Fiber Optics: The Ultimate in Telecommunications
Edward A. Lacy
In the 1980s light wave communications -- photonics -- came of age. The United States and other industrial nations went on a "high-fiber" diet, installing so much fiber-optic cable that the decade was called the "decade of glass." More than 3 million km of these cables were installed in the United States alone. In the process, thousands of miles of copper cables -- both coaxial and twisted-pair -- were made obsolete as far as long distance telecommunications were concerned. Copper cables have been displaced simply because they do not have the tremendous information-carrying capacity, called bandwidth, of fiber-optic cables.
With such bandwidth, fiber-optic systems can transmit thousands of telephone conversations, dozens of television programs, and numerous computer data signals over one or two flexible, hair-thin threads of ultrapure glass called optical fibers. Theoretically, the bandwidth of a fiber-optic cable is 50 THz (50 million MHz). For comparison, a television signal, the biggest user of bandwidth, requires 6 MHz of bandwidth, a telephone conversation just a few kilohertz.
Currently, such phenomenal bandwidths are not available because the input and output equipment to the cables are not capable of such speed. But even present systems, with their bandwidths of a few gigahertz, are able to transmit video conference signals and high definition television at economical prices.
These and other broadband services are not available to homes and businesses at present because of cost and government regulations. The long distance fiber-optic trunks across the nation are like superhighways. While most homes are within 50 miles of these super trunks, the connection between the trunks and homes and businesses is copper cable. In effect, the superhighways of telecommunications are connected to your home with dirt footpaths. To get the advantage of broadband services, the subscriber loop must also be replaced with fiber optics.
Running fiber all the way to a customer's home is presently too expensive. In addition, cable television companies and telephone companies are battling to see who will control the fiber in the home. Obviously, only one fiber would be needed per home. This fiber would carry telephone signals as well as cable TV and other signals. Government bodies will have to decide whether the phone company of the cable TV company will provide the service.
Meanwhile, local area networks (LANs) that interconnect computers at a common geographic site are being connected with optical fibers. In some cases these LANs do not use fiber optics because they have low bandwidth requirements. However, LANs at university campuses throughout the world are using fiber optics to connect host computers with terminals at dozens of buildings and hundreds of dormitory rooms.
In addition to giving extremely high bandwidth, fiber-optic systems have significantly smaller, lighter-weight cables than conventional telephone cables. An optical fiber with its protective jacket may be, typically, 0.635 cm in diameter, yet it can replace a 7.62-cm diameter bundle of copper wires now used to carry the same number of telephone conversations and other signals. The importance of this dramatic decrease in size is not obvious in uncongested rural areas; in major cities, however, where telephone cables must be placed underground, conduits are so crammed that they can scarcely accommodate a single additional copper cable.
Along with the size reduction, there is a corresponding weight reduction. For example, 94.5 kg of copper wire can be replaced with 3.6 kg of optical fiber. Weight reduction is important for the military services as it allows faster deployment of communication cables on battlefields. On the civilian side, it is important in huge jet aircraft, which use a surprising amount of copper cables between the various equipment and instrumentation on board. By replacing these cables with optical cables, up to 1 000 lb of weight may be saved, thereby giving better fuel consumption.
Perhaps of equal importance, fiber-optic systems are immune to electromagnetic interference such as lightning and arcs created in some factories. Because of this immunity, fiber-optic systems give accurate transmission of data -- about 100 times better than transmissions over copper cables. For information transmitted in bits, the accuracy is typically one error in 100 million bits.
Still another advantage of fiber-optic systems is that it is difficult to tap them, as compared to telephone cables, for eavesdropping. In fact, fiber-optic systems are generally considered to be "secure," an important feature for the military. There are numerous other advantages that are important but discussion of them must be reserved for other texts.*
In a fiber-optic communication system, information is carried by lightwaves (photons) rather than by the movement of electrons as in metallic systems.
A fiber-optic communication system has three major components: a transmitter that converts electrical signals to light signals, an optical fiber for transmitting the signals, and a receiver that captures the signals at the other end of the fiber and converts them to electrical signals.
The key part of the transmitter is a light source -- either a semiconductor laser diode or a light-emitting diode (LED). In general, laser diodes have greater capabilities than LEDs but cost more, require more complex circuitry, and are less reliable. Laser diodes are generally used for long-haul routes, LEDs for short-haul loops. With either device, the light emitted is an invisible infrared signal with a wavelength of 1.31 m m (1310 nm). Wavelengths that are either higher or lower are significantly attenuated as they cannot pass through the "window" at 1.31 m m. Older systems had a wavelength of 0.85 m m; future systems may have a wavelength of 1.55 m m. Visible and ultraviolet light are impractical for fiber-optic systems because of high losses in the optical fiber. The laser diodes and LEDs used in this application are miniature units less than half the size of a thumbnail in order to couple the light into the tiny fibers effectively. Even so, a glass lens is frequently used to transfer the emitted light effectively to the fiber.
To transmit an audio, television, or computer data signal by light waves, it is necessary to change or modulate the light waves in accordance with the information in these signals. By varying the intensity of the light beam from the laser diode or the LED, analog modulation is achieved. By flashing the laser diode or LED on and off at an extremely fast rate, digital modulation is achieved.
In digital modulation, a pulse of light represents the number 1, and the absence of light represents 0. In a sense, instead of flashes of light traveling down the fiber, 1s and 0s are moving down the path. With computer-type equipment, any communication can be represented by a particular pattern or code of 1s and 0s. If the receiver is programmed to recognize such digital patterns, it can reconstruct the original signal from the 1s and 0s it receives.
Digital modulation is expressed in bits (short for "binary digit") per second, megabits (1 000 000 bits) per second, or gigabits (1 000 000 000 bits) per second. Engineers have demonstrated a fiber-optic system that can transmit 27 gigabits per second. At this rate, 400 000 phone conversations could be held simultaneously over this system.
As remarkable as these bandwidths are, engineers are pursuing other techniques to give even greater bandwidth. In wavelength division multiplexing (also called color multiplexing) the outputs of two or more lasers with different wavelengths are combined and sent through one optical fiber. In effect, this doubles the bandwidth. In coherent modulation and detection a laser sends a continuous beam whose frequency is varied by the messages that are being transmitted. At the receiver, this light beam is combined with a lightbeam that has been generated at the receiver. The frequencies are designed to be slightly different so that when they are combined there will be a new signal which is the difference of the two. The new signal can be processed, as in superheterodyne radio and television sets, much more easily than the original incoming lightbeam.
As you might suspect, the equipment used in digital modulation, such as encoders, is much more complicated than that used in analog modulation. Digital modulation also requires more bandwidth than analog modulation to send the same message. The former is, however, far more popular because it allows greater transmission distance with the same power and less expensive switching equipment. Thus, even though digital telecommunication is only a minority of present telecommunication now, it is rapidly replacing analog transmission which is used mainly for television signals.
Even though an optical fiber is usually made of glass, it is surprisingly tough; in fact, it can be bent and twisted just like wire. Splicing optical fiber, however, can be difficult: the ends of the fibers can be joined by fusion or with mechanical splices, but not by twisting and soldering as with copper cables. In splicing, great care must be taken to ensure precision mating of the tiny fibers. Otherwise the system will have enough losses to make it inoperable.
Optical fibers have very low transmission losses because of their ultra purity. If a window pane 1 km thick were to be made of such glass, it would be as transparent as an ordinary pane of glass. Despite this purity, the light waves eventually become dim or attenuated because of absorption and scattering. Absorption occurs within the fiber when the light waves encounter impurities and are turned into heat. Scattering occurs primarily at splices or junctions in the fiber where light leaves the fiber because of imprecise connections.
Attenuation is measured in decibels per kilometer (dB/km). In most long haul circuits, the attenuation is less than 1 dB/km for signals being transmitted at a wavelength of 1310 nm. When equipment and fibers now being developed to transmit at a wavelength of 1550 nm are perfected, the losses are expected to be less than 0.25 dB/km.
Because of attenuation, the light signals must eventually be regenerated at intervals by devices called repeaters. A repeater is a combination of a fiber-optic receiver and a fiber-optic transmitter. The receiver decodes the signal and triggers the transmitter to produce an identical version, only now the signal has greater strength and purity. In the case of digital signals, it is also in better time synchronism. Repeaters are typically placed about 30 km apart, but in the newer systems they may be separated as much as 200 km or more. Whereas present repeaters convert the light signals to electrical signals and then back to light signals, all-optical repeaters being developed will convert weak signals directly to strong light signals, skipping the conversion from light signals to electric signals to light signals. By doing this, these repeaters will theoretically be much more sensitive and thus can be placed farther apart. In addition, they will be smaller, cost less, and have greater reliability.
Each fiber has three parts. At the center of the fiber is the core, which carries the light signal. A concentric layer of glass about 125 m m in diameter, called the cladding, surrounds the core. Because the cladding has a different index of refraction than the core, total internal reflection occurs in the core, keeping the light in the core. Surrounding the cladding is a polyurethane jacket that protects the fiber from abrasion, crushing, and chemicals. From one to several hundred fibers are grouped to form a cable.
In large-core fibers, typically with core diameter of 62.5 m m, light pulses can take numerous paths (called modes) as they bounce back and forth down the fiber. Because the different paths are not equal in length, some pulses will take longer to travel down the fiber, causing some of the pulses to overlap and thereby cause distortion. These multimode fibers, which are less expensive than other fibers, were widely used in the early days of fiber optics but now are used mainly for certain short-distance links such as local area networks.
In the newer fibers, the core is much smaller: only 8 m m in diameter, about one sixth the thickness of a sheet of paper. Because of the small diameter, only one light path is possible -- straight down the core with no zigzagging. As there is only one path, there is much less distortion, giving these single-mode fibers a much higher bandwidth than multimode fibers.
At the end of the fiber, a photodiode converts the light signals to electric signals, which are then amplified and decoded, if necessary, to reform the signals originally transmitted.
While long-distance fiber-optic systems have received most of the attention, fiber-optic links are also useful for very short distances, such as between large computer mainframes and their peripheral terminals and printers. Within computers, fiber optics is being used to carry signals between circuit boards.
In an entirely different use of fiber optics, in a nontransmission application, optical fibers are being used as sensors to detect strain, pressure, temperature, and other stimulus. In this function, the fibers offer the advantages of compactness, sensitivity, and immunity to hostile environments, as compared to other sensors. For example, a fiber-optic sensor is being used to measure the temperature of a volcano. In another use, by embedding fiber-optic strain sensors in polymer composites, used to replace metal on some aircraft, it may be possible to give an aircraft a "smart skin" that would warn the pilot of dangerous strains in the wings or fuselage.
In one type of sensor, short lengths of optical fibers are made with intentional small lateral deformations called microbends. At these microbends some of the light radiates from the fiber. The behavior of this light can be influenced by temperature, acceleration, and other parameters to be sensed. In the optical interferometer type of sensor, light from a laser is transmitted down two paths: a reference path and a measuring path. The perturbation being measured will cause the light through the measuring path to be out of phase with the light through the reference path when they are recombined. This phase difference can then be converted to an amplitude change.
While most optical fibers are made of glass, plastic fibers serve a useful role in short-range data links, electronic billboards, automobile displays, and automobile electrical systems. Plastic fibers have excessive losses when used for distances greater than 1 km, but for short distances they have the advantages of being more flexible than glass fibers and less expensive.
Suggested Readings
Books
Lacy, Edward A., Fiber Optics, Prentice-Hall, Inc. Englewood Cliffs, NJ, 1982 (an introductory text).
Busch, E.E. (Editor-in-chief), Optical-Fiber Transmission; Indianapolis, Howard W. Sams & Co., 1987 (for the more advanced student).
Chaffee, C. David, The Rewiring of America: The Fiber Optics Revolution, San Diego, Academic Press, CA, 1987.
Trade Magazines
Lightwave, The Journal of Fiber Optics
Laser Focus/ Electro-optics
Lasers and Applications
Journals
I.E.E.E. Spectrum
I.E.E.E. Journal of Lightwave Technology
Essay Questions
Problems
* E.A. Lacy, Fiber Optics, Englewood Cliffs, N.J., Prentice-Hall, 1982.
Figure Captions
Figure 1 Ultra-pure glass optical fibers carry voice, video and data signals in high-capacity telecommunications networks. (Photo courtesy of Corning Incorporated)
Figure 2 Optical fiber, the transmission medium of choice for telephone company interoffice trunks, proves in against copper for new and rebuild feeder systems. (Photo courtesy of Corning Incorporated)
Figure 3 Interference contrast light micrograph of a fiber optics wave guide of the type used for signal transmission in all aspects of communications. This particular type of fiber is known as a monomode. The faint impression of the innermost core of the fiber is visible as a darker shade of red (its refractive index is different from the surrounding material): it is along this inner core that the light signals travel. (Courtesy of Science Photo Library/Photo Researchers)