Toward deep single-mode cross-aperture nulling

Exoplanets can potentially be observed very close to stars using single-mode cross-aperture nulling interferometry, an optical-fiber based approach in which starlight rejection occurs as a result of a coupling mismatch between an anti-symmetric stellar input field and a symmetric fiber mode. The input stellar point-spread function is first modified by passage through an appropriate upstream pupil-plane phase mask, such as a phase-knife mask, which provides a -radian phase step, or equivalently, a relative field reversal, across a pupil bisector. Using a phase mask with a “crossed” half-wave plate structure to produce the desired relative field reversal, a narrowband null depth of 2.2  10-5 has been demonstrated in the laboratory, a rejection level that is sufficient for the detection of Hot Jupiters with large ground-based telescopes. Moreover, as is shown here, phase masks with slightly more complex spatial patterns can in theory lower the stellar leakage due to finite stellar diameters into the 10-9 to 10-10 range needed for terrestrial exoplanet observations. Because nulling performance improves with wavelength, near-infrared cross-aperture nulling may thus be able to serve as a long-wavelength complement to visible wavelength coronagraphy on future exoplanet missions such as the Habitable Worlds Observatory.