Simulated Atmospheric Turbulence Research Articles

Adaptive Single-Mode Fiber Coupling Method Based on Coarse-Fine Laser Nutation

One of the major restrictions of the free-space optical communication systems is the limitation of single-mode fiber (SMF) coupling efficiency that impacts the system performance. In this paper, an adaptive coupling method for optical communication system based on coarse-fine laser nutation technique was presented. The scanning strategy and algorithm were set up and experimentally examined. The experimental results demonstrate that the coarse-fine laser nutation technique is capable of achieving a good coupling efficiency even the beam spot and the SMF do not coincide with each other at the start. The SMF center calculation accuracy of coarse and fine scanning stage were 0.25 and 0.01 μ m, respectively. And the average coupling efficiency of the system can increase from 0.351 in open loop to 0.624 in closed loop under a simulated atmospheric turbulence condition, the offset compensation error was 0.1245 μ m when turbulence induced. Finally, some factors influencing the systems were also discussed.

Fractal reconstruction of sub-grid scales for large eddy simulation of atmospheric turbulence

We present a fractal sub-grid scale model for large eddy simulation (LES) of atmospheric flows. The fractal model is based on the fractality assumption of turbulent velocity field with a dynamical hypothesis based on energy dissipation. The fractal model reconstruct sub-grid velocity field from the knowledge of its filtered values on LES grid, by means of fractal interpolation, proposed by Scotti and Meneveau (1999). The characteristics of the reconstructed signal depends on the (free) stretching parameters, which is related to the fractal dimension of the signal. In previous studies, the stretching parameters was assumed to be constant in space and time and are obtained from experimental velocity signals of homogeneous and isotropic turbulence. To improve this method and account for the stretching parameter variability, we calculate the probability distribution function of the stretching parameter from direct numerical simulation (DNS) data of stratocumulus-top boundary layer (STBL) (courtesy of Prof. J.-P. Mellado from the Max Planck Institute of Meteorology) using the geometric method proposed by Mazel and Hayes. We perform 1D a priori test and compare statistics of the constructed velocity increment with DNS velocity increments.

High frequencies in random forcing affect turbulence simulations

An investigation into white noise forcing reveals it may not be the best choice for idealized simulations of atmospheric and oceanic turbulence.

Influence of speckle image reconstruction on photometric precision for large solar telescopes

Context. High-resolution observations from large solar telescopes require adaptive optics (AO) systems to overcome image degradation caused by Earth’s turbulent atmosphere. AO corrections are, however, only partial. Achieving near-diffraction limited resolution over a large field of view typically requires post-facto image reconstruction techniques to reconstruct the source image.Aims. This study aims to examine the expected photometric precision of amplitude reconstructed solar images calibrated using models for the on-axis speckle transfer functions and input parameters derived from AO control data. We perform a sensitivity analysis of the photometric precision under variations in the model input parameters for high-resolution solar images consistent with four-meter class solar telescopes.Methods. Using simulations of both atmospheric turbulence and partial compensation by an AO system, we computed the speckle transfer function under variations in the input parameters. We then convolved high-resolution numerical simulations of the solar photosphere with the simulated atmospheric transfer function, and subsequently deconvolved them with the model speckle transfer function to obtain a reconstructed image. To compute the resulting photometric precision, we compared the intensity of the original image with the reconstructed image.Results. The analysis demonstrates that high photometric precision can be obtained for speckle amplitude reconstruction using speckle transfer function models combined with AO-derived input parameters. Additionally, it shows that the reconstruction is most sensitive to the input parameter that characterizes the atmospheric distortion, and sub-2% photometric precision is readily obtained when it is well estimated.

Coherence of simulated atmospheric boundary-layer turbulence

The coherences in a plane perpendicular to incoming flow are measured in wind tunnel simulations of atmospheric turbulent flow. The measured coherences are compared with analytical expressions tailored to field measurements and with theoretical coherence models which assume homogeneous turbulence and the von Kármán’s spectrum. The comparison indicates that the simulated atmospheric boundary layer flow is approximately horizontally homogeneous turbulence. Based on the above assumption and the systematic analysis of lateral coherence, it can be concluded that the lateral coherences of simulated atmospheric boundary turbulence can be determined accurately using the von Kármán spectrum and the turbulence parameters measured by a few measurement points. The measured results also show that the spatial characteristics of vertical coherences are closely related to the dimensionless parameter The vertical coherence at two heights can be roughly estimated by the ratio to The relationship between the phase angles of u-, v- and w-components and the vertical separation distance and the height from the ground is further analyzed. Finally, the roles of the type of land surface roughness, the height from the ground, the turbulence intensity and the integral length scale in lateral and vertical coherences are also discussed in this study.

Statistics of supersonic signatures propagated through simulated atmospheric turbulence

The issue of annoyance caused by acoustical signatures from supersonic aircraft may be quantitatively predicted through propagation simulations. In particular, a model of energy focusing or defocussing due to turbulence in the atmospheric boundary layer is necessary to predict the real variations found whenever measurements are taken at multiple locations or at multiple times. The present algorithm solves the Khoklov-Zobolotskaya-Kuznetzov (KZK) equation and attempts to account for all important atmospheric effects such as refraction, thermoviscous absorption, relaxation processes, and nonlinearity, as well as turbulence effects due to temperature and vector wind variations. The turbulence model uses von Karman spectra, with vertically-varying length scales to better model the full extent of the boundary layer. The presentation will discuss some early results and relevant statistics, as well as the feasibility of comparison to recent overflight tests in NASA's SonicBAT program. [Work supported by NASA via a subcontract from KBRwyle.]

Atmospheric Turbulence Simulator for Adaptive Optics Evaluation on an Optical Test Bench

An adaptive optics system can be simulated or analyzed to predict its closed-loop performance. However, this type of prediction based on various assumptions can occasionally produce outcomes which are far from actual experience. Thus, every adaptive optics system is desired to be tested in a closed loop on an optical test bench before its application to a telescope. In the close-loop test bench, we need an atmospheric simulator that simulates atmospheric disturbances, mostly in phase, in terms of spatial and temporal behavior. We report the development of an atmospheric turbulence simulator consisting of two point sources, a commercially available deformable mirror with a 12×12 actuator array, and two random phase plates. The simulator generates an atmospherically distorted single or binary star with varying stellar magnitudes and angular separations. We conduct a simulation of a binary star by optically combining two point sources mounted on independent precision stages. The light intensity of each source (an LED with a pin hole) is adjustable to the corresponding stellar magnitude, while its angular separation is precisely adjusted by moving the corresponding stage. First, the atmospheric phase disturbance at a single instance, i.e., a phase screen, is generated via a computer simulation based on the thin-layer Kolmogorov atmospheric model and its temporal evolution is predicted based on the frozen flow hypothesis. The deformable mirror is then continuously best-fitted to the time-sequenced phase screens based on the least square method. Similarly, we also implement another simulation by rotating two random phase plates which were manufactured to have atmospheric-disturbance-like residual aberrations. This later method is limited in its ability to simulate atmospheric disturbances, but it is easy and inexpensive to implement. With these two methods, individually or in unison, we can simulate typical atmospheric disturbances observed at the Bohyun Observatory in South Korea, which corresponds to an area from 7 to 15 cm with regard to the Fried parameter at a telescope pupil plane of 500 nm.

Machine learning approach to OAM beam demultiplexing via convolutional neural networks.

Orbital angular momentum (OAM) beams allow for increased channel capacity in free-space optical communication. Conventionally, these OAM beams are multiplexed together at a transmitter and then propagated through the atmosphere to a receiver where, due to their orthogonality properties, they are demultiplexed. We propose a technique to demultiplex these OAM-carrying beams by capturing an image of the unique multiplexing intensity pattern and training a convolutional neural network (CNN) as a classifier. This CNN-based demultiplexing method allows for simplicity of operation as alignment is unnecessary, orthogonality constraints are loosened, and costly optical hardware is not required. We test our CNN-based technique against a traditional demultiplexing method, conjugate mode sorting, with various OAM mode sets and levels of simulated atmospheric turbulence in a laboratory setting. Furthermore, we examine our CNN-based technique with respect to added sensor noise, number of photon detections, number of pixels, unknown levels of turbulence, and training set size. Results show that the CNN-based demultiplexing method is able to demultiplex combinatorially multiplexed OAM modes from a fixed set with >99% accuracy for high levels of turbulence-well exceeding the conjugate mode demultiplexing method. We also show that this new method is robust to added sensor noise, number of photon detections, number of pixels, unknown levels of turbulence, and training set size.

Laboratory simulation of non-Kolmogorov turbulence based on a liquid crystal spatial light modulator

An experimental platform was built for the atmospheric turbulence simulation upon a liquid crystal spatial light modulator. The phase-only spatial light modulator is driven by a series of dynamic phase screens that have non-Kolmogorov turbulence characteristics. After passing through the simulated non-Kolmogorov turbulence, the angle-of-arrival (AoA) and light intensity of the laser beam were analyzed statistically. The variation trends of the AoA fluctuation and the scintillation index exhibited by the experimental results are in accordance with the theoretical predictions. Although there was some divergence between the experiments and theory that needs to be improved further, the experimental data demonstrate the feasibility of using the platform to simulate the turbulence influence on laser communication in the lab.

旋转相位屏的动态大气湍流数值模拟和验证方法

旋转相位屏的动态大气湍流数值模拟和验证方法

Sensitivity of earthquake source inversions to atmospheric noise and corrections of InSAR data

AbstractTropospheric phase delays pose a major challenge to InSAR (interferometric synthetic aperture radar)‐based studies of tectonic deformation. One approach to the mitigation of effects from tropospheric noise is the application of elevation‐dependent corrections based on empirical fits between elevation and interferometric phase. We quantify the effects of corrections with a range of complexity on inferred earthquake source parameters using synthetic interferograms with known atmospheric characteristics. We infer statistical properties of the stratified component of the atmosphere using pressure, temperature, and water vapor data from the North America Regional Reanalysis model over our region of interest in the Basin and Range province of the western United States. The statistics of the simulated atmospheric turbulence are estimated from InSAR and Global Positioning System data. We demonstrate potentially significant improvements in the precision of earthquake magnitude, depth, and dip estimates for several synthetic earthquake focal mechanisms following a correction for spatially variable atmospheric characteristics, relative to cases where the correction is based on a uniform delay versus elevation relationship or where no correction is applied. We apply our approach to the 1992 M5.6 Little Skull Mountain, Nevada, earthquake and demonstrate that the earthquake source parameter error bounds decrease in size after applying the atmospheric corrections. Our approach for evaluating the impact of atmospheric noise on inferred fault parameters is easily adaptable to other regions and source mechanisms.

Simulation of Atmospheric Turbulence for Wind-Tunnel Tests on Full-Scale Light-Duty Vehicles

Simulation of Atmospheric Turbulence for Wind-Tunnel Tests on Full-Scale Light-Duty Vehicles

Gaussian vs non-Gaussian turbulence: impact on wind turbine loads

From large-eddy simulations of atmospheric turbulence, a representation of Gaussian turbulence is constructed by randomizing the phases of the individual modes of variability. Time series of Gaussian turbulence are constructed and compared with its non-Gaussian counterpart. Time series from the two types of turbulence are then used as input to wind turbine load simulations under normal operations with the HAWC2 software package. A slight increase in the extreme loads of the tower base fore-aft moment is observed for high wind speeds when using non-Gaussian turbulence but is insignificant when taking into account the safety factor for extreme moments. Other extreme load moments as well as the fatigue loads are not affected because of the use of non-Gaussian turbulent inflow. It is suggested that the turbine thus acts like a low-pass filter that averages out the non-Gaussian behaviour, which is mainly associated with the fastest and smallest scales. Copyright © 2016 John Wiley & Sons, Ltd.

Design of a single-star optical emulator for a fast telescope

The design of a simple lens system is described capable of projecting a diffraction limited f1/.2 point of light through a variety of plane parallel vacuum windows. The system was built for the purpose of testing prototype CCDs for the Large Synpotic Survey Telescope in which lab testing drove the desire to create a beam that matches the telescope's f-ratio and obstruction, and which would have sufficient back-focal distance to allow imaging onto a sensor at least 50 mm away in various dewars with various window thicknesses. Also used as the final optic in an atmospheric turbulence simulator, the lens can simulate the real-world star PSF as it will appear on the Large Synoptic Survey Telescope (LSST) focal plane.

Two-dimensional simulation of optical wave propagation through atmospheric turbulence.

A methodology for the two-dimensional simulation of optical wave propagation through atmospheric turbulence is presented. The derivations of common statistical field moments in two dimensions, required for performing and validating simulations, are presented and compared with their traditional three-dimensional counterparts. Wave optics simulations are performed to validate the two-dimensional moments and to demonstrate the utility of performing two-dimensional wave optics simulations so that the results may be scaled to those of computationally prohibitive 3D scenarios. Discussions of the benefits and limitations of two-dimensional atmospheric turbulence simulations are provided throughout.

热风对流式大气湍流模拟装置湍流模拟特性分析

热风对流式大气湍流模拟装置湍流模拟特性分析

圆偏振光大气湍流模拟信道传输保偏特性实验研究

圆偏振光大气湍流模拟信道传输保偏特性实验研究

Implementation of a Phase Only Spatial Light Modulator as an Atmospheric Turbulence Simulator at 1550 nm

Modeling and simulating atmospheric turbulence in a controlled environment have been a focus of interest for scientists for decades. The development of new technologies allows scientists to perform this task in a more realistic and controlled environment and provides powerful tools for the study and better understanding of the propagation of light through a nonstatic medium such as the atmosphere. Free space laser communications (FSLC) and studies in light propagation through the atmosphere are areas which constantly benefit from breakthroughs in technology and in the development of realistic atmospheric turbulence simulators, in particular (Santiago et al. 2011). In this paper, we present the results from the implementation of a phase only spatial light modulator (SLM) as an atmospheric turbulence simulator for light propagation in the short-wave infrared (SWIR) regime. Specifically, we demonstrate its efficacy for its use in an FSLC system, at a wavelength of 1550 nm.

A Fast Estimation of the EO/IR System Performances Using the Image Simulation in a Turbulent Atmosphere

The paper presents a fast evaluation method for the image quality acquired with the EO/IR sensors. The principle of the proposed method is based on the analysis of the histograms of the images acquired using atmospheric turbulence simulations and, then, by the analysis ofthe histograms extracted from simulations. It is assumed that the assessment of an EO/IR sensor system performance can be known in poor visibility conditions, where the atmospheric turbulences have an essential contribution.

Study of atmospheric turbulence with Shack Hartmann wavefront sensor

In order to understand adaptive optics technology in detail, it is essential to understand the propagation of light through atmospheric turbulence. In this context, we have attempted to understand this behaviour by simulating atmospheric turbulence environment both numerically and experimentally. Results of such simulation studies are presented. Wavefront sensing is an important component in any adaptive optics system. The performance of the Shack Hartmann wavefront sensor in the presence of atmospheric turbulence is extensively dealt and reported. Finally, the results of laboratory simulation of atmospheric turbulence and its effect on the Shack Hartmann sensor and recalculation of the Fried’s parameter from the laboratory data also presented.