Near- and Mid-IR few-cycle self-defocusing soliton compression in PPLN waveguide

Cascading quadratic nonlinearity is well known as an effective nonlinear phase change (n_2,casc, Kerr-like) during the ultrafast and phase-mismatched second harmonic generation (SHG) process. The sign and intensity of such an equivalent Kerr nonlinearity (electronic cubic nonlinearity) is closely related to the phase mismatch (Ak) between the fundamental wave (FW) and the second harmonic (SH). Therefore, self-defocusing n_2,casc is achievable by using a positive phase mismatch and can be large enough to counterbalance the native material Kerr nonlinearity (n_2,Kerr, usually self-focusing). Then, having an overall self-defocusing nonlinearity, soliton compression may occur with normal dispersion.