wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

2-8 Industrial Communication Systems
TABLE 2.2
Comparison of the Generations of Optical Communication
Serial #
Parameter
First
Generation
Second
Generation
Third
Generation
Fourth
Generation
1 Deploying year 1974 1978 1982 1994
2 Operating
Wavelength
3 Attenuation 20.dB/km
reduced to
5.dB/km
4 Max. Bandwidth for
Communication
820.nm 1,330.nm 1,550.nm 1,550.nm
0.5.dB/km 0.25.dB/km 0.15.dB/km
2.Mbps 140.Mbps 1.Gbps 10.Gbps
expandable to
50.Tbps
5 Repeater Distance 8–10.km 10.km 50.km 100.km EDFA
(erbium doped
fiber amplifier)
2.3.2 types and Media Access
Total internal reflection of rays is a first approximation of light guiding in fibers. Core-cladding structure
and material composition are the key factors in determining fiber properties. Optical fibers are
classified into single and multimode fibers. Furthermore, the fibers are categorized in terms of their
refractive index profile. The most important types of fibers are single-mode (step index), multimode step
index, and multimode graded index.
The thickness of a single-mode fiber today is approximately 8.3–10.μm at the center. The outer cladding
is approximately 125.μm thick. The single-mode fiber is the focus of most of the activities today.
Multimode fiber has given way to single-mode fiber throughout the public carrier networks. Thus,
single-mode fiber has an advantage over all other types since it can handle multiple signals without
any attenuation for long distances. A single-mode step index fiber between the two ends enables the
developers to speed up the input because there is no concern about varying path lengths. If the glass
could be made very thin and very pure in the center, the light would have no choice but to follow the
same path every time.
Multimode fiber, as the name implies, has multiple paths where the light can reach the end of the fiber.
In multimode step index fiber, the core has a uniform index with a sharp change at the boundary of the
cladding. The thickness of the glass is crucial to the passage of the light and the path used to propagate
from one end to the other. This fiber is the thickest form of fiber, having a core of 120–400.μm thick. The
light is both refracted and reflected inside the encased fiber. Light beams in the fiber propagate by using
paths of different lengths causing different delays. These fibers are less in practice because of low carrying
capacity. They have a relatively wide core, but since they are not graded, the light bounces widely
through the fiber exhibiting high levels of modal dispersion.
In a multimode graded-index fiber, the core index is not uniform; it is highest at the center and
decreases until it matches the cladding. By using a grading of the density of the glass from the center
core out, the capacity of the fiber can be increased. The path in the outer part of the glass is longer than
the path in the center of the glass. This means that light arrives at different times because the path
lengths are different. Grading the center core to have a higher level of refraction and the outer parts
of the glass to be thinner (and thus less refractive) can exploit the characteristics of the glass to get
approximately the same length of a wave on the cable and therefore increase the speed of throughput.
The better the grading of the index, the more throughput one can expect. Currently, the two forms
of graded-index fiber either a 62.5 or a 50.μm center core with a 125.μm outer cladding of glass are
being used.
© 2011 by Taylor and Francis Group, LLC