FIBER-OPTIC LASER INTERFEROMETER FOR VISION RESEARCH Timne Bilton PI & Supervisor: Dr. David Williams Collaborators: Julianna Lin, Silvestre Manzanera & Sapna Shroff University of Rochester Center for Visual Science & Dept of Electrical and Computer Engineering

OBJECTIVE To design and calibrate a fiber coupled laser interferometer to be used in conjunction with the University of Rochester’s adaptive optics ophthalmoscope. 1

Laser interferometry avoids blur from diffraction and aberrations WHAT IS INTERFEROMETRY APPLIED TO THE EYE? 2

WHY USEFUL IN VISION? Eye’s optical aberrations primarily limit vision and retinal imaging. Adaptive optics reduces these aberrations, but image quality is still diffraction- limited by the pupil. Laser interferometry can image fringes on that retina that are immune to blur from diffraction as well as aberrations 3

APPLICATIONS Measuring neural visual performance (CSF) Retinal image resolution exceeding the diffraction limit using structured illumination 4

STRUCTURED ILLUMINATION Simulation of imaging interference fringes onto the cone mosaic Retinal images taken with AO should contain moire patterns such as simulated below when the fringe frequency is 130 cycles/deg Such moire patterns may contain valuable information about the cone mosaic as well as other retinal structures Sampling Frequency

DESIGN 690 nm fiber coupled laser is connected to a fiber splitter The splitter feeds directly to two acousto-optic modulators The altered beams are reflected off a right angle prismatic mirror through a series of relay mirrors and a galvanometer Light is sent to the pupil plane via the AO system Fiber modulator Fiber-coupled laser Fiber splitter (1x2) Fiber modulator Lens Mirror on galvanometer Lens Focusing Lens Mirror Movable platform Right Angle Mirror 6

CONTRAST CONTROL WITH ACOUSTO-OPTIC MODULATORS AOMs will pulse the light beams delivered to the retina 500 times a second. Pulses arriving simultaneously generate fringe contrast is 100%. Phase delays bring contrast down towards zero Digital control of pulse overlap alters fringe contrast easily 7

BUILDING Clockwise from the left: Construction and alignment process. Bite bar for head stabilization Assembled (inset) prismatic mirror and U- bench holding fiber cable 8

ZEMAX SIMULATION An approximate fringe image was generated by generating a lens modeling system. Resolution of this image was limited to the ability of the computer’s memory EFL, FFD and BFD determined by designer calculations 9 TOP: Fringe Diagram at retinal plane LEFT: Unfolded Layout of system (after prismatic mirror)

SPATIAL FREQUENCY CALIBRATION Ronchi rulings (transparent plate ruled with black lines and clear spaces of equal width) of known frequency placed in a conjugate retinal plane Frequency match will be made by adjusting the fringe frequency until the Moiré pattern formed with the Ronchi ruling was zero spatial frequency. U bench position will be recorded when interference fringe has identical frequency. 10

RESULTS Low contrast fringes were observed  able to see magnified fringes on a piece of paper, but disappear later in the system Unexpected vibrations made fringe imaging inconsistent Laser issue: temporal coherence required for contrast  current of the laser was below the threshold current for lasing 11

ACKNOWLEDGEMENTS David R. Williams, PhD Center for Visual Science, University of Rochester Center for Adaptive Optics, University of California at Santa Cruz College of Optical Sciences, University of Arizona This project is supported by a Research Experiences for Undergraduates (REU) supplement to the National Science Foundation and Technology Center for Adaptive Optics, managed by the University of California at Santa Cruz under A cooperative agreement No. AST

REFERENCES Publications: Miller, D., Williams, D.R., Morris, G.M., and Liang, J. (1996) Images of cone photoreceptors in the living human eye. Vision Res., 36, DOI link: doi: / (95) DOI link Liang, J., Williams, D.R., and Miller, D.T. (1997) Supernormal vision and high resolution retinal imaging through adaptive optics. J. Opt. Soc. Am. A., 14, H. Hofer, L. Chen, G. Y. Yoon, B. Singer, Y. Yamauchi, and D. R. Williams, (2001) Improvement in retinal image quality with dynamic correction of the eye's aberrations. Optics Express 8, MacRae, S., Williams, D.R., (2001) Wavefront Guided Ablation. American Journal of Ophthalmology, 132:6, Wikipedia-The Free Encyclopedia ( 13