高功率激光装置中的小 尺度自聚焦现象.分析了高功率光束小尺度自聚焦发生的过程中填充因子、调制度、对比度 及高阶对比度随B积分值的变化，指出高阶对比度在光束出现自聚焦成丝前后有更显著 的变化，所以是描述小尺度自聚焦效应的理想参数. 关键词： 小尺度自聚焦 三阶对比度 四阶对比度 功率谱密度相位噪声     

钕玻璃介质中强激光束传输特性的初步研究

 由入射强激光束的强度或相位调制引起的小尺度纹波在经过非线性介质后,将产生强烈的自聚焦效应。在非线性相位延迟(B积分)超过一定值后,将严重影响光束的近场强度分布,甚至造成丝状破坏。数值模拟和实验研究了在一定强度调制下的光束经过钕玻璃介质发生自聚焦成丝效应的阈值条件,以及非线性传输的一般规律。可以通过模拟计算,给出高功率激光系统设计中的D-B判据。     

带宽对强光束非线性成像的抑制效应

 宽频带激光驱动器中光束的传输与常规的窄带系统有很大区别。数值分析了宽带激光的成像这一非线性传输现象，并与窄带激光的成像特性进行了比较，发现宽带激光的热像位置和窄带激光基本相同，但宽带激光的热像强度比窄带激光低很多，说明宽带激光能很好地抑制非线性成像的形成。此外，通过改变脉冲啁啾和脉冲宽度两种方式分别揭示了带宽对非线性成像和光束匀滑的影响，发现不同带宽情形下热像的位置基本相同，均处于共轭位置处；热像的光强和热像处光束的调制对比度随着啁啾值的增大而降低，随脉冲宽度的增大而增大。由于受到群速度色散效应的作用，带宽能够阻碍热像的形成，并提高光束的匀滑度。     

自聚焦激光束光束质量评价的功率谱密度方法

把功率谱密度分析的方法引入到激光束光束质量的评价中，分析了自聚焦激光束的近场分布，研究了强激光非线性自聚焦的一般规律，给出了在一定强度调制下的光束经过钕玻璃介质发生自聚焦成丝效应的B积分阈值条件，实验和理论模拟结果基本一致.     

Numerical and experimental study on coherent beam combining of fibre amplifiers using simulated annealing algorithm

We present the numerical and experimental study on the coherent beam combining of fibre amplifiers by means of simulated annealing (SA) algorithm. The feasibility is validated by the Monte Carlo simulation of correcting static phase distortion using SA algorithm. The performance of SA algorithm under time-varying phase noise is numerically studied by dynamic simulation. It is revealed that the influence of phase noise on the performance of SA algorithm gets stronger with an increase in amplitude or frequency of phase noise; and the laser array that contains more lasers will be more affected from phase noise. The performance of SA algorithm for coherent beam combining is also compared with a widely used stochastic optimization algorithm, i.e., the stochastic parallel gradient descent (SPGD) algorithm. In a proof-of-concept experiment we demonstrate the coherent beam combining of two 1083~nm fibre amplifiers with a total output power of 12~W and 93% combining efficiency. The contrast of the far-field coherently combined beam profiles is calculated to be as high as 95%.     

激光束低频畸变波前模型的计算模拟

 在分析强激光束低频位相噪声特点的基础上,采用位相均方根梯度表征激光束的低频位相噪声并结合随机位相屏构建相应的畸变波前模型。通过模拟计算,给出低频位相噪声的均方根梯度与聚焦光斑尺寸的初步规律。     

激光束低频畸变波前模型的计算模拟

在分析强激光束低频位相噪声特点的基础上 ,采用位相均方根梯度表征激光束的低频位相噪声并结合随机位相屏构建相应的畸变波前模型。通过模拟计算 ,给出低频位相噪声的均方根梯度与聚焦光斑尺寸的初步规律。     

激光通信中大气闪烁引起探测噪声的计算机模拟

大气瑞流效应是光通信系统中不可忽视的一个重要问题，主要表现为大气闪烁，光束抖动、光束漂移等，这些现象都给探测系统带来附加噪声，使系统的接收灵敏度下降，本文主要讨论激光大气传输中强度闪烁效应（大气闪烁效应）的理论基础，以及强度闪烁的计算机模拟。     

Diode-laser noise conversion in an optically dense atomic sample

We report on oscillating complex noise spectra obtained when a diode-laser beam passes through a resonant dense Doppler-broadened cesium-vapor cell. Atomic coherence converts the laser phase noise into amplitude noise in the transmitted beam. We have found that the level of amplitude noise is orders of magnitude above the intrinsic laser noise. As a function of laser detuning, this noise extends over several inhomogeneous widths, depending on the spectral frequency. Numerical calculations based on a simple theory remarkably mimic the details of the experimental noise spectra.     

Analysis on the laser beam pointing instability induced fringe shift and contrast dilution from different interferometers used for writing fiber Bragg grating

This paper presents an analysis on the effect of laser beam pointing instability on the fringe shift and hence the contrast dilution of superposed fringes from two beam interferometers. The interferometers analyzed are those commonly used in writing fiber Bragg grating (FBG) such as phase mask, bi-prism and phase mask based Talbot/Holographic type. The beam pointing instability is incorporated as slight change in the angle between two interfering beams. The relative immunity of different interferometers to laser beam pointing is discussed vs location of FBG writing plane from the beam splitter. The effect of the beam pointing was minimum in proximity FBG writing by phase mask. The effect, in terms of fringes contrast dilution, was worst in case of large length interferometers e.g. phase mask based-Talbot interferometer. For intermediate length prism interferometers, the effect was moderate. For a given length interferometer, the fringe shift was directly proportional to beam pointing angle and inversely proportional to fringe separation. Theoretical analysis is verified experimentally by studying the fringe instability of interference pattern formed by a bi-prism of angle 2^o with the copper vapour laser (CVL, λ = 510 nm) beams of different beam pointing instabilities.     

The spatiotemporal evolution of sinusoidal phase modulated laser and its competitions with noise perturbed laser during small-scale self-focusing

Smoothing by spectral dispersion (SSD) is employed to smooth the laser irradiation in inertial confinement fusion and its effectiveness can be affected by nonlinear effects because of some special modulation of itself. In the paper, the spatial evolution dependent on the time period of the sinusoidal phase modulated laser from SSD is investigated theoretically and numerically when the modulated laser is affected by the small-scale self-focusing(SSSF). Although the effects of SSSF for SSD's modulation and noise can be eliminated by temporal average, the time-dependent modulation intensity of laser with SSD or noise will increased quickly. The results show that the low-frequency spatial modulation from SSD sometimes can affect beam quality bigger than the small-scale noise because of its controlled small initial amplitude. The simulation results agree with the theoretical analysis.     

光阴极微波电子枪中时间抖动对粒子动力学的影响

对光阴极微波电子枪粒子束性能起决定性影响的因素之一就是激光脉冲相对加速微波相位的时间抖动. 对这种时间抖动的测量和反馈控制进行了理论和实验分析, 并对时间抖动对束流品质的影响进行了模拟研究, 得到了利用锁相环回路降低激光脉冲相位噪声的实验经验, 为光阴极微波电子枪中激光与微波相位同步问题提出了解决方法.     

相位对比读出三维光数据存储的实验研究

使用波长800 nm、脉冲宽度150f s的近红外激光脉冲紧聚焦到PMMA（聚甲基丙烯酸甲酯）和熔融石英中实现了三维逐位式光数据存储,分别记录下了5、10、 15和20层数据位点.利用相位对比光学显微原理对各层数据并行读出,实验分析了各层数据位点的读出对比度的变化.结果表明:各层数据位点的折射率对比度由内至外依次增加,记录层数越多,其内部层的对比度下降越明显.由于飞秒激光脉冲与透明介质相互作用中,熔融石英内部产生的残余应力比PMMA内大,因此数据位点参数相同的情况下,利用PMMA材料记录的层数更多.     

Hard X‐ray phase‐contrast imaging with the Compact Light Source based on inverse Compton X‐rays

The first imaging results obtained from a small‐size synchrotron are reported. The newly developed Compact Light Source produces inverse Compton X‐rays at the intersection point of the counter propagating laser and electron beam. The small size of the intersection point gives a highly coherent cone beam with a few milliradian angular divergence and a few percent energy spread. These specifications make the Compact Light Source ideal for a recently developed grating‐based differential phase‐contrast imaging method.