PURPOSE. Adaptive optics scanning laser ophthalmoscopy (AOSLO) under optimized wavefront correction allows for routine imaging of foveal cone photoreceptors. The intersubject variability of foveal cone density was measured and its relation to eye length evaluated. METHODS. AOSLO was used to image 18 healthy eyes with axial lengths from 22.86 to 28.31 mm. Ocular biometry and an eye model were used to estimate the retinal magnification factor. Individual cones in the AOSLO images were labeled, and the locations were used to generate topographic maps representing the spatial distribution of density. Representative retinal (cones/mm 2 ) and angular (cones/deg 2 ) cone densities at specific eccentricities were calculated from these maps. RESULTS. The entire foveal cone mosaic was resolved in four eyes, whereas the cones within 0.03 mm eccentricity remained unresolved in most eyes. The preferred retinal locus deviated significantly (P 0.001) from the point of peak cone density for all except one individual. A significant decrease in retinal density (P 0.05) with increasing axial length was observed at 0.30 mm eccentricity but not closer. Longer, more myopic eyes generally had higher angular density near the foveal center than the shorter eyes, but by 1°, this difference was nullified by retinal expansion, and so angular densities across all eyes were similar. CONCLUSIONS. The AOSLO can resolve the smallest foveal cones in certain eyes. Although myopia causes retinal stretching in the fovea, its effect within the foveola is confounded by factors other than cone density that have high levels of intersubject variability. (Invest Ophthalmol Vis Sci. 2010;51:6858‐6867)

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Intersubject Variability of Foveal Cone Photoreceptor Density in Relation to Eye Length

This paper presents the design, optimization, fabrication, and test results of an electrothermally actuated tip-tilt-piston micromirror with a large optical aperture of 1 mm. The fabrication of the device is a combination of thin-film surface micromachining and bulk silicon micromachining based on silicon-on-insulator wafers. The device has 3-DOF of actuations, including rotations around two axes in the mirror plane, and out-of-plane piston actuation. The micromirror shows an optical scan range of plusmn30deg about both x- and y-axes and displaces 480 mum in the z-axis, all at dc voltages that are less than 8 V. Dynamic testing of the micromirror shows that the thermal response time of each actuator is about 10 ms. Resonant frequencies of the piston and rotation motion are 336 and 488 Hz, respectively. The unique structural design of the device ensures that there is no lateral shift for the piston motion and no rotation-axis shift for the rotation scanning. With the large tip-tilt-piston scan ranges and low driving voltage, this type of device is very suitable for biomedical imaging and laser beam steering applications.

An Electrothermal Tip–Tilt–Piston Micromirror Based on Folded Dual S-Shaped Bimorphs

Adaptive optics (AO) is a technology used in ground-based astronomy to correct for the wavefront aberrations and loss of image quality caused by atmospheric turbulence. Provided some difficult technical problems can be overcome, AO will enable future astronomers to achieve nearly diffraction-limited performance with the extremely large telescopes that are currently under development, thereby greatly improving spatial resolution, spectral resolution and observing efficiency which will be achieved. The goal of this topical review is to present to the inverse problems community a representative sample of these problems. In this review, we first present a tutorial overview of the mathematical models and techniques used in current AO systems. We then examine in detail the following topics: laser guidestar adaptive optics, multi-conjugate and multi-object adaptive optics, high-contrast imaging and deformable mirror modeling and parameter identification.

http://iopscience.iop.org/article/10.1088/0266-5611/25/6/063001/pdf

Inverse problems in astronomical adaptive optics

In the current times, microelectromechanical systems and nanoelectromechanical systems form a major interdisciplinary area of research involving science, engineering, and technology. A lot of work has been reported in the area of modeling and control of these devices, with the aim of better understanding their behavior and improving their performance. This paper presents a review of the emerging advances in the modeling and control of these micro- and nanoscale devices and converges on the exciting research in on-chip control , with a mechatronics and controls perspective and concludes by projecting future trends.

/pdf/a-survey-of-modeling-and-control-techniques-for-micro-and-t30tdz508g.pdf

A Survey of Modeling and Control Techniques for Micro- and Nanoelectromechanical Systems

https://www.eng.ox.ac.uk/som/publications/som_2009_5.PDF

Optimum deformable mirror modes for sensorless adaptive optics

A method is introduced for predicting control voltages that will generate a prescribed surface shape on a MEMS deformable mirror. The algorithm is based upon an analytical elastic model of the mirror membrane and an empirical electromechanical model of its actuators. It is computationally simple and inherently fast. Shapes at the limit of achievable mirror spatial frequencies with up to 1.5 μm amplitudes have been achieved with less than 15 nm rms error.

Open-loop control of a MEMS deformable mirror for large-amplitude wavefront control

We characterize the errors associated with open-loop control of a microelectromechanical system deformable mirror (DM) using an approach that combines sparse calibration of the electrostatic actuator state space with an elastic plate model of the mirror facesheet. We quantify sources of measurement error and modeling error and demonstrate that the DM can be shaped in a single step to a tolerance of ∼8nm of that achievable with iterative feedback-based closed-loop control. Zernike polynomials with up to 2.5μm amplitude were made with this approach and yielded a shape error of <25nm rms in most cases. Residual errors were shown to be due primarily to spatial resolution limits inherent in the DM (e.g., uncontrollable errors).

Open-loop shape control for continuous microelectromechanical system deformable mirror

We report on the development of a new class of electrostatic MEMS deformable mirror (DM) fabricated through a combination of bulk micromachining, wafer bonding, and surface micromachining. The combination of these fabrication technologies introduces four major improvements over previous MEMS DMs, which are fabricated using surface micromachining alone. First, the MEMS DM structural components (mirror surface and actuator array) are made entirely of single crystalline silicon by use of the device layer of a whole 4-inch silicon-on-insulator (SOI) wafer bonded together via anodic bonding. Unlike current MEMS DMs fabricated entirely using surface micromachining, bulk micromachining steps in this fabrication process require no etch access holes, print through is inexistent, and no polishing steps are required. This leads to reduced diffraction of light from the mirror surface, improved mirror surface optical quality, and elimination of manufacturing processing steps. Second, through-wafer interconnects are used to connect the densely-packed electrostatic actuator array to driver electronics. This eliminates the need for wirebonding at the periphery of the DM, increasing the surface area available for actuators and removes the need for bulky wire bundles to connect the device to its driver. Third, by using the full area of a silicon wafer for each mirror, these MEMS DMs offer a larger optical aperture than any previously-reported MEMS DM. The larger aperture will achieve higher angular resolution, providing larger wavefront correction. Finally, the mirror and actuator thicknesses are not limited to several micrometers, unlike in surface micromachining. The thickness limits using this fabrication process is prescribed by the device layer thickness in SOI wafers, which vary between several micrometers to several hundred of microns.

Fabrication of single crystalline MEMS DM using anodic wafer bonding

The development of an assembly and packaging process for MEMS deformable mirrors (DMs) with
through wafer via (TWV) interconnects is presented. The approach consists of attaching a DM die with
high-density TWV electrostatic actuator interconnects to an interposer substrate that fans out these
connections for interfacing to conventional packaging technology.

Through-wafer interconnects for high degree of freedom MEMS deformable mirrors

Improvements for open-loop control of MEMS deformable mirror for large-amplitude
wavefront control are presented. The improvements presented here relate to measurement
filtering, characterization methods, and controlling the true, non-differential shape of the
mirror. These improvements have led to increased accuracy over a wider variety of
deflection profiles including flattening the mirror and Zernike polynomials.

http://people.bu.edu/tgb/PDF_files/Scholarly%20Papers/End_note_attached/Diouf_2010c.pdf

Alioune Diouf

Papers

Open-loop control of a MEMS deformable mirror for large-amplitude wavefront control

Open-loop shape control for continuous microelectromechanical system deformable mirror

Fabrication of single crystalline MEMS DM using anodic wafer bonding

Through-wafer interconnects for high degree of freedom MEMS deformable mirrors

Open loop control on large stroke MEMS deformable mirrors