大气湍流畸变相位屏的数值模拟方法研究

利用功率谱反演法和Zernike多项式展开法对符合Kolmogonov统计规律的大气湍流畸变波前相位屏进行了数值模拟研究。通过对比模拟相位屏的相位结构函数与理论结果的差异分析模拟相位屏的准确性。结果表明,功率谱反演法产生的相位屏在高空间频率部分与理论相符,在低空间频率部分明显偏离理论值,通过次谐波补偿有效改善低频不足,次谐波级数达到4级足够;Zernike多项式展开法产生的相位屏的低空间频率成分与理论相符,而高空间频率成分不足随着所用Zernike阶数的增加而有所改善,但同时也带来计算量增大的问题。     

大气湍流随机相位屏的数值模拟和验证

本文使用傅立叶变换法对符合Kolmogorov谱的大气随机相位屏进行了数值模拟,采用三层随机相位屏叠加模拟大气湍流.通过比较模拟相位屏的相位结构函数和理论值的符合度对相位屏的统计特性进行了验证,利用长曝光传递函数和相位结构函数计算大气相干长度r0.结果表明,用傅立叶变换法模拟的随机相位屏是正确的,但相位结构函数存在明显的低频空间频率成分不足,采用三层随机相位屏叠加对相位结构函数和理论值的符合度有所改善,实际r0值比设计值偏大.     

大气湍流随机相位屏的数值模拟和验证大气湍流随机相位屏的数值模拟和验证

本文使用傅立叶变换法对符合Kolmogorov谱的大气随机相位屏进行了数值模拟，采用三层随机相位屏叠加模拟大气湍流。通过比较模拟相位屏的相位结构函数和理论值的符合度对相位屏的统计特性进行了验证，利用长曝光传递函数和相位结构函数计算大气相干长度r0。结果表明，用傅立叶变换法模拟的随机相位屏是正确的，但相位结构函数存在明显的低频空间频率成分不足，采用三层随机相位屏叠加对相位结构函数和理论值的符合度有所改善，实际r0值比设计值偏大。     

Optimal subharmonics selection for atmosphere turbulence phase screen simulation using the subharmonic method

We investigate the optimal subharmonics selection for atmospheric turbulence phase screen simulation with a given precision using the fast Fourier transform-based subharmonic method. A criterion appropriate for the Kolmogorov model, the von Karman model and the modified von Karman model – called the structure function increment ratio – is defined to determine the optimal subharmonics. Moreover, the integrated power ratio criterion is improved, an expression is presented to calculate the optimal subharmonics to add, and the scope of application is wider than in the past. More accurate optimal subharmonics can be achieved by using either the improved criterion or the structure function increment ratio.     

Extruding Kolmogorov-type phase screen ribbons

A phase screen ribbon extrusion process is presented that allows a phase screen ribbon of any specified width to be extruded, one column at a time, producing a ribbon of any desired length, with Kolmogorov statistics (i.e., having a five-thirds power-law-dependent structure function) for all separations up to some selected upper limit--which upper limit can be as large as desired. The method is an adaptation of the method described by [Assémat et al. Opt. Express 14, 988 (2006)].     

Heterodyne measurements of laser light scattering by a turbulent phase screen

We report experiments in which a fiber-coupled heterodyne laser system operating at a wavelength of 1.5 microm is used to measure the phase fluctuations induced on a laser beam by passage through a thin layer of turbulent air and subsequent propagation through free space. We investigate the statistical properties and power spectra of the phase and its rate of change, in addition to the intensity statistics. We find that the power spectrum of the rate of change of phase has a simple negative exponential form. We discuss our results in the context of the problem of detection of phase variations over an extended turbulent atmospheric path.     

Fast phase recovery from a single closed-fringe pattern

A new framework for phase recovery from a single fringe pattern with closed fringes is proposed. Our algorithm constructs an unwrapped phase from previously computed phases with a simple open-fringe-analysis algorithm, twice applied for analyzing horizontal and vertical oriented fringes, respectively. That reduces the closed-fringe-analysis problem to that of choosing the better phase between the two oriented computed phases and then of estimating the local sign. By propagating the phase sign [and a tilewise constant (DC) term] by regions [here named tiles] instead of a pixelwise phase propagation, our analysis of closed-fringe patterns becomes more robust and faster. Additionally, we propose a multigrid refinement for improving the final computed phase.     

Fast electromagnetic shower simulation

We present a method for fast simulation of showers based on the recombination of Monte Carlo pre-generated showers. Results of the new method are presented for e.m. showers in BGO crystals in the energy range 1–10 GeV, indicating an accuracy comparable with usual simulations with a required time reduced by more than one order of magnitude.     

Optimized phase screen modeling for optical turbulence

An alternative method for statistical interpolation is formalized. A new theorem is proved, providing theoretical basis for optimizing statistical accuracy in successively conditioned rendering applications. The theorem is empirically validated by two simulations, each comparing two different statistical interpolators. The interpolators are used to model high-resolution phase fluctuations over finite apertures. The theorem correctly predicts which interpolator is more optimal, based on empirical trials with greater than 99.9% certainty. The theorem is suitable as a quick alternative to the Monte Carlo optimization techniques used previously.     

大气相位屏的协方差冲激函数产生法

提出正交基为冲激函数的协方差法,在Matlab中仿真实现符合大气湍流统计分布(vonKarman谱和Kolmogorov谱)的大气相位屏。协方差冲激函数产生法对连续波前相位进行冲激函数正交分解,根据正交分解系数的协方差模拟平行光通过大气后的瞬时波前相位。计算多帧相位屏的波前结构函数,与理论值比对,验证产生大气相位屏的正确性。根据比对结果得,该方法产生的相位屏精度高,对于Kolmogorov谱,该方法产生的大气相位屏的波前结构函数误差约为理论波前结构函数均值的1%。     

Modeling scintillation from an aperiodic Kolmogorov phase screen

We propose a technique for the accurate modeling and simulation of scintillation patterns that are due to Kolmogorov statistics without assuming periodic boundary conditions. We show how the more physically justifiable assumption of smoothness results in a propagation kernel of finite extent. This allows the phase screen dimensions for an accurate simulation to be determined, and truncation can then be used to eliminate the unwanted spectral leakage and diffraction effects usually inherent in the use of finite apertures. A detailed outline of the proposed technique and comparison of simulations with analytic results are presented.     

Fast ultrasound imaging simulation in K-space

Most available ultrasound imaging simulation methods are based on the spatial impulse response approach. The execution speed of such a simulation is of the order of days for one heart-sized frame using desktop computers. For some applications, the accuracy of such rigorous simulation approaches is not necessary. This work outlines a much faster 3-D ultrasound imaging simulation approach that can be applied to tasks like simulating 3-D ultrasound images for speckle-tracking. The increased speed of the proposed simulation method is based primarily on the approximation that the point spread function is set to be spatially invariant, which is a reasonably good approximation when using polar coordinates for simulating images from phased arrays with constant aperture. Ultrasound images are found as the convolution of the PSF and an object of sparsely distributed scatterers. The scatterers are passed through an anti-aliasing filter before insertion into a regular beam-space grid to reduce the bandwidth and significantly reduce the amount of data. A comparison with the well-established simulation software package field II has been made. A simulation of a cyst image using the same input object was found to be in the order of 7000 times slower than the presented method. Following these considerations, the proposed simulation method can be a rapid and valuable tool for working with 3-D ultrasound imaging and in particular 3-D speckle-tracking.     

用于可靠性导向相位展开的快速算法

在目前的可靠度导向相位展开算法中,排序操作非常费时。为此提出了一种采用查找表的可靠度导向相位展开快速算法。将可靠度值以一定等级量化从而建立一张查找表。在相位展开过程中,通过查表将相位待展开的点放到一个虚拟队列(等同于洪水算法中的有序队列)中,再进行相位展开。由于采用查表代替了排序操作,相位展开速度获得了极大的提高。实验表明:查表法的速度通常比洪水算法(典型的可靠度导向相位展开算法)快20多倍,而相位展开成功率与洪水算法相差无几。尤其在可靠度分布比较分散情况下,查表法比洪水算法快100多倍。     

Probing of a random phase screen by a focused spatially modulated laser beam: Deflection of interference fringes

Deflection of interference fringes in a diffraction field was investigated under conditions in which a laser beam with a regular interference structure was finely focused onto a random phase screen. It was established that the contrast of the average-intensity fringes depends analytically on the statistical parameters of the screen. Pis’ma Zh. Tekh. Fiz. 25, 56–61 (January 12, 1999)     

Fast algorithms for the simulation of polygonal particles

Three algorithms to speed up discrete-element simulations for granular matter are presented in this paper. The first algorithm allows to determine neighborhood relations in polydisperse mixtures of particles of arbitrary shape, either discs, ellipses, or polygons. The second algorithm allows to calculate the distance of two polygons in constant time, independently of the complexity of the shape of the polygons. This makes fast simulations of polygonal assemblies possible. The third method is a special type of parallelization technique which is optimized for workstations with shared memory.     

Comparison of phase noise simulation techniques on a BJT LC oscillator

The phase noise resulting from white and flicker noise in a bipolar junction transistor (BJT) LC oscillator is investigated. Large signal transient time domain SPICE simulations of phase noise resulting from the random-phase flicker and white noise in a 2 GHz BJT LC oscillator have been performed and demonstrated. The simulation results of this new technique are compared with Eldo RF and Spectre RF based on linear circuit concepts and experimental result reported in the literature.     

Probing of a random phase screen by a focused spatially modulated laser beam. Diffraction at a large number of inhomogeneities

The contrast of the Young’s interference fringes formed in the diffraction field when a dynamic random phase screen is illuminated by a focused, spatially modulated laser beam is obtained analytically as a function of the statistical parameters of the screen. A threshold relative bleaching effect is established for a highly dispersive medium when a low-divergence illuminating beam is used. Pis’ma Zh. Tekh. Fiz. 23, 47–53 (October 12, 1997)     

Fast multipole method for micromagnetic simulation of periodic systems

We have developed a new method for computing the magnetostatic field of an infinite array of images of an arbitrary system of charges or multipoles. The result takes the form of a Taylor expansion of the potential of a "cored array" of distant images, as is required by the fast multipole method, the most efficient method known for calculating magnetostatic fields in very large systems. The new method is much faster and simpler to implement than the usual Ewald summation or fast Fourier transform, and does not require a regular grid inside the central cell.     

Fast simulation of the propagation of a spatially non-uniform laser beam through apertured ABCD systems

Based on the treatment that the rectangular function can be expanded into an approximate sum of complex Gaussian functions with finite numbers, the analytical expressions for the propagation of partially non-uniform laser beams through apertured ABCD optical systems were derived. The typical numerical examples were given and were compared with those obtained from numerically integral calculations. It can be shown from the results of our study that our method can significantly improve the numerical calculation ejciency.