Stable, Ultrafast Pulse Mode-Locked by Topological Insulator Nanosheets Interacting With Photonic Crystal Fiber: From Anomalous Dispersion to Normal Dis

We demonstrate two kinds of stable ultrafast fiber lasers mode-locked by topological insulator (TI) Bi₂Se₃ nanosheets interacting with photonic crystal fiber (PCF). The saturable absorber (SA) produced by drying TI solution filled in 5.5 cm PCF provides a modulation depth of 5%, and its damage threshold is highly enhanced due to the large evanescent field interaction length. When the SA is incorporated in an erbium-doped fiber ring cavity with anomalous dispersion, a conventional soliton with 908-fs duration is obtained, and no wave-breaking and SA damage is ever shown, even for pump power at 900 mW. Through tuning polarization controller, as high as tenth harmonic mode-locking, namely, 202.7 MHz, is obtained, and it operates stably for 8 hours under lab conditions. Dissipative soliton with a duration of 7.564 ps is generated when the dispersion of cavity changes from anomalous to normal, which can be further compressed to 245 fs by additional 15-m single mode fiber, and a maximum average output of 75 mW is obtained. This TI nanosheets-PCF structure possesses advantages of stable saturable absorption performance, high damage threshold, and freedom from special package, which make it an effective choice for a passively mode-locked laser.