Topological Insulator-Assisted Dual-Wavelength Fiber Laser Delivering Versatile Pulse Patterns

We report on the generation of dual-wavelength mode-locked pulses in an erbium-doped fiber laser incorporating a topological insulator (TI):Bi2Se3/Polyvinyl Alcohol (PVA) film which could operate as both an excellent saturable absorber (SA) for mode-locking and a high-nonlinear medium for mitigating the mode competition of erbium-doped fiber and stabilizing the dual-wavelength oscillation. The TI:Bi2Se3/PVA film is prepared through a liquid-phase exfoliation/spin-coating method. Its saturating intensity and modulation depth are measured to be about 31.5 MW/cm(2) and 3.4%, respectively. Taking advantage of the high nonlinearity and saturable absorption of the TI:Bi2Se3, three dual-wavelength pulse patterns, namely bright pulse, bright-dark soliton pair, and hybrid step-like and dark pulse, could be achieved under different lengths of single mode fiber by properly adjusting the pump power and the polarization state. For the bright pulse operation, we obtain its fundamental repetition rate of 9.75 MHz. For the bright-dark soliton pair operation, we find that it contains two distinct parts in the whole optical spectrum: one is at shorter wavelength corresponds to bright soliton and the other one is at longer wavelength corresponds to dual-wavelength dark soliton. In addition, we also achieved the harmonic mode-locking (HML) of the bright-dark soliton pair. The highest obtained repetition rate is about 433.8 MHz, which corresponds to the 280th harmonic of the fundamental repetition rate. For the hybrid step-like and dark pulse operation, we achieve its fundamental repetition rate of 8.805 MHz. Our findings suggest that TI:Bi2Se3 can operate as both an excellent SA for short pulse generation and a promising highly nonlinear photonic material for the related nonlinear optics applications.