High contrast demonstrations with the vortex coronagraph
The optical vortex coronagraph (OVC) has great potential both for carrying out high-contrast exoplanet searches around nearby stars, and for reducing the size of the space telescopes needed for exoplanet imaging and spectral characterization missions. This is because using a coronagraphic mask to modify the phase of a focal-plane stellar point spread function, instead of its intensity, leaves transmissions high even at small angular offsets from the center of the mask. Here we discuss recent progress in demonstrating the capabilities of the OVC, including the production of vector vortex phase masks, and recent contrast measurements obtained for optical vortex masks in the Jet Propulsion Laboratory's High Contrast Imaging Testbed (HCIT). Specifically, monochromatic contrasts below 10-9 have recently been demonstrated across wavefront-corrected “dark hole” regions extending from 2–7 λ/D from a laboratory simulated “stellar” point source, where λ is the wavelength, and D is the aperture diameter. Initial broadband (10% bandwidth) contrasts are not far behind, at approximately the 10−8 level. Further work is aimed at deepening and broadening wideband contrasts in the dark hole. It is anticipated that further development of broadband vortex phase masks should be able to provide the capabilities required by future space-based coronagraphic instruments aimed at exoplanet imaging and spectroscopy.