The aim of this thesis is to quantify the impact of atmospheric turbulence on optical signal used in free space optic communication systems. The first part is associated with atmosphere as transmission medium. Following part deals with interferometry and components of interferometers. Various methods of analysis of trasmission environment for optical beam are introduced theoretically and experimentally. Mathematical apparatus based on Kolmogorov’s cascade theory, Rytov’s variance was used to find value of structural parameter of refraction as a main measure of turbulence intensity. Experimentally obtained data were subjected to statistical analysis. The visualization of interference pattern fluctuations under turbulent conditions is shown in the last section. Interferometric method was realized with Michelson interferometer.

The diploma thesis is focused on the optical beam propagating in the atmosphere in a wireless communication optical link. The first part of the work explains the atmospheric transmission media with turbulence and its effects on reliability of the optical system. The second part introduces methods for turbulence determination based on a statistical approach to turbulence quantification are introduced. In the third part, method of equivalent temperature gradient is described with the advantage of immediate turbulence evaluation. The output of this thesis is the model of turbulent environment formed by the optical elements. Analysis of turbulent properties and non-reciprocal nature of turbulent channel is processed by a 2D simulator based on the mentioned model and method of equivalent temperature gradient.

Un lien optique basé sur un multiplex de longueurs d'onde autour de 1,55μm est une alternative intéressante pour pallier la saturation des bandes radio-fréquences classiquement utilisées et pour répondre aux besoins de liens haut débit par satellite géostationnaire de la prochaine génération de télécommunication. Compte-tenu de la puissance limitée des lasers envisagés, la divergence du faisceau doit être considérablement réduite. Par conséquent, le pointage du faisceau devient un paramètre critique. Au cours de sa propagation entre la station sol et un satellite géostationnaire, le faisceau optique est dévié et éventuellement déformé par la turbulence atmosphérique. Cela induit de fortes fluctuations du signal de télécommunication détecté, réduisant le débit disponible. Un miroir de basculement est utilisé pour pré-compenser la déviation mesurée à partir d'un faisceau provenant du satellite. Du fait de l'angle de pointage en avant entre la liaison descendante et la liaison montante, les effets de turbulence subis par les deux faisceaux sont légèrement différents, ce qui induit une erreur dans la correction.Le critère de performance de la liaison est l’intensité minimale détectable 95% du temps. Un modèle rapide, nommé WPLOT, prenant en compte les erreurs de pointage et leur évolution temporelle, est proposé pour évaluer cette intensité minimale en fonction des paramètres de la station sol et de la qualité de la correction. Les résultats obtenus avec ce modèle sont comparés avec ceux obtenus par un modèle physique mais plus couteux en temps de calcul ; le code TURANDOT. Grâce à ce modèle, une étude de sensibilité a été réalisée et a permis de proposer un dimensionnement de la station sol. Ce modèle permet également de générer des séries temporelles afin d’optimiser les codes de correction d’erreur et optimiser le débit (1Terabit/s d'ici 2025). / An optical link based on a multiplex of wavelengths at 1.55µm is foreseen to be a valuable alternative to the conventional radio-frequencies for the feeder link of the next-generation of high throughput geostationary satellite. Considering the limited power of lasers envisioned for feeder links, the beam divergence has to be dramatically reduced. Consequently, the beam pointing becomes a key issue. During its propagation between the ground station and a geostationary satellite, the optical beam is deflected and possibly distorted by atmospheric turbulence. It induces strong fluctuations of the detected telecom signal, thus reducing the capacity. A steering mirror using a measurement from a beam coming from the satellite is used to pre-compensate the deflection. Because of the point-ahead angle between the downlink and the uplink, the turbulence effects experienced by both beams are slightly different, inducing an error in the correction. The performance criteria is the minimum detectable irradiance 95% of the time. A fast model, named WPLOT, taking into account pointing errors and their temporal evolution, is proposed to evaluate the minimum irradiance as a function of the ground station parameters and quality of the correction. The model’s results are compared to those obtained with a more physical but requiring more computation power: TURANDOT. A sensitivity study has been realized and led to a sizing of a ground station. The model also enables the generation of time series in order to optimize the forward error correction codes in order to be compliant with the targeted capacity (1Terabit/s by 2025).

Thesis. 1975. M.S.--Massachusetts Institute of Technology. Dept. of Meteorology. / Bibliography: leaf 94. / M.S.

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<p> The performance of long-range imaging systems often suffers due to the presence of atmospheric turbulence. One way to alleviate the degradation caused by atmospheric turbulence is to apply post-processing mitigation algorithms, where a high-quality frame is reconstructed from a single degraded image or a sequence of degraded frames. The image processing algorithms for atmospheric turbulence mitigation have been studied for decades, yet some critical problems remain open.</p> <p><br></p> <p>This dissertation addresses the problem of image reconstruction through atmospheric turbulence from three unique perspectives: 1) Reconstruction with the presence of moving objects using an improved classical image processing pipeline. 2) A fast simulation scheme for efficiently generating large-scale turbulence-degraded datasets for training deep neural networks. 3) A deep learning-based single-frame reconstruction method using Vision Transformer. </p>

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The performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. Fade statistics for lasercom systems are determined from the probability density function (PDF) of the irradiance fluctuations. The expected number of fades per second and their mean fade time require the joint PDF of the fluctuating irradiance and its time derivative. Theoretical integral expressions, as well as closed form, analytical approximations, were developed for the joint PDF of a gamma-gamma distributed irradiance and its time derivative, and the corresponding expression for the expected number of fades per second. The new approximation for the conditional PDF of the time derivative of a gamma-gamma irradiance is a zero mean Gaussian distribution, with a complicated irradiance depending variance. Fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gamma-gamma distributions. A Gaussian beam wave was propagated through the atmosphere along a horizontal path, near ground, in the moderate-to-strong optical turbulence. To characterize the propagation path, a new method that infers atmospheric propagation parameters was developed. Scintillation theory combined with a numerical scheme was used to infer the structure constant, Cn2, the inner scale and the outer scale from the optical measurements. The inferred parameters were used in calculations for the theoretical PDFs. It was found that fade predictions made by the gamma-gamma and lognormal distributions provide an upper and lower bound, respectively, for the probability of fade and the number of fades per second for irradiance data collected in the moderate-to-strong fluctuation regime. Aperture averaging effects on the PDF of the irradiance fluctuations were investigated by comparing the irradiance distributions for the three receiver apertures at two different values of the structure parameter and, hence, different values of the coherence radius. For the moderate-to-strong fluctuation regime, the gamma-gamma distribution provides a good fit to the irradiance fluctuations collected by finite-sized apertures that are significantly smaller than the coherence radius. For apertures larger than or equal to the coherence radius, the irradiance fluctuations appear to be lognormally distributed.

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