Passive harmonic mode-locking of a fiber laser at controllable repetition rates from fundamental to eighth-order harmonic operation

We have experimentally reported a passively harmonic mode-locked fiber laser at controllable repetition rates. The pulse train is continuously tunable and controllable from the fundamental cavity frequency to the eighth-order harmonic frequency. The pulses in the proposed laser disappear one by one when the pump power is decreased and, at the same time, the remaining pulses self-organize into a new, stable, uniform distribution in the cavity at each state. The experimental results indicate a supermode suppression of more than 40 dB. By appropriately increasing the coupler output ratio and lengthening the laser cavity, the enhanced mutual influence of the global and local soliton interactions induced by the CW components and the dispersive waves plays the key role in separating pulses stably and uniformly in the cavity. Our laser has the potential to generate chirp-free pulses at even higher rates and shorter pulses for use in future time-division multiplexing systems.