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

TL;DR

This study investigates the stroke saturation behavior of MEMS deformable mirrors used in adaptive optics for high-contrast imaging, highlighting the importance of empirical testing for system design.

Contribution

It provides empirical data on MEMS DM saturation rates under turbulence conditions, informing the design of AO systems for exoplanet imaging.

Findings

MEMS DM saturation occurs ~4% of the time in solitary operation.

Simulating a woofer DM reduces saturation to less than 1%.

Mid-to-high-frequency stroke saturation is more frequent than predicted.

Abstract

High-contrast imaging of extrasolar planet candidates around a main-sequence star has recently been realized from the ground using current adaptive optics (AO) systems. Advancing such observations will be a task for the Gemini Planet Imager, an upcoming "extreme" AO instrument. High-order "tweeter" and low-order "woofer" deformable mirrors (DMs) will supply a >90%-Strehl correction, a specialized coronagraph will suppress the stellar flux, and any planets can then be imaged in the "dark hole" region. Residual wavefront error scatters light into the DM-controlled dark hole, making planets difficult to image above the noise. It is crucial in this regard that the high-density tweeter, a micro-electrical mechanical systems (MEMS) DM, have sufficient stroke to deform to the shapes required by atmospheric turbulence. Laboratory experiments were conducted to determine the rate and circumstance of saturation, i.e. stroke insufficiency. A 1024-actuator 1.5-um-stroke MEMS device was empirically tested with software Kolmogorov-turbulence screens of r_0=10-15cm. The MEMS when solitary suffered saturation ~4% of the time. Simulating a woofer DM with ~5-10 actuators across a 5-m primary mitigated MEMS saturation occurrence to a fraction of a percent. While no adjacent actuators were saturated at opposing positions, mid-to-high-spatial-frequency stroke did saturate more frequently than expected, implying that correlations through the influence functions are important. Analytical models underpredict the stroke requirements, so empirical studies are important.