Detailed experimental investigations are presented on investigating mode instability and stimulated Raman scattering in 30μm core diameter fiber amplifier by varying the pumping light wavelength using 915/976nm LD sources. Results reveal that for 976nm pumping light, the highest power can reach to 3570W; and for 915nm pumping light, the highest power can reach to 3771W. Physical characteristics of mode instability do not perform in a traditional form both in output power and time domain traces, such as roll-over point in output power curve. However, the beam quality of output laser clearly reveals there are some more high-order modes which have an effect in mode instability characters. Compared to 976nm LD pumping source, laser of amplifier using 915nm LD pumping source shows stronger non-linear effect. When the output power is 3512W, the Raman light is 13dB below the signal light. By shortening the passive fiber before the endcap, when the output power is 3520W, the Raman light is 20dB.
Detailed experimental investigations are presented on suppressing mode instability and stimulated Raman scattering by varying the pumping power distribution in a large mode area all-fiber amplifier with fiber core diameter of 20μm using bi-directional configuration. Results reveal that compared to employing co-directional pumping scheme, the fiber amplifier employing counter-directional pumping scheme can enhance the MI threshold power from 1250W to 1447W. Optimizing the pumping power distribution can further strength the mitigation of mode instability, such as the ratio of 57% and 66%, the threshold power is 2176W (highest output power) and 2150W respectively. For the ratio of 49%, which means almost identical scale of co-pumped light and counter-pumped light, the threshold power is 1934W. On the other hand, raising the proportion of backward pumping power can also mitigate the stimulated Raman scattering. 66% of backward pumping power can acquire 2150W output power and 20dB signal-to-noise ratio of the Raman peak, which indicates that optimizing the pumping power ratio can suppress mode instability and stimulated Raman scattering simultaneously.