This paper reports the measurement and analysis of signal power-induced changes in the refractive index spectrum of an aluminosilicate erbium-doped fiber amplifier (EDFA). For a constant 980 nm pump power of 11 mW, a 1553 nm signal power of 4.6 mW caused a peak-to-trough change in refractive index of (3.8\;\pm\;0.4) \times 108 across the EDFA gain spectrum. A Kramers-Krnig transform of the signal-induced change in absorption coefficient agreed well with the measured change in refractive index. This result conflicts with the behavior of the system predicted by a homogeneously broadened two-level model and suggests limitations in the representation of the Er3+:glass system by a discrete set of homogeneously broadened transitions.
Mark Janos and Stephen C. Guy, "Signal-Induced Refractive Index Changes in Erbium-Doped Fiber Amplifiers," J. Lightwave Technol. 16, 542- (1998)