Abstract
In order to investigate the formation and evolution of galaxies, stars and planetary systems, it is necessary to carry out astronomical observations in the far-infrared portion of the electromagnetic spectrum. Missions such as the Herschel Space Observatory (European Space Agency) have already completed observations in this region with great success. Proposed high resolution spectrometer instruments such as SAFARI (a joined European/Japanese (ESA/JAXA) proposal as part of the SPICA mission), aim to build upon the work of previous missions by carrying out observations in the 1.5–10 THz band with unprecedented levels of sensitivity. Spica (SPace Infrared telescope for Cosmology and Astrophysics) is currently a candidate mission as part of ESA’s Cosmic Vision 2015–2025.Future far-IR missions must realise higher levels of sensitivity, limited only by the cosmic microwave background. One solution in achieving these sensitivity goals is to use waveguide coupled Transition Edge Sensor (TES) detectors, arranged in a densely packed focal plane. Additionally, multi-mode pixels can be used in order to maximise the optical throughput and coupling while still defining a definite beam shape. For the SAFARI instrument multimoded horns coupling into integrating waveguide cavities that house the TES detectors and associated absorbing layer are envisioned. This represents a significant technological challenge in terms of accurate manufacture tolerances relative to the short wavelength, however in the case of the SAFARI instrument pixel much work has already been carried out, with prototype pixels having undergone extensive testing at SRON (Space Research Organisation of the Netherlands) Groningen. In order to fully characterise the experimental results, it is necessary also to carry out comprehensive electromagnetic modelling of these structures which is also computationally intensive and requires novel approaches. These waveguide structures (horn and cavity) are typically electrically large however, and so analysis techniques using commercial finite element software prove inefficient (particularly as the structures are multimoded).The mode-matching technique with new analytical features offer a computationally efficient and reliable alternative to full electromagnetic solvers, and in this paper we outline the additions to this technique that were necessary in order to allow typical SAFARI far-infrared pixels to be modeled, including the complete optical coupling calculation of the measurement test setup at SRON and the inclusion of the free space gap within the horn antenna and the integrating cavity. Optical coupling efficiencies simulated using this developed technique show excellent agreement with the experimental measurements.
Highlights
Future far infrared space telescopes have a wide variety of scientific objectives, aiming to build upon the work carried out by previous missions such as the Herschel Space Observatory [1]
SAFARI is optimised to operate in the 1.5–10 THz band, utilising the low background environment provided by the cooled SPICA telescope that is limited only by the cosmic microwave background (CMB) radiation to study the formation and evolution of galaxies, stars and planetary systems
If the results obtained from previous missions are to be improved upon, it is necessary for future instruments to realise higher levels of sensitivity, allowing the complete exploitation of the extraordinary sensitivity permitted by the cold telescope optics and sensitive superconducting detectors (Transition Edge Sensors (TES))
Summary
Future far infrared space telescopes have a wide variety of scientific objectives, aiming to build upon the work carried out by previous missions such as the Herschel Space Observatory [1]. Sensitivity can be maximised by utilising a focal plane with a high packing density and by using multi-mode horns to feed the transition edge sensors and associated absorber in a waveguide cavity in order to maximise optical coupling This has been demonstrated at SRON (Space Research Organisation of the Netherlands) Groningen, where extremely sensitive detector pixels have been developed and characterised for the SAFARI instrument, as reported in [4,5]. This means that a higher percentage of the power incident on the pixel is potentially absorbed in such a waveguide cavity Such systems are challenging to model electromagnetically, in particular due to the necessity to include additional features such as a free space gap that exists between the metal horn array and the cavity array that is manufactured separately on a silicon wafer (which exists as the two arrays clearly cannot be manufactured monolithically, and to maintain structural integrity during launch). We apply this model of the pixels to the testbed that was used in SRON Groningen and outline the additional steps that were necessary to fully simulate the test setup, extracting simulated values for the detected power and comparing them to the measured values in an integrated frequency band
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