Fig. 1. Time-frequency representation of a frequency comb pulse train shows the structure of a discrete frequency comb^[2]

Fig. 2. Mode-locked laser-based mid-infrared optical frequency comb: (a) Particle number inversion and laser emission are realized by using strong light pumped mid-infrared laser gain medium, and mode-locked pulse output is realized by saturable absorber; (b) Typical setup of ring fiber mode-locked lasers in mid-infrared band; (c)-(e) Typical optical spectrum, pulse autocorrelation traces and repetition rate signal of the mid infrared mode-locked laser^{[3, 28]}

Fig. 3. Difference-frequency based mid-infrared optical frequency comb: (a) Longitudinal mode mixing frequency of pump light and signal comb produces different difference frequencies in the middle wave with second order nonlinear χ⁽²⁾, thus forming a mid-infrared idle frequency comb. The pump can also be an optical frequency comb; (b) A typical system for a difference generation based mid-infrared optical frequency comb; (c) A mid-infrared optical frequency comb nonlinear process that reverses the difference frequency process and the comb excitation process, and (d) the corresponding device structure^{[3, 7, 36]}

Fig. 4. Optical parametric oscillation based mid-infrared optical frequency comb: (a) Under the action of strong pump light, the signal light in the optical resonant cavity with χ⁽²⁾ nonlinear medium obtains gain, and when the gain exceeds the loss, the signal light will produce coherent oscillation. Due to the conservation of energy, the mid-infrared idle frequency comb is generated at the same time; (b) Typical system of mid-infrared frequency comb generated by optical parametric oscillation; (c) Mid-infrared optical frequency comb excitation system of OPO based on continuous seed light scheme; (d) Signal and idle frequency comb spectrum based on CW seeded OPO (top) and the schematic diagram of corresponding laser mode (bottom)^{[3, 11, 45]}

Fig. 5. Supercontinuous generation based mid-infrared optical frequency comb:(a) Schematic diagram of soliton induced dispersion wave (DW) generation; (b) Typical systems for supercontinuous generation of mid-infrared optical frequency combs; (c) Typical supercontinuous mid-infrared optical frequency-comb spectrum; (d) Supercontinuous spectral evolution of pump combs in nonlinear media^{[12, 54]}

Fig. 6. Quantum cascade laser based MIR-OFC: (a) Working principle of QCLs; (b) QCLs generates MIR-OFC by injection locking the resonant modes dispersed in the FP cavity; (c) Mid-infrared coherent and stable optical frequency comb device produced by electrical injection locking; (d) Intensity spectrum (blue line) after coherent injection locking, SWIFTS spectrum (red line), fully coherent expected SWIFTS amplitude (blue dot) and phase difference between adjacent comb teeth (green line)^[58-59]

Fig. 7. Microcavity Kerr effect based mid-infrared optical frequency comb: (a) Schematic diagram of soliton induced dispersion wave (DW) generation; (b) Typical system of microcavity generating mid-infrared frequency comb spectrum; (c) Transmission and effective pump-cavity detuning when scanning pump laser over a resonance cavity; (d) Optical spectra and intracavity temporal behavior at different positions (i–vi) in the scanning^{[3, 16, 75]}

Fig. 8. Typical application based on mid-infrared optical frequency combs: (a) Schematic of dual-comb absorption spectroscopy test. A continuous-wave optical parametric oscillator pumps two separate silicon microresonators, which generates two mode locked comb structure; (b) Characterization of dual-comb source. Spectra for each mode locked comb (red, black) combined Michelson-FT spectrum (blue); (c) Absorption spectra; (d) Experimental setup of the GHz-mid-IR DCS system. Two counter-propagating (CP) solitons at 1.55 μm are generated in a silica microcavity to provide two comb signals. These solitons are photo-detected and the resulting signals are processed by electro-optic modulation at 1.06 μm. These near-IR combs are combined in pairs to pump PPLN crystals for generation of GHz line spacing mid-IR frequency combs by interleaved difference frequency generation; (e) Optical spectra of 1.55 μm soliton comb (top) and 1.06 μm EO-comb(bottom); (f) Absorbance spectrum of the methane P(3) branch in the ν3 band together with the ethane rovibrational transitions in the ν7 band. Since ethane has a narrower absorption linewidth, iDFG with N = 16 was also used to further improve the spectral resolution^{[9, 17]}

Fig. 8. [in Chinese]