Doing It with Erbium
The term “wavelength division multiplexing” reeks of engineering jargon, but the concept 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 two values are inextricably bound by their relationship to the speed of light. (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.
If WDM is both straightforward and an idea that’s been around-why has it only recently become practical? 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. The key piece of technology is something called an “erbium-doped fiber amplifier.” These devices, 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 amplifiers work well across the near-infrared region of the spectrum at which fiber-optic systems operate.