Page 200 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

194 Advanced Topics

considering the Teledesic satellite communications system. This com-
pany proposed to launch satellites, several hundred in all, in low earth
orbit. Telephone conversations could then be relayed to any point on
the globe. The capacity of satellite network was large, in the hundreds
of gigabits per second. However, this is still less than the capacity of a
single optical fiber, which is in the terabit per second regime. You can
imagine that it would be very expensive to launch and maintain all
these satellites. The entire communications network would have the
same capacity as one optical fiber, but real telephone companies work
with millions of fiber optic links. Teledesic was not a financial success.

9.2 Glass
Optical fibers are made from glass. Glass is made from silicon and
oxygen in the form of silicon dioxide (SiO 2 ). Silicon dioxide is sand. It
is far the most plentiful compound in the earth’s crust. Glass is an as-
tonishing material, a true gift from nature to the human race. Some
properties of glass are:

It is chemically inert
It is transparent to light over a broad wavelength range
It can be blown into arbitrary shapes
It can be colored to make beautiful artwork
It can be molded
It holds liquids without leaking
It can hold nuclear waste without leaking
It is stronger than steel
It is an excellent electrical insulator
It is a heat insulator
It is the key element in the SiO /Si MOSFET, which makes the in-
2
tegrated circuit possible
It is the basis of optical fibers, making the telecommunications rev-
olution possible

Clearly, so to speak, glass should be worth much more than gold.
But, on top of everything else, glass is one of the cheapest primary
materials there is. Basically, glass is a miracle.
Glass engineering has a lot to do with introducing desired impuri-
ties into SiO 2 and suppressing unwanted elements. An important un-
wanted impurity is water. There is already oxygen in glass. Hydrogen
can easily diffuse into glass and form water-like complexes of O–H
molecules. Understanding how to keep water out of glass has been an
important part of optical fiber research since 1970.

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