Estimation of refractive index profile of a GeO₂–SiO₂ graded-index fiber from the elemental composition

We propose methods for the estimation of the refractive index profile of a GeO₂-SiO₂ graded-index fiber (GIF) from its elemental composition. We measured the mole fraction of GeO₂ as a function of radial position in a commercially available GIF, Thorlabs GIF625, using energy dispersive spectroscopy (EDS) and electron probe microanalysis (EPMA). Its refractive index profiles were also measured at multiple wavelengths. We found that the GeO₂ mole fractions estimated from the measured refractive index profiles show a discrepancy with the elemental composition obtained from the EDS/EPMA measurement. Two empirical approaches are proposed to bridge the elemental composition and the index profiles over the discrepancy: (1) effective mole fraction approach and (2) density parameter approach. Using these approaches, we provide index profile parameters at important wavelengths from visible to infrared near 2.0 µm so that the index profile can be generated for applications of the GIF. The proposed approaches can be applied to other binary glass system GIFs.