A Zeroth-Order Modification of Coupled Mode Theory for Waveguide Gratings

This letter demonstrates the importance of selecting the correct unperturbed structure for coupled mode theory calculations of waveguide gratings. Neglecting the spatial dependence of the average value of the perturbation can cause considerable inaccuracies in the analysis of many common structures. Here, we show the improvement that can be achieved by including this zeroth-order modification and how to choose the proper unperturbed structure through several practical device examples.