Correlation between mode coupling and light scattering caused by microscopic heterogeneous structure in graded-index plastic optical fiber

We investigated the effect of mode coupling due to microscopic heterogeneities in poly(methyl methacrylate) (PMMA)-based graded-index plastic optical fiber (GI POF). To evaluate the correlation characteristics of microscopic heterogeneities inside an actual GI POF, we analyzed scattering loss of the GI POF with light scattering theory for an amorphous solid. This allows for approximate estimation of various pairs of correlation length and mean square of dielectric constant fluctuation for measured scattering loss. Taking into account the evaluated correlation characteristics, we estimated the correlation length of POF core polymer by analyzing the effect of mode coupling on optical pulse response with developed coupled power equation. The results showed that the correlation length of POF core polymer was longer than that of bulk polymer. This indicates that the microscopic heterogeneous structure could be changed in fabrication process of GI POF and become to induce strong mode coupling and large scattering loss.