Fig. 1. Schematic of optical parametric effects. (a) OPG and OPA; (b) OPO

Fig. 2. Process of QPM. (a) Intensity of nonlinear frequency conversion light in optical superlattices versus interaction length; (b) schematic of polarization direction of lithium niobate domain after periodic polarization in external electric field

Fig. 3. Main progress and performance parameters of 2-5 µm CW mid-infrared OPOs based on optical superlattice crystals

Fig. 4. 2--5 μm CW mid-infrared OPOs based on optical superlattice crystals. (a) LD-pumped Nd∶ YVO₄ intracavity single resonant three-mirror L-type composite cavity OPO structure and (b) corresponding output power curves of 3.66 μm idle light ^[57]; (c) LD-pumped Nd∶ YVO₄ intracavity single resonator two-mirror linear composite cavity OPO structure and (d) output power and wavelength tuning curves of idle light ^[60]; (e) VECSEL-pumped fan-shaped PPLN intracavity OPO structure and (f) output power curves of idle light ^[64]; (g) structural diagram of narrow-linewidth PPLN-OPO pumped by DFB fiber laser and (h) CO₂ absorption spectral lines measured by it ^[71]

Fig. 5. 2--5 μm narrow-linewidth CW OPO based on multi-period PPLN^[28]. (a) Experimental setup of OPO; (b) experimentally obtained output spectrum of idle light with longest wavelength; (c) wavelength tuning curves of narrow-linewidth CW OPO; (d) output power and photon conversion efficiency of idle light versus wavelength at pump power of 27.5 W; (e) linewidth of signal light measured by delay self-zero difference method; (f) beam quality of 3497.1 nm idle light at output power of 3.3 W

Fig. 6. Main progress and performance parameters of 2-5 µm ns mid-infrared OPOs based on optical superlattice crystals

Fig. 7. 2--5 μm ns mid-infrared OPOs based on optical superlattice crystals. (a) Experimental setup of four-wavelength near-mid-infrared OPO and (b) corresponding wavelength tuning range of double-period OPO^[107]; (c) experimental setup of high power OPO+OPA and (d) corresponding output power curve of OPA^[108]

Fig. 8. High power 2.1 μm degenerate point ns OPO based on 2 mm thick PPLN^[28]. (a) Experimental setup of OPO; (b) output power curves of OPO; (c) output wavelength of OPO versus temperature and pump power; (d) beam quality at highest output power; (e) test result of power stability @5 h at maximum output power

Fig. 9. 2--5 μm wide-tuning ns OPO based on multi-period PPLN^[28]. (a) Experimental setup of OPO; (b) wavelength-tuning curves of OPO; (c) output power of idle light versus wavelength at pump power of 18 W

Fig. 10. 2--5 μm ps mid-infrared OPO based on optical superlattice crystal. (a) Experimental setup of synchronously-pumped OPO and (b) corresponding output power curves^[129]; (c) experimental setup of synchronously-pumped OPO based on standing-wave cavity and (d) corresponding output power curves^[134]

Fig. 11. Main progress and performance parameters of 2-5 µm ps mid-infrared OPOs based on optical superlattice crystals