Fiber Optics

The principles behind fiber optics

Fabrication of optical fibers

Fiber classifications

Other applications

Resources

Optical fiber is a very thin, transparent strand of glass or plastic capable of transmitting light from one point to another. Optical fiber can also be called an optical waveguide, since it is a device that guides light.

Optical fiber is used in telecommunication systems, imaging optics, and illumination sensors. It is used in such commonly seen consumer products such as holiday lights and in commercial products such as signs and art.

Fiber optics was first used by researchers from the University of Michigan when they patented a fiber optic gastroscope, in 1956, for use in medicine. Upon its success, Charles Kao and George Hockham, while working for Standard Telephones and Cables (in England), demonstrated, in 1965, the usefulness of optical fiber in the field of communications. The first commercially usable optical fiber was designed, in 1970, by U.S. researchers with Corning Glass Works. Then, in 1977, engineers and scientists with General Telephone and Electronics, in California, succeeded in sending the first telephone message through optical fiber.

Optical fibers consist of a light-carrying core and a cladding surrounding the core. There are generally three types of construction: glass core/cladding, glass core with plastic cladding, or all-plastic fiber. Optical fibers typically have an additional outside coating, which surrounds and protects the fiber (Figure 1).

Commonly available glass fiber diameters range from 8 micron core/125 micron cladding to 100 micron core/140 microns cladding, whereas plastic fibers range from 240 micron core/250 micron cladding to 980 micron core/1,000 micron cladding. The human hair, by comparison, is roughly 100 microns in diameter.

The principles behind fiber optics

Fiber optics work on the principle of total internal reflection. Light reaching the boundary between two materials is reflected such that it never leaves the first material. In the case of fiber optics, light is reflected from the optical fiber core-cladding interface in such a way that it propagates down the core of the fiber. This can be explained by a brief discussion of Snell’s law of

refraction and law of reflection, and a physical quantity known as index of bottom material. According to Snell’s law, the light will be bent from its original path to a larger angle in the second material. As the incoming, or incident angle increases, so does the refracted angle. For the properly chosen materials, the incident angle can be increased to the point that the ray is refracted at 90 degrees and never escapes the first medium. The equation can be solved to give the incoming, or incident, angle, which will result in a refracted angle of 90 degrees.

q₂ = sin^-1(n₂/n₁)

This is known as the critical angle (Figure 2).

Light hitting the boundary or interface at angles greater than or equal to this value would never pass into the second material, but undergoes total internal reflection.

Now change the model slightly so that the higher index material is sandwiched between two lower index layers (Figure 3).

Light enters the higher index material, hits the upper interface and is reflected downward, then hits the second interface and is reflected back upward, and so on. Like a marble bouncing off the internal walls of a tube as it travels through the tube, light will make its way down the waveguide. This picture essentially corresponds to an optical fiber in cross-section. Light introduced to the fiber at the critical angle will reflect off the interface, and propagate down the fiber.

The second law of thermodynamics cannot be disregarded, however. Light will not travel down the fiber indefinitely. The strength of the signal will be reduced, or attenuated. Some light will be absorbed by impurities in the fiber or scattered out of the core.

Modern fibers are made of very pure material so that these effects are minimized, but they cannot be entirely eliminated. Some light will be diverted by microbends and other imperfections in the glass. Recall the law of reflection: if a microbend is encountered by light traveling through the fiber, the light may hit the interface at an angle smaller than the critical angle. If this happens, the light will be reflected out of the core and not continue propagating (Figure 4).

Fabrication of optical fibers

Optical fibers are fabricated in a multi-step process: preform fabrication, fiber drawing, fiber coating, and spooling. A preform is a giant-sized version of the final fiber, with central core and cladding refractive indices equal to those of desired product. Preform diameters are typically 0.4 to 1.0 in (1.0 to 2.5 cm). They are produced by one of several variations on chemical vapor deposition, in which chemicals (primarily silica, with other exotic compounds) are vaporized and allowed to deposit on a tube or rod. The porous form produced is heated to release trapped gases and water vapor that might otherwise compromise the performance of the final fiber.

In the drawing stage, the end of the preform is lowered into a furnace heated to roughly 3,632°F (2,000°C). The tip softens until it is drawn down by gravity, shrinking in diameter. Sensors constantly monitor the fiber diameter and concentricity to assure optimal results. An acrylic coating is applied to protect the fiber from damage and preserve its strength. Finally, it is wound onto a take-up spool.

Fiber classifications

Optical fiber falls into three basic classifications: step-index multimode, graded-index multimode, and single mode. A mode is essentially a path that light can follow down the fiber. Step-index fiber has a core with one index of refraction, and a cladding with a second index.

A graded-index fiber has a varying core index of refraction, and a constant cladding index (Figure 6).

In general, the beam diameters of light sources for optical fibers are larger than the diameter of the fiber itself. Each fiber has a cone of light that it can propagate, known as the cone of acceptance of the fiber. It is driven by the critical angle of the fiber, which in turn varies according to the refractive index of the material. Light outside the cone of acceptance will not undergo total internal reflection and will not travel down the fiber.

Now, if light in the cone of acceptance is entering the fiber at a variety of angles greater than or equal to the critical angle, then it will travel a number of different paths down the fiber. These paths are called modes, and a fiber that can support multiple paths is classified as multimode. Notice that the light hitting at the smallest possible angle travels a longer path than the light at the largest angle, since the light at the largest angle is closest to a straight line. For step-index multimode fiber in which light travels the same speed everywhere, the rays running the longest path will take longer to get to the destination than the light running the shortest path. Thus a sharp pulse, or packet of light, will be spread out into a broad packet as it travels through the fiber. This is known as modal dispersion and can be a disadvantage in many applications. This type of fiber is used for in-house phone lines and data links (Figure 5).

Graded-index fiber offers one method for minimizing dispersion. The index of refraction of the core of graded index fiber increases toward the center. Remember, the refractive index of a material controls the speed of light traveling through it. Light propagating in the center of the fiber thus goes more slowly than light on the edges. This reduces the pulse spread caused by differing path lengths. While not a perfect transmission, the transmitted pulse is dramatically improved over the step-index multimode fiber output. Graded-index fiber requires very specialized fabrication and is more expensive than step-index multimode. It is commonly used for mid-length communications (Figure 6).

The best way to avoid modal dispersion, however, is to restrict transmission to only one mode. Single-mode fiber is very narrow, with core diameters typically 8 microns, allowing light to propagate in only one mode (see Figure 7). The cone of acceptance is dramatically decreased, however, which makes light injection difficult. Splicing fiber together is more challenging, as well. Single-mode fiber is more costly than step-index multimode but less so than graded-index multimode. Single-mode fiber is used for long distance communication such as transoceanic telephone lines.

Plastic fiber is available in all three types. It is less expensive and lightweight but experiences more signal attenuation. It is practical for very short distance applications such as in automobiles.

Fiber optic communications

Why is the propagation of pulses of light through optical fibers important? Voice, video, and data signals can be encoded into light pulses and sent across an optical fiber. Each time someone makes a telephone call, a stream of pulses passes through an optical fiber, carrying the information to the person on the other end of the phone line.

A fiber optic communication system generally consists of five elements: the encoder or modulator, the transmitter, the fiber, the detector, and the demodulator (Figure 8).

Electrical input is first coded into a signal by the modulator, using signal processing techniques. The transmitter converts this electrical signal to an optical signal and launches it into the fiber. The signal experiences attenuation as it travels through the fiber, but it is amplified periodically by repeaters. At the destination, the detector receives the signal, converting it back to an electrical signal. It is sent to the demodulator, which decodes it to obtain the original signal. Finally, the output is sent to the computer or to the handset of a telephone, where electrical signals cause the speaker to vibrate, sending audio waves to the listening ear.

Advantages of fiber optic cable

Communication via optical fiber has a number of advantages over copper wire. Wires carrying electrical current are prone to crosstalk, or signal mixing

between adjacent wires. In addition, copper wiring can generate sparks, or can overload and grow hot, causing a fire hazard. Because of the electromagnetic properties of current carrying wires, signals being carried by the wire can be decoded undetectably, compromising communications security. Optical fiber carries light, no electricity, and so is not subject to any of these problems.

The biggest single advantage that optical fiber offers over copper wire is that of capacity, or bandwidth. With the rising popularity of the Internet, the demand for bandwidth has grown exponentially. Using a technique called wavelength division multiplexing (WDM), optical networks can carry thousands of times as much data as copper-based networks.

Most copper networks incorporate a technique known as time division multiplexing (TDM), in which the system interleaves multiple conversations, sending bits of each down the line serially. For example, the system transmits a few milliseconds of one conversation, then a few milliseconds of the next, a few milliseconds of the next, then returns to transmit more of the first conversation, and so on. For many years, network designers increased carrying capacity by developing electronics to transmit shorter, more closely spaced data pulses.

Electronics can operate so quickly, however, and eventually copper wire hit a maximum carrying capacity. To increase bandwidth, network operators had to either lay additional copper cable in already packed underground conduits, or seek another method. Thus, fiber optics entered the market.

The electrons in copper wire can only carry one stream of time-division multiplexed data at a time. Optical fiber, on the other hand, can transmit light at many wavelengths simultaneously, without interference between the different optical signals. Fiber optic networks can thus carry multiple data streams over the same strand of optical fiber, in a technique known as wavelength division multiplexing. A good analogy is a that of a ten-lane expressway compared to a one-lane county road.

Wavelength division multiplexing is an incredibly powerful technique for increasing network capacity. Transmitting data over two wavelengths of light instantly doubles the capacity of the network without any additional optical fiber being added. Transmitting over sixteen wavelengths of light increases the capacity by sixteen times. Commercially deployed WDM systems feature 64 wavelengths, or channels, spaced less than 1 nanometer (nm) apart spectrally. Researchers have built WDM networks that operate over hundreds of channels, sending the equivalent of the amount of data in the Library of Congress across the network in a single second.

Attenuation, dispersion, and optimal communications wavelengths

As mentioned previously, signals carried by optical fiber eventually lose strength, though the loss of attenuation is nowhere near as high as that for copper wire. Single-mode fiber does not incur as much attenuation as multimode fiber. Indeed, signals in high quality fiber can be sent for more than 18.6 mi (30 km) before losing strength. This loss of signal strength is compensated for by installing periodic repeaters on the fiber that receive, amplify, and retransmit the signal. Attenuation is minimized at 1,550 nm, the primary operating wavelength for telecommunications.

Signals in optical fiber also undergo dispersion. One mechanism for this is the modal dispersion already discussed. A second type of dispersion is material dispersion, where different wavelengths of light travel through the fiber at slightly different speeds. Sources used for fiber optics are centered about a primary wavelength, but even with lasers, there is some small amount of variation. At wavelengths around 800 nm, the longer wavelengths travel down the fiber more quickly than the shorter ones. At wavelengths around 1,500 nm, the shorter wavelengths are faster. The zero crossing occurs around 1,310 nm: shorter wavelengths travel at about the same speed as the longer ones, resulting in zero material dispersion. A pulse at 1,310 nm sent through an optical fiber would arrive at its destination looking very much like it did initially. Thus, 1,310 nm is an important wavelength for communications.

A third kind of dispersion is wavelength dispersion, occurring primarily in single-mode fiber. A significant amount of the light launched into the fiber is leaked into the cladding. This amount is wavelength dependent and also influences the speed of propagation. High volume communications lines have carefully timed spacings between individual signals. Signal speed variation could wreak havoc with data transmission. Imagine a telephone call mixing with another person’s call! Fortunately, wavelength dispersion can be minimized by careful design of refractive index.

Based on dispersion and attenuation considerations, then, the optimal wavelengths for fiber-optic communications are 1,300 and 1,550 nm. Despite the dispersion advantages of operating at 1,310 nm, most modern fiber optic networks operate around 1,550 nm. This wavelength band is particularly important to the WDM networks that dominate the major crosscountry fiber optic links because the erbium-doped fiber amplifiers (EDFAs) incorporated in the repeaters provide signal amplification only across a range of wavelengths around 1,550 nm. Thus, most modern fiber optic networks operate around the so-called EDFA window. These signals are in the infrared region of the spectrum, that is, these wavelengths are not visible. Diode lasers are excellent sources at these wavelengths.

Telecommunications companies have developed single-mode optical fiber that addresses the problem of dispersion. Dispersion-shifted fiber is designed so that the region of maximum dispersion falls outside of the so-called telecommunications window. Although dispersion-shifted fiber is sufficient for basic transmission, in the case of WDM systems with tightly spaced channels, the fiber triggers nonlinear effects between channels that degrades signal integrity. In response, fiber manufacturers have developed non-zero dispersion-shifted fiber that eliminates this problem.

Other applications

Optical fiber has a variety of other applications. Fiber-optic stress and strain sensors are in common use on structures, bridges, and in monitoring industrial processes. Researchers have developed fiber-optic lasers that are tunable throughout the visible and fiber-optic amplifiers that will further increase capacity in the communications network. Fiber-optic endoscopes allow doctors to perform non-invasive internal examinations, and fiber-optic chemical sensors allow researchers to monitor pollution levels remotely.

Fiber-optic technology is continually improving and growing more and more an invisible part of daily lives. In

KEY TERMS

Attenuation —Loss of energy in a signal as it passes through the transmission medium.

Cladding —The outer layer of an optical fiber.

Core —The inner portion of an optical fiber.

Dispersion: modal, material, wavelength —Spreading of a signal pulse in an optical fiber.

Index of refraction —The ratio of speed of light in a vacuum to speed of light in a given material.

Modulation —Variation, a method of varying a signal such that information is coded in.

Refraction —The bending of light that occurs when traveling from one medium to another, such as air to glass or air to water.

Total internal reflection —When light reaches an interface between two materials and is reflected back into the first material.

1854, when John Tyndall (1820–1893) demonstrated light guided in a curved path by a parabolic stream of water, he could never have guessed at the ramifications of his discovery. By the same token, humans living in 2006, for instance, can only guess what applications will be found for optical fiber in the future.

Resources

BOOKS

DeCusatis, Casimer. Fiber Optic Essentials. Amsterdam, Netherlands, and Boston, MA: Elsevier/Academic Press, 2006.

Downing, James N. Fiber-optic Communications. Clifton Park, NY: Thomson/Delmar Learning, 2005.

Hecht, Jeff. Understandng Fiber Optics. Upper Saddle River, NJ: Pearson/Prentice Hall, 2006.

Kristin Lewotsky

Fiber optics

Optical fiber is a very thin strand of glass or plastic capable of transmitting light from one point to another. Optical fiber can also be called an optical waveguide, since it is a device that guides light.

Optical fibers consist of a light-carrying core and a cladding surrounding the core. There are generally three types of construction: glass core/cladding, glass core with plastic cladding, or all-plastic fiber. Optical fibers typically have an additional outside coating which surrounds and protects the fiber (see Figure 1).

Commonly available glass fiber diameters range from 8 micron core/125 micron cladding to 100 micron core/140 microns cladding, whereas plastic fibers range from 240 micron core/250 micron cladding to 980 micron core/1,000 micron cladding. The human hair, by comparison, is roughly 100 microns in diameter.

The principles behind fiber optics

Fiber optics work on the principle of total internal reflection. Light reaching the boundary between two materials is reflected such that it never leaves the first material. In the case of fiber optics, light is reflected from the optical fiber core-cladding interface in such a way that it propagates down the core of the fiber. This can be explained by a brief discussion of Snell's law of refraction and law of reflection, and a physical quantity known as index of bottom material. According to Snell's law, the light will be bent from its original path to a larger angle in the second material. As the incoming, or incident angle increases, so does the refracted angle. For the properly chosen materials, the incident angle can be increased to the point that the ray is refracted at 90 degrees and never escapes the first medium. The equation can be solved to give the incoming, or incident, angle which will result in a refracted angle of 90 degrees.

This is known as the critical angle (see Figure 2).

Light hitting the boundary or interface at angles greater than or equal to this value would never pass into the second material, but would rather undergo total internal reflection.

Now change the model slightly so that the higher index material is sandwiched between two lower index layers (see Figure 3).

Light enters the higher index material, hits the upper interface and is reflected downward, then hits the second interface and is reflected back upward, and so on. Like a marble bouncing off rails , light will make its way down the waveguide. This picture essentially corresponds to an optical fiber in cross-section. Light introduced to the fiber at the critical angle will reflect off the interface, and propagate down the fiber.

The second law of thermodynamics cannot be disregarded, however. Light will not travel down the fiber indefinitely. The strength of the signal will be reduced, or attenuated. Some light will be absorbed by impurities in the fiber, or scattered out of the core. Modern fibers are made of very pure material so that these effects are minimized, but they cannot be entirely eliminated. Some light will be diverted by microbends and other imperfections in the glass. Recall the law of reflection. If a microbend is encountered by light traveling through the fiber, the light may hit the interface at an angle smaller than the critical angle. If this happens, the light will be reflected out of the core and not continue propagating (see Figure 4).

Fabrication of optical fibers

Optical fibers are fabricated in a multi-step process: preform fabrication, fiber drawing, fiber coating, and spooling. A preform is a giant-sized version of the final fiber, with central core and cladding refractive indices equal to those of desired product. Preform diameters are typically 0.4-1 in (1-2.5 cm). They are produced by one of several variations on chemical vapor
deposition, in which chemicals (primarily silica, with other exotic compounds) are vaporized and allowed to deposit on a tube or rod. The porous form produced is heated to release trapped gases and water vapor that might otherwise compromise the performance of the final fiber.

In the drawing stage, the end of the preform is lowered into a furnace heated to roughly 3,632°F (2,000°C). The tip softens until it is drawn down by gravity, shrinking in diameter. Sensors constantly monitor the fiber diameter and concentricity to assure optimal results. An acrylic coating is applied to protect the fiber from damage and preserve its strength. Finally, it is wound onto a takeup spool.

Fiber classifications

Optical fiber falls into three basic classifications: step-index multimode, graded-index multimode, and single mode . A mode is essentially a path that light can follow down the fiber. Step-index fiber has a core with one index of refraction, and a cladding with a second index.

A graded-index fiber has a varying core index of refraction, and a constant cladding index (see Figure 6).

In general, the beam diameters of light sources for optical fibers are larger than the diameter of the fiber itself. Each fiber has a cone of light that it can propagate, known as the cone of acceptance of the fiber. It is driven by the critical angle of the fiber, which in turn varies according to the refractive index of the material. Light outside the cone of acceptance will not undergo total internal reflection and will not travel down the fiber.

Now, if light in the cone of acceptance is entering the fiber at a variety of angles greater than or equal to the critical angle, then it will travel a number of different paths down the fiber. These paths are called modes, and a fiber that can support multiple paths is classified as multimode. Notice that the light hitting at the smallest possible angle travels a longer path than the light at the largest angle, since the light at the largest angle is closest to a straight line. For step-index multimode fiber in which light travels the same speed everywhere, the rays running the longest path will take longer to get to the destination than the light running the shortest path. Thus a sharp pulse, or packet of light, will be spread out into a broad packet as it travels through the fiber. This is known as modal dispersion and can be a disadvantage in many applications. This type of fiber is used for in-house phone lines and data links.

Graded-index fiber offers one method for minimizing dispersion. The index of refraction of the core of graded index fiber increases toward the center. Remember, the refractive index of a material controls the speed of light traveling through it. Light propagating in the


center of the fiber thus goes more slowly than light on the edges. This reduces the pulse spread caused by differing path lengths. While not a perfect transmission, the transmitted pulse is dramatically improved over the step-index multimode fiber output. Graded-index fiber requires very specialized fabrication and is more expensive than step-index multimode. It is commonly used for mid-length communications (see Figure 6).

The best way to avoid modal dispersion, however, is to restrict transmission to only one mode. Single mode fiber is very narrow, with core diameters typically 8 microns, allowing light to propagate in only one mode (see Figure 7). The cone of acceptance is dramatically decreased, however, which makes light injection difficult. Splicing fiber together is more challenging, as well. Single-mode fiber is more costly than step-index multimode but less so than graded-index multimode. Single-mode fiber is used for long distance communication such as transoceanic telephone lines.

Plastic fiber is available in all three types. It is less expensive and lightweight but experiences more signal attenuation. It is practical for very short distance applications such as in automobiles.

Fiber optic communications

Why is the propagation of pulses of light through optical fibers important? Voice, video, and data signals can be encoded into light pulses and sent across an optical fiber. Each time someone makes a phone call, a stream of pulses passes through an optical fiber, carrying the information to the person on the other end of the phone line.

A fiber optic communication system generally consists of five elements: the encoder or modulator, the transmitter, the fiber, the detector, and the demodulator (see Figure 8).

Electrical input is first coded into a signal by the modulator, using signal processing techniques. The transmitter converts this electrical signal to an optical signal and launches it into the fiber. The signal experiences attenuation as it travels through the fiber, but it is amplified periodically by repeaters. At the destination, the detector receives the signal, converting it back to an

electrical signal. It is sent to the demodulator, which decodes it to obtain the original signal. Finally, the output is sent to the computer or to the handset of your telephone, where electrical signals cause the speaker to vibrate, sending audio waves to your ear .

Advantages of fiber optic cable

Communication via optical fiber has a number of advantages over copper wire. Wires carrying electrical current are prone to crosstalk, or signal mixing between adjacent wires. In addition, copper wiring can generate sparks, or can overload and grow hot, causing a fire hazard. Because of the electromagnetic properties of current carrying wires, signals being carried by the wire can be decoded undetectably, compromising communications security. Optical fiber carries light, no electricity , and so is not subject to any of these problems.

The biggest single advantage that optical fiber offers over copper wire is that of capacity, or bandwidth. With the rising popularity of the Internet, the demand for bandwidth has grown exponentially. Using a technique called wavelength division multiplexing (WDM), optical networks can carry thousands of times as much data as copper-based networks.

Most copper networks incorporate a technique known as time division multiplexing (TDM), in which the system interleaves multiple conversations, sending bits of each down the line serially. For example, the system transmits a few milliseconds of one conversation, then a few milliseconds of the next, a few milliseconds of a the next, then returns to transmit more of the first conversation, and so on. For many years, network designers increased carrying capacity by developing electronics to transmit shorter, more closely spaced data pulses.

Electronics can operate so quickly, however, and eventually copper wire hit a maximum carrying capacity. To increase bandwidth, network operators had to either lay more copper cable in already packed underground conduits, or seek another method. Enter fiber optics.

The electrons in copper wire can only carry one stream of time-division multiplexed data at a time. Optical fiber, on the other hand, can transmit light at many wavelengths simultaneously, without interference between the different optical signals. Fiber optic networks can thus carry multiple data streams over the same strand of optical fiber, in a technique known as wavelength division multiplexing. A good analogy is a that of a ten-lane expressway compared to a one-lane county road.

Wavelength division multiplexing is an incredibly powerful technique for increasing network capacity. Transmitting data over two wavelengths of light instantly doubles the capacity of the network without any additional optical fiber being added. Transmitting over sixteen wavelengths of light increases the capacity by sixteen times. Commercially deployed WDM systems feature 64 wavelengths, or channels, spaced less than 1 nanometer (nm) apart spectrally. Researchers have built WDM networks that operate over hundreds of channels, sending the equivalent of the amount of data in the Library of Congress across the network in a single second.

Attenuation, dispersion, and optimal communications wavelengths

As mentioned previously, signals carried by optical fiber eventually lose strength, though the loss of attenuation is nowhere near as high as that for copper wire. Singlemode fiber does not incur as much attenuation as multimode fiber. Indeed, signals in high quality fiber can be sent for more than 18.6 mi (30 km) before losing strength. This loss of signal strength is compensated for by installing periodic repeaters on the fiber that receive, amplify, and retransmit the signal. Attenuation is minimized at 1,550 nm, the primary operating wavelength for telecommunications.

Signals in optical fiber also undergo dispersion. One mechanism for this is the modal dispersion already discussed. A second type of dispersion is material dispersion, where different wavelengths of light travel through the fiber at slightly different speeds. Sources used for fiber optics are centered about a primary wavelength, but even with lasers, there is some small amount of variation. At wavelengths around 800 nm, the longer wavelengths travel down the fiber more quickly than the shorter ones. At wavelengths around 1,500 nm, the shorter wavelengths are faster. The zero crossing occurs around 1,310 nm: shorter wavelengths travel at about the same speed as the longer ones, resulting in zero material dispersion. A pulse at 1,310 nm sent through an optical fiber would arrive at its destination looking very much like it did initially. Thus, 1,310 nm is an important wavelength for communications.

A third kind of dispersion is wavelength dispersion, occurring primarily in single-mode fiber. A significant amount of the light launched into the fiber is leaked into the cladding. This amount is wavelength dependent and also influences the speed of propagation. High volume communications lines have carefully timed spacings between individual signals. Signal speed variation could wreak havoc with data transmission. Imagine your telephone call mixing with someone else's! Fortunately, wavelength dispersion can be minimized by careful design of refractive index.

Based on dispersion and attenuation considerations, then, the optimal wavelengths for fiber-optic communications are 1,300 and 1,550 nm. Despite the dispersion advantages of operating at 1,310 nm, most modern fiber optic networks operate around 1,550 nm. This wavelength band is particularly important to the WDM networks that dominate the major cross-country fiber optic links because the erbium-doped fiber amplifiers (EDFAs) incorporated in the repeaters provide signal amplification only across a range of wavelengths around 1,550 nm. Thus, most modern fiber optic networks operate around the so-called EDFA window. These signals are in the infrared region of the spectrum , that is, these wavelengths are not visible. Diode lasers are excellent sources at these wavelengths.

Telecommunications companies have developed singlemode optical fiber that addresses the problem of dispersion. Dispersion-shifted fiber is designed so that the region of maximum dispersion falls outside of the socalled telecommunications window. Although dispersion-shifted fiber is sufficient for basic transmission, in the case of WDM systems with tightly spaced channels, the fiber triggers nonlinear effects between channels that degrades signal integrity. In response, fiber manufacturers have developed non-zero dispersion-shifted fiber that eliminates this problem.

Other applications

Optical fiber has a variety of other applications. Fiber-optic stress and strain sensors are in common use on structures, bridges, and in monitoring industrial processes. Researchers have developed fiber-optic lasers that are tunable throughout the visible and fiber-optic amplifiers that will further increase capacity in the communications network. Fiber-optic endoscopes allow doctors to perform non-invasive internal examinations, and fiber-optic chemical sensors allow researchers to monitor pollution levels remotely.

Fiber-optic technology is continually improving and growing more and more an invisible part of our daily lives. In 1854, when John Tyndall demonstrated light guided in a curved path by a parabolic stream of water, he could never have guessed at the ramifications of his discovery. By the same token, we can only guess what applications will be found for optical fiber in the future.

Resources

books

Sterling, D. Technician's Guide to Fiber Optics (AMP). Albany, NY: Delmar Publishers Inc., 1987.

Kristin Lewotsky

KEY TERMS

Attenuation

—Loss of energy in a signal as it passes through the transmission medium.

Cladding

—The outer layer of an optical fiber.

Core

—The inner portion of an optical fiber.

Dispersion: modal, material, wavelength

—Spreading of a signal pulse in an optical fiber.

Index of refraction

—The ratio of speed of light in a vacuum to speed of light in a given material.

Modulation

—Variation, a method of varying a signal such that information is coded in.

Refraction

—The bending of light that occurs when traveling from one medium to another, such as air to glass or air to water.

Total internal reflection

—When light reaches an interface between two materials and is reflected back into the first material.

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. While optical 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 sources are 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

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.

see also Digital Logic Design; Networks; Telecommunications; Transmission Media.

Richard A. Thompson

Bibliography

Green, Paul E. Fiber Optic Networks. Upper Saddle River, NJ: Prentice Hall, 1993.

Palais, Joseph C. Fiber Optic Communications, 4th ed. Upper Saddle River, NJ: Prentice Hall, 1998.

FIBER OPTICS

Fiber optics is the transmission of data via light waves passed through glass threads. Most major telephone companies have replaced, or are in the process of replacing, traditional copper telephone lines with fiber optic cables. Additionally, local-area networks often use fiber optic technology. Single-mode fiber is used in conjunction with laser light to transfer data more than five miles in distance. Multi-mode fiber is used with a lower frequency light-emitting diode (LED) for shorter transmissions.

Fiber optic cables can carry significantly more data at a much greater speed than metal cables. For this reason, companies across the globe became interested in the technology, starting as early as the 1970s. For example, several Japanese companies, including Furukawa Electric Company Ltd., worked cooperatively to develop fiber optic cables capable of transmitting more information faster and more reliably than conventional microwave cable. Furukawa's developments throughout the 1980s included the first single-mode fiber optic connector using high-heat fusion splicing methods; a stronger, more heat resistant fiber optic cable; and a flexible fiber optic scope for use in examining the inside of pipes.

Western Electric engineers started experimenting with fiber optics in 1979. In 1980, AT&T Corp. sought permission from the U.S. Federal Communications Commission to build a 611-mile fiber optic network connecting major cities in the Northeastern United States. By 1984, fiber cables in the United States had reached 250,000 miles. Other leading telecommunications players, such as Nippon Telegraph and Telephone Corp., also began to focus on fiber optic technologies in the early 1980s. To bolster its fiber optic efforts, MCI Communications Corp. bought 100,000 kilometers of fiber optic cable from Corning Inc., which invested $87 million on new fiber optic plant facilities in 1986. At roughly the same time, the Williams Companies created Williams Telecommunications, a telecommunications unit which developed a fiber optic cable network that could be run inside unused steel pipelines; AMP Inc. spent more than $100 million in the development of fiber optics technology; and NYNEX Corp. entered the international long distance business by forming a $400 million joint venture to lay a transatlantic fiber optic cable. In 1988, GTE Laboratories developed the first fiber optic amplifier, and Bell Laboratories sent light pulses over fiber optic cables for 2,480 miles, setting a distance record. That year, the first transatlantic fiber optic cable was completed. In 1989, AT&T and Kokusai Denshin Denwa brought the first transpacific fiber optic cable into use.

Advances in fiber optics continued into the next decade as an increasing number of telecommunications companies, as well as firms in other industries, began embracing the technology. MCI Communications Corp. and British Telecom began working together to lay a transatlantic fiber optic cable in 1990. Cable company Cox Enterprises Inc. acquired a 50-percent stake in fiber optics vendor Teleport Communications Group. In 1992, Nynex Corp. revealed its intent to lay a fiber optic cable connecting the eastern United States with Japan via England and the Middle East. LDDS Communications, the predecessor to WorldCom, gained access to its first nationwide fiber optic network in 1995 when it paid $2.5 billion for WilTel Network Services, a unit of the Williams Companies. Chevron Corp. pioneered the use of fiber optic cables to monitor oil field production in 1996. Simplex Technologies Inc. partnered with Tyco into 1997 to form Tyco Submarine Systems Ltd., an undersea fiber optic telecommunication cable system. The following year, ADC Telecommunications Inc. introduced the EtherRing switch, which allowed less expensive implementation of Ethernet technology over fiber optic networks. Furukawa began developing and marketing fiber optic products in North America in 1999 via its FITEL Technologies Inc. subsidiary.

Fiber optic developments continued to improve telecommunications in 2000 and 2001. To improve the speed and quality of their networks, many organizations began upgrading to optical Ethernet systems. Nortel Networks, for example, started converting its North American ATM systems to optical Ethernet networks. Canadian financial giant CICB also began using optical Ethernet networking in Toronto. According to an October 2001 article in Business Communications Review, "the rationale for these activities is straightforward: simpler, faster and more reliable networking opportunities for rethinking server and storage distribution, and increased knowledge-worker productivity. The reason these are taking place now is the maturing of Ethernet transmission and switching, and the increased investment in metropolitan optical networking." Many industry analysts believe that all communications eventually will use fiber optic technology in one form or another.

FURTHER READING:

"About Fiber Optics." Port Huron, MI: AboutFiberOptics.com, 2001. Available from www.aboutfiberoptics.com.

"Fiber Optics." In Webopedia. Darien, CT: Internet.com, 2001. Available from e-comm.webopedia.com.

"Fiber Optics to the Fore." Washington, DC: National Academy of Sciences, 2001. Available from www4.nas.edu.

Rybczynski, Tony. "Optical Ethernet—Preparing for the Transition." Business Communications Review. October 2001.

SEE ALSO: AT&T Corp.; Bandwidth; Connectivity, Internet; Internet Infrastructure; Photonics

FIBER OPTICS

FIBER OPTICS. Narinder Kapany did not believe a high school teacher who told him that light could only travel in a straight line. His fascination with the idea set off a lifetime of research into fiber optics, which involves the use of reflection to transmit light through fibers of glass or plastic. In 1954, Kapany reported in the British journal Nature that he had successfully transmitted images through fiber optic bundles of transparent glass or plastic rods. Kapany's research built on more than 200 years of research and investigation into sending communications over translucent devices.

The American inventor Alexander Graham Bell dreamed of sending communications signals through the air via light impulses. He patented an optical telephone system in 1880, called the Photophone, but his invention of the landline telephone was more practical, thus receiving the lion's share of his time and effort. Further innovation in fiber optics was uneven until the 1920s when Clarence W. Hansell of the United States and John Logie Baird in England patented the idea of using hollow rods to transmit images for television systems. Despite the patent, the first person that established image transmission through a bundle of optical fibers was Heinrich Lamm, a medical student in Germany, who later moved to the United States to avoid persecution by the Nazis.

In 1955, after receiving a doctorate, Kapany journeyed to the United States to teach at the University of Rochester, in New York. In 1960, he moved to California's Silicon Valley and founded Optics Technology, taking it public in 1967. Another Northern California team, this one based at Stanford University, also worked on fiber optic research. Antoni E. Karbowiak and Charles K. Kao led a team examining the properties of fiber and concluded that impurities led to loss of transmission. The team attempted to figure out why light dimmed only a few yards down fiber optic strands, called "fiber attenuation." In 1966, after Karbowiak left Stanford, Kao developed a proposal for long-distance fiber optic communications over single-mode fibers. Although skeptics doubted Kao's research, he proved that fiber could be used for communications systems.

In the 1960s, Kao continued his theoretical and practical research, receiving twenty-nine patents for ideas on manufacturing pure glass fibers to splicing fibers to form communications lines. For their important early work, many observers have dubbed either Kapany or Kao as "the father of fiber optics."

Corning Glass Works produced the first commercial fiber optic cable in 1970. Company scientists used fused silica, an extremely pure material with a high melting point, to perfect fiber optic cable. Less than a decade later, in 1978, communications giant AT&T demonstrated the first fiber communications system. From this humble beginning, several million miles of fiber have been installed around the world, both on land and undersea.

In the early 1980s, when deregulation opened the telecommunications industry, telephony carriers built the national backbone of the industry on fiber optics. Soon, the technology spread from long-distance to other applications, ultimately setting the stage for nationwide fiber systems and the Internet.

In the mid-to late-1990s, the growth of the Internet and a "New Economy" based online solidified the idea that future communications networks would be built on fiber optics, or "broadband" technology. At the height of dot-com mania, companies rushed to connect Internet users to vast broadband networks, which offered the kind of high-speed access needed to fuel the growth of the wired economy.

After the dot-com economic bubble burst, however, the fiber optics industry virtually collapsed. Many formerly solid companies, such as Lucent and Nortel, foundered and startup money for new companies vanished. The fiber optic industry successfully increased bandwidth around the world, but was spread too thin in an effort to build new systems. When an economic recession hit the United States in the early 2000s, many companies were extended beyond their means.

Fiber optic data transmissions carried over silica fiber is at the heart of worldwide communications. The high bandwidth, light-carrying medium transports voice, video, and data and is the keystone of the Internet. Since the 1980s, communications companies have placed more than 300 million miles of fiber optic cable in the ground. However, less than 10 percent of this wiring is being used, eliminating any hope for profitability among many companies. These companies overextended their credit limits to install the fiber optic lines, but could not get enough users "lit" to justify the expense.

BIBLIOGRAPHY

Hecht, Jeff. City of Light: The Story of Fiber Optics. New York: Oxford University Press, 1999.

Hitz, Breck, James J. Ewing, and Jeff Hecht. Introduction to Laser Technology. 3d ed. New York: Wiley-IEEE Press, 2001.

Palais, Joseph C. Fiber Optic Communications. 4th ed. Garden City, N.J.: Prentice-Hall, 1998.

BobBatchelor

See alsoComputers and Computer Industry ; Telecommunications .