Fig. 1. Experimental setup of the mode-locked laser and the DFT device. DM, plane dichroic mirror; M1, concave mirror (R = 200 mm); M2, plane mirror; GTI1 and GTI2, Gires–Tournois interferometer mirrors; OC, output coupler; SESAM, semiconductor saturable absorber mirror; Col, collimator; PD, photo-detector; BS, beam splitter; SMF, single-mode fiber; OSA, optical spectrum analyzer.

Fig. 2. Output characteristics of the mode-locked Yb:KGW laser. (a) Output power versus pump power. (b) RF spectrum at the fundamental frequency (resolution bandwidth ∼1 kHz) and longer-term RF spectrum at a span of 1-GHz (resolution bandwidth ∼100 kHz). (c) Autocorrelation trace of the mode-locked pulse with a sech² fitting, indicated by a dark green curve. (d) Spectrum of the mode-locked pulse.

Fig. 3. Real-time buildup dynamics of mode locking in the Yb:KGW laser. (a) Entire buildup dynamic process obtained by dispersing the pulses in 10-km SMF before detection. (b) Single frame of mode locking spectral information. (c) and (d) show the data from (a) for the last spike and reveal the detailed information of this period. (e) Dominant pulse with a subordinate pulse together in the cavity.

Fig. 4. Spectra derived from the DFT results during the mode locking. (a) Spectral evolution of the buildup process for 120,000 round trips. (b) Spectral broadening process near the mode locking. (c) The last frame extracted from (a). (d) Spectrum of the mode-locked pulse detected by the OSA.

Fig. 5. Real-time extinction dynamics of the mode-locked Yb:KGW laser. (a) Entire extinction dynamic process obtained by dispersing the pulses in 10-km SMF before detection. (b) Single frame of spectral information before the disruption of mode locking. (c) and (d) show the data extracted from (a) for the first spike after the mode-locking state disappears. (e) Several pulses which involve subordinate pulses and a dominant pulse together in the cavity.

Fig. 6. Real-time spectral evolution of the extinction dynamics of the Yb:KGW laser. (a) Spectral evolution of the extinction process for 100,000 round trips. (b) Spectrum for the moment when stable mode locking transforms into post-lasing oscillations. (c) Narrowing of the spectrum when the mode locking is affected.