非均匀组织医学超声发射声场仿真

为了分析医学超声在非均匀组织中的分布特性,建立了超声发射声场的计算模型。采用交错网格有限差分法对耦合超声非线性方程进行数值求解,获得了声速、密度及非线性参数非均匀分布情况下人体腹壁组织内的超声声场分布数据。同均匀介质相比:当声速均匀而密度非均匀时,声束仍聚焦良好,焦点处声能下降了1.8 dB;当密度均匀而声速非均匀时,声束发散严重,焦点处声能下降了3.8 dB,下降程度与非均匀组织接近。组织声速在空间分布的非均匀性是导致聚焦声束能量分布畸变的主要原因。      

混凝土声发射信号源定位精度的细观模型计算分析*

混凝土内部在断裂时会产生相应的声发射信号,通过这一信号,混凝土内部损伤断裂区域可以被定位。然而,混凝土是一种多相非均匀材料,而声发射定位算法是基于均匀介质假设进行计算的,因此采用该算法对其定位会产生一定的误差,有必要从细观的角度研究混凝土非均匀性对定位精度的影响。本文基于随机骨料模型以及时差定位算法（基于遗传算法）,建立了一种用于估计混凝土声发射定位误差的定量估算模型。本文采用该模型定量估算分析了混凝土细观组分对其声速的影响,并在该分析结构的基础上继续分析了骨料含量以及声速取值偏差对定位精度的影响。     

功率谱反演大气湍流随机相位屏采样方法的研究

从产生大气湍流随机相位屏的功率谱反演法原理出发,分析了均匀采样造成的随机相位屏大量低频信息泄漏的不足,提出利用非均匀采样方法对功率谱进行滤波产生随机相位屏.建立了非均匀采样模型,并分析比较了两种采样方法覆盖的采样频率范围和单边采样频率区域的湍流功率,理论证明了非均匀采样功率谱反演产生大气湍流随机相位屏的可行性和有效性.针对大气湍流的Kolmogorov谱,分别仿真计算了两种采样模型下功率谱反演生成的大气湍流随机相位屏.仿真结果表明,在不增加采样点和计算量条件下,非均匀采样方法生成的大气湍流随机相位屏具有丰富的低频和高频信息,有效改善了传统功率谱反演大气湍流随机相位屏时低频信息严重不足的缺陷.     

基于时变相位屏的非等晕成像数值模型及验证

基于多层薄相位屏光传输理论,建立了包含光传输过程、大气分层、相位屏实时连续变化、太阳扩展目标非等晕成像的数值模型,并从相位结构函数、大气相干长度及等晕角等方面进行了分析验证.在H-V 5/7和H-V 10/10两种强弱湍流模型下,大气相干长度及等晕角的绝对误差分别在0.02m和0.3″以内.结果表明,建立的非等晕成像数值模型在时间和空间上的统计特性符合实际应用需求,为研究非等晕误差在太阳稳像及自适应光学系统中的影响提供了可靠的模型支持.     

非均匀湍流路径光传播数值模拟中相位屏间C_n~2的选取

针对非均匀湍流路径光传播的数值模拟,为减小相位屏间湍流强度均匀的假设所引起的模拟误差,数值分析了五种选择相位屏间折射率结构常数Cn2的可能方案。通过计算平面波和球面波的Rytov指数和Fried参数,对比五种方案得到的Rytov指数和Fried参数与理论结果的相对误差。结果表明,当相位屏间C2n选择为相位屏间湍流大气实际的折射率结构常数的路径平均值时,模拟误差最小,该选择方案为相位屏间C2n选取的最佳方案;当根据相位屏间湍流大气的Rytov指数相等的原则来设置相位屏分布时,选择相位屏间C2n为两相邻相位屏处折射率结构常数的几何平均值时,在一定的误差允许范围内,该选择方案具有可取性。     

非均匀湍流路径光传播数值模拟中相位屏间C2n的选取

针对非均匀湍流路径光传播的数值模拟,为减小相位屏间湍流强度均匀的假设所引起的模拟误差,数值分析了在种选择相位屏间折射率结构常数C2n的可能方案.通过计算平面波和球面波的Rytov指数和Fried参数,对比五种方案得到的Rytov指数和Fried参数与理论结果的相对误差.结果表明,当相位屏间C2n选择为相位屏间湍流大气实际的折射率结构常数的路径平均值时,模拟误差最小,该选择方案为相位屏间C2n选取的最伟方案;当根据相位屏间湍流大气的Rytov指数相等的原则来设置相位屏分布时,选择相位屏间C2n为两相邻相位屏处折射率结构常数的儿何平均值时,在一定的误差允许范围内,该选择方案具有可取性.     

非Kolmogorov湍流相位屏仿真及对光束传输模拟的影响

采用正态分布模型来描述传播路径上功率谱幂值的非均匀性,利用功率谱反演法构建了基于等效结构常数的非Kolmogorov湍流相位屏,并利用多重相位屏法进行了高斯光束在均匀各向同性湍流与非Kolmogorov湍流两种模型下的传输模拟。通过观察模拟光束的光强均匀度、光束漂移以及闪烁指数等,研究了非Kolmogorov湍流对光束传输模拟的影响。结果发现,当光束通过单个非Kolmogorov相位屏时,光强最大值与光强均匀度随着幂值先增大后减小,光束漂移随幂值在一定范围内单调变化。当光束通过多重相位屏时,发现模拟光束的光强闪烁指数会受到相位屏数量以及湍流模型的影响,当相位屏数量较多时,均匀各向同性湍流模型下模拟得到的光强闪烁指数会大于非K湍流模型下的结果,且非K湍流模拟的光束漂移与均匀各项同性湍流模拟得到的光束漂移的相对误差会随着相位屏数量增多而趋于0。     

非均匀采样的功率谱反演大气湍流相位屏的快速模拟

对大气湍流功率谱非均匀采样可以有效改善传统功率谱反演法低频采样严重不足的缺陷, 实现高精度的大气湍流相位屏的模拟. 但采用的直接求和运算计算复杂度高, 相位屏的模拟速度极慢. 将非均匀快速傅里叶变换(NUFFT)引入到大气湍流相位屏的模拟, 可以实现相位屏的快速模拟. 从随机过程的谱分解出发, 将大气湍流相位随机过程表示为有限谐波分量叠加和的均方极限. 通过一个高斯核函数的卷积, 将非均匀分布的谐波复振幅映射到均匀网格空间, 进而利用快速傅里叶变换, 降低计算复杂度, 加快大气湍流相位屏的模拟速度. 以大气湍流的Kolmogorov 谱为例, 利用NUFFT仿真得到大气湍流相位屏, 并对相位屏的模拟精度、模拟速度和误差进行统计分析. 结果表明, NUFFT的引入可以实现快速、高精度的大气湍流相位屏的模拟.     

基于随机数据元扩张的大气湍流相位屏数值模拟

准确模拟大气湍流相位屏是建立大气湍流数值模型的核心问题。从大气湍流的统计学特性入手,利用大气湍流波前相位结构函数建立了一种新的大气湍流相位屏数值模拟方法,通过随机数据元扩张对大气湍流波前畸变相位分布的尺度随机性和高频分量随机性进行了模拟,并以此为基础通过相位屏近似构建了满足Kolmogorov统计规律的大气湍流数值模型。计算结果表明,随机数据元扩张法生成的相位屏在统计特性上与理论值基本吻合,在低频部分相对于功率谱反演法有明显的提升。同时,随着相位屏网格数的增加,计算结果的高频特性逐渐呈现并逼近理论值。对于由相位屏构建的大气湍流数值模型,在此通过光强闪烁率作为判据进行了验证,结果表明对于弱湍流和中等强度的湍流,模拟结果与理论计算基本相同;对于强湍流则误差较为明显,相对误差最大可达40%。     

非均匀湍流路径上光传播数值模拟的相位屏分布

针对非均匀湍流路径上光传播的数值模拟,研究了相位屏分布的三种不同设置方案,即均匀分布相位屏（UDPS）、等Rytov指数间隔的相位屏（ERPS）和等Fried参数间隔的相位屏（EFPS）.结果表明,ERPS方案根据折射率起伏的强弱设置密疏相间的相位屏,实现了对不同折射率起伏区的充分采样,是非均匀路径上相位屏分布的首选方案.在满足相位屏间距基本要求的前提下,决定相位屏间隔的Rytov指数间隔的选取具有较大的任意性,统计结果显示,尽管相位屏间Rytov指数变化超过一个数量级,但光束有效半径、能量Strehl比的变化均小于2.1%,模拟结果与理论结果符合很好,表明了ERPS方案的可靠性和稳定性. 关键词： 光传播 非均匀湍流路径 数值模拟 相位屏分布     

Influence of the abdominal wall on the nonlinear propagation of focused therapeutic ultrasound

This article theoretically studies the influence of inhomogeneous abdominal walls on focused therapeutic ultrasound based on the phase screen model. An inhomogeneous tissue is considered as a combination of a homogeneous medium and a phase aberration screen. Variations of acoustic parameters such as peak positive pressure, peak negative pressure, and acoustic intensity are discussed with respect to the phase screen statistics of human abdominal walls. Results indicate that the abdominal wall can result in energy loss of the sound in the focal plane. For a typical human abdominal wall with correlation length of 7.9~mm and variance of 0.36, the peak acoustic intensity radiated from a 1~MHz transmitter with a radius of 30~mm can be reduced by about 14% at the focal plane.     

Statistical description of wave-front aberration in the human eye

The wave aberration of the human eye has been measured by means of a Hartmann-Shack wave-front sensor in a population of normal subjects. The set of data has been used to compute the phase distribution, the power spectrum, and the structure function for the average eye to analyze the statistics of the ocular aberration considered as a phase screen. The observed statistics fits the classical Kolmogorov model of a statistically homogeneous medium. These results can be of use in understanding the average effect of aberrations on the retinal image and can serve as a tool to analyze the consequences of ocular-aberration compensation by adaptive optics, customized ophtalmic elements, or refractive surgery.     

Statistical model of clutter suppression in tissue harmonic imaging

A statistical model is developed for the suppression of clutter in tissue harmonic imaging (THI). Tissue heterogeneity is modeled as a random phase screen that is characterized by its correlation length and variance. With the autocorrelation function taken to be Gaussian and for small variance, statistical solutions are derived for the mean intensities at the fundamental and second-harmonic frequencies in the field of a focused sound beam that propagates through the phase screen. The statistical solutions are verified by comparison with ensemble averaging of direct numerical simulations. The model demonstrates that THI reduces the aberration clutter appearing in the focal region regardless of the depth of the aberrating layer, with suppression of the clutter most effective when the layer is close to the source. The model is also applied to the reverberation clutter that is transmitted forward along the axis of the beam. As with aberration clutter, suppression of such reverberation clutter by THI is most pronounced when the tissue heterogeneity is located close to the source.     

Phase-Locked Fibre Array for Coherent Combination and Atmosphere Aberration Compensation

We report a fibre amplifier array that not only achieves coherent beam combination by compensation of phase noises of fibre amplifier, but also accomplishes correction of atmosphere aberration. It is of master-oscillatormultiple-amplifier （MOPA） configuration, which can be phase-locked by the multidither principle or heterodyne detection principle. First laboratory experiments of atmosphere aberration compensation of a three-element fibre amplifier array are reported. The atmosphere aberration is created by a phase screen in the experiment. The phase changes of the beam, which are introduced by the fibre amplifier and the phase screen, are both detected by the heterodyne detection method. Phase modulators are controlled to compensate for the phase in the three paths. No matter whether there is a phase screen producing atmosphere aberration or not, the dim dynamic interference fringes in the far field turn to a clear and stable pattern, and the peak intensity is maximized. It is indicated that the fibre amplifier array is phase-locked, and coherent combination of the three beams is achieved. It can be used not only to obtain high power fibre laser array but also in laser space communication.     

近场头相关传递函数的数值计算与特性分析

建立了近场头相关传递函数(HRTF)的边界元算法,针对刚性圆球人头简化模型,利用解析计算结果对其进行了验证,然后依据GB/T2428-1998建立了一个椭球人头模型,利用边界元算法计算了中低频多个空间方位的近场HRTF,并对其随方位和距离的变化特性进行了分析。     

低频位相误差新校正方法模拟研究

 就多程放大系统中产生的低频位相噪声，提出用位相补偿反射镜来进行补偿的静态校正方法。用随机位相屏模拟光学元件产生的低频位相噪声，建立光传输模拟模型，引入等效位相片的概念，将数个光学元件对位相的影响简化为等效位相片，在此基础上对位相补偿反射镜的校正原理进行阐述。进而对双程光路校正过程进行了模拟研究，得到了很好的校正效果。     

高功率激光装置的线性传输特性

在菲涅耳数的基础上重新定义了衍射特征量,并将其定义扩展至聚焦或发散光路,定义了一个光学系统中各光学元件衍射特征量的符号。通过全新的定义和扩展,研究了波前畸变对近场调制度的影响以及正弦调频光谱色散平滑光束衍射传输引入的幅频调制的大小。以神光-Ⅲ主机主放大级为例,研究了元件波前畸变对近场调制度的影响以及正弦调频光谱色散平滑光束衍射传输引入的幅频调制的大小。衍射特征量与能流分布F相结合,可以作为评价高功率激光装置线性传输效应的一项受限指标,指导高功率激光装置的设计。     

色分离光栅对输入波前畸变宽容度的研究

 采用频域分段的方法，对强激光系统的波前畸变进行了理论分析；使用随机相位屏构建低频畸变波前模型，并提出采用迭代算法构建中高频畸变波前模型。使用构建的畸变波前模型作为输入，通过模拟计算，给出了色分离光栅对输入波前畸变宽容度的初步规律。     

Application of optical diffraction method in designing phase plates

Continuous phase plate(CPP),which has a function of beam shaping in laser systems,is one kind of important diffractive optics.Based on the Fourier transform of the Gerchberg-Saxton(G-S) algorithm for designing CPP,we proposed an optical diffraction method according to the real system conditions.A thin lens can complete the Fourier transform of the input signal and the inverse propagation of light can be implemented in a program.Using both of the two functions can realize the iteration process to calculate the near-field distribution of light and the far-field repeatedly,which is similar to the G-S algorithm.The results show that using the optical diffraction method can design a CPP for a complicated laser system,and make the CPP have abilities of beam shaping and phase compensation for the phase aberration of the system.The method can improve the adaptation of the phase plate in systems with phase aberrations.     

Phase plates for wave-aberration compensation in the human eye

We present a method for manufacturing phase plates to compensate for the wave aberration in the human eye. The wave aberration of the eye is measured in vivo by a new laser ray-tracing method and then compensated for by a phase plate placed in front of the eye. This plate is made from a gray-level single-mask photosculpture in photoresist. Two experiments were carried out, first with an artificial eye and then with a human eye: 80% compensation for the wave aberration was achieved in both cases.