This paper presents MEMS deformable-mirror technology under development at Iris AO. The hybrid approach uses surface-micromachining techniques to fabricate actuator arrays. High-fill-factor mirror arrays are flip-chip bonded on top of these actuator arrays. The single-crystal-silicon mirror segments provide robust substrates for optical coating with excellent surface quality (6-20 nm rms surface-figure errors). The hexagonally close-packed segments are 350 μm on a side, and can thus provide high-spatial frequency corrections in a small form factor.
High-stroke actuation of greater than >7.5 μm has been experimentally verified while keeping actuation voltages within reasonable bounds (<130 V). Three electrodes under each actuator allow for piston/tip/tilt motion. An open-loop controller has been demonstrated to position a 37-segment array resulting in a flattened array with only 19 nm rms of surface figure error.
This paper presents a MEMS DM that is a hybrid of surface micromachining and bulk micromachining. The combination of fabrication techniques resulted in a DM that has demonstrated 7.6 μm stroke at 125 V, 98.6% fill factor, and excellent optical quality of better than 16 nm rms after packaging. Preliminary cyclic testing over 110 hours and 107 cycles showed no noticeable changes to the actuator positions after cycling.
The inherent ability of silicon micromachining to provide a multitude of precision aligned optical components on a single die naturally facilitates optical communication trends towards installing larger cross-connects and transmitting many channels on individual fibers. A micromachined mirror array has been designed and fabricated using the Analog Devices, Inc. (ADI) Optical iMEMS (R) process. The single-chip mirror die consists of 84 mirrors arranged in a linear array with an average pitch of 95 μm. Each mirror is equipped with a pair of polysilicon actuation electrodes located beneath the mirror. These two electrodes allow each mirror to be independently rotated around the axis parallel to the long dimension of the array using off-chip voltage commands. An operating mirror tilt of +/- 2 degrees is achieved with less than 130 volts of actuation. The design objectives including high mirror fill factor, optimal air damping, low mirror-to-mirror cross talk, acceptable voltage levels, and robustness posed significant challenges. This paper will describe how these challenges were overcome using an interdigitated mirror layout. The mirrors were successfully fabricated with good yield and characterized through both customer and ADI testing.