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optical fiber bundle
Optical Fiber

Description

Optical fiber ("fiber optics") has revolutionized communications and remote sensing, and yet, it is essentially what the name describes, a thread or fiber of glass. A high-quality fiber can be a kilometer or more in length, and it transmits light from one end to the other.

An optical fiber works on the nature of light to bend (refract) when it travels from water to air, for example. Light travels at different speeds in different media, and it changes direction when going from one environment to another. From water to air, for example, a beam of light bends away from vertical, and at a shallow enough angle, it can't bend far enough. In other words, the light simply can't escape. A person immersed in a swimming pool sees this phenomenon. Viewed at a shallow angle, the underside of the surface is reflective, not transparent.

An optical fiber uses this principle. A beam of light traveling down the glass fiber is totally reflected off the sides and remains inside. The original fibers relied on the difference between glass and air, but in modern designs the core of a fiber is encased in a second kind of glass for the same effect. The core is a dense, highly-refractive glass, and the outer layer is a less dense material.

Optical fiber end sliceFor practical reasons, a modern optical fiber is also encased in layers of protective resins, polymers, or other materials, making its design more complex. Additionally, the two ends require sophisticated connectors where the signal enters and leaves. The end connectors may include light-emitting diodes or may simply be polished to mirror finishes for joining other optical equipment.

History

By the 17th century, a number of Arab and European scientists had investigated the behavior of light. In 1621, Dutch astronomer Willebrord Snell distilled their discoveries into a simple mathematical law of refraction, now called Snell's Law. One consequence of this law is the total reflection described above. In the 1840s, Jean Colladon (swiss) used this effect to guide sunlight through a stream of water, and Jacques Babinet (french) reflected candlelight through a glass rod.

In the early 20th century, glass fibers were starting to be used to illuminate small mechanisms, dental work, and other tight quarters. This evolved into using bundles of glass tubes to form images of inaccessible locations, such as with the gastroscope inspecting a patient's stomache. Glass fibers would also make possible a flexible periscope. These implements were typically made of bare glass fibers until the 1950s when the core fibers were clad in a suitable second glass material as described above. This cladding prevented scratches, fibers touching fibers, and other problems.

A parallel use of light tubes for communication may begin with Alexander Graham Bell. In 1880, he and fellow inventor Charles Tainter invented a device to transmit voice over an optical beam in air. Though this was possible, it was not commercially practical. Not until the 1960s were fiber-optic communications devices developed and patented. The 1960s were also the era of the laser and of purer glass, both crucial for very long distance optical cables.

In the modern era, we see widespread use of fiber optics across continents and throughout major cities. Engineering and manufacturing advances rather than new discoveries in basic science have made this possible.

Production

optical fibersOptical fibers are made of pure silicon dioxide (quartz) doped carefully with impurities to slightly alter the properties. A block of quarts is drawn into perhaps 40 kilometers of fiber by a powerful machine pulling at a speed of 100 kilometers per hour. Bundles of fibers are clad successively during production in a second kind of glass, then a third, dark, glass to suppress light leakage, then coatings for strength. The assemblage is then encased in a strong outer sheath.

A key weakness of optical fibers, literal and for performance, is bending. Bending can cause light leakage and fracturing, so most fiber cables must be laid straight. Coatings within a fiber cable strengthen the glass against bending. Special bendable fiber cables are available for use in the tight quarters of offices and homes.

The ends of a cable's fibers will be cut and polished or else fitted to connectors, depending on the intended use. An optical fiber cable is either intended to join another optical system or it connects to a normal electrical data system. The latter use requires some kind of optical-electrical connector.

Uses

The most common current use of optical fibers, as alluded to, is in data communications. A fiber-optic system can transmit data at rates of 10-100 gigabits per second, much faster than copper wire.

Fiber-optic lampOptical fibers transmitting light are used in medicine, for exploring the human body. Optical fibers direct light to tight or remote quarters. This can be decorative (right).

Optical fibers function as other kinds of sensors as well. Changes in the temperature, strain, or pressure of one end of the fiber will affect the light transmission measured at the other end. Therefore, a properly designed optical fiber can function as a remote thermometer.

Other Considerations

Unlike so much other technology, optical fibers do not use electricity. This is an advantage in environments where a spark would be dangerous, such as inside a fuel tank. It also means fiber-optic sensors can function around high-voltage electrical transformers and near other intense electrical fields.

In recent years, copper wire in public spaces has been subject to theft for resale. Fibers are only glass and do not offer that temptation.

For high-speed and high-volume communications, optical fiber is vastly superior to copper wire. Yet, only 3% of the US population was connected by fiber at the end of 2013, and countries in Western Europe were in the range of 5-10% (data from the OECD). The large disincentive to laying optical fiber is the expense, particularly when traditional telephone and data lines already exist. In the long run, the new cables will be laid, but expansion will likely be gradual and irregular.

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