Development of a low voltage and large stroke MEMS-based lorentz force continuous deformable polymer mirror system

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Park, Byoungyoul
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The image resolution of modern optical systems is restricted by wavefront aberrations due to turbulence and diffraction in optical media and instruments. Adaptive optics (AO) is a technology that improves image resolution by compensating distorted wavefronts by using deformable mirror system (DMs). In this research, the design and fabrication of miniaturized Lorentz DM for AO applications is studied. The Lorentz actuator arrays and the continuous polymer deformable mirror are manufactured using bulk micromachining techniques developed for the fabrication of Microelectromechanical Systems (MEMS). In many AO applications, a massive array of actuators (100s – 5000s) and flexible mirrors are required to control the topology of the mirror surface. Sometimes, DMs with a large vertical stroke (over 5 μm) are required depending on the application. The stroke of conventional and MEMS electrostatic, piezoelectric, and electrostrictive actuated DMs is limited. This is because the stroke is proportional to the actuation voltage that can rise to dangerously high level (over 100V), which can cause problems with integrating the DM with on-chip driving circuits. The high voltage DM requires multiple wire connections to external high voltage amplifiers. This, in turn, results in significant space occupancy, high power consumption, and increased complexity of the system. Therefore, the development of a flexible continuous mirror and actuator array with high stroke and low driving voltage is highly desirable. In this study, a flexible polymer-based continuous deformable mirror is developed. The mirror uses the epoxy-based SU-8 as a structural material. The actuator arrays that drive the mirror are constructed as flexible serpentine springs on either side of a central thick and rigid crossbar above permanent magnets. There were several challenges in the development of the new Lorentz force actuated DMs. One of the significant challenges in designing the DMs was to determine the correlation between the spring constant of the mirror and actuator, and the inter-actuator coupling of the DM. The inter-actuator coupling represents the range of allowable tolerances for mirror deformation to correction of wavefront aberrations which may vary depending on the applications. In this research, we were able to predict the correlation between the spring constant of the mirror and actuator, and the inter-actuator coupling of the DMs simultaneously using COMSOL Multiphysics software. As a result of this work, we have developed a technique for varying the inter-actuator coupling of the DMs by changing the dimensions of the actuator. One problem encountered during the course of the research was the random local deformation of the DM after releasing the mirror. This problem was caused by localized stress and strain. The double side coating technique allowed us to build a sufficiently flat continuous mirror membrane. Lastly, KOH bulk micromachining, which was necessary for fabrication the actuator, provided a rugged etch profile. We were able to etch the substrate uniformly by the regular rotation of the substrate during the KOH etch. Using the technologies developed in this research, we demonstrate a 12 mm × 12 mm Al/SU-8/Al mirror with a 5 × 5 serpentine spring Lorentz actuator array. The mirror substrate was bonded with an actuator substrate, tested, and observed a significant ± 17 μm deformation at ± 3 mA actuation current, requiring an operational voltage of less than 1 V. This Lorentz DM would operate at a frequency over 1 kHz with a 40 μm large stroke at operation current of 9 mA. Such parameters can be suitable for large deformation adaptive optics applications. Furthermore, the Lorentz DM can potentially be applicable in correcting high order wavefront distortion once the development of a suitable bonding technique is realized.
Adaptive Optics, Deformable Mirror, MEMS, Lorentz Actuator