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

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 telescope's coronagraphic regime by employing nulling interferometry across a telescope's 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 "splitring" 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 p-radian phase shift applied to half the telescope pupil prior to focusing the telescope's point spread function (PSF) into a single-mode fiber. The theoretical peak efficiency of the phase-knife fiber coronagraph, 35.2% 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%.