SUMMARY

Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas you have learned. You should have a thorough understanding of these principles before advancing to chapter 2.

FIBER OPTICS is the branch of optical technology concerned with the transmission of radiant power (light energy) through fibers.

A FIBER OPTIC DATA LINK has three basic functions: to convert an electrical input signal to an optical signal, to send the optical signal over an optical fiber, and to convert the optical signal back to an electrical signal. It consists of three parts: transmitter, optical fiber, and receiver.

The TRANSMITTER consists of two parts, an interface circuit and a source drive circuit. The transmitter converts the electrical input signal to an optical signal by varying the current flow through the light source.

The RECEIVER consists of two parts, the optical detector and signal conditioning circuits. The receiver converts the optical signal exiting the fiber back into the original form of the electrical input signal.

SCATTERING, ABSORPTION, and DISPERSION MECHANISMS in the fiber waveguides cause the optical signal launched into the fiber to become weakened and distorted.

NOISE is any disturbance that obscures or reduces the quality of the signal.

SIGNAL LOSS is the decrease in the amount of light reaching the end of the fiber. Impurities in the fiber material cause the signal loss in optical fibers. By removing these impurities, construction of low-loss optical fibers was possible.

The TWO BASIC TYPES OF OPTICAL FIBERS are multimode fibers and single mode fibers.

A LOW-LOSS MULTIMODE OPTICAL FIBER was developed in 1970.

A SMALL AREA LIGHT-EMITTING DIODE (LED) was developed in 1971. This light source was suitable for low-loss coupling to optical fibers.

FIBER OPTIC SYSTEM DESIGN has centered on long-haul communications and the subscriber-loop plant. Limited work has also been done on short-distance applications and some military systems.

FIBER PERFORMANCE depends on the amount of loss and signal distortion introduced by the fiber when it is operating at a specific wavelength. Single mode fibers tend to have lower loss and produce less distortion than multimode fibers.

The LARGER FIBER CORE and the <emphasis type="b.GIF">HIGHER NUMERICAL APERTURE (NA)</emphasis> of multimode fibers reduce the amount of loss at fiber connections.

In MILITARY and SUBSCRIBER-LOOP APPLICATIONS, system designers consider trade-offs in the following areas: fiber properties, types of connections, optical sources, and detector types.

The ADVANTAGES of fiber optic systems include improved system performance, immunity to electrical noise, signal security, and electrical isolation. Advantages also include reduced size and weight, environmental protection, and overall system economy.

The DISADVANTAGES of fiber optic systems include problems with the relative newness of the technology, the relatively expensive cost, and the lack of component and system standardization. However, these disadvantages are already being eliminated because of increased use and acceptance of fiber optic technology.