Diverse passive harmonic mode-locking were observed in an all-normal-dispersion Yb-doped passively mode-lockedfiber laser with a long cavity based on the nonlinear polarization rotation (NPR). Using a cascaded long-period fibergrating as a spectral filter, several modes of passively harmonic mode-locking including passive high-order harmonicmode-locking, passive multiple-pulse harmonic mode locking and passive large-pulse-width harmonic mode locking(over 300 ns ) were obtained by adjusting the tunable CW semiconductor laser (pumping source in the experiment) andpolarization controller, in which the intense nonlinear effects, large fiber dispersion and phase bias played an importantrole. It is expected that the observed various modes of passive harmonic mode-locking may find applications in industrialfields where different pulse widths and repetition rates are needed and provide experimental evidence to the research ofultrafast lasers.
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