Dual-beam polarimeter based on nonachromatic wave plates with high polarimetric efficiency over a broad band

The design and test of a dual-beam polarimeter to be applied to the Fiber Array Solar Optical Telescope of the second generation is described. The polarimeter consists of a half and a quarter nonachromatic wave plate, followed by a polarizing beam splitter as a polarization analyzer. It has the features of simple structure, stable operation, and temperature insensitivity. The most outstanding feature of the polarimeter is that a combination of commercial nonachromatic wave plates is used as a modulator to have the high polarimetric efficiency of the Stokes polarization parameters over 500-900 nm; the efficiency balance among the linear and circular polarization parameters is also taken into account. In order to see the stability and reliability of this polarimeter, we perform the actual measurement of the polarimetric efficiencies of the assembling polarimeter in the laboratory. It is found that the lowest linear polarimetric efficiency is over 0.46, the lowest circular polarimetric efficiency is above 0.47, and the total polarimetric efficiency is greater than 0.93 over 500-900 nm. The measured results are basically consistent with the theoretical design. Thus, the polarimeter guarantees observers to choose freely spectral lines, formed in different layers of the solar atmosphere. It can be concluded that such a dual-beam polarimeter based on nonachromatic wave plates has excellent performance and can be extensively applied in astronomical measurement.