The original coupled-mode theory for the nonlinear directional coupler (NLDC) and two later improved theories are reviewed and compared with the analysis using the segmentation method proposed in this work. The segmentation method is more accurate in analyzing the NLDC because it takes into account the real situation that the local nonlinear guided modes in the NLDC will vary along the propagation direction due to the accompanying variation in the local power distribution. In the coupled-mode formulations reviewed, however, either linear guided modes or nonlinear modes at a fixed power level are employed when evaluating related coupling quantities.
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