Page 146 - Materials Chemistry, Second Edition
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2.4. The Amorphous State
a
protective plastic jacket
doped SiO core b
2
undoped glass cladding
c
Figure 2.94. Cross-section image of an optical fiber. Shown is the propagation of light waves through an
(a) step-index multimode fiber, (b) graded-index multimode fiber, and (c) single-mode fiber.
fiberglass insulation that may be purchased from home improvement stores. In fact,
insulation represents the leading application for fibrous glass materials.
In contrast to fiberglass, which consists of a disordered array of needle-like fibers,
extremely long, one-dimensional cylindrical glass structures may be carefully fab-
ricated to allow the transmission of light from one end to the other. Although the
diameters of optical fibers are less than a human hair (i.e., 8–10 mm), these fibers are
stronger than steel and are able to withstand being buried underground. The fiber
consists of a core surrounded by a cladding layer (Figure 2.94). The silica core is
doped with other oxides such as B 2 O 3 , GeO 2 , and P 2 O 5 , resulting in a slightly higher
(ca. 0.4%) refractive index than the cladding. The boundary between the core and
cladding may either be abrupt (step-index fiber), or gradual (graded-index fiber).
There are two types of optical fibers: single-mode or multiple-mode, referring
to the simultaneous transmission of single or multiple light rays, respectively.
Single-mode fibers have much smaller diameters than multimode analogues
(Figure 2.94a–c), resulting in a simpler pathway for light through the fiber. Whereas
multimode fibers are used to transmit information short distances (e.g., LAN appli-
cations), single-mode fibers are used for the long-distance transmission of cable
television and telephone signals. It is not hard to see why fiber optics are desirable
for transmission applications. The amount of information transmitted through
0.25 lb of optical fiber would take 33 t of copper wire! Further, optical fibers are
able to transmit signals at lightning speeds – unmatched by any other material. For
example, three half-hour T.V. episodes may be transmitted in 1 s.
In a step-index fiber, wavelengths of light are transmitted through the fiber core by
total internal reflection (Figure 2.94a). Depending on the angle of incidence of the
light, the rays may either be transmitted through the fiber to the detector, or refracted
from the core into the cladding resulting in signal loss. The desired angle
(acceptance angle) required for total internal reflection is determined by the