Optimization of the response time measuring method for liquid crystal variable retarders

Abstract

Liquid crystal variable retarders (LCVRs) have been extensively used as light polarization modulators for ground-based polarimetric applications. Shortly, LCVRs will be used as polarization state analyzers in two instruments onboard the Solar Orbiter mission of the European Space Agency. Both ground- and space-based polarimeters require LCVR response time values that fulfill the required image acquisition rate of the polarimetric measurements. Therefore, it is necessary to have a reliable method to measure the LCVR optical retardance response times. Response times are usually estimated via optical methods using crossed or parallel polarizers. Nevertheless, these methods measure light intensity transitions to infer the response time instead of directly measuring the changes in the optical retardance. In this work, an experimental setup that uses a Soleil-Babinet variable compensator is proposed. On one hand, this allows one to study the effect of the nonlinear dependence of the light intensity on the optical retardance in the response time determination, which is neglected in most works. On the other hand, the use of the variable compensator allows one to measure the LCVR response times in the highest sensitivity areas of the system that minimizes the uncertainty of the measurement. The six transitions for the Polarimetric and Helioseismic Imager instrument modulation scheme of a representative LCVR have been measured. Based on the results, the optimized conditions to measure response times are found, which can be achieved by using the variable compensator and an IR wavelength (λ = 987.7 nm) as proposed in the experimental setup.