Page 199 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

Optical Fibers and Optical FIber Amplifiers 193

pulses/sec, hardly the stuff of high-bandwidth telecommunications.
The telephone companies were still focused on communications via
copper cables.
In 1970, almost all telephone conversations were carried by elec-
trons moving in wires. For high-speed transmission, coaxial cable was
used. There were some point-to-point radio links to relay signals over
long distances, but only wire cable was capable of going around cor-
ners, passing through ducts, and connecting people living or working
in buildings with their counterparts around the world. The difficulty
with this technology is fundamentally related to electrons. Electrons
have mass, and they become harder to move as the frequency increas-
es. Eventually, only the outer skin of a wire can carry the current, and
the resistance of the wire is much higher than it was at lower fre-
quencies. Resistance means loss, and loss means that the signal can-
not travel as far.
Photons, on the other hand, have no mass. There is no analogous
loss mechanism for photons when the frequency is increased. Optical
fibers are ideally adapted to carry very high bandwidth communica-
tions, right up to the frequency of the light beam itself. Coaxial elec-
tric cable can be used to transmit electrical signals at high frequency.
However, “high” means perhaps 1 GHz for distances of a few meters.
Optical fibers can carry signals with three more orders of bandwidth,
in the terahertz regime, over distances of hundreds of kilometers. An
easy way to appreciate the limits of coaxial cable is to look around
your neighborhood for the boxes where the cable TV vendor has to in-
stall amplifiers to boost the TV signals, which are sent at approxi-
mately 10 MHz. There are lots of these boxes, because the signals
have to be amplified every few hundred meters. Basically, transmis-
sion of a modulated electrical current becomes more and more diffi-
cult as the frequency of modulation goes up. On the other hand, send-
ing more information means going to higher frequencies. Using
electrons to accomplish this is a losing battle. Transmission of optical-
ly modulated signals does not have this problem. The introduction of
optical fiber communications changed the rules (see Fig. 1.1). This is
what we call a “killer technology.” Since 1980, telephone companies
around the world have been mining copper as they pull thousands of
kilometers of copper cable out of the ground in order to replace it with
optical fiber.
Two components of optical fibers that distinguish this technology
from the other options are the ability to carry very high bandwidth
communications and the ability to confine the communication in a fiber
cable so that lines can be installed in buildings or passed under the
ocean. This latter feature is what distinguishes optical fiber communi-
cations from radio communications. A good comparison can be made by

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.