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题名:
液晶空间光相位调制器特性研究及在自适应光学中的应用
作者: 蔡冬梅
学位类别: 博士
答辩日期: 2008-01-04
授予单位: 中国科学院光电技术研究所
授予地点: 光电技术研究所
导师: 凌宁
关键词: 液晶空间光调制器 ; 相位调制 ; 波前校正 ; 自适应光学
其他题名: Characteristics of Liquid-Crystal Spatial Lighter Modulator and its application in Adaptive Optics
学位专业: 光学工程
中文摘要: 自适应光学系统能够实时测量并补偿受动态扰动所造成的波前畸变,使光学系统具备自动适应外界条件变化,保持最佳工作状态的能力。其中波前校正器是自适应光学系统的核心器件。近年来,随着自适应光学应用领域的扩展,具有批量生产能力的微小型波前校正器成为自适应光学技术研究的热点。液晶空间光调制器具有尺寸小、单元数多可以批量生产的优点成为微小型波前校正的一种候选方案。但是作为自适应光学的校正器件,还要有一系列时间、空间特性、光谱特性和光能利用率,液晶空间光调制器在这些方面存在严重不足,限制了它在自适应光学系统中的适用性。为了全面深入了解液晶空间光调制器作为波前校正器的特性,判断其限制因素和适用范围,本文着重研究液晶空间光调制器作为波前校正器的各种特性。文中分别对相位型液晶空间光调制器的时间、空间调制特性、像差校正能力、像差校正中器件的光能利用率以及波长对器件应用的影响进行研究。并利用液晶空间光调制器作为波前校正器构成自适应光学闭环系统校正波前像差。此外还将液晶空间光调制器作为光学衍射元件生成程控微透镜阵列实现动态H-S波前传感器。 本文具体内容如下: (1) 利用Jones 算法对液晶空间光调制器的相位调制特性进行理论计算,讨论不同扭曲角向列液晶的相位调制特点。以零扭曲向列液晶空间光调制器为例对它的相位调制特性、相位调制的时间稳定性、空间均匀性以及相邻像素边缘电场造成的相位交连进行理论分析和实验研究。 (2) 时间特性决定了液晶空间光调制器能否满足快速动态波前校正。以电寻址液晶空间光调制器为研究对象,对其响应速度以及和相位调制量的关系进行实验研究。改变控制信号频率,研究液晶空间光调制器的响应频率及对不同频率信号的相位延迟量。 (3) 分析液晶空间光调制器产生的Zernike像差,评价它对低阶和高阶像差校正能力。通过仿真计算和实验测量,分析填充比、相位交连以及Phase Wrapping方法对不同像差校正性能的影响。在不同入射波长条件下,研究了液晶空间光调制器实现像差校正性能的光谱特性。 (4) 光能利用率是决定液晶空间光调制器能否在微弱光照条件正常工作的决定因素。论文对偏振入射条件、光栅结构的衍射损失以及Phase Wrapping方法对液晶空间光调制器校正像差的光效率进行理论分析和实验研究。 (5) 论文对液晶空间光调制器在自适应光学中的应用进行讨论。建立用液晶空间光调制器和哈特曼传感器构成的自适应光学系统,通过合并像素单元解决了高空间分辨率液晶空间光调制器与哈特曼的子孔径几何匹配。采用埃尔米特插值方法对确定合并单元像素相位值的“区域拟合方法”进行改进,改进后像差校正效果提高,尤其适用高阶像差。研究了液晶空间光调制器对静态波前像差的校正效果,分析了入射波长对校正性能的影响。 (6) 利用液晶空间光调制器各个像素可编程控制的特点,构建口径、位置、焦距动态可变的衍射透镜和微透镜阵列,分析液晶衍射微透镜的性能和微透镜阵列设计的限制因素。 通过本文的研究,对液晶空间光调制器在自适应光学系统应用方面,得到如下结论: 1) 相位型液晶空间光调制器可对波前像差进行校正,然而校正范围限制为一个波长,对超过一个波长的校正量需要利用Phase Wrapping方法扩大,但Phase Wrapping方法只能对特定波长实施,对宽光谱校正将带来很多的误差。 2) 液晶空间光调制器具有明显的色散性能,对宽光谱的白光,将影响校正效果。 3) 液晶空间光调制器的响应速度不够,特别是恢复过程响应速度更慢,普通向列液晶的响应时间达到几十毫秒。而大气校正Greenwood频率常常超过100Hz,液晶空间光调制器对大型望远镜这样的高端应用,还有很大差距。 4) 液晶空间光调制器,只能对偏振光起作用,对非偏振光,将损失50%光能。虽然采用光轴互相垂直的液晶盒,可以实现非偏振光相位调制,但液晶层厚度增加,同时增加了复杂性。另外液晶像素光栅结构造成的衍射损失使光能损失仍然较大,限制了它在微弱光照条件下的应用。 5) 液晶空间光调制器的像素单元数远远超过目前的哈特曼传感器子孔径数。构建液晶空间光调制器和哈特曼传感器组成的自适应系统,其波前探测和信号处理方法是还没有解决的问题。 液晶空间光调制器由于上述物理限制,目前只能在要求成本低、像差变化缓慢、光谱范围窄、光照条件较强的普及性自适应系统中找到应用。
英文摘要: The Adaptive optics system (AO) can compensate the dynamical wave-front aberration in real time. Using AO system, the optical system has an ability working with environment to achieve the best condition. Wave-front corrector plays a key role in AO system. In recent years, micro wave-front correctors have emerging. Liquid crystal spatial modulator (LC-SLM) has a serious of attractive characteristics as novel wave-front corrector such as compactness, high density integration, low cost and possibility of batch production. However it also suffers from several drawbacks to limit its using in atmospheric turbulence correction. In this paper, the characteristics of LC-SLM and its application in AO are investigated across the board. The temporal and space characteristics, the wave-front generation performance, optical efficiency and the dispersion of LC-SLM was measured and analysis. A close-loop AO system was developed using phase-only spatial light modulator as wave-front corrector and Hartmann-Shack sensor (H-S) to correct wave-front aberration. Apart from that, we also demonstrate an adaptive H-S sensor using LC-SLM generating micro-lens array. (1) Base on Jones algorithm, the phase modulation characteristics of LC-SLM were analyzed in theory. The relationship between twisted angle of nematic LC and phase modulation was also discussed in theory. For a zero-twisted nematic LC-SLM, its stability and uniform of phase modulation, fringing field effect of neighboring pixels were analyzed and measured. (2) The temporal characteristic of LC-SLM is the key factor for LC-SLM to compensate dynamical wave-front aberration. For an electrically addressed LC-SLM, its response time with phase stroke was measured. Apart from that, the relationship between response time and symbol frequency, the phase delay to sine wave of different frequency was also measured. (3) As a novel aberration corrector, the wave-front generation performance of a phase-only LC-SLM is a governing factor that determines its ability for wave-front correction in an AO system. The performance of wave-front generation of LC-SLM is demonstrated by the production and quantification of the first 36 Zernike terms with different values. The factors, such as the fill factor, fringe field effect, pixel structure, Phase Wrapping method and wavelength, to impact wave-front generation are analyzed. (4) The optical efficiency of a nematic LC-SLM was studied using as a wave-front corrector in AO system. The impacts of polarized sensitive, fill factor, the diffraction of pixel structure and phase wrapping method to optical efficiency are discussed and measured. (5) A close loop adaptive optical system was configured to compensate wave-front aberration, in which LC-SLM was employed as wave-front corrector and H-S sensor as wave-front senor. The matching between H-S sensor and LC-SLM was settled through incorporating some pixels into one drive unit. The cubic Hermite interpolation method was adopted to determine the phase value of pixel including one drive unit. Compared to the linear interpolation method, the phase pattern was more smooth and continuous, especially for high order aberration. The performance of LC-SLM to compensate wave aberration with different wavelength was analyzed. (6) LC-SLMs are pixilated that allowed the implementation of a diffractive optical element by controlling the pixels individually. The micro-lens array with different size, focal length and location were generated using phase only LC-SLM. The strategies for efficient generation of micro-lens by LC-SLM was deduced and discussed. In this paper, we investigated the performance of LC-SLM using as wave-front corrector, the conclusion are following. (1) The phase-only liquid crystal spatial modulator can accomplish wave-front compensation as wave-front corrector. However the phase wrapping method should be adopted for its limited phase stroke. For designed wavelength, its phase stroke is only one wave. Phase wrapping method is exactly used only for designed wave. So phase wrapping is not applied to broad spectrum. (2) The dispersion of LC limits its using under white light. (3) The response speed of CL-SLM is very slow, especially for the decay time. For nematic LCs, their response time is decades of milliseconds. However the Greenwood frequency is usually more than 100Hz. The LC-SLM is not suit for using in telescope to compensating atmospheric turbulence. (4) The LC-SLM is only sensitive to polarized light. The un-polarized light intensity will reduce to 50% after polarizer. The method is useful for un-polarized light modulation two orthogonal LC cells. However, a LC wave-front corrector will have many elements in which case aligning two orthogonal devices would be much more complicated. The light loss of diffraction effect of pixel structure is graveness. The low optical efficiency limits its using under weak light. (5) The pixel number is much more than the number of micro-lens including in H-S sensor. So wave-front sensing and processing adjusting AO configured by LC-SLM and H-S sensor will be a complicated problem. To summarize, the drawbacks of LC-SLM are huge drawbacks to limit it using a practical adaptive optical imaging system. For now, it is only useful slowly varying wave-front aberration control of narrow spectral bandwidth.
语种: 中文
内容类型: 学位论文
URI标识: http://ir.ioe.ac.cn/handle/181551/211
Appears in Collections:光电技术研究所博硕士论文_学位论文

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蔡冬梅. 液晶空间光相位调制器特性研究及在自适应光学中的应用[D]. 光电技术研究所. 中国科学院光电技术研究所. 2008.
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