Interference effect of dual cylindrical microlenses with the closed boundary in dielectric incident space

The focal characteristics of closed-boundary cylindrical microlenses with dielectric incident space are investigated based on rigorous electromagnetic theory and the boundary element method. For single cylindrical microlens, focal performance is improved with the use of the dielectric incident space, for instance, its diffraction efficiency can be increased. Moreover, the interference effect of dual cylindrical microlenses embedded in a uniform dielectric material is studied in detail. Numerical results reveal that the interference effect between the dual microlenses can be suppressed substantially by the dielectric incident material environment. The interference effect can also be depressed by adjusting some other parameters, such as the distance between the dual microlenses, and the incident wavelength.     

Effect of illumination types on focusing performance of closed-boundary cylindrical microlenses

The effect of illumination types on closed-boundary cylindrical microlenses (CBCMs) is investigated by rigorous electromagnetic theory and boundary element method. As a result, the focusing performance of CBCMs with different illumination types are different to each other, while the optics elements cannot be considered as pure phase element for small f-number.     

Broadband dispersion characteristics of diffractive microlenses based on the finite-difference time-domain method

The broadband dispersion characteristics of diffractive microlenses are studied using the finite-difference time-domain (FDTD) method. The distributions of the diffracted electric fields obtained through a diffractive microlens are presented for illumination wavelengths ranging from 0.35 to 1.30 μm, along with the corresponding broadband dispersion curves. It is shown that both the principal focal length and the diffracted order of the principal focal point are dependent on the illumination wavelengths. The broadband dispersion characteristics of a diffractive microlens may be used to optimise light coupling for a broadband optical fibre source.     

Rigorous electromagnetic analysis of metallic cylindrical focusing micromirrors with high diffraction efficiency, achromatic aberration and long focal depth

In this paper, we propose a kind of metallic cylindrical focusing micromirrors (MCFMs) with parabolic profiles, and the focal performance is investigated based on rigorous electromagnetic theory and the boundary element method (BEM). Numerical results by the BEM show that the proposed MCFM possesses a high diffraction efficiency and axially achromatic aberration. In addition, through designing a modulating phase function, the MCFM holds long focal depth and high transverse resolution simultaneously. It is believed that the MCFM should have wide application prospects in micro-optical systems, such as micro solar concentrators.     

基于时间反转理论的聚焦Lamb波结构损伤成像

从理论和实验上研究了时间反转法在频散和多模式的Lamb波结构健康检测方面的应用.当Lamb波在包含有损伤的板类结构中传播时,损伤的存在表现为一个被动波源.采用分布式传感器网络,基于传递函数的观点,通过推导由损伤这个被动波源产生的时间反转波场幅值的表达式,证实了当观察点位于损伤位置时,时间反转波场的幅值最大.为验证时间反转方法的聚焦效应,提出了一种适合于分布的激励/接收传感器网络的成像方法,该方法可以对损伤定位并近似确定损伤尺寸.结合有限元的实验结果显示了Lamb波检测信号的能量可在损伤处聚焦,表明时间反转     

Combined implementing the hole-drilling method and reflection hologram interferometry for residual stresses determination in cylindrical shells and tubes

Main features inherent in simplified approach to residual stresses determination in cylindrical shells and tubes, external diameter of which is not less than 60 mm, by combing the hole-drilling method and reflection hologram interferometry are discussed in detail. Initial experimental information in a form of hole diameter increments in principal stress directions is derived from high-quality reflection holograms recorded near cylindrical objects of intermediate curvature value. Converting measured parameters into required stress values is based on the transition model that corresponds to plane stress conditions of pure membrane type. The technique developed is capable of determining residual stress component values within 5% accuracy in an absence of stress gradients over the probe hole diameter when a type of residual stress field corresponds to the transition model adopted. The accuracy analysis involved is based on matrix formulation of conventionally direct problem and an assumption on a pure membrane character of residual stress field under study for thin-walled shell. Required error estimations in a case of inspecting thick-walled cylindrical tube are obtained by combining the above-mentioned approach and an analogy of reconstructed fringe patterns with actual and artificial interferograms, which follow from drilling blind hole of the same geometrical parameters in thick-walled plates. Experimental verification of the developed approach is founded upon a determination of actual stresses in thin-walled cylindrical shell and obtaining residual stress distributions at the proximity of welded joint in thick-walled cylindrical tube.     

基于一维金属光子晶体平凹镜的柱矢量光束亚波长聚焦

柱矢量光束具有柱对称性的偏振分布,其独特的光场分布和聚焦特性被广泛应用于光学微操纵及光学成像等领域,并迅速向亚波长尺度拓展.通常,亚波长尺度聚焦采用等离激元透镜实现,但存在光场调控的偏振态局限性.而借助光子晶体的负折射效应,不仅能够实现亚波长聚焦或成像,而且应对正交偏振态同时有效.采用对电磁波具有更强调控能力的一维金属光子晶体结构,计算得到的能带结构和等频曲线表明其负折射效应在特定波段对正交偏振态同时有效.在此基础上设计出一维金属光子晶体柱对称平凹镜结构,通过有限元算法模拟显示了可见光波段的径向和旋向偏振光的同时亚波长聚焦行为.进一步的结果表明,改变柱矢量光束的偏振组分能够直接有效地调节焦场空间分布及偏振分布特性.所提出的平凹镜结构能够实现对任意偏振组分的柱矢量光束的亚波长尺度聚焦,且该结构的设计对于各波段情况均有参考意义.该研究结果对小尺度粒子的光学微操纵、超分辨率成像等相关领域具有潜在的应用价值.     

Studies on conical and cylindrical resonators

Spectral features of conical and cylindrical Hartmann resonators are compared in this work through a systematic parametric study. Experiments have been conducted by varying the following parameters: stand-off distance, nozzle pressure ratio and cone angle. Resonance frequencies of conical cavities are found to be higher than those of cylindrical cavities of the same length. Low (∼kHz) and high frequency (∼10 kHz) modes are observed in the spectra. Low frequency modes show an oscillatory trend with stand-off distance. The high frequency tones are found to be independent of cavity geometry and cavity length, and are similar to jet impingement tones.     

Negative refraction and focusing of electromagnetic wave through two-dimensional photonic crystals

The negative refraction of electromagnetic waves in photonic crystals was recently demonstrated experimentally, and the physical properties were analyzed. Microsuperlenses based on two-dimensional photonic crystals were designed and the subwavelength images were observed. In this review, after providing a brief history of the research related to the above phenomena, we will summarize our research works in this field including the method of creating a negative refraction region, generating an absolute negative refraction, the focusing of unpolarized electromagnetic waves, and the effect of interface and disorder on the image by the two-dimensional photonic crystal flat lens. The discussion on the negative refraction and the focusing by high symmetric quasicrystals is also presented.     

时间反演镜对时间反演电磁波聚焦特性影响因素的研究

时间反演电磁波聚焦特性的主要影响因素来自于时间反演镜(time reversal mirror,TRM).利用电磁仿真工具,对各种影响因素进行分析,给出了改善时间反演电磁波聚焦特性的方法.最后,通过实验对改善方法的有效性进行了验证. 关键词： 时间反演镜 时间反演电磁波 聚焦特性 影响因素     

正交电磁场中粒子束的质量聚焦及粒子轨迹

正交电磁场中粒子的运动轨迹在拐点处按指数规律变化,使不同质量离子的轨迹有明显区别,由此可创建新的电磁核素分离法。从牛顿运动方程出发,基于单粒子轨道法阐述被分离核素在正交电磁场中的运动规律,重点找出轨迹的拐点。给出了多质量粒子束在正交电磁场中的质量聚焦特性,以Li离子为例,应用MATLAB模拟粒子束的运动轨迹,结果表明正交电磁场中可实现多质量束流的质量分离。总结出粒子轨迹方程的特点,为多质量束流分离结构的工程化实现提供参数。研究成果可应用到质量分离器、质谱分析仪及材料提纯等装置的研制中,同时对于特殊位形电磁场控制多质量束流等相关领域的研究亦有一定的参考价值。     

调焦方式对测绘相机主点位置的影响

考虑测绘相机调焦会影响相机的主点位置,从而改变相机的内方位元素,降低立体测绘精度,故本文分析了空间相机常用的3种调焦方式（镜组调焦、平面反射镜调焦、焦面调焦）对测绘相机主点位置的影响。简要介绍3种调焦方式的工作原理,研究了理想情况下3种调焦方式对主点位置的影响。在考虑系统装凋误差的情况下,建立了主点位置变化量与调焦量之间的数学模型,通过实例计算得出焦面调焦易满足测绘相机主点定位精度〈0．2pixel,是最适合的调焦方式。最后,以某一型号相机调焦机构为实验对象,对平面反射镜方式对主点的位置变化进行了实验量测,结果证明了提出的主点位置变化量理论计算公式的准确性和分析得到的不同调焦方式对主点位置影响的正确性,该公式可以用于指导测绘相机调焦方式的选择,以满足高精度测绘的需要。     

基于ZEMAX的二次反射式激光聚焦系统设计与模拟

反射式激光聚焦系统使激光推力器具备了推力矢量控制能力,并且聚焦系统不存在球差,具有广阔的应用前景。针对三种二次反射聚焦方式,抛物面-双曲面、平面-抛物面、抛物面-平面,在建立的聚焦系统坐标系下,通过几何光学方法分析了两个反射面母线参数之间的数学关系,以及聚焦特征参数与两个反射面母线参数的关系,并建立了程序设计流程。针对特定的聚焦特征参数设计要求,在光学软件ZEMAX中进行了反射面的建模和聚焦效果的验证,为下一步开展聚焦性能的研究打下了基础。     

基于Zemax的He-Ne激光光束聚焦物镜的设计

 为了得到一个合理的He-Ne激光光束聚焦物镜,采用以正前凸型为基础的高折射率双片结构,应用Zemax软件进行优化设计,获得了弥散斑直径为0.002mm的He-Ne激光聚焦物镜,该镜头只需校正轴上点球差。实验结果表明：设计的镜头比低折射率单片透镜得到的弥散圆直径更小,达到0.0019mm,球差被控制在-0.05mm～+0.05mm范围内,MTF曲线所围面积变大,中心点亮度增高,符合实际需要。     

经颅超声聚焦方法和声场特性研究

基于高分辨的CT数据建立了非均匀颅骨仿真模型,该模型引入了颅骨的声衰减系数,深入研究和分析了声波时间反转法和超声相控阵法在颅脑中的聚焦方法及效果。颅骨具有较强的声波衰减特性,使用时间反转聚焦时需要进行幅度补偿,对于0.7MHz的频率信号,幅度补偿后的时间反转聚焦声场主瓣宽度窄、旁瓣低,焦点处声场比无幅度补偿的时间反转法提高8.86dB,比超声相控阵聚焦法提高7.89dB,具有很好的空间聚焦精度和聚焦效率。研究了颅骨衰减系数、声场焦点位置、声波频率、换能器阵列位置和方位等参数对聚焦声场的影响,结果表明,幅度补偿时间反转法比相控阵法具有更低的旁瓣,且高频时的聚焦效果比相控阵好,相控阵聚焦对换能器阵列的位置和方位比较敏感,而时间反转经颅超声聚焦对声传播路径和入射角具有更高的鲁棒性。     

利用磁聚焦法测量电子比荷的误差分析

在利用磁聚焦法测量电子比荷的实验中,测量误差与示波管的加速电压和聚焦电压存在较大的相关性．通过对测量误差与电聚焦程度、加速电压和聚焦电压之间关系的分析,指出了误差产生的原因,给出了加速电压和聚焦电压的最佳取值范围．     

数字图像一阶矩的自动聚焦区域选择算法

聚焦区域选择是自动聚焦算法中的重要模块,直接影响评价函数曲线的尖锐性、准确性和稳定性。针对中心取窗和多点取窗等传统聚焦区域选择算法的局限性,提出了一种基于数字图像一阶矩的自动聚焦区域选择算法,它是通过图像灰度或边缘一阶矩定位聚焦区域的。传统聚焦区域选择是一种定点估计,实验证明一阶矩方式选择可以跟踪偏离中心的主体景物,是一种具有自适应性的前景位置估计算法,提高了聚焦精确度和速度。     

Numerical simulations of laser light guiding and focusing using an open hollow cone

An open hollow cone that can optically guide and focus a laser light into a small beam spot is proposed. The shaping and focusing effects are numerically demonstrated by finite-difference time-domain simulations. The results reveal that the cone-focused laser originates from the multi-beam interference and squeeze of two cone side walls acting like oblique targets. Moreover, the influences of oblique incident angle of laser light, cone angle and cone tip size on the focusing properties are discussed and the optimum cone geometry with cone angle of 16° and tip size of 4.5 wavelengths to achieve high energy concentration at the cone tip is also presented. It is shown that with an open hollow cone a laser light can be focused into a tiny highly localized beam spot of 1 µm diameter and keeps propagating forward for a distance of about 8 wavelengths with a tiny spot diameter remaining almost unchanged. The intensity almost increases up to twentyfold in a 1 µm focal spot.     

基于多重结构的平显视差仪中内调焦物镜设计

根据平显视差测量原理中自动调焦的要求,设计了一个相对孔径与调焦范围均较大的内调焦物镜。针对内调焦传统设计方法的不足,在对内调焦各镜组选型后将它们组合成为一个系统,采用多重数据结构设计的方法,对不同物距下的结构进行同时优化,从而解决了近距离的成像质量难以控制的问题。所设计的内调焦物镜在不同的物距下成像质量稳定,无穷远时光学传递函数(MTF)值在空间频率为120 lp/mm时轴上与轴外均达到0.4以上,物距在300 mm时轴上视场MTF为0.3,轴外视场弧矢MTF在0.3以上,子午MTF达到了0.2;相对畸变均控制在1%以内,有利于后续的图像处理工作。     

基于近场聚焦逆波束形成的UUV平台噪声自适应抵消

UUV平台自噪声会对舷侧阵产生干扰,从而影响其探测性能,本文针对平台尾部自噪声源的直达噪声和海面反射噪声,提出了基于近场聚焦逆波束形成的平台噪声自适应抵消方法。文中利用UUV平台设计参数、深度以及运动姿态信息对尾部自噪声传播到舷侧阵的路径建立几何模型,并分析了影响该路径的因素;通过该模型,平台尾部自噪声能够实时自适应地通过聚焦波束形成被估计出来,从而在舷侧阵的接收信号中实现干扰抵消。仿真结果与海试实验数据处理结果表明,该方法能够有效抑制尾部自噪声干扰,显著提高UUV探测弱目标的能力。