Fig. 1. Schematic of the mode-locked EDF laser. SMF, single mode fiber; COL, collimator; QWP, quarter-wave plate; ISO, polarization-dependent isolator.

Fig. 2. Different optical spectra of dark solitons are obtained with the different polarization states when we rotate HWPs and QWPs in the fiber laser. (a) The optical spectrum is smooth with the 4.8 nm spectral width at 1530 nm. (b) The optical spectrum is a bit rough with the 4.1 nm spectral width. (c) and (d) both have the 0.16 nm spectral width.

Fig. 3. Due to the different intracavity power, the different pulse sequences corresponding to Fig. 2 are presented. The oscilloscope trace of dark solitons is in the time scales of 10 ns/div.

Fig. 4. (a) Radio frequency (RF) spectrum of dark solitons. (b) Phase noise measurement at 116.5 MHz, and the timing jitter is about 1 ps.