Fig. 1. Schematic diagram of optical parameter down conversion. (a) DFG; (b) OPO

Fig. 2. Experimental setup of typical mid-infrared femtosecond OPO

Fig. 3. Experimental setup and results of the few-cycle mid-infrared optical parametric oscillator^[25]

Fig. 4. Experimental setup and results of high-power mid-infrared femtosecond OPO pumped by a Yb thin-disk femtosecond laser^[26]

Fig. 5. Yb femtosecond laser synchronously pumped KTA-OPO experimental setup and the corresponding wavelength-power curves of signal and idler

Fig. 6. Mid-infrared femtosecond laser with a pulse width as short as three optical cycles produced by a cascaded degenerate OPO^[49]

Fig. 7. Summary of the tunable mid-infrared femtosecond OPO (dashed line represents its tuning range) and broadband OPO (solid line represents its output spectral coverage)

Fig. 8. Typical absorption spectra for ZGP, CSP, GaAs, and GaP^[53]