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WDM (Wavelength Division Multiplexing)

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WDM is an FDM (frequency division multiplexing) technique for fiber-optic cable in which multiple optical signal channels are carried across a single strand of fiber at different wavelengths of light. These channels are also called lambda circuits. Think of each wavelength as a different color of light in the infrared range that can carry data.

A fiber-optic cable guides light from end to end. A signal is injected in one end by an LED (light-emitting diode) or by semiconductor lasers. Lasers for silica-based fiber-optic cables produce light in a range called a "window." These windows occupy the near infrared range at wavelengths of 850 nm (nanometer or billionths of a meter), 1,320 nm, 1,400 nm, 1,550 nm, and 1,620 nm. For example, you may see a system described as a 1,550-nm system. Optical multiplexers divide the window into many individual lambdas. Figure W-1 illustrates the output of a 16-channel WDM system operating in the 1,530- to 1,565-nm range. Each lambda circuit is capable of transmitting 2.5 Gbits/sec for a total of 40 Gbits/sec.

As mentioned, optical systems are discussed in terms of their wavelengths (in nanometers). For comparison, red blood corpuscles are about the same size as the wavelengths in the infrared range. A wavelength of 1,550 nm has a frequency of 194,000 GHz (194,000 billion cycles/sec). The frequency increases as the wavelength is shortened. A decrease of only 1 nm increases the frequency by 133 GHz. This is used to advantage by Avanex in its PowerMux optical multiplexer. The PowerMux can put over 800 channels on a single fiber. It separates channels by 12.5 GHz or 0.1 nm. The Avanex Web site ( provides some interesting information about optical systems.

WDM is employed by carriers such as MCI to boost the data rates of their networks dramatically. MCI incorporated Quad WDM (four-wavelength WDM) in its backbone several years ago, instantly quadrupling its network capacity. The backbone operated at 2.5 Gbits/sec before Quad-WDM and at 10 Gbits/sec after installing Quad-WDM multiplexer devices. Since then, MCI has been upgrading to higher-capacity systems.

There are three categories of wavelength division multiplexing:

  • WDM (wavelength division multiplexing)    Two to four wavelengths per fiber. The original WDM systems were dual-channel 1310/1550 nm systems.

  • CWDM (coarse wavelength division multiplexing)    From four to 8 wavelengths per fiber, sometimes more. Designed for short to medium-haul networks (regional and metropolitan area).

  • DWDM (dense wavelength division multiplexing)    A typical DWDM system supports eight or more wavelengths. Emerging systems support hundreds of wavelengths.

The spacing between wavelengths in CWDM is about 10 to 20 nm, while the spacing in DWDM is about 1 to 2 nm. Due to the tight spacing and number of lasers, DWDM systems require elaborate cooling systems. Also, precision light sources and complex optical multiplexers are required to ensure that channels do not interfere with one another. In contrast, CWDM systems are simple and easy to manufacture, and cost much less than DWDM systems. They are also smaller. A CWDM device can be held in your hand, while a DWDM device is a large box that requires rack mounting.

The development of EDFAs (erbium-doped fiber amplifiers) provided a boost in cable distance and capacity for fiber-optic networks. EDFAs can amplify optical signals directly by injecting light into the cable via a light pump. Weak signals enter the amplifier and stimulate excited erbium atoms in the erbium-doped fiber to emit more light, thus preserving the original signal and boosting its output signal. Best of all, EDFAs can simultaneously boost the signals of multiple wavelengths in the same cable. EDFAs work in the 1,500- to 1,600-nm range, so a typical DWDM system has a range of lambda circuits operating in this range.

Prior to the development of EDFAs, optoelectronic amplifiers were used to boost optical signals. The process is often called "3R" regeneration, referring to reamplify, regenerate, and retime. Weak incoming light is converted to a voltage signal, amplified, and then converted back to light. This is impractical in high-speed core networks.

With the potential of hundreds of lambdas per fiber, it is practical for carriers to lease entire optical circuits to businesses. For example, a television network could lease lambda circuits to transmit video signals among media centers and stations. Recently, MPLS (Multiprotocol Label Switching) has been considered an ideal protocol for controlling optical switches in DWDM networks. It already controls LSPs (label switched paths) across routed networks and can also be used to control optical paths. This topic continues under "Optical Networks."

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.