S.T. Peng

Analysis and Design of Grating Couplers

Based on an accurate perturbation analysis of the guiding properties of dielectric gratings, simple design criteria are developed for grating couplers which transfer the energy of a beam into or out of an optical waveguide. Gratings having arbitrary groove shapes are considered and explicit formulae are given for the leakage parameters of gratings with symmetric profiles. The results cover TEv and TMv modes and they apply to both shallow and deep grating grooves. The variation of the leakage parameter α in rectangular gratings is examined in detail; these rectangular gratings are then used as basic configurations for predicting the characteristics of other grating profiles. Particular attention is given to trapezoidal and triangular profiles and gratings with asymmetric profiles are also discussed.

Transmission characteristics of finite-width conductor-backed coplanar waveguide

This paper presents theoretical and experimental results for a finite-width conductor-backed coplanar waveguide (FW-CBCPW). The guiding characteristics of FW-CBCPW are investigated first by the rigorous method of mode matching. An FW-CBCPW through line is then placed within a test fixture commonly used in laboratories, and the scattering parameters of the through line are obtained theoretically by approximating the FW-CBCPW as a simple system of coupled transmission lines. Experimental results are shown to agree very well with the theoretical ones. In particular, the anomalous behavior observed in the transmission characteristic of the through line is related to the resonant phenomenon of the terminated side planes which are short-circuited at both input and output ends due to the test fixture. Finally, a technique of mode suppression in the side-plane regions is suggested for the improvement of signal transmission over a broad band of frequency spectrum. The effects of extra higher order modes on the transmission characteristics at high frequencies are also discussed. >

TM-mode perturbation analysis of dielectric gratings

An improved perturbation procedure that was previously presented for TE modes guided by dielectric gratings is extended here to TM modes. In both cases, the fields are described in terms of a transverse transmission-line network, which lends considerable physical insight into the behavior of dielectric gratings. However, TM modes require equivalent sources of both the voltage and current varieties, in contrast to TE modes which involve only current sources. By comparing with an exact solution, the numerical results obtained by the present perturbation procedure are found to be very accurate.

Resonant phenomena in conductor-backed coplanar waveguides (CBCPW's)

The resonant phenomena found in CBCPW integrated through line are investigated in detail both experimentally and theoretically. Two CBCPW through-line test circuits have been built and tested. One has uniform side planes and the other contains two slits in the middle of the side planes. Three techniques are used to investigate the resonant phenomena, namely, the patch antenna cavity model, the multimode model, and the full-wave space-domain integral equation approach. The measured transmission ( mod S/sub 21/ mod ) and reflection ( mod S/sub 11/ mod ) characteristics of the through lines are reported. At a representative resonant frequency of the measured data, current distributions are displayed, demonstrating that the side planes of the CBCPW contribute to the resonance in a way similar to planar patch antenna or 2D planar circuits. >

Leakage from a Gap in NRD Guide

Results are presented for the effect of an air gap between the top metal plate and the dielectric strip in nonradiative dielectric (NRD) guide, a promising candidate for millimeter wave integrated circuits. The leakage from small gaps can cause crosstalk between components, and that from larger gaps can furnish a new type of leaky wave antenna.

Full-wave analysis of lossy quasi-planar transmission line incorporating the metal modes

The authors present a novel full-wave mode-matching approach to accurately analyze the dispersion characteristics of millimeter-wave and microwave transmission lines with finite conductivity, metallization thickness, and holding grooves. The approach is quite general, but only the results for a unilateral finline are presented. The accuracy of the solution depends primarily on the correct and complete description of eigenfunction expansions in each of the uniform (stratified) or nonuniform layer regions. The latter consists of metallized strips of finite conductivity, which in turn produce the so-called metal modes (eigenmodes). The metal mode exists in the metallized region with high conductivity for the most part and decays sharply in the air region. Without incorporating the metal modes, the convergence studies will fail and the accuracy of the field theory solution deteriorates. Since the accuracy of the proposed approach is established, the composite effects of the finite conductivity and metallization thickness can be studied rigorously. >

Full-wave analysis of lossy transmission line incorporating the metal modes

A full-wave mode-matching analysis of a unilateral finline with finite conductivity, metallization thickness, and holding grooves is presented. A novel class of metal modes exists in the metallized region for the most part and decays sharply in the air region. The relative convergence criterion for a lossless waveguide is also observed for the finline with good metal coating. A criterion is given in order to determine the relative numbers of air and metal modes needed in the mode-matching formulation. It is shown that there exist the so-called metal modes in each metallized region with finite conductivity. For the particular case studied, the omission of the metal modes results in an inaccurate propagation constant, of which the attenuation constant can be two orders of magnitude smaller than that obtained by including the metal modes.<<ETX>>

Effects of substrate anisotropy on the dispersion of transient signals in microstrip lines

The analysis of dispersion characteristics of transient signal in microstrip lines with anisotropic substrate is developed here, with particular attention directed toward the effects of arbitrary orientations of the principal optical axis in anisotropic substrates. Numerical simulations are carried out for the propagation of transient signals, square or Guassian pulses, along microstrips with anisotropic substrates. It is shown that the dispersion characteristics is substantially affected by the change of the orientation angle of the principal optical axis in the substrate.

Guidance characteristics of two-dimensionally periodic impedance surface

We have modeled a general two-dimensionally periodic structure by a planar surface impedance that is periodic in two dimensions. For the first time, a set of dispersion curves is rigorously constructed to explain new and interesting phenomena that do not occur in the one-dimensional case.

Full-wave and experimental investigations of resonant and leaky phenomena of microstrip step discontinuity problems with and without top cover

In this paper we present both theoretic and experimental investigations of the resonant and leaky phenomena caused by microstrip step discontinuity with and without top cover. The rigorous two-dimensional and three-dimensional full-wave analyses are applied to obtain the dispersion characteristics of leaky behavior of the microstrip and the scattering parameters of the microstrip step discontinuity problems with symmetric and asymmetric excitations. The theoretic scattering analyses show that great transmission loss is closely related to the excitation of the first higher-order leaky wave in the strong leakage region when the microstrip is excited asymmetrically.<<ETX>>