On the power threshold of mode instabilities

The exponential growth of the average output power of fiber laser systems seen over the last decade has recently come to an abrupt halt due to the onset of a new phenomenon: mode instabilities [1]. In this effect, the stable Gaussian output beam of a fiber amplifier starts to fluctuate suddenly once that a certain average power threshold has been reached. This effect is particularly damaging because it undermines one of the most valued characteristics of fiber laser systems: their power-independent beam quality. Thus, in the fiber laser community there is a consensus that solutions are urgently needed. This explains the amount of attention that this effect has attracted over the last two years. In this period of time, the theoretical basis for the understanding of the effect has been set. Thus, the physical origin of mode instabilities has been identified to be a thermally induced long-period grating caused by the interference between two fiber modes [2]. It has been demonstrated that this thermally-induced grating is able to transfer energy between the otherwise orthogonal fiber modes that ultimately give rise to it [2-4].