High-efficiency Yb³⁺-doped fiber laser with highly optical nonlinear Bi₄Br₄-based saturable absorber

Recently, Bi₄Br₄ is proved to be a member of topological insulators and is expected to be a promising candidate for ultrafast photonic device. However, experimental studies on the nonlinear optical properties of Bi₄Br₄ are limited, and its broadband absorption capabilities have not been validated. This study presents the first preparation of Bi₄Br₄ samples using the chemical vapor transport method, resulting in a saturable absorber (SA) with a high modulation depth (46.23%) and low non-saturable loss (6.5%). The optical nonlinearity ranks among the best in similar studies. Additionally, this work applies Bi₄Br₄-SA for the first time in 1-μm fiber laser, developing a ring-cavity mode-locked fiber laser with a central wavelength of 1029.79 nm, a pulse duration of 442 fs, and a maximum output power of 90.83 mW. And a linear-cavity mode-locked fiber laser with a central wavelength of 1031.24 nm, a pulse duration of 511 fs, and a maximum output power of 92.81 mW is constructed. It is worth noting that the optical-to-optical conversion efficiency has reached about 11.54% and 33.58%. This study verifies Bi₄Br₄-SA’s modulation effectiveness for 1-μm pulse lasers and provides a powerful reference for the design of high-efficiency fiber lasers.