WDM and DWDM.
WDM represents the second major fiber-optic revolution in telecommunications. The first came during the 1980s, when telephone companies laced the United States and other countries with fibers to create a global backbone of information pipelines that could carry vastly more data than the copper wires and microwave links they replaced. A decade ago, phone companies had laid cables containing 24 to 36 fibers, many held in reserve as "dark fibers." Each fiber carried hundreds of megabits per second at a single wavelength. Since then, carriers have raised data rates to 2.5 Gbit/s and lit most of the dark fibers.
WDM takes this advantage a giant step further—multiplying the potential capacity of each fiber by filling it with not just one but many wavelengths of light, each capable of carrying a separate signal.
Taking advantage of WDM, long-distance carriers such as AT&T and MCI have been able to avoid laying expensive new cables; instead, they simply pump additional wavelengths through existing fibers.
The concept of "wavelength division multiplexing" is simple:
simultaneously send separate signals through the same fiber at different wavelengths. Essentially the same idea forms the foundation of radio and television broadcasting, where each station sends its signal out on an assigned wavelength in the radio-frequency spectrum. Of course, most people think in terms of frequency instead, but the Frequency x Wavelength = Light speed (for instance, 100 megahertz on the FM dial corresponds to a wavelength of about 3 meters.)
The same principles work for the light going through an optical fiber as for radio waves transmitted through air. Optical fibers transmit best at the invisible, near-infrared-light wavelengths between 1.3 and 1.6 micrometers—roughly double the wavelength of red light.
The biggest obstacle has been the lack of suitable amplifiers. Light signals traveling through even the most transparent optical fibers fade to undetectable levels after a couple hundred kilometers. For most of the time fiber optics have been in place, the only way to span fibers longer than that was to regenerate the signal through an optoelectronic process: A photodetector would convert the stream of weakened light pulses into a voltage signal that could be amplified electronically; this boosted signal modulated a laser transmitter.
The problem is that light detectors don’t discriminate between wavelengths,
they scramble signals at different colors, much the way your ears have trouble discerning what is being said if two people talk at once. For optoelectronic systems to work with multiple wavelengths, they must have a way to separate the wavelengths optically, using filters or other similar elements, enabling each signal to pass through its own regenerator. Until recently, though, that has proved impractical.
This limitation disappeared with the invention of a technique for boosting the signal light’s intensity directly, without the need for an intermediate electronic step. Devices called an "erbium-doped fiber amplifier" developed in the late 1980s, made the WDM revolution possible.
Unlike a regenerator, a fiber amplifier operates directly on light. Light in the feeble input signal stimulates excited erbium atoms in the fiber to emit more light at the same wavelength. Chains of optical amplifiers can combine to carry signals through thousands of kilometers of fiber-optic cable on land or under the ocean—without regenerators. Because they preserve the wavelength of the optical signals, erbium fiber devices can amplify several different wavelength channels simultaneously without scrambling them. Erbium doped fiber amplifiers (EDFA) work well across the near-infrared region of the spectrum at which fiber-optic systems operate.
Long-distance telephone companies were the first to realize that WDM could cut the cost of bandwidth. Compared with the alternative of adding new fiber, WDM technology provides a much more effective way to add capacity. Laying new cable is expensive and time-consuming. And burying new cable along the same route already occupied by an older cable is risky—new excavation invites cable breaks that could put the whole system out of service.
The telecommunications carriers’ desire to save time and money has driven a rapid development in WDM techniques. In the mid-1990s, the carrier companies began using systems transmitting at four wavelengths, and soon upped the count to eight. Developers quickly sliced the spectrum even more finely to squeeze 16 wavelength channels through a single fiber for what has become known as "dense" WDM (DWDM).
When the carriers saw the need, manufacturers were equally quick to sense the market.
The major WDM systems manufacturers include Lucent Technologies, Ciena, Cambrian, Pirelli, Nortel, Alcatel, Fujitsu, NEC, Ericsson, Siemens, and Hitachi. For components, there are many firms, including JDS Fitel, E-Tek Dynamics, GPT Optical Corp. of America, Advanced Optronics, Alliance Fiber Optics, AMP, ATI Electronique, Bosch Telecom, Corning, DiCon Fiberoptics, Gould Electronics, Instruments SA, Mitsubishi, and MP Fiberoptics.
Lucent Technologies of Murray Hill, N.J., adapted technology developed at its Bell Labs subsidiary.
Ciena, a Linthicum, Md., company founded in 1992, charged ahead faster, delivering its first commercial 16-channel system in 1996—at nearly the same time as the AT&T spinoff. Over the past two to three years, several companies—including Ciena, Lucent and Nortel of Saint-Laurent, Que.—have begun to market systems that slice the erbium-amplifier spectrum into 32 or 40 slivers, each only 0.8 nanometer wide. September 1999, Lucent delivered its first 80-channel system to AT&T. Pirelli Cable of Lexington, S.C., followed by promising a 128-channel version.
Telecommunications carriers don’t need all those channels today
and thanks to WDM’s inherent modularity, they don’t need to buy more channels until they’re ready. A carrier installing a WDM system can start with only the transmitters and receivers needed for the few initial channels. Later, as demand for capacity grows, additional equipment can be plugged in to open up new wavelengths.
Taking full advantage of WDM often requires upgrading older cables by adding components that compensate for a troublesome effect called chromatic dispersion. This refers to the tendency of a short light pulse to stretch out as it travels through a fiber owing to the fact that some wavelengths travel faster than others. Dispersion smears light pulses together and therefore limits transmission speed. Avoiding this phenomenon is especially important in submarine cables, where light signals must travel through several thousand kilometers of fiber from shore to shore. New installations can exploit fibers designed for optimum WDM performance, recently developed both by Lucent and by Corning (Corning, N.Y.).
Last year, the first big submarine cable designed for multiwavelength operation
called "Atlantic Crossing 1" began sending 2.5 Gbit/s at four wavelength channels on each of its four fiber pairs. The capacity of this system can be upgraded to 16 wavelengths per fiber at that speed, says Patrick R. Trischitta, director of technical marketing at Tyco Submarine Systems Laboratories in Holmdel, N.J. That promises a total of 160 Gbit/s through the cable, a loop connecting the United States with Britain, the Netherlands and Germany.
Newer WDM technology will carry 10 Gbit/s at each of 16 wavelengths across the ocean in four fiber pairs, a total capacity of 640 Gbit/s per cable. That’s more than 1,000 times the capacity of the first transatlantic fiber-optic cable, which began service just a decade ago. The whole system will ultimately include 168,000 kilometers of cable—enough to circle the globe four times.
On land, regional telephone companies have just begun to adopt wavelength multiplexing. Last year, Bell Atlantic began testing WDM on a 35-kilometer cable between Brunswick and Freehold, N.J.
Four channels each carried signals at speeds to 2.5 Gbit/s—the top rate between company switching offices—and the Ciena-built system has slots for up to 16 wavelength channels. Bell South tested three of 16 channels in a similar system on a cable spanning 80 kilometers between Grenada and Greenwood, Miss. The economics are clear: "It’s cheaper to add WDM capacity than to add new fiber."
Different rules apply to the shorter cables linking switching offices to major business customers. Here, in the so-called "metro" market, "the cost of increasing fiber count is not as big an issue because the runs are so much shorter."
Still, WDM improves signal transmission in other important ways. One is by carrying signals in their original digital formats rather than converting them into the digital coding used within the telephone network. Because such conversion requires costly electronics, it can be cheaper to dedicate a wavelength for end-to-end transmission in the original format.
The ability to sort signals by wavelength should streamline the operation of future fiber-optic networks. Traditionally, phone companies organize digital signals in a hierarchy of bit rates, merging many low-bit-rate tributaries into mighty digital rivers carrying gigabits per second. This packs bits efficiently onto transmission lines, but requires unpacking the whole bit stream to extract individual signals. If the signals are organized by wavelength, however, simple optics can tease out the desired wavelength channel without disturbing the others. Engineers speak of adding a new "optical layer" to the telecommunications system. Customers might lease a wavelength in this optical layer instead of leasing the right to transmit at a specific data rate. A television station, for instance, could reserve one wavelength from its studio to its transmitter and another to the local cable company—and transmit both signals in digital video formats not used on the phone network.
Since the demand for bandwidth shows no sign of slowing down, the developers of WDM systems are already thinking about how to pack more wavelengths into the same fiber. At the moment, there are two basic approaches being investigated—and limits to both are apparent.
One approach is to reduce the "space" between wavelengths, by choosing wavelengths that are closer together to carry the multiplicity of signals. Packing wavelengths closer works well up to a point, but it ultimately clashes with basic physics. As bit rates increase, optical pulses get briefer, and—following the dictates of Heisenberg’s Uncertainty Principle—this shortening forces the light signal to spread over a broader range of wavelengths. This spreading can cause interference between closely spaced channels. Lucent’s highest-capacity system handles 10 Gbit/s on wavelength channels separated by 0.8 nanometer but only 2.5 Gbit/s when channel spacing is halved. And few experts think channels can be squeezed much tighter. Among major vendors, only Hitachi Telecom of Norcross, Ga., talks about modulating individual channels at 40 Gbit/s—and admits that those signals could span only limited distances.
Prospects look better for the second option: expanding the range of transmission wavelengths.
Pirelli, for example, uses three erbium-fiber amplifiers, optimized for separate bands between 1,525 and 1,605 nanometers, to squeeze 128 wavelength channels at 10 Gbit/s each into a single fiber.
Lucent has demonstrated erbium amplifiers covering a similar range in the laboratory, and last year introduced a new optical fiber that opens up a long-neglected block of the spectrum around 1,400 nanometers.
CIENA's vice president of architecture Joe Berthold expains that "there are two windows in the optical fiber transmittion where the losses hit a minimum: one is 1300 nm, other is 1550 nm. That gives you two channel on one fiber. People started exploiting 1550 band and some system put 4 channels in this band, which were widely spaced. When you begin to pack those channels much more closely together, that is dense WDM (DWDM)."
Ciena started their system with 16 channels and then jumped up to 40 channels.
Good optical amplifiers are not yet available for other wavelengths.
For WDM to reach its full potential, though, more will be needed than simply packing in additional wavelengths. It will also be necessary to develop better equipment for switching and manipulating the various wavelengths after the signal emerges from the optical "pipe." To fully emulate what happens in digital cross-connects, you need to reallocate and reassign wavelengths. It’s impossible to allocate the same wavelength to one customer throughout an entire system because the huge network has far more customers than it has wavelengths.
All-optical conversion approaches, while demonstrated in the lab, have yet to reach commercial practicality.
Even if these technical problems are solved, however, that won’t be enough for the technology to really spread its wings. For that, the price will also have to come down. Through the next year or two
WDM will be economical only for backbone networks. Once cost drops to $100,000 a node, the technology will make sense for metropolitan and regional networks, starting with service to large businesses. Residential access in large apartment buildings will follow after costs drop to $10,000 a node in about 2005, with WDM reaching individual homes once costs decline to about $1,000 in 2010. The real information revolution won’t come until cheap WDM pipelines reach individual residences. Today’s modem connections remain bottlenecks, forcing us to sip the torrent of data through the electronic equivalent of a thin plastic straw. But get ready: As fiber reaches the home, your very own wavelength could deliver a bubbling fountain of bits.
WDM can carry multiple data bit rates, allowing multiple channels to be carried on a single fiber. The technique quite literally uses different colors of light down the same fiber to carry different channels of information, which are then separated out at the distant end by a diffraction grating that identifies each color. All optical networks employing WDM with add/drop multiplexers and cross-connects permit this. Dense WDM (DWDM) systems multiplex up to 8, 16 or more wavelengths in the 1550 nanometer (nm) window, increase capacity on existing fiber, and are data rate transparent.
WDM was first developed to increase the distance signals could be transported in long distance networks, from 35-50 km to as much as 970 km or more with optical amplifiers. Subsequently companies such as Ciena and Cambrian discovered that DWDM would work in metropolitan networks just as well. These DWDM ring systems can be connected with Asynchronous Transfer Mode (ATM) switches and Internet Protocol (IP) routers. ATM networks are expected to use SONET/SDH physical layer interfaces with OC-12 add/drop multiplexers. ATM can carry voice, video and data communications in the same transport and switching equipment.
WDM systems require non-zero dispersion fiber that is produced by vendors such as Corning (SMF-LS) and Lucent (TrueWave). This type of fiber introduces a small amount of dispersion that decreases nonlinear component effects.
WDM also may be integrated into OC-12, OC-48 and OC-192 networks, as long as vendors provide standard wavelengths in the 1550 nm window. The mix of OC-48 and OC-192 WDM architecture requires fewer rings and saves on cost. In some cases WDM cannot be placed over the SONET layer, and instead must use transponders, which are costly. In general, though, WDM will be the most cost effective option that provides the necessary bandwidth without installation of more fiber.
"The DWDM market will grow from $980 million in 1998 to $5.2 billion in 2003" (Insight). The first deployments of WDM systems were in the US, but this has now expanded to the UK, Italy, France, Norway, Finland, Japan, China and Korea. Undersea submarine cables are almost entirely relying on DWDM. DWDM is also being deployed in broadband networks using new fiber technologies, optical amplifiers and SONET and SDH transport terminals.