Polarimetric performance of a polarization modulator based on liquid crystal variable retarders for wide acceptance angles

Pilar García Parejo,Gerardo Capobianco,Silvano Fineschi,Alberto Álvarez-Herrero

doi:10.1117/1.jatis.5.3.034002

Abstract

Liquid crystal variable retarders (LCVRs) will be used in the polarization modulation packages (PMPs) of the instruments SO/PHI (Polarimetric and Helioseismic Imager) and METIS/COR (Multielement Telescope for Imaging and Spectroscopy, Coronagraph) of the Solar Orbiter Mission of the European Space Agency (ESA). Optical retarders are dependent on the angle of incidence (AOI). Since the optical retardances during the polarization modulations are optimized for a particular AOI, other angles increase the polarimetric measurement error. Coronagraphs, such as METIS, are characterized by having wide field-of-view (FoV), which involves large incidence angles through the entire instrument. METIS PMP will work with collimated beams and an AOI up to ±7.0 deg. For this reason, a double LCVR configuration with molecular tilts in opposite directions was selected for METIS PMP, which provides lower angular dependence. The polarimetric performance of the METIS PMP flight model was measured at different AOIs and compared to a single LCVR PMP. The results shown in this paper demonstrate that the functional concept used in METIS guarantees the polarimetric performances at the wide FoV expected in METIS coronagraph. Moreover, a detailed theoretical model is showed and compared to the experimental data, finding successful agreement, which can be very helpful for the design of instruments characterized by wide FoV.

Highlights

Liquid crystal variable retarders (LCVRs) are optical devices that provide a variable optical retardance achieved by the application of electric fields
Up to Æ7.0 deg of the angle of incidence (AOI), which are the higher working incident angles of METIS coronagraph, the polarimetric performances of the double-cell polarization modulation packages (PMPs) keep stable with slight variations, and a significant change is observed for the case of the single-cell configuration
The METIS PMP concept using two LCVR cell with their molecular tilts in opposite directions guarantees an excellent maintenance of the polarimetric performances at the wide FOV expected in METIS coronagraph

Summary

Introduction

Liquid crystal variable retarders (LCVRs) are optical devices that provide a variable optical retardance achieved by the application of electric fields They have been used as light polarization modulators for polarimetric applications in ground telescopes.[1] For space applications, LCVRs provide many advantages with respect to the traditional rotatory polarizing optics, in particular, low voltage and power requirements, low mass, low volume, large clear apertures, and they avoid the utilization of mechanisms. This mission is a medium-class mission of the Cosmic Vision Program of the European Space Agency (ESA), being developed in collaboration with NASA The objective of this space mission is to study the Sun and its inner heliosphere at an approach point to the Sun as close as 0.28 AU. In Solar Orbiter, the LCVRs will be used as polarization modulators[2] in 2 of the 10 scientific instruments: SO/PHI (Polarimetric and Helioseismic Imager) and METIS (Multi Element Telescope for Imaging and Spectroscopy, Coronagraph)

Objectives

Results

Conclusion