QUBIC VI: cryogenic half wave plate rotator, design and performances

Abstract

Inflation Gravity Waves B-Modes polarization detection is the ultimate goal of modern large angular scale cosmic microwave background (CMB) experiments around the world. A big effort is undergoing with the deployment of many ground-based, balloon-borne and satellite experiments using different methods to separate this faint polarized component from the incoming radiation. One of the largely used technique is the Stokes Polarimetry that uses a rotating half-wave plate (HWP) and a linear polarizer to separate and modulate the polarization components with low residual crosspolarization. This paper describes the QUBIC Stokes Polarimeter highlighting its design features and its performances. A common systematic with these devices is the generation of large spurious signals synchronous with the rotation and proportional to the emissivity of the optical elements.A key feature of the QUBIC Stokes Polarimeter is to operate at cryogenic temperature in order to minimize this unwanted component. Moving efficiently this large optical element at low temperature constitutes a big engineering challenge in order to reduce friction power dissipation. Big attention has been given during the designing phase to minimize the differential thermal contractions between parts. The rotation is driven by a stepper motor placed outside the cryostat to avoid thermal load dissipation at cryogenic temperature. The tests and the results presented in this work show that the QUBIC polarimeter can easily achieve a precision below 0.1°in positioning simply using the stepper motor precision and the optical absolute encoder. The rotation induces only few mK of extra power load on the second cryogenic stage (∼ 8 K).

QUBIC I: Overview and science program

“…for a detector close to the center of the focal plane. As studied in details in D'Alessandro et al [37] from this series of articles, we have found a median measured cross-polarization of 0.12% among our detectors and a 0.61% median 95% upper-limit. 77% of our detectors have a measured cross-polarization compatible with zero at the one-sigma level.…”

“…The next optical component is a stepped rotating Half-Wave-Plate which modulates incoming polarization. This sub-system is described in D'Alessandro et al [37] from this series of articles. A single polarization is then selected thanks to a polarizing grid.…”

“…The scientific overview and expected performance of the instrument are addressed here. The other papers in the special issue address the following topics: the ability of bolometric interferometry to do spectral imaging [33], the calibration and performance in the laboratory of the TD [34], the performance of the detector array and readout electronics [35], the cryogenic system performance [36], the Half Wave Plate rotator system [37], the back-to-back feedhorn-switch system [38], and the optical design and performance [39].…”

QUBIC II: Spectro-Polarimetry with Bolometric Interferometry

“…Detailed information about QUBIC can be found in the companion papers: Scientific overview and expected performance of QUBIC [23], Characterization of the Technological Demonstrator (TD) [22], Transition-Edge Sensors and readout characterization [24], Cryogenic system performance [25], Half Wave Plate rotator design and performance [26], Feedhorn-switches system of the TD [27], and Optical design and performance [28].…”

A Polarization Modulator Unit for the Mid- and High-Frequency Telescopes of the LiteBIRD mission

“…A key component of LiteBIRD is its polarization modulator unit (PMU), an essential feature to suppress the 1/f noise contribution (low-frequency system drifts induced by thermal variations or detector gain drift) and and mitigate systematics uncertainties induced by detector gain drifts. The polarization modulation methodology based on HWP is already used by a large number of experiments and can be divided in two families: a stepand-integrate strategy (SPIDER, 7 QUBIC 8,9 ) and a continuously-rotating HWP (ABS, 10 EBEX, 11 ACT-pol, 12 POLARBEAR-2, 13,14 LSPE/SWIPE 15,16 ). Each of the 3 telescopes (high, mid and low frequency: HFT, MFT, LFT) is equipped with a cryogenic continuously-rotating half-wave plate (HWP) based on a superconducting magnetic bearing (SMB), an emerging technology with a low technology readiness level (TRL).…”