Characterisation of Fresnel Interactions in a Novel Full-Stokes Spectropolarimeter Design

Abstract

Polarimetry in space is an increasingly prominent field in space research. Polarimetry, especially spectropolarimetry, have the potential to provide a substantial amount of information about the Earth as well as distant interstellar objects. In this field, full-Stokes spectropolarimetry is ideal for providing comprehensive polarimetric information. An instrument capable of making such measurements is often too large or complex for satellite use. A method which can theoretically perform full-Stokes spectropolarimetry while avoiding these drawbacks is currently being researched by Dr. Bogdan Vasilescu in collaboration with TU Delft. This new method makes use of a modulator formed out of 3 birefringent prisms, and a linear polariser, which together produces fluctuations in intensity. These fluctuations can be used to obtain information about any incident light.

The aim of this study was to investigate unexpected intensity fluctuations which occur even without the linear polariser, and determine the extent to which these can be explained by Fresnel behaviour at the boundaries of the prisms. To achieve this, Jones and Stokes models were produced in Python, simulations were performed in ZEMAX, and experimental measurements were taken to validate the results. For each of these methods, only the modulator first and third prisms were used as the second is designed as a structural component rather than an active element. From the models, simulations and experimental results, it was found that including Fresnel behaviour to models and simulations does result in output intensity fluctuations. Furthermore, it was determined that the periodicity of the fluctuations is very closely linked to Fresnel phenomena. The Python models and ZEMAX simulations especially returned results which appeared consistent, with easily explainable differences. On the other hand, further study is required to explain the differences in the amplitudes and average transmission of the intensity patterns between the different models, especially compared to the experimental results. The developed models and simulations can therefore demonstrate that Fresnel behaviour does explain the presence of the fluctuations but also suggests that the methods used in this thesis do not achieve a full representation of the phenomena, or that there are additional factors which must still be investigated.