# The short wavelength instrument for the polarization explorer balloon-borne experiment: Polarization modulation issues

## TL;DR

This paper examines the use of a polarization modulator in the SWIPE instrument of LSPE, focusing on its impact on polarization measurement accuracy and the trade-offs between different modulation strategies.

## Contribution

It provides a detailed analysis of polarization modulation techniques, especially the choice between stepped and spinning HWPs, and their effects on data quality in CMB polarization measurements.

## Key findings

- Spinning HWP chosen over stepped for better modulation.
- Modulation at 4 times the spin frequency effectively shifts the signal away from 1/f noise.
- Characterization of spurious signals at 2f and 4f is essential for data accuracy.

## Abstract

In this paper we investigate the impact of using a polarization modulator in the Short Wavelenght Instrument for the Polarization Explorer (SWIPE) of the Large Scale Polarization Explorer (LSPE). The experiment is optimized to measure the linear polarization of the Cosmic Microwave Background at large angular scales during a circumpolar long-duration stratospheric balloon mission, and uses multi-mode bolometers cooled at 0.3 K. The 330 detectors cover 3 bands at 140 GHz, 220 GHz and 240 GHz. Polarimetry is achieved by means of a large rotating half-wave plate (HWP) and a single wire-grid polarizer in front of the arrays. The polarization modulator is the first polarization-active component of the optical chain, reducing significantly the effect of instrumental polarization. A trade-off study comparing stepped vs spinning HWPs drives the choice towards the second. Modulating the CMB polarization signal at 4 times the spin frequency moves it away from $1/f$ noise from the detectors and the residual atmosphere. The HWP is cooled at 1.6 K to reduce the background on the detectors. Furthermore its polarized emission combined with the emission of the polarizer produces spurious signals modulated at $2f$ and $4f$. The $4f$ component is synchronous with the signal of interest and has to characterized to be removed from cosmological data.