Biomedical Engineering Reference
In-Depth Information
125
m
140
μ
m
μ
125
μ
m
62.5
μ
m
100
μ
m
9
μ
m
n
1
≈
1.470
n
2
≈
n
1
(1-
Δ
)
n
1
≈
1.470
n
1
≈
1.460
n
2
≈
n
1
(1-
Δ
)
n
2
≈
n
1
(1-
Δ
)
Δ ≈
Δ ≈
0.002
n
1
= 1.0
0.01
n
1
=1.0
Δ ≈
0.01
n
1
= 1.0
Refractive
Index
Refractive
Index
Refractive
Index
r
r
r
Multimode Fiber
Step Index
Single Mode
Fiber
Graded Index
Multimode Fiber
Figure 6.2-9 Dimensions and refractive indexes for commonly used optical fibers.
refractive index
n
2
that is slightly lower than the re-
fractive index of the core region. The cladding of the fiber
is made of a high-silica-content glass or a multicompo-
nent glass.
Figure 6.2-9
shows the dimensions and re-
fractive indexes of commonly used telecommunication
fibers.
Figure 6.2-10
enumerates some of the advantages,
constraints, and applications of the different types of
fibers. In general, when the transmission medium must
have a very high bandwidth
such as short data links in which lower bandwidth re-
quirements are placed on the transmission medium,
either a graded-index or a step-index multimode fiber
can be used.
Because of their low-loss and high-bandwidth capa-
bilities, optical fibers have the potential for being used
wherever twisted-wire pairs or coaxial cables are used as
the transmission medium in a communication system. If
an engineer were interested in choosing a transmission
medium for a given transmission objective, he or she
would tabulate the required and desired features of al-
ternative technologies that may be available for use in the
applications. With that process in mind, a summary of
the attractive features and the advantages of optical fiber
for example, in an undersea
or long-distance terrestrial system
d
a single-mode fiber is
used. For intermediate system bandwidth requirements
between 150 MHz-km and 2 GHz-km, such as found in
local-area networks, either a single-mode or graded-index
multimode fiber would be the choice. For applications
d
Single-Mode
Fiber
Graded-Index
Multimode Fiber
Step-Index
Multimode Fiber
Plastic Coating
Core
Cladding
Source
Laser Preferred
Laser or LED
Laser or LED
Very very large
>2 GHz-km
Very Large
150 MHz to 2 GHz-km
Large
<200 MHz-km
Bandwidth
Submarine cable
system
Example of
application
LANS
Data Links
Figure 6.2-10 Advantages, constraints, and applications of optical fibers.
