Antenna arrays offer notable superiority over a single antenna element. By weighting the signals before combining them, antenna arrays offer several enhanced features such as beam steering or beam switching without physically moving the aperture. While in general the array platform can take any given shape, cylindrical arrays offer many advantages compared to linear and planar arrays due to their azimuthal symmetry, and as such find numerous applications in radar, sonar, etc.. This research proposes a Wi-Fi band cylindrical switched beam array that is capable of full azimuthal beam switching for direction finding applications. Six microstrip patch antennas are arranged in a hexagonal platform to create a cylindrical array. The antenna beam is switched electronically using a SP6T RF switch providing azimuthal coverage with 60-degree resolution. Multiple antennas and arrays were designed, analyzed, and fabricated. The devices and the full array system were measured, and their RF performance was characterized. The experimental results validate the feasibility and practicality of the proposed design, and demonstrate a high-speed array platform for direction finding that can be used for search and rescue operations in emergency situations.

Understanding, predicting, and controlling electromagnetic field interactions on and between complex RF platforms requires high fidelity computational electromagnetic (CEM) simulation. The primary CEM tool within NASA is GEMINI, an integral equation based method-of-moments (MoM) code for frequency domain electromagnetic modeling. However, GEMINI is currently limited in the size and complexity of problems that can be effectively handled. To extend GEMINI’S CEM capabilities beyond those currently available, primary research is devoted to integrating the MFDlib library developed at the University of Kentucky with GEMINI for efficient filling, factorization, and solution of large electromagnetic problems formulated using integral equation methods. A secondary research project involves the hybrid parallelization of GEMINI for the efficient speedup of the impedance matrix filling process. This thesis discusses the research, development, and testing of the secondary research project on the High Performance Computing DLX Linux supercomputer cluster. Initial testing of GEMINI’s existing MPI parallelization establishes the benchmark for speedup and reveals performance issues subsequently solved by the NASA CEM Lab. Implementation of hybrid parallelization incorporates GEMINI’s existing course level MPI parallelization with Open MP fine level parallel threading. Simple and nested Open MP threading are compared. Final testing documents the improvements realized by hybrid parallelization.

<p>In this study we propose a fast hybrid spectral-element time-domain (SETD) / finite-element time-domain (FETD) method for transient analysis of multiscale electromagnetic problems, where electrically fine structures with details much smaller than a typical wavelength and electrically coarse structures comparable to or larger than a typical wavelength coexist.</p><p>Simulations of multiscale electromagnetic problems, such as electromagnetic interference (EMI), electromagnetic compatibility (EMC), and electronic packaging, can be very challenging for conventional numerical methods. In terms of spatial discretization, conventional methods use a single mesh for the whole structure, thus a high discretization density required to capture the geometric characteristics of electrically fine structures will inevitably lead to a large number of wasted unknowns in the electrically coarse parts. This issue will become especially severe for orthogonal grids used by the popular finite-difference time-domain (FDTD) method. In terms of temporal integration, dense meshes in electrically fine domains will make the time step size extremely small for numerical methods with explicit time-stepping schemes. Implicit schemes can surpass stability criterion limited by the Courant-Friedrichs-Levy (CFL) condition. However, due to the large system matrices generated by conventional methods, it is almost impossible to employ implicit schemes to the whole structure for time-stepping.</p><p>To address these challenges, we propose an efficient hybrid SETD/FETD method for transient electromagnetic simulations by taking advantages of the strengths of these two methods while avoiding their weaknesses in multiscale problems. More specifically, a multiscale structure is divided into several subdomains based on the electrical size of each part, and a hybrid spectral-element / finite-element scheme is proposed for spatial discretization. The hexahedron-based spectral elements with higher interpolation degrees are efficient in modeling electrically coarse structures, and the tetrahedron-based finite elements with lower interpolation degrees are flexible in discretizing electrically fine structures with complex shapes. A non-spurious finite element method (FEM) as well as a non-spurious spectral element method (SEM) is proposed to make the hybrid SEM/FEM discretization work. For time integration we employ hybrid implicit / explicit (IMEX) time-stepping schemes, where explicit schemes are used for electrically coarse subdomains discretized by coarse spectral element meshes, and implicit schemes are used to overcome the CFL limit for electrically fine subdomains discretized by dense finite element meshes. Numerical examples show that the proposed hybrid SETD/FETD method is free of spurious modes, is flexible in discretizing sophisticated structure, and is more efficient than conventional methods for multiscale electromagnetic simulations.</p> / Dissertation

Many systems designed by electrical & computer engineers rely on electromagnetic (EM) signals to transmit, receive, and extract either information or energy. In many cases, these systems are large and complex. Their accurate, cost-effective design requires high-fidelity computer modeling of the underlying EM field/material interaction problem in order to find a design with acceptable system performance. This modeling is accomplished by projecting the governing Maxwell equations onto finite dimensional subspaces, which results in a large matrix equation representation (Zx = b) of the EM problem. In the case of integral equation-based formulations of EM problems, the M-by-N system matrix, Z, is generally dense. For this reason, when treating large problems, it is necessary to use compression methods to store and manipulate Z. One such sparse representation is provided by so-called H^2 matrices. At low-to-moderate frequencies, H^2 matrices provide a controllably accurate data-sparse representation of Z. The scale at which problems in EM are considered ``large'' is continuously being redefined to be larger. This growth of problem scale is not only happening in EM, but respectively across all other sub-fields of computational science as well. The pursuit of increasingly large problems is unwavering in all these sub-fields, and this drive has long outpaced the rate of advancements in processing and storage capabilities in computing. This has caused computational science communities to now face the computational limitations of standard linear algebraic methods that have been relied upon for decades to run quickly and efficiently on modern computing hardware. This common set of algorithms can only produce reliable results quickly and efficiently for small to mid-sized matrices that fit into the memory of the host computer. Therefore, the drive to pursue larger problems has even began to outpace the reasonable capabilities of these common numerical algorithms; the deterministic numerical linear algebra algorithms that have gotten matrix computation this far have proven to be inadequate for many problems of current interest. This has computational science communities focusing on improvements in their mathematical and software approaches in order to push further advancement. Randomized numerical linear algebra (RandNLA) is an emerging area that both academia and industry believe to be strong candidates to assist in overcoming the limitations faced when solving massive and computationally expensive problems. This thesis presents results of recent work that uses a random sampling method (RSM) to implement algebraic operations involving multiple H^2 matrices. Significantly, this work is done in a manner that is non-invasive to an existing H^2 code base for filling and factoring H^2 matrices. The work presented thus expands the existing code's capabilities with minimal impact on existing (and well-tested) applications. In addition to this work with randomized H^2 algebra, improvements in sparse factorization methods for the compressed H^2 data structure will also be presented. The reported developments in filling and factoring H^2 data structures assist in, and allow for, the further pursuit of large and complex problems in computational EM (CEM) within simulation code bases that utilize the H^2 data structure.

No description available.

<p>The main contributions of this thesis include two points: firstly, we originally establish Complex STM to semi-analytically calculate the mode profiles of multi-layer planar waveguide terminated with both PML and PRB ; secondly, although CMMM has been generally applied to the simulation of waveguide facets, Bragg gratings, etc[52-53], we for the first time demonstrate that CMMM can also be utilized for the modeling of couplings of radiation field outgoing perpendicularly to the waveguide axis with an incident wave launched in the examples of high-index-contrast waveguide crossings and corners. CMMM is proved to be able to estimate the field profiles and power flows accurately through the validation with FDTD.</p> / <p>Optical waveguides are basic building blocks of high-density photonic integrated circuits and play crucial roles in optical access networks, biomedical system, sensors and so on. Various kinds of dielectric waveguides apply the total internal reflection condition to transmit optical field [9] and even more complicated structures based on waveguide interconnects, Bragg grating, photonic crystals are actively developed by corporations and academic institutes. Especially, the fast developing pace of Metal-Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE) and other fabrication techniques has predicted the increasing complication and thus more advanced function of modern optics integrated circuits. Under such circumstances, convenient and accurate modeling and simulation schemes are necessary for the exploration, designing and optimization of photonic devices, systems and networks before the time-consuming and expensive fabrication process.</p> <p>The thesis summarizes several frequency-domain modeling schemes for the calculation of mode profile or beam propagation in 2D dielectric waveguide. The thesis mainly covers conventional Smooth Transition Method (STM), High Order Finite Difference (HOFD) scheme, Complex STM, and Complex Mode Matching Method (CMMM) based on the 2D waveguide model terminated with Perfect Matching Layer (PML) and Perfect Reflection Boundary (PRB). The mode spectrums and modal patterns obtained from Complex STM are compared with those of HOFD, and the simulation of waveguide crossings and corners with CMMM is validated with Finite-Difference-Time-Domain (FDTD) Method.</p> <p><strong> </strong></p> / Master of Applied Science (MASc)

<p>Semiconductor laser diode has been a popular research topic for longer than half a century and plays a crucial role in optical communication systems. The work in this thesis focuses on the development of the semiconductor laser diode with rapid-evolving nanotechnologies: by incorporating specific semiconductor or metal structures in the nanometer scale into the laser cavity, several key advantages are achieved.</p> <p>One category of the nano-materials is semiconductor quantum dots (QD). QD laser is a promising product by providing three-dimensional confinement to the injected electrons and holes. However, in order to realize the single-longitudinal-mode operation, which is critical to optical communications in purpose of reducing the dispersion and partition noise, the Fabry-Perot (FP) QD laser still needs further development to suppress the gain-broadening effects; otherwise the mode-selective structure must be adopted, such as the distributed feedback (DFB) cavity. In this thesis, the QD FP laser and QD DFB laser are both researched by advanced modelling techniques and the work is summarized as follows.</p> <p>1) For the QD FP laser, a comprehensive rate-equation model has been applied for simulation, with the emphasis on describing the interplay of inhomogeneous and homogeneous gain-broadening effects. According to the laser-behaviour simulations, it is found that for each given inhomogeneous broadening, the optimum homogeneous broadening can be obtained for the single longitudinal-mode selectivity. Based on the optimal gain-broadening parameters, the single-mode QD FP laser is designed and analysed. The quantitative conditions for the performance feasibility are examined with respect to the gain-broadening parameters.</p> <p>2) A one-dimensional (1D) standing wave model is developed for the QD DFB laser. This model can provide more information for the laser operation and better describe the dynamic behaviour compared with the rate-equation model. Based on it, the statistic operation and output spectrum of a typical QD DFB laser are simulated; and then the dynamic properties of the laser are analysed.</p> <p>The other category is the metal nano-structure, including the metal nano-particle and the metal nano-strip Bragg grating. The related work is summarized as follows.</p> <p>1) The optical properties of a single metal nano-particle with different size, composition and shape are researched by Mie theory, with respect to the localized surface plasmon polariton (LSPP) effect. It shows that both the resonance wavelength and Q-factor can be tuned in a large scale by proper methods.</p> <p>2) A novel metal nano-strip distributed Bragg grating (DBR) laser is proposed and investigated theoretically. Firstly the metal nano-strip Bragg grating is simulated by the couple-mode theory and the mode-matching method. It shows that the coupling constant and reflection spectrum can be tuned to meet different requirements when varying the grating parameters. Then for the designed metal-grating DBR laser, the rate-equation simulation results show that it works under the single-mode operation for a broad range of the design parameters.<br /> <strong></strong></p> / Doctor of Philosophy (PhD)

<p>This thesis explores the ablation and texturing of synthetic single crystal diamond under ultrashort laser pulse irradiation in rough vacuum ambient conditions. Experiments were performed with a 1 kHz repetition rate titanium sapphire laser system producing pulses at 800 nm central wavelength and 150 fs pulse duration. First, the production of ordered periodic nanostructures (ripples) on the diamond surface was characterized and the periodicity of these structures was analyzed against irradiation parameters. Ripples were usually found to fall into one of two categories: one with a spatial period near the irradiation wavelength and one with a spatial period near λ/2n, where λ is the central laser wavelength in air and n is the refractive index of diamond at λ. The spatial periods were plotted against irradiation conditions including the peak fluence, number of incident pulses, and the angle of incidence. Unique outcomes arising out of complex irradiation conditions were also explored. Second, the ablation thresholds for single pulse up to 1000 pulse irradiation were determined using the <em>D</em>² method at both the fundamental central wavelength (800 nm) and the second harmonic (400 nm). The single 800 nm pulse ablation threshold for synthetic single crystal diamond was determined to be 2.3 J/cm² with an incubation coefficient of 0.54. The single 400 nm pulse ablation threshold was determined to be 1.5 J/cm² with an incubation coefficient of 0.73.</p> / Master of Applied Science (MASc)

<p>A glass-clad, slow-light photonic-crystal waveguide is proposed as a solution to sub-bandgap light detection in silicon photonic circuits. Such detection in silicon is perceived as a challenge owing to silicon's indirect band gap and transparency to 1550nm wavelengths, yet is essential for achieving low-cost, high-yield integration with today's microelectronics industry. Photonic crystals can be engineered in such a way as to enhance light-matter interaction over a specific bandwidth via the reduction of the group velocity of the propagating wave (i.e. the slowing of light). The interaction enhanced for light detection in the present work is electron-hole pair generation at defect sites. The intrinsic electric field of a p-i-n junction enables light detection by separating the electron-hole pairs as a form of measurable current. The photonic-crystal waveguides are designed to have bandwidths in the proximity of a wavelength of 1550nm. Refractive indices of over 80 near the photonic-crystal waveguide's Brillouin zone boundary are measured using Fourier transform spectral interferometry and are found to correspond to numerical simulations. Defect-induced propagation loss was seen to scale with group index, from 400dB/cm at a group index of 8 to 1200dB/cm at a group index of 88. Scaling was sublinear, which is believed to be due to the spreading of modal volume at large group index values. Photodetectors were measured to have responsivities as high as 34mA/W near the photonic-crystal waveguide's Brillouin zone boundary for a reverse bias of 20V and a remarkably short detector length of 80um. The fabrication of each device is fully CMOS-compatible for the sake of cost-effective integration with silicon microelectronics.</p> / Master of Applied Science (MASc)

This dissertation presents techniques for high-order simulation of electromagnetic fields, particularly for problems involving ships with ferromagnetic hulls and active corrosion-protection systems. A set of numerically constrained hexahedral basis functions for volume integral equation discretization is presented in a method-of-moments context. Test simulations demonstrate the accuracy achievable with these functions as well as the improvement brought about in system conditioning when compared to other basis sets. A general method for converting between a locally-corrected Nyström discretization of an integral equation and a method-of-moments discretization is presented next. Several problems involving conducting and magnetic-conducting materials are solved to verify the accuracy of the method and to illustrate both the reduction in number of unknowns and the effect of the numerically constrained bases on the conditioning of the converted matrix. Finally, a surface integral equation derived from Laplace’s equation is discretized using the locally-corrected Nyström method in order to calculate the electric fields created by impressed-current corrosion protection systems. An iterative technique is presented for handling nonlinear boundary conditions. In addition we examine different approaches for calculating the magnetic field radiated by the corrosion protection system. Numerical tests show the accuracy achievable by higher-order discretizations, validate the iterative technique presented. Various methods for magnetic field calculation are also applied to basic test cases.