Optical Characterisation of Astronomical Submillimetre Receivers including ALMA Bands 5 and 9.
PhD thesis, National University of Ireland Maynooth.
The primary concern of this thesis is the analysis of long wavelength quasioptical
receiver systems operating within the Terahertz and submillimetre wavebands. Specific
attention is paid to the front-end coupling optics of the Band 5 and Band 9 receiver
channels of the Atacama Large Millimetre Array (ALMA). The theory of Gaussian Beam
Mode Analysis (GBMA) is expanded and developed as the basic analytical tool for the
work presented. This technique is utilised to model both classical optics diffraction and
interference patterns. An alternate method of describing these diffraction patterns is
developed using the Angular Spectrum of Plane Waves (ASPW). The general GBMA
technique is supplemented by the commercially available Physical Optics (PO) package
GRASP9 developed by TICRA. A comprehensive analysis of the ALMA Band 5 front
end optics was conducted in conjunction with the Group for Advanced Receiver
Development (GARD). This analysis was developed to investigate the efficiency of
various configurations of the optics and was supplemented by a rigorous measurement
campaign at GARD. As part of our ongoing collaboration with the Space Research
Organisation of the Netherlands (SRON) a series of theoretical and experimental
analyses were performed with the aim of improving the cross polar efficiency of the
ALMA Band 9 receiver.
In addition to these investigations of long wavelength receiver optics the theory of
GBMA was combined with mode-matching theory to describe the behaviour of
standing waves in typical submillimetre receiver systems. The reflection and
transmission amplitude response patterns of several standing wave cavities were
predicted with this technique and compared with experimental measurements. The
effect of minor alterations to the feed horn structures on the resonance profiles are
studied in depth. The eigenmodes of these resonant cavities are also analysed. A
complex stray light baffle structure is introduced within the cavity and its effect upon
the system is quantified.
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