Polarization Characterization of a Multi-Moded Feed Structure

The  Polarization Characterization of a Multi-Moded Feed Structure projects characterize the polarization response of a multi-moded feed horn as an innovative approach to polarized measurements in the far-infrared.  Multi-moded optics provide a significant increase in instrument sensitivity per detector, but the polarization response of such quasi-optical structures is not well understood. We will measure the co-polar and cross-polar beam patterns of a multi-moded feed horn.

A simple, focused investigation will measure the co-polar and cross-polar response of a multi-moded feed. We will fabricate a larger version of a far-IR feed, mount it within the Goddard Electromagnetic Anechoic Chamber (GEMAC), and measure the co-polar and cross-polar response using wire grid polarizers mounted at the source and multi-mode detector.

The polarization response of a multi-moded feed is difficult to model.  Geometric optics (ray tracing) yields the response in the high-frequency (short wavelength) limit, but ignores effects of beam diffraction at the edges of the feed. Solution of Maxwell's equations within the feed structure provides an analytic expansion of the response using waveguide theory, but is computationally tractable only in the few-mode (long wavelength) regime.  Astrophysical signals in the far-infrared (e.g. the cosmic microwave background) are bright at  frequencies near 300 GHz (1 mm wavelength), squarely between the two limits.  Diffraction is important at these wavelengths, yet tens to hundreds of electromagnetic modes propagate within the feed structure, making direct calculation extremely difficult. Measurements of a first-generation feed horn demonstrate basic functionalit and provide a simple model to guid future design.  Based on the initial results, we plan to design and test a second-generation feed to maximize throughput while minimizing the cross-polar response.