A cryogenic waveplate rotator for polarimetry at mm and sub-mm wavelengths

TL;DR

This paper presents a cryogenic waveplate rotator system capable of automated, precise rotation of a birefringent crystal at 4K, essential for polarimetry in mm and sub-mm wavelengths in space experiments.

Contribution

The paper introduces a novel cryogenic waveplate rotator system with automated control and minimal power dissipation, suitable for space-based polarimetry applications.

Findings

Successful rotation of birefringent crystal at 4K with high precision

Minimal power dissipation of a few milliwatts in cryogenic environment

Compatibility with space experiments like PILOT, BOOMERanG, and LSPE

Abstract

Mm and sub-mm waves polarimetry is the new frontier of research in Cosmic Microwave Background and Interstellar Dust studies. Polarimeters working in the IR to MM range need to be operated at cryogenic temperatures, to limit the systematic effects related to the emission of the polarization analyzer. In this paper we study the effect of the temperature of the different components of a waveplate polarimeter, and describe a system able to rotate, in a completely automated way, a birefringent crystal at 4K. We simulate the main systematic effects related to the temperature and non-ideality of the optical components in a Stokes polarimeter. To limit these effects, a cryogenic implementation of the polarimeter is mandatory. In our system, the rotation produced by a step motor, running at room temperature, is transmitted down to cryogenic temperatures by means of a long shaft and gears running on custom cryogenic bearings. Our system is able to rotate, in a completely automated way, a birefringent crystal at 4K, dissipating only a few mW in the cold environment. A readout system based on optical fibers allows to control the rotation of the crystal to better than 0.1{\deg}. This device fulfills the stringent requirements for operation in cryogenic space experiments, like the forthcoming PILOT, BOOMERanG and LSPE.