Calibration and data extraction in a Stokes polarimeter employing three wavelengths simultaneously.

Abstract

We present a method for calibration and data extraction for a Stokes polarimeter working with three different wavelengths simultaneously. In the Stokes polarimeter considered in this work, we use two liquid crystal variable retarders (LCVRs) combined with a Glan-Thompson linear polarizer. A recently developed fitting calibration procedure is used. We use the same calibration samples and LCVR voltages for all three wavelengths, giving simultaneous measurement and calibration. We compare the performance of the polarimeter, after calibration, using four or six calibration samples in our experiment. To generate the four known calibration beams, we use a linear polarizer oriented at 130° and 30° with respect to the horizontal, a horizontal linear polarizer followed by a half-wave plate (at 632 nm) with its fast axis at 30°, and a horizontal linear polarizer followed by a quarter-wave plate (at 632 nm) with its fast axis at 30°. For calibration with six reference beams, we add two known calibration beams by setting the fast axis of the half- and quarter-wave plates at 130°. Experimental results show good agreement with the expected results, with the fitting calibration procedure giving an approximately 50% reduction in total RMS error with four calibration samples. There is a negligible reduction in the error when six calibration samples are used compared to the case with four samples.