FIGURE 4 Types of optical fibers.

United States and Sweden. The commercial breakthrough came in 1984. By 1988 the Atlantic cable will be in operation. For the 1990s we expect a breakthrough in subscriber networks, the so-called local area networks (LAN), especially for high-definition television (HDTV) and data communications.

The technical development of optical fiber includes the step-index, the graded-index, and the single-mode fibers (see Figure 4). For advanced applications and broad bandwidth networks, the single-mode fiber will be used because of its remarkable capacity to transport narrow signals.

Dramatic achievements have been reached in the practical use of fiber-optic transmission systems. We will soon have the ability to carry a million or more telephone calls on one fiber over distances of more than 1,000 kilometers without repeaters (see Figure 5). By the end of this century, a fiber pair will probably handle 100–1,000 channels, each with a bandwidth of 1 gigabit per second (Gbit/s). This can be contrasted with coaxial cable transmission technology, which permits a maximum of 10,000 telephone calls per pair, with a maximum distance of 4 kilometers between repeaters.

To build a fully optical network, it is necessary to be able to switch light. One technology is the electro-optical directional coupler (EDC), with which light can be switched in frequencies of many gigabits per second. This coupler is made of lithium niobate (LiNbO3), in which certain optical properties, such as the refractive index, change as a function of a magnetic field induced into the material.

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