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First Demonstration of a Bidirectional Tandem-Pumped High-Brightness 8 kW Level Confined-Doped Fiber Amplifier

In this paper, we demonstrated a high-brightness bidirectional tandem-pumped confined-doped fiber amplifier with nearly 8 kW output power. The technical difficulties for realizing the bidirectional tandem-pumping scheme are comprehensively analyzed, according to which, the homemade high signal-isolation side-coupled pump and signal combiner as well as stable 1018 nm pump sources were developed and applied in the experimental study, successfully enabling the first demonstration of the bidirectional tandem pumping scheme. Moreover, benefiting from the confined-doped fiber design and the bidirectional tandem pumping technique, the transverse mode instability was successfully mitigated and the beam quality was well preserved during the power scaling process. Eventually, an output power of 7.88 kW was obtained with the beam quality factor of ∼1.97 and the optical-to-optical efficiency of ∼84.76%. This work provides a good reference for realizing large-mode-area fiber lasers with high output power and good beam quality by utilizing the confined-doped fiber design and bidirectional tandem-pumping technique.