Page 33 - Optical Communications Essentials

Optical Communication Systems Overview

Optical Communication Systems Overview 23

significant attenuation and distortion. Glass is an obvious material for such
applications. The earliest known glass was made around 2500 B.C., and glass
already was drawn into fibers during the time of the Roman Empire. However,
such glasses have very high losses and are not suitable for communication appli-
cations. One of the first known attempts of using optical fibers for communication
purposes was a demonstration in 1930 by Heinrich Lamm of image transmission
through a short bundle of optical fibers for potential medical imaging. However,
no further work was done beyond the demonstration phase, since the techno-
logy for producing low-loss fibers with good light confinement was not yet
mature.
Further work and experiments on using optical fibers for image transmission
continued, and by 1960 glass-clad fibers had an attenuation of about 1dB/m.
This attenuation allowed fibers to be used for medical imaging, but it was still
much too high for communications, since only 1 percent of the inserted optical
power would emerge from the end of a 20-m-long fiber. Optical fibers attracted
the attention of researchers at that time because they were analogous in theory
to plastic dielectric waveguides used in certain microwave applications. In 1961
Elias Snitzer published a classic theoretical description of single-mode fibers
with implications for information transmission use. However, to be applicable
to communication systems, optical fibers would need to have a loss of no more
than 10 or 20dB/km (a power loss factor of 10 to 100).
In the early 1960s when Charles Kao was at the Standard Telecommunication
Laboratories in England, he pursued the idea of using a clad glass fiber for an
optical waveguide. After he and George Hockman painstakingly examined the
transparency properties of various types of glass, Kao made a prediction in 1966
that losses of no more than 20dB/km were possible in optical fibers. In July
1966, Kao and Hockman presented a detailed analysis for achieving such a loss
level. Kao then went on to actively advocate and promote the prospects of fiber
communications, which generated interest in laboratories around the world to
reduce fiber loss. It took 4 years to reach Kao’s predicted goal of 20dB/km, and
the final solution was different from what many had expected.
To understand the process of making a fiber, consider the schematic of a typ-
ical fiber structure, shown in Fig. 2.1. A fiber consists of a solid glass cylinder
called the core. This is surrounded by a dielectric cladding, which has a different
material property from that of the core in order to achieve light guiding in the
fiber. Surrounding these two layers is a polymer buffer coating that protects the

Figure 2.1. A typical fiber structure consists of a core,
a cladding, and a buffer coating.

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