5.2 kW bidirectional-pumping fiber oscillator based on confined doped fiber

In this manuscript, we theoretically analyze the impact of bend-induced mode distortion and Yb-doping ratio on the gain distribution for modes and transverse mode instability (TMI), which indicates the advantages of using confined-doped fibers in elevating the TMI threshold of fiber oscillators. Based on simulations, we provide an appropriate range for selecting bending radii and doping ratios. Experimentally, we conduct comparisons using different bending conditions and pump ratios for the confined doped fibers, as well as a comparison with traditional fully doped fibers. Lastly, we present a design for a 5.2-kW directly pumped fiber oscillator achieved by tightly bending a piece of 25/500 µm confined doped fiber. At the maximum output power, the beam quality factor M 2 is 2.05. These findings offer valuable insights for enhancing the power output of confined doped fiber oscillators with high beam quality.