Fig. 1. Schematic diagram of the experimental setup for fabricating the ULPG sample. The green frame is the diagram of the ULPG sample and a micrograph of a fiber taper.

Fig. 2. Typical optical characteristics and saturable absorption property of the as-used ULPG: (a) transmission spectrum (inset: the photograph when the yellow light is passing out); (b) nonlinear saturable absorption behavior.

Fig. 3. Experimental setup. LD, laser diode; WDM, wavelength-division multiplexer; EDF, erbium-doped fiber; ISO, isolator; OC, optical coupler; SMF, single-mode fiber; PC, polarization controller; ULPG, ultra-long-period grating.

Fig. 4. Three-wavelength soliton mode-locking operation with a length of SMF of 93.5 m and a corresponding cavity dispersion of ∼−2.01ps2. (a) Optical spectrum and (b) the corresponding pulse train (inset: the zoom-in image).

Fig. 5. Four-wavelength soliton mode-locking operation with a length of SMF of 93.5 m and a corresponding cavity dispersion of ∼−2.01ps2. (a) Optical spectrum and (b) the corresponding pulse train (inset: the zoom-in image).

Fig. 6. Multi-wavelength mode-locking operation with a length of SMF of 93.5 m and a corresponding cavity dispersion of ∼−2.01ps2. (a) Four-, (b) five-, (c) six-wavelength, and (b) the corresponding pulse train.

Fig. 7. Seven-wavelength mode-locking operation with a length of SMF of 20 m and a corresponding cavity dispersion of ∼−0.35ps2. (a) Optical spectrum, (b) the corresponding pulse train, and (c) long-term optical spectra measured at 2 h intervals over 14 h.

Fig. 8. Seven-wavelength and hybrid soliton operation with a length of SMF of 135 m and a corresponding cavity dispersion of ∼−2.94ps2. (a), (c) The optical spectra and (b), (d) the corresponding pulse trains, respectively.

Fig. 9. Average output power versus the pump power of the laser with different lengths of SMF.

Fig. 10. Mode-locking principle of ULPG. CW, continuous wave.