多频光栅物体高精度廓形三维测量及重建研究

针对目前实际系统中,由于噪声的存在,单频光栅逐点解相时无法获得全场相位值,导致对廓形复杂、体积小物体的重建精度低,提出多频结构光栅投影实现体积较小、形貌复杂物体的高精度重建。系统算法基于随时间序列变化改进的四步相移法对多频光栅进行解相,多频光栅解相是对每种频率的光栅单独展开相位,每种频率的光栅解相是逐点进行的,单频光栅中因噪声得到不连续的相位可通过其他频率光栅拟合修补,得到一个全场范围内连续的相位。经实验单频结构光测量体积小、形貌复杂物体的误差范围为0.1 mm～0.5 mm,而文中所提出方法精度达到了0.03 mm～0.05 mm,该方法精度提高了10倍左右。     

非线性二次曝光法制作三角形光纤Bragg光栅

提出了一种利用非线性二次曝光技术制作三角形光纤Bragg光栅的新方法.该方法只使用普通Bragg位相模板,采用两次曝光技术,通过控制光纤光栅的本地谐振波长和反射率,达到控制光纤光栅谱形的目的.第一次直接对光纤曝光,用来控制光纤光栅谐振波长;第二次通过位相模板曝光,用来控制光纤光栅的反射率.考虑到光敏光纤感光特性曲线的多项e负指数变化规律,按照"分步写入光纤光栅设计软件"进行曝光分布设计,利用"分步曝光光纤光栅写入系统"进行曝光控制,使用信息产业部第46研究所生产的DCS-01型光敏光纤,得到了线性范围1.24 nm、反射率为0～64%的三角形光纤Bragg光栅.三角形光纤Bragg光栅作为光纤光栅传感解调器件,在光纤传感领域具有重要应用价值.     

啁啾sinc取样光纤光栅研究

提出了一种特殊的取样光纤光栅：啁啾sinc取样光纤光栅.研究表明,对于均匀光纤光栅进行啁啾sinc取样,可以实现色散斜率补偿和获得精确的反射波长数控制;对啁啾光纤光栅进行啁啾sinc取样,则能够实现色散和色散斜率的同时补偿,并且可以进行精确的波长数控制及获得各信道一致的反射率.利用啁啾sinc取样光纤光栅,可以对各种传输光纤进行色散和色散斜率的补偿.     

Optimized staircase profiles for diffractive optical devices made from absorbing materials

We report on the optimization of staircase grating profiles for the case of absorbing grating materials. Using a simple numerical algorithm, we determined the grating parameters, maximizing the first-order diffraction efficiency for different numbers of staircase steps. The results show that there is a significant difference between the staircase profiles for nonnegligible and negligible absorption. The obtained solutions are of importance for diffractive optics in the soft-x-ray and extreme-ultraviolet ranges.     

彩色多通道光栅投影轮廓术

研究了一种双通道光学相位测量轮廓术。利用彩色空间中红色与蓝色互相独立、互不干扰的特性,将两个相位相反的线性周期变化的光栅分别放在红以与蓝色通道,形成彩色双通道投影光栅。用彩色ＣＣＤ获取图像,并和图像采集卡将图像送入计算机。从彩色图像的红色与蓝色通道可分别获得两个互相反相位的线性周期光栅的调制图。在一次采样中即可获得两个通道的测量结果,因而特别适用于动态测量。     

Enhancing the phase profile and contrast of an interferogram by polarization recording in bacteriorhodopsin

Interferometry is a technique for reconstructing the profiles of phase objects. We present a novel interferometric setup for generating interferograms with doubled phase profile and enhanced contrast compared with the standard interferogram. The proposed system consists of a two-beam interferometer in which the reference and test waves are circularly polarized orthogonally to each other. They are superposed upon a bacteriorhodopsin film, creating a polarization grating that is distorted by the phase of the test object. This polarization pattern is read by a polarized He-Ne beam. We show analytically and experimentally that, when the zero diffraction order is removed, an interferogram with doubled phase profile and enhanced contrast is obtained.     

Spatial phase coding for incoherent optical processors

In this paper we introduce spatial phase coding of incoherent optical signals for representing signed numbers in optical processors and present an experimental demonstration of this coding technique. If a diffraction grating, such as an acousto-optic cell, modulates a stream of light, the image of the grating can be recovered from the diffracted beam. The position of the grating image, or more precisely its phase, can be used to denote the sign of the number represented by the diffracted light. The intensity of the light represents the magnitude of the number. This technique is more economical than current methods in terms of the number of information channels required to represent a number and the amount of post processing required.     

点阵绝对相位彩色多频光栅傅里叶变换轮廓术

提出一种将三角法测距与多频光栅相结合的方法,充分利用数字投影的RGB通道,将多频光栅与点阵同时投影在物体表面,综合控制受高度调制的相位信息的提取,最终获取物体的三维形貌。仿真结果显示,在高度跳变使高频光栅位移达102量级个周期时,可以准确还原原始形貌。通过实验验证,本方法能够同时复原169mm的陡峭高度跳变以及3mm的微小细节,提高了传统双频光栅的性能,具有处理较大截断面和保证细节部分的测量能力,较好地改善了傅里叶变换轮廓术(FTP)的应用性能。     

Analysis and design of grating couplers

Based on an accurate perturbation analysis of the guiding properties of dielectric gratings, simple design criteria are developed for grating couplers which transfer the energy of a beam into or out of an optical waveguide. Gratings having arbitrary groove shapes are considered and explicit formulae are given for the leakage parameters of gratings with symmetric profiles. The results cover TE_v and TM_v modes and they apply to both shallow and deep grating grooves. The variation of the leakage parameter α in rectangular gratings is examined in detail; these rectangular gratings are then used as basic configurations for predicting the characteristics of other grating profiles. Particular attention is given to trapezoidal and triangular profiles and gratings with asymmetric profiles are also discussed.     

免分析光栅一次曝光相位衬度成像方法

X射线光栅微分相位衬度成像技术可以观察到常规吸收衬度成像难以分辨的弱吸收物质的精细结构信息,因而在医学、材料学等研究领域具有巨大的应用前景.但传统的X射线光栅微分相位衬度成像技术由于采用分析光栅作为空间滤波器,需要采用相位步进法扫描分析光栅来获得样品的多张投影图像才能够分离出样品的吸收、折射和散射信息,因此存在样品曝光时间长、辐射剂量高以及X射线光通量利用率低等问题,限制了其在各个学科领域的应用研究.为克服上述问题,本文提出一种基于免分析光栅相位衬度成像系统的一次曝光样品信息提取算法.该算法只需要利用一块相位光栅,进而采用高分辨探测器进行样品投影数据的一次采集即可提取样品的吸收、折射和散射信息.理论和模拟研究结果表明:与传统相位步进法相比,该算法具有样品信息提取精度高,且不受光栅的自成像周期需为探测器像素尺寸的整数倍条件的限制.此外,该算法还能够有效地减少对生物样品的辐射损伤,因此在生物医学成像等研究领域中具有广泛的应用前景.     

Theoretical models of diffraction efficiencies of arc profiled bar transmission grating

Diffraction efficiencies of transmission grating play important roles in accurate measurement of soft X-ray and grating applications. Circular, horizontal elliptical, and vertical elliptical arc profiled bar models were established from Kirchhoff diffraction theory with the Fraunhofer diffraction approximation to calculate the diffraction efficiencies of arc profiled bar transmission grating. The calculated results were compared with the available data from X-ray Optics web site of Lawrence Berkeley Laboratory's Center. Excellent agreements have been obtained. With these modes the diffraction efficiencies of the particular type of transmission grating having arc profiled bars can be simulated accurately and the best fit bar profiles can be obtained.     

The amplitude and phase of a transmission holographic grating due to four-wave interaction in a photorefractive medium

Based on a previously developed two-level model of four-wave interaction in photorefractive crystals, expressions for the amplitude and phase of a transmission holographic grating have been obtained. The analysis of these expressions points to the dependence of both the amplitude and the phase of the grating on the intensity of the readout beam.     

Nonlinear acoustic waves in channels with variable cross sections

The point symmetry group is studied for the generalized Webster-type equation describing nonlinear acoustic waves in lossy channels with variable cross sections. It is shown that, for certain types of cross section profiles, the allowed symmetry group is extended and the invariant solutions corresponding to these profiles are obtained. Approximate analytic solutions to the generalized Webster equation are derived for channels with smoothly varying cross sections and arbitrary initial conditions.     

The modeling and the optimization of a grating optical low-pass filter

Through the one-dimensional analysis of the diffraction of a phase grating, we calculated the optical transfer function (OTF) of a phase grating. By a 0th-order approximation of the optical transfer function, we analyzed the feasibility of using it as a spatial frequency filter and obtained the structure of a grating for the optimum-filtering characteristics. The OTF of a phase grating as an optical low-pass filter is optimal when the incoming light is equally divided into three beams. The distance of those beams must be equal to the pixel size. The theoretical development was verified through an experiment. Received: 21 May 2001 / Revised version: 16 July 2001 / Published online: 15 October 2001     

Phase locked loop profilometry

An innovative technique for obtaining automatic three-dimensional shape information of a diffuse surface is presented. The technique is based on a new approach of phase measuring from a fringe pattern. The target to be studied is first modulated by projecting a linear grating onto its surface. The linear fringes are deformed according to the surface shape. Demodulation is carried out by determining the phase of these deformed fringes using a novel digital phase locked loop demodulation algorithm. The technique has the main advantage that the traditional phase unwrapping process is not required. The phase is determined continuously as the algorithm scans the two-dimensional fringe pattern. Owing to its sequential nature, this phase detection technique can be implemented using a special purpose video processor for phase determination at video rates, making it a very fast algorithm. The algorithm along with experimental results of real surface profiles are presented.     

全息光栅实时显影监测曲线的理论模拟

在特定的工艺条件下,光刻胶的非线性效应非常显著;合理地利用非线性效应,能够制作出近似矩形的全息光栅掩模。为了分析清楚非线性效应对光栅沟槽成形的作用机理,有必要建立一个显影理论模型。模拟得到的光栅轮廓和实验样片的扫描电子显微镜照片结果很吻合。根据这个模型,进而使用光栅严格理论,得到其主要特征与实时监测实验曲线一致的理论模拟监测曲线。理论分析和实验证实,该模型基本表征了工艺条件对光栅沟槽形状的影响,并揭示了光刻胶呈现显著非线性效应时,必然对应着明暗条纹中心位置之间的显影刻蚀速率相差很大。这个模型为全息光栅的工艺研究提供了一个有效的理论分析工具。     

Diffraction properties of ultrashort pulsed beams with arbitrary temporal profiles by a volume holographic grating

In this paper, the diffraction properties of an ultrashort optical pulse with arbitrary profiles in time diffracted by a volume holographic grating are investigated using the modified Kogelnik coupled-wave theory. Simple analytical expressions for the spectrum and spatial profiles of the transmitted and diffracted beams are obtained. The dependences of the diffraction bandwidth, the Bragg selectivity bandwidth and the total diffraction efficiency of the volume grating on the temporal profiles of the input ultrashort pulse are investigated. For three different temporal profiles, numerical results of diffraction properties are given. It is shown that the temporal shapes of the input pulsed beams have been found to be an important factor in the analyses of the propagation characteristics.     

制作具有预期位相调制度的卤化银乳胶位相型薄全息光栅的研究

提出了一种制作具有预期位相调制度的卤化银乳胶位型薄全息光栅的方法。它通过分析所拍摄的第一张全息光栅的数据来获得最佳曝光参量，从而使继后的拍摄，在同样的实验条件下能获得所预期的位相调制度。文中给出了这方法的理论分析和某些应用。     

Guided-mode resonant Brewster filter consisting of two homogenous layers and a single grating with an equal refractive index

In this paper, a new type of resonant Brewster filter (RBF) consisting of two homogenous layers and a single grating with an equal refractive index is presented. The properties are studied by using the plane waveguide method (PWM) and rigorous coupled-wave analysis (RCWA). It is found that the variation of the grating thickness does not effectively change the position of the resonant wavelength, however it has a remarkable effect on the line width, and the resonant peak can be adjusted back to its original position by slightly tuning the grating period. Moreover, by simultaneously tuning the thicknesses of the homogeneous layers above and beneath the grating structure, multiple channels can also be obtained when the RBF is illuminated at the Brewster angle calculated with the effective medium theory (EMT) of subwavelength grating. The adjacent optical thickness for acquiring the multiple channels is about three-quarters of the resonant wavelength. Furthermore, it is demonstrated that the line width at the operating resonant wavelength can be appreciably narrowed by tuning the thickness of the homogenous layer to its corresponding thickness without fine tuning the grating period or the thickness. Therefore, it is very useful for designing filters with different line widths at the desired wavelength. In addition, it is shown from our calculations that the symmetrical line feather can be obtained if the total optical thickness for the homogeneous layer meets the special condition.     

采用空间—角度复用的盘式三维全息存储

提出基于空间－角度复用技术的盘式三维全息存储方案，给出这一技术的基本原理，并采用简化的光线分析方法对该方案的可行性进行分析。研究表明，这是一种能提供高密度和高数据传输率，而读写系统又相对简单的盘式三维全息存储方案，具有潜在的应用前景。