City of Light: The Story of Fiber Optics

A VISION OF THE FUTURE 81
forgotten. The optical telegraph was essentially a series of relay towers, where
operators peered through telescopes to spot semaphore signals from a tower
on one side, then displayed the same signals for the next operator to read.
The signal schemes were ingenious, but the technology was crude—men
looked through telescopes and pulled ropes to set flaps and flags. 7 Laborintensive,
expensive to run, and unusable at night or in bad weather, the
optical telegraph was replaced by the electrical telegraph. (Semaphores and
signal flags survived to send messages between ships at sea.) Electricity could
send telegraph signals farther through wires than people could see through
the air.
The telephone followed over wires, but after the dust and the patent lawsuits
had settled, Alexander Graham Bell wasn’t quite satisfied with the results.
He wanted to send signals without wires, and at the time the only
wireless transmission he could imagine was by light. On an earlier trip to
England, Bell had seen that exposure to light changed how much current
could pass through the element selenium. In early 1880, he realized that
selenium might detect changes in a beam of light modulated by voices. He
doubted the idea would prove practical but was nonetheless intrigued. 8 Thomas
Edison also envisioned a wireless optical telephone about the same time
but never built one, perhaps because he was more practical. 9
Within a month, Bell demonstrated his ‘‘photophone,’’ which operated by
focusing sunlight onto the surface of a flat mirror that vibrated when moved
by sound waves from voices (figure 7-2). The vibrations modulated the
amount of sunlight reflected onto a selenium cell in a telephone receiver
circuit, reproducing the voice. The excited Bell wrote: ‘‘I have heard articulate
speech produced by sunlight! I have heard a ray of the sun laugh and cough
and sing. ...Ihave been able to hear a shadow, and I have even perceived
by ear the passage of a cloud across the sun’s disk.’’ 10 He dispatched his first
photophone to the Smithsonian Institution in a sealed box, and packed off
another several weeks later after reaching a distance of 700 feet (213 meters).
He proudly described his invention to the then-young American Association
for the Advancement of Science at the end of August. 11
Yet the photophone came to naught. Wires were messy, but they carried
signals in fair weather or foul, day or night, farther than Bell could send light.
He talked some about the idea in later years but never worked seriously on
it again, and it was soon forgotten. 12 In the decades that followed, the only
serious work on optical communications was by military agencies worried
about enemy eavesdropping. Those systems never got very far. 13
The Triumph of Radio
It was radio that filled the need for wireless transmission and it did the job
so well that dictionaries still define wireless as another word for radio. The
first radio signals were sparks spanning so many frequencies that they blocked
each other, sometimes with disastrous consequences for ships frantically tele-