City of Light: The Story of Fiber Optics

80 CITY OF LIGHT
Figure 7-1: Modulation of light waves.
to millions of hertz, megahertz. By World War II, they were testing microwave
frequencies, at billions of hertz or, equivalently, gigahertz.
As Alec Reeves watched the radio spectrum grow crowded in the 1950s,
he considered the possibility of moving to even higher frequencies. He knew
that radio waves and microwaves are part of the broad spectrum of electromagnetic
waves. All are essentially the same phenomenon, waves composed
of oscillating electric and magnetic fields. 6 Their properties differ with their
frequency. Move beyond the microwave spectrum and you reach millimeter
waves, infrared radiation, and visible light; ultraviolet light, X rays, and
gamma rays have even higher frequencies. For visible light, the frequencies
approach 900 trillion hertz, or 900 million megahertz in Reeves’s terms. He
estimated maximum possible bandwidth as roughly the carrier frequency, and
that meant visible light had a truly staggering capacity.
The years around 1960 were the heyday of technological optimism, when
the space frontier seemed within reach, nuclear power promised a glittering
future, and pollution was unknown. Any down-to-earth engineer could have
done the same calculations, but it took a visionary like Reeves to believe them.
Optical Telegraphs and Photophones
When Reeves turned to light, he was putting a new spin on an old idea. The
first telegraphs were optical, part of a system developed in France at the end
of the eighteenth century and copied by many other countries but now largely