Dynamics of pump/signal-induced index change in Yb-doped fiber amplifier

The refractive index change in a Yb-doped fiber amplifier under diode pumping and pulse amplification is investigated using Mach-Zehnder interferometer operating in the spectral range far from Yb-ion transitions. A strong phase shift which indicate the change in the refractive index is observed both under pulsed and continuous wave pumping.