This chapter includes the following sections:
A Brief History of Fiber-Optic Communications—This section discusses the history of fiber optics, from the optical semaphore telegraph to the invention of the first clad glass fiber invented by Abraham Van Heel. Today more than 80 percent of the world's long-distance voice and data traffic is carried over optical-fiber cables.
Fiber-Optic Applications—Telecommunications applications of fiber-optic cable are widespread, ranging from global networks to desktop computers.
The Physics Behind Fiber Optics—This section discusses the physics behind the operation of fiber-optic cables.
Optical-Cable Construction—This section discusses fiber-optic cable construction. Fiber-optic cables are constructed of three types of materials: glass, plastic, and plastic-clad silica (PCS).
Propagation Modes—There are two main modes of fiber-optic propagation: multimode and single mode. These two modes perform differently with respect to both attenuation and chromatic dispersion.
Fiber-Optic Characteristics—Fiber-optic system characteristics include linear and nonlinear characteristics. Linear characteristics include attenuation and interference. Nonlinear characteristics include single-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS).
Fiber Types—This section discusses various multimode and single-mode fiber types currently used for premise, metro, aerial, submarine, and long-haul applications.
Fiber-Optic Cable Termination—Removable and reusable optical termination in the form of metal and plastic connectors plays a vital role in an optical system.
Splicing—Seamless permanent or semipermanent optical connections require fibers to be spliced. Fiber-optic cables might have to be spliced together for a number of reasons.
Physical-Design Considerations—When designing a fiber-optic cable plant, you must consider many factors. First and foremost, the designer must determine whether the cable is to be installed for an inside-plant (ISP) or outside-plant (OSP) application.
Fiber-Optic Communications System—This section discusses the end-to-end fiber-optic system.
Fiber Span Analysis—Optical loss, or total attenuation, is the sum of the losses of each individual component between the transmitter and receiver. Loss-budget analysis is the calculation and verification of a fiber-optic system's operating characteristics.
A Brief History of Fiber-Optic Communications
Optical communication systems date back to the 1790s, to the optical semaphore telegraph invented by French inventor Claude Chappe. In 1880, Alexander Graham Bell patented an optical telephone system, which he called the Photophone. However, his earlier invention, the telephone, was more practical and took tangible shape. The Photophone remained an experimental invention and never materialized. During the 1920s, John Logie Baird in England and Clarence W. Hansell in the United States patented the idea of using arrays of hollow pipes or transparent rods to transmit images for television or facsimile systems.
In 1954, Dutch scientist Abraham Van Heel and British scientist Harold H. Hopkins separately wrote papers on imaging bundles. Hopkins reported on imaging bundles of unclad fibers, whereas Van Heel reported on simple bundles of clad fibers. Van Heel covered a bare fiber with a transparent cladding of a lower refractive index. This protected the fiber reflection surface from outside distortion and greatly reduced interference between fibers.
Abraham Van Heel is also notable for another contribution. Stimulated by a conversation with the American optical physicist Brian O'Brien, Van Heel made the crucial innovation of cladding fiber-optic cables. All earlier fibers developed were bare and lacked any form of cladding, with total internal reflection occurring at a glass-air interface. Abraham Van Heel covered a bare fiber or glass or plastic with a transparent cladding of lower refractive index. This protected the total reflection surface from contamination and greatly reduced cross talk between fibers. By 1960, glass-clad fibers had attenuation of about 1 decibel (dB) per meter, fine for medical imaging, but much too high for communications. In 1961, Elias Snitzer of American Optical published a theoretical description of a fiber with a core so small it could carry light with only one waveguide mode. Snitzer's proposal was acceptable for a medical instrument looking inside the human, but the fiber had a light loss of 1 dB per meter. Communication devices needed to operate over much longer distances and required a light loss of no more than 10 or 20 dB per kilometer.
By 1964, a critical and theoretical specification was identified by Dr. Charles K. Kao for long-range communication devices, the 10 or 20 dB of light loss per kilometer standard. Dr. Kao also illustrated the need for a purer form of glass to help reduce light loss.
In the summer of 1970, one team of researchers began experimenting with fused silica, a material capable of extreme purity with a high melting point and a low refractive index. Corning Glass researchers Robert Maurer, Donald Keck, and Peter Schultz invented fiber-optic wire or "optical waveguide fibers" (patent no. 3,711,262), which was capable of carrying 65,000 times more information than copper wire, through which information carried by a pattern of light waves could be decoded at a destination even a thousand miles away. The team had solved the decibel-loss problem presented by Dr. Kao. The team had developed an SMF with loss of 17 dB/km at 633 nm by doping titanium into the fiber core. By June of 1972, Robert Maurer, Donald Keck, and Peter Schultz invented multimode germanium-doped fiber with a loss of 4 dB per kilometer and much greater strength than titanium-doped fiber. By 1973, John MacChesney developed a modified chemical vapor-deposition process for fiber manufacture at Bell Labs. This process spearheaded the commercial manufacture of fiber-optic cable.
In April 1977, General Telephone and Electronics tested and deployed the world's first live telephone traffic through a fiber-optic system running at 6 Mbps, in Long Beach, California. They were soon followed by Bell in May 1977, with an optical telephone communication system installed in the downtown Chicago area, covering a distance of 1.5 miles (2.4 kilometers). Each optical-fiber pair carried the equivalent of 672 voice channels and was equivalent to a DS3 circuit. Today more than 80 percent of the world's long-distance voice and data traffic is carried over optical-fiber cables.