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Modeling, characterization and computational image processing for fiber- coupled imaging

  • Author(s): Motamedi, Nojan
  • Advisor(s): Lomakin, Vitaliy
  • et al.
Abstract

High-contrast imaging Fiber Bundles (FB) have been used in recent years to integrate high resolution, compact and wide field-of-view imagers into small volumes. Imaging fiber bundles are composed of arrays of individual high-contrast optical fibers that are fused together to form a uniform and reliable image-transfer medium. Monocentric lenses have been designed to demonstrate superior diffraction-limited performance in the visible spectrum with few optical elements which enables integration of robust imagers into a small volume. However the curved image plane formed by these lenses can not be detected with conventional flat image sensors. Additional optical elements can be introduced to flatten the image plane but it would lead to the complexity of the system design and loss of compactness. One can instead take advantage of imaging fiber bundles to overcome this limitation. FBs can be milled and polished to form a curved input facet and a flat output facet to map the spherical image plane onto the flat image sensor plane. Proper understanding of imaging fiber bundles performance is therefore crucial for designing a low cross-talk image-transfer medium.

In this dissertation the Rigorous Coupled Wave Analysis (RCWA) method is used for 2D (1D+propagation) modeling of deep periodic dielectric gratings (straight imaging fiber bundles) and a numerical method is presented to efficiently calculate the electromagnetic beam transmission through the arrays of fibers. The Scanning Electron Microscope (SEM) image of the straight FB cross-section is captured and processed to extract the actual core boundaries. The measured core boundaries are then used for 3D (2D+propagation) modal analysis of fiber bundles. The effect of irregularity is also investigated in both 2D and 3D models and it is shown to improve the light confinement and image transfer fidelity in optical fiber bundles. The straight fiber bundles with various pitches are then experimentally characterized to measure the performance under different illumination conditions. A performance metric is used to quantify cross-talk in fiber bundles and demonstrate the poor performance in the fibers with core pitches of only a few wavelengths of the guided light. The experimental measurements are then compared to numerical modeling results and are shown to be in good agreement. The numerical and experimental modeling is followed by an experimental measurement of a fiber-coupled imager (FCI) in order to restore the lost resolution due to problems associated with fiber-sensor coupling. The Point Spread Function (PSF) of the strongly shift-variant FCI is first measured as a function of input location with sub pixel spatial resolution. Various computational image reconstruction methods are then used to recover the captured image resolution to that of the fiber bundle pitch.

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