Berberine: A Natural Saturable Absorber for Accelerating Mode-Locking in Ultrafast Fiber Laser

Currently, the performance, cost, and environmental sustainability of saturable absorbers (SAs) represent major bottlenecks in the development of ultrafast fiber lasers. However, the development of high-performance SAs remains challenging due to intricate fabrication processes and inadequate nonlinear performance. In this context, this work presents the natural alkaloid berberine as a promising alternative for mode-locked erbium-doped fiber lasers. The berberine-based SA exhibits excellent nonlinear optical properties, including a modulation depth of 24.40 % and a saturation intensity of 1.281 MW/cm², and successfully enables stable femtosecond pulse generation. By employing time-stretched dispersive Fourier transform technology, the transient dynamics results indicate that the berberine-based SA significantly shortens the relaxation oscillation time and effectively suppresses pulse intensity fluctuations, thereby accelerating the self-starting process of mode-locking. This work provides a new strategy for developing high-performance, low-cost, and environmentally friendly ultrafast photonic devices, significantly advancing the practical application of green SAs.