Hello alistu!
yes i agree , no doubt it has three ports ( one input and two output ports) but if you try to cascade two fiber bragg gratings for add function , you simply cannot..
It would be easy for you to understand if you do it on optiwave.. Let me put it this way, say you do cascade two fiber bragg gratings, at the first output (transmittance side) of first fiber bragg grating you will be having a signal with three signals with high power (transmitted signals) and on the second output of the first fiber bragg grating you will have the reflected signal (dropped signal) with high power and other three with low power..Now, at the input of the 2nd fiber bragg grating which is the cascaded , we will have the signal from the first output of the first fiber bragg grating.From this signal at the input , we can only drop a wavelength. There is no way that we can add another wavelength.
Hope i answered your query.