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This research began as a continuation of work on the propagation of planar electromagnetic (EM) waves through a turbulent atmosphere, specifically a form of refractive index based phase turbulence modeled by the Modified von Karman Spectrum (MVKS). In the previous work within our group, EM propagation through a turbulent atmosphere under the MVKS model was investigated for essentially isoplanatic propagation, whereby the propagation from the source to the receiver progressed along a horizontal path, such that the effective structure parameter associated with the turbulence remained unchanged along the propagation. The problem was numerically set up by using the split-step propagation model, whereby the EM wave from the source (sometimes interpreted as a planar aperture) propagates alternately through non-turbulent regions (governed by standard Fresnel-Kirchhoff diffraction), and thereafter through MVKS regions where the phase turbulence occurs. A narrowly turbulent layer is described by a random 2D phase screen in the transverse plane; extended turbulence is modeled by a series of planar phase screens placed along the propagation path. In the above analyses, propagation of both uniform as well as profiled plane waves was considered. The present research commenced with investigating uniform, Gaussian and Bessel beam propagation along a turbulent path, and detailed numerical simulations were carried out relative to infinite as well as finite apertures in the source plane (including single and double slits, and single and double circular apertures), considering both non-turbulent and turbulent paths for comparison. Results were obtained in the near, far and deep far fields. The problem was further developed to include the case of anisoplanatic plane EM wave propagation over a slanted path. The turbulence structure function (Cn2) in this environment was considered to be altitude dependent, and for this purpose the Hufnagel-Valley (HV) model for the structure function. A standard prototype tested for this system consisted of propagation along a slanted path with a fixed horizontal distance, and made up along the propagation path of a diffractive (LD) and a turbulent (LT) section. The effect of turbulence was examined for test 2D images/transparencies under two environments: (a) the 2D image, under digital encoding converted to time signals, being used to modulate a carrier (typically optical) wave, which is thereafter propagated across the LD+LT path, and recovered in the "image" plane using heterodyne-type communications strategies. Of special note here is the fact that since MVKS and most other turbulence models are intrinsically spatial in nature, a method has been developed within the group whereby the time-statistics of the turbulence is derived from received intensities (typically on-axis) as the phase screen(s) is/are varied at a specific rate corresponding to the average turbulence frequency (in the range 20 Hz-200 Hz). Using this statistical information, the modulated wave propagation across the turbulence is examined; and (b) the source image/transparency is treated as a spatial amplitude distribution through which an unmodulated carrier wave (in the phasor domain) is propagated, and later the object transparency is recovered via a positive thin lens in its back focal plane (assuming thereby that the object transparency is essentially located at infinity relative to the lens). Of the two strategies, it was found that the carrier modulation method yielded better image cross-correlation (CC) products than the method using the thin lens, in the presence of turbulence. Overall, it is seen that recovered EM signals (2D object transparencies, modulated plane waves, and also dynamic/video scenes) are adversely affected by MVKS turbulence (which incidentally is limited in its applicability to only cases where in the Rytov criterion is satisfied, and therefore in many cases works for only weak to moderate levels of turbulence; some cases involving strong turbulence have been investigated nevertheless), and the degree of drop in the CC product goes up as the strength of the turbulence increases. In view of this, a strategy was adopted later whereby the goal was to ascertain if by "packaging" the incident signal/digitized image inside a chaotic carrier, and thereafter propagating the encrypted chaos wave across the turbulent path might help mitigate the loss of CC product (leading to image distortion) during propagation through (MVKS) turbulence. This concept has thereafter been tested for several 2D image scenarios, using an acousto-optically generated chaotic carrier for the encryption prior to turbulent propagation. The corresponding recovered signals (obtained via two levels of demodulation) consistently indicate improvements in the CC products of the recovered images relative to the source. Additionally, the MVKS turbulent system used along the slanted path is also examined under an interchanging of the source and receiver positions. Following extended examinations of the altitude-dependent propagation along an MVKS turbulent path, this work next focused on an alternative turbulence model, viz., the gamma-gamma turbulence (also refractive index) model, which it turns out actually is valid for all atmospheric turbulence (weak through strong) conditions. The use of the HV model for Cn2 assumes, however, that much of the turbulence considered is within a relatively low-altitude limit. For the gamma-gamma problem as well, applications similar to those used for the MVKS cases (i.e., propagation of modulated EM carriers with message signals transmitted along a turbulent (LT) path) using the gamma-gamma time statistics. This problem was analyzed via numerical simulation for both non-chaotic and chaotic carriers. Once again, use of a chaotic carrier is consistently found to improve the bit error rates (BERs) of the recovered image relative to the source image.
Since publication of the first edition of this text in 1998, there have been several new, important developments in the theory of beam wave propagation through a random medium, which have been incorporated into this second edition. Also new to this edition are models for the scintillation index under moderate-to-strong irradiance fluctuations; models for aperture averaging based on ABCD ray matrices; beam wander and its effects on scintillation; theory of partial coherence of the source; models of rough targets for ladar applications; phase fluctuations; analysis of other beam shapes; plus expanded analysis of free-space optical communication systems and imaging systems.
This book provides an in-depth understanding of free space optical (FSO) communication with a particular emphasis on optical beam propagation through atmospheric turbulence. The book is structured in such a way that it provides a basic framework for the beginners and also gives a concise description from a designer’s perspective. The book provides an exposure to FSO technology, fundamental limitations, design methodologies, system trade-offs, acquisition, tracking and pointing (ATP) techniques and link-feasibility analysis. The contents of this book will be of interest to professionals and researchers alike. The book may also be used as a textbook for engineering coursework and professional training.
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"Written in the full heat of the Great Depression, Jack Conroy'sA World to Winbears the marks of the labor struggles and union strikes he witnessed in the early 1930s. Like Dickens, Conroy evokes compassion and warmth for his absurd, comic, tragic characters through caricature, using parody to extract humor from their gray, circumscribed lives.Set in St. Louis, the narrative centers on Leo and Robert Hurley, two half brothers who are divided by education and aspirations. Leo is an unlikely proletarian hero who finally gains political consciousness in spite of himself. Robert has literary pretensions (but little talent) and a head clogged with scraps of genteel romance and Victorian poetry. As Leo and Robert grope toward reconciliation, they come into contact with hybrids of artistic milieus, radical politics, and labor activism that were particular to the Popular Front era.The introduction by Douglas Wixson shows how Conroy's writing is embedded in his experiences and also how his flawless ear for language shapes both form and character in the novel. Moving readers from a sentimentalized concern for the poor to a more concrete contemplation of the social and political conditions that characterize their lives,A World to Winserves as a reminder of the continuing importance of a dedication, like Conroy's, to giving voice to the voiceless."
Blind deconvolution is a classical image processing problem which has been investigated by a large number of researchers over the last four decades. The purpose of this monograph is not to propose yet another method for blind image restoration. Rather the basic issue of deconvolvability has been explored from a theoretical view point. Some authors claim very good results while quite a few claim that blind restoration does not work. The authors clearly detail when such methods are expected to work and when they will not. In order to avoid the assumptions needed for convergence analysis in the Fourier domain, the authors use a general method of convergence analysis used for alternate minimization based on three point and four point properties of the points in the image space. The authors prove that all points in the image space satisfy the three point property and also derive the conditions under which four point property is satisfied. This provides the conditions under which alternate minimization for blind deconvolution converges with a quadratic prior. Since the convergence properties depend on the chosen priors, one should design priors that avoid trivial solutions. Hence, a sparsity based solution is also provided for blind deconvolution, by using image priors having a cost that increases with the amount of blur, which is another way to prevent trivial solutions in joint estimation. This book will be a highly useful resource to the researchers and academicians in the specific area of blind deconvolution.
This book examines the University of Dayton Cameroon Immersion program, with comparative evidence from the Peace Corps. It analyzes different aspects of experiential learning, contextualizes them within the broader issues of modern American and African history, and offers important conclusions for an understanding of the benefits of study abroad.
In recent years, photonics has found increasing applications in such areas as communications, signal processing, computing, sensing, display, printing, and energy transport. Now, Fundamentals of Photonics is the first self-contained introductory-level textbook to offer a thorough survey of this rapidly expanding area of engineering and applied physics. Featuring a logical blend of theory and applications, coverage includes detailed accounts of the primary theories of light, including ray optics, wave optics, electromagnetic optics, and photon optics, as well as the interaction of light with matter, and the theory of semiconductor materials and their optical properties. Presented at increasing levels of complexity, these sections serve as building blocks for the treatment of more advanced topics, such as Fourier optics and holography, guidedwave and fiber optics, photon sources and detectors, electro-optic and acousto-optic devices, nonlinear optical devices, fiber-optic communications, and photonic switching and computing. Included are such vital topics as: Generation of coherent light by lasers, and incoherent light by luminescence sources such as light-emitting diodes Transmission of light through optical components (lenses, apertures, and imaging systems), waveguides, and fibers Modulation, switching, and scanning of light through the use of electrically, acoustically, and optically controlled devices Amplification and frequency conversion of light by the use of wave interactions in nonlinear materials Detection of light by means of semiconductor photodetectors Each chapter contains summaries, highlighted equations, problem sets and exercises, and selected reading lists. Examples of real systems are included to emphasize the concepts governing applications of current interest, and appendices summarize the properties of one- and two-dimensional Fourier transforms, linear-systems theory, and modes of linear systems. An Instructor's Manual presenting detailed solutions to all the problems in the book is available from the Wiley editorial department.