Construction of a Large Diameter Reflective Half-Wave Plate Modulator for Millimeter Wave Applications

TL;DR

This paper presents the design and initial testing of a large 60 cm diameter reflective half-wave plate modulator for millimeter wave polarization measurements, aiming to improve measurement stability and reduce systematic errors.

Contribution

It introduces a novel large-diameter RHWP design with a custom drive system suitable for 77-108 GHz applications, advancing polarization modulation technology.

Findings

Successful laboratory performance of the prototype

Precise flatness and alignment of wires achieved

Effective rotation mechanism implemented

Abstract

Polarization modulation is a powerful technique to increase the stability of measurements by enabling the distinction of a polarized signal from dominant slow system drifts and unpolarized foregrounds. Furthermore, when placed as close to the sky as possible, modulation can reduce systematic errors from instrument polarization. In this work, we introduce the design and preliminary drive system laboratory performance of a new 60 cm diameter reflective half-wave plate (RHWP) polarization modulator. The wave plate consists of a wire array situated in front of a flat mirror. Using \mbox{50 $\mu$m} diameter wires with \mbox{175 $\mu$m} spacing, the wave plate will be suitable for operation in the millimeter wavelength range with flatness of the wires and parallelism to the mirror held to a small fraction of a wavelength. The presented design targets the 77--108 GHz range. Modulation is performed by a rotation of the wave plate with a custom rotary drive utilizing an actively controlled servo motor.