ROADM and wavelength-selective switch (WSS)
A reconfigurable optical add-drop multiplexer routes individual WDM channels through a network node, adding, dropping or passing each one under software control. Its core component is the wavelength-selective switch, which sends each wavelength to any of several output ports.
In a WDM network each fiber carries dozens of wavelength channels, and at every node some channels end, some begin, and most pass straight through toward other destinations. A reconfigurable optical add-drop multiplexer (ROADM) does this routing in the optical domain, without converting the through traffic to electronics, and lets an operator change which channels go where from software rather than by rewiring. Every modern long-haul and metro network is built from ROADM nodes connected by amplified fiber spans.
The component that makes a ROADM reconfigurable is the wavelength-selective switch. A WSS takes the WDM signal from one input fiber, disperses it with a diffraction grating so that each wavelength lands at a different position on a switching array, and steers each wavelength independently toward one of N output fibers, typically 1×9 up to 1×32. The switching array is either a set of MEMS micromirrors, one per channel, or a liquid-crystal-on-silicon (LCoS) spatial light modulator, which acts as a programmable phase grating. LCoS divides the spectrum into fine pixels and so supports a flexible grid, in which a channel's width can be set in 12.5 GHz steps under ITU-T G.694.1 to fit its symbol rate, where fixed-grid systems use slots of 50 or 100 GHz. An extended C-band from 191.35 to 196.10 THz, 4.75 THz wide, holds 95 slots of 50 GHz, and fewer, wider slots for high-rate coherent channels such as 400ZR and beyond.
A node is built by connecting a WSS on each incoming fiber direction (degree) to a WSS on each outgoing one, with add and drop ports leading to the local transponders. Early nodes fixed each add or drop port to one wavelength and one direction. Colorless, directionless and contentionless (CDC) designs remove those restrictions, so any transponder can use any wavelength toward any direction without blocking another, which is what makes wavelength routing fully software-defined and relies on tunable lasers in the transponders.
Optical transparency carries costs that accumulate along a path. Each WSS passband is slightly rounded, and a channel crossing many nodes sees the product of all of them, so its usable bandwidth narrows with the number of nodes passed; this filtering penalty is one reason channels are given some guard band. Each node also adds insertion loss, which the amplifiers must make up, and a little polarization-dependent loss and crosstalk. Planning tools track these impairments together with fiber noise to decide how far a channel can travel before it must be regenerated electronically.
References: T. A. Strasser, J. L. Wagener, IEEE J. Sel. Top. Quantum Electron. 16, 1150 (2010); ITU-T Recommendation G.694.1, Spectral grids for WDM applications: DWDM frequency grid.