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.
Liquid crystals on silicon spatial light modulator (LCOS-SLM) combine the potential of reflection type spatial light modulators with the compactness and robustness of a single chip. They are used today for beam steering applications, optical beam shaping and laser processing. These devices have a high potential for space applications due to the fact that they allow to introduce any tailored wavefront distortion in an imaging instrument. Then, image reconstruction methods as phase diversity can be used to determine the Point Spread Function (PSF) inflight and, later, to introduce a corrective wavefront distortion to correct possible deviations of the expected optical quality.Among other aberrations, the beam phase control can act on the level of focus. In space optical applications image refocusing is usually performed by means of mechanisms, either by using linear displacement of lenses or rotating wheels with plates with different thicknesses. The compactness and absence of mechanical parts of LCOS-SLM can be of great advantage for these applications. LCOS-SLM can save complexity and weight. It also reduces the risk associated to the wear of moving parts.However, this technology has not been qualified for space applications. Liquid crystal leaks as well as outgassing issues may result as a consequence of a low pressure environment. Thermal issues can also result in loss of device homogeneity and the radiation tolerance should be analyzed. In any case, an exhaustive space simulation test is mandatory to increase the Technological Readiness Level of these devices for their use in space systems.In our work we are showing preliminary test of a commercial LCOS-SLM under thermo-vacuum conditions. These tests are basic calibrations used to evaluate performance and degradation in a simulated space environment. Different calibration procedures are also discussed. This technology, with potential to greatly simplify an instrument design, was included in a proposal for the instrument IMaX+ spectro-polarimeter, to be onboard the mission Sunrise III, within the NASA Long Duration Balloon program.
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.
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