Estimation and correction of wavefront aberrations using the self-coherent camera: laboratory results

TL;DR

This paper introduces a novel method using the self-coherent camera to estimate and correct wavefront aberrations in high-contrast imaging systems, demonstrating laboratory success in achieving extremely high contrast levels for exoplanet detection.

Contribution

The paper presents a new technique employing the self-coherent camera for direct measurement and correction of wavefront aberrations upstream of a coronagraph, validated through laboratory experiments.

Findings

Achieved contrast better than 10^-6 between 2 and 12 λ/D.

Demonstrated stable closed-loop correction of aberrations.

Identified limitations due to deformable mirror surface defects.

Abstract

Direct imaging of exoplanets requires very high contrast levels, which are obtained using coronagraphs. But residual quasi-static aberrations create speckles in the focal plane downstream of the coronagraph which mask the planet. This problem appears in ground-based instruments as well as in space-based telescopes. An active correction of these wavefront errors using a deformable mirror upstream of the coronagraph is mandatory, but conventional adaptive optics are limited by differential path aberrations. Dedicated techniques have to be implemented to measure phase and amplitude errors directly in the science focal plane. First, we propose a method for estimating phase and amplitude aberrations upstream of a coronagraph from the speckle complex field in the downstream focal plane. Then, we present the self-coherent camera, which uses the coherence of light to spatially encode the focal plane speckles and retrieve the associated complex field. This enable us to estimate and compensate in a closed loop for the aberrations upstream of the coronagraph. We conducted numerical simulations as well as laboratory tests using a four-quadrant phase mask and a 32x32 actuator deformable mirror. We demonstrated in the laboratory our capability to achieve a stable closed loop and compensate for phase and amplitude quasi-static aberrations. We determined the best-suited parameter values to implement our technique. Contrasts better than 10-6 between 2 and 12 lambda/D and even 3.10-7 (RMS) between 7 and 11 lambda/D were reached in the focal plane. It seems that the contrast level is mainly limited by amplitude defects created by the surface of the deformable mirror and by the dynamic of the detector. These results are promising for a future application to a dedicated space mission for exoplanet characterization. A number of possible improvements have been identified.