QUBIC VI: Cryogenic half wave plate rotator, design and performance

Abstract

Setting an upper limit or detection of B-mode polarization imprinted by gravitational waves from Inflation is one goal of modern large angular scale cosmic microwave background (CMB) experiments around the world. A great effort is being made in the deployment of many ground-based, balloon-borne and satellite experiments, using different methods to separate this faint polarized component from the incoming radiation. QUBIC exploits one of the most widely-used techniques to extract the input Stokes parameters, consisting in a rotating half-wave plate (HWP) and a linear polarizer to separate and modulate polarization components. QUBIC uses a step-by-step rotating HWP, with 15° steps, combined with a 0.4°s-1 azimuth sky scan speed. The rotation is driven by a stepper motor mounted on the cryostat outer shell to avoid heat load at internal cryogenic stages. The design of this optical element is an engineering challenge due to its large 370 mm diameter and the 8 K operation temperature that are unique features of the QUBIC experiment. We present the design for a modulator mechanism for up to 370 mm, and the first optical tests by using the prototype of QUBIC HWP (180 mm diameter). The tests and results presented in this work show that the QUBIC HWP rotator can achieve a precision of 0.15° in position by using the stepper motor and custom-made optical encoder. The rotation induces <5.0 mW (95% C.L) of power load on the 4 K stage, resulting in no thermal issues on this stage during measurements. We measure a temperature settle-down characteristic time of 28 s after a rotation through a 15° step, compatible with the scanning strategy, and we estimate a maximum temperature gradient within the HWP of ≤ 10 mK. This was calculated by setting up finite element thermal simulations that include the temperature profiles measured during the rotator operations. We report polarization modulation measurements performed at 150 GHz, showing a polarization efficiency >99% (68% C.L.) and a median cross-polarization χPol of 0.12%, with 71% of detectors showing a χPol + 2σ upper limit <1%, measured using selected detectors that had the best signal-to-noise ratio.

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 V: Cryogenic system design and performance

“…In this paper, which is part of a set describing the current status of the QUBIC experiment [8,19,[24][25][26][27][28] we describe the design, fabrication, experimental optimization and validation of the QUBIC cryogenic system in the Technological Demonstrator (TD) configuration. This configuration differs from the Full Instrument (FI) configuration because the optical apertures are reduced in size, only one of the two focal planes (the transmitted one) is installed, and only 1/4 of the detectors (one wafer) is present.…”

Polarization Modulator Unit Harness Thermal Design for the Mid- and High-Frequency Telescopes of the LiteBIRD Space Mission

“…A critical and powerful component in CMB polarization experiments is the polarization modulator unit (PMU). The polarization modulation methodology based on half-wave plate (HWP) can be divided in two families: step-and-integrate HWPs (SPIDER [5], QUBIC [6][7][8]) and continuously-rotating HWPs (ABS [9], EBEX [10], ACT-pol [11], POLARBEAR-2 [12], LSPE/SWIPE [13][14][15]).…”