Laboratory demonstration of real-time focal plane wavefront control of residual atmospheric speckles

Abstract

Current and future high contrast imaging instruments aim to detect exoplanets at closer orbital separations, lower masses, and/or older ages than their predecessors. However, continually evolving speckles in the coronagraphic science image limit contrasts of state-ofthe-art ground-based exoplanet imaging instruments. For ground-based adaptive optics (AO) instruments, it remains challenging for most speckle suppression techniques to attenuate both the dynamic atmospheric as well as quasistatic instrumental speckles on-sky. We have proposed a focal plane wavefront sensing and control algorithm to address this challenge, called the fast atmospheric selfcoherent camera (SCC) technique (FAST), which in theory enables the SCC to operate down to millisecond timescales even when only a few photons are detected per speckle. Here, we present the first experimental results of FAST on the Santa Cruz Extreme AO Laboratory (SEAL) testbed. In particular, we illustrate the benefit of "second stage" AO-based focal plane wavefront control, demonstrating up to 5× contrast improvement with FAST closedloop compensation of evolving residual atmospheric turbulence-both for low and high order spatial modes-down to 20-ms timescales.

Various Wavefront Sensing and Control Developments on the Santa Cruz Extreme AO Laboratory (SEAL) Testbed

“…These results are consistent with the FAST findings in Ref. 4, where we demonstrated how four different modal groups enabled good linearity and closedloop performance: TTF, LO Zernike modes (labeled "Zernikes"), LO Fourier modes (labeled "LO Fourier," still diffracting sine spots within the FAST focal plane mask inner working angle), and high order Fourier modes (labeled "HO Fourier," using the conventional modal basis to generate a half dark hole, or DH). The former three modes are controlled with the LODM, where as the latter high order Fourier modal group is controlled with the HODM.…”

“…• §3: Multi-WFS SCAO applied to FAST-SHWFS operations. This demonstration uses the Fast Atmospheric Self-coherent camera (SCC) Technique (FAST) 4,10 and Shack Hartmann WFS (SHWFS) sensors to control our LO (LO) ALPAO 97 actuator DM (hereafter referred to as the LODM, for low-order DM) and Boston Micromachines Corporation (BMC) high order DM (34 actuators across the beam diameter, hereafter referred to as the HODM, for high order DM, and in combination WT, for "woofer-tweeter").…”

“…We also note here that in this paper we utilize SEAL calibration procedures described in Ref. 4, both specifically for FAST and more generally for SEAL use (e.g., LODM And HODM voltage-to-wavefront conversion).…”

“…We also implement the same LO acquisition procedure as in Ref. 4 and as in Fig. 2: first closing TTF, then closing LODM Zernike and Fourier modes, and lastly closing HODM Fourier modes, which we found was necessary to enable stable closed-loop DHs over time.…”

High-speed Focal Plane Wave Front Sensing with an Optical Chopper

“…Our laboratory setup, mimicking Figure 1(a), uses SEALthe Santa Cruz Extreme AO Laboratory (Gerard et al 2022a;Jensen-Clem et al 2021). We use a Thorlabs KLS635 laser (set at 0.15 mW unless otherwise noted), ALPAO 97 actuator DM (which also serves as the system pupil stop), Andor Zyla 5.5 sCMOS detector, Thorlabs WFS-20 SHWFS (for this paper used only to flatten the ALPAO DM), a Thorlabs MC2000B optical chopper with blade MC1F10, and a Stanford Instruments DG535 controller and an in-house electrical doubler so that the Andor detector readout is synchronized with the optical chopper (with the chopper as the leader and the Andor camera as the follower 5 ) to produce a chopped and unchopped pupil every other frame, respectively.…”