Coupled-mode formulation for double-strip NRD waveguides based on singular perturbation technique

Analyzes a double-strip NRD waveguide by using the coupled-mode formulation based on singular perturbation technique. The results of the present formulation are in good agreement with the exact one, and the error rates decrease according to the successive order of perturbation. Also, we compared the results with those of conventional and improved theory and it is shown that the present formulation with second-order corrections is superior to both CCMT and ICMT. The analytical steps of the present formulation are straightforward, and therefore the coupled-equations are systematically derived in respective order of perturbation