Calculation results of linear phase mismatching of the degenerate FWM process in this waveguide. (a) Linear phase mismatching Δβ under different pump wavelengths and signal-pump wavelength differences Δ_λ, (b) linear phase mismatching under different signal wavelengths when the pump wavelength is set near the shorter ZDW, (c) linear phase mismatching under different signal wavelengths when the pump wavelength is set near the longer ZDW.