Fig. 1. Applications of MWFL: (a) DWDM technology for an optical communication system, and (b) the multi-wavelength Raman fiber laser for long-distance simultaneous measurement of strain and temperature selected from Ref. [12]. (c) Phased array antenna system selected from Ref. [14]. (d) Microwave signal generation based on a multi-wavelength Brillouin fiber laser selected from Ref. [16].

Fig. 2. Multi-wavelength EDFL based on a phase modulator: (a) the schematic of the experimental setup; the output spectrum characteristics (b) without modulation feedback and (c) with modulation feedback. Selected from Ref. [24].

Fig. 3. Multi-wavelength operation based on the MZI filter effect: (a) the experimental schematic of an EDFL; (b) the comb filter transmission spectra; (c) the spectral characteristics of 14-wavelengths operation; (d) the spectral characteristics of 29-wavelengths operation. Selected from Ref. [27].

Fig. 4. MWFL based on the SMS interferometer: (a) the experimental schematic diagram of dual-wavelength EDFL; (b) the output spectral tunable dual-wavelength fiber laser. Selected from Ref. [30].

Fig. 5. Multi-wavelength fiber laser and the output characteristics: (a) the schematic diagram of dual-wavelength EDFL; (b) optical spectral evolution with different pump power; (c) the stability measurement of optical spectra. Selected from Ref. [66]. (d) The schematic diagram of multi-wavelength TDFL; (e) the stable tri-wavelength operation. Selected from Ref. [72].

Fig. 6. MWFL based on two types of intensity-dependent loss structures: (a) schematic of the NPR mode-locked TDFL; (b) working principle of the NPR structure. Selected from Ref. [78]. Two cases of output spectrum of MWFL based on NPR structures: (c) 22-wavelength operation; (d) 28-wavelength operation. Selected from Ref. [73]. (e) The experimental setup of the NALM structure. Output spectrum characteristics of EDFL based on the NALM structure at two different states by adjusting the PCs. Selected from Ref. [75]. (f) 41 wavelengths; (g) 50 wavelengths. Selected from Ref. [76].

Fig. 7. Multi-wavelength operation in the ring EDFL: (a) the experimental setup of backward pumping; (b) the experimental setup of forward pumping; (c) the output spectrum of forward and backward pumping. Selected from Ref. [83]. The multi-wavelength Brillouin–Raman fiber laser: (d) the experimental setup; (e) and (f) illustrations of multi-wavelength lasing spectra at different DCF lengths. The magnified views are shown in graphs on the right. Selected from Ref. [34].

Fig. 8. Spectrum characteristic of the dual-wavelength TDFL: (a) the three-states switchable dual-wavelength conventional soliton; (b) the numerical simulation transmission spectrum of the NPR; (c) the comparison between simulative and experimental results. Selected from Ref. [78].

Fig. 9. Schematic and laser characteristics of the NALM fiber laser: (a) the schematic diagram of a mode-locked Tm/Ho-doped fiber laser; (b)–(e) tunable multi-wavelength spectrum (left), corresponding pulse trace (middle), and single pulse (right); (f) and (g) show CW operation characteristics. Selected from Ref. [126].

Fig. 10. 2D materials. (a) The 2D family members. Selected from Ref. [135]. (b) The current dominant SAs for ultrashort-pulse generation. Selected from Ref. [136]. (c) The sketch map of the saturable absorption process in the BP. Selected from Ref. [154].

Fig. 11. Diverse methods of integration of CNT-/graphene-SAs into the resonant cavity: (a) sandwiched film between two fiber connectors; (b) in-fiber microfluidic channels; (c) PCFs filled by the SA; (d) D-shaped fiber; (e) tapered fiber; (f) fully integrated monolithic fiber laser. Selected from Ref. [177].

Fig. 12. Characteristics of dual-wavelength YDFL-based graphene SA (GSA): (a) microscopy image of tapered fiber-based GSA; (b) the schematic diagram of dual-wavelength YDFL; (c) the spectrum of dual-wavelength CW operation; (d) the spectrum of mode-locked operation; (e) the oscilloscope trace, inset: single-pulse envelope; (f) the RF spectrum. Selected from Ref. [196].

Fig. 13. TI-SA and characteristics of MWMLFL: (a) the solution of Bi2Se3/PVA; (b) Raman spectrum of Bi2Se3/PVA, inset: scanning electron microscope (SEM) image; (c) optical deposition process, inset: photo of the end of the fiber; (d) the saturable absorption characteristic of TI-SA; (e) the output spectrum under 116.2 mW pump power; (f) long-time output wavelength stability measurement of the tri-wavelength mode-locking operation over 9 h. Selected from Ref. [212].

Fig. 14. Output properties of dual-wavelength EDFL: (a) the spectrum of the dual-wavelength EDFL; (b) the pulse traces; (c) long-term output spectrum stability measurement. Selected from Ref. [224].

Fig. 15. Characteristics of BP nanoparticles (NPs): (a) the atomic force microscope (AFM) image; (b) height profiles of the sections marked in (a); (c) Raman spectrum; (d) the linear absorption spectrum; (e) the Z-scan measurements of BP-PMMA film; (f) the relation of normalized transmittance and intensity. Selected from Ref. [154].

Fig. 16. Output characteristics of tri-wavelength mode-locking based on the BP-SA: (a) the schematic of the EDFL; (b) the characteristics of the pulse trace (up) and spectrum (down); the emission spectrum of the EDF (c) without and (d) with BP-SA. Selected from Ref. [233].

Fig. 17. Schematic diagram and laser output characteristics of the fiber laser: (a) the schematic of the tri-wavelength mode-locked fiber laser; (b) the measured reflection spectra of three CFBGs; (c) the normalized absorption characteristic of the SWCNT-SA; (d) linear absorption characteristic of the SWCNT-SA; (e)–(g) the output spectrum and corresponding autocorrelation intensity trace of λ1, λ2, and λ3, respectively. Selected from Ref. [48].

Fig. 18. Switchable multi-wavelength mode-locked TDFL: (a) the experimental setup; the spectrum of the switchable tri-wavelength of (b) pair-by-pair and (c) one-by-one. Selected from Ref. [124]. (d) The schematic of the YDFL based on a graphene-oxide (GO)-SA, and spectral characteristics of tunable multi-wavelength DS; (e) the tunable single-wavelength spectra; (f) the wavelength-tunable dual-wavelength DSs; (g) the spectrum of spacing-tunable dual-wavelength DSs; (h) the switchable spectrum dynamics of tri-wavelength DSs by adjusting the orientation of the PC. Selected from Ref. [132].

Fig. 19. Laser characteristics of a bright–dark soliton pair based on NALM structures: (a) oscilloscope pulse traces and (b) the corresponding optical spectrum. Selected from Ref. [257]. The laser characteristics of the bright–dark pulse based on the ReS2–SA: (c) the pulse trace of a bright pulse (up) and dark pulse (down) and (d) corresponding optical spectrum, respectively. Selected from Ref. [72].

Fig. 20. Schematic of cross-absorption modulation in graphene.

Fig. 21. Passively synchronized two-color fiber laser with the aid of SWCNTs: (a) the experimental setup of the fiber laser; (b) linear transmission of SWCNTs; (c) the intensity autocorrelations of the Er laser; (d) the intensity autocorrelations of the Yb laser; (e) the corresponding spectrum of the Er laser; (f) the corresponding spectrum of the Yb laser. Selected from Ref. [257].

Fig. 22. Passively synchronized two-color fiber laser based on the XPM effect: (a) the schematic diagram of the fiber laser; (b), (c) intensity autocorrelation trace (inset: corresponding spectrum) of the Er laser and Yb laser. Selected from Ref. [258].

Fig. 23. Dual-wavelength dual-loop cavity passively synchronized mode-locked fiber laser: (a) the schematic diagram of the experimental setup; the relation between repetition rates of Er- and Tm-doped cavities and Er-cavity length offset (b) with a common GSA in the public area and (c) with two independent GSAXPM in the different loops; (d) the central wavelengths versus the offset of Er-cavity length based on a common GSA; (e) the RF spectrum. Selected from Ref. [197].

Fig. 24. Synchronized dual-cavity two-color Q-switched EYDF laser: (a) the schematic of the experimental setup; (b) the energy level diagram of the EYDF; Q-switched traces under different pumps of (c) 1 μm and (d) 1.5 μm; optical spectra of (e) 1 μm and (f) 1.5 μm; the corresponding RF spectra of (g) 1 μm and (h) 1.5 μm. Selected from Ref. [260].

Fig. 25. Dual-wavelength Ho3+-doped fluoride fiber laser: (a) the experimental setup; (b) the energy level of the cascade transition process; (c) the illustration of laser upper-level populations of I65 and I57, respectively, and the temporal domain evolution of pulse intensity; the characteristics of optical and corresponding RF spectra (inserted) at the different pump powers of (d), (e) at 3.76 W and (f), (g) at 6.47 W, respectively. Selected from Ref. [261].

Fig. 26. Ultrashort-cavity fiber laser. Selected from Ref. [280].

Fig. 27. Schematic diagram of the intelligent MWMLFL.

Table 1. Summary of the Multi-Wavelength Pulsed Lasers Based on a Real SA

Table 2. Summary of the MWFL Based on the Filter Effect in the Cavity

Table 3. Summary of the Multi-Wavelength Mode-Locked Lasers Based on NPR or NALM

Table 4. Summary of the Dual-Cavity Two-Color Mode-Locked Lasers