Compared with conventional uniform fibers, long tapered fibers provide an effective way to maintain beam quality and suppress the generation of nonlinear effects in fiber lasers. In this work, the transverse mode instability (TMI) of two amplifiers based on ytterbium-doped fiber (YDF) with uniform core diameter and tapered core diameter has been investigated experimentally. For a reasonable comparison of TMI in these two fiber amplifiers, the same effective core diameters and design parameters are applied to both. In the uniform YDF amplifier, the TMI threshold is around 1046 W, and the beam quality M 2 factor increases with the power and reaches 3.2 when the power is around 1500 W. In a tapered YDF (T-YDF) amplifier, the maximum output power is up to 2170 W with a beam quality of M 2 ~ 2.2 and no sign of TMI is observed. To the best of our knowledge, this is the first reported all-fiber tapered fiber laser with a 2 kW-level average output power. The experimental results and theoretical analysis indicate that the TMI threshold of the T-YDF amplifier is higher than the uniform YDF amplifier in spite of sharing the same effective core diameter.

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In this Letter, we demonstrate a monolithic high-power master oscillator power amplifier by using a home-made double-clad tapered Yb-doped fiber (T-YDF) with an input end of ∼20/400 µm and an output end of ∼30/600 µm. Thanks to perfect core/cladding matching with the fiber components at both ends of the T-YDF, the laser is pumped bidirectionally and an output power of over 4 kW with a high slope efficiency of 84.1% and excellent beam quality M2 ∼ 1.46 is achieved. In contrast to previous work on common fiber lasers, experimental results also reveal that the co-pump scheme has a higher transverse mode instability (TMI) threshold and power-boosting capability than that of a counter-pump scheme. To the best of our knowledge, this is the highest output power demonstrated to date from such a T-YDF with excellent beam quality. This work indicates the great potential of the T-YDF to realize further power scaling, high laser efficiency, and excellent beam quality in high-power fiber lasers.

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