ROADM

A reconfigurable optical add-drop multiplexer (ROADM) is a device used in wavelength-division multiplexed (WDM) optical networks to dynamically add, drop, or pass through selected wavelengths on a fiber link. ROADMs enable flexible bandwidth provisioning without converting optical signals to electrical form, allowing operators to reconfigure lightpaths remotely.

Most ROADMs use wavelength selective switching (WSS) to route individual wavelengths to designated output ports. Incoming wavelengths may be dropped to a customer or network node, inserted from a transponder or local source, or simply forwarded through to another segment. The add/drop ports, in combination with the WSS, determine how each wavelength is handled along a link. Modern ROADMs support channels on ITU grid frequencies (e.g., 50 GHz) and, in flexible-grid systems, variable channel spacing for finer spectrum use.

Architectures vary, including fixed-grid and flexible-grid (flex-grid) ROADMs. Flexible-grid ROADMs support variable channel widths and nonstandard spacing, improving spectral efficiency. Features such as colorless, directionless, and contentionless (CDC) design aim to improve operational flexibility: colorless means any wavelength can be added or dropped at any port; directionless allows adding/dropping in either direction on a link; contentionless reduces wavelength contention. Many ROADMs also incorporate monitoring, protection switching, and automation interfaces.

Control and management typically use a control plane based on GMPLS or software-defined networking (SDN) to provision lightpaths and maintain network state. ROADMs are central to dynamic optical networks, enabling rapid provisioning, traffic engineering, and scalable capacity in metropolitan and long-haul deployments. Limitations include cost, power consumption, and the complexity of coordinating multiple ROADM nodes across a network.