Fig. 1. Technical routes for generating broadband idler-waves based on DFGs. (a) Using narrowband pump-wave, broadband signal-wave, and crystal with broad signal phase-matching bandwidth; (b) using broadband pump-wave, narrowband signal-wave, and crystal with broad pump acceptance bandwidth

Fig. 2. Group velocity in PPLN crystal versus wavelength

Fig. 3. Phase-matching efficiency curves for 1-mm-long PPLN crystal with poling period of 30 μm. (a) Signal wave；(b) (d) idler wave; (c) pump wave

Fig. 4. Experimental setup of mid-infrared DFG system

Fig. 5. Measured interferometric autocorrelation traces and spectra of incident waves. (a) Interferometric autocorrelation trace of pump pulse; (b) interferometric autocorrelation trace of signal pulse; (c) spectrum of pump wave; (d) spectrum of signal wave

Fig. 6. Experimentally measured spectra of idler waves generated from DFGs under different conditions. (a) Λ＝29.5 μm，L＝1 mm; (b) Λ＝30 μm，L＝1 mm; (c) Λ＝30.5 μm，L＝1 mm; (d) Λ＝29.5 μm，L＝3 mm; (e) Λ＝30 μm，L＝3 mm; (f) Λ＝30.5 μm，L＝3 mm

Fig. 7. Spectra of incident waves and idler wave generated from DFG. (a) Spectrum of incident pump wave; (b) spectrum of incident signal wave; (c) spectrum of idler wave generated from DFG