傅立叶变换用于全息三维物体面形测量

介绍了将傅立叶变换用于全息三维面形测量的基本原理和实验装置。首先用全息干涉条纹替代投影条纹投影到待测物体上 ,在相位解调中 ,结合Gerchberg迭代算法对变形条纹进行处理 ,然后用汉宁窗口进行数字加权滤波。结果表明 ,该测量系统实验装置简单 ,测量精度高 ,速度快 ,适用于复杂大物体三维面形测量     

Voltage-controlled optical switch in planar nematic liquid crystal film

This study presents an integrated device that consists of a directional coupler and an electro-optic switch. The device is designed to include a nematic liquid crystal cell, comprising a grating-like electrode. Applying the appropriate voltage to the cell yields a periodically distributed refractive index. An incident polarized beam will couple to an adjacent channel if it is parallel to the channel. The coupling efficiency is controlled by applied voltage. An obliquely injected polarized beam will be reflected and refracted in the channel, and propagated along a curved path. The route of the beam can be controlled by applying the voltage. A multiport routing was achieved for voltage modulation. In addition, the distribution of refractive index is also investigated by employing conoscopic technique experimentally and numerically.     

傅里叶变换条纹位相分析技术中移频消去载频法对位相分析的影响

在理论上分析了傅里叶变换位相分析技术中的移频消去载频法,由于采样的离散化,不同的条纹周期对位相分析将产生不同的影响。利用计算机仿真,模拟了不同周期的条纹图以及形变条纹图,根据傅里叶变换频移法,获得了不同周期条纹图的位相分布及误差,同时与"减基准条纹法"获得的位相分布及误差进行比较,验证了不同条纹周期对位相分析产生的影响。以花瓶为目标物体,在实验中进一步验证了采样的离散化对位相分析带来的误差和影响。并且讨论了两种减弱这种误差的方法。     

Thermally enhanced optical nonlinearity in nematic liquid crystal close to phase transition temperature

This study investigates the beam profile and the liquid crystal (LC) arrangement affected by an optical field on LC thin films at a temperature close to nematic-isotropic phase transition temperature (T_NI). A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal (NLC). An optical field combined with thermal enhancement enhances molecular reorientation and causes additional molecular excitation along the axis of propagation of the beam. The reorientational nonlinearity yields an undulating structure with multi-foci; the length between each pair of foci increases with time, as described.     

基于傅里叶变换轮廓术的动态液膜测量

动态液膜三维形貌的高速测量及重建,对于能源动力领域的工业过程优化十分重要。基于傅里叶变换轮廓术,对缓变表面和非缓变表面分别进行了模拟仿真,研究了物体表面形貌重建精度的影响因素,包括物体表面高度变化率、环境随机噪声以及条纹频率。并根据模拟结果对实验参数进行了优化,研发构建了高速三维结构光测量系统,对竖直壁面下降液膜表面形貌进行了动态测量。实验结果表明:随着液膜沿竖直壁面向下流动,液膜厚度呈现先增大后减小的趋势,高度方向的平均误差为0.1 mm,傅里叶变换轮廓术能够精确地应用于动态液膜高速测量。     

Applications of windowed Fourier fringe analysis in optical measurement: A review

The applications of windowed Fourier fringe analysis in the past decade are reviewed. Because fringe patterns from different optical measurement systems are similar, the reviewed applications are classified according to the functions of the windowed Fourier transform being used in fringe pattern analysis: denosing exponential phase fields, demodulating carrier fringe patterns, getting phase derivatives, and utilizing local properties. From these applications, the windowed Fourier transform is shown to be effective and versatile for fringe pattern analysis.     

液晶光阀用于光学傅里叶变换

结合我校开设的液晶光阀实验，介绍了几个典型的静态傅里叶变换实验内容，以液晶光阀在计算全息扫描器中的应用为例，对其在动态傅里叶变换中的应用进行了讨论。     

Optical properties and optimized conditions for polymer dispersed liquid crystal containing UV curable polymer and nematic liquid crystal

Polymer dispersed liquid crystal was synthesized by combining a UV curable polymer and a nematic liquid crystal. Optimized conditions for the optical properties of the PDLC were found to be the concentration ratio of LC and polymer at 7:3, UV curing time of 18 min, and the thickness less than 25 μm. In the case of the high LC concentration (≥70%) sample, the amount of liquid crystal segregated in the polymerization process was enough to form a spherical shape of droplet, and the threshold driving voltage was reduced. The response time for the turn-on process was nearly independent of the concentration, while the turn-off process was almost proportional to the concentration. From microscopic image and UV–visible spectrum analysis, the relation between LC droplet morphology and optical properties were explained.     

Collimation testing using wedge plate lateral shearing interferometry and Fourier fringe analysis

For checking the collimation of an optical beam Fourier fringe analysis has been incorporated into the wedge plate interferometric setup. Typical interferograms corresponding to ‘in-focus’, ‘at-focus’ and ‘out-of-focus’ positions of an optical beam have been recorded. As per the testing procedure, FFT of the recorded interferometer is computed digitally, and necessary processing for direct determination of phase is undertaken. Finally, the phase data is unwrapped and plotted as a function of pixel position along the direction perpendicular to the shear. The slope of the phase provides the information regarding collimation position of the collimator. As the collimation position is detected by the direct measurement of the phase over the whole area of the interferogram, high accuracy, reliability and precision are achieved.     

Nonlinear experimental dye-doped nematic liquid crystal optical transmission spectra estimated by neural network empirical physical formulas

In this paper, two complementary objectives related to optical transmission spectra of nematic liquid crystals (NLCs) were achieved. First, at room temperature, for both pure and dye (DR9) doped E7 NLCs, the 10-250 W halogen lamp transmission spectra (wavelength 400-1200 nm) were measured at various bias voltages. Second, because the measured spectra were inherently highly nonlinear, it was difficult to construct explicit empirical physical formulas (EPFs) to employ as transmittance functions. To avoid this difficulty, layered feedforward neural networks (LFNNs) were used to construct explicit EPFs for these theoretically unknown nonlinear NLC transmittance functions. As we theoretically showed in a previous work, a LFNN, as an excellent nonlinear function approximator, is highly relevant to EPF construction. The LFNN-EPFs efficiently and consistently estimated both the measured and yet-to-be-measured nonlinear transmittance response values. The experimentally obtained doping ratio dependencies and applied bias voltage responses of transmittance were also confirmed by LFFN-EPFs. This clearly indicates that physical laws embedded in the physical data can be faithfully extracted by the suitable LFNNs. The extraordinary success achieved with LFNN here suggests two potential applications. First, although not attempted here, these LFNN-EPFs, by such mathematical operations as derivation, integration, minimization etc., can be used to obtain further transmittance related functions of NLCs. Second, for a given NLC response function, whose theoretical nonlinear functional form is yet unknown, a suitable experimental data based LFNN-EPF can be constructed to predict the yet-to-be-measured values.     

向列相液晶缺陷光子晶体透射光谱偏振敏感特性研究

研究了向列相液晶缺陷TiO2和SiO2交替的光学多层膜一维光子晶体透射谱偏振敏感特性。外电压下透射光谱测试和模拟结果显示,对于平行取向的向列相液晶,当自然光垂直入射时,禁带中两处出现e(TE模式)光和o光(TM模式)透射峰,具有偏振敏感性。随着电压增大,e光透射峰蓝移与o光透射峰合二为一,光谱可调谐范围分别为31和34nm;而对于取向混乱的向列相液晶,禁带中两处出现独立的透射峰,无偏振敏感性。随着电压增大峰位也蓝移,光谱可调谐范围分别为64和15nm。通过混乱取向液晶分子,可以使o光和e光有效折射率值相等,获得偏振不敏感特性。     

不均匀温度场中空气密度分布实验研究

利用基本的光学元件和光源,搭建Mach-Zehnder(M-Z)干涉仪,测量压力对空气折射率的影响和不同热源的热场分布.将传统光学实验与计算机的图像处理技术相结合,求得温度场为等密度线,并对实验现象进行了分析与研究.本实验可作为各类高校的设计性实验.     

单弹光调制傅里叶偏振光谱测量研究

提出一种基于弹光调制器(photoelastic-modulator, PEM)的复色光偏振光谱的傅里叶测量方法,被测光经过0°(-45°)弹光调制器调制,通过45°(0°)检偏器并由探测器探测调制后形成干涉信号,对干涉信号进行傅里叶变换,由所得结果实部和虚部得到被测光Stokes矢量中的S1、S2和S3的光谱。该方法降低了偏振光光程测量的复杂度,给出理论推导过程,并结合仿真结果与理论结果对比,证实了该方法复原后的偏振光谱与原始光谱可很好地吻合。     

单弹光调制傅里叶偏振光谱测量研究

提出一种基于弹光调制器（photoelastic-modulator, PEM)的复色光偏振光谱的傅里叶测量方法,被测光经过0(-45)弹光调制器调制,通过45(0)检偏器并由探测器探测调制后形成干涉信号,对干涉信号进行傅里叶变换,由所得结果实部和虚部得到被测光Stokes矢量中的S1、S2和S3的光谱。该方法降低了偏振光光程测量的复杂度,给出理论推导过程,并结合仿真结果与理论结果对比,证实了该方法复原后的偏振光谱与原始光谱可很好地吻合。     

Direct measurement of group dispersion of optical components using white-light spectral interferometry

We present a simple white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the group dispersion of optical components over a wide wavelength range. The technique utilizes an unbalanced Mach-Zehnder interferometer with a component under test inserted in one arm and the other arm with adjustable path length. We record a series of spectral interferograms to measure the equalization wavelength as a function of the path length difference. We measure the absolute group refractive index as a function of wavelength for a quartz crystal of known thickness and the relative one for optical fiber. In the latter case we use a microscope objective in front and a lens behind the fiber and subtract their group dispersion, which is measured by a technique of tandem interferometry including also a Michelson interferometer.     

傅里叶变换光谱分析系统及其测试

构建了一套改进的光的时间相干性以及傅里叶光谱分析实验系统,该系统利用光机电以及计算机软件编程技术,实时采集以He-Ne激光器、钠光灯和白炽灯为代表的各种光源在迈克耳孙干涉仪上产生的干涉条纹信号,将得到的光信号经光电倍增管转换为电信号进入电路系统,经滤波、A/D转换后,再输送到计算机进行傅里叶变换以及频谱分析.实验结果证实了该系统的有效性和实用性.     

Two-dimensional windowed Fourier transform for fringe pattern analysis: Principles, applications and implementations

Fringe patterns from optical metrology systems need to be demodulated to get the desired parameters. Two-dimensional windowed Fourier transform is chosen for the determination of phase and phase derivatives. Two algorithms, one based on filtering and the other based on similarity measure, are developed. Some applications based on these two algorithms are explored, including strain determination, phase unwrapping, phase-shifter calibration, fault detection, edge detection and fringe segmentation. Various examples are given to demonstrate the ideas. Finally implementations of these algorithms are addressed. Most of the work has appeared in various papers and its originality is not claimed. Instead, this paper gives an overview and more insights of our work on windowed Fourier transform.     

基于窗口傅里叶变换剪切干涉法波前检测

提出了一种利用二维窗口傅里叶变换从径向剪切干涉条纹中准确得到波前的重建技术。首先对剪切干涉条纹做二维窗口傅里叶变换,设置阈值和频率积分范围后,进行二维窗口傅里叶逆变换,然后对包裹相位做去载频和相位展开处理得到相位差分布,最后使用波前迭代算法从相位差中复原实际波前。模拟计算表明,使用该方法最大相位复原误差为0.82%,均方根值为0.020 9 rad,实验结果验证了该方法的有效性。同时也对窗口傅里叶变换的关键参数,如窗函数的选择、窗口大小的确定以及阈值的选取等进行了简要讨论。与传统傅里叶变换法(FFT)相比,基于窗口傅里叶变换的剪切干涉波前检测法有更高的精度和稳定性,为波前检测提供一种新的处理方法。     

利用联合付里叶变换谱干涉法测量透镜焦距

本文提出了采用联合付里叶变换频谱干涉法测量透镜焦距的方法。输入面上两个分布完全相同的图像经光学付里叶变换在频谱面上得到二者付里叶变换频谱的干涉条纹，通过测量干涉条纹的间距即可得到透镜的焦距。该方法装置简单、操作调整方便；易于采用实时空间光调制器、ＣＣＤ及微机实现快速自动测量，且具有较高的精度。文中进行了原理分析和实验验证。     

Photonic crystal optical switch using a new slow light waveguide and heterostructure Y-junctions

In this paper we propose a photonic crystal Mach-Zehnder interferometer (MZI) based on a new coupled-cavity waveguide, where its advantage over the previous type of photonic crystal coupled-cavity waveguide is discussed. A heterostructure Y-junction is comprised of square and hexagonal lattices of the new coupled-cavity waveguide. It is designed by applying the temporal coupled-mode theory and is optimized, where an overall throughput efficiency of more than 90% is achieved at the wavelength of 1.55 μm. The design of the Y-junction is performed using finite-difference time-domain method. The designed heterostructure Y-junction is used in the structure of a MZI, where its arms are approximately as short as 24 μm. Finally, the transient response of the proposed optical switch is investigated.