Laboratory demonstration of wavefront control through a single-mode fiber over a 20% bandwidth for the characterization of exoplanet atmospheres

Abstract

To address the fundamental questions of exoplanetary science, future space-based observatories will have to obtain quality spectra of a large enough set of Earth-like planets around main-sequence stars. Although coronagraph instruments provide the necessary observational efficiency to probe many systems with respect to a starshade observation, they typically suffer from a limited achievable bandwidth at the necessary contrast and relatively poor throughput to off-axis sources. This is mainly due to the fact that the starlight is suppressed within the optical system, so the quasi-static aberrations from optical imperfections are the dominant term and need to be dealt with deformable mirrors (DMs). The DMs have limited capabilities to achieve large bandwidths, and their high stroke after corrections is highly detrimental to the Strehl ratio of off-axis sources. A technological path to overcome these issues is the use of single-mode fibers (SMFs). Coupling the planet light into an SMF to feed a high-resolution spectrograph has been shown to improve the final signal-to-noise ratio. Furthermore, it has been shown that it is more favorable to do broadband wavefront control with SMFs when exploiting their modal selectivity; the DMs have to work less so the bandwidth is improved and the off-axis throughput is better. Here, we demonstrate the potential of this technology by performing wavefront control through an SMF in a two-step process: first, by digging a small dark hole around the position of the SMF, and second, performing an innovative version of the electric field conjugation algorithm modified for SMFs. We perform these experiments with 20% bandwidth light at the high-contrast spectroscopy testbed achieving 2.5 × 10⁻⁸ raw contrast.