OptiGrating will let you select only one mode for input. You can select as many modes as you like for the simulation. If the grating causes coupling from the input mode to any of those selected modes, the power lost from the input mode to the selected mode will be taken from the transmission spectrum of the input mode. The interaction will be apparent from observing the transmission spectrum of the input mode. The graphics do not permit display of all selected modes. If only two modes are selected, all 4 curves will be shown (forward and back waves of each of the two modes). If more than two modes are selected, only forward and back waves of the input mode are shown.