Fig. 1. Operating schematic of semiconductor lasers and quantum cascade lasers^[32]

Fig. 2. Schematic diagrams of energy level transitions of Tm³⁺ and Ho^3+[44]

Fig. 3. Schematic of typical second-order nonlinear frequency conversion processes^[75]. (a) OPG; (b) OPO; (c) DFG; (d) OPA

Fig. 4. Differential frequency generation process through nonlinear crystals^[82]

Fig. 5. Schematic of MIR pulse generated by DFG technology^[92]

Fig. 6. Schematic of inter-pulse DFG^[80]

Fig. 7. Structural diagram of MIR fs laser^[93]

Fig. 8. Experimental setup of coincidence-up-conversion MIR time-stretch spectroscopy^[94]

Fig. 9. MIR laser generation involves the implementation of IP-DFG technology^[104]. (a) Principle of IP-DFG process; (b) schematic of an IP-DFG experimental setup

Fig. 10. Spectral coverage of different nonlinear crystals in IP-DFG

Fig. 11. MIR laser generation based on a 1.5 μm Er∶fiber mode-locked laser^[124]

Fig. 12. Experimental setup of MIR pulse generation and offset frequency (f₀) detection^[110]

Fig. 13. Experimental setup of MIR electro-optical comb^[51]

Fig. 14. Frequency comb generation for six octaves ^[111]

Fig. 15. IP-DFG MIR frequency comb generation with 1 GHz bandwidth^[112]

Fig. 16. Schematic of MIR optical frequency comb^[126]

Fig. 17. Schematic of MIR optical frequency comb generation^[127]

Fig. 18. Schematic of the experimental setup for IP-DFG^[20]

Fig. 19. Schematic of optical fiber CPA using three parallel spectral broadening techniques to generate infrared spectra^[130]

Fig. 20. Experimental layout for IP-DFG optical frequency comb generation^[105]

Fig. 21. Schematic of 1-µm-pumped IP-DFG system based on Yb-doped fiber laser^[134]

Fig. 22. Schematic of experimental setup of IP-DFG optimization system^[136]

Fig. 23. Schematic of experimental setup for generating high-energy MIR pulse^[104]

Fig. 24. Waveform synthesis via cascaded nonlinear processes^[115]

Fig. 25. Experimental setup of single-cycle Cr∶ZnS laser source^[115]

Fig. 26. Fourier transform spectroscopy based on optical frequency comb. (a) Fourier transform spectroscopy based on Michelson frequency comb^[145]; (b) dual-comb spectroscopy^[146]