Fiber Optics

Optical fibers have a highly transparent core of glass or plastic surrounded by a covering called a cladding. Light impulses from a laser, a light bulb, or some other source enter one end of the optical fiber. As light travels through the core, it is typically kept inside it by the cladding. The cladding is designed to bend or reflect--inward--light rays that strike its inside surface. At the other end of the fiber, a detector, such as a photosensitive device or the human eye, receives the light.

Kinds of optical fibers. The two basic kinds of optical fibers are single-mode fibers and multi-mode fibers. Single-mode fibers are used for long-distance transmissions. They have extremely small cores, and they accept light only along the axis of the fibers. As a result, single-mode fibers require the use of laser light because it travels in a narrow beam in only one direction. Single-mode fibers also need to be precisely connected to the laser, to other fibers in the system, and to the detector. Multi-mode fibers have cores larger than those of single-mode fibers, and they accept light from a variety of angles. Multi-mode fibers can use more types of light sources and cheaper connectors than can single-mode fibers, but they cannot be used over long distances.

Uses of optical fibers. Optical fibers have a number of uses. Various industries use optical fibers to measure temperature, pressure, acceleration, and voltage. In fiber-optic communication systems, lasers transmit coded messages by flashing on and off at high speeds. The messages travel through optical fibers to interpreting devices that decode the messages, converting them back into the form of the original signal. Fiber-optic communication systems have a number of features that make them superior to systems that use traditional copper cables. For example, they have a much larger information-carrying capacity and are not subject to electrical interference. In addition, signals sent over long-distance fiber-optic cables need less amplification than do signals sent over copper cables of equal length. Many telephone and other communication companies have installed large networks of fiber-optic cables. Underwater fiber-optic cables carry signals across the Atlantic and Pacific oceans.

Optical fibers are well-suited for medical use. They can be made in extremely thin, flexible strands for insertion into the blood vessels, lungs, and other hollow parts of the body. Optical fibers are used in a number of techniques that enable physicians to look and work inside the body through tiny incisions.

Instruments called endoscopes carry two bundles of optical fibers inside a long tube. One bundle directs light at the tissue being examined. The other bundle transmits light reflected from the tissue, producing a detailed image of the tissue. Endoscopes may be designed to look into specific areas. For example, physicians use an arthroscope to examine knees, shoulders, and other joints. A slender angioscope can pass through arteries that carry blood to the heart.

Optical fibers also can be used to measure such properties as pressure and temperature in specific areas. They can be inserted into blood vessels to give a quick, accurate analysis of blood chemistry. Another use is to direct intense laser light that stops bleeding or burns away abnormal tissue.

Contributor: Nathan M. Denkin, Ph.D., Distinguished Member of Technical Staff, AT&T Bell Laboratories.

See also Laser; Telephone.

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