All-normal-dispersion passive harmonic mode-locking 220  fs ytterbium fiber laser

We report a stable passive second-harmonic mode-locked all-normal-dispersion ytterbium fiber laser based on nonlinear polarization evolution. This fiber laser has two polarization beam splitter output ports for optimizing the output spectrum 5 ps duration pulses with 187 mW average power being generated at the harmonic repetition rate of 99.6 MHz. By use of a pair of gratings to extracavity compensate the chirp, the pulse is further compressed to 220 fs. We measured that the peak-to-pedestal extinction of the radio frequency is about 80 dB corresponding to a pulse-to-pulse energy fluctuation of 0.32% and timing jitter of 3.2 ps.