City of Light: The Story of Fiber Optics

86 CITY OF LIGHT
long distance. They decided to shield millimeter waves inside hollow pipes
called waveguides.
The workings of waveguides are more subtle than those of William
Wheeler’s light pipes or early optical fibers. Waveguides started as the sort of
abstract problem that intrigues theoretical physicists facile with advanced calculus.
They wondered what would happen to an electromagnetic wave inside
various structures, such as long tubes made either of electrically conductive
materials or of nonconductive insulators.
Waveguide behavior depends on ‘‘boundary conditions’’—how the walls
affect the electric and magnetic fields that make up radio waves, light, and
other electromagnetic waves. Conductive metal walls reflect electromagnetic
waves, so metal tubes guide waves along their lengths. Grind through the
mathematics, and you find that waveguides don’t work for wavelengths
longer than a particular cutoff value. In essence, those waves don’t fit inside,
although the details are more complicated and depend on the waveguide
shape. The minimum wavelength is half the wider dimension of rectangular
waveguides, and a little longer for round ones. Filling the waveguide with
plastic or something else more substantial than air increases the cutoff wavelength
further.
This restriction meant that waveguides were strictly of academic interest
in the early days of radio. A 100-megahertz signal has waves three meters
long, so it would require an impractical 1.5-meter (5-foot) waveguide. Only
when engineers pushed frequencies to several gigahertz, where wavelengths
are ten centimeters (four inches) or less, did waveguides become practical.
The technology spread quickly with the development of radar during World
War II, and with postwar advances in microwave communications, making
waveguides seem attractive for the new generation of high-capacity longdistance
systems.
In America, Bell Telephone Laboratories settled on circular hollow waveguides
with inner diameter of five centimeters (two inches) to carry signals
at 60 gigahertz, with wavelength of five millimeters (0.2 inch). In 1950,
management put Stewart E. Miller in charge of millimeter waveguide development.
26 In England, Harold E. M. Barlow, a professor at University College
London, proposed a slightly different circular millimeter waveguide. 27 The
British Post Office, which ran the country’s phone system, began work at its
Dollis Hill Research Laboratory in London. Standard Telecommunication Laboratories
quickly followed, the first British company in the field.
The technical challenges didn’t daunt mid-century providers of telephone
service. They were government or private monopolies, and regulations assured
that customers or the government would pay the bill. AT&T led the
way in America, generously funding basic research at handsome Bell Labs
facilities scattered about suburban New Jersey. In 1956, Bell Labs put the
new technology to the test at its Holmdel development lab, burying 3.2 kilometers
(2 miles) of a millimeter waveguide made by embedding a tightly
wound coil of copper wire in protective plastic. The experiment confirmed one
growing concern—signals leaked from any kinks or bends, even small ones