Ph.D. Dissertation
of
Research Advisor
Professor and Chairman of Manufacturing Engineering
Thesis Committee
John Baillieul
Professor and Chairman of Aerospace and Mechanical Engineering
Mark Horenstein
Associate Professor of Electrical Engineering
Department of Electrical Engineering
Boston University, Boston MA 02215
This dissertation describes the development of kilo-pixel micro-electro-mechanical optical-quality surface-micromachined deformable mirrors and spatial light modulators along with scaleable control electronics. These silicon based deformable mirrors have the potential to modulate spatial and temporal features of an optical wavefront, and have applications in imaging, beam-forming, and optical communication systems. Techniques to improve the manufacturing, quality, and capability of these mirrors have been developed. The new mirror system was characterized and a scalable control system has been developed to coordinate and control a large array of mirrors.
Three types of kilo-pixel deformable mirrors were created: continuous membrane, segmented membrane, and a hybrid stress-relieved membrane mirrors. This new class of mirrors, deformed using electrostatically actuated surface-normal actuators, have an aperature of 10 mm, a stroke of 2 mm, position repeatability of 3 nm, surface roughness of 12 nm, reflectivity of 91%, and a bandwidth in air of 7 kHz. Previous work uncovered problems with yield, print-through, and residual stress, each of which limited their optical quality. A custom fabrication process was developed in tandem with a new mirror design to address these challenges in manufacturing. Design and layout issues addressed include packaging, stress reduction, reliability, yield, fill factor, and surface topography.
A chemo-mechanical polishing process was used to improve the surface quality of the mirrors by decreasing surface roughness by almost a factor of 4 from 46nm to 12nm. A gold coating process was also developed to almost double the reflectivity from 42% to greater than 91% without introducing a significant amount of stress in the mirror membrane. An alternative actuator design and layout was also developed that achieved an increased stroke of 6 mm, with the potential for even longer stroke with stress reduction. The long stroke capability was realized through introduction of split electrodes, actuation membrane cuts, and a double stacked anchor architecture.
A computer-driven electronic system was developed to aid in the electro-mechanical testing of these deformable mirrors. Quasi-static mirror characterization included voltage versus deflection measurements and optical surface quality measurements. Dynamic measurements included frequency response in air and vacuum, actuator repeatability, and the real-time imaging of a single spatial light modulator pixel.
A single channel controller was developed to dynamically control a single pixel in a spatial light modulator array. Results from this controller demonstrated the feasibility for a larger parallel processor system. A newly-developed multi-processor parallel gradient descent control algorithm is demonstrated in the controller. This multichannel system is inherently scalable to a kilo-pixel control system, and can be used as a tool for future research into high speed AO control.