Polarization properties of a broadband multi-moded concentrator

We present the design and performance of a non-imaging concentrator for use in broadband polarimetry at millimeter through submillimeter wavelengths. A rectangular geometry preserves the input polarization state as the concentrator couples f/2 incident optics to a 2π sr detector. Measurements of the co-polar and cross-polar beams in both the few-mode and highly over-moded limits agree with a simple model based on mode truncation. The measured co-polar beam pattern is nearly independent of frequency in both linear polarizations. The cross-polar beam pattern is dominated by a uniform term corresponding to polarization efficiency of 94%. After correcting for efficiency, the remaining cross-polar response is -18  dB.