Optically Actuated Deformable Micro-Mirrors for Adaptive Optics
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Optically Actuated Deformable Micro-Mirrors for Adaptive Optics Troy Ribaudo1, Jin Li1, Bahareh Haji-saeed2, Jed Khoury3, and William Goodhue1 1 Physics, University of Massachusetts at Lowell, 1 University ave, Lowell, MA, 01854 2 2 Solid State Scientific Corporation, Hollis, NH, 03049 3 Air Force Research Laboratory, Hanscom AFB, MA, 01731 ABSTRACT In this paper we report on our latest efforts to fabricate and characterize optically actuated deformable micro mirrors for wavefront correction in an adaptive optics system. The optically actuated DMM device consists of a Silicon Nitride (Si3N4) thin film patterned into a spring plate array, an SU-8 photoresist supporting structure that provides the space through which the mirror is allowed to deform, and a PIN photodiode which allows an optical control signal to actuate the DMM. INTRODUCTION MEMS based deformable micro mirrors (DMM’s) have been employed in a variety of optical processing systems. They are used for phase correction in adaptive optics, as switches in fiber optics communications and as image production mechanisms in DLP technology. Adaptive optics technology has found applications in a multitude of fields including astronomy, retinal imaging, and free space optical communications. Wavefront correction is essential to enhance the performance of optical systems and to approach the diffraction limit of geometrical optics. As light propagates through a turbulent medium, wavefront distortion may be introduced. This aberration gives rise to a blurring of the image that can be restored with an adaptive optics system. One of the key components in such a system is a deformable mirror that can deflect under appropriate control and correct the distortions. Currently, the main technologies for addressing membrane mirror light modulation are electron beam addressing, optical addressing, and electrical addressing with integrated circuit technology. In the group’s previous work, optically actuated deformable micro mirrors were fabricated to serve as wavefront correction mechanisms. The use of aluminized Mylar films demonstrated the concept, but large applied voltages were required due to high spring constants [1]. To overcome this problem, spring plate arrays that were patterned in a Si3N4 thin film were employed which successfully reduced these voltages [2]. However, the fabrication procedure for this device included a difficult manual alignment step that hindered its repeatability. In this work, we demonstrate the improved fabrication of a deformable micro mirror that can be optically actuated with low voltage requirements. The layer supporting the Si3N4 micro-mirror arrays is replaced with a web of SU-8 photoresist (Microchem) and is deposited directly around the mirrors. SU-8 is a versatile negative photosresist that has been employed in MEMS fabrication procedures as both mechanically actuated [3], and passive components. This change allows for precise alignment, and a stronger, more
durable device. In addition, a GaAs PIN photodiode is used for opt
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