Electronic and thermal refractive index changes in Ytterbium-doped fiber amplifiers

We develop a theoretical framework to analyze the mechanism of refractive index changes (RIC) in double-clad Yb³⁺ doped optical fibers under resonant core or clad pumping, and with signal amplification. The model describes and compares thermal and electronic contributions to the phase shifts induced on the amplified signal at 1064 nm and the probe signal at 1550 nm, i.e. located inside and outside of the fiber amplification band, respectively. The ratio between the thermal and electronic phase shifts is evaluated as a function of the pump pulse duration, the gain saturation, the amplified beam power and for a variety of fiber parameters.