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.
Highlights
Liquid crystal variable retarders (LCVRs) have been extensively used as light polarization modulators for groundbased polarimetric applications.1–6 The Solar Orbiter mission of the European Space Agency will be the first space mission that will use LCVRs onboard its payload
An important feature of ferroelectric liquid crystals (FLCs) is that these devices present faster response than nematic liquid crystals (NLCs), and, they become the best option in applications where the acquisition velocity is a crucial factor
We present an experimental setup that improves the standard method to measure LCVR response times
Summary
Liquid crystal variable retarders (LCVRs) have been extensively used as light polarization modulators for groundbased polarimetric applications. The Solar Orbiter mission of the European Space Agency will be the first space mission that will use LCVRs onboard its payload. The sensitivity to determine the response times of the optical retardance extracted from the light intensity measurement is different along the curve and must be taken into account For this reason, an experimental setup that uses a Soleil-Babinet compensator is proposed in this work. The experimental setup uses a longer wavelength (λ = 987 nm) than the PHI operative wavelength (λ = 617.3 nm) In this way, we reduce the optical retardance change of all the LCVR transitions studied below π radians, avoiding overtaking a maximum in the light intensity sinusoidal signal and simplifying the transitions measurement analysis. A deeper understanding of the method sensitivity and how this affects the response time measurements allow defining experimental setups and optimal conditions for the measurement and inferring realistic response values of the complete instrument for the final application
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