Low-noise 2-GHz figure-9 fiber laser based on passive harmonic mode-locking.

We have proposed and demonstrated the generation of a high-repetition-rate ultrashort pulse with long-term stability and low noise based on a harmonic mode-locked (HML) figure-9 fiber laser. Different HML orders from the 2nd to the 13th are generated by adjusting the pump power, net dispersion, and wave plate angles. A 2-GHz HML pulse is obtained with a 155-MHz fundamental repetition rate in a Yb-doped fiber laser, and the corresponding supermode suppression level is as high as 50 dB. The average power of the output pulse is nearly 200 mW, and the compressed pulse duration is 535 fs. To the best of our knowledge, 2 GHz and 196 mW represent, respectively, the highest repetition rate and output power in a figure-9 fiber laser. This work offers a potential pathway to achieve high-repetition-rate ultrashort pulses in the GHz range with high power and low noise levels.