Fig. 1. (a) Instantaneous angular frequencies and the bandwidth of each interacting wave (, , ) of the oppositely dual-chirped DFG scheme; (b) sketch of the ideal broadband PM condition.

Fig. 2. The wavelength-dependent in various commonly used nonlinear crystals under different PM conditions. A well-developed Ti:sapphire ultrafast pulsed laser is employed as the pump laser. The special cases where the generated idler wave is absorbable by the employed nonlinear crystal are also included (dashed lines), in consideration of its other potential applications^{[27, 28]}. The shadowed area indicates the desired values of , which may realize the optimum idler broadening.

Fig. 3. (a), (b) The calculated conversion efficiency and idler spectral bandwidth versus the ratio of in a weak-conversion situation . (c), (d) The gain in the spectrum is introduced to evaluate the performance of such a difference frequency generator as the spectrum broader.

Fig. 4. Schematic of a tunable mid-IR spectrum broader based on the oppositely dual-chirped DFG scheme. BS, beam splitter; DM, dichroic mirror; DL, delay line; and , lenses; C, sapphire plate.

Fig. 5. The calculated conversion efficiency and TL pulse duration of the generated idler wave as a function of the signal wavelength, which is continuously tuned from 980 nm to 1100 nm and from 1100 nm to 1500 nm, respectively. Note that, the TL pulse duration of the initialized 790 nm pump laser is . Solid line, a Type-II PPLN crystal is employed and the is optimized for various signal wavelengths; dashed line, the ideal cases, where the material dispersions are absent; dotted line, the comparison results based on a conventional Type-0 PPLN crystal. (a) ; (b) ; (c) ; (d) .

Table 1. Nonlinear optical parameters for the 5% doped MgO:PPLN and the YCOB crystal at (, , ).