Optical Add Drop Multiplexer
As shown in the figure-1, optical multiplexer combines multiple wavelength signals into a single optical fiber. The same device acts as a de-multiplexer in the reverse direction and separates the signals of different wavelengths.
As shown in the figure-2, optical ADM drops and adds a selective number of Wavelengths from a WDM signal, while permitting remaining wavelengths to pass through itself.
Optical ADM architectures are mainly of three types viz. parallel, serial and band drop. . Typically optical ADMs are deployed in linear or ring topologies.
Figure-2 depicts basic function of Add-Drop Multiplexer where in 16 channels are fed as input to PORT-1 and 8 channels are dropped at PORT-4 and rest of the channels pass through PORT-2. At the same time, 4 channels are fed into PORT-3 and are added with signal stream coming out from PORT-2.
Optical Add Drop Multiplexer (OADM) types
There are two main types of Optical ADMs viz. fixed and reconfigurable OADM used in WDM optical networks. Fixed OADMs are used to add or to drop signals on dedicated WDM channels. Reconfigurable OADMs possess capability to alter the selected channel route through electronic control along the fiber optic network.
The reconfigurable second type of OADM helps in re-routing the optical streams in the case of faults and hence minimizes service disruption. It can also be used in upgrading optical network based on new and different WDM technologies.