A reconfigurable optical add-drop multiplexer (ROADM) is a device that can add, block, pass or redirect modulated infrared (IR) and visible light beams of various wavelengths in a fiber optic network. ROADMs are used in systems that employ wavelength division multiplexing.
Before the development of optical multiplexing devices such as ROADMs, signal routing in fiber optic networks was done by converting the IR or visible beams to electrical signals and routing those signals using conventional electronic switches. The rerouted electrical signals were then converted back into IR or visible beams.
In a conventional ROADM, switching is accomplished without optical-to-electrical or electrical-to-optical conversion using three operations called add, drop and cut-through. An outgoing IR or visible beam can be generated (the add operation) or an incoming beam terminated (the drop operation). A beam can also be passed through the device without modification (the cut-through operation). In combination, these functions allow optical signal routing of considerable complexity. The configuration of the system can be changed remotely.
Two major ROADM technologies are in current use. They are called wavelength blocking (WB) and planar light-wave circuit (PLC). Wavelength blocking, also called first-generation ROADM technology, is the older of the two. When a wavelength change is necessary for a particular channel, the IR or visible light beam at the original wavelength is filtered out and its data extracted. Then the data is impressed onto a beam of another wavelength. PLC or second-generation ROADM technology in effect combines these steps, streamlining the process and reducing the cost.
Neither the WB nor the PLC ROADM designs facilitate true optical branching, in which beams of any wavelength can be directly routed to any desired port without the need to perform multiple intermediate operations. Optical branching capability is important in the deployment of efficient, reliable, high-volume optical networks designed to provide advanced services such as video on demand (VoD). An evolving technology called enhanced ROADM (eROADM) makes true optical branching possible.
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