New method to achieve the proper polarization state for a vector vortex coronagraph

TL;DR

This paper introduces a practical method to achieve the correct polarization state for a vector vortex coronagraph, enhancing its performance in high-contrast imaging systems like those proposed for space telescopes.

Contribution

The authors present a novel polarization control technique using only a deformable mirror and optical rotations, simplifying the process for VVC implementation.

Findings

Achieved raw contrast levels of 1e-2 for broadband light at HCST.

Demonstrated effective polarization state control with minimal additional optics.

Improved VVC performance in laboratory conditions.

Abstract

The vector vortex coronagraph (VVC) performance in the laboratory and in ground-based observatories has earned it a spot on the NASA mission concepts HabEx and LUVOIR. The VVC induces a phase ramp through the manipulation of the polarization state. Left- and right-circular polarizations get imprinted a phase ramp of opposite signs, which prevents model-based focal plane wavefront sensing and control strategies in natural light. We thus have to work with a polarization state than ensures circularly polarized light at the VVC mask. However, achieving this polarization state can be non trivial if there are optics that add phase retardance of any kind between the circular polarizer and the focal plane mask. Here we present the method currently used at the Caltech high contrast spectroscopy testbed (HCST) to achieve the proper circular polarization state for a VVC, which only uses the deformable mirror and appropriate rotation of the circular polarizer and analyzer optics. At HCST we achieve raw contrast levels of \tentoe~for broadband light with a VVC.