Howard R. Stuart

Lower Bounds on the Q of Electrically Small Dipole Antennas

General expressions are obtained for the lower bounds on the quality factor (Q) of electrically small electric- and magnetic-dipole antennas confined to an arbitrarily shaped volume V and excited by general sources or by global electric-current sources alone. The lower-bound expressions depend only on the direction of the dipole moment with respect to V , the electrical size of V , and the static electric and magnetic polarizabilities per unit volume of hypothetical perfectly electrically conducting and perfectly magnetically conducting volumes V . The lower bounds are obtained directly from the electromagnetic field expressions for Q with the help of current equivalence principles and the uncoupling of Maxwells equations for electrically small volumes into quasi-electrostatic and quasi-magnetostatic fields.

Limitations in Relating Quality Factor to Bandwidth in a Double Resonance Small Antenna

We examine the behavior of a matched electrically small antenna that exhibits two impedance resonances within its defined voltage standing wave ratio (VSWR) bandwidth. The exact quality factor (Q), computed indirectly from the antennas input reactance and far field, is compared to twice the inverse of the matched half-power VSWR bandwidth (Q BW) and to an approximate quality factor (Qz) determined from the frequency derivative of the antennas input impedance. The well-established approximate equalities Q ap Q BW ap Q z for antennas exhibiting an isolated single impedance resonance within their operating band are shown to become highly inaccurate for an electrically small antenna exhibiting two closely spaced impedance resonances.

Dispersive multiplexing in multimode fiber

A novel multiplexing technique for multimode fiber is presented. The technique uses modal dispersion to dramatically increase the information capacity of multimode fiber. A simple proof-of-principle experiment is shown that demonstrates the feasibility of this idea.

Approaching the Lower Bounds on Q for Electrically Small Electric-Dipole Antennas Using High Permeability Shells

We study the effect of surrounding electrically small, top-loaded, electric-dipole antennas with a thin shell of high-permeability magnetic material. The magnetic polarization currents induced in the thin shell of magnetic material reduce the internal stored energy, resulting in a lower as compared to conventional designs. The simulated s of thin-magnetic-shell cylindrical and spherical antennas are compared to recently derived lower bounds on . The high permeability shells reduce the of these antennas to values below the lower bounds for purely global electric current sources. In the case of the spherical electric-dipole antenna, a sufficiently large value of permeability enables the to be reduced to a value that is only 1.11 times the Chu lower bound.

Eigenmode Analysis of Small Multielement Spherical Antennas

A numerical eigenmode solver is used to analyze the behavior of two-, four-, and six-arm small multielement spherical antennas. The resonant frequency and radiation Q-factor of the natural modes of the shorted antenna structure are determined from eigenmode simulations using perfectly matched layer outer boundaries. The radiation resistance of each eigenmode at resonance is determined using a multiarm feed geometry that excites the modes individually. The impedance response of the antenna when driven on a single arm can then be described as a multipole RLC network, where the component values of the network are directly related to the parameters and relative excitation amplitudes of the various eigenmodes of the antenna. A good match between the modal analysis and the full harmonic simulation is observed, and the analysis also predicts the far-field cross-polarization levels arising from energy radiated by the higher-order modes. The analysis provides physical insight into the multiresonant impedance behavior of these antennas, clearly illustrating their ultimate performance limits and yielding ideas for improved designs. The analysis can be applied towards understanding the behavior of other multielement antenna structures.

Eigenmode Analysis of a Two Element Segmented Capped Monopole Antenna

The behavior of a two element segmented capped monopole antenna is described in terms of three natural resonances of the antenna structure. A numerical eigenmode solver is used to derive the resonant frequency and Q-factor of the natural resonances, as well as the impedance properties of these modes when excited individually. The impedance of the antenna is described as a lumped element network, where the component values of the network are determined from the impedance parameters of the resonant modes and the static properties of the antenna structure. The model is applied to predict the behavior of the antenna when a lumped element inductor is used to connect the two segments. The inductor tunes the properties of one of the three resonances, enabling the impedance bandwidth to be optimized. The analysis is then extended to predict the behavior of the antenna when the two vertical elements have different radii, a configuration that enables wide bandwidth operation without the use of an inductor. The modal analysis accurately predicts the impedance and far-field properties of the antennas. Analyzing the antenna in terms of its natural resonant modes provides physical insights into both its behavior and the fundamental limitations of its performance.

Ultra-sensitive optical sampling by coherent-linear detection

Optical sampling is achieved using an all-optical linear technique with a record sensitivity of 10/sup -3/ mW/sup 2/, 1000 times better than nonlinear techniques. Phase sensitivity is removed by simultaneous measurement of orthogonal phase quadratures.

A small wideband multimode antenna

This paper presents a multi-resonant spherical antenna with a normalized size ka~0.54 and excellent bandwidth performance (k is the wavenumber and a is the radius of the sphere circumscribing the maximum dimension of the antenna). The spherical geometry is chosen in order to establish a baseline Q-factor against which to compare the bandwidth: a Q at 1.5x the lower bound is known to be achievable for optimized small, single resonance spherical antenna designs. The antenna presented here is designed to support two natural resonances, one low-Q and one high-Q . It postulates that the high-Q mode acts in a manner analogous to a higher-order matching circuit.

Signal-to-noise ratio monitoring of optical data using narrowband RF analysis at the half-clock frequency

A novel technique for monitoring the in-band SNR of an optical data channel is demonstrated. The technique utilizes an inherent symmetry present the RF spectrum at half the data clock frequency, is highly sensitive and potentially low-cost.

Nanophotonics — Quantum dots, photonic crystals, and optical silicon circuits: An excursion into the optical behavior of very small things

This paper covers four specific areas — diverse in their scope and approach — of nanophotonics research at Bell Labs, ranging from nanosized light emitters to passive and active optical circuits fabricated or actuated with nanometer precision. Our first example involves the dynamics of carriers confined in quantum dots fabricated via self-assembly on semiconducting substrates; we show that the evolution of carrier populations is a result of the interplay of interactions between photons, phonons, electrons, and holes. A second example involves a nanosize light-emitting quantum dot system that is fabricated by embedding passivated spherical semiconducting quantum dots in a polymer matrix; the fabrication steps and subsequent optical measurements, including the observation of net optical gain in this system, are described. In the area of passive and active optical circuits, we report on efforts to optimize the transmission efficiencies of photonic crystal circuit-building elements (e.g., waveguide bends and waveguide tapers in passive two-dimensional photonic crystal circuits). Finally, we describe active optical silicon circuits fabricated on a silicon-on-insulator platform; these devices comprise waveguide interferometers and electrostatically actuated piston micromirrors, and they require nanometer-size displacements to function as optical switches with inherently low power consumption.