Fig. 1. Schematic setup of the MO for generating GHz harmonic mode-locking. Col, collimator; YDF, Yb-doped fiber; BPF, bandpass filter; QWP, quarter-wave plate; HWP, half-wave plate; PBS, polarization beam splitter; ISO, isolator; LD, laser diode.

Fig. 2. Experiment results. (a) Spectra: red is the mode-locked spectrum when the ∆λ_f is 0 nm; blue is the mode-locked spectrum when the ∆λ_f is 3 nm; inset shows the zoom-in of the CW state. (b) Pulse train. (c) RF spectrum.

Fig. 3. Evolution of the pulses with increasing pump power.

Fig. 4. HML pulse train and RF spectrum: (a) 11th, (b) 14th, (c) 35th, (d) 45th, (e) 51st and (f) 59th harmonic orders.

Fig. 5. Experimental results under different ∆λ_f. (a)–(e) Pulse train; inset, RF spectrum. (f)–(j) Spectra; inset, zoom-in of the CW state.

Fig. 6. Experimental results. (a) Pulse train; inset, RF spectrum. (b) Spectrum. (c) Autocorrelation trace.

Fig. 7. Power stability measurement of the mode-locked MO within 5 hours.

Fig. 8. Overview of GHz harmonic mode-locking in different configurations^[33–48].