Experimental demonstration of a dark hole at the diffraction limit

Abstract

High-contrast imaging at very small angular separations is essential for detecting and characterizing exoplanets orbiting close to their host stars. Future segmented telescopes will play a key role in direct exoplanet imaging by enabling the large apertures required to achieve high angular resolution and sensitivity. However, their complex pupils introduce challenging diffraction effects. We experimentally demonstrate the formation of a dark hole (DH) at the diffraction limit on a segmented pupil. Experiments were conducted on the SPEED test bed, a high-contrast imaging platform reproducing the diffraction environment of segmented apertures with a small inner working angle coronagraph. The setup implements a novel multi-deformable mirror (DM) architecture, in which two widely separated out-of-pupil DMs provide efficient amplitude and phase control at the diffraction limit. Focal-plane wavefront control was performed using implicit electric field conjugation (iEFC). We demonstrate a stable DH from 1 to 4 łambda/D in the H band, providing, to our knowledge, the first experimental validation of diffraction-limited DH formation reaching 1 łambda/D. Beyond this achievement, the demonstration is performed on a segmented telescope, representing the first realization of a diffraction-limited DH with a segmented pupil and iEFC focal-plane wavefront control. Wavefront control and shaping down to (1,łambda/D) is challenging because of the PSF-core proximity and increased sensitivity to various factors. Rather than claiming record contrast or superiority over conventional DMs, this work validates the proposed two-DM architecture for close-in DH control on a segmented aperture, advancing the technology readiness of two-DM DH generation.