Harmonic mode-locking and wavelength-tunable Q-switching operation in the graphene–Bi₂Te₃heterostructure saturable absorber-based fiber laser

We have investigated the broadband saturable absorption property of graphene–Bi2Te3 heterostructures and demonstrated their applications for stable harmonic mode-locking operation in a Yb-doped fiber laser and wavelength-tunable Q-switching operation in an Er-doped fiber laser. The modulation depth of a graphene–Bi2Te3 heterostructure saturable absorber (G-Bi2Te3-SA) is dependent on the coverage of Bi2Te3 on the graphene. By using 15%-Bi2Te3-covered G-Bi2Te3-SA with a modulation depth of 23.28% and saturable intensity of 3.32 MW/cm2, the harmonic mode-locked Yb-doped fiber laser outputs the mode-locked pulses with a pulse duration down to 189.94 ps, spectral bandwidth of 3.5 nm, and repetition rate of 79.13 MHz (21st order of the fundamental frequency). After inserting the G-Bi2Te3-SA with 85% coverage of Bi2Te3 on graphene into Er-doped fiber laser cavity, whose modulation depth and saturable intensity are about 40.79% and 12.48 MW/cm2, respectively, the wavelength-tunable Q-switched pulse with tunable wavelength range over 13.2 nm has been obtained by adjusting the intracavity fiber filter. These results suggest that the graphene–Bi2Te3 heterostructure could serve as a high nonlinear photonic device for practical applications.