Optimized configuration for liquid crystal Stokes polarimeters in the presence of fast-axis orientation errors.

We present an optimal configuration for Stokes polarimeters based on liquid crystal variable retarders, with the minimum number of measurements. Due to the inherent variations of the director orientation of the liquid crystal molecules, we propose a configuration that minimizes the sensibility of the polarimeter to fast-axis variations. For the optimization we consider a scheme that maximizes the volume of a tetrahedron inscribed in the Poincare sphere, to address additive and Poisson noise, with one of the vertices invariant to changes in the axis positions.

Effects of typical liquid-crystal retarder errors on optimized Stokes polarimeters.

Liquid crystal variable retarders (LCVRs) are often used in Stokes polarimeters as they allow the measurement of different polarization components by applying an electric field that manipulates the induced retardance. However, the optical retardance introduced by these devices is in general not homogenous across the aperture. Another problem with this type of devices is that the fast-axis orientation is not homogenous, and it changes with the applied voltage.

Characterization of retardance spatial variations over the aperture of liquid-crystal variable retarders.

We present a comparison of two experimental methods to measure retardance as a function of applied voltage and as a function of position over the aperture of liquid-crystal variable retarders. These measurements are required for many applications, particularly in polarimetry. One method involves the scan of an unexpanded laser beam over the aperture, and the other uses an expanded beam from a LED and a CCD camera to measure the full aperture with a single measurement.

Method to calibrate a full-Stokes polarimeter based on variable retarders.

We present a calibration method for a full-Stokes polarimeter. The polarimeter uses two liquid-crystal variable retarders (LCVR) and a linear polarizer to measure the four Stokes parameters. The calibration method proposed in this paper calculates the errors in the experimental setup by fitting the experimental intensity measurements for a set of calibration samples to a theoretical polarimeter with errors.