Hi Dhiman,
Optical add-drop multiplexer is a component that can be used in WDM systems. In WDM systems, different channels have different wavelengths, are multiplexed in transmitter using WDM multiplxer and can all be demultiplexed at receiver. Sometimes it is desired to access or “drop” the information of one certain channel before the end receiver and/or add a new channel there. In these cases OADM can be used. You can find some examples in OptiSystem samples in “Add and Drop” folder.
Regards