Electrically tunable holographic polymer templated blue phase liquid crystal grating

In this paper, we demonstrate an alternative approach to fabricating an electrically tunable holographic polymer tem- plated blue phase liquid crystal grating. This grating is obtained by preforming a polymer template comprised of periodic fringes, and then refilling it with a blue phase liquid crystal. Compared with conventional holographic polymer dispersed liquid crystal gratings, our grating can remarkably reduce its switching voltage from 200 V to 43 V while maintaining a sub-millisecond response time. The holographic polymer templated blue phase liquid crystal （HPTBPLC） grating is free from electrode patterning, thus leading to a lower cost and more flexible applications.

Fast generation of controllable equal-intensity four beams based on isosceles triangle multilevel phase grating realized by liquid crystal spatial light modulator

Liquid crystal spatial light modulator （LCSLM） realizing equal-intensity multiple beams often has some features, i.e., phase valley between two adjacent pixels, flybaek region when phase decreases immediately from 2~r to 0, and inevitable backplane curvature, which are different from those of most conventional diffractive optical elements （DOEs）, such as static DOEs. For optimal intensity uniformity, equal-intensity multi-beam generation must be considered for these artifacts. We present a tunable-grating method in which the intensity uniformity can be improved by considering the LCSLM artifacts. For instance, tuning phase modulation depth of the grating, called isosceles triangle multilevel phase grating （ITMPG）, can be used not only to improve the intensity uniformity, but also to fast steer four beams with narrow beamwidths, determined by the same effective aperture of ITMPG. Improved intensity uniformity and high relative diffraction efficiency are demonstrated through experiments with phase-only LCSLM.

Kinetics investigations for holographic Bragg grating based on polymer dispersed liquid crystal

This paper investigates the monomer kinetics of polymer dispersed liquid crystal （PDLC） grating. Fourier transform infrared （FTIR） spectra are used in the studies of photoreaction kinetics. The results indicate that there is a relative stable stage arises after a very short initial stage. Based on FTIR studies, the monomer diffusion equation is deduced and necessary numerical simulations are carried out to analyse the monomer conversion which is an important point to improve phase separation structure of PDLC grating. Some simulation results have a good agreement with experimental data. In addition, the effects induced by monomer diffusion constant D and diffusion-polymerization-ratio rate R are discussed. Results show that monomer conversion can be improved by increasing value of D. Besides, a good equilibrium state （R = 1） is more beneficial to the diffusion of monomer which is important in the process of phase separation.

Asymmetric dynamic phase holographic grating in nematic liquid crystal

A new scheme for recording a dynamic phase grating with an asymmetric profile in C60-doped homeotropically aligned nematic liquid crystal（NLC） was presented.An oblique incidence beam was used to record the thin asymmetric dynamic phase holographic grating.The diffraction efficiency we achieved is more than 40%,exceeding the theoretical limit for symmetric profile gratings.Both facts can be explained by assuming that a grating with an asymmetric saw-tooth profile is formed in the NLC.Finally,physical mechanism and mathematical model for characterizing the asymmetric phase holographic grating were presented,based on the photo-refractive-like（PR-like） effect.

Dynamic shaping of vectorial optical fields based on two-dimensional blazed holographic grating

We propose a vectorial optical field generation system based on two-dimensional blazed grating to high-efficiently generate structured optical fields with prescribed amplitude,phase,and polarization.In this system,an optimized blazed grating hologram is written on a spatial light modulator(SLM)and can diffract the majority of the incident light into the first-order diffractions of the x and y directions,which then serve as base vectors for synthesizing desired vector beams.Compared with the conventional cosine grating used in the previous work,the proposed two-dimensional,blazed grating has a much higher efficiency.Both computer simulation and optical experiment validate that a conversion efficiency up to 5 times that of the former work is achieved.Our method can facilitate applications of the optical field manipulation.

Design of an augmented reality display based on polarization grating

A new optical system for an augmented reality(AR) display is proposed in this paper.The optical system mainly includes a ray deflector, coupling input grating, optical waveguide, and coupling output grating.Both the ray deflector and the coupling input grating are designed based on the diffraction characteristics of the polarization grating, and the coupling output grating is the Bragg reflection grating.Compared with other AR schemes, this AR optical system not only reduces the number of projections from two to one, but also improves the efficiency of light coupling into the optical waveguides.The energy loss is reduced by utilizing the single-order diffraction characteristics of the polarization grating in its coupling input structure.The light deflector uses the polarization selectivity of the polarization grating and the characteristics of the rotating light of the twisted nematic liquid crystal layer to realize beam deflection.The working principle of the optical system is experimentally and theoretically demonstrated.

应用于彩色光波导显示中的液晶偏振体光栅结构设计与制备

本文设计并实现了一种三层结构液晶偏振体光栅,对其衍射特性及其在光波导中的耦合情况进行了研究。理论上,选取横向周期为424 nm,根据特定工作波长450 nm、532 nm、633 nm在满足布拉格衍射条件的情况下计算出了相对应的3个纵向周期以实现其厚度方向的叠加。采用有限元法对该器件进行了模拟仿真,计算出了其光谱特性及角度特性,根据红绿蓝三色光色散关系仿真出了其在波导内耦合的入射角与衍射角之间的关系曲线。实验上,本文实现了该液晶偏振体光栅器件的制备并对其进行了系统性能表征,给出了一个初步的光波导耦合实验,演示了其耦合效果。结果表明,所制备的三层液晶偏振体光栅器件在450 nm、532 nm、633 nm下衍射效率最高可达80%以上,对应波长下的角带宽分别为12°、8°、6°,其在波导内耦合的视场经模拟可达到27.81°,是一种良好的光波导耦合器件。

基于可调剪切干涉曝光方法的大口径液晶偏振光栅制备

为了解决传统偏振全息干涉方法受限于过长的光路难以制备长周期的大口径液晶偏振光栅的问题,提出了基于液晶偏振光栅模板的可调剪切干涉曝光方法,并通过理论分析了制备偏振光栅的周期随模板方位角及沿光轴轴向位置的变化。利用小口径高效液晶偏振光栅作为模板搭建可调剪切干涉曝光光路,并基于此制备了数十微米周期的200 mm口径液晶偏振光栅,平均衍射效率达到99%以上,平均周期约为59.86μm,周期分布不均匀性约为0.07%。实验结果表明,调剪切干涉曝光方法能够制备大口径的长周期偏振光栅,周期和效率分布均匀,还具备周期易于调节、结构紧凑和抗振动干扰强等优势。

基于液晶偏振光栅的大口径激光光束扫描系统

非机械式激光光束扫描系统具有体积小、重量轻、易集成等优势,在激光雷达、激光武器和激光通信等领域具有较大的应用前景,其通光口径决定着设备的工作距离、扫描精度和目标摧毁能力。基于液晶偏振光栅设计了一种全电控制的大口径激光光束扫描系统。采用液晶偏振光栅和液晶空间光调制器的协同工作方式,实现光束的大角度连续扫描;设计了光束扫描系统,并重点分析了级联液晶偏振光栅组的控制策略,给出了具体控制方法。最后,进行了激光光束的初步扫描实验,实现了70 mm口径光束在−16.575°~16.575°的偏转。该研究结果为激光光束的非机械式扫描控制提供了技术支撑,并在大口径激光扫描系统应用方面进行了初步探索。

基于液晶光栅的大视场红外凝视成像系统设计

分视场成像的中波红外凝视成像系统为解决红外光学系统很难同时满足大视场、高分辨率的问题,将多个子视场分时成像于同一个探测器中,然而目前该系统还存在着无法实现无缝拼接和液晶快门阵列结构复杂的缺点。本文在此基础上提出了一种新的基于液晶光栅的大视场中波红外凝视成像系统设计。通过在一次像面处加入矩形视场光阑并将其后移,使原有的0.28°的视场缺失减小至0°,实现图像的无缝拼接。快门阵列采用双光栅快门结构,使其无需复杂的位置关系,并将液晶快门阵列放置于平行光路中,可消除其所引入的色差与偏振带来的主光线间纵向位移影响。采用所提出的方法,设计出了F数为3.25,焦距为130 mm,波长范围为4.25~4.75μm,全视场对角线为10.8°的红外凝视成像系统,其各子视场单元MTF均为0.3@30 lp/mm以上。最终通过仿真结果表明,该系统具有分视场成像的功能,且成像质量良好。

液晶调控下的亚波长金光栅反射相位

空间光调制器(Spatial Light Modulator,SLM)是实现光场调控的关键器件。本文研究了一种将亚波长金光栅嵌入硅基液晶(Liquid Crystal on Silicon,LCoS)构成的SLM(G-LCoS)器件模型的反射相位特性,该器件模型以ITO透明导电层和液晶构成G-LCoS中亚波长金光栅的非对称边界。首先运用散射矩阵原理给出了亚波长金光栅反射相位的计算表达式,其次给出了下表面液晶的有效折射率计算公式。在此基础上,利用TechWiz 3D软件模拟加电情况下金光栅下表面液晶指向矢的变化,给出金光栅下表面液晶有效折射率的变化值。将得到的有效折射率数据导入到FDTD Solutions软件,对该器件结构开展了相关的仿真实验和结果分析。综合考虑了上表面的ITO色散情况、金光栅高度变化以及不同电压下液晶有效折射率值,实现了可见光范围内多波长的近2π相位调制。本文研究的器件也为AR/VR显示系统以及全息视频显示的优化提供了一种新思路。

双液晶偏振光栅系统的倾斜误差特性分析

为了精确控制激光光束指向,本文基于双液晶偏振光栅系统的光束指向算法,分析了系统中因光栅倾斜产生的误差。首先,采用一种基于衍射光栅方程的光线追迹方法求解出射光束指向,引入了入射光束指向和光栅倾斜角,通过与仿真结果进行对比,验证了该方法的正确性和精度。其次,通过对光栅不同倾斜情况的分析,本文给出了不同倾斜情况下光栅姿态的表达式,并结合光线追迹法得到了相应的出射光束指向表达式,并进一步分析了光栅倾斜产生的调零误差和旋转误差。研究结果表明,在0°~0.3°光栅倾斜角范围内,调零误差分别在0.25 mrad和2 mrad以内,旋转误差分别在85 mrad和430 mrad左右。本文方法可实现对双液晶偏振光栅系统中出射光束指向和光栅倾斜误差的精确计算。

非机械指向空间激光通信接收系统设计与分析

采用非机械指向结构能够极大缩小空间激光通信系统尺寸与重量,由于现有器件性能局限,目前尚未见到成熟的非机械指向空间激光通信接收系统。提出基于正交液晶级联偏振光栅与四象限探测器的空间激光通信接收光学系统,利用了正交液晶级联偏振光栅体积小、口径大、偏转范围大,及四象限探测器接收角度范围大的优势,根据正交液晶级联偏振光栅与四象限探测器的理论与器件性能,设计了光学系统,分析了探测器噪声、背景噪声、系统噪声对于通信与入射角度测量的影响,计算了可实现的通信速率,给出了系统的最大响应角速度。

采用丙烯酸酯双单体的高空间频率透射式体全息H-PDLC光栅的研制

全息聚合物分散液晶(holographic-polymer dispersed liquid crystal, H-PDLC)体全息光栅因其化学状态稳定、制作简单、价格低廉等优势为全息光波导耦合器件提供了一个更优的思路。本文提出了采用复合型丙烯酸酯类高/低折射率匹配调制的H-PDLC透射式体光栅的研制。首先结合耦合波理论及分子扩散理论介绍了H-PDLC光栅的形成机理。然后,通过实验对不同官能度单体的光栅材料进行筛选,并研究其全息衍射特性。在此基础上,进一步采用提高引发剂含量的方式优化了高频体全息H-PDLC光栅性能。实验结果表明,该双单体的H-PDLC配方可实现在可见光波段透过率大于90%,在空间频率为973 lp/mm下,衍射效率达到90%,响应带宽为99 nm;在2 941 lp/mm下,衍射效率为75.4%,响应带宽为29 nm。该H-PDLC光栅作为波导显示系统的耦合器件有可观的应用前景。

液晶偏振体光栅及其在增强现实波导显示中的应用

液晶偏振体光栅是一种基于几何相位的布拉格光栅,具有偏振选择性强、衍射效率高、衍射角大以及制备工艺简单等优点,在增强现实系统中具有潜在的应用前景。本文详细介绍了液晶偏振体光栅的工作原理和光学特性,并对其近年来的研究进展进行了总结。重点讨论了液晶偏振体光栅在解决增强现实波导显示中各种关键性问题的研究,展望了其在增强现实波导显示中的应用前景。最后,总结了目前液晶偏振体光栅应用于波导显示领域存在的问题和发展趋势。

基于双液晶偏振光栅的机载光电跟瞄平台技术研究

为实现新型机载光电跟踪平台轻量化、小型化与高精度化设计,提出一种应用于机载平台下的基于双液晶偏振光栅光束偏转机构的光电跟瞄方法。建立了基于视轴解耦的双液晶偏振光栅光束跟瞄反向公式模型,该模型只需输入跟踪点方位与俯仰角度坐标即可计算出两共轴光栅旋转位置,通过两共轴光栅的旋转来进行光束偏转;相比传统双棱镜光束偏转模型反向公式,该模型具有模型简单、运算快速、精度较高等优点;将基于视轴解耦的双液晶偏振光栅光束偏转反向公式模型与线性二次型调节器控制算法相结合嵌入控制板以实现跟踪功能。通过试验检测了系统跟踪性能结果表明:该双液晶偏振光栅跟踪模型在载体扰动的情况下可以实现目标跟瞄,在动态跟瞄实验中,采用视轴解耦与线性二次型调节器相结合的控制策略使得系统跟瞄精度得到明显提升,相比比例积分微分控制器系统方位与俯仰轴实时跟踪残差峰峰值均下降40%以上,在2°@0.5 Hz与5°@0.2 Hz载体扰动情况下双轴脱靶量均方根值综合统计数值分别达到328μrad与289μrad,验证了所提跟瞄模型的有效性。

Arbitrary and reconfigurable optical vortex generation:a high-efficiency technique using director-varying liquid crystal fork gratings

A high-efficiency technique for optical vortex(OV) generation is proposed and demonstrated. The technique is based on liquid crystal fork gratings with space-variant azimuthal orientations, which are locally controlled via polarization-sensitive alignment layers. Thanks to the optical rewritability of the alignment agent and the dynamic image generation of the digital micro-mirror device, fork gratings can be instantly and arbitrarily reconfigured.Corresponding optical vortices carrying arbitrary azimuthal and radial indices are demonstrated with a conversion efficiency of 98.5%, exhibiting features of polarization control and electrical switching. The technique may pave a bright road toward OV generation, manipulation, and detection.

Optical edge detection with adjustable resolution based on liquid crystal polarization gratings

Optical edge detection, a part of image processing, plays an important role in extracting image information used in optical analog computation. In this Letter, we raise a new way to realize optical edge detection. This design is based on two liquid crystal polarization gratings with a period of 2.2 mm, which function as a spatial differentiator. We experimentally demonstrate broadband optical detection and real-time adjustable resolution.The proposed method takes advantage of the convenience to use, simple fabrication process, and real-time tunable resolution. It may guide more significant applications in the optical field and other practical scenarios like machine vision in computers.

液晶电扫描主动成像视场扩展技术研究

针对场景监视、视觉导航等领域亟需解决的大视场高分辨率成像问题,提出了一种液晶电扫描主动成像视场扩展技术,基于液晶电扫描原理设计了基于液晶偏振光栅和铁电液晶的液晶电扫描模块,并搭建了一套主动成像视场扩展系统。通过激光主动照明方式,采用在光源发射端和图像接收端引入液晶电扫描模块的方法,结合液晶偏振时序控制和拼接融合得到了大视场高分辨率的目标图像。实验结果表明,基于液晶电扫描的主动照明成像,实现了分辨率为3840×480、视场角为52°的大视场高分辨率成像,成像视场扩大了6倍。工作实现了非机械式的主动扫描成像,为大视场成像扩展技术提供了一种很好的方法。

偏振光复用的液晶体光栅波导AR近眼显示系统

光波导组合器在增强现实显示的出瞳扩展和外观薄化方面颇具优势。出瞳扩展技术是通过在耦出区域诱导多次衍射,将光线引导至人眼。然而,这种多次衍射可能导致出瞳亮度在沿光路径的空间分布上逐渐减弱,从而造成亮度不均。特别是当使用液晶制备而成的偏振体全息光栅作为耦合元件时,由于光栅仅对一种圆偏振光敏感,与之正交的圆偏振光会被直接透射,导致波导系统的能量利用率并不高。针对这一挑战,本文提出了一种实现左旋和右旋圆偏振光复用的偏振体全息光栅光波导AR显示光学系统,同时实现了大出瞳尺寸和良好入眼均匀性。在理论设计和仿真方面,首先通过k空间理论设计了大视场角下的光栅结构,再以微型发光二极管(Micro-LED,μLED)作为微显示屏,设计了一套与之匹配的投影镜头,并对整个波导系统进行了成像仿真。在实验部分,制备了中心波长为532 nm、出瞳尺寸为45 mm×25 mm的偏振体全息光栅波导。实验结果显示,该波导系统在基板折射率为1.51下的对角线视场角达到了预期的32.86°。相比于仅响应单一圆偏振光的方案,采用九点法测得的均匀性提升了48.8%,整体均匀性提升了34.1%,波导组合器的光学效率也实现了1.2倍的提升。本文工作为偏振体全息光栅波导的设计提供了宝贵的指导和参考。