MEMS Deformable Mirrors in Astronomical AO Thomas Bifano
MEMS Deformable Mirrors in Astronomical AO Thomas Bifano Director, Boston University Photonics Center (BUPC) Chief Technical Officer, Boston Micromachines Corporation (BMC) Paul Bierden President, BMC Steven Cornelissen, VP, BMC AO 4 ELT, Paris, 25 June 2009
Microelectromechanical (MEMS) DMs Over the past decade, we’ve led an academic program at the Boston University Photonics Center (BU), and a technology development program at Boston Micromachines Corporation (BMC), to pioneer and demonstrate DMs made with semiconductor foundry processes. Attachment post Mirror + Silicon wafer Electrostatic actuator array
Two DMs described in this talk 4096 actuator continuous membrane DM for Gemini Planet Imager 331 segment (993 actuator) hexagonal tip-tilt-piston DM for NASA TPF-C visible nulling coronagraph
Application: Gemini Planet Imaging (4 K DM) Gemini Planet Imager: 4096 actuator DM (BMC), with 3. 5µm stroke, for Jovian exoplanet detection. Engineering mirror delivered, science mirror due. B. Macintosh, J. Graham, D. Palmer et al. , “Adaptive optics for direct detection of extrasolar planets: the Gemini Planet Imager, ” Comptes Rendus Physique, vol. 8, no. 3 -4, pp. 365 -373, Apr-May, 2007.
Some DM Requirements for 4 K GPI DM Description Requirement Actuators 4096 (64 x 64 array) Stroke 3µm, after mirror is flattened Active Aperture 19. 2 mm (48 actuator diameter @ 400µm pitch) Local nonflatness <10 nm. RMS Bandwidth ~2. 5 k. Hz Inter-Actuator Stroke >1µm Yield 100% of actuators on a 48 actuator aperture Operating Temperature -30 C to +25 C
4 K DM Prototype Results 40 4 x 4 acutator poke 4000 35 Influence function 3000 30 2000 1000 25 0 20 400 0 200 Influence fnc, % 5000 2. 6 mm 4. 32µm Deflection, nm >4µm stroke achieved @ 210 V Voltage, V High spatial frequency print-through reduced to <10 nm RMS Previous DM: 21. 5 nm RMS 1. 15µm Interactuator stroke achieved Phase I DM: 5 nm RMS 175 nm 80 nm 0 nm 1150 nm
Measured Optical Quality Measured surface Filtered surface (uncontrollable) 6µm 200 nm 0µm 0 nm 4. 06µm PV 707 nm RMS 48 m ROC 16 RW 013#001 Top right zone (showing scallop at periphery) ~50 nm PV 4 nm RMS Center zone 100 nm 50 nm 0 nm ~25 nm PV
DM Static Cold Test @ 24. 7 C @ -20. 2 C
Cycling & Hysteresis
Package and Driver Form factor 3 U Chassis (5. 25” x 19” x 14”) Frame rate 34 k. Hz / 60 k. Hz (Low Latency) Resolution 14 -bit
This MEMS DM architecture permits ultraprecise, repeatable control 144 nm Initial 12 nm Controlled 1024 actuator MEMS DM • Controllable flatness <12 nm • Actuator repeatability <1 nm • Hysteresis <1 nm J. W. Evans et al. , Optics Express 14, 5558 (2006) Three research groups have developed precise models of MEMS DM behavior, including mechanical coupling through the mirror and nonlinear actuation electromechanics. Result: We can now achieve open-loop shape control within 25 nm error in one step. J. B. Stewart, A. Diouf, Y. P. Zhou, T. G. Bifano, Journal of the Optical Society of America 24, 3827 (Dec, 2007).
331 Element Tip-Tilt-Piston MEMS DM +/-6 mrad tip-tilt 2 um piston 600µm
Hex Mirror Segments Use thick, eptiaxial-grown polysilicon layer (6 -10µm) to achieve surface figure requirement 5. 9 nm ± 0. 2 nm RMS over DM aperture 35 nm 0 nm Actual Segment Thickness: 7. 5µm
Acknowledgements MEMS DM Students: Y. Zhou, J. Stewart*, J. Perreault, R. K. Mali, Andrew Le. Gendre BMC Technical Research Staff: A. Hartzell, P. Bierden, S. Cornelissen, J. Stewart, P. Woskov, C. Lam Funding: Cf. AO, Gemini, NASA, DARPA
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