Nonideal optical response of liquid crystal variable retarders and its impact on their performance as polarization modulators

Abstract

Liquid crystal variable retarders (LCVRs) will be used for the first time in a space instrument, the Solar Orbiter mission of the European Space Agency, as polarization states analyzers (PSAs). These devices will determine the Stokes parameters of the light coming from the Sun by temporal polarization modulation, using the so-called modulation matrix O. This is a matrix constituted by the first rows of properly selected PSA Mueller matrices. Calibrating a space instrument, in particular, finding O, is a critical point because in a spacecraft there is no possibility of physical access. Due to the huge difficulty in calibrating the complete instruments in all possible scenarios, a more complete calibration of the individual components has been done in ground in order to make extrapolations to obtain O in-flight. Nevertheless, apart from the individual calibrations, the experimental errors and nonideal effects that inhibit the system to reach the designed and theoretical values must be known. In this work, description and study of these effects have been done, focusing on the nonideal effects of the LCVRs and the azimuthal misalignments between the optical components of the PSA during the mechanical assembly. The Mueller matrix of a representative LCVR has been measured and mathematically decomposed by logarithm decomposition, looking for values of circular birefringence and fast axis angle variations as a function of voltage. These effects, in the absence of other nonidealities, affect the polarimetric performance, reducing the polarimetric efficiencies in some cases until 11%. Nevertheless, in this case, they are negligible if compared to the other nonideality studied, which are the azimuthal misalignments between the PSA optical components. The study presented in this work is key to extrapolate the PSA O matrix if the expected instrumental set-point temperatures are not reached in flight and can be used for the design and implementation of other polarimetric instruments.