Mode instability in Yb3+-doped fiber amplifiers of continuous and pulsed signal

Spatio-temporal instability of fundamental mode in Yb³⁺-doped large mode area polarization-maintaining fiber amplifiers was analyzed. Limitations of the amplifier output power and gain were investigated depending on the input signal parameters: spectral width, power level and polarization. The influence of backward reflection of the optical waves on the mode-instability threshold was also examined. The traveling electronic and thermal refractive index gratings following the optically-induced population gratings were found to provide the energy transfer from the main mode to the higher-order modes.