Geo₂concentration dependence of nonlinear refractive index coefficients of silica‐based optical fibers

The nonlinear refractive index coefficient n 2 dependence on the GeO 2 concentration in optical fibers is quantitatively shown for SiO 2 core fibers and GeO 2 ‐SiO 2 core/SiO 2 clad fibers with Δ = 0.35–2.2% using an XPM measurement method where the phase change in the fiber of a probe signal caused by a pump signal is found from the delay self‐heterodyne spectrum. The GeO 2 concentration distribution which determines the n 2 distribution in the fiber is not uniform, and the measured n 2 is the average of the product of the local n 2 and the fourth power of the electric field in the fiber. In this work, this relationship of the measured n 2 , the cross sectional distribution of n 2 , and the electric field distribution in the fiber is used to define an effective GeO 2 concentration, η eff . This effective GeO 2 concentration level is used to evaluate n 2 . The measured n 2 increases proportionally to η eff by the relationship n 2 [x 10–20 m 2 /W] = 0.0552° eff [mol%] + 2.44. Furthermore, the measured n 2 in the fiber agrees well with the n 2 of bulk glass obtained a semiempirically for the nonlinear refractive index coefficient derived from linear refractive index data.