311.7-W high-beam-quality single-frequency fiber laser based on heavy boron doping for SBS suppression

In this Letter, Yb-doped Al₂O₃-P₂O₅-B₂O₃-SiO₂ fibers are first studied with different B₂O₃ doping levels to suppress stimulated Brillouin scattering (SBS). It is revealed that by heavy B₂O₃ doping, the Brillouin gain spectrum broadens, and the gain coefficient decreases substantially, which leads to a significant SBS suppression effect. For Yb-doped fibers (YDFs) of 6.43 mole% B₂O₃, the Brillouin gain coefficient suppression ratio improves by more than 5.0 dB, and the SBS threshold of single-frequency (SF) laser emission enhances by 4.87 times, compared to YDF of weak B₂O₃ doping. Based on the optimal B₂O₃-doped YDF with a step-index 20-μm-diameter core, high-beam-quality single-frequency laser emission has been achieved with 311.7 W output power. The laser linewidth is measured to be 2.9 kHz. The beam quality M² factor is 1.20, and the fundamental mode ratio is calculated to be above 97% in the output beam at the maximum output. This work provides an effective material-based strategy for power scaling of single-frequency fiber lasers with strong potential for practical applications.