Influence of laser polarization on ocular refractive parameters after refractive surgery

We provide an adjustment factor for ablation algorithms used in photorefractive laser surgery that takes into account how laser polarization in reflection losses affects the cornea. We evaluate the influence of this factor on corneal radius and asphericity after surgery, showing that it is significant for visual performance (effective visual acuity is reduced) and for the correction of eye aberrations. Our data indicate that this adjustment factor should be included in the ablation algorithms (depending on the polarization state of each laser device) that are proposed for customized corneal ablation, which need great accuracy for minimization of eye aberrations.     

Evaluation of the meridional longitudinal spherical aberration from corneal topography measurements

This paper shows how corneal topographic data can be used to determine the value of the longitudinal spherical aberration. We have obtained the corneal profiles and the values of the longitudinal spherical aberration for the rays propagating within the steepest and flattest meridional planes, by using a real raytracing algorithm. These corneal profiles have been also fitted to conicoids and the asphericity parameter has been calculated. We have found that the longitudinal spherical aberration follows a parabolic dependence for a circular region of 5 mm in diameter. This parabolic dependence has been fitted with a polynomial function. The data provided by commercial topographic systems can be used to obtain the longitudinal spherical aberration along the selected meridians.     

随机取向非球形沙尘气溶胶粒子的光学特性

利用离散偶极子近似法分析了一种随机取向旋转椭球体沙尘气溶胶粒子模型在尺度参数变化范围为0.1~23时(波长0.55!m对应有效半径为0.01~2!m)的光学特性,研究了沙尘粒子非球形性程度对其光学特性的影响,并考察了非球形粒子的随机取向能否用等体积球体来代替。就随机取向单分散和多分散旋转椭球体沙尘气溶胶而言,粒子非球形特征越明显,消光效率因子、不对称因子和单次散射反照率基本上偏离其等体积球体越大;对于相同的非球形,不对称因子偏离其等体积球体的相对偏差要比消光效率因子和单次散射反照率要大。非球形粒子的随机取向并不能使其光学特性严格等效为其等体积球体的光学特性。如果粒子形状偏离球体较小,则非球形粒子的随机取向的平均效果能使其消光效率因子、不对称因子和单次散射反照率近似用等体积球体的对应光学参量来等效;而如果粒子形状偏离球形较大,仅有单次散射反照率可以近似用等体积球体的单次散射反照率来等效,例如,轴半径比为16的旋转椭球体沙尘粒子的单次散射反照率偏离其等体积球体仅在3%以内。     

Influence of local inhomogeneities induced in corneal ablation on the evolution of contrast sensitivity

The presence of local inhomogeneities in corneal tissue after refractive surgery has an influence on visual performance. Here we focus on the corneal ablation associated with Lasik surgery and its effect on the modulation transfer function (MTF) that we obtained by modifying a personalized Kooijman model. Inhomogeneities induced by the ablation occur in the form of Gaussian-distributed refractive-index variations of a given correlation length. We show how variation of refractive-index deviation and correlation length (size) of the inhomogeneities allows us to obtain pairs of values that are able to achieve a MTF evolution similar to that observed for contrast sensitivity in the same patients. An estimate of the characteristics of the local effects is obtained.     

A new method to calculate corneal ablation depth based on optical individual eye model

The corneal shape, axial lengths and the wavefront aberrations of the eye were all considered to calculate the ablation depth based on the individual eye model. The optimization method for the curves surface replaced the direct calculation from only the optical path difference method (OPD). We analyzed the eye's optical system on its exit pupil and offered the optimum corneal anterior surface. And the ablation depth was the difference between the pre- and post-optimization along the optical axis of the eye. In our experiment, the maximum ablation depth decreases by 8.5% and the mean ablation depth decreases by 8.2% compared with the OPD method.     

Minimization of cavitation effects in pulsed laser ablation illustrated on laser angioplasty

Cavitation effects in pulsed laser ablation can cause severe deformation of tissue near the ablation site. In angioplasty, they result in a harmful dilatation and invagination of the vessel walls. We suggest to reduce cavitation effects by dividing the laser pulse energy into a pre-pulse with low and an ablation pulse with high energy. The pre-pulse creates a small cavitation bubble which can be filled by the ablation products of the main pulse. For suitable energy ratios between the pulses, this bubble will not be enlarged by the ablation products, and the maximal bubble size remains much smaller than after a single ablation pulse. The concept was analyzed by numerical calculations based on the Gilmore model of cavitation dynamics and by high-speed photography of the effects of single and double pulses performed with a silicone tube as vessel model. The use of double pulses prevents the deformation of the vessel walls. The concept works with an energy ratio of up to about 1:30 between the pulses. For the calculated optimal ratio of 1:14.6, the bubble volume is reduced by a factor of 17.7. The ablation pulse is best applied when the pre-pulse bubble is maximally expanded, but the timing is not very critical.     

Estimation of the degree of asphericity of a glass sphere using a vectorial shearing interferometer

The degree of asphericity is estimated by determining the average radius of curvature in different sections, at various points on the surface of a sphere, and the deviation from it. We employ the vectorial shearing interferometer (VSI) as the instrument to determine the radius of curvature from two subapertures of the transparent glass sphere. We incorporate the sphere as a thick lens into the interferometric setup, illuminating it with an expanded beam. The spherical aberration, introduced by the sphere in the wave front, depends on the local sphere radius, on the refraction index of the glass, and on the cone angle of the source. The wave front aberrated by the sphere impinges on the VSI. Here, the wave front is divided in two in amplitude, it is sheared vectorially, and it is superimposed with itself. The fringe pattern is formed in the intersection of the wave fronts. The shape of the resulting fringe pattern is directly related to spherical aberration. We estimate qualitatively the degree of asphericity, comparing the phase gradients in different sections of the sphere. Here, we report on the experimental setup to test the asphericity, the results with different vectorial shearing (magnitude and direction). Finally, we perform a comparison with the theoretical predictions.     

激光治疗近视眼中角膜曲率修正量的理论计算

本文从示意眼模型出发,利用自治的单透镜近似模型,根据矩阵光学原理,用计算机算出不同近视眼度数焦点的前移量以及在三种不同原因引起的近视眼的情况下角膜需要切除的数量。计算告诉我们即使1000°近视眼,角膜最大切除量也仅50μ,不到角膜总厚度的1/10。 理论上探讨了根据激光束的高斯分布经过光学系统改变光强分布来实现近视眼需要校正的曲率。     

Effects of the amorphous layer on laser-induced subwavelength structures on carbon allotropes

By micro-Raman spectroscopy, we show that the structured surfaces of highly oriented pyrolytic graphite and diamond induced by 800 nm, 125 fs or 532 nm, 30 ps laser pulses are capped by thin amorphous carbon layers. Based on the results, we propose that for multiphoton ablation the thin amorphous layer with a reduced bandgap can facilitate surface ionization, raise free electron density, bring on plasmonic effects, and thus promote the growth of subwavelength structures. Therefore, concerning multipulse laser ablation of wide bandgap materials, we should take into account the effects of the superficial amorphous layer produced by preceding pulses instead of the intrinsic surface.     

A model of nanosecond laser ablation of compound semiconductors accounting for non-congruent vaporization

We report a model of nanosecond laser ablation of compound semiconductors taking into account stoichiometry loss as a result of different volatilities of the material components. The model is based on the heat-flow equation for the bulk material and the diffusion equation for its atomic constituents and takes into account variations of material properties as functions of temperature and composition. Changing the optical response which results from stoichiometry violation is described within the concept of an effective medium and a multi-layer reflection model is applied. For cadmium telluride, as an example, the processes of ablation, melting, and resolidification under the action of a KrF laser have been studied in dynamics for particular experimental conditions in a wide range of laser fluences from the ablation threshold to the plasma shielding regimes described by the effective plasma plume representation. Multi-shot irradiation regimes have been investigated and the mechanism of the irradiation-controlled stoichiometry reversal has been elucidated.     

电磁轨道炮性能的数值模拟

 本文介绍一种简单的数值计算模拟，它可以模拟轨道炮的主要性能。模型中考虑了由于炮膛壁烧蚀引起的电枢等离子体质量的增加和各种阻力的影响。这些阻力包括弹丸与壁之间的摩擦力，等离子体粘性及空气阻力等。用一个经验关系表征它们的影响。我们把等离子体等效成一个可变电阻负载，而不管它与电离度有关性质的细节。数值计算结果与实验结果比较表明符合程度是满意的。     

Effects of waveguide behavior during femtosecond-laser drilling of metals

We investigate the dynamics of femtosecond-laser drilling of metals, both theoretically and experimentally, by taking into account waveguide-like behavior of ablated cavities. In particular, we show that cylindrical holes generated during laser ablation of metals act like hollow optical waveguides. Since the drilling is generally achieved by a large number of consecutive pulses, each pulse is first guided through the channel formed by the previous pulses, and at the end of the channel, it is absorbed by the metal, making its own contribution to ablation. The ablation stops at maximum depth when attenuation in the cavity reduces the pulse fluence to the ablation threshold. We use waveguide theory to calculate attenuation constants, and perform an iterative calculation to model pulse-by-pulse ablation. We also performed detailed experiments and compare the results with the theoretical findings. When only absorption losses are included, the waveguide model predicts significantly deeper structures. On the other hand, when we include scattering losses caused by nanostructures formed on the cavity walls, quantitative agreement with experiments is achieved. The waveguide model is particularly effective at fluences close to ablation threshold, and it can explain several behaviors such as evolution of the depth per pulse and effect of the incoming pulse energy in different focusing configurations.     

Simulations of light scattering spectra of a nanoshell on plane interface based on the discrete sources method

The resonance properties of nanoshells are of great interest in nanosensing applications such as surface enhanced Raman scattering or biological sensing. In this paper the discrete sources method has been applied to analyze the spectrum of evanescent light scattering from a nanoshell particle deposited near a plane surface. Based on the rigorous theoretical model, which allows to take into account all features of the scattering problem as: medium with frequency dispersion, presence of the interface, the objective aperture and its location and core-shell asphericity, the scattering spectrum of nanoshells was calculated. The dependence of the local nanoshell spectral density behavior on its properties is discussed.     

Research of cornea section''s shape ablated by 193-nm ArF laser spots

The ablation theory of cornea and biology effect by 193-nm ArF excimer laser are introduced. The ablation tracks model is put forward to make laser spots scan around cornea by many steps and many areas to change cornea curvature. The corneal average ablation curve is calculated by software so as to explain the feasibility of the ablation tracks model. By analyzing the actual ablation shapes of many arbitrary cornea sections, the optimal ablation method for deciding the random position of every laser spot in every ablation track is obtained. Experiments combining the ablation model with the device testify the energy stability of laser spots and the accuracy of rectifying anisometropia.     

Influence of dynamic screening on the conductivity of hydrogen plasma including electron–electron collisions

We investigate the dynamical conductivity of dense hydrogen plasmas on the basis of the effective interaction potential (taking into account static or dynamic screening and diffraction effects). We apply a generalized Drude–Lorentz formula related to the dynamical conductivity with collision frequency. The influence of electron–ion collisions as well as the influence of both electron–ion and electron–electron collisions were taken into account via a renormalization factor. All calculations were performed on the basis of the effective potential with static or dynamic screening.     

脉冲激光沉积动力学研究进展

脉冲激光沉积技术是现代常用的先进薄膜材料制备技术之一.文章在简要介绍脉冲激光沉积技术及其进展的基础上,较全面地介绍了脉冲激光沉积动力学的基本物理图像和动力学构架,深入地探讨了激光烧蚀靶材过程、等离子体膨胀过程和薄膜沉积过程的动力学规律,阐述了我国学者在脉冲激光沉积动力学研究方面的贡献,例如包括脉冲激光沉积三个工艺过程自洽的统一模型,等离子体膨胀的冲击波模型,基于局域能量动量守恒定律的新等离子体演化动力学模型,包括热源项、蒸发项、等离子体屏蔽效应和动态物性参数的烧蚀热传导模型,考虑电子碰撞效应和能带结构变化的修正双温模型,能统一描写从纳秒级到飞秒级脉冲激光烧蚀规律的统一双温模型等.     

特大非球面度离轴非球面补偿器结构设计与装调

针对快焦比特大非球面度离轴非球面反射镜,设计了3片式Offner补偿器。为应对3片式补偿器对中心偏差及镜间隔严格的公差要求,设计了相应的补偿器镜筒结构。该结构使透镜中心倾斜及平移调整相分离,实现补偿器的高精度装调。根据中心偏差测量仪的测量结果,2片补偿镜之间倾斜误差4.4″,平移误差3.5 μm, 镜间隔误差3.8 μm;补偿镜组与场镜之间倾斜误差5.3″,平移误差4.2 μm, 镜间隔误差7.2 μm,满足检测使用要求。利用该补偿器及4D动态干涉仪对精抛光阶段的离轴非球面进行检测,面形结果PVq值达到0.135λ,RMS值达到0.019 5λ,优于设计要求。     

分形理论在碳化材料三维烧蚀热防护计算中的应用

烧蚀过程中的传热传质对烧蚀防护工程具有重要意义,准确的温度评估可以为高超声速飞行器的热防护结构和烧蚀材料的设计提供有效的支持.由于材料烧蚀形成的碳化物是一种典型的多孔介质,其结构具有自相似性,可以用分形理论来描述.在烧蚀计算中引入了分形渗透率张量模型进行计算.针对烧蚀过程中热解气体在碳化层中的扩散方程、温度、材料等参数...     

磁绝缘线振荡器阴极烧蚀与电压波形的关系研究

通过粒子模拟和实验研究相结合的方法,研究了磁绝缘线振荡器的阴极烧蚀与输入电压波形的关系,找到了造成阴极烧蚀的根本原因,给出了电子束回轰阴极的清晰图像,并通过对低阻抗加速器平台输出电压波形的调整,抑制了反向电压,解决了器件的阴极烧蚀问题.研究结果表明:在正向电压作用下已经发射出来的电子束在超过一定幅度的反向电压作用下发生反向运动并回轰阴极,产生反向电流并造成器件阴极的烧蚀,因此注入电压下降沿之后的反向电压是引起磁绝缘线振荡器的阴极烧蚀的最根本因素.     

Role of laser-induced plasma in ultradeep drilling of materials by nanosecond laser pulses

Radiative effects of the laser-induced ablative plasma on the heating and ablation dynamics of materials irradiated by nanosecond laser pulses are studied by the example of graphite ablation. On the basis of combined thermal and gas dynamic modeling, the laser-induced plasma plume is shown to be a controlling factor responsible for ultradeep laser drilling due to plasma radiation, both bremsstrahlung and recombinative. We demonstrate that plasma radiative heating of the target considerably deepens the molten layer, thus explaining the observed crater depths.