Mahon, Ronan John
Modal Analysis of Millimetre-wave
and Terahertz Imaging Systems.
PhD thesis, National University of Ireland Maynooth.
This thesis presents the theory and applications of electromagnetic field
calculation using orthogonal Gaussian beam modes within the context of far-infrared
imaging systems. Laguerre and Hermite-Gaussian modes have been frequently
reported in the analysis of paraxial millimetre-wave propagation in astronomical
optical systems. Here the method of Gaussian beam mode analysis (GBMA) is
extended to fields of optical research that have until recently been associated with
wavelengths in the visible band.
Using recently derived expressions for the non-paraxial diffraction of
Hermite-Gaussian modes, the author demonstrates the modal calculation of far-field
intensity distributions with less angular restriction on the accuracy of the method
compared to the conventional paraxial description of orthogonal Gaussian modes.
This method shows excellent agreement with predictions from more rigourous fullwave
numerical methods such as the finite-difference time-domain algorithm, which
is also described as a software tool in the modelling of horn and lens antennas.
The properties of diffraction limited Bessel beams is described using the
Laguerre-Gaussian expansion of conical lenses, and experimental measurements of a
conical lens is presented to explore the validity of the use of these optical elements as
horn coupled devices in millimetre wave imaging systems.
A study of diffractive Fresnel lenses has been undertaken with a comparison
of experimentally measured fields with those predicted by the modal techniques. The
effects of such lenses on ultrashort paraxial pulses are also investigated using a novel
numerical description of few-cycle fields as a superposition of pulsed Laguerre-
The application of digital holography in the far-infra red band has the prospect
of diffraction limited imaging systems without creating distortions and aberrations
which is a common problem in conventional techniques using lenses and mirrors. The
author presents results from a simple proof-of-concept system which exhibits the
potential of this technique for application in, for example, mm-wave security
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