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An in-flight unmanned aerial vehicle (UAV) free-space optical (FSO) communication channel model is proposed by considering the beam deviation of the UAV under different motion states and the phase distortion caused by atmospheric turbulences. The influence of the different motion states and turbulences on the communication quality is evaluated through phase screen and Monte Carlo methods. When the average bit error rate (BER) is 10−5, the signal-to-noise ratio (SNR) should be increased from 13 dB to 20 dB when the tilt angle of the UAV increases from 0 to 5 mrad. An SNR of up to 20 dB is required when the variance of the wind σα2 is 2 mrad. The performance of the in-flight UAV FSO link can be effectively improved through spatial diversity receiving technology. The BER of lower than 10−5 can be obtained just with an SNR of 13 dB if the spatial diversity array with four receivers is used.

A comprehensive multiparameter model is proposed for underwater wireless optical communication (UWOC) channels to integrate the effects of absorption, scattering, and dynamic turbulence. The simulation accuracy is further improved by the combined use of the subharmonic method and the strict sampling constraint method by comparing the phase structure function with the theoretical value. The average light intensity and scintillation index are analyzed using the channel parameters of absorption, scattering, turbulence, flow velocity, and transmission distance. Under weak or medium turbulence, the bit error rate (BER) performance can be effectively improved by increasing the transmitting light power. The power penalty of a 50 m UWOC channel is 5.8 dBm from pure seawater to ocean water and 1.0 dBm from weak turbulence to medium turbulence, with the BER threshold of 10−6.

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 ap...

Here, we theoretically demonstrate that if a Gaussian optical vortex is distorted by a random phase screen (diffuser) then the average intensity distribution in the focus of a spherical lens has a form of a ring with a nonzero value on the optical axis. The radius of the average-intensity ring depends on both the topological charge of an optical vortex and on the diffusing power of the diffuser. Therefore, the value of the topological charge cannot be unambiguously determined from the radius of the average intensity ring. However, the value of the topological charge of the optical vortex can be obtained from the number of points of phase singularity that can be determined using a Shack-Hartmann wavefront sensor. It is also shown that if we use a linear combination of two optical vortices, then the average intensity distribution has local maxima, the number of which is equal to the difference of the topological charges of the two original vortices. The number of these maxima no longer depends on the scattering force of the diffuser and can serve as an indicator for optical vortex identification. Modeling and experiments confirm the theoretical conclusions.

We present mathematical and computer modelling of ghost imaging technique based on broadband pulsed radiation of terahertz spectrum range, modulated by a random phase screen. The modelling is performed in both frequency and time domains. The quality of the reconstructed images is discussed and estimated.

We demonstrate analytically, numerically, and experimentally that the fractional topological charge shifting in perfect vortices having a ring-shaped distribution is precisely determined by the correlation coefficient of speckle distributions obtained by the scattering of these perfect vortices through a random phase screen. The scattering of elliptic perfect optical vortices is also analyzed and discussed. Fil: Bianchetti, Arturo Abel. Instituto Nacional de Tecnología Industrial; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Etchepareborda, Pablo Gonzalo. Instituto Nacional de Tecnología Industrial; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Federico, Roque Alejandro. Instituto Nacional de Tecnología Industrial; Argentina

During the last decade, optical memory effects have been explored extensively for various applications. In this letter we propose phase screen models to facilitate the analysis and the simulation of wave propagation through optical media that exhibits memory effects. We show that the classical optical memory effect, which implies tilt wave correlations of the input and the scattered fields, can be readily modeled by a single random phase screen. For the recently discovered generalized optical memory effect, which implies the existence of shift wave correlations in addition to the tilt correlation, we propose an appropriate generalized random phase screen model.

Wave Optics Simulations (WOS) are a valuable tool for exploring the effects of atmospheric turbulence on laser beam propagation and imaging. Obtaining accurate results from WOS requires careful selection of simulation parameters. Specifically, the inter-screen spacing, spatial sampling of source and receiver planes, and phase screen sampling are all important parameters. The parameter space for WOS in Kolmogorov turbulence is well characterized and while it is often assumed that similar requirements hold for simulation of non-Kolmogorov atmospheres, we find this is not the case. In this paper we will discuss the practical limits of modeling optical propagation in non-Kolmogorov turbulence using the split-step phase screen method underlying WOS.

We propose a fast technique to simulate the effect of ionospheric scintillation on GNSS signals. The method uses a single-layer phase screen model of the ionosphere with the Huygens- Fresnel integral being determined analytically. The adopted model of the ionosphere assumes an infinite train of plasma bubbles drifting eastward with constant velocity. By modifying the total electron content of the plasma bubbles different values of the scintillation index $S_{4}$ can be obtained. This model is useful for the simulation of equatorial scintillation scenarios.

The majority of the works are dedicated to the specific problem, for instance, scattering from the rough surface although, the general theoretical bases of the statistical and spectral characteristics of the scattered light for the light of non-Gaussian statistics have not been studied so far. The signals with such statistics are produced when the characteristic scales of the refraction factor is in comparison with the size of illuminated region. In the present paper, we discuss the method of signal processing for analyzing first and second order statistical moments of electromagnetic radiation.

High fidelity Monte Carlo simulation of atmospheric turbulence phase screens is important for performance testing of astronomical adaptive optics systems. With a sparse spectrum model and an optimal sampling method, it is possible to generate an atmospheric turbulence phase screen with high fidelity. However, the phase screen generation speed is limited by the algorithm structure of this technique. A non-uniform fast Fourier transform technique is proposed in this paper to accelerate phase screen generation speed. This method is able to generate huge atmospheric turbulence phase screens with high fidelity and an acceptable time-cost enabling practical adaptive optics simulations of forthcoming Extremely Large Telescopes.

Secure communications are critical for many applications. While natural ionospheric effects will be negligible on frequencies above 30 GHz, amplitude scintillation caused by a high altitude nuclear event (HANE) could degrade communication links. Amplitude scintillation is modeled using the multiple phase screen method. The results show that after a HANE, amplitude scintillation will degrade signals at frequencies below 60 GHz but the duration of the effect is only for a few minutes after the event.

By using the method of the power-spectrum inversion, the turbulence phase screen has been built up, and the propagation characteristics of high-frequency phase of laser beam in atmospheric turbulence have been analyzed; in addition, the phase correction effect of laser beams by using the adaptive deformable mirror has been simulated, and its affecting factors in turbulence have also been analyzed quantitatively. The results show that the phase correction effect of laser beams in turbulence is mostly determined by the percent of high-frequency phase in distorted wavefront. With the increase of the intensity of atmospheric turbulence and the propagation distance in turbulence, the percent of high-frequency phase in distorted wavefront increases, resulting in the degradation of the phase correction effect.

In order to restrain the scintillation effect in the course of laser transmitting through the atmosphere, the relation between the laser coherence and the scintillation index was researched. First, the mathematic model correlating with laser coherence was established based on the generation mechanism of scintillation effect. Then, the laser was divided into four beams, and the coherence was destroyed. On this basis, the atmosphere turbulence emulation experiment device was made using the random phase screen as core element, and the far field spot scintillation index with different laser coherence was evaluated. Experimental results indicate that the scintillation index with four beams reduce about 75% in comparison with single beam. So the availability of multiple partially coherent beams limiting laser intensity scintillation effect has been approved by the result of theory analyzing and experiment.

The new techniques in adaptive optics, free space optical(FSO) communication rely on the use of numerical simulations for atmospheric turbulence to evaluate the performance of the system. The simulation of turbulence phase screen is the heart of numerical simulations which produces random wavefront phase perturbations with the correct statistical properties corresponding to models of optical propagation through atmospheric turbulence. The phase-screen simulation techniques can be roughly divided into fast Fourier transform (FFT) method and matrix-based method. Because of a better performance in computation time, the FFT method is generally used for modeling the performance of a real system. But the classical FFT method has a main deficiency of oversample in low frequency region, which leads to the lost of accuracy. To overcome this deficiency, many methods have been proposed for compensating for the oversample of low frequency components, in the last decades. Essentially, these methods achieve a higher accuracy at the expense of computation time. A good compensation method should take into consideration both accuracy and computation time. To achieve higher accurcy and lower computational cost simultaneously, we develop a hybrid method to generate turbulence phase screen, i.e. the classical FFT model is mixed with the sparse spectrum model. We first extract the low frequency region from the frequency grid of FFT model, and resample this region with 16 samples. It is found that the accuracy of phase screen is related to the distribution of these samples, and there must be an optimum distribution that can minimize the relative error between expected structure function and theoretical structure function in the low frequency region. So it permits one to use optimization algorithm to find the optimized distribution of low frequency samples. Here an improved gravity search algorithm is adopted in which the memory of each particle is taken into consideration. The optimization parameters are determined after a lot of tests, and the robustness testing shows that the algorithm is effective. To compare with existing subharmonic method, we choose the same parameters of phase screen as those used in the expanded subharmonic method, generate 1000 phase screens for each method, compute the phase structure function, and we also compare our results with those from the theoretical structure function. The comparison result shows that the curve of phase structure function generated by our method is nearly consistent with the theoretical one, the maximum relative error in low frequency region is about 0.063% which is much better than that from the expanded subharmonic method 5%. Finally in this paper, the computational cost is analyzed, showing that the generation speed for our method is at least 4.5 times as fast as that for the Johansson’s method.

We analyze the radius of curvature of Bessel Gaussian (BG) and modified Bessel Gaussian (mBG) beams. The study is based on the results of analytic derivation as well as those of the random phase screen approach. Our results are displayed in graphs as variations of radius of curvature against propagation distance at various settings of beam order, width parameter, source focal length, wavelength, refractive index structure constant. Our findings indicate that mBG beams, in general will have larger radius of curvature values than BG beams. It is further observed that increases in beam order will lead to greater radius of curvatures. Rises in the width parameter will reveal more the differentiations between BG and mBG beams. At small focal lengths, the difference between BG and mBG beams is hardly noticeable. Higher wavelengths will initially cause a reduction in the radius of curvature, but at longer propagation distances, the reverse will happen. Increases in the refractive index structure constant will lead to smaller radius of curvature values. A general agreement is found in comparing the analytic results of BG beams with those of phase screen approach.

The new detector of acoustic oscillations of the medium suggested. The basic laws of detecting acoustic oscillations were investigated by means of computer and physical modeling.

A statistical mechanical approach to random phase screens is adapted to describe telegraph-wave phase screens with a bias. The results agree with those found by Jakeman and Renshaw [ J. Opt. Soc. Am. A4, 1206 ( 1987)] for an asymmetrically correlated random-walk model for scattering.

An atmospheric turbulence phase screen generated using a fractal method is introduced. It is etched onto fused silica and tested in the laboratory. The etched screen has relatively low cost, high resolution, and can be used in the broad waveband under severe temperature conditions. Our results are shown to agree well with the theory.

Gaussian models without intermittency are extensively used in estimating the effect of turbulence, but it also brings some puzzles, for example the observed pulse shape that disagrees with the result of the standard theory of interstellar scintillations. Indeed the property of intermittency is inherent in turbulence, i.e., all the quantities that characterize it suffer from strong fluctuations. So it is necessary to consider turbulent intermittency in many applications. In this paper we propose a non-Gaussian phase screen, which obeys log-Poisson statistics, and also offers the corresponding point spread function (PSF). These results describe that intermittency leads to the more extent and different directional distribution of PSF. Theoretical analysis is made under the hypothesis of the phase difference satisfying log-Poisson statistics, and the average point spread function, which accord qualitatively with the result of the above generated phase screen, is derived.

Use of large-eddy simulation (LES) data in electromagnetic wave propagation modeling is not very common because of the high computational cost involved. A new phase-screen method is proposed to model radio wave propagation, in the atmospheric turbulence, using the resolved scale refractivity obtained from LES. The proposed method offers the same level of accuracy, as the one already existing in the literature, at much cheaper cost.

The numerical-analytical phase screen method is used to analyze the statistics of the density of caustics in an underwater sound channel with large-scale random inhomogeneities. Different cases of wave propagation direction with respect to the channel axis are considered, and the influence of the inhomogeneity correlation radius is investigated.

Enhanced backscatter effects have long been predicted theoretically and experimentally demonstrated. The reciprocity of a turbulent channel generates a group of paired rays with identical trajectory and phase information that lea ds to a region in phase space with double intensity and scintillation index. Though simulation work based on phase screen models ha s demonstrated the existence of the phenomenon, few experimental results have been published describing its characteristics , and possible applications of the enhanced backscatter phenomenon are still unclear. W ith the development of commerciall y available high powered lasers and advanced cameras with high frame rates, we have successfully captured the enhanced backscatter effect s from different reflection surfaces. In addition to static observations , we have also tilted and pre -distorted the transmitted beam at various frequencies to track the dynamic properties of the enhanced backscatter phenomenon to verify its possible applic ation in guidance and beam and image correction through atmospheric turbulence. In this paper, experiment al results will be described, and discussions on the principle and applications of the phenomenon will be included. Enhanced backscatter effects are be st observed in certain levels of turbulence (C

We study theoretically the statistical properties of one-dimensional wave-field inversions. We show that the real and imaginary parts of the logarithm of the normalized coherence function are the invariants of the inverted field if the field is measured on the statistical symmetry axis. Using these invariants, one can easily reconstruct two-point statistical moments of the phase distribution on the screen. We derive equations for the reconstruction of phase-distribution moments in the general case. Numerical simulations show that these equations can be solved by an iterative technique. The convergence range of the iteration method with variation in the parameters is studied.

It is shown that the validity of the random phase screen approximation in coherent wave scattering from random media can be determined from the correlation between symmetric sidebands in the scattered field. Analysis of doublet correlations in microwave reflectometer data taken on the Tokamak Fusion Test Reactor confirms the validity of the model and allows for the numerical back propagation of measurements without the use of imaging optics.

The purpose of the research presented in this paper is to do an error analysis of the phase screen method for forward and inverse wave propagation calculations in 3D heterogeneous acoustic media. The differential operator formalism approach is used, which provides new insight into the phase screen method. Under the assumptions that backscattering and commutator term of operators R and Q are negligible, we derive a partial differential equation and its solution corresponding to the phase screen method. The errors introduced in these derivations are also obtained. For a one-step (δz) phase screen calculation, the leading error term is in the first order of k0Δz and proportional to the error from splitting the square-root operator Q. The propagation angle should be less than 40 degrees to control the error from the splitting operator Q under 5%.

Beamlet migration is a wave equation based migration approach. The use of local background velocity and local perturbations results in a two-scale decomposition of beamlet propagators: a background propagator for largescale structures and a local phase screen correction for small-scale local perturbations. The beamlet propagator can handle strong lateral velocity variations with improved accuracy. Local cosine basis beamlet propagator was proposed and developed recently. It has been successfully applied to poststack and prestack depth migration for media with large lateral velocity variations. Here we extend the beamlet propagator into transversely isotropic medium with vertical symmetric axis (VTI) medium. We tested impulse responses, a synthetic dataset and a field dataset. The results demonstrated the validity of the VTI beamlet propagator.

A significant improvement to the recently introduced complex screen (CS) method for generating partially coherent Schell-model sources is presented. The method, called the modified phase screen (MPS) technique, applies a deterministic amplitude and the phase portion of a CS to an initially coherent light source using a single phase-only spatial light modulator. The MPS technique, unlike the CS approach from which it is derived, does not produce a fully developed speckle pattern in the source plane, and therefore converges faster and more uniformly to the desired partially coherent source. The analytical development of the MPS method is presented. Experimental results of a Bessel-Gaussian-correlated Schell-model source, generated using the CS and MPS approaches, are compared to demonstrate the validity and utility of the MPS technique.

On the basis of the analogy between the density evolution of a heated gas of noninteracting particles and the diffraction of monochromatic optical waves by a random phase screen, a formula is derived for the correlation function of intensity fluctuations behind the screen. A hypothesis on the statistical splitting of the geometric and diffraction means allows the multiplicity of the integral that determines this function to be reduced by half.

We present a new approach for simulating time-evolving speckle patterns, by combining the shift of a rigid phase screen with its evolution. Thus, the two time-scales associated with speckle boiling and speckle motion can be adjusted independently. The statistical properties of the phase perturbation and the speckle pattern are investigated by using the temporal phase structure function and the temporal intensity correlation. We found a very good agreement with experimental results.

We consider the problem of multiple light scattering from a random surface set nearly perpendicular to a plane mirror. With an approach based on the thin phase screen model we show that this scattering system exhibits the phenomenon of enhanced backscattering. The predictions are supported by experimental results.

An investigation of non-Gaussian speckle produced by a random phase screen which has Gaussian probability density function and correlation function is presented using a 1-D computer simulation. Rms phase fluctuations around and greater than 2pi rad are considered. The results significantly differ from previous theoretical results. Comparison between the results of the computer simulation and those of early experiments using a ground glass is also given.