熔石英损伤修复坑下游光场调制的数值模拟与实验研究

系统研究了熔石英激光损伤修复后的形貌特征,根据测量数据建立了典型的损伤修复坑三维模型,利用标量衍射理论并结合快速傅里叶变换算法研究了修复坑在351 nm激光辐照下游光场调制的分布规律.研究表明,修复坑引起的光场调制会使得下游不同距离位置处出现环形光场增强区和轴上位置光场增强点;环形光场增强区位置距离修复元件较近,其环形调制极大值主要受修复坑深度的影响,且随修复坑深度的增大而逐渐增加;轴上位置光场增强点位置距离修复元件较远,其轴上调制极大值主要受修复坑边缘凸起高度的影响,且随凸起高度的增大而快速增加;环形调制极大值或轴上调制极大值增大的同时,其分布位置与修复元件之间的距离均会逐渐减小.实验验证表明,利用三维修复坑模型得到的下游光场调制数值模拟结果与实验测量结果具有较好的一致性.本研究结果对控制熔石英元件损伤修复形貌特征以抑制调制增强效应给出了具体的控制方向,对修复工艺的改进与完善提供了非常有意义的参考.     

不同类型修复坑形貌对光传输的影响北大核心CSCD

建立了不同类型熔石英元件损伤修复坑形貌的三维模型,利用标量衍射理论并结合快速傅里叶变换算法研究了在351nm激光辐照下修复坑形貌对光传输的影响规律。研究表明,无论是锥形、抛物形或是高斯形修复坑形貌,均会产生离轴位置的环形光场调制增强区,其环形调制极大值均主要受修复坑深度影响,随着修复坑深度的增大而增大,而其分布位置随着修复坑深度的增加逐渐靠近修复元件后表面,随着修复坑底面半径的增大而远离修复元件后表面;同时,当修复坑深度与底面半径一定时,三类修复坑形貌中以抛物形修复坑形貌最为理想。     

级联介质的空间排布对光场特性的影响

基于菲涅尔衍射原理，应用傅里叶变换方法，从理论上推导了高斯光束经级联非线性介质传输后任意位置处光场分布的解析表达式,研究了光场峰值强度、峰值出现的位置以及光束的束宽和调制因子等参量在级联介质中的变化规律。结果表明：随着非线性效应的增强，光场峰值强度逐渐增大，峰值距离逐渐缩短，光束束宽不断减小而调制因子逐渐增大。在介质排布中要尽量错开光束峰值位置，采用均匀排布方案虽然可以降低元件受损的风险，但还需对介质均匀配置的间距进行有效优化。     

高功率激光传输过程中散射点对光束质量的影响

为了研究高功率激光传输过程中不透明颗粒引起的光束调制,通过分析这些不透明颗粒的形状和分布特点,建立了位置呈随机分布的高斯状散射点模型,从光束的衍射理论和干涉叠加理论出发,得到该模型下散射点对传输光束质量影响的解析式。数值分析了高斯状散射点的大小、密度、散射面积比及其传输距离对输出光束的近场分布、位相分布和光束透过率的影响,结果显示亚毫米量级散射点的衍射效应引起最大调制可达1．4,光学元件散射面积比小于0．003时才能满足元件透过率大于99．5％的需求。该结果可用于评价高功率激光装置光学元件的加工状况,并对光学元件加工要求和激光装置的洁净度要求有指导意义。     

透镜光轴上的光场分布

透镜作为一种特殊简单的二元衍射光学元件,推导计算了它带有硬边孔径光阑和高斯激光束入射情况下的光轴上光场复振幅分布的一段表达式,并对几种不同参数的高斯光束情况给出了具体计算结果.当高斯光波退化为平面波和透镜相对孔径较大时,它所描述的光轴上的场分布与《光学原理》.一书上的结果一致. 关键词：     

基于偏振位相调制的位相轮廓测量

本文介绍了一种用来测量三维物体面形的位相轮廓方法，它是通过将正弦光场投影到被测物面，该光场被物体表面调制后发生位相改变，利用偏振位相调制及位相检测算法计算物面的位相分布，再根据几何关系实现对物体三维形貌的测量。实验装置采用一种偏振位相调制的干涉光场投影装置对光场进行简便而精确的移相，采用ＣＣＤ摄像机记录畸变光场，并用计算机处理和显示测量结果。文中给出了有关实验结果。     

基于响应函数的大口径衍射元件光场调制分析

针对大口径衍射光学元件快速高精度分析难题，提出了基于响应函数的光场调制分析方法。首先，将衍射元件表面的微结构分解成多个台阶结构；然后，使用严格矢量理论方法计算台阶对入射光场的调制作用，并转化成阶跃响应函数；最后，利用相干合成原理将每个台阶的阶跃响应函数合成为微结构对入射光场的响应。分析了微结构最小线宽、响应函数作用范围和台阶定位误差等对计算精度的影响，并利用所提方法计算了不同口径衍射透镜的近场及远场分布。结果表明，即使存在25 nm的台阶定位误差，透镜远场的最大光强和衍射效率与严格矢量理论计算结果的差异仍然小于2%，同时计算效率提升了至少3个数量级，可见，所提方法可兼顾大口径衍射元件分析的精度与速度。     

TIE和角谱迭代用于光学元件表面划痕深度检测

针对散射法在检测光学元件表面划痕时只能得到其光场分布而无法直接得到划痕深度信息的问题,将角谱迭代算法、光强传输方程（TIE）和角谱迭代结合的算法应用到光学元件表面划痕深度检测中。首先,采集光学元件表面的光场分布,分别利用两种重建算法得到表面划痕的相位分布,通过表面划痕对相位的调制特性计算出划痕深度;然后,从强度误差、相关系数及相对均方根误差来对两种算法的有效性进行评价;最后,通过实验验证了光学元件表面划痕深度重建结果的准确性。结果表明,与角谱迭代算法相比,TIE和角谱迭代相结合的算法重建划痕深度的相对误差更小,重建效果更好,重建精度更高。     

4π聚焦系统中衍射光学元件对聚焦场多光球结构的影响

主要研究了调制入射光场的衍射光学元件(DOE)对聚焦场的影响.以贝塞尔高斯光束为例,利用Richards-Wolf矢量衍射积分理论,分析几种不同DOE结构调制的径向偏振光束的4 π聚焦场.数值模拟结果表明径向偏振光束经DOE调制后的4π聚焦,在焦平面附近形成了具有潜在应用价值的多光球结构,并且发现形成的光球的个数与DOE的环数有关,而DOE的透射函数对其影响较小,且所形成光球的个数会随着DOE结构环数的增加而增加.     

相位共轭技术抑制散射及还原光场研究

采用数字相位共轭技术研究了散射介质中光场的还原特性。将散射介质简化为相幅随机调制面,推导了三种相幅调制情况下的光场还原效果,并给出数值模拟结果。结果表明在纯相位随机型调制或高斯曲线足够宽的高斯振幅相位随机型调制下,可以得到理想的光场还原效果。在纯相位随机型调制下,共轭波前记录精度越高、波前探测孔径越大,光场还原效果越好。光场的还原过程可以容纳一定的噪声,相对于光场相位的还原,强度还原的抗噪能力更强。     

Incident laser modulation of a repaired damage site with a rim in fused silica rear subsurface

Local CO2 laser treatment has proved to be an effective method to prevent the 351-nm laser-induced damage sites in a fused silica surface from exponentially growing, which is responsible for limiting the lifetime of optics in high fluence laser systems. However, the CO2 laser induced ablation crater is often surrounded by a raised rim at the edge, which can also result in the intensification of transmitted ultraviolet light that may damage the downstream optics. In this work, the three-dimensional finite-difference time-domain method is developed to simulate the distribution of electrical field intensity in the vicinity of the CO2 laser mitigated damage site located in the exit subsurface of fused silica. The simulated results show that the repaired damage sites with raised rims cause more notable modulation to the incident laser than those without rims. Specifically, we present a theoretical model of using dimpled patterning to control the rim structure around the edge of repaired damage sites to avoid damage to downstream optics. The calculated results accord well with previous experimental results and the underlying physical mechanism is analysed in detail.     

Numerical simulation of the modulation to incident laser by the repaired damage site in a fused silica subsurface

One of the main factors of laser induced damage is the modulation to incident laser which is caused by the defect in the subsurface of the fused silica.In this work,the repaired damage site irradiated by CO 2 laser is simplified to a Gaussian rotation according to the corresponding experimental results.Then,the three-dimensional finite-difference time-domain method is employed to simulate the electric field intensity distribution in the vicinity of this kind of defect in fused silica front subsurface.The simulated results show that the modulation is notable,the E max is about 2.6 times the irradiated electric field intensity in the fused silica with the damage site (the width is 1.5 μm and depth is 2.3 μm) though the damage site is repaired by CO 2 laser.The phenomenon and the theoretical result of the annular laser enhancement existed on the rear surface are first verified effectively,which agrees well with the corresponding experimental results.The relations between the maximal electric field intensity in fused silica with defect depth and width are given respectively.Meanwhile,the corresponding physical mechanism is analysed theoretically in detail.     

Study on incident laser modulation using surface micro-defects on KH₂PO₄ crystal

KH2PO4 crystal is a crucial optical component of inertial confinement fusion.Modulation of an incident laser by surface micro-defects will induce the growth of surface damage,which largely restricts the enhancement of the laser induced damage threshold.The modulation of an incident laser by using different kinds of surface defects are simulated by employing the three-dimensional finite-difference time-domain method.The results indicate that after the modulation of surface defects,the light intensity distribution inside the crystal is badly distorted,with the light intensity enhanced symmetrically.The relations between modulation properties and defect geometries(e.g.,width,morphology,and depth of defects) are quite different for different defects.The modulation action is most obvious when the width of surface defects reaches 1.064 μm.For defects with smooth morphology,such as spherical pits,the degree of modulation is the smallest and the light intensity distribution seems relatively uniform.The degree of modulation increases rapidly with the increase of the depth of surface defects and becomes stable when the depth reaches a critical value.The critical depth is 1.064 μm for cuboid pits and radial cracks,while for ellipsoidal pits the value depends on both the width and the length of the defects.     

激光脉冲诱导损伤点对激光传输的影响

高能重复激光脉冲对光学元件损伤点的产生和扩展与损伤点对后续激光脉冲光强的调制作用密切相关。观察激光诱导K9玻璃的损伤点,发现损伤点是从里到外呈现辐射状分布,内部损伤程度最大,可致充分断裂;向外损伤程度减小,呈现辐射的应力相变,引起折射率变化;对损伤点的透射光谱检测发现其透过率下降大于20%,但是下降的幅度与波长无关,说明充分断裂的材料会对激光进行充分吸收,类似黑体吸收,其对入射光的吸收只与损伤点的面积有关。CCD对激光通过损伤点后的光强分布探测发现： 在激光能量传输过程中,损伤点会导致光强分布的不均匀,存在明显的散射效应,这会引起激光光强分布的不均匀性,导致损伤区域的进一步扩散。     

Study of modulation property to incident laser by surface micro-defects on KH2PO4 crystal

KH₂PO₄ crystal is a crucial optical component of inertial confinement fusion. Modulation of an incident laser by surface micro-defects will induce the growth of surface damage, which largely restricts the enhancement of the laser induced damage threshold. The modulation of an incident laser by using different kinds of surface defects are simulated by employing the three-dimensional finite-difference time-domain method. The results indicate that after the modulation of surface defects, the light intensity distribution inside the crystal is badly distorted, with the light intensity enhanced symmetrically. The relations between modulation properties and defect geometries (e.g., width, morphology, and depth of defects) are quite different for different defects. The modulation action is most obvious when the width of surface defects reaches 1.064 μ. For defects with smooth morphology, such as spherical pits, the degree of modulation is the smallest and the light intensity distribution seems relatively uniform. The degree of modulation increases rapidly with the increase of the depth of surface defects and becomes stable when the depth reaches a critical value. The critical depth is 1.064 μ for cuboid pits and radial cracks, while for ellipsoidal pits the value depends on both the width and the length of the defects.     

Diffraction of a modulated laser beam in 3ω optics system

Based on diffraction optical theory, diffraction of a laser beam with periodic amplitude modulation and phase distortion is derived in 3ω optics system. Influence of defocus distance and focal length of a focusing lens on intensity distribution of diffraction light is investigated by numerical simulation. The results show that appropriate distance away from the focus spot and increase the focal length in final optical systems are beneficial to control the modulation of light intensity fluctuations and reduce the optical components damage caused by small-scale self-focusing effect.     

Measurement of the frequency modulation transfer function of a laser using a Mach–Zehnder interferometer

A technique is presented for determining the frequency modulation transfer function of a laser. The method is based on a Mach–Zehnder interferometer, with a significant difference in the optical path lengths of the two arms. A frequency-modulated laser beam incident on the interferometer produces a phase-modulated photocurrent signal with an effective modulation index that is related to the amplitude of the optical frequency modulation. Techniques for determining both the amplitude and the phase of the optical frequency modulation from the photocurrent signal are described.     

Laser picosecond acoustics in a two-layer structure with oblique probe light incidence

A theory for the analysis of experiments involving laser picosecond acoustics with obliquely incident probe light in a two-layer structure is outlined. The reflectance and phase changes of the reflected light are calculated with a theory that takes into account the effects of multiple optical reflections. The sample consists of a single partially transparent layer on a substrate, both with arbitrary optical constants. We discuss the conditions in which one may discriminate between components of the optical modulation of a probe beam arising from the photoelastic effect and from the displacement of the sample interfaces induced by the acoustic strain.     

Study of the near-field modulation property of microwaviness on a KH2PO4 crystal surface

The KH 2 PO 4 crystal is a key component in optical systems of inertial confinement fusion (ICF).The microwaviness on a KH 2 PO 4 crystal surface is strongly related to its damage threshold which is a key parameter for application.To study the laser induced damage mechanism caused by microwaviness,in this paper the near-field modulation properties of microwaviness to the incident wave are discussed by the Fourier modal method.Research results indicate that the microwaviness on the machined surface will distort the incident wave and thus lead to non-uniform distribution of the light intensity inside the crystal;in a common range of microwaviness amplitude,the light intensity modulation degree increases about 0.03 whenever the microwaviness amplitude increases 10 nm;1 order diffraction efficiencies are the key factors responsible for light intensity modulation inside the crystal;the light intensity modulation is just around the microwaviness in the form of an evanescent wave,not inside the crystal when the microwaviness period is below 0.712 μm;light intensity modulation degree has two extreme points in microwaviness periods of 1.064 μm and 1.6 μm,remains unchanged between periods of 3 μm and 150 μm,and descends above the period of 150 μm to 920 μm.     

Evolution of the hot image effect in high-power laser system with cascaded Kerr medium

Based on the diffraction theory model for hot image formation, the evolution of the hot image effect in a high-power laser system structured by cascaded Kerr medium with gain and loss is numerically investigated. For a sample simulation system with five pieces of Kerr medium plates arrayed in parallel, the dependences of the peak intensity and the location of the hot image on different parameters of the scatterer and the system are analyzed. It is shown that the possible damage spots caused by the beam peak intensity would be repetitious in the laser system with cascaded medium. Arranging the Kerr medium plates and the downstream optical components in a rational way (for instance increasing the interval between two adjacent medium plates, arranging the Kerr medium plates aperiodically or certain optical components properly) may be helpful to relax or avoid the optical damage caused by the hot image effect.