It is clarified for the first time that the effect of the mode coupling on the bandwidth characteristic is very small in the graded-index polymer optical fiber (GI POF) compared with in the step-index (SI) POF. Although there have been several trials regarding the theoretical prediction of bandwidth characteristics of the GI POF, a disagreement between measured and estimated properties was observed. The origin of disagreement has been generally explained as the mode coupling effect. However, it is clarified in this paper that the effect of the differential mode attenuation is the dominant factor of disagreement between measured and predicted bandwidth characteristics, while the effect of the mode coupling is small. In order to clarify the reason why the calculated bandwidth characteristics of the GI POF show a disagreement with that measured,differential mode delay and differential mode attenuation were investigated. It was found that the highest order mode in the poly methyl methacrylate (PMMA) base GI POF attenuated 17 dB compared with the lowest order mode through 100-m transmission. By taking into account this differential mode attenuation, an excellent agreement is observed between calculated and measured results of both the bandwidth characteristics and group delay, in which no mode coupling was considered.
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