Generation and Evolution of Harmonic Mode- Locked Square Pulse in an Yb-Doped Fiber Laser

The generation and evolution of harmonic mode-locked (HML) square pulse have been numerically investigated in an all-normal-dispersion Yb-doped figure-eight passively mode-locked fiber laser. By carefully adjusting the polarization controllers with fixed pump power, the square pulse HML have also been experimentally obtained. Further study showed that the square pulse HML nanosecond square pulses had characteristics identical to those of the single square pulse, with pulse energies and widths all increasing continuously with the pump power, while their peak power remained almost constant.