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

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Revised Pages

Summary • 785

Intensity

Ti m e

(a) (b) (c) (d)

Intensity

Ti m e

(e)

Cladding

Core

Radial position

Index of

refraction

Output

impulse

Input

impulse

Figure 19.22 Graded-index optical fiber design. (a) Fiber cross section. (b) Fiber radial

index of refraction profile. (c) Input light pulse. (d) Internal reflection of a light ray.

(e) Output light pulse. (Adapted from S. R. Nagel,IEEE Communications Magazine,

Vol. 25, No. 4, p. 34, 1987.)

periphery of the core travel faster in this lower-index material, and arrive at the out-

put at approximately the same time as undeviated rays that pass through the center

portion of the core.

Exceptionally pure and high-quality fibers are fabricated using advanced and

sophisticated processing techniques, which will not be discussed here. Impurities

and other defects that absorb, scatter, and thus attenuate the light beam must be

eliminated. The presence of copper, iron, and vanadium is especially detrimental;

their concentrations are reduced to on the order of several parts per billion. Like-

wise, water and hydroxyl contaminant contents are extremely low. Uniformity of

fiber cross-sectional dimensions and core roundness are critical; tolerances of these

parameters to within a micrometer over 1 km (0.6 mile) of length are possible. In

addition, bubbles within the glass and surface defects have been virtually eliminated.

The attenuation of light in this glass material is imperceptibly small. For example,

the power loss through a 16-kilometer (10-mile) thickness of optical fiber glass is

equivalent to the power loss through a 25-millimeter (1-inch) thickness of ordinary

window glass!

SUMMARY

Electromagnetic Radiation

Light Interactions with Solids

The optical behavior of a solid material is a function of its interactions with elec-

tromagnetic radiation having wavelengths within the visible region of the spectrum.

Possible interactive phenomena include refraction, reflection, absorption, and trans-

mission of incident light.

Optical Properties of Metals

Metals appear opaque as a result of the absorption and then reemission of light

radiation within a thin outer surface layer. Absorption occurs via the excitation of

electrons from occupied energy states to unoccupied ones above the Fermi energy

level. Reemission takes place by decay electron transitions in the reverse direction.

The perceived color of a metal is determined by the spectral composition of the

reflected light.