A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Laser beam control, combining, and propagation in atmospheric turbulence(2016) Nelson, William; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation is concerned with the control, combining, and propagation of laser beams through a turbulent atmosphere. In the first part we consider adaptive optics: the process of controlling the beam based on information of the current state of the turbulence. If the target is cooperative and provides a coherent return beam, the phase measured near the beam transmitter and adaptive optics can, in principle, correct these fluctuations. However, for many applications, the target is uncooperative. In this case, we show that an incoherent return from the target can be used instead. Using the principle of reciprocity, we derive a novel relation between the field at the target and the scattered field at a detector. We then demonstrate through simulation that an adaptive optics system can utilize this relation to focus a beam through atmospheric turbulence onto a rough surface. In the second part we consider beam combining. To achieve the power levels needed for directed energy applications it is necessary to combine a large number of lasers into a single beam. The large linewidths inherent in high-power fiber and slab lasers cause random phase and intensity fluctuations occurring on sub-nanosecond time scales. We demonstrate that this presents a challenging problem when attempting to phase-lock high-power lasers. Furthermore, we show that even if instruments are developed that can precisely control the phase of high-power lasers; coherent combining is problematic for DE applications. The dephasing effects of atmospheric turbulence typically encountered in DE applications will degrade the coherent properties of the beam before it reaches the target. Finally, we investigate the propagation of Bessel and Airy beams through atmospheric turbulence. It has been proposed that these quasi-non-diffracting beams could be resistant to the effects of atmospheric turbulence. However, we find that atmospheric turbulence disrupts the quasi-non-diffracting nature of Bessel and Airy beams when the transverse coherence length nears the initial aperture diameter or diagonal respectively. The turbulence induced transverse phase distortion limits the effectiveness of Bessel and Airy beams for applications requiring propagation over long distances in the turbulent atmosphere.Item Design and Analysis of Advanced Free Space Optical Communication Systems(2006-04-12) Trisno, Sugianto; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Free space optical (FSO) communication has emerged as a viable technology for broadband wireless applications. FSO technology offers the potential of high bandwidth capacity over unlicensed optical wavelengths. On long-range FSO links, atmospheric turbulence causes intensity fluctuations, which degrades links performance. The performance of an optical link can be improved by the use of a time delayed diversity technique, which takes advantage of the fact that the atmospheric path from transmitter to receiver is statistically independent for time intervals beyond the coherence time of the intensity fluctuations. Communications performance is improved because the joint probability of error is less than the probability of error from individual channels. Theoretical analysis and experimental investigation were conducted to assess and characterize the performance of a time delayed diversity FSO system. Two experiments were conducted: inside our laboratory under simulated convective turbulence and inter-building in clear atmospheric turbulence. In both cases, time delayed diversity system is shown to offer a notable performance improvement compared to a non-diversity FSO system, where the signal-to-noise ratio (SNR) performance can gain up to 4.7 dB and the bit error rate (BER) performance is doubled. These experimental studies confirm the effectiveness of a time delayed diversity technique to mitigate turbulence induced fading, and its optimality in a dual diversity scheme. This is the first published report of theoretical and experimental performance characteristics of FSO communication system utilizing time delayed diversity technique. FSO technology has also emerged as a key technology for the development of rapidly deployable and secure communication and surveillance networks. In networking applications, broadcasting capability is frequently required to establish and maintain inter-node communications. One approach to deal with the broadcasting issue in FSO networking is the use of omnidirectional FSO links, which is based on non-directed line-of-sight (LOS) technique. Prototype omnidirectional FSO transceiver had been constructed and their performance investigated. Although omnidirectional FSO links cannot provide the performance of directional ones, the results suggest that they could be used in sensor networks or as alternative for traditional wireless networks, when the use of radio frequency (RF) technology is prohibited.Item Techniques to Mitigate the Effects of Atmospheric Turbulence on Free Space Optical Communication Links(2004-10-15) Wasiczko, Linda Marie; Davis, Christopher C; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Free space optical communication links are an attractive technology for broadband communications when fiber optic links are unavailable or simply not feasible. Atmospheric turbulence, aerosols, and molecular absorption all affect the propagation of optical waves in the atmosphere. Since atmospheric turbulence is the major source of errors on free space optical communication links, this dissertation investigates two techniques to reduce the impact of atmospheric turbulence on such links. These two techniques are aperture averaging and the incorporation of nonimaging optical elements into optical receiver systems. Aperture averaging is the process by which atmospheric turbulence-induced intensity fluctuations are averaged across a receiver aperture of sufficient size. We investigate the behavior of aperture averaging in weak and strong turbulence conditions by comparing experimental data with available models for plane and spherical wave propagation. New expressions for the aperture averaging factor in weak turbulence are given. In strong turbulence conditions, aperture averaging is analyzed with special attention to the various wavenumber spectrum models. This is the first report of experimental strong fluctuation aperture averaging data acquired in non-saturated conditions. Nonimaging optical elements are particularly useful for the mitigation of atmospheric turbulence-induced beam wander in the focal plane of a free space optical communication receiver. Experimental results of the bit error ratio enhancement due to the incorporation of a nonimaging optical element, specifically a compound parabolic concentrator, are presented. Two link ranges were tested, a 1.7 km link at the University of Maryland experiencing weak turbulence, and a 32.4 km link at the Naval Research Laboratory's Chesapeake Bay Detachment experiencing saturated, strong turbulence. These results are the first reported experimental test of a nonimaging optical element integrated into an outdoor free space optical communications system.