Fiber Optics (Basic Overview)

Electronic communication over conducting wires or by atmospheric radio transmission began in the latter part of the 19th century and was highly developed by the middle part of the 20th century. Extensive communication via beams of light traveling over thin glass fibers is a relative new technology, beginning in the 1970s, reaching acceptance as a viable technology in the early 1980s, and continuing to evolve since then. Fibers now form a major part of the infrastructure for a national munication information highway in the U.S. and all around the world. Not since Alexander Graham Bell’s invention of the telephone has communications experienced such revolutionary development, and nearly all of it has been made possible by electro-optics. Ironically, it was Bell himself who invented one of the earliest light-wave communications devices in 1880. Bell’s “photo phone” used a flexible diaphragm to modulate a beam of sunlight and a selenium photo detector as a receiver. Current fluctuations generated in the photoconductive selenium by the modulated light beam were fed through a transducer to recreate the original sound. But what made the photo phone impractical for long-distance communications were the incoherent light and its free-space transmission through the atmosphere. The advent of lasers in 1960 and low-loss optical fiber in 1970 eliminated all these barriers.

The applications of optical fiber communications have increased rapidly, since the first commercial installation of a fiber-optic system in 1977. Telephone companies began early on, replacing the old copper wire systems with optical fiber lines. Today’s telephone companies use optical fiber throughout their system as the backbone architecture and as the long-distance connection between city phone systems. Cable television companies have also begun integrating fiber optics into their cable systems. The trunk lines that connect central offices have generally been replaced with optical fiber. Some providers have begun experimenting with fiber fiber/coaxial hybrid. Such a hybrid allows for the integration of fiber and coaxial at a neighborhood location. This location, called a node, would provide the optical receiver that converts the light impulses back to electronic signals. The signals could then be fed to individual homes via coaxial cable. In recent years it has become apparent that fiber optics are steadily replacing copper wire as an appropriate means of communication signal transmission. They span the long distances between local phone systems as well as providing the backbone for many network systems. Other system users include cable television services, university campuses, office buildings, industrial plants, and electric utility companies.

There are several advantages that have been established with the development and implementation of fiber-optic cable systems. Fiber optic networks operate at speeds up to 2.5 gigabits per second, as opposed to 1.54 megabits per second for copper. There is a great advantage in terms of bandwidth: fiber optics has a greater capacity for information, which means smaller cables can be used. Signals in a fiber optic cable can be transmitted further without needing to be “refreshed” or strengthened. Fiber optic cables have a greater resistance to electromagnetic noise such as radios, motors or other nearby cables. Because optical fibers carry beams of light, they are free of electrical noise and interference. Finally, from a maintenance perspective, fiber optic cables costs much less to maintain. A disadvantage of the fiber-optic system is its incompatibility with the electronic hardware systems that compose today’s world. This inability to interconnect easily requires that current communication hardware systems be somewhat retrofitted to the fiber-optic networks. Much of the speed that is gained through optical fiber transmission can be lost at the conversion points of a fiber-optic chain. When a portion of the chain experiences heavy use, information becomes jammed in a bottleneck at the points where conversion to, or from, electronic signals is taking place. Bottlenecks like this should become less frequent as microprocessors become more efficient and fiber-optics reach closer to a direct electronic hardware interface.

A fiber-optic system is similar to the copper wire system that fiber optics is replacing. The difference is that fiber-optics use light pulses to transmit information down fiber lines instead of using electronic pulses to transmit information down copper lines. At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then processes and translates that information into equivalently coded light pulses. A light-emitting diode (LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses are funneled into the fiber-optic medium where they transmit themselves down the line. Light pulses move easily down the fiber-optic line because of a principle known as “total internal reflection”. The principle of total internal reflection states that when the angle of incidence exceeds a critical value, light cannot get out of the glass; instead, the light bounces back in. When this principle is applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines in the form of light pulses.

There are two basic types of fiber used today and many different types of Fiber Optic Cable. The two types of fiber are called Single-Mode (SM) and Multi-Mode (MM); SM fiber is more expensive but more efficient than MM fiber. Single-Mode fiber is generally used where the distances to be covered are greater. There are generally five elements that make up the construction of a fiber-optic strand, or cable: the optic core, optic cladding, a buffer material, a strength material and the outer jacket. The optic core is the light-carrying element at the center of the optical fiber. It is commonly made from a combination of Silica and Germania. Surrounding the core is the optic cladding made of pure silica. It is this combination that makes the principle of total internal reflection possible. The difference in materials used in the making of the core and the cladding creates an extremely reflective surface at the point in which they interface.

Light pulses entering the fiber core reflect off the core/cladding interface and thus remain within the core as they move down the line. Surrounding the cladding is a buffer material used to help shield the core and cladding from damage. A strength material surrounds the buffer, preventing stretch problems when the fiber cable is being pulled. The outer jacket is added to protect against abrasion, solvents, and other contaminants. Once the light pulses reach their destination they are channeled into the optical receiver. “The basic purpose of an optical receiver is to detect the received light incident on it and to convert it to an electrical signal containing the information impressed on the light at the transmitting end. The electronic information is then ready for input into electronic based communication devices, such as a computer, telephone, or TV.

LAN’s developed as a result to the information explosion that occurred in the late 1980’s. The need for office, laboratory, and factory computers to share information became essential. Since the 1980’s, many media have been developed for use in LAN’s, each meeting a specific user demand. Glass fiber was developed for use in long distance, high bandwidth applications, but the high cost of hardware and installation has discouraged users. Twisted pair, on the other hand, was developed as a low cost alternative medium for use in shorter distance applications. Twisted pair seemed to be an ideal medium for LAN’s, but as computer graphics have become more “graphic intensive”, LAN’s have been required to transmit a much greater volume of information, requiring a media with a much higher bandwidth. Since twisted pair is not capable of supporting the higher signaling rates, there has been an increased interest in developing a low cost fiber solution.