Simultaneous phase and amplitude retrieval with COFFEE: from theory to laboratory results

Abstract

The final performance of current and future instruments dedicated to exoplanet detection and characterization (such as SPHERE on the VLT, GPI on Gemini North or future instruments on the ELTs) is limited by intensity residuals in the scientific image plane, which originate in uncorrected optical aberrations. After correction of the atmospheric turbulence, the main contribution to these residuals comes from the quasi-static aberrations introduced upstream of the coronagraph which create long-lived speckles in the detector plane that can easily be mistaken for a planet. In order to reach very high contrast such as the ones required to image earth-like planets, these aberrations needs to be compensated for. We have recently proposed a dedicated focal-plane wave-font sensor called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet detection), which consists in an extension of conventional phase diversity to a coronagraphic system: aberrations both upstream and downstream of the coronagraph are estimated using two coronagraphic focal-plane images, recorded from the scientific camera itself, without any differential aberration. Such a system has been successfully validated on the SPHERE instrument, where COFFEEs estimation has been used to compensate for the phase aberration upstream of the coronagraph, leading to a contrast optimization in the whole focal plane area controlled by the AO loop. If compensating for phase aberrations only was acceptable to reach levels of contrast of 10􀀀6, it will no longer be the case for instruments that aim at imaging earth-like planets. Such targets, which would be the ones of a planet-finder instrument integrated on an ELT, require a level of contrast better than 10􀀀9. To reach this level, neglecting amplitude aberrations (inhomogeneous intensity in the pupil, Fresnel propagation effect) is no longer possible. In this communication, we present an extension of COFFEE able to perform a simultaneous estimation of both phase and amplitude aberration from three focal plane images. After a theoretical presentation, we present a study of its performances. Notably, we analyze the contrast that can be achieved in a compensation process when this estimation method is combined with our non-linear dark hole method, demonstrating that the nanometric precision estimation that can be achieved with COFFEE allow one to reach very high contrast levels. It is worth mentioning that both estimation (COFFEE) and compensation (the nonlinear dark hole) methods are model based, and thus easily adaptable to a broad class of coronagraphic device. Lastly, we validate our complex field estimator on the LAM (Laboratoire dAstrophysique de Marseille) XAO test bench, described in this communication. We introduce calibrated phase and amplitude aberration in the entrance pupil plane. Then, we demonstrate the ability of our extended version of COFFEE to estimate both phase and amplitude aberration from three coronagraphic focal plane images that differs from a known aberration.