Ultrafast fiber lasers have demonstrated outstanding performance in LIDAR, spectroscopy, medicine as well as material processing owing to their advantages of good beam quality, compact structure, and easy thermal management. High output power and diffraction-limited beam quality achieved by ultrafast fiber lasers make this technique one of the most remarkable and promising laser technologies existing nowadays. For example, several thousand watts average power, up to 26 mJ pulse energy, and 22 GW peak power have been reported in recent years. Nevertheless, the progress of power scaling of fiber lasers is currently under challenge. Nonlinear effects and mode instabilities have become two main factors hindering the further development of ultrafast fiber lasers. In general, increasing the mode field area and reducing the power intensity of the core are the most effective strategies to circumvent the nonlinear effects in the optical fiber. However, fibers with large mode area (LMA) usually support multimode propagation which may lead to transverse mode instabilities (TMIs) at high average power. This phenomenon would drastically degrade the emitted beam quality of the fiber laser system.
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