The Optimisation and Analysis of Multi-moded Feed Horn Structures at Terahertz Frequencies

Carrying out astronomical observations at far-infrared wavelengths is critical in enabling further progress in the flelds of cosmology and astrophysics. Such observations will allow additional insight into the birth and evolution of the Universe. To allow progress in these areas, it is necessary to improve the sensitivity and resolution of the instrumentation that is used to carry out these observations. At the high frequencies in question (terahertz), the instruments typically make use of horn antenna fed detector systems. To achieve the required performance, the horn designs must be highly optimised. Full electromagnetic solvers (CST, HFSS, COMSOL etc.) struggle to predict the performance of horn antennas at such high frequencies in a timely manner due to the large electrical size of the structures. It is therefore very challenging to perform the optimisation using such solvers particularly for multi-mode systems where each mode would have to be considered individually. In this paper we outline an alternative technique for modelling multi-mode (partially coherent) horn antennas based on the mode-matching technique, which allows electrically large structures to be modelled in a highly e-cient manner. This technique returns a set of scattering matrices which gives a full vector deflnition of the transmission and re ection characteristics of the resulting design at a given frequency. We demonstrate how this can be used to extract fleld patterns and other flgures of merit that are important for evaluating the electromagnetic performance of a horn design. An e-cient genetic algorithm based optimisation technique (using mode-matching) is also pre- sented. The optimisation process is based on a piecewise conical proflle horn design and produces a geometry that is optimised with respect to some flgure of merit that is of critical importance for the application in question. This allows the instrument to realise the high levels of optical performance that are required for astronomical applications. In order to further progress in the flelds of cosmology and astrophysics, it is necessary to carry out astronomical observations at far-infrared wavelengths. Observations carried out at these wave- lengths will allow insight into the birth and conditions necessary for the evolution of galaxies, stars, planetary systems and the evolution of matter. Such observations will allow further constraints to be placed on the theories and models that govern the origin and evolution of the Universe. In order to make progress in these areas, it is necessary to improve the sensitivity and resolution of the instruments that are being used. One method that is used to increase the sensitivity is to make use of multi-moded horns. These horns provide additional channels of power relative to their single-modes counterparts, resulting in increased throughput to the incoherent detectors that they feed, which results in increased sensitivity. Since in incoherent systems these modes will couple independently to the detector, a higher level of control over the beam deflnition is possible, resulting in an increase in the packing factor when arrays of such detector pixels are used, which will further increase sensitivity. Multi- moded horns have already been used on the European Space Agency Planck satellite, allowing the mapping of the Cosmic Microwave Background radiation with hitherto unseen levels of sensitivity and angular resolution (1). The horns were used on the HFI instrument (2,3) for the 545GHz and 857GHz channels. These channels were used to remove foreground sources from the data, and were an excellent example of how such horns can be used to retain high levels of sensitivity, resolution and beam pattern control. In terms of application to far infrared missions, multi-mode horn antennas are being investigated as candidates to feed transition edge sensor based detector arrays, for example in the case of the SAFARI instrument on board the proposed SPICA satellite (4). In this case, smooth-walled proflled horns are being examined as the feeds. At the high frequencies in question for far infrared missions, the horn antennas must be carefully optimised in order to comply with the stringent optical performance requirements that are enforced.