The 3-D finite difference time domain (FDTD) cut-back method is used to study losses in nondisordered photonic crystal silicon membrane waveguides. Losses above the light-line have been shown to be in good agreement with other methods. Below the light-line, however, FDTD is predicting a rapid increase in losses. This paper studies the possible causes for this effect, including meshing effects, back reflections, and finite thickness sidewalls. It is found that since below the light-line the group index becomes very high and the loss becomes very low, strong Fabry–Perot oscillations dominate the cut-back results. The paper also discusses the impact of operating near to the cut-off wavelength of the photonic crystal waveguide Bloch mode and the implications this has for loss calculation.
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