Bautista Fernandez

Adaptive optics confocal microscopy using direct wavefront sensing

Optical aberrations due to the inhomogeneous refractive index of tissue degrade the resolution and brightness of images in deep-tissue imaging. We introduce a confocal fluorescence microscope with adaptive optics, which can correct aberrations based on direct wavefront measurements using a Shack-Hartmann wavefront sensor with a fluorescent bead used as a point source reference beacon. The results show a 4.3× improvement in the Strehl ratio and a 240% improvement in the signal intensity for fixed mouse tissues at depths of up to 100 μm.

Design, processing, and materials for large-stroke actuators

Adaptive optics (AO) applications in astronomy and vision science require deformable mirrors with larger stroke, higher packing density and at lower cost than currently available technology. The use of high-aspect ratio Micro-Electro- Mechanical Systems (MEMS) processing techniques to fabricate large-stroke actuators that can meet stroke, packing density and cost specifications for AO applications have been explored. Different actuator designs, materials and postprocessing procedures fabricated in two different high-aspect ratio processes have been investigated. These manufacturing processes allow high-precision multilayer fabrication, and both parallel plate and comb drive actuator deformable mirror designs have been created. Multilayer fabrication has reduced pull-in voltage requirements for large stroke comb-drive actuators. The design, modeling and simulation of these actuators are compared to experimental measurements of their pull-voltages, which characterizes their stiffness and maximum stroke.

Simulation and interferometer results of MEMS deformable mirrors

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.

High-aspect-ratio microelectromechanical systems deformable mirrors for adaptive optics

Adaptive optics (AO) applications in astronomy and vision sci- ence require deformable mirrors (DMs) with high-stroke, high-order pack- ing density at a lower cost than the currently available technology. The required AO specifications are achievable with microelectromechanical systems (MEMS) devices fabricated with LIGA (lithographie galvanofo- mung abformung) high-aspect-ratio processing techniques. Different ac- tuator designs and a bonded faceplate fabricated in a LIGA process, enabling multilayer fabrication of MEMS devices, are investigated. Vari- ous types of high-stroke gold actuators for AO consisting of folded springs with rectangular and circular membranes as well as x-beam actuators sup- ported diagonally by fixed-guided springs are designed, simulated, and fabricated individually and as part of a continuous-face-sheet DM system. The actuators and DM displacement versus voltage are measured with an interferometer and the corresponding results are compared to finite element analysis simulations. Simulations and interferometer scans show the ability of the actuators to achieve displacements of greater than 1/3 of the initial gap. A stroke of ∼9.4 μm is achieved, thus showing that this fabrication process holds promise in the manufacture of future MEMS DMs for the next generation of extremely large telescopes. C 2010 Society of

High-stroke, high-order MEMS deformable mirrors

Abstract. Monolithic fabrication of continuous facesheet high-aspect ratio gold microelectromechanical systems (MEMS) deformable mirrors (DMs) onto a thermally matched ceramic–glass substrate (WMS-15) has been performed. The monolithic process allows thick layer deposition (tens of microns) of sacrificial and structural materials thus allowing high-stroke actuation to be achieved. The fabrication process does not require wafer bonding to achieve high aspect ratio three-dimensional structures. A gold continuous facesheet mirror with 3.4 nm surface roughness has been deposited on a 16×16 array of X-beam actuators on a 1-mm pitch. A stroke of 6.4 μm was obtained when poking two neighboring actuators. Initial electrostatic actuation displacement results for a high-aspect ratio gold MEMS DM with a continuous facesheet will be discussed.

Adaptive optics confocal fluorescence microscopy with direct wavefront sensing for brain tissue imaging

Recently, there has been a growing interest in deep tissue imaging for the study of neurons. Unfortunately, because of the inhomogeneous refractive index of the tissue, the aberrations degrade the resolution and brightness of the final image. In this paper, we describe an adaptive optics confocal fluorescence microscope (AOCFM) which can correct aberrations based on direct wavefront measurements using a point source reference beacon and a Shack-Hartmann Wavefront Sensor (SHWS). Mouse brain tissues with different thicknesses are tested. After correction, both the signal intensity and contrast of the image are improved.

Characterization and annealing of high-stroke monolithic gold MEMS deformable mirror for adaptive optics

Adaptive optics for the next generation of extremely large telescopes (30 - 50 meter diameter primary mirrors) requires high-stroke (10 microns), high-order (100x100) deformable mirrors at lower-cost than current technology. Lowering the cost while improving the performance of deformable mirrors is possible using Micro-Electro-Mechanical Systems (MEMS) technology. In this paper the fabrication and testing of an array of high-stroke gold MEMS X-beam actuators attached to a continuous gold facesheet will be described. Both the actuator and the facesheet were fabricated monolithically in gold plated onto a thermally matched ceramic-glass substrate (WMS-15) using a high-aspect ratio fabrication process. Continuous facesheets that are deformed due to stress gradients have been annealed at high temperature and for an extended amount of time. The facesheet was flattened to the point where features such as etch holes and support post topography were easily distinguishable. Initial root-mean-square (RMS) topography at center of facesheet attached to a 16x16 X-beam actuator array with 1mm pitch was measured to be ~13.8μm. After annealing, the surface topography was measured to be ~1.0μm.

Prototyping of MWIR MEMS-based optical filter combined with HgCdTe detector

In the past decades, there have been several attempts to create a tunable optical detector with operation in the infrared. The drive for creating such a filter is its wide range of applications, from passive night vision to biological and chemical sensors. Such a device would combine a tunable optical filter with a wide-range detector. In this work, we propose using a Fabry-Perot interferometer centered in the mid-wave infrared (MWIR) spectrum with an HgCdTe detector. Using a MEMS-based interferometer with an integrated Bragg stack will allow in-plane operation over a wide range. Because such devices have a tendency to warp, creating less-than-perfect optical surfaces, the Fabry-Perot interferometer is prototyped using the SOI-MUMPS process to ensure desirable operation. The mechanical design is aimed at optimal optical flatness of the moving membranes and a low operating voltage. The prototype is tested for these requirements. An HgCdTe detector provides greater performance than a pyroelectic detector used in some previous work, allowing for lower noise, greater detection speed and higher sensitivity. Both a custom HgCdTe detector and commercially available pyroelectric detector are tested with commercial optical filter. In previous work, monolithic integration of HgCdTe detectors with optical filters proved to be problematic. Part of this work investigates the best approach to combining these two components, either monolithically in HgCdTe or using a hybrid packaging approach where a silicon MEMS Fabry-Perot filter is bonded at low temperature to a HgCdTe detector.

Nonlinear plate equation analysis for the design of large stroke deformable mirror

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 Galerkins 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.

Fabrication and testing of MEMS-based optical filter combined with a HgCdTe detector

The Mid-wave infrared (MWIR) spectrum has applications to many fields, from night vision to chemical and biological sensors. Existing broadband detector technology based on HgCdTe allows for high sensitivity and wide range, but lacks the spectral decomposition necessary for many applications. Combining this detector technology with a tunable optical filter has been sought after, but few commercial realizations have been developed. MEMS-based optical filters have been identified as promising for their small size, light-weight, scalability and robustness of operation. In particular, Fabry-Perot interferometers with dielectric Bragg stacks used as reflective surfaces have been investigated. The integration of a detector and a filter in a device that would be compact, light-weight, inexpensive to produce and scaled for the entire range of applications could provide spectrally resolved detection in the MWIR for multiple instruments. We present a fabrication method for the optical components of such a filter. The emphasis was placed on wafer-scale fabrication with IC-compatible methods. Single, double and triple Bragg stacks composed of germanium and silicon oxide quarter-wavelength layers were designed for MWIR devices centered around 4 microns and have been fabricated on Silicon-On-Insulator (SOI) wafers, with and without anti-reflective half-wavelength silicon nitride layers. Optical testing in the MWIR and comparison of these measurements to theory and simulations are presented. The effect of film stress induced by deposition of these dielectric layers on the mechanical performance of the device is investigated. An optimal SOI substrate for the mechanical performance is determined. The fabrication flow for the optical MEMS component is also determined. Part of this work investigates device geometry and fabrication methods for scalable integration with HgCdTe detector and IC circuitry.