Electrical & Computer Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2765
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Item EXPERIMENTAL CHARACTERIZATION OF ATMOSPHERIC TURBULENCE SUPPORTED BY ADVANCED PHASE SCREEN SIMULATIONS(2020) PAULSON, DANIEL A; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Characterization of optical propagation through the low turbulent atmosphere has been a topic of scientific investigation for decades, and has important engineering applications in the fields of free space optical communications, remote sensing, and directed energy. Traditional theories, starting with early radio science, have flowed down from the assumption of three dimensional statistical symmetry of so-called fully developed, isotropic turbulence. More recent experimental results have demonstrated that anisotropy and irregular frequency domain characteristics are regularly observed near boundaries of the atmosphere, and similar findings have been reported in computational fluid dynamics literature. We have used a multi-aperture transmissometer in field testing to characterize atmospheric transparency, refractive index structure functions, and turbulence anisotropy near atmospheric boundaries. Additionally, we have fielded arrays of resistive temperature detector probes alongside optical propagation paths to provide direct measurements of temperature and refractive index statistics supporting optical turbulence observations. We are backing up these experimental observations with a modified algorithm for modeling optical propagation through atmospheric turbulence. Our new phase screen approach utilizes a randomized spectral sampling algorithm to emulate the turbulence energy spectrum and improve modeling of low frequency fluctuations and improve convergence with theory. We have used the new algorithm to investigate open theoretical topics, such as the behavior of beam statistics in the strong fluctuation regime as functions of anisotropy parameters, and energy spectrum power law behavior. These results have to be leveraged in order to develop new approaches for characterization of atmospheric optical turbulence.Item Forward Scattering Meter for Visibility Measurements(2019) Ferlic, Nathaniel A.; Davis, Christopher C; Van Iersel, Miranda; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atmospheric aerosols, containing water, constitute most of the air during non-ideal weather conditions including fog, haze, and mist. These aerosols cause light to be attenuated while propagating through the atmosphere causing the effective visibility to decrease. The visibility is dependent on the extinction coefficient of the aerosol distribution that can be found using Mie scattering theory. In the case of a real environment a distribution of particle sizes must be considered where the particles present are described by a weighted value relative to the number density. In this thesis a forward scattering meter is devised that measures the amount of scattered light at a specific forward scattering angle under the assumption that the scattered light is linearly related to the extinction coefficient of different weather conditions. To validate the design, it will be compared against a commercial visibility meter along with using a fog chamber to simulate various weather conditions.Item Characterizing Atmospheric Turbulence with Conventional and Plenoptic approaches(2017) Ko, Jonathan; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Atmospheric turbulence is a phenomenon of interest in many scientific fields. The direct effects of atmospheric turbulence can be observed in everyday situations. The twinkling of stars is an indicator of weak atmospheric turbulence while the shimmering of objects above a hot surface is an indicator of strong atmospheric turbulence. The effects of atmospheric turbulence are generally considered a nuisance to optical applications. Image blurring effects are often present when observing distant objects through atmospheric turbulence. Applications that require maintaining the coherence of a laser beam, such as in free space optical communication, suffer from poor link quality in the presence of atmospheric turbulence. Attempts to compensate for the effects of atmospheric turbulence have varied in effectiveness. In astronomical applications, weak cases of atmospheric turbulence have been successfully compensated with the use of a Shack-Hartmann wavefront sensor combined with adaptive optics. Software techniques such as “Lucky Imaging” can be useful when clear images briefly appear through the presence of weak turbulence. However, stronger cases of atmospheric turbulence often found in horizontal or slant paths near the Earth’s surface present a much more challenging situation to counteract. This thesis focuses primarily on the effects of strong or “deep” atmospheric turbulence. The process of compensating for the effects of strong atmospheric turbulence begins with being able to characterize it effectively. A scintillometer measures the scintillation in the intensity of a light source to determine the strength of current turbulence conditions. Thermal fluctuation measurements can also be used to derive the strength of atmospheric turbulence. Experimental results are presented of a developed large aperture scintillometer, thermal probe atmospheric characterization device, and a transmissometer. While these tools are effective in characterizing atmospheric turbulence, they do not provide for a means to correct for turbulence effects. To compensate for the effects of atmospheric turbulence, the development of the Plenoptic Sensor is presented as a wavefront sensor capable of handling strong turbulence conditions. Theoretical and experimental results are presented to demonstrate the performance of the Plenoptic Sensor, specifically in how it leads to adaptive optics algorithms that can rapidly correct for the effects of turbulence.