Fig. 1. Optical frequency comb. (a) Temporal and (b) spectral features of optical frequency comb; (c) optical frequency comb stabilized by f-2f self-reference technique; (d) optical frequency comb stabilized by two ultrastable continuous-wave (CW) lasers

Fig. 2. Dual combs features. (a) Optical frequency domain; (b) radio frequency domain; (c) time domain; (d) temporal interferogram

Fig. 3. Generation of dual combs. (a) Phase locking of two independent mode-locked fiber lasers; (b) electro-optic modulation on a CW laser with different radio frequencies; (c) generation of two stable Kerr frequency combs formed in the microring resonator; (d) generation of two mode-locked pulses from a single laser cavity with different repetition rates

Fig. 4. Bidirectionally multiplexed mode-locked fiber lasers. (a) Bidirectionally erbium-doped mode-locked fiber laser (ED-MLFL)^[36]; (b) repetition rate difference-tunable bidirectionally ED-MLFL^[38]; (c) bidirectionally ytterbium-doped MLFL^[39]; (d) all-polarization-maintaining bidirectionally ED-MLFL^[40]

Fig. 5. Dual-wavelength MLFLs. (a) Dual-wavelength ED-MLFL based on Lyot filter^[19]; (b) dual-wavelength thulium-doped MLFL based on Sagnac loop filter^[50]; (c) all-polarization-maintaining dual-wavelength ED-MLFL^[51]; (d) dual-wavelength ytterbium-doped MLFL based on mechanical filter^[52]

Fig. 6. Polarization-multiplexed MLFLs. (a) Polarization-multiplexed thulium-doped MLFL based on weak fiber birefringence^[63]; (b) polarization-multiplexed ED-MLFL by inserting a polarization-maintaining fiber in the laser cavity^[20]; (c) all-polarization-maintaining polarization-multiplexed ED-MLFL^[65]; (d) all-polarization-maintaining and all-fiber polarization-multiplexed ED-MLFL^[67]

Fig. 7. Buildup dynamics of asynchronous vector solitons in a polarization-multiplexed mode-locked fiber laser^[88]. (a) Schematic of the laser and the single-shot spectral measurement; (b) buildup dynamics of horizontally and vertically polarized vector solitons

Fig. 8. Collision dynamics of asynchronous vector solitons in a polarization-multiplexed mode-locked fiber laser^[91]. (a) Moderate collision; (b) extreme collision

Fig. 9. Application of dual combs from a laser cavity. (a) Measurement of static strain sensing of FBG by didirectionally multiplexed dual combs from a laser cavity^[55]; (b) measurement of wearable and skin-mountable strain sensing by polarization-multiplexed dual combs from a laser cavity^[100]