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

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Revised Pages

784 • Chapter 19 / Optical Properties

Cladding

Coating

Core

Figure 19.20 Schematic cross section of an optical fiber.

core, whereas the surrounding cladding constrains the light rays to travel within the

core; the outer coating protects core and cladding from damage that might result

from abrasion and external pressures.

High-purity silica glass is used as the fiber material; fiber diameters normally

range between about 5 and 100μm. The fibers are relatively flaw free and, thus,

remarkably strong; during production the continuous fibers are tested to ensure that

they meet minimum strength standards.

Containment of the light to within the fiber core is made possible by total internal

reflection; that is, any light rays traveling at oblique angles to the fiber axis are

reflected back into the core. Internal reflection is accomplished by varying the index

of refraction of the core and cladding glass materials. In this regard, two design types

are employed. With one type (termed “step-index”), the index of refraction of the

cladding is slightly lower than that of the core. The index profile and the manner of

internal reflection are shown in Figures 19.21band 19.21d. For this design, the output

pulse will be broader than the input one (Figures 19.21cande), a phenomenon that is

undesirable since it limits the rate of transmission. Pulse broadening results because

various light rays, although being injected at approximately the same instant, arrive

at the output at different times; they traverse different trajectories and, thus, have a

variety of path lengths.

Pulse broadening is largely avoided by utilization of the other or “graded-index”

design. Here, impurities such as boron oxide (B 2 O 3 ) or germanium dioxide (GeO 2 )

are added to the silica glass such that the index of refraction is made to vary parabol-

ically across the cross section (Figure 19.22b). Thus, the velocity of light within the

core varies with radial position, being greater at the periphery than at the cen-

ter. Consequently, light rays that traverse longer path lengths through the outer

Ti m e

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

Ti m e

(e)

Index of

refraction

Output

impulse

Input

impulse

Cladding

Core

Radial position Intensity Intensity

Figure 19.21 Step-index optical fiber design. (a) Fiber cross section. (b) Fiber radial index

of refraction profile. (c) Input light pulse. (d) Internal reflection of light rays. (e) Output light

pulse. (Adapted from S. R. Nagel,IEEE Communications Magazine,Vol. 25, No. 4, p. 34,

1987.)