Transmission and Reflection Characterization of Polarizing Beam Splitters at Submillimeter Wavelengths

This paper presents a comparison of the transmission and reflection performance of different free-standing wire grids and photolithographic polarizers. This study has been carried out in the frame of the development work of the submillimeter-wave instrument (SWI) as part of the JUpiter and ICy moons Explorer mission of the European Space Agency. SWI is a passive heterodyne radiometer that is operating in the frequency bands from 530 to 625 GHz and from 1080 to 1275 GHz. The optical coupling network of SWI includes a polarizing beam splitter to separate the signal into the two channels. Simulations of the transmittance and reflectance in copolarization and in cross polarization have been performed at different incidence angles and polarizations to support the selection of the optimum beam splitter for SWI. The nominal and actual design parameters of the investigated polarizing beam splitters, measured with a confocal laser microscope, have been considered in the simulations. Measurements of the transmission and reflection losses in co- and in cross polarization have been performed at 636.5 GHz to validate these numerical calculations. The very low losses of the beam splitters in copolarization were accurately measured by means of a free-space cavity resonator setup. These measurements are in very good agreement with the simulations and radiometric measurements of the transmission loss which were performed with a breadboard model of the SWI receiver unit at 590.0 GHz. The lowest transmission losses among the studied beam splitters are observed for a free-standing wire grid with a single plane of gold-coated wires. The highest reflectance in copolarization and the lowest levels of cross-polarization leakage are determined for the photolithographic polarizers.