Mode instabilities in large-mode-area fiber amplifiers

The onset of mode instabilities is an effect occurring in high-average-power fiber lasers and amplifiers when their output average power exceeds a certain threshold value [1]. This effect is characterized by a sudden change of the output beam from an initially stable fundamental mode to a regime with more or less chaotic fluctuations between the fundamental mode and one or more higher-order modes (shown in Fig. 1). Mode instabilities occur in different fiber architectures and they can additionally be observed both in continuous wave and in pulsed operation. Only a few years after the first mention of this effect in the literature, it has become one of the most severe limitations for the further average-power scaling of fiber lasers and amplifiers.