A Laboratory Method for Measuring the Cross-Polarization in High-Contrast Imaging

Abstract

Existing Electric Field Conjugation (EFC) methods are not suited for treating small polarization effects, referred to here as cross polarization. EFC utilizes a deformable mirror (DM) to nullify the electric fields from the host star within a \emph{dark hole} region in the image plane, allowing the detection of planets. This article presents realistic numerical simulations of a laboratory method for measuring the cross-polarization electric field in a stellar coronagraph. This novel method uses linear polarizers to isolate the cross polarization component and probing procedures similar to those in existing EFC methods to measure the electric field corresponding to cross polarization. The proposed nonlinear probing scheme is specifically designed to address polarizer leakage. The method is not a generalization of existing EFC procedures in order to null the cross polarization field; rather, it is a non-iterative laboratory calibration procedure with applications in instrument characterization and validation of numerical models. However, lab-validated numerical models of the cross-polarization are directly applicable to on-sky observations. The proposed experimental method is not significantly more demanding than existing testbed procedures, requiring little additional equipment. Yet, the simulation results demonstrate highly accurate estimates of the electric field corresponding to the cross polarization in a dark hole. These encouraging results suggest that laboratory implementation is viable. Simulations are performed for a randomly aberrated Lyot coronagraph with a dark hole size of $\sim 4 \lambda/D \times 4 \lambda/D$ in the image plane with a contrast of $\sim 10^{-10}$ in contrast (i.e., the planet-to-star intensity ratio) units. The cross-polarization electric fields accurately estimated by the method correspond to intensities of $\sim 10^{-11}$ in contrast units.