Mitigation strategies for mode instabilities in high-power fiber-laser systems

Fiber-based laser systems and amplifiers have shown a rapid evolution of average output-power scaling reaching the kilowatt level both for cw- and pulsed-operation [1]. However, this fast development has led to the observation of a new, detrimental effect: mode instabilities (MI) [2]. Hereby after reaching a certain threshold power level a former Gaussian-like beam profile becomes suddenly unstable and, simultaneously, its beam quality is degraded. This phenomenon is currently the most limiting effect when it comes to further scaling the average power of fiber laser systems. Thus it is of great importance to learn how to overcome this limitation. The most recent investigations of MI (experimental and theoretical) have shown that MI are related to a thermally induced long period grating that mimics the intensity interference pattern of the guided fiber modes [3,4,5]. Consequently, controlling, weakening or ultimately washing out this grating seems a promising approach to suppress or even avoid MI.