Biomedical Engineering Reference
In-Depth Information
22 Gauge Twisted-Pair
Transmission Line
11
10
9
8
0.375 in. Coaxial Cable
7
6
5
4
3
2
Low-Loss Optical Fiber
1
0
10 kHz
100 kHz
1 MHz
10 MHz
100 MHz
FREQUENCY
Figure 6.2-11 Signal attenuation vs frequency for different transmission media.
transmission will be given. These advantages include
(1) low loss and high bandwidth; (2) small size and
bending radius; (3) nonconductive, nonradiative, and
noninductive; (4) light weight; and (5) providing natural
growth capability.
To appreciate the low- and high-bandwidth capabil-
ities of optical fibers, consider the curves of signal at-
tenuation vs frequency for three different transmission
media shown in
Figure 6.2-11
. Optical fibers have a ''flat''
transfer function well beyond 100 MHz. Compared with
wire pairs or coaxial cables, optical fibers have far less loss
for signal frequencies above a few megahertz. This is an
important characteristic that strongly influences system
economics, because it allows the system designer to in-
crease the distance between regenerators (amplifiers) in
a communication system.
The small size, small bending radius (a few centime-
ters), and light weight of optical fibers and cables are very
important where space is at a premium, such as in air-
craft, on ships, and in crowded ducts under city streets.
Because optical fibers are dielectric waveguides, they
avoid many noise problems such as radiated interference,
ground loops, and, when installed in a cable without
metal, lightning-induced damage that exists in other
transmission media.
Finally, the engineer using optical fibers has a great
deal of flexibility. He or she can install an optical fiber
cable and use it initially in a low-capacity (low-bit-rate)
system. As the system needs to grow, the engineer can
take advantage of the broadband capabilities of optical
fibers and convert to a high-capacity (high-bit-rate)
system by simply changing the terminal electronics.
The proper design and operation of an optical com-
munication system using optical fibers as the trans-
mission medium require a knowledge of the transmission
characteristics of the optical sources, fibers, and in-
terconnection devices (connectors, couplers, and splices)
used to join lengths of fibers together. The transmission
criteria that affect the choice of the fiber type used in
a system are signal attenuation, information transmission
capacity (bandwidth), and source coupling and inter-
connection efficiency. Signal attenuation is due to
a number of loss mechanisms within the fiber, as shown
in
Table 6.2-1
, and due to the losses occurring in splices
and connectors. The information transmission capacity of
a fiber is limited by dispersion, a phenomenon that causes
light that is originally concentrated into a short pulse to
spread out into a broader pulse as it travels along an op-
tical fiber. Source and interconnection efficiency depends
on the fiber's core diameter and its numerical aperture,
a measure of the angle over which light is accepted in
the fiber. Absorption and scattering of light traveling
Table 6.2-1 Loss mechanisms in optical fibers
1.
Intrinsic material absorption loss
(a) Ultraviolet absorption tail
(b) Infrared absorption tail
2. Absorption loss due to impurity ions
3. Rayleigh scattering loss
4. Waveguide scattering loss
5. Microbending loss
