Fiber Optics

Fiber Optics, the technique of transmitting light through transparent, flexible fibers of glass or plastic. The fibers, called optical fibers, can channel light over a curved path. Bundles of parallel fibers can be used to illuminate and observe hard-to-reach places. Optical fibers of very pure glass are able to carry light over long distances ranging from a few inches or centimeters to more than 100 miles (160 km) with little dimming. Cables containing such fibers are used in certain types of communications systems. Some individual fibers are thinner than human hair and measure less than 0.00015 inch (0.004 mm) in diameter.

Fiber optics is based on the optical phenomenon known as total internal reflection. With the simplest form of optical fiber, light entering one end of the fiber strikes the boundary of the fiber and is reflected inward. The light travels through the fiber in a succession of zigzag reflections until it exits from the other end of the fiber. Other forms of optical fibers are designed in such a way that the zigzagging of the light is greatly reduced or virtually eliminated.

Most optical fibers made today consist of at least two parts: a core through which the light is transmitted and a protective cladding (either glass or plastic) that surrounds the core and helps prevent light from leaking from the core. The cladding bends or reflects inward the light rays that strike its inside surface. A detector, such as a photosensitive device or the human eye, receives the light at the other end of the fiber.

Optical fiber bundles are either coherent or incoherent. In a coherent bundle, the fibers are arranged so that images, as well as illumination, can be transmitted. In incoherent bundles, the fibers are not arranged in any particular way and can transmit only illumination. There are two basic types of optical fibers: single-mode fibers and multi-mode fibers. Single mode fibers are designed for the transmission of a single ray as a carrier and is used for high-speed signal transmission over long distances. They have much smaller cores than multi-mode fibers, and they accept light only along the axis of the fibers. Tiny lasers send light directly into the fiber.

Low-loss connectors may be used to join fibers within the system without reducing the light signal. Such connectors also join fibers to the detector. Multi-mode fibers are designed to carry multiple light rays. They have much larger core diameter compared to those of single-mode fibers, and they accept light from a variety of angles. Multi-mode fibers use more types of light sources and cheaper connectors than single-mode fibers. They are mostly used for communication over shorter distances.

The uses of optical fibers are numerous. In medicine, optical fibers enable physicians to look and work inside the body through tiny incisions without having to perform surgery. They are used for endoscopesinstruments for viewing the interior of hollow organs in the body. Most endoscopes have two sets of fibers: an outer ring of incoherent fibers that supplies the light, and an inner coherent bundle that transmits the image. Endoscopes may be designed to look into specific areas. For example, physicians use an arthroscope to examine knees, shoulders, and other joints. In some models, a third set of fibers transmits a laser beam that is used to stop bleeding or to burn away diseased tissue. Body temperatures can be measured using optical fiber. They can also be used for insertion into blood vessels to give a quick, accurate analysis of blood chemistry.

In scientific research and in manufacturing, fiber optic devices carry light to and from hazardous areas, vacuum chambers, and confined spaces within machines. Some instruments use optical-fiber coils as a sensing device; changes in the fiber produced by changes in pressure, temperature, or some other condition cause a measurable change in the properties of the light transmitted through the fiber. Optical fibers are used to measure temperature, pressure, acceleration, and voltage in industries.

Fiber-optic communication systems have a number of advantages that make them more efficient than systems that use traditional copper cables. They have a much larger information-carrying capacity, are not bothered by electrical interference, and require fewer amplifiers than copper-cable systems. As part of a communications system, an optical fiber transmits information in the form of light signals usually as flashes of light. The signals are generated by a small semiconductor laser or light-emitting diode (LED) at one end of the fiber and detected by a light-sensitive device at the other end. An optical-fiber cable can transmit much more information than an electrical cable of the same size. A major application of optical-fiber cable is in linking telephone switching offices. Many communication companies have installed large networks of fiber-optic cables across the continents and under the oceans to provide information worldwide.

The first studies of fiber optics were made in the late 1800s, but practical development did not begin until the early 1950s. The development of fiber optics was spurred by the introduction of lasers in the early 1960s and by the production of the first optical fibers of very pure glass in 1970. The commercial use of fiber optics, especially in communications systems, developed rapidly in the 1980s.