Abstract
We present the design and characterization of a ground-based absolute polarization angle calibrator accurate to better than 0.1° for use with polarization sensitive cosmic microwave background (CMB) experiments. The calibrator's accuracy requirement is driven by the need to reduce upper limits on cosmic polarization rotation, which is expected to be zero in a large class of cosmological models. Cosmic polarization effects such as cosmic birefringence and primordial magnetic fields can generate spurious B-modes that result in non-zero CMB TB and EB correlations that are degenerate with a misalignment of detector orientation. Common polarized astrophysical sources used for absolute polarization angle calibration have not been characterized to better than 0.5°. Higher accuracy can be achieved through self-calibration methods, however these are subject to astrophysical foreground contamination and inherently assume the absence of effects like cosmic polarization rotation. The deficiencies in these two calibration methods highlight the need for a well characterized polarized source. The calibrator we present utilizes a 76 GHz Gunn oscillator coupled to a frequency doubler, pyramidal horn antenna, and co-rotating wire-grid polarizer. A blackened optics tube was built around the source in order to mitigate stray reflections, attached to a motor-driven rotation stage, then mounted in a blackened aluminum enclosure for further reflection mitigation and robust weatherproofing. We use an accurate bubble level in combination with four precision-grade aluminum planes located within the enclosure to calibrate the source's linear polarization plane with respect to the local gravity vector to better than the 0.1° goal. In 2017 the calibrator was deployed for an engineering test run on the POLARBEAR CMB experiment located in Chile's Atacama Desert and is being upgraded for calibration of the POLARBEAR-2b receiver in 2018. In the following work we present a detailed overview of the calibrator design, systematic control, characterization, deployment, and plans for future CMB experiment absolute polarization calibration.
