Time-Domain Electromagnetic Wave Propagation in Confined Environments

International audienceConfined environments like tunnels are electrically large structures for guided wave propagation. They can have arbitrary cross sections, and the design and optimization of antenna for communication system requires the knowledge of a "full-wave" solution in nearby zones. Current models based on asymptotic approaches do not describe adequately the wave propagation under the above conditions. In addition, a complete "full-wave" analysis of the tunnel propagation performances is not feasible in terms of computer expenditure. After a survey of the most commonly used techniques for propagation in tunnels, some investigation regarding an appropriate approach to find the fields is proposed. It is based on a modal decomposition of the wave propagation that allows an optimization of the coupling with the antenna. To find the mode characteristic for arbitrary cross section, a full-wave method, namely, the transmission-line matrix (TLM), is modified to a so-called 2.5-dimensional TLM algorithm and presented in details. This approach is validated for a canonical structure. Then, it is applied to study the wave propagation in a realistic rectangular tunnel. The concept of surface impedance boundary condition (SIBC) is introduced to reduce the TLM computational domain and model the tunnel walls that can be considered as lossy dielectric. Results show that guided structures with lossy dielectric walls of arbitrary cross section can be studied with this approach

Designing coupled-resonator optical waveguides based on high-Q tapered grating-defect resonators

We present a systematic design of coupled-resonator optical waveguides (CROWs) based on high-Q tapered grating-defect resonators. The formalism is based on coupled-mode theory where forward and backward waveguide modes are coupled by the grating. Although applied to strong gratings (periodic air holes in single-mode silicon-on-insulator waveguides), coupled-mode theory is shown to be valid, since the spatial Fourier transform of the resonant mode is engineered to minimize the coupling to radiation modes and thus the propagation loss. We demonstrate the numerical characterization of strong gratings, the design of high-Q tapered grating-defect resonators (Q>2 × 10^6, modal volume = 0.38•(λ/n)^3), and the control of inter-resonator coupling for CROWs. Furthermore, we design Butterworth and Bessel filters by tailoring the numbers of holes between adjacent defects. We show with numerical simulation that Butterworth CROWs are more tolerant against fabrication disorder than CROWs with uniform coupling coefficient

Time-Domain Macromodeling of High Speed Distributed Networks

With the rapid growth in density, operating speeds and complexity of modern very-large-scale integration (VLSI) circuits, there is a growing demand on efficient and accurate modeling and simulation of high speed interconnects and packages in order to ensure the signal integrity, reliability and performance of electronic systems. Such models can be derived from the knowledge of the physical characteristics of the structure or based on the measured port-to-port response.In the first part of this thesis, a passive macromodeling technique based on Method of Characteristics (referred as Passive Method of Characteristics or PMoC) is described which is applicable for modeling of electrically long high-speed interconnect networks. This algorithm is based on extracting the propagation delay of the interconnect followed by a low order rational approximation to capture the attenuation effects. The key advantage of the algorithm is that the curve fitting to realize the macromodel depends only on per-unit-length (p.u.l.) parameters and not on the length of the transmission line. In this work, the PMoC is developed to model multiconductor transmission lines.Next, an efficient approach for time domain sensitivity analysis of lossy high speed interconnects in the presence of nonlinear terminations is presented based on PMoC. An important feature of the proposed method is that the sensitivities are obtained from the solution of the original network, leading to significant computational advantages. The sensitivity analysis is also used to optimize the physical parameters of the network to satisfy the required design constraints. A time-domain macromodel for lossy multiconductor transmission lines exposed to electromag¬netic interference is also described in this thesis based on PMoC. The algorithm provides an efficient mechanism to ensure the passivity of the macromodel for different line lengths. Numerical examples illustrate that when compared to other passive incident field coupling algorithms, the proposed method is efficient in modeling electrically long interconnects since delay extraction without segmentation is used to capture the frequency response.In addition, this thesis discusses macromodeling techniques for complex packaging structures based on the frequency-domain behavior of the system obtained from measurements or electromagnetic simulators. Such techniques approximate the transfer function of the interconnect network as a rational function which can be embedded with modern circuit simulators with integrated circuit emphasis (SPICE). One of the most popular tools for rational approximations of measured or simulated data is based on vector fitting (VF) algorithms. Nonetheless, the vector fitting algorithms usually suffer convergence issues and lack of accuracy when dealing with noisy measured data. As a part of this thesis, a methodology is presented to improve the convergence and accuracy issues of vector fitting algorithm based on instrumental variable technique. This methodology is based on obtaining the “instruments” in an iterative manner and do not increase the complexity of vector fitting to capture the frequency response and minimize the biasing

Plasmonic-organic hybrid electro/optic Mach-Zehnder modulators: from waveguide to multiphysics modal-FDTD modeling

Plasmonic organic hybrid electro/optic modulators are among the most innovative light modulators fully compatible with the silicon photonics platform. In this context, modeling is instrumental to both computer-aided optimization and interpretation of experimental data. Due to the large computational resources required, modeling is usually limited to waveguide simulations. The first aim of this work to investigate an improved, physics-based description of the voltage-dependent electro/optic effect, leading to a multiphysics-augmented model of the modulator cross-section. Targeting the accuracy of full-wave, 3D modeling with moderate computational resources, the paper presents a novel mixed modal-FDTD simulation strategy that allows us to drastically reduce the number and complexity of 3D-FDTD simulations needed to accurately evaluate the modulator response. This framework is demonstrated on a device inspired by the literature