The two-dimensional (2-D) iterative finite difference beam-propagation method (IFD-BPM) is modified to model the cylindrically symmetric three-dimensional (quasi-3-D) second-order nonlinear wavelength conversion in quasi-phase-matched condition. The study shows that the difference between the 2-D and 3-D schemes is small for the guided waves but large for the nonguided beams. The comparison with experimental results shows that the quasi-3-D IFD-BPM is closer to reality than the 2-D scheme. In addition, simulation using the quasi-3-D IFD-BPM reveals that plane-wave and Gaussian-beam assumptions are not sufficient for estimating the nonlinear conversion and beam propagation in second-order nonlinear devices.
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