City of Light: The Story of Fiber Optics

94 CITY OF LIGHT
Maiman used ruby because he understood its properties, but it is not an
ideal laser material. It only fires pulses, and converts only a small fraction of
the input energy—supplied by a flashlamp—into laser energy. Bell Labs
wanted a laser that oscillated continuously, emitting a steady light beam that
could be modulated with a signal like a radio carrier frequency. Bell had a
young Iranian-born physicist hard at work toward that goal, Ali Javan, who
had studied under Townes at Columbia.
Javan was trying to generate a laser beam by passing an electric current
through a gas. Others used the vapors of alkali metals like sodium and potassium
as the gas, but Javan picked the rare gases helium and neon, which
are simpler to study and much easier to handle. He filled glass tubes with
helium, to capture energy from electrons passing through the gas, and a dash
of neon, which borrowed energy from the helium and turned it into light. He
mounted highly reflective mirrors on both ends, with one allowing a small
fraction of light to escape. On a snowy Monday afternoon, December 12,
1960, he was elated when his helium-neon laser emitted an invisible infrared
beam at 1.15 micrometers. 13 It was the first laser to emit a continuous beam,
and the first laser to operate in a gas. Management, which was growing
impatient, thought it was about time.
The helium-neon laser proved as important as Javan hoped; it soon became
the standard laboratory laser and remains the most common gas laser. However,
it did need some modifications. Other Bell Labs scientists developed a
version that emitted at 633 nanometers in the red part of the spectrum. That
was much better for communication experiments, because the beam was visible
as well as stable, and very coherent. Output powers were milliwatts to
tens of milliwatts, fine for research; external devices could modulate the beam
by changing their transparency over time. Few developers thought the
helium-neon laser was ideal for communications, but they didn’t have ideal
millimeter-wave sources either, and they thought laser technology had plenty
of time to grow.
If they had an ideal laser, it was the solid-state semiconductor type, first
demonstrated in the fall of 1962. The transistor age was on a roll; semiconductors
were swiftly replacing vacuum tubes in electronic circuits. But doubts
remained about how far semiconductor technology could go, and progress on
semiconductor lasers soon stalled, leaving the best devices able to operate for
only a short time at the �196�C (�321�F) temperature of liquid nitrogen. A
few were used in experiments, but helium-neon lasers remained far more
practical.
Clouds in the Picture
Communications engineers started sending laser beams through the atmosphere
as soon as they got their hands on lasers. Air looked like an ideal