Browsing by Author "Lu, Liang-Ju"

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    Analysis and design of broadband single-mode multi-clad fibers
    Lu, Liang-Ju (Virginia Polytechnic Institute and State University, 1989)
    ln the last several years, considerable attention has been paid to the study of dispersion-flattened single-mode fibers which offer a high transmission capacity with low losses through a wide range of wavelengths. However, the existing designs are sensitive to bending and manufacturing tolerances, and are not truly single-mode at most wavelengths of interest. To remedy these problems a new series of broadband dispersion-flattened truly single-mode fiber designs are proposed. These fibers have both dispersion-shifted and dispersion-flattened features with low splice and bend losses. Results demonstrating a total dispersion of ±0.97 ps/km-nm over the entire spectral range between 1.31 μm to 1.66 μm are presented. Such dispersion-flattening is achieved while simultaneously maintaining a mode-field radius of 3 μm to 5 μm in the dispersion-flattened wavelength range. The most significant achievement is that the proposed muIti-clad fiber design is strictly single-mode and splice and bend losses are smaller than those of double-clad, triple-clad, and quadruple-clad fibers with the same value of dispersion. Ultralow dispersion fibers, whose chromatic dispersion and the first and second-order derivatives of the chromatic dispersion are zero at 1.5 μm or 1.55 μm, are described. This effectively increases the laser emission tolerance. Ultralow dispersion fibers open the way to wavelength multiplexing with currently available inexpensive multifrequency lasers, either in local or long distance networks. These fibers also have low splice and bend losses compared to double-clad, triple-clad, and quadruple-cIad fibers. An inverse waveguide synthesis program, which can trace multiple objective functions and optimize multiple parameters simultaneously, is developed. An objective function is applied, for the first time, to optimize the dispersion-flattened single-mode fiber index profile with respect to: (1) minimum dispersion, (2) the wavelengths of zero-dispersion, (3) maximum width of dispersion-flattened window, (4) maximum layer index difference less than 0.8%, and (5) layer thickness larger than 3.5 μm. The accuracy of chromatic dispersion calculations in dispersion-flattened fibers is evaluated. lt has been shown that the accuracy of approximate methods is influenced not only by the index differences, but also by their derivatives with respect to wavelength. The matrix method and direct numerical integration of the wave equation are used to compute the mode propagation constants, cutoff frequencies, field distributions, mode-field radius, and splice loss, and carry out production tolerance analysis for multi-clad step-index fibers and graded-index fibers, respectively. Detailed analysis and optimized fiber data are presented.
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    HESCAD - an interface between HESCOMP and CADAM for the generation of helicopter models
    Lu, Liang-Ju (Virginia Polytechnic Institute and State University, 1985)
    3-D Interactive CADAM allows for easier construction, modification, analysis, and display of 3-D geometry surfaces and wire-frames. This research forms a basis for preliminary aircraft geometric design using the CADAM system. The helicopter design program, HESCOMP, originally a batch mode program, was coupled with CADAM via the CADAM data base such that the analysis, design, and redesign of the helicopter geometry and interior equipment geometry can be accomplished interactively. HESCAD, a program which produces the helicopter preliminary design model and enables the interior equipment design process, is developed. It provides a capability to evolve rapidly and refine helicopter configurations generated automatically using output from HESCOMP or interior equipment design by graphically and numerically defining helicopter components through interactive, on line, computer graphic display devices. Helicopter 3-D wireframes are automatically produced for any HESCOMP helicopter geometry output. A method which directs CADAM to analyze the helicopter components and produce weights, centers of gravity, moments and products of inertia and to review the results of the analyses directly on the screen is provided. This research was sponsored by IBM Corporation Federal Systems Division under contract No. 417503-DE. This thesis describes and illustrates the HESCAD program. Detailed graphical results are also presented.