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Characterization of a Bimorph Deformable Mirror in a Closed Loop Adaptive Optics System for Vision Science Purposes Zachary Graham1 Sophie Laut2, David Horsley3, John Werner2 1 Hartnell Community College, Salinas, CA of Ophthalmology and Psychophysics, UC Davis 3 Department of Mechanical and Aeronautical Engineering, UC Davis 2 Department 1 AO in vision science Removes aberrations in the eye Increases resolving power Allows for more thorough and advanced study of the eye and brain (psychophysics) 2 My Project To help characterize a mirror for use in a next generation Adaptive Optics imaging system Helped with the setup of the system Wrote a program in MATLAB to generate Zernike mode aberrations Took data on the mirror 3 Next Gen. AO System Will operate in 2 modes Scanning Laser Ophthalmoscope (SLO) Optical Coherence Tomography (OCT) 2 Deformable Mirrors MEMS and Bimorph will be cascaded in one system Bimorph will replacethe role trial lenses Will remove more aberrations Computer automated Much more flexible 4 The Boston Micromachines MEMS mirror Specification : • Active area : 4 mm x 4 mm • No of actuators : 100 • Continuous surface • Stroke (wavefront) : +/- 2 m • Response speed : ~3.5 kHz • Operative voltage : 200 V • Cost : ~ $25,000 Relatively small Stroke ! for high-order aberration correction Slide Courtesy of Sophie Laut, UC Davis Medical Center 5 The AOptix Bimorph deformable mirror Specification : • Active area : 12 mm, round • No of actuators : 35 • Continuous surface • Stroke (wavefront) : +/- 40 m • Maximum deflection : +/- 20 m • Response speed : ~4 kHz • Operative voltage : 15-30 V Actuator geometry Usual applications : Optical telecommunication system High Stroke ! for low-order aberration correction Slide Courtesy of Sophie Laut, UC Davis Medical Center 6 Imaging Setup System Information total g = 1.00 flatness = l/13 Hartmann – Shack Wave front sensor Laser Diode Telescope 2 g=1 Pupil Plane Telescope 1 g=1 Bimorph DM 7 Characterization Process Place aberration into system Control Loop closes and mirror corrects wave front Before/After data analyzed 8 Characterization Process Place aberration into system Control Loop closes and mirror corrects wave front Before/After data analyzed 9 Aberrations Lower order aberrations were introduced using trial lenses. Cylinder and Sphere Higher ordered aberrations Trial lenses cannot be used Generated in MATLAB 10 Centroid Displacement Algorithm a1 S x 1 .. S xn , S y 1 .. S yn A ( x, y ) 2 an ij xij x lf ( x, y ) 2 yij y l f Matrix of partial Solution Vector (Slope) ij derivatives for all used zernike modes •Starts with a file of reference positions •Reads the value of each reference Centro id from a matrix of partial derivatives for the particular Fernike mode. and calculates the slope in x and y •The slope is direcly proportional to the displacement •The displacement is added to the reference position and logged Vector of normalized zernike coefficients 11 Preparing Simulated Aberrations 1. 2. 3. 4. A specific Zernike mode is picked Maximum detectable amplitude is determined Aberrations are generated Aberrations introduced to the system 12 Characterization Process Place aberration into system Control Loop closes and mirror corrects wave front Before/After data analyzed 13 AO in Action 14 Problems with trial lenses The lenslet array could not resolve more than 1.8 diopters of error (defocus) If aberration too strong the WFS spots will be displaced outside their sub-aperture Occurs on physically introduced aberrations only Limits testing to resolution of lenslet and not stroke of mirror. 15 Dealing With Loss of WFS Spots Some aberrations are so strong that the computer cannot find all of the WFS spots Using MATLAB we can correct for this by using an extrapolation algorithm 16 Characterization Process Place aberration into system Control Loop closes and mirror corrects wave front Before/After data analyzed 17 Results The group are continuing to work on data analysis algorithms and are implementing them in MATLAB Will be presented at Optics East 2005 SPIE Conference in Boston1 The OCT / SLO set-up is under construction 1 Bimorph deformable mirror; an appropriate wavefront corrector for retinal imaging? –Sophie Laut, Steve Jones, Hyunkyu Park, David Horsley, Scot Olivier, John Werner 18 OCT / SLO Schematic MEMS DM Talk about the SLO system Bimorph DM Slide Courtesy of Sophie Laut, UC Davis Medical Center 19 Acknowledgements This project is supported by the National Science Foundation Science and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under cooperative agreement No. AST - 9876783. Dr. Scot Olivier and Dr. Steven Jones at LLNL Dr. Sophie Laut and Prof. John Werner at UCDMC Prof. David Horsley at UCD Everyone at the CfAO, LLNL, and UCD for a great internship experience 20