GEO2 CONCENTRATION-DEPENDENCE OF NONLINEAR REFRACTIVE-INDEX COEFFICIENTS OF SILICA-BASED OPTICAL FIBERS

The nonlinear refractive index coefficient n(2) dependence on the GeO2 concentration in optical fibers is quantitatively shown for SiO2 core fibers and GeO2-SiO2 core/SiO2 clad fibers with Delta = 0.35-2.2% usi ng 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-hete rodyne spectrum. The GeO2 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 powe r 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 th e electric field distribution in the fiber is used to define an effect ive GeO2 concentration, eta(eff). This effective GeO2 concentration le vel is used to evaluate n(2). The measured n(2) increases proportional ly to eta(eff) by the relationship n(2)[x 10-20 m(2)/W] = 0.0552 eta(e ff)[mol%] + 2.44. Furthermore, the measured n(2) in the fiber agrees w ell with the n(2) of bulk glass obtained a semiempirically for the non linear refractive index coefficient derived from linear refractive ind ex data.