Page 53 - Optical Switching And Networking Handbook

03_200023_CH02/Batesx 1/17/01 8:17 AM Page 38

38 Chapter 2

Fiber is produced using silicon dioxide (glass). Silica is good for the
lower end of the operating frequencies, in the range of 800 to 1,100
nanometers. Other forms of fiber use a germanium dioxide mixture
in the glass. This fiber type operates better in the range of 1,300 to
1,500 nanometers. Thus one can extrapolate that the quality of the
various glasses is different and that by using different glass compo-
sitions, better results can be achieved. The dopants added to the
glass mix create higher speeds and less loss.

Fiber Cable Types

The differences between the fibers come down to multimode and sin-
gle-mode arrangement. However, stepped-index cables use a differ-
ent form of composition to create a fiber cable. The three forms of
cables are

1. Multimode stepped index
2. Multimode graded index
3. Single mode

Multimode Stepped Index Fiber When fiber was first being
used in the communications area, the technology was still develop-
ing. Multimode fiber was used as a means of carrying communica-
tions signals on glass more reliably. Multimode fiber, as the name
implies, has multiple paths by which the light can reach the end of
the fiber. Single-mode fiber, on the other hand, has only one path
over which the light can travel to get to the other end. Using a large
piece (chunk) of pure glass, the developers extruded the fiber into
much thinner glass strands.
Multimode fibers are thicker glass by today’s standards. These
multimode fibers were developed in two different types, step index
and graded index, both operating differently. Figure 2-6 illustrates a
multimode fiber using a step index mode. In this case, the glass is
very thick (by today’s standards) at the center core (approximately
120 to 400 microns ). The thickness of the glass is crucial to the pas-
1

1 A micron or micrometer is one-millionth of a meter.