Stroke saturation on a MEMS deformable mirror for woofer-tweeter adaptive optics

Area of Science:

• Astronomy and Astrophysics
• Optical Engineering

Background:

• Ground-based adaptive optics (AO) systems enable high-contrast imaging of exoplanets around main-sequence stars.
• The Gemini Planet Imager (GPI) is an advanced "extreme" AO instrument designed to improve these observations.
• Achieving high Strehl ratios (>90%) requires specialized deformable mirrors (DMs) and coronagraphs to suppress starlight.

Purpose of the Study:

• To investigate the stroke limitations and saturation of micro-electrical mechanical systems (MEMS) deformable mirrors (DMs).
• To determine the conditions under which MEMS DMs experience stroke insufficiency during high-contrast imaging simulations.
• To evaluate the impact of woofer DMs on mitigating MEMS DM saturation.

Main Methods:

• Laboratory experiments using a 1024-actuator, 1.5-micrometer stroke MEMS DM.
• Simulation of atmospheric turbulence using Kolmogorov screens with r(0) = 10-15 cm.
• Testing the MEMS DM both in isolation and in conjunction with a simulated woofer DM.

Main Results:

• The MEMS DM alone experienced saturation approximately 4% of the time.
• Incorporating a simulated woofer DM reduced MEMS DM saturation to less than 1%.
• Mid-to-high-spatial-frequency stroke saturation occurred more often than predicted, highlighting the importance of influence function correlations.

Conclusions:

• Empirical studies are crucial as analytical models underpredict stroke requirements for MEMS DMs.
• Sufficient DM stroke is critical for minimizing residual wavefront error and enabling exoplanet detection.
• The combination of woofer and tweeter DMs is essential for achieving the required performance in extreme AO systems.