The History of Fiber Optics

Optical communication systems date back two centuries to the "optical telegraph" that French engineer Claude Chappe invented in the 1790s. His system was a series of semaphores mounted on towers, where human operators relayed messages from one tower to the next. It reduced the need in hand-carried messages, but by the mid-19th century it was replaced by the electric telegraph.

Alexander Graham Bell patented an optical telephone system, which he called the Photophone, in 1880, but his earlier invention, the telephone, proved far more practical. He dreamed of sending signals through the air, but the atmosphere didn't transmit light as reliably as wires carried electricity. In the decades that followed, light was used for a few special applications, such as signalling between ships, but otherwise optical communications, like the experimental photophone Bell donated to the Smithsonian Institution, languished on the shelf.

In the intervening years, a new technology slowly took root that would ultimately solve the problem of optical transmission, although it was a long time before it was adapted for communications. It depended on the phenomenon of total internal reflection, which can confine light in a material surrounded by other materials with lower refractive index, such as glass in air. In the 1840s, Swiss physicist Daniel Collodon and French physicist Jacques Babinet showed that light could be guided along jets of water for fountain displays.

Optical fibers went a step further. They were essentially transparent rods of glass or plastic stretched so they were long and flexible. 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. However, the first person known to have demonstrated image transmission through a bundle of optical fibers was Heinrich Lamm, then a medical student in Munich. His goal was to look inside inaccessible parts of the body. During his experiments, he reported transmitting the image of a light bulb.

By 1960, glass-clad fibers fine for medical imaging were made, but they didn’t match communication purposes.

Meanwhile, telecommunications engineers were seeking more transmission bandwidth. Radio and microwave frequencies were in heavy use, so they looked to higher frequencies to carry loads they expected to continue increasing with the growth of television and telephone traffic.

The next step towards optical communications was the invention of laser. The July 22, 1960 issue of Electronics magazine introduced its report on Theodore Maiman's demonstration of the first laser by saying "Usable communications channels in the electromagnetic spectrum may be extended by development of an experimental optical-frequency amplifier." But rain, haze, clouds, and atmospheric turbulence limited the reliability of long-distance atmospheric laser links. Optical wave-guides were proving to be a problem.

Optical fibers had attracted some attention because they were analogous in theory to plastic dielectric wave-guides used in certain microwave applications. In 1961, Elias Snitzer demonstrated the similarity by drawing fibers with cores so small that they carried light in only one wave-guide mode. However virtually everyone considered fibers too lossy for communications.

1964, a critical (and theoretical) specification was identified by Dr. C.K. Kao for long-range communication devices, the 10 or 20 decibels of light loss per kilometer standard. Kao also illustrated the need for a purer form of glass to help reduce light loss.

In 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" 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 problems presented by Dr. Kao.

The first optical telephone communication system was installed about 1.5 miles under downtown Chicago in 1977, and each optical fiber carried the equivalent of 672 voice channels. Today more than 80 percent of the world's long-distance traffic is carried over optical fiber cables. About 25 million kilometers of the cable Maurer, Keck and Schultz designed has been installed worldwide.

 

Fiber Optic Systems

In recent years it has become apparent that fiber optics are steadily replacing copper wire as an appropriate means of communication signal transmission. Fiber optic systems are currently used most extensively as the transmission link between terrestrial hardwired systems. They span the long distances between local phone systems as well as other system users which include cable television services, university campuses, office buildings, industrial plants, and electric utility companies.

Fiber Optic Technology

A fiber-optic system can generally be seen as a system with three main components: a transmitter, a transmission medium and a receiver. As a model it 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. Looking at the three main components in the fiber optic chain will give a better understanding of how the system works in conjunction with wire based systems.

At the head end of the chain is a transmitter. This is a 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 tunneled 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. This 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 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 germanium. 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. In other words the coded light pulse information is translated back into its original state as coded electronic information. The electronic information is then ready for input into electronic based communication devices such as a computer, telephone or TV.

 

 






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