Tapered Yb-doped fiber enabled a 4 kW near-single-mode monolithic fiber amplifier

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 similar to 20/400 mu m and an output end of similar to 30/600 mu m. 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 M-2 similar to 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. (C) 2022 Optica Publishing Group