The Single-mode Complex Amplitude Refinement (SCAR) coronagraph: I. Concept, theory and design

The discovery of an Earth-mass exoplanet around the nearby star Proxima Centauri provides a prime target for the search for life on planets outside our solar system. Atmospheric characterization of these planets has been proposed by blocking the starlight with a stellar coronagraph and using a high-resolution spectrograph to search for reflected starlight off the planet. Due to the large flux ratio and small angular separation between Proxima b and its host star ($\lesssim10^{-7}$ and $\lesssim2.2\lambda/D$ respectively; at 750nm for an 8m-class telescope) the coronagraph needs to have a high starlight suppression at low inner-working angles. We aim to find the global optimum of an integrated coronagraphic integral-field spectrograph. We present the Single-mode Complex Amplitude Refinement (SCAR) coronagraph that uses a microlens-fed single-mode fiber array in the focal plane downstream from a pupil-plane phase plate. The mode-filtering property of the single-mode fibers allows for the nulling of starlight on the fibers. The phase pattern in the pupil plane is specifically designed to take advantage of this mode-filtering capability. Second-order nulling on the fibers expands the spectral bandwidth and decreases the tip-tilt sensitivity of the coronagraph. The SCAR coronagraph has a low inner-working angle ($\sim1\lambda/D$) at a contrast of $<3\times10^{-5}$ for the 6 fibers surrounding the star using a sufficiently-good adaptive optics system. It can operate over broad spectral bandwidths ($\sim20\%$) and delivers high throughput ($>50\%$ including fiber injection losses). Additionally, it is robust against tip-tilt errors ($\sim0.1\lambda/D$ rms). We present SCAR designs for both an unobstructed and a VLT-like pupil. The SCAR coronagraph is a promising candidate for exoplanet detection and characterization around nearby stars using current high-resolution imaging instruments.