Structured light in atmospheric turbulence—a guide to its digital implementation: tutorial

Real time simulation of atmospheric turbulence based on GPU

In this letter, an atmospheric turbulence simulation equipment based on a liquid crystal spatial light modulator is studied. We use fast Fourier transform to generate a phase screen, and uses the subharmonic method to compensate for its low-frequency components. The influence of different turbulence intensities on laser atmospheric transmission is successfully-demonstrated. The far-field spot and received power of Gaussian beams under simulated atmospheric turbulence with different intensity are measured through actual experiments. The experimental results show that the spot will be distorted, and the received power at the receiving end will be jittered in varying degrees.

AOLI, Adaptive Optics Lucky Imager, is the next generation of extremely high resolution instruments in the optical range, combining the two more promising techniques: Adaptive optics and lucky imaging. The possibility of reaching fainter objects at maximum resolution implies a better use of weak energy on each lucky image. AOLI aims to achieve this by using an adaptive optics system to reduce the dispersion that seeing causes on the spot and therefore increasing the number of optimal images to accumulate, maximizing the efficiency of the lucky imaging technique. The complexity of developments in hardware, control and software for in-site telescope tests claim for a system to simulate the telescope performance. This paper outlines the requirements and a concept/preliminary design for the William Herschel Telescope (WHT) and atmospheric turbulence simulator. The design consists of pupil resemble, a variable intensity point source, phase plates and a focal plane mask to assist in the alignment, diagnostics and calibration of AOLI wavefront sensor, AO loop and science detectors, as well as enabling stand-alone test operation of AOLI.

We propose an adaptive optics (AO) pre-compensation scheme to improve the transmission quality of orbital angular momentum (OAM) beams in atmospheric turbulence. The distortion wavefront caused by atmospheric turbulence is obtained with the Gaussian beacon from the receiver. The AO system imposes the conjugate distortion wavefront onto the outgoing OAM beams at the transmitter, tto achieve the pre-compensation. Using the scheme, we conducted transmission experiments with different OAM beams in the simulated atmospheric turbulence. The experimental results indicated that the AO pre-compensation scheme can improve the transmission quality of the OAM beams in the atmospheric turbulence in real-time. It is found that the turbulence-induced crosstalk effects on neighboring modes are reduced by an average of 6 dB, and the system power penalty is improved by an average of 12.6 dB after pre-compensation.

With the rapid development of modern science and technology in astronomical imaging, optical communications, optical radar, optical information processing, high-precision ranging, tracking, guidance, and remote sensing, light waves propagating in the medium, especially in the turbulent atmosphere spread more and more important. Atmospheric turbulence is one of the main factors which have influence on the performance of a laser communication system. Adaptive optics technology is an important means to solve the problem of atmospheric turbulence. This paper states how adaptive optics technique can be used in space laser communication system to compensate atmospheric turbulence when laser beam transmission through it. The core content of adaptive optics is correct laser beam wave-front disturbance in real-time，with it to enhance optical system imaging quality and the next aim is reach the level of diffraction limitation. Adaptive optics system consists of wave-front detection， wave-front control and wave-front correction . The demo platform including: atmospheric turbulence simulation unit、adaptive correction unit、signal transmitting and receiving unit. Liquid crystal spatial light modulator applications in adaptive optics system and the turbulence simulation system introduced. And used zernike polynomials method to produce atmospheric turbulence phase screen simulation analysis. Simulation results show that: in the low spatial frequency components, the atmospheric turbulence phase screen generated by Zernike polynomial method consistent with the theoretical values, but in the high spatial frequency components, the simulation results with large difference between the theoretical values. In addition, the simulation results also show that: we can change the distribution of turbulence in the atmospheric turbulence phase screen by increasing the Zernike polynomials of orders or change the receiving apertures, but to calculate the large calculate the complex and other shortcomings. If adaptive optics technique can be applied in space laser communication system and is proved by practice， must make a space laser communication system performance has greatly improved.

To study the effects of atmospheric turbulence on the quality of entanglement of photons propagating from a ground station to a satellite and to verify if coincidence measurements of entangled photons can be used to back out atmospheric turbulence parameters for long-distance propagation or moderate-to-strong turbulence , we built an atmospheric turbulence simulator (ATS) in a laboratory setting. The ATS comprised of two afocal systems with a Lexitek phase wheel and a Meadowlark spatial light modulator representing discrete layers of atmospheric turbulence. The ATS could represent propagation distances on the order of 1km and could theoretically simulate Rytov variances as high as 5.16 and Fried parameters as low as 0.5cm for a 1m telescope. The design parameters, numerical simulations, and experimental setup is detailed in this proceeding.

A temporal model was proposed for simulating the atmospheric turbulence varying with the time in the adaptive optics system testing.The relationship between the number of interpolated frames for the random phase screen,refurbish frequency and the mean wind speed was analyzed.The analysis result demonstrates that it is necessary to smooth the random phase screen for characterizing the temporal gradual variation of the random wavefront in order to make the change of the random wavefront better aligned with the influence of atmospheric turbulence on continuous smoothing gradients of incident wavefront.The interpolated frames of the random phase screen is only related to the aperture diameter and the atmospheric coherence length,but not related to the wind speed,and the refurbish frequency of the random phase screen increases with the mean wind speed,and the refurbish frequency smoothed increases with the number of interpolated frames.A atmospheric turbulence simulator was constructed in laboratory and the analysis of the power spectrum density of experimental result demonstrates that temporal model of the atmospheric turbulence simulation is valid.

The Air Force Institute of Technology (AFIT) is building a quantum optics and quantum information laboratory to study single photon phenomena and its applications. We propose an experiment to characterize the effects of atmospheric turbulence on single photons and entangled pair of photons as a function of statistical quantities that define a turbulent atmosphere (such as the Fried parameter, Greenwood frequency, Rytov variance, ect).This presentation details the initial experiment studying single photon propagation using an atmospheric turbulence simulator that statistically represents ground-to-space quantum communications.

Videos captured in long-distance horizontal imaging through the atmosphere suffer from dynamic spatiotemporal movements and blur caused by the air turbulence. Simulations of atmospheric turbulence in such videos, which have been conducted in the past, are difficult to compute. Our goal in this research is to develop an effective simulation algorithm of videos affected by atmospheric turbulence characterized by spatiotemporally varying blur and tilt, when supplied with a given image. We accomplish this via extending an already established method that simulates atmospheric turbulence in a single image, by incorporating turbulence properties in the time domain that include both the tilts and blurring effects. This study also extends our previous work that simulated turbulence, but did not consider the space-varying property of the blur. This is done by employing the relationship between turbulence image distortions and the intermodal correlations of the Zernike coefficients in time and space, and also via analyzing the spatiotemporal matrix that represents the spatial correlation of movements between different frames. The proposed method can facilitate the production of simulations, given turbulence properties that include turbulence strength, object distance, and height. The simulation is applied to videos with low and high frame rates, and the differences between them are analyzed. The proposed method can prove useful when generating machine-learning algorithms that apply to videos affected by atmospheric turbulence, which require large labeled video datasets (with controlled turbulence and imaging parameters) for training.

Three types of turbulence fields were investigated using a research method combining wind tunnel tests and theoretical analysis to further explore the spatial structure of atmospheric boundary layer turbulence, which was passively simulated by a wind tunnel. The fundamental theory of turbulence is introduced, and some traditional theoretical coherence models based on isotropic turbulence theory are derived. The difference between the theoretical results and the passive simulation of atmospheric boundary layer turbulence was compared and discussed. The analysis results show that the passively simulated atmospheric turbulence basically conformed to the homogeneous isotropic turbulence assumption on the horizontal plane, but the interference of the nonisotropic turbulence components cannot be ignored either. Finally, some improvements were made to the traditional coherence function model based on the experimental results to apply the passively simulated atmospheric boundary layer turbulence.

In the present communication, a systematic characterization of atmospheric turbulence simulator (ATS) based on near-index-matched optics is reported. Characteristics of the propagating laser beam in actual turbulence can be realized in the laboratory by employing such types of turbulence simulators. Such simulators are necessarily required to evaluate the performance of an adaptive optics sensor and compensator modules in the laboratory. Various strengths of atmospheric turbulence can be generated by selecting the different speeds of rotation and the diameters of the beam-interacting area of ATS. A MATLAB-based high-speed video processing method is developed and used for estimating the various turbulence parameters such as angle-of-arrival fluctuations, Fried parameter, Hurst exponent, turbulence frequencies and the scintillation index from the generated turbulence. Also, the maximum transmitted wavefront error produced by this turbulence simulator is measured by an in-house developed Shack–Hartmann wavefront sensor.

The starting point of wind turbine operation is the incoming wind. Wind turbines are positioned in the atmospheric boundary layer (ABL), the lower approximately 1 km of the atmosphere; here the wind tends to be dominated by turbulent structures generated through the transfer of momentum and heat with the Earth's surface, as well as interaction with the free atmosphere above governed by large-scale motion. In this chapter, we will look at the turbulence affecting wind turbines from a turbulence -simulation point of view. This means that the focus will be on the properties of atmospheric turbulence which directly affect the performance and operation of wind turbines. In the ABL, turbulence is produced by mean wind shear and enhanced or destructed by buoyancy effects. This results in profiles of the various turbulence quantities across wind turbine rotors. Examples include the mean wind speed itself; second order moments, like variances and stresses; and turning of the mean wind speed and even length scales of turbulence. The degree to which a wind turbine will be affected by the turbulence in the ABL depends on its size such as rotor diameter and hub height, its power generating properties such as thrust coefficient, as well as on the applied controller which ultimately decides the operation window of the turbine. Simulations of atmospheric turbulence can guide us in quantifying the effects.

The dynamic atmospheric turbulence is simulated in the laboratory upon the phase-only liquid crystal spatial light modulator. Dynamic phase screens are generated by the spline function method and the frozen turbulence method. The average cross-correlation coefficient of the Zernike coefficients between these two methods is 0.6608. Moreover, the laser atmospheric transition experiment is carried on under different turbulence intensities. The logarithmic light intensity probability density distribution is close to the normal distribution, and the fitting determine coefficient is above 0.9. In the weak turbulence (r0=0.1m), the standard deviation of the arrival angle fluctuation is approximately 30 to 40 μrad, while it is 40 to 50 μrad in the moderate turbulence (r0=0.01m). The simulation result is compliant with the turbulence theory. Compared with the frozen method, the arrival angle fluctuation spectrum in high frequency upon the spline method is smoother. It reveals that although two methods have a good consistency with each other, the dynamic simulation of the spline method is supposed to be more favorable in the researching of the free-space laser propagation.

Previous simulations of atmospheric turbulence in videos are computationally complex. The purpose of this study is to develop an efficient algorithm for simulating spatiotemporal videos affected by atmospheric turbulence, given a static image. We extend an existing method for the simulation of atmospheric turbulence in a single image by incorporating turbulence properties in the time domain and the blurring effect. We accomplish this through analysis of the correlation between turbulence image distortions in time and in space. The significance of this method is the ease with which it will be possible to produce a simulation, given properties of the turbulence (including turbulence strength, object distance, and height). We apply the simulation to low and high frame rate videos, and we show that the spatiotemporal cross correlation of the distortion fields in the simulated video matches the physical spatiotemporal cross correlation function. Such a simulation can be useful when developing algorithms that apply to videos degraded by atmospheric turbulence and require a large amount of imaging data for training.

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.