Passive Harmonically Mode-Locked Yb $^{3 +}$-Doped Fiber Laser Free From Anomalous Dispersion

We report on an all normal dispersion harmonically mode-locked ytterbium-doped femtosecond fiber laser. The mode-locking mechanism of the laser is based on nonlinear polarization evolution with strong pulse shaping. Bandpass filtering is achieved with a cascaded long-period fiber grating, forming highly chirped pulses. The laser generates stable 2.5-ps duration pulses at a repetition rate of 125.39 MHz that could be dechirped externally to 310 fs with a maximum pulse energy of 0.36 nJ.