Observing inside the coronagraphic regime with optimized single-mode nulling interferometry

TL;DR

This paper explores optimized nulling interferometry techniques within the coronagraphic regime, introducing a new phase-mask design and identifying the most efficient configuration for high-contrast exoplanet imaging.

Contribution

It provides a unified geometrical framework for nulling interferometers and coronagraphs, introduces the split-ring coronagraph, and identifies the aperture-plane phase knife as the optimal nulling configuration.

Findings

The phase-knife fiber coronagraph achieves a peak efficiency of 35.2%.

The vortex fiber nuller has a maximum efficiency of 19.0%.

A new split-ring phase-mask coronagraph is proposed.

Abstract

The number of terrestrial exoplanets accessible to high-contrast coronagraphic imaging with large telescopes is limited by the smallest angular offset from bright stars at which coronagraphs can observe. However, it is possible to reach inside a telescopes coronagraphic regime by employing nulling interferometry across a telescopes pupil. Indeed, cross-aperture nulling interferometry can observe significantly closer to stars than typical coronagraphs, enabling observations even within the stellar diffraction core. Identifying an optimal nulling coronagraph, i.e., one with both a very small IWA and a high throughput for exoplanet light, would thus be of great interest. A systematic examination of available nulling options has therefore been carried out, which has led to three things. The first is a topological overview that unites both multi-aperture nulling interferometers and single-aperture phase coronagraphs into a common geometrical framework. The second is a new type of phase-mask coronagraph that has emerged from a gap in this framework, called here the split-ring coronagraph. The third is a clear identification of the optimal configuration for a nulling coronagraph, which turned out to be an aperture-plane phase knife, i.e., an achromatic pi-radian phase shift applied to half the telescope pupil prior to focusing the telescopes point spread function into a single-mode fiber. The theoretical peak efficiency of the phase-knife fiber coronagraph, 35.2 percent for a circular telescope aperture, is found to be almost twice that of the next most efficient case, the vortex fiber nuller, at 19.0 percent.