Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

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Revised Pages

19.14 Optical Fibers in Communications • 783

Input

signal

Output

Encoder signal

Electrical/

Optical

Converter

Optical/

Electrical

Converter

Repeater

Fiber optic cable

Decoder

Figure 19.18

Schematic diagram

showing the

components of an

optical fiber

communications

system.

optics. With regard to speed, optical fibers can transmit, in one second, informa-

tion equivalent to three episodes of your favorite television program. Or relative to

information density, two small optical fibers can transmit the equivalent of 24,000

telephone calls simultaneously. Furthermore, it would require 30,000 kg (33 tons) of

copper to transmit the same amount of information as only 0.1 kg (^14 lbm) of optical

fiber material.

The present treatment will center on the characteristics of optical fibers; however,

it is thought worthwhile to first briefly discuss the components and operation of the

transmission system. A schematic diagram showing these components is presented

in Figure 19.18. The information (i.e., telephone conversation) in electronic form

must first be digitized into bits, that is, 1’s and 0’s; this is accomplished in the encoder.

It is next necessary to convert this electrical signal into an optical (photonic) one,

which takes place in the electrical-to-optical converter (Figure 19.18). This converter

is normally a semiconductor laser, as described in the previous section, which emits

monochromatic and coherent light. The wavelength normally lies between 0.78 and

1.6μm, which is in the infrared region of the electromagnetic spectrum; absorption

losses are low within this range of wavelengths. The output from this laser converter is

in the form of pulses of light; a binary 1 is represented by a high-power pulse (Figure

19.19a), whereas a 0 corresponds to a low-power pulse (or the absence of one),

Figure 19.19b. These photonic pulse signals are then fed into and carried through

the fiber-optical cable (sometimes called a “waveguide”) to the receiving end. For

long transmissions, repeaters may be required; these are devices that amplify and

regenerate the signal. Finally, at the receiving end the photonic signal is reconverted

to an electronic one, and is then decoded (undigitized).

The heart of this communication system is the optical fiber. It must guide these

light pulses over long distances without significant signal power loss (i.e., attenuation)

and pulse distortion. Fiber components are the core, cladding, and coating; these are

represented in the cross-section profile, Figure 19.20. The signal passes through the

Intensity

Time

(a)

Intensity

Ti m e

(b)

Figure 19.19 Digital

encoding scheme for

optical communications.

(a) A high-power pulse

of photons corresponds

to a “one” in the binary

format. (b) A low-

power photon pulse

represents a “zero.”