Wave optics simulation of atmospheric turbulence and reflective speckle effects in CO2 lidar

Laser speckle can influence lidar measurements from a diffuse hard target. Atmospheric optical turbulence will also affect the lidar return signal. We present a numerical simulation that models the propagation of a lidar beam and accounts for both reflective speckle and atmospheric turbulence effects. Our simulation is based on implementing a Huygens-Fresnel approximation to laser propagation. A series of phase screens, with the appropriate atmospheric statistical characteristics, are used to simulate the effect of atmospheric turbulence. A single random phase screen is used to simulate scattering of the entire beam from a rough surface. We compare the output of our numerical model with separate CO(2) lidar measurements of atmospheric turbulence and reflective speckle. We also compare the output of our model with separate analytical predictions for atmospheric turbulence and reflective speckle. Good agreement was found between the model and the experimental data. Good agreement was also found with analytical predictions. Finally, we present results of a simulation of the combined effects on a finite-aperture lidar system that are qualitatively consistent with previous experimental observations of increasing rms noise with increasing turbulence level.     

逆向调制无线光通信空间分集分析

在逆向调制无线光通信系统中,大气湍流对系统的影响大于传统的无线光通信系统。本文研究了一种基于逆向端调制器分集的逆向调制无线光通信系统,以减小大气湍流对系统的影响。利用相位屏法,建立了弱湍流下的激光大气的传输模型,对比分析了逆向调制无线光通信系统逆向端逆向调制器分集和不分集分别所受到的大气湍流的影响。结果显示在相同情况下,逆向调制无线光通信系统在逆向端对逆向调制器进行分集能抑制大气湍流对系统的影响,使整个系统的闪烁指数下降。     

基于OFDM的大气激光通信湍流抑制关键技术研究

文章在分析无线激光通信(FSO)存在两种主要的大气信道问题的基础上,针对激光大气信道问题尤其是在复杂湍流环境下的频率选择性衰落问题和多径效应问题,提出了基于正交频分复用(OFDM)的湍流效应抑制方法,构建了FSO-OFDM系统,研究了该系统的基带模型以及信号的多载波调制与解调方法。分析了无线激光通信中存在复杂湍流环境下的大气信道问题,讨论了大气湍流影响下的平面波激光通信系统模型,建立了大气湍流影响下对数正态湍流通道的高斯光束空间光通信系统模型,推导了光波强度的概率密度函数,研究了利用信噪比概率密度函数分析各种大气湍流效应对系统性能影响的方法;设计了无线光通信系统的OFDM多载波调制方案,构建了FSO-OFDM系统基带模式模型,并基于该模型研究了其信号的调制与解调原理。最后,采用MATLAB编程实现FSO-OFDM系统,对多径干扰下的FSO通信系统进行仿真实验,进行了不同保护间隔下的误码率特性实验,验证了FSO-OFDM系统具有很强的抗多径干扰和频谱选择性衰落能力以及良好的BER性能,可有效解决码间干扰大、链路不可靠的问题,具有非常广泛的应用前景和使用价值。     

Atmospheric spectral model and theoretical expressions of irradiance scintillation index for optical wave propagating through moderate-to-strong non-Kolmogorov turbulence

A new atmospheric spectral model and expressions of irradiance scintillation index are derived theoretically for optical wave propagating through moderate-to-strong non-Kolmogorov turbulence. They are developed under Andrews' assumption that small-scale irradiance fluctuations are modulated by large-scale irradiance fluctuations of the wave, and the geometrical optics approximation is adopted for mathematical development. A wide range of turbulence strength is considered instead of a limited range for weak turbulence. The atmospheric spectral model has a spectral power law value in the range of 3 to 4 instead of the standard power law value of 11/3. Numerical calculations are conducted to analyze the influences of spectral power law and turbulence strength.     

Investigation of seeing by means of an atmospheric mesoscale numerical simulation

We intend to show the potential of the numerical simulation of atmospheric turbulence to help find optimal sites for astronomical observation. We present results obtained with an atmospheric model, in which a representation of turbulence has been included. The model simulates the atmospheric flow over any given area, including the gross characteristics of the turbulence, from which maps of the astronomical seeing can be retrieved. A validation of the approach is obtained with actual measurements of the seeing, taken during field campaigns on two different sites. We find a good correlation in time between the observed and simulated values of the seeing, and we argue that this result can be extrapolated to space correlations.     

M-ary pulse position modulation performance with adaptive optics corrections in atmospheric turbulence

ABSTRACT

The performance of M-ary pulse position modulated (PPM) optical wireless communication (OWC) systems in atmospheric weak turbulence medium is evaluated by using adaptive optics corrections. Piston, tilt, defocus and coma components of adaptive optics corrections are applied to the avalanche photodetector (APD) type of receiver and the results are obtained depending on various turbulence and receiver parameters. The lognormal channel distribution is used to model the weak atmospheric turbulence conditions. Adaptive optics correction increases the bit-error-rate (BER) performance of an OWC system operating in atmospheric turbulence conditions. Piston component yields the highest BER performance, followed by the tilt, defocus and coma adaptive optics correction components respectively.     

Generalized modified atmospheric spectral model for optical wave propagating through non-Kolmogorov turbulence

A new generalized modified atmospheric spectral model is derived theoretically for wave propagating through non-Kolmogorov turbulence, which has been reported recently by increasing experimental evidence and theoretical investigation. The generalized, modified atmospheric spectrum considers finite turbulence inner and outer scales and has a spectral power law value in the range of 3 to 5 instead of the standard power law value of 11/3. When the inner scale and outer scale are set to zero and infinity, respectively, this spectral model is reduced to the classical non-Kolmogorov spectrum.     

Analysis of the performance of a wireless optical multi-input to multi-output communication system

We investigate robust optical wireless communication in a highly scattering propagation medium using multielement optical detector arrays. The communication setup consists of synchronized multiple transmitters that send information to a receiver array and an atmospheric propagation channel. The mathematical model that best describes this scenario is multi-input to multi-output communication through stochastic slow changing channels. In this model, signals from m transmitters are received by n receiver-detectors. The channel transfer function matrix is G, and its size is n x m. G(i,j) is the transfer function from transmitter i to detector j, and m > or = n. We adopt a quasi-stationary approach in which the channel time variation has a negligible effect on communication performance over a burst. The G matrix is calculated on the basis of the optical transfer function of the atmospheric channel (composed of aerosol and turbulence elements) and the receiver's optics. In this work we derive a performance model using environmental data, such as documented turbulence and aerosol models and noise statistics. We also present the results of simulations conducted for the proposed detection algorithm.     

Coherence Length of a Gaussian-Schell Beam and Atmospheric Turbulence

Abstract

The theoretical relationship between the coherence length and phase-front randomness is analysed using a Gaussian-Schell source and beam model for light propagating through a turbulent atmosphere. Computer results are given which show that partially coherent sources are less influenced by atmospheric turbulence than are coherent sources.     

热风式大气湍流模拟装置的哈特曼测量

大气湍流是由于大气温度、压强的随机变化引起大气折射率随机改变而产生的。遵循这一原理,热风式湍流模拟装置对传输光束周围的空气加热,用风扇抽吸热空气产生湍流,通过改变加热温度和风扇的转速,模拟不同强弱的湍流。用哈特曼传感器对该装置进行测量并采集数据,通过波前斜率处理复原波前,在此基础上从时间域和空间域等方面分析所产生湍流的特点,并与理论进行比较。结果表明,该装置能产生基本符合大气湍流统计理论的湍流。     

M分布模型下多跳相干OFDM FSO系统的性能研究

本文在涵盖了从弱湍流到强湍流的所有信道条件,能够表征现有大多数湍流信道的M分布模型下,采用QPSK调制方式研究了多跳相干OFDM FSO系统的性能。系统在中继辅助链路的发射机和接收机之间使用DF中继协议。考虑大气湍流、路径损耗以及瞄准误差对大气信道衰落模型的联合作用,分别推导出系统的中断概率和误符号率的Meijer G形式的闭合表达式。通过仿真分析了中继链路长度、中继节点数以及子载波个数等关键因素对系统的中断性能和误符号率性能的影响。本研究为中继系统的实际应用奠定了理论基础。     

Performance of a laser Earth-to-satellite link over turbulence and beam wander using the modulated gamma-gamma irradiance distribution

We present an analytical framework for the performance evaluation of laser satellite uplinks over the major probabilistic impairments, i.e., atmospheric turbulence and beam wander. Specifically, we consider a ground-station-to-space laser uplink with a Gaussian beam wave model, and we focus on the particular regime assuming untracked beams where beam wandering takes place. In that regime, the modulated gamma-gamma distribution has been proposed as an effective irradiance model to characterize the combined effect of turbulence and beam wander. First we provide a closed-form expression of the probability density function and deduce the fundamental statistics of the new model. Then we evaluate the performance of the laser system assuming coherent detection for several modulation schemes. An appropriate set of numerical results is presented to verify the accuracy of the derived expressions.     

Free-space communication through turbulence: a comparison of plane-wave and orbital-angular-momentum encodings

Abstract

Free-space communication allows one to use spatial mode encoding, which is susceptible to the effects of diffraction and turbulence. Here, we discuss the optimum communication modes of a system while taking such effects into account. We construct a free-space communication system that encodes information onto the plane-wave (PW) modes of light. We study the performance of this system in the presence of atmospheric turbulence, and compare it with previous results for a system employing orbital-angular-momentum (OAM) encoding. We are able to show that the PW basis is the preferred basis set for communication through atmospheric turbulence for a system with a large Fresnel number product. This study has important implications for high-dimensional quantum key distribution systems.     

大气湍流对贝塞尔高斯光束轴上光强的影响

针对光束在大气中传播时,大气湍流引起的光强起伏等现象.为了探索寻求减弱或者消除湍流的影响的途径,本文采用数值模拟的方法,分析了湍流大气对(零阶)贝塞尔高斯光束轴上光强的影响.分析发现,在湍流环境中,贝塞尔高斯光束的轴上光强都随着传播距离的增加而减小,湍流越强,光束的轴上光强减小越快.贝塞尔高斯光束的径向波矢量越小,轴上光强的稳定性越好.贝塞尔高斯光束轴上光强的稳定性随着传播距离的增加而变坏.贝塞尔高斯光束跟同样束宽的高斯光束相比较,贝塞尔高斯光束的轴上光强受湍流的影响较大.     

Atmospheric turbulence effects on a partially coherent Gaussian beam: implications for free-space laser communication

A partially coherent quasi-monochromatic Gaussian laser beam propagating in atmospheric turbulence is examined by using a derived analytic expression for the cross-spectral density function. Expressions for average intensity, beam size, phase front radius of curvature, and wave-front coherence length are obtained from the cross-spectral density function. These results provide a model for a free-space laser transmitter with a phase diffuser used to reduce pointing errors.     

A modified model of the atmospheric effects on the performance of FSO links employing single and multiple receivers

In free space optical (FSO) communication links, atmospheric effects including absorption, scattering and turbulence have significant impacts on the quality of the laser beam propagating through the free space channel. Absorption and/or scattering due to the atmospheric particles result in optical losses, whereas turbulence contributes to the intensity scintillation which can severely impair the operation of FSO links. In this paper, using a modified model we analyze the atmospheric effects on the signal-to-noise ratio (SNR) and the bit error rate (BER) performance of an FSO system. We show that for multiple detectors with the same surface area as a single detector there is a critical link range less than which the SNR decreases for larger values of M.     

Analysis of adaptive multi-rate FSO/RF hybrid systems using Málaga-ℳ distribution model in turbulent channels

Precise modeling of the optical channels with different conditions of atmospheric turbulence is an important prerequisite for analyzing the performance of Free Space Optical (FSO) communication systems. Statistical models which have been proposed to describe the optical channels, have some limitations to precisely model the channel. In this paper, The ? distribution which is a relatively new and comprehensive model, introduced by the University of Malaga, has been used to model and evaluate FSO/RF hybrid systems under the effect of atmospheric turbulence of the channels for two modes of operation, classic adaptive rate transmission, and the combination of adaptive rate transmission and automatic repeat request.The simulation results have been compared with the experimental results which have been reported by other researchers. It seems, using ? distribution model, yields more consistency between design objectives and implementation results.     

Effect of atmospheric spectrum models on scintillation in moderate turbulence

Experimental studies have shown that a ‘bump’ occurs in the atmospheric spectrum just prior to turbulence cell dissipation. In weak optical turbulence, this bump affects calculated scintillation. The purpose of this study was to determine if a simpler non-bump atmospheric power spectrum can be used to model scintillation for plane waves and spherical waves in moderate to strong optical turbulence regimes. Scintillation expressions were developed from an ‘effective’ von Karman spectrum using an approach similar to that used by Andrews et al. in developing expressions from an effective modified (bump) spectrum. The effective spectrum extends the Rytov approximation into all optical turbulence regimes using filter functions to eliminate mid-range turbulent cell size effects to the scintillation index. Filter cutoffs were established by matching to known weak and saturated scintillation results. The resulting new expressions track those derived from the effective bump spectrum fairly closely. In extremely strong turbulence, differences are minimal.     

Atmospheric-turbulence-induced power-fade statistics for a multiaperture optical receiver

To estimate the probability distributions of power fades, we consider two basic types of disturbance in electromagnetic wave propagation through atmospheric turbulence: wave-front intensity fluctuations and wave-front distortion. We assess the reduction in the cumulative probability of losses caused by these two effects through spatial diversity by using a multiaperture receiver configuration. Degradations in receiver performance are determined with fractal techniques used to simulate the turbulence-induced wave-front phase distortion, and a log normal model is assumed for the collected power fluctuations.     

Mean-square residual error of a wavefront after propagation through atmospheric turbulence and after correction with Zernike polynomials

The root-mean-square (rms) of the residual wavefront, after propagation through atmospheric turbulence and corrected from Zernike polynomials, has been derived for the von Kármán turbulence model. The rms for any location in the telescope pupil and the pupil average rms have been calculated. It is shown that the residual rms on the edge of the pupil can be up to 35% larger than the pupil average residual rms. The results are useful to estimate the required rms stroke of deformable mirror (DM) actuators when they are used as a second stage of correction either in a tip-tilt, single-DM configuration or in a tip-tilt, two-DM (woofer-tweeter) setup.