It was clarified for the first time that the slight attenuation increment observed in the previous poly methyl methacrylate (PMMA)-dopant system graded-index polymer optical fiber (GI POF) we reported originated in the excess scattering loss induced by the aggregation of absorbed water into the POF. Although the PMMA material generally absorbs two weight% of water at maximum, the low attenuation of the conventional step-index (SI) POF whose core material is PMMA can be maintained at 70°C, 80% relative humidity (RH) atmosphere because the absorbed water might be homogeneously dispersed without any aggregation in the core of POF. On the other hand, addition of dopant material having higher refractive index than that of PMMA is required to form the refractive index distribution in the GI POF, which decreases the amount of water absorption into polymer,because the dopant material is more hydrophobic than PMMA. Therefore, in spite of the small amount of absorbed water such as 0.5 wt.%, the absorbed water molecules can not be uniformly dispersed but must be aggregated to form heterogeneities in the refractive index of the polymer matrix. It was clarified that a dopant material that is as hydrophilic as PMMA was required to maintain the attenuation of the PMMA-dopant system GI POF under high temperature and high humidity atmosphere, and such a suitable dopant system GI POF was proposed.
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