Harmonic soliton mode-locked zirconia-based erbium-doped fiber laser with graphene saturable absorber

Abstract This work demonstrates and characterizes passive harmonic mode-locking (HML) in a zirconia-based erbium-doped fiber (Zr-EDF) laser using a graphene thin film as the saturable absorber. Initially, fundamental mode-locking is attained at a threshold power of 50 mW, producing mode-locked spectrum with a central wavelength of 1563.3 nm and output pulse with a repetition rate of 10.9 MHz. By increasing the pump power beyond the threshold and tuning the polarization controller, the laser system can generate harmonics up to the 11th order of the fundamental frequency, corresponding to 119.9 MHz. This is achieved at a relatively low pump power of 100 mW. The mode-locked spectrum displays Kelly sidebands, indicating operation within the soliton regime, which remains stable up to the maximum harmonic order. Analysis of pulse width reveals slight variations against pump power, which fluctuates between 0.68 ps and 0.74 ps. To the best of our knowledge, this is the first demonstration of harmonic pulse generation in a passively mode-locked Zr-EDFL at low pump power. This capability opens significant potential for advancing research and development in all-fiber HML.