第七章 光全息术 Optical Holography,光波的完整记录与再现Recording and Reconstruction of Light Wavefront,1、全息术发展简史,§7.1 引言 Introduction,丹尼斯·伽柏（Dennis Gabor，1900年～1979年），英国匈牙利裔物理学家，1971年诺贝尔物理奖获得者，全息摄影技术的发明者,1948年 Dennis Gabor 提出 “波前重现” 理论 目的：改善电子显微镜的分辨率 效果：因光源(汞灯)相干性差，效果不明显,1960年 激光器 问世， 提供 理想的相干光源 为全息技术的发展创造了条件 红宝石 Laser — 梅曼（英），美国休斯飞机制造公司 氦—氖 Laser — 霍曼（伊朗），美国贝尔实验室,1962年 离轴全息图 问世 ——— Leith 和 Upatnieks （美） 加速了全息术的发展,40多年来，全息科学和技术得到飞速发展，应用领域几乎无所不及。
§5.1 引言 Introduction 1、全息术发展简史,1964年 氩离子激光器 Ar+ Laser 问世 提供了短波长连续可见光 扩展了全息技术的应用范围… …,Introduction to laser light 激光简介,1 Light is most easily thought of its being trains of waves, which like water waves and sound waves.,4 In comparison to red light, green light has a shorter wavelength,6 Most light, though, like the light from a flashlight, is disorganized or incoherent..,9 This means the light is a single color and monochromic.,11 Together these properties result in a highly uniform and controllable beam of coherent light.,2 They also lay between two extremes called crests and troughs.,3 The color of light depends on the distance between crests and it’s called its wavelength,5 Light comes from many different sources.,7 It’s made of many colors generated random and traveling in many different directions,8 Laser light, however, consists of only one wavelength.,10 The light is also in phase. Crests and troughs are in perfect alignment.,and blue light’s wavelength is shorter still.,单色光波 在z = 0 平面, 复振幅分布: O (x,y),记录此波前要求: 制作出一种薄的光学元件, 其复振幅透过率 t(x,y) = O (x,y),再现：用单位振幅的平面波垂直照明 在z = 0 平面，透射波复振幅: U(x,y) ＝ 1. t(x,y) ＝O(x,y) 在z = 0平面上所有的点都再现出初始的波, 在z > 0 的空间中任意位置都再现出初始的波。
§7-2 全息术原理 — 波前记录与再现,1. 波前记录: 将位相编码为强度 Transform Phase into Intensity,制作透明片t(x,y)= O(x,y)的关键障碍：光学探测器只响应光强度| O(x,y)|2，而对位相arg{O(x,y)}不敏感位相信息不应当丢弃为了记录O(x,y)的位相，应当找到一种编码方法将位相的变化转换为强度变化1. 波前记录: 全息编码 Holographic Code,基本点：在z = 0平面, 将初始光波O（物波）与已知的参考波R叠加(混合).用照相方法记录两个波叠加以后干涉图样的强度得到复振幅透过率 t与曝光强度成正比的透明片.,物光波的振幅和位相信息以干涉条纹的形状、疏密和强度的形式“冻结”在感光的全息干板上这就是波前记录的过程Recording of a hologram 全息图拍摄,Recording of a hologram 全息图拍摄,In holographic laboratory, the laser and the object sit on the massive table that isolates them from vibration of the environment.,This setup is used to make the basic laser transmission hologram,,the type gained by Dennis Gabor and developed by Emmett Leith.,Using a piece of ground glass to show the laser light, let’s follow this path.,The light leaves a laser and hits a partially specular mirror called a beam splitter, which splitters the laser beam in two.,One beam, called the object beam, is directed to the space shuttle. A lens expands the beam to cover the shuttle with light.,This light reflects off the shuttle onto the film holder, which would be loaded with unexposed films.,The other beam, called the reference beam, is directed by mirrors and expanded with a lens to shine directly on the film holder.,From above, the table looks like this.,Now we are ready to shoot the hologram.,The laser light is blocked, the room is darken, and the film holder is loaded with film, which is exposed by the laser light from the shuttle.,Principle of hologram recording 全息图的拍摄原理,During exposure the object beam, whose crests are indicated by lines, strikes the space shuttle,,The light reflecting from the low of space shuttle is shown for simplicity,At the same time, the uniform and featureless wave-front of reference beam arrived at the emulsion.,At the areas with wave crests from the object, and reference beam meet,,and it is greater than the other areas of intersection.,This phenomenon called interference results in an interference pattern of bright and dim regions on the film.,The bright regions exposed film forming microscopic planes.,which reflects its characteristics pattern of light or wave-front to the film.,the brightness or intensity of light energy combines,,1. 波前记录: 数学模型 Mathematical Model,全息干板H上设置x , y坐标，,Complex Amplitude Transmittance of a Hologram 全息图的复振幅透过率,令Ir和Io分别为参考波和物波在z = 0平面的强度，,经线性处理后，底片的透过率函数tH 与曝光光强成正比,这就是全息照片，又称全息图 (Hologram)。
Reconstruction of holograms 全息图再现,The developed hologram looks much like a photographic negative.,The hologram itself seems to contain no image at all.,But if the reference beam shines on the hologram at a correct angle, a replica of the light originally reflected by the space shuttle is played back.,This is caused by an effect known as diffraction.,Principle of hologram reconstruction 全息图的再现原理,When the reference beam shines on the emulsion, the exposed planes in the emulsion behave like tiny prisms,changing the direction or diffracting some of the light.,This diffracted light is the duplicate of the wave-front reflected by the shuttle during exposure.,When you look at the hologram, what you see is the same wave-front,Because the light is transmitted by the hologram during viewing, it is called a transmission hologram.,。