Dissertations, Theses, and Capstone Projects

Date of Degree

9-2016

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Ying-Chih Chen

Committee Members

Azriel Z. Genack

Hyungsik Lim

Yuhang Ren

Edward A. Whittaker

Subject Categories

Optics

Keywords

optics, laser, multimode fiber, wavefront shaping, focusing

Abstract

This thesis focuses on a technique of delivering spatially focused and temporally compressed picosecond laser pulses through multimode fibers. This study was inspired by recent success in focusing light through optically diffusive media of which multimode fibers were a special case in terms of causing scrambled phase distribution in the transmitted light. The approach involved controlling the phase distribution of incoming beam using a deformable mirror prior to its entry into the multimode fiber in order to achieve constructive interference at selected spots in the output. With phase control, the intensity of the focused light at the output can be enhanced by two orders of magnitude and the pulse durations reduced to their initial values compared to the situation without phase control. However, strongly focused and sharpened pulses can be obtained only under the condition that there were a large number of fiber modes overlapping in time and space at the output end of the fiber. Unlike the case of delivering focused light through highly scattered open media which contained thousands of modes, the condition for best performance was not always met in multimode fibers with limited number of modes. In step-index fiber of long length, the number of overlapping modes at a given time can be only a small fraction of all possible modes due to modal dispersion. The participation by a subset of all available modes resulted in considerable broadening in the focused spot sizes and compressed pulse durations. Our study demonstrated that this problem can be largely alleviated in grade-index fibers whose small modal dispersion ensured the overlapping of a large number of modes over a longer distance, resulting in more effective focusing and compression. The experimental observations and the understanding of the underlying mechanisms were in supported by the results of numerical simulations of beam propagation based on a complete sets of known transverse modes in the fibers.

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