helo. I came across this mzi add drop function/device and need some help to to fully understand it. I attached a picture which shows the thing. ---- Ok so, the first 50/50 coupler splits the intensities to the 2 arms with the bragg gratings, bragg gratings are adjusted(made) so that they reflect back lambda1, now as the 2 arms reflect back the lambda1s , these 2 intensities are now at the same 50/50 coupler getting ready to get splitted again. As the picture shows lambda1 is "only" going to "drop port 1". I can guess that the 2 lambda1s are phase shifted and then cancel each other out going to input port 1 and constructively added to drop port 1. But Im not sure. I dont see why they get different phase shifts. Does the coupling give them different phase shifts ? After the bragg gratings, the upper arm has a pi-phase shift function which shifts the upper lambdas, lambda2/3, and a new lambda1' is added from "output port 2". Lambda2/3 get constructively added to "output port1" with the added lambda1'. thx in advance.