Despite advances in astronomy, much of the universe remains hidden behind gas and dust clouds that absorb optical radiation, making it undetectable by conventional telescopes. Far-Infrared (FIR) radiation is capable of propagating through these clouds, and offers a solution for observing these obscured interstellar regions. However, the sensitivity of existing detector systems is insufficient to detect FIR radiation effectively. This thesis focuses on the development of highly sensitive detectors based on Microwave Kinetic Inductance Detectors (MKIDs) that meet the sensitivity requirement for use in future space missions, specifically the NASA led POEMM and PRIMA missions, covering a frequency range from approximately 1.5 THz to 12 THz.

While detectors have been developed previously covering the upper band of this frequency range at 6.98 THz and 12 THz, the lower band remains uncovered. Therefore, lens-absorber coupled MKIDs designed to operate at 1.5 THz are presented, with their performance analysed using models based on the Geometrical Optics-Fourier Optics (GOFO) technique in combination with a Floquet wave approach for periodic absorbers. Additionally, the design methodology of MKIDs to operate at specific readout frequencies is discussed. Absorber coupled MKIDs are limited to lower resonance frequencies with respect to comparable antenna based MKIDs, which offer advantages for space-based missions due to reduced power consumption, but do present challenges for readout systems operating at higher frequencies. In this thesis, two lens-absorber coupled MKID designs were developed and are currently being fabricated as candidates to experimentally demonstrate highly sensitive detector arrays operating at 1.5 THz.

The imaging spectrometers in the POEMM and PRIMA missions require a dispersive element to separate different wavelengths of the incoming radiation. The dispersive element envisioned for these missions, the Virtually Imaged Phased Array (VIPA), suffers from non-idealities, causing phase aberrations in the transmitted field, and part of the power transmitting towards unwanted propagation directions, which can degrade instrument performance. This thesis presents and analyses optimisation techniques to mitigate these effects, thereby enhancing the overall efficiency and degrees of freedom for designing the full instrument.

Furthermore, absorbers, unlike antenna-based devices, are multi-mode detectors. Understanding how absorbers couple to higher-order modes is crucial for optimising their performance. This thesis introduces and validates a model based on a novel experimental technique, Energy Absorption Interferometry (EAI), to extract a modal description of detectors. In the future steps related to the POEMM mission, the findings from this initial investigation into multi-mode absorbers will be used to gain insight and explore possible improvement routes in the coupling of these detectors to complex optical systems.

In the framework of this MSc thesis, the analysis and design guidelines for part of the quasi-optical system of a proposed astronomical instrument, called TIFUUN, are presented. The TIFUUN instrument is an imaging spectrometer, planned to be placed in the ASTE telescope to perform ground-based astronomical observations in the mm-submm wavelength regime. Part of the instrument development involves the design of the quasi-optical system coupling the radiation from the telescope’s main dish to the spectrometer array. In this thesis, the focal plane array of antennas, as well as the first component of the quasi-optical chain are analyzed and two different design approaches are presented, as candidate geometries. Each of them satisfies the requirements of different science surveys targeted by TIFUUN. The first examined architecture is comprised of a focal plane array of on-chip feeding elements under a single hyper-hemispherical lens, which is then coupled geometrically to a hyperbolic lens. The second geometry is instead comprised of an array of integrated elliptical lenses, with a single on-chip feeding element per lens, diffractively coupled to a hyperbolic lens. The methodologies to efficiently analyze these kinds of geometries are Geometrical Optics (GO) combined with analysis in reception for the first design approach and Coherent Fourier Optics (CFO) for the second one. During the design process, the performance of both architectures is optimized throughout the field of view, using methodologies to correct for phase aberrations, such as feed displacement inside the lenses and symmetric shaping of dielectric surfaces. The design guidelines provided and insights obtained during this MSc project will be utilized to develop the quasi-optical system of the TIFUUN instrument, within the limited space of its cryostat.

Recently, there has been an increasing demand for security in public places. As a result, non-destructive and fast millimetre-wave and submillimetre-wave imaging systems have gained more and more attention.
This project is based on Concealed Objects Stand-off Real-Time Imaging for Security (CONSORTIS), which is a European next-generation airport security imaging radar system published in 2017. In this project, we will discuss the design of the lens antenna illuminated by a leaky wave waveguide antenna for a large format focal plane array with wide scanning capabilities in three typical cases. The Coherent Fourier Optics (CFO) and leaky wave antenna design methodologies are used in this project. The system will be analysed in reception mode and then validated in transmission mode. We have a very promising performance with an aperture efficiency of about 80% in the centre and 47% at the edge of the array with a shaped top and AR coating. The directivity of the antenna at the edge is about 50.2 dB. And the scan loss is about -2.3 dB, which means it can scan about 10,000 beams in total.

In this thesis, an efficient optimization method is described for synthesizing lens antennas fabricated in low permittivity plastic materials to achieve wide scanning performances. The method is based on an antenna in reception representation. The optimized parameters are the location and orientation of the antenna feeder, lens angular region and its shape by adding Zernike polynomials to standard ellipsoidal shape. The synthesized lens antennas achieve scan losses lower than 3dB while scanning up to 60°. The presented theoretical results are validated using full wave simulations. Furthermore, an efficient analysis method based on Geometrical Optics and ray tracing is developed. This method is applied to the study of multi lens Quasi-Optical structures. The presented theoretical results are validated by multi surface Physical-Optics approach.

Lens array architectures are being developed for high frequency applications such as phased arrays for 5G point-to-point communications, Fly’s Eye lens arrays for wideband wireless communications and scanning phased arrays at sub-millimeter wavelengths. Each of these applications employ different feed configurations, lens geometries and require specific feed isolation levels. In these architectures, the mutual coupling between feeds is an important design criteria. This project proposes a theoretical model based on Geometrical Optics and Physical Optics to represent the multiple reflections in a dielectric lens antenna and its impact on the mutual coupling between lens feeders. This model is then employed to study the mutual coupling in architectures with different feed models, edge illumination levels, feed positions, lens diameters and dielectric materials.

In this work, a MATLAB based graphical user interface (GUI) tool is built to analyze Quasi-optical (QO) systems in reception with the Fourier Optics (FO) and Geometrical Optics (GO) methods. Five canonical QO components are discussed, namely parabolic reflectors, elliptical lenses, hemispherical lenses, hyperbolic lenses, and elliptical mirrors. The ray tracing technique is implemented to describe reception scenarios and visualize the phase aberrations in the QO systems. The FO method represents the fields focalized by a QO component on its focal plane as a Plane Wave Spectrum (PWS). This spectrum is calculated by using the GO method and can be used in spectral techniques like equivalent Floquet circuit. Moreover, the tool is able to use this spectrum to estimate the power delivered to an antenna placed at the focal plane. By obtaining this delivered power, the performance of the antenna-coupled QO system is analyzed, including pattern, directivity, common efficiency terms, and gain. In addition, the performance calculated in reception is validated by CST and GRASP full-wave simulation software. Therefore, this GUI tool represents a GO/FO based tool that can be used to analyze and design antenna-coupled QO systems in reception.