CommScope Definitions: What is Wavelength Division Multiplexing (WDM)?

This blog post is part of a series called “CommScope Definitions” in which we will explain common terms in communications network infrastructure.

Wave Division Multiplexing (WDM) is a method of combining or separating multiple wavelengths of light in or out of a single strand of fiber with each wavelength of light carrying a different signal. The use of optical filters allows a certain range of wavelengths and let another range of wavelengths pass through. CommScope uses thin-film filter technology (TFF) to obtain this optical effect. Thin layers are stacked together. Consecutive reflections on the interfaces between these layers create interference effects that let light pass through for certain wavelengths and reflect others.

See Also: How Multiplexing Techniques Enable Higher Speeds on Fiber Optic Cabling

WDM is a cost effective way to increase the capacity of a network. There are two different types:

• Coarse Wave Division Multiplexing (CWDM) is standardized to have 18 different wavelength channels with a spacing of 20 nanometers (nm) starting at 1270 nm and ending at 1610 nm. Most systems use the eight channels in the upper band (eight channels from 1470 nm to 1610 nm). The advantage of CWDM systems is that it is always possible to upgrade at a later point in time to limit the installation cost on day one. The wider channel spacing places less stringent requirements on the lasers, which allows use of less expensive lasers without temperature controllers.
• Dense Wave Division Multiplexing (DWDM) devices are mostly used in the core networks to extend over very long distances and where more wavelengths are required between sites. The 40 wavelength channels are distributed in the C-band from 1530 nm to 1570 nm. If required, DWDM can be “over-layed” on a CWDM infrastructure to increase capacity.

Typically CWDM and DWDM have lower insertion losses compared to optical splitters. This significantly increases the reach of the network from the central office. Moreover, every customer has an assigned wavelength(s), providing greater security and making eavesdropping difficult if not impossible.

There are a few different ways to utilize WDMs in your network:

Mux/demux vs. add/drop

A mux (commonly known as a multiplexer) combines multiple wavelength channels on a single fiber, and a demux separates them again at the other end. A mux/demux set-up is especially useful to increase the end-to-end capacity of a deployed fiber. The mux is typically located in the central office, and the demux unit located in either a cabinet or splice closure from which point the fibers go to their destination in a star-shaped topology.

Instead of separating the wavelengths at one end, it is also possible to add or drop individual wavelengths at different points across the line while the other wavelengths remain untouched. This is often the preferred choice when the topology is such that the sites are grouped in a ring structure or when the distance between them is long.

One fiber vs. two fibers

The upstream and downstream signals can be sent through the same fiber at different wavelengths or on two separate fibers. Most CWDM systems are built as two-fiber systems where one fiber is used for the upstream and the other fiber is used for the downstream. In that case, each customer has two separate fibers and one wavelength. If only one fiber is used, every customer will have two separate wavelengths.

Passive optical components can have a significant impact on the efficiency of communication network roll-outs. The incorporation of passive WDM devices reduces the amount of fibers in the network, decreasing both the footprint and investment cost of network roll-outs. In existing networks, these components allow capacity upgrades at a relatively low cost without additional construction works.