Optical EngineeringFerroelectric retarders as an alternative to piezoelastic modulators for use in solar Stokes vector polarimetry
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We present a concept to modulate all three polarized Stokes components Q, U and V using two standard ?/2 ferroelectric liquid- crystal modulators. Thereby the complete Stokes vector (two linear polarizations and circular polarization) can be deduced from four intensity measurements taken at different phase intervals of the modulation. With ferroelectric liquid crystals, modulation frequencies in the kilohertz range are easily possible, which is necessary for applications in solar astronomy to overcome the influence of turbulence in the earth's atmosphere (seeing). No moving parts that could introduce spurious optical effects are used. In combination with fast demodulators, high-precision vector polarimetry in the visible can be performed. In contrast to conventional modulation schemes using one ?/2 and one ?/4 variable retarder, the technique presented here allows higher polarimetric efficiency and accuracy over a broad spectral range. A theoretical description is presented and compared with first test results on the system's performance.