A Three Dimensional Gaussian Beam Diffraction Approach to Analysis of Quasi-optical Networks.

Abstract

Millimetre and sub-millimetre wave systems have been applied in many areas, such as radio astronomy, remote sensing of the atmosphere, plasma diagnostics, material exploration. A Quasi-optical Network (QoN) is the most efficient approach to transport signals within such systems. The Gaussian beam summation method has proven to be a useful ray-tracing-based solution in designing the complex configurations of QoNs. An efficient approach, diffracted Gaussian beam analysis (DGBA) has been developed at Queen Mary, University of London. However, this version of DGBA can be only used for a 2.5-D analysis, for the following reasons: (1) the input Gaussian beams (GBs) should be stigmatic (circular); (2) all incident beams are approximated as normally incident, unable to treat oblique polar angles of incidence. So in this regard, there is a need to develop a 3D treatment of diffraction in DGBA. In this thesis, a 3D diffracted GB approach (3D-DGBA) to the analysis of multireflector QoNs is presented. This new approach expands the input beam/field into a set of elementary GBs by using windowed Fourier transforms (WFT). The ensuing GBs propagate to the reflector. Reflected and diffracted beams are respectively handled by a phase-matching method and a 3D GB diffraction method. In addition, it is modular and suitable for analysis of large, multi-element, QoNs. A specific design procedure of multi-path, multi-reflector, QoNs is presented and two different kinds of dual-path QoNs are built and assessed. By analysing one of these QoNs, the new 3D-DGBA approach is verified experimentally, as well as compared to the original DGBA and GRASP predictions. It is observed that 3D-DGBA is more accurate than the original DGBA. For example, in one of the simulated cases, the farfield deviation between 3D-DGBA and GRASP within 20˚ stays in the range of ±8.0 dB in comparison with ±23.0 dB of the original DGBA.