Optical and RF beam propagation in turbulent media
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Date
2017-01-01
Type of Work
Department
Computer Science and Electrical Engineering
Program
Engineering, Electrical
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This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
Distribution Rights granted to UMBC by the author.
Distribution Rights granted to UMBC by the author.
Abstract
The optical beam spread and the optical beam quality factor in the presence of both an initial quartic phase aberration and atmospheric turbulence is analyzed. We obtain analytical expressions for both the mean-square beam radius and the beam quality factor using the moment method. We compare these expressions to the results from Monte Carlo simulations, which allows us to mutually validate the theory and the Monte Carlo simulation codes. We also discuss the reason for the discrepancy between the moment method and the classical approach for calculating the ensemble-averaged mean-square beam radius in a turbulent atmosphere that is described by Andrews and Phillips and by Fante. We analyze the first and second-order statistical moments of the fluctuating intensity of a propagating laser beam and the probability density function versus intensity as the beam propagates through a turbulent atmosphere. At the end, we compare our analytical expression and our simulations to field test experimental results, and we find good agreement. We simulate the propagation of both a partially spatially coherent infra-red (IR) and a visible laser beam through a turbulent atmosphere, and we compare the intensity fluctuations produced in the simulation to the intensity fluctuations that are observed in both maritime and terrestrial environments at the US Naval Academy. We focus on the effect of the level of turbulence and the degree of the beam's spatial coherence on the receiver scintillations, and we compare the probability density function (PDF) of the intensity in our simulation to the experimental data. We also investigate the effect of optical beam spreading on the coherent and partially coherent laser beams along the propagation path. Finally, we investigate the evolution of a radio frequency (RF) X-band signal as it propagates through the solar corona turbulence in superior solar conjunction at low Sun-Earth-Probe (SEP) angles. Our analysis of the data that was obtained during several MESSENGER (MErcury Surface, Space ENivornment, GEochmeisty, and Ranging) conjunctions reveals a short term and long term effect. Amplitude scintillation is a smaller effect and is evident on a short time scale. Phase scintillations are stronger, but occur over a longer time duration. We examine different possible phenomena in the solar plasma that could be the source of the different time scales of the amplitude and phase scintillations. We propose a theoretical model in which the amplitude scintillations are due to local fluctuations of the index of refraction that scatter the RF signal. These rapidly varying fluctuations randomly attenuate the signal without affecting its phase. By contrast, we propose a model in which phase fluctuations are due to long ducts in the solar plasma, streaming from the sun, that trap some parts of the RF signal. These ducts act as waveguides, changing the phase velocity of the RF beam as it travels a zigzag path inside a duct. When the radiated wave exits from a duct, its phase is changed with respect to the signal that did not pass through the duct, which can lead to destructive interference and carrier suppression. The trapping of the wave is random in nature and can be either a fast or slow process. The predictions of this model are consistent with observations.