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Title: A holographic modal wave front sensor for ocular adaptive optics
Author: Corbett, Alexander David
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2008
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In this dissertation, the design and construction of a sensor that can rapidly detect the aberrations of the eye is reported. This sensor was specifically designed for use in an adaptive correction system according to the properties of ocular aberrations seen in a normal population. A holographic modal wave front sensor was adapted to significantly increase its dynamic range to encompass the broad disruption of ocular aberrations seen in a normal population. Calculations were performed to determine the range of ‘biasing’ aberrations that could successfully be encoded into a hologram with a resolution of NxN without aliasing. Holograms were displayed on a 320x320 reconfigurable binary phase liquid crystal spatial light modulator to validate the calculations by measuring the aberrations generated holographically using interferometry. The dynamics of the ocular aberrations required that the sensor be used to drive an adaptive optical aberration correction with a minimum sampling rate of 30Hz. These requirements were met by the use of a binary ferroelectric liquid crystal device with a 0.7ms switching speed in tandem with a CCD camera sampling at 60Hz. Simulations demonstrated that in order to maintain the sensitivity of the sensor over its full working range, the size of the spatial filter used in the sensor must be increased in direct proportion to the bias aberration amplitude. A new metric, the sensitivity to noise ratio was used to identify that for a 320x320 resolution hologram, measuring the population of aberrations under study, the sensitivity to noise ratio is maximised when the aberration is at the aliasing limit of the hologram. Experiments confirmed that the sensor was capable of measuring an ocular aberration in closed adaptive loop to within 0.2 wavelengths after only two correction cycles.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available