Abstract

The phenomenon of mode instabilities refers to the output beam of a fiber laser system becoming suddenly unstable once that a certain output power threshold has been reached. Shortly after the earliest reports on this effect [1] came out, the first hypothesis on its origin was published. In this explanation, which still holds, the interference pattern that appears along a fiber due to the beating of two transverse modes gives rise to a long period index grating via the thermo-optic effect [2, 3]. Up to now, mode instabilities remain an unsolved problem in optical fiber technology and are currently the main effect preventing a further scaling of the output average power of fiber laser systems. Therefore, there is an enormous interest worldwide in understanding and overcoming mode instabilities. In this context our latest investigations and experimental results strongly suggest a link between photodarkening and the mode instability threshold. Based on this knowledge in this work we provide further insights on the physics of the mode instability threshold and give some guidelines to significantly increase its value in the near future.

© 2015 IEEE