We investigated the way to reduce the mode coupling (the energy transfer among the propagating modes) in a multi-mode fiber. It strongly influences the bandwidth in the graded-index plastic optical fiber (GI POF), and it was found that the mode-coupling strength in the GI POF depended on the numerical aperture of the fiber. Moreover, we propose a new optimum waveguiding design of GI POFs when only small mode groups are launched [under filled launch condition (UFL)], which is expected in a real high-speed GI POF link. We demonstrate greater than 1-GHz transmission by a 250-m polymethylmethacrylate (PMMA-d8)-based GI POF under the UFL condition.
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