Following the maser development, there was much speculation about the
possibility of extending the principle to the optical region. Indeed the
first lasers—_l_ight _a_mplification by _s_timulated _e_mission of
_r_adiation—were called “optical masers”.
The difficulty, of course, was that optical wavelengths are so
tiny—about ¹/₁₀,₀₀₀ that of microwaves. The maser principle depended
upon a physical resonator, a box a few centimeters (or even millimeters)
in length. But at millimeter wavelengths, such resonators are already so
small that they are hard to make accurately. Making a box ¹/₁,₀₀₀ that
size was out of the question. Another approach was necessary.
In 1958 A. L. Schawlow of Bell Telephone Laboratories and Dr. Townes
outlined the theory and proposed a structure for an optical maser. They
suggested that resonance could be obtained by making the waves travel
back and forth along a relatively long, thin column of amplifying
substance that had parallel reflectors at the ends.
After their theory of the optical maser had been published, the race to
build the first actual device began in earnest. The winner, in 1960, was
Dr. T. H. Maiman, then with Hughes Aircraft Company. (He is now
president of Maiman Associates.) The active substance he used was a
single crystal of ruby, with the ends ground flat and silvered.
Ruby is an aluminum oxide in which a small fraction of the aluminum
atoms in the molecular structure, or lattice, have been replaced with
chromium atoms. These atoms absorb green and blue light and hence impart
a red color to the ruby. The chromium atoms can be boosted from their
ground state into excited states when they absorb the green or blue
light. This process, by which population inversion is achieved, has been
given the name pumping.[13]
Pumping in a crystal laser is generally achieved by placing the ruby rod
within a spiral flash lamp (Figure 15) that operates like those used in
high-speed (stroboscopic) photography. When the lamp is flashed, a
bright beam of red light emerges from the ruby, shining out through one
end, which has been only partially silvered.
[Illustration: Figure 15 _A ruby laser system._]
Ruby
Flash lamp
Partially silvered end
Laser output
Power
Cooling
The duration of this flash of red light is quite brief, lasting only
some 300 millionths of a second, but it is very intense. In the early
lasers, such a flash reached a peak power of some 10,000 watts.
When Maiman’s device was successfully built and operating, a public
relations expert was called in to help introduce this revolutionary
device to the world. He took one look at the laser and decided that it
was too small and insignificant looking and would not photograph well.
Looking around the lab, he spotted a larger laser and decided that that
one was better.
Dr. Maiman informed him in his best scientific manner that laser action
had not been achieved with that one. But the world of promotion won out,
and Dr. Maiman allowed the larger device to be photographed on the
assumption—or was it hope?—that he would be able to get it to operate in
the future. (He did.)
The device shown in Figure 16 is the true first laser. The all-important
crystal rod is seen at the center. These crystals, incidentally, must be
quite free of extraneous material; hence they are artificially “grown”,
as shown in Figure 17. The single large crystal is formed as it is
pulled slowly from the “melt”, after which it is ground to size and
polished.
[Illustration: Figure 16 _Dr. Maiman’s first laser. Output was
10,000 watts._]
[Illustration: Figure 17 _An exotic crystal of the garnet family is
“grown” from a melt at a temperature of 3400°F._]