Liquid-crystal polarization state generator

Abstract

An important problem in imaging polarimetry occurs when the optical axis of the system and the center of the camera sensor get misaligned. This situation typically occurs after rotating the polarizing element mounts in order to change the input state of polarization. This work presents a liquid-crystal polarization state generator, devoid of moving parts, which can generate any arbitrary state of polarization (SOP) on the Poincare sphere through the phase-shift manipulation of two voltage-controlled variable retarders. The proposed optical system consists of a linear polarizer, cascaded by two liquidcrystal retarders (LCR1 and LCR2) and a quarter-wave plate. We show that by varying the retardance of LCR1 but keeping the LCR2 retardance constant, the SOP moves along the corresponding meridian of the Poincare sphere. When the reverse is done, the SOP follows a trajectory along the given parallel. Experimental results are compared to numerical simulations where we calculate the Stokes parameters and represent the trajectories of the SOP on the Poincare sphere as we change the voltage addressed to the LCRs. Good agreement between theory and experiment is obtained if we take into account the Fabry-Perot interference effects on these variable retarders. This system can also be used as a polarization state analyzer. To verify the performance of this system as analyzer, the Mueller Matrix of a retarder plate is determined by imaging polarimetry.