Dynamic characteristics of nematic liquid crystal variable retarders investigated by a high-speed polarimetry

Abstract

Liquid crystal variable retarders (LCVRs) have been widely used as a transmissive element with electrically tunable optical phase retardance in polarimetric optical systems. Since the responsive characteristics of the LCVRs depend on the modulation processes, accurately and rapidly characterizing LCVRs is of great significance for the performance improvement of the optical system with LCVRs. Firstly, we proposed a Mueller matrix based model to characterize an LCVRs with considerations of both linear birefringence (LB) and circular birefringence (CB), with which the LCVRs is described by several driving voltage-dependent optical parameters such as retardance, azimuth, relative transmittance and optical rotation angle. The experimental results show that the Mueller matrices of the LCVRs measured by a commercial Mueller matrix ellipsometer are consistently fitted by the proposed method, and the improvement of the proposed characterization method can be read from the 15 times reduced average residual errors of the reconstructed Muller matrix compared to the matrix reconstructed with the conventional characterization methods. Through several typical dynamic measurement experiments using a high-speed Stokes polarimeter with a temporal resolution of several nanoseconds, we demonstrated the driving-voltage-dependence of these optical parameters as well as the existence of both LB, CB and depolarization properties in the LCVRs modulation processes. Based on the analysis of the extracted optical parameters, we have obtained the continuous modulation characteristics and step response characteristics of LCVRs. Additionally, a comparison between the Mueller matrices of air measured with the proposed and conventional characterization method has been carried out to demonstrate the fidelity of the proposed method as well.