Title

Practical vortex beam generation

Date of Award

Summer 2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering - (Ph.D.)

Department

Electrical and Computer Engineering

First Advisor

Edip Niver

Second Advisor

Peter G. Petropoulos

Third Advisor

Ali N. Akansu

Fourth Advisor

Gerald Martin Whitman

Fifth Advisor

Ali Abdi

Abstract

Vector vortex beams are monochromatic electromagnetic wave fields carrying spin angular momentum (SAM) and orbital angular momentum (OAM). Spin angular momentum is associated with the polarization of the field, whereas OAM yields an azimuthal field dependence of the form exp( jqφ), where φ is the azimuthal angle, and q is an integer designating the helicity order, which is also known as the topological charge of the vortex beam. Vortex beams owe their names to the characteristic on-axis phase singularity and amplitude null.

In this study, a new method to generate vector vortex beams in the microwave regime is devised based on waveguide modes, where the vortex beam is set to be the aperture field at the open-end of a metallic circular waveguide section. This method takes inspiration from previous work on zero-order Bessel beam generation in the microwave regime. In this design, the launched vortex beam is a transverse electric (TE) electromagnetic field with a truncated Bessel profile. The aperture field is formed by the propagating field of the TEq1 mode of the waveguide. Excitation is provided by means of a single circular loop antenna inserted coaxially inside the waveguide section. The waveguide housing of the large loop antenna is shown to be advantageous in terms of impedance matching, where the input impedance is shown to depend on the antenna location inside the waveguide. A phenomenological simplified analytical expression of the input impedance is derived based on transmission-line theory and verified using multi-level fast multipole method (MLFMM) full-wave simulation. In the far-field region, vortex beams have conical radiation pattern, and by adding an angled flange to the waveguide, the radiation cone angle can be altered. In particular, the effect of the flange angle on the direction of the maximum radiation is studied to provide valuable insight into using the launcher in practical communication links. Furthermore, a parametric sensitivity analysis is performed to model the effect of small perturbations in antenna position and tilt on the performance of the launcher.

This research aims to provide a practically feasible method for vortex beam generation in the microwave regime; however, due to practical limitations, the results of this research are not yet compared to experimental data, but are numerically verified using full-wave simulations.

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