利用回波相干性的医学超声成像相位畸变校正方法

为了提高相位畸变条件下的医学超声成像的横向分辨率和对比度,提出了一种利用回波信号相干性的校正方法。首先给出平均相干系数的概念,并将其作为相位误差校正的度量,然后通过最大化平均相干系数逐步校正各个通道的延时误差,最后再利用经过校正的数据计算出一组新的相干系数并对回波信号进行加权优化,从而得到最终用于成像的数据。对点散射目标及斑散射目标的仿真结果分别显示,利用所提出的方法横向分辨率提高了约0.24 mm,对比度提高了约18 dB,且要优于邻近阵元互相关方法和相干系数直接加权的处理方法。利用回波相干性的相位畸变校正方法结合了相位误差校正和加权处理的优点,可以有效地改善医学超声成像的质量。      

全息声谱显示

本文提出了一种全息声谱的编码和显示方法，全息声谱图是在二维屏幕上显示四维的声谱函数语图，它同时表征了语声的幅度谱和相位谱，在显示坐标上，除了时间轴和频率轴两维外，还分别用亮度来表示幅度谱，用色彩来表示相位谱，通过编码，我们成功地实现了这一显示系统。     

基于相位差实时计算法实现相控式高强度超声聚焦

利用时间反转法对相控阵高强聚焦超声系统进行自适应聚焦,需要计算出各阵元的初始发射相位。锁相环电路初始工作时频率有震荡,用常规计算相位差的方法当截取数据的时间段不同时计算出的相位差不相同。提出一种计算相位差的方法,可以实时显示相位差的变化,将稳定后的相位差作为相控阵聚焦系统各发射阵元的初始相位。实验结果表明,用此方法计算的相位差进行聚焦可以在一定空间范围内得到较好的声场分布。      

基于微分法的新傅里叶测量轮廓术

在傅里叶测量轮廓术中,为了得到真实相位值,必须进行相位解包裹处理的繁锁过程,为此提出一种可以直接获得真实相位值的新算法.这种算法的原理是空域中的微分(一阶)等效于频域中乘以一个线性因子,由此得到相位微分与光强的关系,求出相位的微分值后将其积分就获得相位的真实值.计算机仿真显示该方法确实有效可行.     

超声平面波经颅成像相位校正方法

超声平面波经颅成像时,由于颅骨对超声传播的影响引起图像质量下降,需要对颅骨造成的超声相位畸变进行校正.为此,该文研究了两种相位补偿方法:基于近似射线声学的理论方法和基于时间反转的数值计算方法,并利用数值仿真对比了两种方法的补偿和成像效果.结果表明:无论使用近似射线法还是时间反转法,都能够有效地校正因颅骨造成的相位畸变;...     

圆环形聚焦声场的构建与调控

提出了一种由径向振动模式的圆环形压电换能器晶片组成的圆柱形阵列换能器结构, 依据阵元激励信号的相位调控机理, 推导了圆环形聚焦声场的调控公式, 在三维空间中构建了圆环形聚焦声场, 实现了对其聚焦区域大小、聚焦圆环半径以及轴向位置移动的调控. 理论分析和仿真研究表明, 所提出的圆柱形阵列换能器实现了对圆环形聚焦声场的调控. 为检测超声、功率超声、医学超声等应用领域提供了一种可实现的新型圆环形可调控聚焦声场.     

基于正弦相位编码的相机离焦标定

分析了光学系统离焦状态对基于正弦光栅的结构光相位的影响.基于正弦结构光相位与相机对焦状态无关的性质,提出了一种利用相移法正弦结构光编码显示面板对离焦相机进行标定的方法.使用Floyd-Steinberg Dithering算法消除了显示面板gamma变换带来的结构光相位计算误差.实现了相机在离焦状态下对标定物特征点的准确提取.标定结果重投影误差为0.17pixels.相机焦距的标定误差在0.39%以内.与传统方法相比提高了标定精度,为具有特殊成像范围的相机的标定提供了一种解决方案.     

基于随机相位实值编码的光学图像加密

提出了一种用于图像加密的随机相位实值编码方法,待编码的纯相位图像与一个随机相位掩膜一起作傅里叶变换,取其实部作为编码图像.已编码的图像和随机相位掩膜的傅里叶变换相加作傅里叶反变换,反变换的光强可以准确地重建原图像.该编译码方法简单,编码图像是一个实值图像,便于计算机打印或显示输出.     

紧凑型纯相位全息近眼三维显示

提出了一种紧凑型纯相位全息近眼显示方法，避免了4f系统的使用且通过滤波可消除零级光和高级衍射光的干扰，能够实现近眼虚拟现实（VR）和增强现实（AR）三维显示。在进行全息图计算时，三维物体被分割成多层图像，利用每一层图像相对于投影透镜焦平面的距离和投影透镜的焦距获得该深度对应的补偿相位因子，采用迭代优化算法获得三维物体的纯相位全息图并在所提系统中进行显示。通过数值模拟和光学实验，证明了所提方法的有效性。     

一种用于缺陷超声检测的相位标准差分离谱新算法

在粗晶材料超声无损检测中，分离谱处理技术可用来降低材料内部的相干噪声、但分离谱处理对滤波器阵列参数较敏感、造成性能不稳，且至今对滤波信号有效利用的解释尚不明了．本文从理论上讨论了超声检测信号滤波输出的相位统计特性，据此提出了一种基于相位标准差的分离谱新算法，用计算机模拟及实验数据对该算法的世能进行了测试并与常用的极小值分离谱算法作了对比。结果证实，相位标难差的分离谱算法有好的信噪比增强能力，且对滤波器数不敏感．     

光束穿过超声驻波及行波后的相干现象显示技术

提出了一种光通过超声驻波及超声行波后能产生高衬比度相干条纹的新术,利用超声驻波调制光束与超声行波作用后的相关衬比度来选定相干波束.从理论上分析了其相关叠加的结果,并在实验中得到了证实.     

Edge enhancement of phase objects through complex media by using transmission-matrix-based spiral phase contrast imaging

The wavefront shaping based technique has been introduced to detect the edges of amplitude objects through complex media, but the extraction of the boundary information of invisible phase objects through complex media has not been demonstrated yet. Here, we present a phase contrast imaging technique to overcome the scattering, aiming to achieve the edge detection of the phase object through the complex media. An operator based on the experimentally measured transmission matrix is obtained by numerically adding a spiral phase in the Fourier domain. With the inverse of the filtered transmission matrix, we can directly reconstruct the edge enhanced images for both amplitude object and phase object beyond scattering. Experimentally, both digital and real objects are imaged, and the results verify that isotropic edge detection can be achieved with our technique. Our work could benefit the detection of invisible phase objects through complex media.     

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.     

Enhanced wavefront reconstruction by random phase modulation with a phase diffuser

A phase retrieval technique for enhanced wavefront reconstruction using random phase modulation and a phase diffuser is proposed. The speckle field generated is sampled at multiple axially displaced planes and the speckle patterns are used in an iterative algorithm based on the optical wave propagation in free space. The presentation of this technique is carried out using two setups. In the first setup, a diffuser plate is placed at the image plane of a metallic test object. The benefit of randomizing the phase of the object wave is the enhanced intensity recording due to high dynamic range of the diffusely scattered beam. The use of demagnification optics will also allow the investigations of relatively large objects. In the second setup, a transparent object is illuminated using a wavefront with random phase and constant amplitude by positioning the phase diffuser close to the object. The benefit of phase-only modulation is the increased resolution of the phase structures of the transparent test objects.     

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。     

Polymers on disordered trees,spin glasses,and traveling waves

We show that the problem of a directed polymer on a tree with disorder can be reduced to the study of nonlinear equations of reaction-diffusion type. These equations admit traveling wave solutions that move at all possible speeds above a certain minimal speed. The speed of the wavefront is the free energy of the polymer problem and the minimal speed corresponds to a phase transition to a glassy phase similar to the spin-glass phase. Several properties of the polymer problem can be extracted from the correspondence with the traveling wave: probability distribution of the free energy, overlaps, etc.     

湍流大气中斜程传输光场的相位特性

运用功率谱反演法对斜程传输大气湍流扰动相位屏进行了数值模拟。通过建立激光束初始畸变相位模型,从波前峰谷值和波前功率谱密度函数两个方面,对激光束通过大气湍流后的相位特性进行了研究,重点分析了湍流传输距离和天顶角对激光束波前相位分布的影响。研究结果表明:激光束在湍流大气斜程传输后,其波前相位会发生明显畸变,且空间高频相位较低频相位所占比例明显增加;通过湍流后的波前相位与传输距离及天顶角密切相关,传输距离越长,天顶角越大,相位畸变程度越大,高频相位所占比例越多。     

Surface profiling using fringe projection technique based on Lau effect

In this paper, the surface profile of objects is determined by using a grating projection system. The method is based on the concept of self-imaging, namely, the Lau effect. Periodic fringe patterns are generated from the projection of a grating illuminated by multiple mutually incoherent quasi-plane wavefront. The fringe patterns are then projected on to the object surface and the deformed grating image is captured by a CCD camera for subsequent analysis. Phase shifting techniques have been employed to determine the phase value. With suitable calibration of the system and the phase value obtained, the surface profile of the objects can be determined. The phase variation is achieved by using a linear translation stage incorporated to the grating. In this study, two specimens are tested to demonstrate the validity of the method. One is a spherical cap with a height of 4 mm, and the other is a coin. The experimental results are compared with results obtained by mechanical stylus method. In the case of the spherical cap the results are also compared with fringe projection method based on Talbot effect.     

Ultrasonic wave fluctuations through tissue: an experimental pilot study

The fundamental role of the assumption of plane wave ultrasonic propagation in attenuation and scattering measurements on soft tissues is discussed. The evidence for and against the validity of the assumption is shown to be inconclusive. An experimental procedure for direct assessment, by measurement of the ultrasonic amplitude and phase fluctuations occurring after an interrogating plane wave has passed through a tissue specimen, is described. The phase measurements impose significant demands on the mechanical precision, temperature stability and electronic timing capabilities of the measurement system. Measurements are reported on two specimens of fresh beef liver. One exhibited significant fluctuations of amplitude and phase which were reduced by 50% when the effects of thickness variations in the specimen were removed. The other specimen exhibited relatively small fluctuations. The feasibility of making such measurements has been demonstrated and the detailed features of the experimental procedure-which requires considerable care-have been outlined. Although more measurements are needed for definitive conclusions, these will require the development of a reliable technique for determining the presence of gaseous microbubbles in tissue in vitro. The preliminary conclusion that some samples of liver may exhibit low levels of wavefront disruption is consistent with recent evidence in the literature.     

近红外瞬态脉冲波前高精度干涉检测技术

惯性约束聚变(ICF)系统中高能瞬态脉冲激光由于脉冲时间短、能量高、波前畸变大,通常的检测方法难于检测脉冲激光波前。提出了一种基于空间相位调制技术可用于近红外瞬态波前高精度检测的环形径向剪切干涉仪。该系统可以以30~150 mm的圆瞳和方瞳口径、对波长为1064 nm的近红外纳秒级脉宽的脉冲激光波前实现共路、无参考面的瞬态、高精度的检测。系统的波前重构理论经过计算机仿真验证,精度达1/1000λ以上;检测结果与ZYGO数字波面干涉仪进行了比对,峰谷值、均方根值均优于1/15λ,并具有很好的可重复性。该系统目前已用于惯性约束聚变系统的脉冲检测,并且该技术适用于各种可见光和红外波段激光。