Frequency domain analysis of dynamic refractive index changes in fiber amplifiers

Gain dynamics and corresponding refractive index changes in fiber amplifiers are important for low noise fiber amplifiers and when using doped fibers as phase actuators. Furthermore self-induced refractive index changes are a possible explanation for mode fluctuations in high power fiber amplifiers, which currently present a major challenge for high power fiber lasers. Since these processes are all time dependent, a dynamic model of the refractive index change is very important. We will present measurements of phase shift dynamics in a low power ytterbium fiber amplifier. Using a simplified analytic model, which was originally developed to predict power modulation in telecom amplifiers, we were able to model the part attributed to the Kramers-Kronig-Relations and to obtain a much better understanding of the involved timescales. This will help modeling of phase noise in low noise amplifiers and possibly mode fluctuations in fiber amplifiers.