Adaptive model of the gradient index of the human lens. I. Formulation and model of aging ex vivo lenses

A simple, parametric adaptive model of the refractive index distribution of the ex vivo crystalline lens is presented. It assumes conicoid (or nonrevolution quadric in 3D) iso-indical surfaces, concentric with the external surfaces of the lens. The model uses a minimum number of internal structural parameters, while the shape of the iso-indical surfaces adapts automatically to the external geometry. In this way, it is able to adapt and fit individual distributions as well as adapt to the changes of the lens shape and structure with age and accommodation. The model is fit to experimental data for individual eyes spanning ages 7 to 82 years, where for each eye the crystalline lens dimensions and iso-indical index data are known. The analysis demonstrates that only one age-dependent structural parameter is needed to replicate the internal iso-indical index structure, given age-dependent models for the external surfaces. An age-dependent-parameter global model is derived and is shown to predict age-dependent changes in the ex vivo lens power and longitudinal spherical aberration with age.     

Tomographic method for measurement of the gradient refractive index of the crystalline lens. II. The rotationally symmetrical lens

In the first part of this paper we presented a tomographic method to reconstruct the refractive index profile of spherically symmetrical lenses. Here we perform the generalization to lenses that are rotationally symmetrical around the optical axis, as is the ideal human lens. Analysis of the accuracy and versatility of this method is carried out by performing numerical simulations in which different magnitudes of experimental errors and two extreme case scenarios for the likely shape of the refractive index distribution of the human lens are considered. Finally, experimental results for a porcine lens are shown. Conceptually simple and computationally swift, this method could prove to be a valuable tool for the accurate retrieval of the gradient index of a broad spectrum of rotationally symmetrical crystalline lenses.     

Tomographic method for measurement of the gradient refractive index of the crystalline lens. I. The spherical fish lens

We present an iterative tomographic algorithm to reconstruct refractive-index profiles for meridional planes of the lens of the spherical fish eye from measurements of deflection angles of refracted rays. Numerical simulations show that the algorithm allows accuracy up to the fourth decimal place, provided that the refractive index can be regarded as an analytical function of the radial coordinate and the experimental errors are neglected. An experimental demonstration is given by applying the algorithm to retrieve the refractive-index profile of a spherical fish lens. The method is conceptually simple and does not require matching of the index of the surrounding medium to that of the surface of the lens, and the related iterative algorithm rapidly converges.     

Broadband microwave Luneburg lens made of gradient index metamaterials

Luneburg lenses are able to form perfect focus that is free of aberration. Because of the varying refractive index throughout the lens, incoming electromagnetic waves can travel in a curved path and be guided to focus at the back of the lens. The implementation of Luneburg lenses is often difficult due to the challenges in creating a medium with varying refractive index using normal materials. This problem can be overcome with the use of gradient index metamaterials. We report a two dimensional Luneburg lens made of gradient index metamaterials. It consists of 17 concentric shells with etched patterns on a printed circuit board working in microwave X band frequency. The broad properties of the Luneburg lens are then discussed.     

Single dispersive gradient-index profile for the aging human lens

We provide a single gradient-index (GRIN) profile for the crystalline lens in an updated age-dependent emmetropic-eye model. The parameters defining the GRIN profile include their variation with age and the dispersion of the refractive index in order to account for the increase in the positive-wave spherical aberration, for the constant chromatic difference in the refraction of the human eye, as well as for the decrease in the retinal-image quality with aging. In accounting for these ocular properties, the results show that first, the value of the dispersion parameters are invariant with age. Second, those parameters defining the distribution of the lens index cause the lens-center-index value to decrease slightly, and its position along the lens axis changes with age. Furthermore, these findings are in agreement with the lens paradox.     

光电技术研究所的自适应光学技术

自适应光学是近二十年发展起来的光学新技术，它使光学系统具有能动地适应外界变化自动克服动态干扰的能力。十多年来光电技术研究所的自适应光学研究室建立了自适应光学技术的基础，研制成功多套自适应光学系统。带动了能动光学元件、微驱动技术和光束动态诊断技术等单元技术的发展。本文综合报导了这些技术进展。     

Low chromatic aberration of gradient refractive index rod lens for Na+/Li+ ion exchange using oxide compositions model

In this paper, the oxide compositions model (OC model) is established for discussing the chromatic aberrations of gradient refractive index rod lens. The chromatic aberration for Na⁺/Li⁺ ion exchanges is discussed based on the OC model and the Huggins–Sun–Davis (HSD) model. Theoretical result indicates that the function value mainly depends on base glass properties and nature of exchanging ion pairs, and hardly depends on the quantity of ion exchanging. Experimental results show that the chromatic aberrations using the OC model have smaller errors than them using the HSD model. The estimating average error between the OC model and the HSD model is ?0.0067 for the Na⁺/Li⁺ ion exchanges.     

多层共轭自适应光学的进展与展望

该文论述了多层共轭自适应光学的原理和主要模式，波前畸变的探测与重构是多层共轭自适应光学的研究核心，文中分析了层析、定向层和多视场定向层等实现波前畸变探测与重构的方法；介绍了系统中导星的使用：总结并分析了近年来国内外在理论及实践中的研究进展和取得的成果．提出多层共轭自适应光学的发展前景．     

Numerical analysis of astigmatism correction in gradient refractive index lens based optical coherence tomography catheters

Endoscopic optical coherence tomography (OCT) catheters comprise a transparent tube to separate the imaging instrument from tissues. This tube acts as a cylindrical lens, introducing astigmatism into the beam. In this report, we quantified this negative effect using optical simulations of OCT catheter devices, and discuss possible compensation strategies. For esophageal imaging, the astigmatism is aggravated by the long working distance. For intracoronary imaging, the beam quality is degraded due to the liquid imaging environment. A nearly circular beam profile can be achieved by a curved focusing optics. We also consider the method of matching refractive indices, and it is shown to successfully restore a round beam.     

Nested shell optical model of the lens of the human eye

A nested shell model of the human lens is developed based on the known anatomical construction of the lens, on the known way in which the lens grows throughout its life, on the measured characteristics of the lens surfaces as a function of the age of the lens, on the measured changes in the shape of the lens during accommodation, and on measured material characteristics of the lens materials, such as density and index of refraction throughout. The observed changes in central surface curvature and thickness force the shell thicknesses to vary in a predictable way and in turn force the shell surface asphericity to take certain values. Thus, in addition to giving the shape of each shell, the model predicts the change expected in the asphericity of the lens surfaces as the lens ages and adds cortical cell layers. Two examples are given, one for a 25-year-old lens and one for a 40-year-old lens, to show how the cortical layers change their shapes throughout the cortex and over time as the lens ages. The performance of the model of this paper is compared to that of two other nested shell models, one where the layers have constant thickness and one where the lens posterior is fixed within the eye over time, to show the superior performance of this model with respect to maintaining a constant refractive error for the eye as the lens ages and grows.     

Coupling efficiency of optical coherence tomography system with gradient refractive index rod lens as reference arm

The coupling efficiency equation of an optical coherence tomography system with a gradient refractive index rod lens as the reference arm is established and discussed theoretically and experimentally. The experimental result is consistent with the theoretical result. The higher coupling efficiency of the OCT system can be obtained if θ, which is the angle between the mirror and the y axis, is smaller than 0.5°.     

Optical analysis of time-averaged multiscale Bessel beams generated by a tunable acoustic gradient index of refraction lens

Current methods for generating Bessel beams are limited to fixed beam sizes or, in the case of conventional adaptive optics, relatively long switching times between beam shapes. We analyze the multiscale Bessel beams created using an alternative rapidly switchable device: a tunable acoustic gradient index (TAG) lens. The shape of the beams and their nondiffracting, self-healing characteristics are studied experimentally and explained theoretically using both geometric and Fourier optics. By adjusting the electrical driving signal, we can tune the ring spacings, the size of the central spot, and the working distance of the lens. The results presented here will enable researchers to employ dynamic Bessel beams generated by TAG lenses.     

自适应光学在空间光学遥感器上的应用

阐述了在空间光学遥感器上应用自适应光学技术的必要性,介绍了国外的ＳＴ－ＭＴ,ＬＤＲ和Ｈｕｂｂｌｅ等几种空间光学遥感器应用自适应光学技术的情况,论述了轻小型和集成化的星载自适应光学系统的技术现状和发展趋势。进行了星载相机主镜温度变形的自适应光学校正实验,改善系数大于１０。这说明,对大口径、高分辨率空间相机采用自适应光学技术校正主镜由于温度场变化而产生的变莆,可以有效地改善波前质量。     

二维正交柱透镜列阵光学系统及优化设计

提出二维正交凸柱透镜列阵光学系统,可实现靶面光强二维均匀辐照,应用矩阵光学和衍射积分理论,详细分析了工作原理、焦斑强度分布特性,并从衍射、干涉等角度分析了系统参数与强度分布的关系,给出了详细的系统优化设计参数和数值计算结果。     

自适应光学系统SPGD控制算法的FPGA硬件实现

针对随机并行梯度下降(SPGD)算法实时性强,同时具有一定的灵活性的要求,本文提出了一种基于FPGA的SPGD算法硬件实现方法.该方法首先划分了各功能模块,然后对关键模块进行了实时化处理,并使用"流水线"和RAM技术设计了可升级和扩展的变形镜控制模块.最后将该算法实现并应用到6l单元自适应光学激光实验中,结果表明本文的设计可使用不同的性能指标实现变形镜的SPGD算法闭环控制,并能同时完成倾斜镜的控制,达到了实时性和灵活性的要求.     

Metamaterials with gradient negative index of refraction

We propose a new metamaterial with a gradient negative index of refraction, which can focus a collimated beam of light coming from a distant object. A slab of the negative refractive index metamaterial has a focal length that can be tuned by changing the gradient of the negative refractive index. A thin metal film pierced with holes of appropriate size or spacing between them can be used as a metamaterial with the gradient negative index of refraction. We use finite-difference time-domain calculations to show the focusing of a plane electromagnetic wave passing through a system of equidistantly spaced holes in a metal slab with decreasing diameters toward the edges of the slab.     

自适应光学系统时间带宽和空间带宽的匹配

自适应光学系统存在的时间带宽和空间带宽。为了有效地利用自适应光学系统的系统资源，必须使这两种带宽匹配。本文通过经系统校正后的剩余波前扰动相位的均方差与系统时间带宽和空间带宽的关系，研究了在不同条件下系统时间带宽和空间带宽的匹配问题。     

Refractive index gradient of the atmosphere at near ground levels

We experimentally investigated the mean refractive index gradient and its fluctuations due to atmospheric turbulence. The horizontal and vertical gradient components were directly measured by the corresponding displacements of thin laser beams propagating horizontally near the ground over short paths. A strong anisotropy was found when average values were observed as a function of time. This anisotropy can be related to the local convective air movement from the ground, irregularly perturbed by the wind. As a test, wind velocity was simultaneously measured and correlated to refractive index gradient measurements. The fluctuation PDFs (probability density functions) of the gradient horizontal component fluctuations show marked exponential tails, independently of the time-scales τ over which the averages and the corresponding fluctuations are evaluated. A similar behaviour is found for the vertical component only when very small values of τ are considered, showing that isotropy conditions hold for very short time-scales.