A High-Efficiency and High-Accuracy Polarimeter for Solar Magnetic Field Measurements

Abstract

Solar activity is dominated by the magnetic field. Nowadays, a polarimeter is a mandatory tool to measure solar magnetic fields, which are generally faint and correspond to a polarization of an order of $10^{-2}$ – $10^{-4}$ . As such, polarization measurements of high efficiency with a high accuracy are crucial to investigate faint magnetic fields. Here we propose a high-efficiency and high-accuracy polarimeter, which is based on a pair of nematic liquid crystal variable retarders (LCVRs) and a Wollaston prism (WP). It uses a dedicated Stokes modulation strategy to achieve high efficiency. A calibration unit (CU) is developed to measure the polarimeter response matrix, which provides a high-precision calibration to correct possible systematic errors. Compared with other traditional polarimeters, the modulation scheme of our polarimeter is flexible. In addition to be able to measure all the three Stokes polarization components ( $Q$ , $U$ , or $V$ ) simultaneously, it can also measure one or two of these polarization components alone, with high polarization efficiency. Dedicated alignment and calibration techniques optimized for our polarimeter are developed and high measurement accuracy is achieved. In our laboratory experimental test, our two-image based polarization measurement delivers an overall measurement accuracy of the order of $10^{-4}$ , which is about 10 times better, compared with our previous polarimeters that use the traditional four-image polarization modulation. This work provides a new option for high-efficiency and high-accuracy polarization measurement for future solar synoptic observations.