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1917
Albert Einstein, Zur Quantentheorie der Strahlung (On the
Quantum Theory of Radiation), laid the foundation for the
invention of the laser by rederiving Planck’s law of radiation using
the concepts of probability coefficients ( Einstein coefficients )
for the absorption, spontaneous, and stimulated emission.

1928
Rudolph W. Landenburg confirmed the existence of stimulated
emission and negative absorption.

1939
Valentin A. Fabrikant predicted the use of stimulated emission
to amplify "short" waves.

1947
Willis E. Lamb and R. C. Retherford found apparent stimulated
emission in hydrogen spectra and made the first demonstration of
stimulated emission.

1950
Alfred Kastler proposed the method of optical pumping, which
was experimentally confirmed by Brossel, Kastler and Winter two
years later.
Development of the Idea of the Laser
1953
Charles Townes and graduate students James P. Gordon and
Herbert J. Zeiger produced the first microwave amplifier, a device
operating on similar principles to the laser, but amplifying microwave
rather than optical radiation. Townes s maser was incapable of
continuous output.

1955
Nikolay Basov and Aleksandr Prokhorov worked independently on
the quantum oscillator and solved the problem of continuous output
systems by using more than two energy levels and produced the first
maser. They suggested an optical pumping of multilevel system as a
method for obtaining the population inversion, which later became
one of the main methods of laser pumping.

1964
Townes, Basov, and Prokhorov shared the Nobel Prize
in physics "For fundamental work in the field of quantum
electronics, which has led to the construction of oscillators
and amplifiers based on the maser-laser principle".

1957
Charles Hard Townes and Arthur Leonard Shawlow published their
theoretical calculations on infrared maser. [Physical Review, Volume
112, Issue 6]. As ideas were developed, infrared frequencies were
abandoned with focus on visible light instead. The concept was
originally known as an "optical maser". Bell Labs filed a patent
application for their proposed optical maser a year later.

1957
After graduating from Brooklyn PolytechnicUniversity, Gordon
Gould, a graduate student at Columbia University, was working on a
doctoral thesis under supervision of Townes. Gould and Townes had
conversations on the general subject of radiation emission.
Afterwards Gould made notes about his ideas for a "laser", including
suggesting using an open resonator, which became an important
ingredient of future lasers.

1958
Prokhorov independently proposed using an open resonator, the
first published appearance of this idea. Schawlow and Townes also
settled on an open resonator design, apparently unaware of both the
published work of Prokhorov and the unpublished work of Gould.
16

1959-60
The term "laser" was first introduced to the public in
Gould s 1959 conference paper "The LASER, Light Amplification by
Stimulated Emission of Radiation". Gould intended "-aser" to be a
suffix, to be used with an appropriate prefix for the spectra of light
emitted by the device (x-ray laser = xaser, ultraviolet laser = uvaser,
etc.). None of the other terms became popular, although "raser" was
used for a short time to describe radio-frequency emitting devices.
He continued working on his idea and filed a patent application in
April 1959. The U. S. Patent Office denied his application and
awarded a patent to Bell Labs in 1960. This sparked a legal battle
that ran 28 years, with scientific prestige and much money at stake.
Gould won his first minor patent in 1977, but it was not until 1987
that he could claim his first significant patent victory when a
federal judge ordered the government to issue patents to him for
the optically . pumped laser and the gas discharge laser.
17

1960 The first working laser was made by Theodore Maiman at
Hughes Research Laboratories, beating several research teams
including those of Townes at Columbia University, Arthur L.
Schawlow at Bell Labs, and Gould at a company called TRG (Technical
Research Group). Maiman used a solid-state flashlamp-pumped
synthetic ruby crystal to produce red laser light at 694 nm.
Maiman s laser, however, was only capable of pulsed operation due to
its three energy lev