Increasing Mode Instability Threshold by Mitigating Driver Noise in High Power Fiber Amplifiers

Reduction of mode instability threshold by driver noise has been observed in a 3 kW level high power fiber amplifier system for the first time, which has been confirmed by monitoring the temporal characteristic of the beam profile and the output current. After the onset of mode instability induced by driver noise, the laser beam profiles fluctuated at a characteristic frequency of 0.3 kHz with 50 Hz-spaced frequency lines in the background. Obvious fluctuation in the signal peak of the optical spectrum has also shown up with the onset of mode instability. By employing an alternative driver with built-in filter, the diver noise has been suppressed by nearly 30 dB, and the mode instability threshold has been increased by 25.8%. The evidence of electrical noise lines was absent from the temporal characteristic of the beam profile, only regular frequency component, which demonstrate that the driver noise can be suppressed efficiently by properly designing the driver. The results revealed that the driver noise should be taken into consideration in designing the high power fiber amplifier systems.