FOA Guide

Wavelength Division Multiplexing (WDM)

Wavelength division multiplexing is a technique that sends signals down optical fibers at different wavelengths, using the physical property of light that different wavelengths do not mix when transmitted together.

Why Is WDM Used?

With the exponential growth in communications, caused mainly by the wide acceptance of the Internet, FTTH, wireles, etc., expanding bandwidth is a major concern. Although most cable plants included many spare fibers when installed, bandwidth growth has used many of them and new capacity is needed. Three methods exist for expanding capacity: 1) installing more cables, 2) increasing system bitrate to multiplex more signals or 3) wavelength division multiplexing.

Installing more cables is the most expensive method in most cases, even though fiber has become much less inexpensive and installation methods more efficient (like microtrenching and mass fusion splicing.) But if conduit space is not available or major construction is necessary, this may not be the most cost effective.

Increasing system bitrate may not prove cost effective either. Many systems are already running at 100G, mostly coherent, with 200G, 400G or higher becoming available, but the cost is high.

The third alternative, wavelength division multiplexing (WDM), has proven more cost effective in many instances. It allows using current systems and current fibers, but simply shares fibers by transmitting different channels at different wavelengths of light. Systems that already use fiber optic amplifiers as repeaters also do not require upgrading for most WDM systems.

How Does WDM Work?
It is easy to understand WDM. Consider the fact that you can see many different colors of light - reg, green, yellow, blue, etc. all at once. The colors are transmitted through the air together and may mix, but they can be easily separated using a simple device like a prism, just like we separate the "white" light from the sun into a spectrum of colors with the prism.


Separating a beam of light into its colors

This technique was first demonstrated with optical fiber in the early 80s when telco fiber optic links still used multimode fiber. Light at 850 nm and 1300 nm was injected into the fiber at one end using a simple fused-fiber coupler. At the far end of the fiber, another coupler split the light into two fibers, one sent to a silicon detector more sensitive to 850 nm and one to an InGaAs detector more sensitive to 1300 nm. Filters removed the unwanted wavelengths, so each detector then was able to receive only the signal intended for it. This sytem could work with two wavelengths in one direction on a fiber in a system with two fibers for full duplex operation or in opposite directions on a single fiber for a duplex link.

WDM with couplers and filters

By the late 80s, all telecom links were singlemode fiber, and coupler manufactures learned how to make fused couplers that could separate 1300nm and 1550 nm signals adequately to allow WDM with simple, inexpensive components. However, these had limited usefulness, as fiber was designed differently for 1300nm and 1550 nm, due to the dispersion characteristics of glass. Fiber optimized at 1300 nm was used for local loop links, while long haul and submarine cables used dispersion-shifted fiber optimized for performance at 1550 nm. This simple version of WDM is widely used in fiber to the home (FTTH) applications. Signals are sent downstream to the subscriber at 1490 nm (and 1550 for analog CATV if used) and upstream at 1310 n. Read more on FTTH.

With the advent of fiber optic amplifiers for repeaters in the late 80s, emphasis shifted to the 1550 nm transmission band. WDM only made sense if the multiplexed wavelengths were in the region of the fiber amplifiers operating range of 1520 to 1560 nm. It was not long before WDM equipment was able to put 4 signals into this band, with wavelengths about 10 nm apart.

The input end of a WDM system is really quite simple. It is a simple coupler that combines all the inputs into one output fiber. These have been available for many years, offering 2, 4, 8, 16, 32 or even 64 inputs.

WDM demultiplexer

It is the demultiplexer that is the difficult component to make.The demultiplexer takes the input fiber and collimates the light into a narrow, parallel beam of light. It shines on a grating (a mirror like device that works like a prism, similar to the data side of a CD) which separates the light into the different wavelengths by sending them off at different angles. Optics capture each wavelength and focuses it into a fiber, creating separate outputs for each separate wavelength of light.

WDM In Fiber Optic Data Links

Here are two examples of how WDM can be used in fiber optics.

First a link that transmits several signals at different wavelengths in the same direction.

WDM Link

Then a link can transmit data in both directions simultaneously at different wavelengths.

datalink bidirectional

FTTH Passive Optical Networks

Fiber to the home passive optical networks use a simple version of WDM to allow bidirectional communications over a single fiber to reduce costs. The signals downstream and upstream are at different wavelengths to prevent interference. PON standards use 1490 and 1550nm downstream and 1310nm upstream from the subscribers.


Current systems offer up to 96 or 128 channels of wavelengths in two versions over the wavelength range of ~1270 to 1600nm - CWDM and DWDM for "coarse" and "dense" wavelength division multiplexing. CWDM lasers are spaced 20nm apart while DWDM lasers are spaced 0.8nm apart. The technical requirement is only that the lasers be of very specific wavelengths and the wavelengths are very stable, since the DWDM demultiplexers must be capable of distinguishing each wavelength without crosstalk.

WDM Wavelengths

WDM wavelengths over the fiber bands - O. E. S. C. L, U.

Fiber Amplifiers As Repeaters

Another technology that facilitates DWDM is the development of fiber optic amplifiers for use as repeaters. They can amplify numerous wavelengths of light simultaneously, as long as all are in the wavelength range of the FO amplifier. They work best in the range of 1520-1560 nm, so most DWDM systems are designed for that range. Now that fiber has been made with less effect from the OH absorption bands at 1400 nm and 1600 nm, the possible range of DWDM has broadened considerably. Technology needs development for wider range fiber amplifiers to take advantage of the new fibers.

(C) 2003-20, The Fiber Optic Association, Inc.

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