Fiber Optics

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Optical fiber Summary

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Fiber Optics

Fiber optics is the set of technologies that enables the point-to-point transmission of signals in the form of light—instead of in the form of electricity. The main component is optical fiber, the thread of glass-like material that carries the optical signal. Two related components are: (1) the light emitting diode (LED) and its advanced cousin, the semiconductor diode laser, which convert electrical signals to optical signals and couple them into the fiber; and (2) the photodiode, which receives optical signals from the fiber and converts them back to electrical signals.

Although fiber optics has many applications, including its use in sensors, its greatest impact has been in telecommunications. For millennia, humans used optical technology to send signals over distance—for example, as smoke puffs, reflected sunlight, or flares. Remember American Revolutionary War hero Paul Revere's ride and the warning signal: "one if by land and two if by sea?" But, these techniques are limited to what can be seen by human beings within a finite line of sight. Wired and wireless electrical technologies allow global, even interplanetary, transmission. But, these technologies have high attenuation, high noise susceptibility, and low bandwidth. While coaxial cable helps alleviate these problems, fiber optics provides a better point-to-point transmission medium than any form of wired electronics.

Description

An optical fiber's diameter is typically only one-eighth of a millimeter (0.005 inches), but one rarely sees a bare fiber. When it is covered by protective plastic, an individual fiber looks like an insulated wire. Or, many fibers are incorporated into a cable that includes an internal plastic spine, for strength and rigidity, and a hard outer jacket that protects the fibers from external damage (including bites from gophers or sharks). Like wires or coaxial cable (co-ax), fibers can be quite long but, unlike wires or co-ax, an 80-kilometer (50-mile) fiber span may not need an intermediate repeater.

Optical fiber is not uniform in its cross-section. A concentric cylindrical region, called the "core," lies inside the fiber. The core has a slightly different chemistry from the fiber's outer layer, called the "cladding." Light, launched into the fiber's core, travels the length of the fiber, staying inside the core by ricocheting off its walls.

Operation

When an electrical signal moves along a wire, individual electrons move slowly, shifting from atom to atom. But, optical signals are carried by photons, which are launched into the fiber and carry the signal as they traverse the fiber. Electrical signals move along a wire or co-ax at a bandwidth-dependent rate, typically around 20 percent of the speed of light. Whileoptical signals, and electrical signals in free space, move at the speed of light, light moves slower in glass than in air. So, optical signals traverse fiber at about two-thirds the speed of light, which is still three times as fast as electrical signals move along wires.

In multi-mode fiber, different ricochet angles (called "modes") have different velocities —so, a narrow optical pulse spreads as it moves. In more expensive single-mode fiber, the smaller core diameter (eight microns or 0.0003 inches, instead of 62.5 microns or 0.0025 inches) supports only one mode, which eliminates this modal distortion and allows pulses to be more closely spaced, giving a higher data-rate. Since different wavelengths have slightly different velocities, even single-mode pulses can spread. Using a light source with a narrow range of wavelength reduces this consequence, known as chromatic dispersion, which allows pulses to be even more closely spaced, resulting in an even higher data-rate. Many commercial long-distance optical fibers carry 2.5 gigabits per second (Gbps) today, and new transmitters and receivers support ten Gbps—over the same fiber.

Techniques

Since a digitized voice signal requires 64 kilobits per second (Kbps), a single fiber at 2.5 Gbps carries more than 30,000 voice channels. A process called "time-division multiplexing" interleaves the individual signals. Another technology, called "wavelength division multiplexing" (WDM), has recently become practical. WDM allows several channels, each at 2.5 Gbps, to use the same fiber by using different wavelengths. These wavelengths must be far enough apart to be practically separable at the receiver, but close enough together to reside within a fiber's low-attenuation wavelength windows.

Fiber optics is highly nonlinear. When analog signals (like conventional television channels) are transmitted over fiber, the fiber can not be pushed to its limits. So, state-of-the-art transmission is digital, because digital signals are not as affected by nonlinearities. One such nonlinearity, which causes light to move faster through a lit fiber than through a dark fiber, imposes practical limits on the number of WDM channels on a single fiber. Data rate and WDM are both being intensely researched.

Characteristics

The maximum span of any transmission link is determined by the signal-to-noise ratio (SNR) at the receiver. Increasing a wire's length increases both the received noise power and the signal's attenuation. So, wire's SNR is a strong inverse function of length. The property that keeps an optical signal inside a fiber's core also keeps external interference outside it. Since fiber's received noise power is practically independent of length, fiber's SNR depends on attenuation only, making it a relatively weak function of length. So, fiber spans can be longer than wire spans.

Different wavelengths not only have different velocities, but they also suffer different attenuation. The practical attenuation needed in a short span of optical fiber requires the light source's wavelength to be in the infrared range of 0.7 to 1.6 microns. Fortunately, cheap LEDs operate at 0.8 microns. The very low attenuation needed in a long span occurs over two narrow regions of wavelength: around 1.3 and 1.5 microns, where light sourcesare expensive. The lowest attenuation occurs at 1.5 microns, but chromatic dispersion is minimized at 1.3 microns. Not surprisingly, long-distance optical transmission occurs around these two wavelengths.

Although low attenuation and low noise immunity are important, fiber's most important characteristic is its huge bandwidth. Comparing information transmission to water flow, bandwidth corresponds to pipe diameter. On a scale where a telephone channel (4 kHz) corresponds to 1-centimeter (3/8-inch) copper tubing, a co-ax carrying 70 television channels (350 MHz) corresponds to a 2 meter (6-foot) sewer pipe. Fiber's long-span attenuation requirement allows about 15 THz (terahertz) in each of the 1.3- and 1.5-micron windows. This 30 THz of ultimate capacity corresponds to a pipe with 1.6-kilometer (one-mile) diameter. Researchers have only begun to figure out how to use it all.

Cost

Carrying 100 Mbps (megabits per second) over a short span, where multi-mode fiber and LEDs are used, fiber optics costs only a little more than wired electronics. For high rates over long spans, where single-mode fiber and semiconductor diode lasers must be used, fiber optics is expensive. But, the huge bandwidth makes it cost-effective. While fiber's material (silica) is cheaper than wire's (copper), fiber is more expensive to manufacture— especially single-mode fiber. However, since new installation cost is typically

	Access infrastructure	Backbone network
Broadcast application	I	II
Point-to-point apps	III	IV

much higher than the cost of what is being installed, it is common practice to include dark fiber in any wire installation, even if there are no current plans for it.

There are other cost issues, as well. Fiber optics is more difficult to use than wire, and technicians need to be trained. While wire can be soldered or wrapped around a terminal, optical fiber must be carefully spliced. Fiber connectors, especially for single-mode fiber, are more expensive than wire connectors.

Application

Consider Table 1. Users get access (left column) to information signals by several competing media. People access (I) commercial broadcast television signals by local antenna, co-ax, or direct dish, and (II) point-to-point applications, like telephony or connecting to an Internet service provider, by wire or local wireless (cellular). But, the backbone infrastructures (right column), which distribute these signals over wide areas, use an application-dependent medium-of-choice. Commercial television is effectively (III) broadcast using geo-synchronous satellites, and the wide-area networks for point-to-point applications, like long-distance networks for telephony and the Internet for data, typically use fiber optics.

This may all change, of course, as the technology, the industry, the applications, and the economics evolve. Although technically feasible, fiber-to-the-home and fiber-to-the-desktop are economically difficult to justify. If video-conferencing becomes popular, perhaps it will be the so-called "golden service" that makes it happen.

Future

Because of the nonlinearity that causes light to go faster through a lit fiber than a dark fiber, the photons at the back of a pulse can actually catch up to the photons at the front. A soliton is a pulse whose shape is retained because this effect carefully balances the effects that widen pulses—and researchers are trying to make them practical. With all that unused potential bandwidth, fiber optics is the logical technology for making networks that must scale easily, like the Internet. If research efforts in photonic switching and optical computing are fruitful, there will be wonderful synergies with fiber optic transmission. If researchers learn to master solitons and these other research efforts are fruitful, fiber optics has a "bright" future.

Richard A. Thompson