directional coupler


In the example of directional coupler, the length of the coupler depends on the difference in refractive indices. why did the example use the indices of the first two modes of the structure (knowing that the first mode in the list is not propagating)?


The first mode is needed to guide the mode from one guide to the other. Without the evanescent mode, I doubt if the coupling would work. As you said, the difference in refractive index is needed to estimate the coupling length required. This is why we will need to use the first mode. This is explained in the “How does this work” section on this page


Thank you for your reply. I got it.