Optical modelling and experimental verification of few-mode spectrometers in the far-infrared
| PhD ceremony: | B.N.R. Lap |
| When: | September 23, 2025 |
| Start: | 16:15 |
| Supervisors: | S.C. (Scott) Trager, Prof, prof. dr. S. Withington, prof. dr. D.A. Naylor |
| Co-supervisor: | dr. ir. W. Jellema, PhD |
| Where: | Academy building RUG / Student Information & Administration |
| Faculty: | Science and Engineering |
The next generation of far-infrared (FIR) spectroscopic missions will use state-of-the-art direct detectors with a partially coherent, or equivalently, few-mode, reception pattern to achieve unprecedented levels of observing sensitivities.
However, highly sensitive few-mode spectrometers, particularly grating spectrometers, are likely to exhibit troublesome few-mode artefacts that must be addressed pre-flight to ensure the success of these future spectroscopic missions. Although classical optical modelling approaches provide valuable insights, they do not account for few-mode behaviour. Whereas experimental techniques are available for characterising few-mode optics, the researchers are unaware if these techniques have ever been applied to determine the modal properties of FIR grating spectrometers.
To address these issues, Bram Lap present the Huygens-Fresnel Modal Framework (HFMF) for the partially coherent modelling of few-mode optics. Lap uses the HFMF to simulate two characteristic examples, a grating spectrometer and a Fourier Transform Spectrometer, that are representative of future FIR missions currently under consideration and illustrative to highlight few-mode artefacts observed in-flight.
These simulations show that the HFMF accurately describes the partially coherent behaviour in few-mode FIR spectrometers, including thermal straylight and the few-mode behaviour seen in Herschel-SPIRE. Next, Lap describes the development of a THz-VNA for the optical mode characterisation of FIR optical systems, with a high dynamic range, well-behaved Gaussian beams, and good optical coupling over the specified bandwidth. He uses this THz-VNA to measure the modal characteristics of a representative few-mode FIR grating spectrometer and verify the HFMF experimentally, which he achieves because the measured and simulated modal characteristics show good agreement.