Topological Designs for Scalar Vortex Coronagraphs

TL;DR

This paper explores scalar vortex coronagraphs for direct exoplanet imaging, presenting experimental results, comparative design analysis, and a new optimization tool to enhance broadband performance and chromatic correction.

Contribution

It introduces a new scalar vortex coronagraph design, compares multiple topologies for broadband contrast, and develops an optimization tool for achromatic performance.

Findings

Achieved 2e-7 raw contrast in 10% broadband light with staircase SVC.

Compared several SVC designs to optimize broadband contrast.

Developed a new tool for predicting and optimizing SVC performance.

Abstract

The detection and characterization of Earth-like exoplanets around Sun-like stars for future flagship missions requires coronagraphs to achieve contrasts on the order of 1e-10 at close angular separations and over large spectral bandwidths (>=20%). We present our progress thus far on exploring the potential for scalar vortex coronagraphs (SVCs) in direct exoplanet imaging. SVCs are an attractive alternative to vector vortex coronagraphs (VVCs), which have recently demonstrated 6e-9 raw contrast in 20% broadband light but are polarization dependent. SVCs imprint the same phase ramp on the incoming light and do not require polarization splitting, but are inherently limited by their chromatic behavior. Several SVC designs have been proposed in recent years to solve this issue by modulating or wrapping the azimuthal phase function according to specific patterns. For one such design, the staircase SVC, we present our best experimental SVC results demonstrating raw contrast of 2e-7 in 10% broadband light. Since SVC broadband performance and aberration sensitivities are highly dependent on topology, we conducted a comparative study of several SVC designs to optimize for high contrast across a range of bandwidths. Furthermore, we present a new coronagraph optimization tool to predict performance in order to find an achromatic solution.