| In this thesis, we report the results of our experimental and theoretical studies of mode coupling in multi-mode optical fibers. We demonstrate that strong mode coupling in poly(methyl methacrylate) (PMMA) based multi-mode plastic optical fibers (POFs) effectively increases the bandwidth performance by a factor of as much as an order of magnitude. We investigate in detail the physical mechanisms of mode coupling.; The most important feature of an optical fiber waveguide is its bandwidth, which defines its information carrying capacity. A major limitation on the bandwidth of multi-mode glass and plastic optical fibers is modal dispersion, in which different optical modes propagate at different velocities and the dispersion grows linearly with length.; Modal dispersion in an optical fiber waveguide is reduced through mode coupling by allowing the energy packets of a signal pulse at different times to occupy different modes as they are propagating down the waveguide. With mode coupling, pulse broadening varies only as a characteristic square root function of the fiber length as opposed to following the usual linear dependence; The bandwidths of both step index (SI) and graded index (GI) POF samples are carefully measured in the time domain and determined to be 80 ± 10 MHz per 100m for SI POF and 3.0 0.4 GHz per 100m for GI POF. The index profiles are first directly measured by standard high accuracy techniques, such as the near field method, the refracted near field ray method, and the transverse interferometric method (TIM), and then probed with the highly sensitive differential mode delay (DMD) measurement.; We developed a highly efficient numerical methodology based on the Streifer-Kurtz WKB theory for solving the scalar wave equation in fiber waveguides and implemented the new method in MathematicaTM to simulate fiber optical transmission characteristics for an arbitrary given index profile, such as the DMD profile, impulse response, and bandwidth performance. Material dispersion, in addition to waveguide dispersion, are found to have a huge impact on the optical transmission behavior of a fiber waveguide. The theoretical bandwidth values for the SI and GI POF samples are calculated from the measured index profiles and the corresponding DMA results and determined to be 27 MHz per 100m for SI and 0.43 GHz per 100m for GI.; Work is underway to further study the origin and nature of index perturbations with advanced microscopy and light scattering measurements. The confocal microscopic images obtained recently have already revealed the trace of density fluctuations in SI POF. (Abstract shortened by UMI.)... |
