Abstract: We propose a new way to obtain so-called quasi-phase-matching（QPM） condition in the second harmonic generation（SHG） in the real space.The QPM-SHG can be deduced by solving the coupled wave equation.The mismatched angle as a function of x is discussed.We find that a phase shift of 180° can be obtained from the reversion of ferroelectric domain,this restores the phase relation between the interacting waves,thus the SHG conversion efficiency grows continuously.

Key words: nonlinear optic, second harmonic generation, coupled wave equation, quasi-phase-matching