This paper confirms that numerical aperture (NA) is a key factor in mode coupling [the energy transfer among propagating modes in multimode fibers (MMF)] and that providing a high NA is a viable solution to reduce mode coupling in graded-index plastic optical fibers (GI POFs). Furthermore, the authors propose a new refractive-index-profile design of GI POFs when only small mode groups are launched (restricted-launch condition), which is a combined profile with the index exponents lower and higher than optimum value for the core center and periphery, respectively. The advantage of the new index profile is investigated both experimentally and theoretically. Furthermore, it is verified that the high-bandwidth performance of GI POF with the new index profile under the restricted-launch condition is maintained even when statistical fiber bendings are added to the GI POF and when misalignment is caused at the optical coupling between the light source and the GI POF.
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