A Low-Threshold Nonlinear-Amplifying-Loop-Mirror Mode-Locked Bismuth-Doped Fiber Laser Using A 3×3 Coupler

Passive mode-locking plays a crucial role in generating ultrafast laser pulses and high-quality beams. However, traditional figure-of-eight laser cavities often require additional intra-cavity amplitude modulators, tapping or a two-gain-segment design to initiate mode-locking, leading to increased complexity and loss within the cavity. Here we propose a novel approach utilizing a 3×3 optical coupler to address these challenges. This work was performed at the novel communication O-band using bismuth-doped phosphosilicate glass fiber, where most rare-earth-doped silica fibers are inaccessible. By integrating a bismuth-doped nonlinear-amplifying-loop-mirror with a 3×3 optical coupler operating at 1310 nm, we achieve stable mode-locking at a significantly reduced pump power of 119.9 mW. Notably, the incorporation of a 120-degree (2π/3) phase difference results in an initiation threshold reduction of at least by 45% for NALM laser cavity using a symmetrical 2×2 optical coupler, lower pump power that maintain stable mode-locking by 26% compared to using an asymmetrical 2×2 optical coupler (70:30), and at least a twofold increase in output power of up to 6.839 mW. These findings underscore the potential of our optimized structure for improving performances of fiber lasers operating at the optical communication O-band.