Which is a more serious factor to the bandwidth of GI POF: Differential mode attenuation or mode coupling?

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 opti cal 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 estimat ed properties was observed. The origin of disagreement has been generally e xplained as the mode coupling effect. However, it is clarified in this pape r that the effect of the differential mode attenuation is the dominant fact or of disagreement between measured and predicted bandwidth characteristics , while the effect of the mode coupling is small. In order to clarify the r eason why the calculated bandwidth characteristics of the GI POF show a dis agreement 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 accou nt this differential mode attenuation, an excellent agreement is observed b etween calculated and measured results of both the bandwidth characteristic s and group delay, in which no mode coupling was considered.