Page 218 - Photonics Essentials an introduction with experiments

Optical Fibers and Optical Fiber Amplifiers

212 Advanced Topics

tion; that is, one simple device could replace literally thousands of dif-
ferent components: detectors, transistors, power supplies, lasers, and
modulators. Needless to say, the introduction of laser amplifier tech-
nology caused a quantum leap in the growth of telecommunications
networks, and was instrumental in enabling the worldwide installa-
tion of the Internet.

9.6 Optical Amplifiers
The laser was originally proposed as a “maser,” with the M standing
for microwave. It was first used as a microwave amplifier for radio as-
tronomy, and was based on atomic transitions in ammonia gas. The
big advantage of the maser amplifier was its lower noise compared to
conventional electronic amplification (via vacuum tubes). Shortly af-
terward, it was shown that the maser could be made to work at short-
er wavelengths, in the optical regime. The first solid-state lasers were
made by introducing isolated impurities in a transparent host, for ex-
ample, chromium in aluminum oxide (known as ruby). In order to
function, these lasers needed to be pumped by an external light
source, typically a flashlamp. With the addition of mirrors to form an
optical cavity, the amplifier could be made into a source of light rather
than just an amplifier. About this time, in 1961, Elias Snitzer, now a
professor emeritus at Rutgers, introduced the idea of putting rare-
earth ions like erbium in a glass host, and developed an optical ampli-
fier. He showed that a large number of these rare-earth ions could be
used, each having a characteristic wavelength. One in particular,
neodymium (Nd), was developed into a high-powered laser source at
1060 nm and is still a workhorse of the laser industry. Laser engi-
neers for the next twenty years focussed on making sources of light
with higher output power. When the room-temperature semiconduc-
tor laser was developed in 1970, people began to think about smaller
devices that could be pumped electrically instead of by a flashlamp.
With this background, Julian Stone, working in my department Bell
Labs, demonstrated an optical fiber laser based on Nd-doping of a
glass fiber in 1973. Shortly afterward, he was able to show that a
GaAs laser diode could be used to pump the fiber laser. His discovery
was treated as a big nonevent because optically pumped lasers were
old technology; everyone else was concentrating on new compact
semiconductor laser diode sources. As we shall see shortly, his inven-
tion was key to the commercial success of optical communications.
About 15 years later, in 1987, research temas in the U.K. and at
Bell Labs in the U.S. rediscovered the fiber-based optical amplifier.
They were using erbium-doped glass, because Snitzer had shown that
erbium was the rare-earth element to use if you were interested in

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