Characterization of transverse mode instability in fiber-laser systems using a position-sensitive detector

A new approach to characterize the effect of transverse mode instability (TMI) in high-power fiber-laser systems is presented. A position-sensitive detector is employed to detect the trajectories of the center of gravity of the fluctuating beam (in a short time-window) at different average power levels. With an increasing average output power, the area covered by the trajectories increases which is used as a measure for the stability of the system. The new concept allows for a simple, fast, and detailed characterization of TMI, which accurately determines its threshold even in complex operating regimes. This new technique can easily distinguish between spatial and power fluctuations of the beam. Furthermore, the trajectories contain information about the movement of the center of gravity of the beam, which can be used to gain additional insight about the dynamics of TMI. The technique robustness was tested by characterizing the TMI behavior in a complex operating regime. e.g., using a pump-power modulation.