【作者】 杨磊；

【导师】 张晓兵； 夏军；

【作者基本信息】 东南大学 ， 光学工程， 2016， 博士

【摘要】 三维显示技术能够提供身临其境的视觉感受,是信息显示技术发展的必然要求。全息技术能够在空间中显示真实的立体影像,是最受关注的三维显示技术。计算全息是通过空间光调制器(spatial light modulator, SLM)承载计算全息图,实现对于物光波前的振幅与相位的重构而进行三维显示。在众多空间光调制器中,硅基液晶器件(liquid crystal on silicon, LCoS)具有器件集成度高,填充率高,容易实现更高分辨率与较快的响应速度等优势而被广泛采用。计算全息的发展受制于三个关键科学问题：1、三维场景描述信息计算量巨大。2、器件空间带宽积的限制。3、噪声影响成像质量。因此针对这些关键科学问题的解决方案就成为了硅基液晶全息成像的关键技术。本论文紧紧围绕这些关键技术,从硅基液晶器件的调制特性与全息成像系统的成像特性两个方面展开研究。本论文的主要研究成果总结如下：1、提出了一种具有等效电容结构的高分辨率液晶光栅器件,提高了相位调制深度,使得衍射效率达到理论极限值33.86%,提高了光能利用率。提出了具有电介质隔断与局域场增强的高分辨率硅基液晶器件,有效的阻隔了像素间液晶分子的带动作用和补偿了边缘场效应造成的液晶排列紊乱,将平均灰度误差由67灰度降低到30灰度,显著的改善了成像质量。对于计算全息,器件的像素间距制约了视场角,而液晶器件微型化的瓶颈问题是边缘场效应。论文对于边缘场效应进行了仿真研究,针对一维相位光栅与二维相位阵列提出了新的器件结构,优化了边缘场效应。论文设计的新型器件是在光栅寻址电极之间设置等效电容结构,其中条状接地电极与光栅寻址电极间产生横向电场,抵制边缘电场；同时三层接地电极构成两个串联的等效电容结构。由于等效电容结构的调压作用,上基板与寻址电极之间的电压得到进一步减小。仿真结果显示相位调制深度达到了2π,在电压U=3.7V时相位调制深度达到3.68 rad,从而使衍射效率达到理论极限值,并且使远场衍射类型中零级能量从82.31%降低到0.983%,大幅减少了光能损失。对于二维阵列的仿真,针对边缘场效应的产生原因：液晶分子的黏滞力与像素电场的扩展,分别提出在像素间设置光敏墙壁结构隔断液晶分子与使用狭缝-台阶电极提高局域电场的方法予以改善。仿真结果显示成像质量有了很大的提高。2、提出了一种实现波前精确控制的方法,利用数字全息显微的方法对SLM进行精确的相位表征,并在此基础上使用伽马校正的方法对器件的动态误差畸变进行了矫正,将相位光栅的均方根误差从1.6319 rad降低到0.2132 rad,优化了波前相位分布,提高了全息成像质量。硅基液晶器件的动态非线性相位响应,即相位调制量与驱动电压(加载灰度)的非线性对应关系,造成了动态误差畸变,进而降低了成像质量。论文分别利用双缝干涉方法与数字全息显微方法进行了全局相位测量与局部相位测量。从测量结果中找出相移与查找表值之间的最佳拟合多项式方程,将上述关系转化为灰度与查找表值之间的关系即为校正的伽马曲线。先后对非线性电光效应与非线性相关效应进行了伽马校正,得到了较好的矫正效果。3、提出了分时多平面与分区域多平面两种空间成像方法,实现了三维全息显示。大幅减少了全息计算量,降低了对于运算处理设备与显示器件的要求。全息技术描述三维物体模型复杂,计算量大,对显示器件分辨率、空间带宽积等性能要求高。利用分数阶傅里叶变换全息图描述光的传播过程,运用硅基液晶加载全息图,配合透镜及屏幕的使用,实现了多个平面不同深度信息的显示。提出的两种多平面成像方法：分时多平面,将含有不同深度信息的全息图成像在透镜后的不同平面上。使用SLM时序加载全息图,并用同步控制信号控制聚合物分散液晶屏的开关,利用人眼的视觉暂留效应,实现了多平面三维显示。分区域多平面,将三维光场分布离散化为相干光在三个二维平行平面上的同时调制,在三个屏幕上依次得到三个不同景深的图像,提供了调焦与重叠两种深度感知信息,实现了对于人眼调焦过程的模拟。更多还原

【Abstract】 Three dimensional display (3D) is an inevitable demand with the development of information display technology, since it can provide immersive visual perception. Holographic display is the most attractive 3D display technology for its capability of recording and reconstructing real-existing 3D images. With computer-generated-hologram (CGH) exerted on the spatial light modulator (SLM), computer holography can reconstruct the amplitude and phase of wavefront of the object.Liquid crystal on silicon (LCoS) is widely used for its advantages, like high-level integration, high pixel fill factor, and easy to achieve higher resolution and shorter response time over other types of spatial light modulators.The development of computer holography based on SLM is limited by three key scientific issues: 1. Huge computing loads of 3D scences.2. Limitation of device space-bandwidth product.3. Noises deteriorate image quality. The key technologies of holographic imaging based on LCoS provide solutions to the above three problems. The research on these key technologies is composed of modulation characteristics of LCoS and imaging characteristics of holographic imaging system. The main achievements are summarized as follows:1. For computer holography, pixel pitch limits the field of view, while the bottleneck of the micro devices is the fringing field effect. The simulation research on the fringing field effect was done. Novel device configurations of one dimensional phase grating and two dimensional phase array were proposed respectively to optimize the fringing field effect.Firstly, simulation models of LCoS based on the finite element method (FEM) was built. Secondly, the influence factors of the fringing field effect were studied. Then, the sinusoidal phase grating was simulated. Owing to the fringing field effect, the phase modulation depth of the conventional configuration doesn’t increase with the increase of the driving voltage and the phase profile of the grating electrodes expand to neighboring electrodes. Correspondingly, the diffraction efficiency doesn’t increase with the increasing of the voltages and the main energy of the diffraction pattern focuses on the zero order which cannot be used. In this thesis, a liquid crystal grating with equivalent capacitance configuration is proposed, where two layers of periodical ground electrodes are interlaced and aligned with the addressing electrodes. The generated electric fields between the stripe ground electrodes and grating electrodes resist the fringing field. Meanwhile the equivalent capacitance configuration is constructed with three layers of ground electrodes. By proper structure design and selection of dielectric material, distribution of the equivalent voltage exerting on the liquid crystal layer can be adjusted to further reduce the fringing field effect. The simulation results show that phase modulation depth of 2π is achieved. And phase modulation depth of 3.68 rad is obtained when the voltage is 3.7V, which means that the theoretical upper limit diffraction efficiency of the first-order diffraction 33.86% is achieved. The energy of the first order in the diffraction pattern is lower than 0.1, which indicates the efficiency is greatly enhanced. The simulation of two dimensional phase grating used chessboard pattern for the crosstalk of neighboring pixels is quite obvious. With regard to the cause of the fringing field effect:the inherent viscosity of LC molecules and expansion of the pixel electric field, a novel structure using dielectric separation to confine the LC molecules and slit-step electrodes to enhance local electric field where disclination lies is proposed. The simulation results show that the image quality is greatly improved.2. The dynamic nonlinear phase response of LCoS, namely the nonlinear relationship between phase modulation depth and driving voltage (grayscale), causes deviation between actual and ideal phase retardation, which reduces the image quality. Accuate phase modution characterization methods are proposed based on which gamma curve calibration is used to optimize the dynamic phase aberration.In this thesis, global phase measurement by double slits interference and local phase measurement by digital holographic microscopy were done. Based on the accurate characterization, gamma curve calibration was used for phase aberration, namely to find the best fitting polynomial equation between phase shift and LUT (look-up table) value from measurement results, then transform it to the relationship between gray level and LUT value which is the calibrated gamma curve. Calibrations for the nonlinear electro-optics effect and the nonlinear correlation effect were made respectively. Good results of calibrations were obtained.3.Holographic display has to use complex models to describe 3D objects and also has problems of massive computation, high requirements of device resolution and space-band width. Fractional Fourier transformation is used for CGH of LCoS. Time-sequence multi-plane method and multi-plane iteration algorithm were proposed to realize three dimensional holographic display.In true 3D techniques, volume display has complex system; holographic display has to use complex models to describe 3D objects and also has problems of massive computation, high requirements of device resolution and space-band width. Two multi-plane imaging methods were proposed to solve these problems. By using fractional Fourier transformation, different images are projected at different depth in the same system. By using synchronization information sent by the spatial light modulator, the circuits control the switch of the polymer doped liquid crystal screens.3D display by holographic display imaging on multiple focal planes is realized as a result of visual staying phenomenon. For multi-plane iteration algorithm, 3D light distribution is realized by simultaneously modulating coherent light in several parallel 2D planes. The system is simple and doesn’t have extra requirements for power or display devices. By using a phase-only spatial light modulator on which the computer generated holograms load, images at different depth planes are sharply projected onto the closely spaced screens, which mimic the accommodation of natural vision.更多还原