Laser ultrasonic propagation imaging method in the frequency domain based on wavelet transformation

A wavelet-transformed ultrasonic propagation imaging method capable of ultrasonic propagation imaging in the frequency domain was developed and applied as a new structural damage or flaw visualization algorithm. Since the wavelet-transformed ultrasonic propagation imaging method has strong frequency selectivity, it can visualize the propagation of ultrasonic waves of a specific frequency (for example, to isolate ultrasonic mode of interest and a damage-related ultrasonic wave). The strong frequency selectivity of the wavelet-transformed ultrasonic propagation imaging method was demonstrated, isolating only the zeroth-order asymmetrical mode of the fundamental Lamb wave modes in an anisotropic carbon fiber-reinforced plastic plate with a thickness of 5 mm. The wavelet-transformed ultrasonic propagation imaging method can also convert a complex time domain multiple wavefield into a simple frequency domain single wavefield. This feature enables easy interpretation of the results, and facilitates the precise evaluation of the location and size of structural damage or flaws. We demonstrated this capability by detecting a disbond in a sandwich structure made of Al-alloy skins and a foam core. A disbond with a diameter of 20 mm, which is representative of a common manufacturing flaw, was successfully detected, localized, and evaluated. Since a method to determine the allowable maximum pulse repetition frequency depending on target materials and structures was found by investigating the residual wave caused from the previous laser impinging, our laser ultrasonic system can scan rapidly the target with an optimal pulse repetition rate. In addition, the proposed wavelet-transformed ultrasonic propagation imaging method can visualize damage or flaw without the need for reference data from the intact state of the structure. Hence, we propose the wavelet-transformed ultrasonic propagation imaging approach for automatic inspection of in-service engineering structures, or in-process quality inspection in manufacturing.     

灰阶编码掩模制作微光学元件

提出一种基于改变灰阶编码掩模的单元形状和位置的掩模设计新方法,并根据成像过程中的非线性因素,用这种方法对掩模图形进行了预畸变校正,根据部分相干光成像理论和抗蚀剂曝光显影模型,模拟计算了这种灰阶编码掩模产生的空间光强分布和光刻胶上的浮雕结构,采用电子束曝光系统制作了这种掩模,并在光刻胶上获得具有连续面形的微透镜的阵列。     

TE0导模干涉刻写周期可调亚波长光栅理论研究

通过棱镜耦合激发非对称金属包覆介质波导结构中的TE₀导波模式, 利用两束TE₀模的干涉从理论上实现了周期可调的亚波长光栅刻写. 分析了TE₀模式的色散关系, 刻写亚波长光栅的周期与激发光源、棱镜折射率、光刻胶薄膜厚度及折射率之间的关系. 用有限元方法数值模拟了金属薄膜、光刻胶薄膜和空气多层结构中TE₀导模的干涉场分布. 研究发现, 激发光源波长越短, TE₀ 模干涉刻写的亚波长光栅周期越小; 光刻胶越厚, 刻写的亚波长光栅周期越小; 高折射率光刻胶有利于更小周期亚波长光栅的刻写. 相较于表面等离子体干涉光刻, 基于TE₀ 模的干涉可在厚光刻胶条件下通过改变激发光源、棱镜折射率、光刻胶材料折射率、特别是光刻胶薄膜的厚度等多种方式实现对亚波长光栅周期的有效调控.     

Partially coherent beam and its applications

The recent progresses on the propagation, generation and application of partially coherent beam (PCB) are reviewed in this paper. A new tensor ABCD law for treating the propagation of partially coherent Gaussian-Schell model (GSM) beams through a paraxial optical system is derived. The focusing, spectral shift and fractional Fourier transform of the GSM beam are investigated by using the tensor method. The generation of PCB with special optical resonator is studied both theoretically and experimentally. Furthermore, the ghost imaging and superluminal propagation of the PCB is discussed. The results show clearly that the coherence of light have strong influences on the ghost imaging and superluminal propagation.      

高质量光刻胶微小透镜阵列的制作

本文对光刻胶熔融法制作微小透镜阵列的机理进行了进一步深入的理论和实验研究,特别是提出和研究了非球面表面的形成问题。对微小透镜,特别是非球面形成过程的参数控制,例如对光刻胶小圆柱的高度、熔融光刻胶和基片的接触角的控制进行了较详细的讨论。我们制出了具有较好性能的微小透镜列阵,并给出了测量结果以及实物和成象照片。最后,给出了一种建议采用的实时控制高质量微小透镜列阵形成的装置方案。     

Fabrication and application of imaging holographic gratings (chirped gratings) for integrated optics

Holographic chirped gratings with parallel grating lines have been fabricated by recording the interference structure of a plane wave and a cylindrical wave in a photoresist layer on a monomode glass waveguide. These gratings will be useful elements for the imaging of guided waves in integrated optics. A theoretical approach for treating their imaging properties is presented. The experimental results are described theoretically.     

基于数字微反射镜灰度光刻的成像模型

在深入探讨DMD(Digital Mirror Device)灰度图形传递的特点基础上,把曝光量分布作为分析数字光刻成像的物理量,建立了数字灰度光刻的成像模型.以制作微米量级微透镜为例,通过模拟分析,讨论了数字光刻过程中DMD的工作方式、成像系统参量对抗蚀剂曝光量分布的影响,揭示了数字灰度光刻成像机理,为开展数字光刻实验研究提供了理论根据.     

一种大F数微透镜阵列的制备方法

针对大F数(大于10)微透镜阵列难以制备的现状,提出了一种制备大F数微透镜阵列的方法.首先采用传统光刻胶热熔法及刻蚀技术制作出成形的微透镜阵列,再将一层具有较高粘滞系数的光刻胶均匀地涂覆在该微透镜阵列上,在光刻胶的粘滞作用以及烘烤过程中光刻胶自身表面张力的共同作用下,微透镜阵列的F数得到提高.采用该方法制备的二氧化硅微透镜阵列的F数达40,与传统大F数微透镜阵列的加工方法相比,该方法简便易行、制备的微透镜阵列面形良好,且只需调节光刻胶的粘滞系数,即可获得F数不同的微透镜阵列.     

极紫外光刻离轴照明技术研究

离轴照明技术(OAI)是极紫外光刻技术中提高光刻分辨率的关键技术之一。为了实现考虑掩模阴影效应情况下离轴照明的优化选择,构造了一种新型实现OAI曝光的成像模型。将照射到掩模上的非相干光等效为一系列具有连续入射方向的等强度平行光,基于阿贝成像原理分别对掩模进行成像,最终在像面进行强度叠加实现OAI方式下空间成像的计算;并通过向投影系统函数添加离焦像差项实现不同离焦面上空间成像计算。该模型极大地简化了OAI条件下对掩模阴影效应的计算,提高了成像质量计算效率。结合光刻胶特性及投影曝光系统焦深设计要求,以显影后光刻胶轮廓的侧壁倾角为判据,获得了采用数值孔径为0.32的投影系统实现16 nm线宽黑白线条曝光的最优OAI参数。     

Wavefront sensing with random amplitude mask and phase retrieval

A light beam with an ideal wavefront that is transmitted or reflected from an object is modified by different characteristics of the object such as shape, refractive index, density, or temperature. Wavefront sensing therefore yields valuable information about the system or the changes happening to the system. A new method for wavefront sensing using a random amplitude mask and a phase retrieval method based on the Rayleigh-Sommerfeld wave propagation equation is described. The proposed method has many potential applications ranging from phase contrast imaging and measurement of lens aberration to shape measurement of three-dimensional objects.     

Holographic imaging with a nanometer resolution using compact table-top EUV laser

Holographic 2D/3D imaging with nanometer resolution using short wavelength extreme ultraviolet (EUV) light is presented in this paper. Gabor’s holograms were recorded with a highly coherent table top EUV laser with different numerical apertures demonstrating ultimately a spatial resolution of 46+/−2 nm, comparable with the illumination wavelength, in 2D holographic imaging. Three dimensional images were obtained from a single high numerical aperture hologram recorded in a high resolution photoresist and numerically reconstructed at different image planes, allowing numerical optical sectioning with a lateral resolution ∼170 nm and depth resolution of 2.4 μm. The holograms were recorded in a high resolution photoresist and digitized with an atomic force microscope. To assess the spatial resolution of the numerical reconstructions of the holograms a correlation method was used. The algorithm allows for simultaneous estimation of the resolution and the feature size of the image under analysis.     

Nonlinear propagation delay and pulse distortion resulting from dual frequency band transmit pulse complexes

A method of acoustic imaging is discussed that potentially can improve the diagnostic capabilities of medical ultrasound. The method, given the name second order ultrasound field imaging, is achieved by the processing of the received signals from transmitted dual frequency band pulse complexes with at least partly overlapping high frequency (HF) and low frequency (LF) pulses. The transmitted HF pulses are used for image reconstruction whereas the transmitted LF pulses are used to manipulate the elastic properties of the medium observed by the HF imaging pulses. In the present paper, nonlinear propagation effects observed by a HF imaging pulse due to the presence of a LF manipulation pulse is discussed. When using dual frequency band transmit pulse complexes with a large separation in center frequency (e.g., 1:10), these nonlinear propagation effects are manifested as a nonlinear HF propagation delay and a HF pulse distortion different from conventional harmonic distortion. In addition, with different transmit foci for the HF and LF pulses, nonlinear aberration will occur.     

Three-dimensional photonic crystal of negative refraction achieved by interference lithography

A three-dimensional bicontinuous photonic crystal of a bcc lattice with all-angle negative refraction that can be achieved at optical frequencies by interference lithography is proposed. A numerical simulation of the focusing imaging of a slab of this crystal was performed to verify the property of all-angle negative refraction. The dependence of the negative-refraction frequency range on the threshold of photoresist development and on the dielectric constant is also discussed.     

水下声成像中旁瓣抑制方法及其实验研究

提出了一种近场条件下采用不等间隔阵并进行孔径变迹处理的水下声成像旁瓣抑制方法,并进行了理论和实验研究。考虑以球面波传播理论为基础的聚焦波束形成,首先通过阵元位置微调,设计了可实现低旁瓣的不等间隔阵,从单程波束响应上降低旁瓣;然后,将孔径变迹处理方法应用于水下声成像中,全部阵元用于接收,部分阵元用于发射,从双程波束响应上进一步降低旁瓣。通过水池实验对所提出的方法进行了验证。结果表明:采用不等间隔阵并进行孔径变迹处理可以更为有效地降低旁瓣,而主瓣仅有小量展宽,且该方法工程应用简便易行,在改善成像质量的同时降低了系统复杂度。      

Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Application to actual damages in composite wing

Laser ultrasonic wave propagation imaging methods have great potential for integrated structural health management and non-destructive evaluation. However, application of these techniques to complex structures in the field is difficult because they give rise to complicated wave propagation patterns. We developed an anomalous wave propagation imaging method with adjacent wave subtraction using laser ultrasonic scanning to solve this problem. The proposed method is suitable for non-destructive evaluation of complex structures because it highlights the propagation of anomalous waves related to structural discontinuities, and suppresses complex incident waves without the need of pre-stored reference data. In this study, the method was applied to a real composite wing subjected to bending and impact tests. The method enhanced the visibility of the anomalous waves related to damages such as stringer tip debonding, skin-spar debonding, and invisible impact damage. Based on these anomalous waves, variable time window amplitude mapping was performed to show the damage location, size, and shape resemble to the actual damage. Comparisons showed that the methods performed better than the ultrasonic A-scan in terms of damage detection and sizing accuracy. The presence of structural elements such as spars, stringers, ribs, and surface-mounted PZT elements did not adversely affect the inspection. The proposed wing test setup with a built-in ultrasonic propagation imaging system for automatic NDE could be easily expanded throughout a hanger for maintenance inspection.     

激光直写光刻中线条轮廓的分析

考虑了光刻胶对光吸收作用,在已有描述胶层内光场分布模型的基础上,较为准确地推导出光刻胶层内不同深度位置的光场分布.使用迭代方法计算得到了胶层内曝光量空间分布曲线,分析了不同曝光量下胶层内的线条轮廓,为直写光刻中曝光量的选择提供了依据.实验结果分析与理论分析的结果一致.     

扩展微透镜数值孔径范围的阶梯光刻热熔法研究

在微透镜阵列的光刻热熔制作法中，临界角效应严重影响了微透镜的制作范围和面形质量。在对临界角效应定性研究的基础上，提出了用阶梯光刻熔法来扩展热熔型微透镜阵列的数值孔径范围。实验结果表明，采用这一方法制作的微透镜，其单元孔径范围扩展为５０￣９００μｍ，相对口径范围扩大到为Ｆ／１￣Ｆ／１０，并有效地改善了临界角效应对大孔径微透镜面形质量的影响。     

SU-8光刻胶应变分布光学全场检测方法

由于SU-8光刻胶的内应力将会影响高深宽比结构的全金属光栅的制作质量,本文针对近年来SU-8光刻胶应力测量困难的情况,提出了一种基于激光剪切散斑干涉技术的SU-8光刻胶应变分布测量的新方法。该方法通过对被测胶体加载前后两幅干涉图像的处理,直接得到被测胶体结构的全场应变分布情况,由胶体的应变变形数据即可反映出内应力的变化和分布趋势。同时使用ANSYS有限元分析软件对同一被测胶体进行应变仿真模拟研究,获得胶体结构的变形场仿真数据。组建了实验系统,进行了实验验证,结果表明:实际测量变形量约为1.189μm,仿真的最大变形量为1.088μm,测量误差在允许范围内,且测量的形变趋势与仿真模拟结果相一致,表明激光剪切散斑干涉技术可应用于SU-8光刻胶的应变分布全场无损检测。     

Patterning of oxide-hardened gold black by photolithography and metal lift-off

A method to pattern infrared-absorbing gold black by conventional photolithography and lift-off is described. A photo-resist pattern is developed on a substrate by standard photolithography. Gold black is deposited over the whole by thermal evaporation in an inert gas at ∼1 Torr. SiO₂ is then deposited as a protection layer by electron beam evaporation. Lift-off proceeds by dissolving the photoresist in acetone. The resulting sub-millimeter size gold black patterns that remain on the substrate retain high infrared absorption out to ∼5 μm wavelength and exhibit good mechanical stability. This technique allows selective application of gold black coatings to the pixels of thermal infrared imaging array detectors.     

Depth of focus estimation based on exposure dose distribution

A defocusing exposure dose distribution model is established with the integral effect of light intensity on time taken into account for laser direct writing on a thin photoresist with total reflection substrate. Exposure dose distribution curves are established using the established model for different photoresist depths. A side slope angle is established for each defocusing amount in accordance with the exposure dose distribution curves, and so depth of focus can be estimated by simply checking to see if the maximum side slope angle with the horizontal is in the range of 80-100°. Simulation results indicate that when laser direct writing is done on a thin photoresist with total reflection substrate using a laser with wavelength equal to 442 nm and a lens with numerical aperture equal to 0.5, the depth of focus estimated using the proposed method is 1 μm, which is just 1/3 of the depth of focus estimated using the method based on intensity distribution. It is therefore concluded that it is the integral effect of light intensity on time that causes the depth of focus estimation error, and the proposed method can be used to achieve a more accurate depth of focus estimation compared to the intensity distribution based method.