This PDF file contains the front matter associated with SPIE Proceedings Volume 6888, including the Title Page, Copyright information, Table of Contents, the Conference Committee listing, and Plenary Paper.

The Applied Optics group at the National University of Ireland, Galway, is engaged in research into various aspects of the application of adaptive optics to both ocular and atmospheric wavefront correction. A large number of commercially available deformable mirrors have been selected by the group for AO experiments, and these mirrors have been carefully characterised to determine their suitability for these tasks. In this paper we describe the approach we have used in characterising deformable mirrors and present results for several MEMs mirrors, including membrane mirrors from AgilOptics and Flexible Optical BV, a segmented micromirror from IrisAO and a 140-actuator mirror from Boston micromachines.

This work briefly reviews the achievements of adaptive optics scanning laser ophthalmoscopy to date. Then, an instrument designed for testing phase imaging modalities is described, and finally, the requirements for MEMS devices in scanning ophthalmic devices are discussed.

The MEMS-AO/Villages project consists of a series of on-sky experiments that will demonstrate key new technologies for the next generation of adaptive optics systems for large telescopes. One of our first goals is to demonstrate the use of a micro-electro-mechanical systems (MEMS) deformable mirror as the wavefront correcting element. The system is mounted the 1-meter Nickel Telescope at the UCO/Lick Observatory on Mount Hamilton. It uses a 140 element (10 subapertures across) MEMS deformable mirror and is designed to produce diffraction-limited images at wavelengths from 0.5 to 1.0 microns. The system had first light on the telescope in October 2007. Here we report on the results of initial on-sky tests.

The Naval Prototype Optical Interferometer (NPOI) is the longest baseline at visible wavelengths interferometer in the world. The astronomical capabilities of such an instrument are being exploited and recent results will be presented. NPOI is also the largest optical telescope belonging to the US Department of Defense with a maximum baseline of 435 meter has a resolution that is approximately 181 times the resolution attainable by the Hubble Space Telescope (HST) and 118 times the resolution attainable by the Advanced Electro-Optical System (AEOS). It is also the only optical interferometer capable of recombining up to six apertures simultaneously. The NPOI is a sparse aperture and its sensitivity is limited by the size of the unit aperture, currently that size is 0.5 meters. In order to increase the overall sensitivity of the instrument a program was started to manufacture larger, 1.4 meter, ultra-light telescopes. The lightness of the telescopes requirement is due to the fact that telescopes have to be easily transportable in order to reconfigure the array. For this reason a program was started three years ago to investigate the feasibility of manufacturing Carbon Fiber Reinforced Polymer (CFRP) telescopes, including the optics. Furthermore, since the unit apertures are now much larger than r₀ there is a need to compensate the aperture with adaptive optics (AO). Since the need for mobility of the telescopes, compact AO systems, based on Micro-Electro-Mechanical-Systems (MEMS), have been developed. This paper will present the status of our adaptive optics system and some of the results attained so far with it.

In a recent effort, researchers from Wellman Center of Photomedicine use fluorescence signal provided by single- or two-photon excitation, second harmonic generation and coherent anti-Stokes Raman spectroscopy (CARS) to illustrate the cell level detail of mouse bone marrow [1]. However, the several non-linear imaging techniques suffered on a common base: signal degradation with deeper light penetration. The fluorescence signal weakening from the mouse skull is caused by the decreased excitation light intensity. With deeper imaging depth, the excitation light suffers tissue scattering, absorption and optical aberration. The last one of the causes spreads the light intensity away from its diffraction limited focal spot. In consequence, less fluorescence light is produced in the enlarged focal volume. In this paper, I will introduce Adaptive Optics (AO), a system for real time optical aberration compensation, to improve the non-linear fluorescence signal in the mouse bone marrow imaging. A parallel stochastic gradient decent algorithm based on Zernike polynomial is employed to control the deformable mirror in real time aberration compensation.

Depth aberrations are a major source of image degradation in three-dimensional microscopy, causing a significant loss of resolution and intensity deep into the sample. These aberrations occur because of an inevitable mismatch between the sample refractive index and the immersion medium index. We have built a wide-field fluorescence microscope that incorporates a large-throw deformable mirror to correct for depth aberrations in 3D imaging. We demonstrate a corrected point spread function imaging beads in water with an oil immersion lens and a twofold improvement in peak signal intensity. We apply this new microscope to imaging biological samples, and show sharper images and improved deconvolution.

We present an overview of a wavefront sensor-less adaptive optics scheme for microscopy based upon the optimisation of a metric related to the spatial frequency content of images. Aberrations are expanded as a series of suitable functions that permit the independent optimisation of each aberration mode. A general scheme to derive these modes theoretically and experimentally is presented. Resulting aberration correction is demonstrated in an incoherent transmission imaging system and in a structured illumination microscope.

We report progress on a nulling coronagraph intended for direct imaging of extrasolar planets. White light is suppressed in an interferometer, and phase errors are measured by a second interferometer. A 1020-pixel MEMS deformable mirror in the first interferometer adjusts the path length across the pupil. A feedback control system reduces deflections of the deformable mirror to order of 1 nm rms.

An all reflective Shack Hartmann style wavefront sensor has been developed using a Sandia National Laboratory segmented Micro-Electro-Mechanical (MEM) deformable mirror. This wavefront sensor is presently being explored for use with adaptive optics systems at the Naval Prototype Optical Interferometer and other experimental adaptive systems within the Naval Research Laboratory. The 61 MEM mirror segments are constructed in a hexagonal array and each segment can be constructed with either flat or optically powered surfaces. The later allows each mirror segment to bring its subaperture of light to a focus on an imaging array, creating an array of spots similar to a Shack Hartmann. Each mirror segment has tip, tilt and piston functionality to control the position of the focused spot such that measurement of the applied voltage can be used to drive a deformable mirror. As the system is reflective and each segment is controllable, this wavefront sensor avoids the light loss associated with refractive optics and has larger dynamic range than traditional Shack Hartmann wavefront sensors. This wavefront sensor can detect large magnitude aberrations up to and beyond where the focused spots overlap, due to the ability to dither each focused spot. Previous publications reported on this novel new technique and the electrical specifications, while this paper reports on experiments and analysis of the open-loop performance, including repeatability and linearity measurements. The suitability of using the MEM deformable mirror as a high dynamic range reflective wavefront sensor will be discussed and compared to current wavefront sensors and future work will be discussed.

This paper describes design, fabrication, and characterization of a miniaturized, Fourier transform infrared (FTIR) spectrometer for the detection and identification of toxic or flammable gases. By measuring the absorption by the target material of IR radiation, unambiguous detection and identification can be achieved. The key component of the device is a micromachined Michelson interferometer capable of modulating light in the 2 - 14 μm spectral region. Two major technical achievements associated with developing a MEMS interferometer module are discussed: development of a micromirror assembly having an order of magnitude larger modulation stroke to approach laboratory instrument-grade spectral resolutions; and assembly of monolithic, millimeter-scale optical components using multi-layer surface micromachining techniques to produce an extremely low cost MEMS interferometer, which has an unprecedented optical throughput. We have manufactured and tested the device. Reported optical characterization results include a precisely aligned, static interferogram acquired from an assembled Michelson interferometer using visible light wavelengths, which promises a high sensitivity FTIR spectrometer for its size.

Adaptive optics calibration of a novel wide-field scanning microscope is described, comparing relevant parameters for several optimization techniques. Specifically, comparisons of the optimization algorithm, image quality metrics, and the calibration image target are detailed. It is shown that stochastic parallel gradient descent (SPGD) algorithm using image intensity as a metric provides robust, repeatable system optimization. Results also show that optimization performance improves when the feature sizes on the calibration target approach the diffraction limit and are more uniformly distributed. This paper further compares stochastic, image-based optimization performance to that of conventional adaptive optics optimization with a point source object and a Shack Hartmann wavefront sensor.

We describe a compact MEMS-based adaptive optics (AO) optical coherence tomography (OCT) system with improved AO performance and ease of clinical use. A typical AO system consists of a Shack-Hartmann wavefront sensor and a deformable mirror that measures and corrects the ocular and system aberrations. Because of limitations on current deformable mirror technologies, the amount of real-time ocular-aberration compensation is restricted and small in previous AO-OCT instruments. In this instrument, we incorporate an optical apparatus to correct the spectacle aberrations of the patients such as myopia, hyperopia and astigmatism. This eliminates the tedious process of using trial lenses in clinical imaging. Different amount of spectacle aberration compensation was achieved by motorized stages and automated with the AO computer for ease of clinical use. In addition, the compact AO-OCT was optimized to have minimum system aberrations to reduce AO registration errors and improve AO performance.

Adaptive optics (AO) and optical coherence tomography (OCT) are powerful imaging modalities that, when combined, can provide high-resolution, 3-D images of the retina. The AO-OCT system at UC Davis has been under development for 2 years and has demonstrated the utility of this technology for microscopic, volumetric, in vivo retinal imaging. The current system uses a bimorph deformable mirror (DM) made by AOptix Technologies, Inc. for low-order, high-stroke correction and a 140-actuator mirco-electrical-mechanical-system (MEMS) DM made by Boston Micromachines Corporation for high-order correction. We present our on-going characterization of AO system performance. The AO-OCT system typically has residual wavefront error of 100 nm rms. The correctable error in the system is dominated by low-order error that we believe is introduced by aliasing in the control loop. Careful characterization of the AO system will lead to improved performance and inform the design of future systems.

The Gemini Planet Imager (GPI) Adaptive Optics system will use a high-order MEMS deformable mirror for phase compensation. The MEMS mirror will be used in a Woofer-Tweeter configuration, with a frequency-domain based splitting of the phase between the two mirrors. Precise wavefront control depends on the ability to command them MEMS to make the exact phase desired. Non-linearities in the MEMS may prevent this. We determine that influence-function pre-compensation can remove most, but not all, open-loop error. We use simulation and a simulation of a non-linear MEMS to address the issue of how much non-linearity can be tolerated in closed-loop by GPI.

Several high-contrast imaging systems are currently under construction to enable the detection of extra-solar planets. In order for these systems to achieve their objectives, however, there is considerable developmental work and testing which must take place. Given the need to perform these tests, a spatially-filtered Shack-Hartmann adaptive optics system has been assembled to evaluate new algorithms and hardware configurations which will be implemented in these future high-contrast imaging systems. In this article, construction and phase measurements of a membrane "woofer" mirror are presented. In addition, results from closed-loop operation of the assembled testbed with static phase plates are presented. The testbed is currently being upgraded to enable operation at speeds approaching 500 hz and to enable studies of the interactions between the woofer and tweeter deformable mirrors.

Micro-electrical-mechanical-systems (MEMS) deformable mirrors (DMs) are under study at the Laboratory for Adaptive Optics for inclusion in possible future adaptive optics systems, including open loop or extreme adaptive optics (ExAO) systems. MEMS DMs have several advantages in these areas because of low (to zero) hysterisis and high actuator counts. In this paper, we present work in the area of high-contrast adaptive optics systems, such as those needed to image extrasolar planets. These are known to require excellent wavefront control and diffraction suppression. On the ExAO testbed we have already demonstrated wavefront control of better than 1 nm rms within controllable spatial frequencies, however, corresponding contrast measurements are limited by amplitude variations, including variations introduced by the MEMS. Results from experimental measurements and wave optic simulations on the ExAO testbed will be presented. In particular the effect of small scale MEMS structures on amplitude variations and ultimately high-contrast far field measurements will be examined. Experimental results include interferometer measurements of phase and amplitude using the phase shifting diffraction interferometer, direct imaging of the pupil, and far-field imaging.

We present a novel technique for the design of DM controllers in high spatial resolution adaptive optics systems, operating in open-loop. It consists of a Shack-Hartmann (SH) figure sensor and multiple overlapping MIMO controllers based on the H_∞ synthesis method. The controller synthesis can be carried out periodically using a linearized representation of a continuously adjusted model that accounts for varying physical or ambient conditions and incorporates the spatial geometry of the SH. The figure sensor uses a bright reference source and a fast CMOS detector to sample the DM surface sequentially with an optical arrangement that does not interfere with the main corrected beam. Taking full advantage of such robust techniques, the controller can successfully handle the dynamics and non-linearity of the DM, allowing one to decouple, from the main AO control loop standpoint, the turbulence estimation errors from those originating in the DM servo-loop. It can also implement noise and vibration rejection without compromising the loop stability, pushing the control bandwidth to the physical limits imposed by hardware and software components. By splitting the control function into several overlapping controllers, implementation complexity is reduced and continuous updating of the controller can be easily achieved. Simulations show its ability to successfully control the DM shape, in spite of partial and non-simultaneous sampling of the SH figure sensor due to detector speed limitations.

A new method is introduced for predicting control voltages that will generate a prescribed surface shape on a 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 1.5μm amplitudes have been achieved with less than 15nm RMS error.

Adaptive Optics (AO) improves the quality of astronomical imaging systems by using real time measurement of the turbulent medium in the optical path. The measurements are then taken and applied to a deformable mirror (DM) that is in the conjugate position of the aberrations in the optical path. The quality of the reconstructed wavefront directly affects the images obtained. One of the limiting factors in current DM technology is the amount of stroke available to correct the wavefront distortions which can be as high as 20 microns of optical path difference. We have developed a simulation analysis using Galerkin's method to solve the nonlinear plate equation. The analysis uses a set of orthogonal equations that satisfied the boundary condition to solve for the linear deformation on the mirror surface. This deformation is used to iteratively converge to the final solution by applying the nonlinear plate equation and the nonlinear actuator forces. This simulation was used to design a microelectromechanical DM with 10 μm of stroke.

Various types of large stroke actuators for Adaptive Optics (AO) were simulated individually and as part of a mirror system consisting of actuators bonded to face plates with different boundary conditions. The actuators and faceplate were fabricated using a high aspect ratio process that enables the fabrication of 3-dimensional Micro-Electro-Mechanical System (MEMS) devices. This paper will review simulation results along with measurements of the displacement of the actuators utilizing a white-light interferometer. Both simulations and interferometer scans have shown the ability of the actuators to achieve displacements of 1/3 of the initial gap between the spring layer and the counter electrode.

New concepts for astronomical adaptive optics are enabled by use of micro-electrical mechanical systems (MEMS) deformable mirrors (DMs). Unlike traditional DMs now used in astronomical AO systems, MEMS devices are smaller, less expensive, and exhibit extraordinarily repeatable actuation. Consequently, MEMS technology allows for novel configurations, such as multi-object AO, that require open-loop control of multiple DMs. At the UCO/Lick Observatory Laboratory for Adaptive Optics we are pursuing this concept in part by creating a phaseto- voltage model for the MEMS DM. We model the surface deflection approximately by the thin-plate equation. Using this modeling technique, we have achieved open-loop control accuracy in the laboratory to ~13-30 nm surface rms in response to ~1-3 μm peak-to-valley commands, respectively. Next, high-resolution measurements of the displacement between actuator posts are compared to the homogeneous solution of the thin-plate equation, to verify the model's validity. These measurements show that the thin-plate equation seems a plausible approach to modeling deformations of the top surface down to lateral scales of a tenth actuator spacing. Finally, in order to determine the physical lower limit to which our model can be expected to be accurate, we conducted a set of hysteresis experiments with a MEMS. We detect only a sub-nanometer amount of hysteresis of 0.6±0.3 nm surface over a 160-volt loop. This complements our previous stability and position repeatability measurements, showing that MEMS DMs actuate to sub-nanometer precision and are hence controllable in open-loop.

A MEMS SLM with an array of 64×64 pixels, each 120 μm ×120 μm in size, with 98% fill-factor, has been developed. Each reflector in the array is capable of 5 μm of stroke, and ±4° tip and tilt. From a prototype array, 14 contiguous pixels have been independently wired-out to off-chip drive electronics. These 14 pixels have been demonstrated to be effective in an off-the-shelf AO system (with requisite modifications to suit the SLM). For a low-order static aberration, the measured Strehl ratio has been improved from 0.069 to 0.861, a factor of 12 improvement.

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.

This paper presents the progress in the development of a 4096 element MEMS deformable mirror, fabricated using polysilicon surface micromachining manufacturing processes, with 4μm of stroke, a surface finish of less than 10nm RMS, a fill factor of 99.5%, and bandwidth greater than 5kHz. The packaging and high speed drive electronics for this device, capable of frame rates of 22 kHz, are also presented.

An array of micromirrors can be used to correct the wavefront aberrations due to atmospheric turbulence. A simple method is presented for estimating the number of piston-only micromirrors needed to correct the seeing. We also compute how many piston-tip-tilt micromirrors are required. The three-actuator micromirrors are found to produce a more efficient solution, requiring 4X fewer actuators for the same improvement in the Strehl ratio.