Design and Performance Analysis of a Highly Efficient Polychromatic Full-Stokes Polarization Modulator for the CRISP Imaging Spectrometer

TL;DR

This paper details the design, analysis, and testing of a broadband polarization modulator for solar spectropolarimetry, demonstrating near-optimal performance over a broad wavelength range since 2015.

Contribution

It introduces a novel design process and tolerance analysis for a polychromatic polarization modulator, applicable to broadband applications in solar observations.

Findings

Performance close to optimal between 500-900 nm

Design differences largely understood and manageable

Operational since 2015 with successful scientific use

Abstract

We present the design and performance of a polychromatic polarization modulator for the CRisp Imaging SpectroPolarimeter (CRISP) Fabry-Perot tunable narrow-band imaging spectropolarimer at the Swedish 1-m Solar Telescope (SST). We discuss the design process in depth, compare two possible modulator designs through a tolerance analysis, and investigate thermal sensitivity of the selected design. The trade-offs and procedures described in this paper are generally applicable in the development of broadband polarization modulators. The modulator was built and has been operational since 2015. Its measured performance is close to optimal between 500 and 900~nm, and differences between the design and as-built modulator are largely understood. We show some example data, and briefly review scientific work that used data from SST/CRISP and this modulator.