Callister - An introduction - 8th edition

864 • Chapter 21 / Optical Properties
Coating
Figure 21.20
Schematic cross section of an optical fiber.
Cladding
Core
These photonic pulse signals are then fed into and carried through the fiber-optic
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 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 shown in Figure 21.20. The signal
passes through the 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 100 m. 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 21.21b and 21.21d. For this design,
the output pulse will be broader than the input one (Figures 21.21c and e), a
Input
impulse
Output
impulse
Cladding
Core
Radial position
Intensity
Intensity
Index of
refraction
Time
Time
(a)
(b)
(c)
Figure 21.21 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.)
(d)
(e)