City of Light: The Story of Fiber Optics

148 CITY OF LIGHT
transistor electronics were pushing vacuum tubes into the grave of obsolescence.
Semiconductor lasers generated light easily, with the intensity controlled
by the electrical currents passing through them. Tiny as grains of salt,
they matched the size of optical fibers, but the potential for compact, solidstate
laser transmitters attracted even the developers of hollow optical waveguides.
Unfortunately, early semiconductor lasers were as useless for practical
communications as the optical fibers of the early 1960s. The lasers operated
only at the cryogenic temperature of liquid nitrogen, �321�F or�196�C.
They burned out quickly and unpredictably. Worse yet, the warmer the laser
got, the more current you needed to make it emit light, and the higher the
current, the faster the laser burned out. It didn’t look good, and after an early
burst of energy, progress stalled in the mid-1960s. Developers needed better
recipes for their grains of salt.
A Curious Class of Materials
Both the allure and the problems of semiconductor lasers came from the
nature of semiconductors. From an electrical standpoint, materials fall into
three classes—conductors, semiconductors, and insulators. The essential difference
among them is the amount of energy needed to free an electron from
the bonds that link atoms in the material. In conductors, notably metals, very
little energy is needed, so electrons flow freely through copper wires. In glass
and other insulators, the electrons are bound so tightly between atoms that
essentially none of them have enough energy to escape.
Semiconductors fall between the two extremes, because a few electrons do
escape from atomic bonds to conduct electricity in the crystal. The bonded
electrons occupy a ‘‘valence band,’’ where they form bonds between atoms
in the crystal. Electrons that get enough energy to escape those atomic bonds
fall into another energy state called the ‘‘conduction band.’’ No energy states
exist between the valence and conduction bands, so electrons have to get
enough energy to jump this ‘‘band gap’’ before they can carry a current.
In a pure semiconductor, only a few electrons have enough energy to
reach the conduction band. Those that escape the valence band leave behind
vacancies called ‘‘holes,’’ which effectively have a positive charge equal to
the negative charge of the electron. Other valence-band electrons can move
to fill the hole, leaving another hole behind, so essentially the holes move as
well as the electrons in the conduction band. For practical purposes, you can
think of holes as positive charge carriers and electrons as negative charge
carriers, although it really isn’t that simple.
One way that engineers can adjust the number of electrons and holes in
a semiconductor is to add impurities to replace some atoms in the crystal
lattice. If the impurity has one more outer electron than the atom it replaces,
that extra electron is free to roam through the crystal, forming an n-type (for