Abstract
Microelectromechanical systems (MEMS) have produced high-quality, high-bandwidth, small form factor, and inexpensive fast steering mirror (FSM) devices potentially suitable for a large variety of applications, such as image stabilization and beam pointing in satellite-based and ground-based, free-space optical communication systems. However, one outstanding question for this application is power handling. The absorption of the mirror substrate is low, but non-negligible, so the question remains of whether thermal loading from laser radiation on a MEMS mirror will deform its surface and, if so, to what extent. We show experimental results of optical performance changes due to thermal loading for MEMS two-axis FSM devices from Mirrorcle Technologies, Inc. Results and reproducible behavior are reported and compared in ambient versus vacuum conditions, where the benefits of convective cooling are absent. Finite element analyses corroborate the experimental results and show that the mirror substrate can deform due to thermal expansion imbalances. The deformation changes the focusing characteristics of the mirror, with a peak to valley defocus (second-order Zernike mode) of up to 50 nm when the mirrors are tested in ambient and up to approximately 450 nm when under vacuum. Such defocusing negatively impacts the link budget for laser-based satellite communications.
Highlights
A variety of laser communications, optical sensor, and imaging applications utilize fast steering mirrors (FSM) to execute precision, high speed pointing, and/or angular error rejection as part of closed-loop feedback systems.[1]
Considering our example case from the introduction session (a 1-W laser being reflected by the microelectromechanical systems (MEMS) mirror surface), these devices have a very poor thermally conductive path to dissipate this constant heat load coming from the laser, as it is going to be explained in this session
We believe that the effects we describe in this paper are of similar nature to the ones encountered in the high reflectivity coatings field, which strengthens our hypothesis that the cause for MEMS mirrors defocus deformation is thermal stress
Summary
A variety of laser communications, optical sensor, and imaging applications utilize fast steering mirrors (FSM) to execute precision, high speed pointing, and/or angular error rejection as part of closed-loop feedback systems.[1] Traditionally, commercial FSMs have come in two varieties based on their actuation mechanism: piezo electric or voice coil.[2] Both technologies have heritage with industry-known advantages and disadvantages. Small size, weight, and power applications demand disruptive solutions. Size, weight, and power consumption using microelectromechanical systems (MEMS) FSM(s) leads to the main investigation of this work. Advances in MEMS have produced a lower cost, small form factor (2 cm × 2 cm × 1 cm), high-bandwidth electrostatic devices that have relatively simple high voltage driver requirements that consume very little power compared to piezo and voice coil-based systems.[3]
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