What is WDM (Wavelength Division Multiplexing) For Fiber Optic Communication? Fiber Optic Tutorial Series

By: Colin Yao

What is WDM?

WDM is the abbreviation for Wavelength Division Multiplexing. What it does is to split the the light in an optic fiber into a number of discrete wavelengths (colors). Each wavelength (color) is a independent channel running at data rate at 2.5Gbit/s, 10Gbit/s, 40Gbit/s or even 100Gbit/s (still under development). So if the light in the fiber is split into 16 wavelengths (colors or channels), and each wavelength is running at 40Gbit/s data rate, we get a total of 40Gbit/s x 16 = 640Gbit/s rate. This is especially true in long haul and ultra long haul fiber optic communication links.

In addition, fibers carrying 64 and more channels (wavelengths) are already available on the market now. Which means we can run 2,560Gbit/s data rate on a single fiber. How about 48 fibers in a single fiber optic cable? That gives us an amazing 2,560Gbit/s x 48 = 122,880Gbit/s link. Of course, this kind of high speed and high fiber count links are usually only deployed for Internet backbones.

From aforementioned samples, you can see the shocking truth about WDM. It dramatically increases capacity of a fiber optic link while minimizes equipment and fiber optic cable cost.

What is DWDM?

DWDM stands for Dense Wavelength Division Multiplexing. Here "dense" means the wavelength channels are very narrow and close to each other. For 100 GHz dense WDM, the interval between adjacent channels are only 100 GHz, (or 0.8nm). For example, the adjacent channels could be 1530.33nm, 1531.12nm and 1531.90nm.

DWDM are widely used for the 1550nm band so as to leverage the capabilities of EDFA (Erbium Doped Fiber Amplifiers). EDFAs are commonly used for the 1525nm ~ 1565nm (C band) and 1570nm ~ 1610nm (L Band).

Why is DWM so important?

The exploitation of DWDM has fueled an explosion in transmission capacity. The amount of information that can be sent over the fiber cables that span the world has increased so much that there is now a glut of available capacity.

In practice, more can be wrung out of DWD systems by extending the upper or lower bounds of the available transmission window or by spacing wavelengths more closely, typically at 50GHz, or even 25 GHz. In doing this, suppliers can double or triple the number of channels. Each optical channel can currently be routinely used for transmission of light pules at 10Gbit/s, or even higher data rates at 100 GHz spacing. With the help of WDM, a pair of fibers can provide data capacity of several hundred gigabits per second.

WDM technology does not require any upgrade or replacement of the fiber infrastructure that has been put in the ground. Hence, we can upgrade links from one capacity level to the next simply by reconfiguring or upgrading terminal equipment and repeaters.

WDM technologies provide the raw transmission capacity. This has to be structured in some way so that it can carry useful traffic and be routed where it needs to go. This is where the next layer of network protocol comes to play. SDH and SONET (They are equivalent. SONET is used in the United States while SDH is used in the rest of the world). We will touch on SDH and SONET in some other tutorials.

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