Fabrication and Characterization of Mems Deformable Mirrors for Adaptive Optics

A bimorph deformable mirror (DM) for use in ophthalmologic adaptive optics is presented. The fabrication process and the results of characterization of the DM are described. Interferometric measurements of the DM surface shape and voltage-to-displacement characteristics are shown. The response of the DM to a step voltage input is measured using a commercial laser Doppler vibrometer (LDV). Experimental measurements of the DM are compared with both finite-element and analytical models. Analysis of the experimental measurements compared to the theoretical model will be used to design and fabricate an optimized DM for vision science. INTRODUCTION Adaptive Optics (AO), originally developed for astronomical telescopes, is a technique in which an activelycontrolled optical element, such as a deformable mirror (DM), is used to correct for optical aberrations in an imaging path. A decade of successful use in astronomy has motivated the search for new applications, for example, ophthalmologic instruments and free-space optical communications systems. Since the existing DMs are too large and expensive for these applications, recent research has focused on using micro electro-mechanical systems (MEMS) technology to create a more compact, lowcost DM. Several different MEMS-based DM designs have been presented by earlier researchers: membrane-based (OKO Technologies Inc.) [1]; polysilicon surface-micromachined (Boston Micromachines Inc.) [2]; bulk silicon (Iris AO Inc.) [3]; and piezoelectric monomorphs (JPL) [4]. The ability of a DM to correct for optical aberrations is a function of both the total number of actuators and the stroke that each actuator can achieve. Segmented DMs, which are composed of an array of individually-actuated micro-mirrors, have the advantage that there is little interaction between adjacent actuators, so the maximum actuator stroke is available at all spatial frequencies. Continuous face-sheet and bimorph DMs, on the other hand, are simpler to construct but suffer from reduced stroke at higher spatial frequencies. We are aiming to develop MEMS-based DM technology for use in retinal imaging instruments. In this application, the DM is used to correct for aberrations in the human eye, allowing highresolution retinal images to be acquired. The ideal DM for vision science should achieve a large stroke (> 10 μm), have roughly a 10 mm diameter pupil with around 100 actuators and be capable of a relatively modest closed-loop bandwidth (100 Hz) [5]. A few commercial MEMS DMs exist and have been successfully used to correct for low-order aberrations in the human eye [6]. Among the different types of MEMS DMs, bimorph devices show particular promise for low-cost applications since they are simple to construct, reliable in operation, and require little special packaging [7, 8]. This class of DM is capable of correcting very large amplitude, low-order aberrations, and is simple to construct, a fact which should ultimately result in a low-cost DM [9]. The drawback of the bimorph design is that the maximum correctable amplitude diminishes strongly with increasing spatial frequency [10]. However, the aberrations present in the human eye are dominated by low spatial frequency components. Defocus and astigmatism, which are second-order aberrations, represent 92% of the total wavefront aberrations found in subjects with normal vision [11], and the aberration magnitude diminishes with increasing radial order. For this reason, the bimorph DM may satisfy the requirements of opthalmological AO, where