It is almost self-evident that no single device, even one as incredible
as the laser, could accomplish all the feats mentioned in the preceding
paragraphs. After all, some of these applications require high power but
not extremely high monochromaticity, while in others the reverse may be
true. Yet, by its very nature, any laser produces a beam with one, or at
the most a few, wavelengths, and many different materials would be
needed to provide the many different wavelengths required for all the
tasks listed.
Also, the first laser was a pulsed device. Light energy was pumped in
and a bullet of energy emerged from it. Then the whole process had to be
repeated. Pulsed operation is fine for spot-welding and for applications
such as radar-type rangefinding, where pulses of energy are normally
used anyway. With lasers smaller objects can be detected than when using
the usual microwaves. But a pulsed process is not useful for
communications. In other words, pulsing is good for certain applications
but not for others.
And of course solid crystals are difficult to manufacture. Hence, it was
natural for laser pioneers to look hopefully at gases. Gas lasers would
be easier to make—simply fill a glass tube with the proper gas and seal
it.
But other advantages would accrue. For one thing the relatively sparse
population of emitting atoms in a gas provides an almost ideally
homogeneous medium. That is, the emitting atoms (corresponding to
chromium in the ruby crystal) are not “contaminated” by the lattice or
host atoms. Since only active atoms need be used, the frequency
coherence of a gas laser would probably be even better than that of the
crystal laser, they reasoned.
It was less than a year after the development of the ruby laser that Ali
Javan of Bell Telephone Laboratories proposed a gas laser employing a
mixture of helium and neon gases. This was an ingeniously contrived
partnership whereby one gas did the energizing and the other did the
amplifying. Gas lasers now utilize many different gases for different
wavelength outputs and powers and provide the “purest” light of all. An
additional advantage is that the optical pumping light could be
dispensed with. An input of radio waves of the proper frequency did the
job very nicely.
But most significant of all, Javan’s gas laser provided the first
continuous output. This is commonly referred to as CW (continuous wave)
operation. The distinction between pulsed and CW operation is like the
difference between baking one loaf of bread at a time and putting the
ingredients in one end of a baking machine and having a continuous loaf
emerge at the other.
When a non-expert thinks of a laser, he is apt to think of
power—blinding flashes of energy—as illustrated in Figure 26. As we
know, this is only a small part of the capability of the laser.
Nevertheless, since lasers are often specified in terms of power output
it may be well to discuss this aspect.
The two units generally used are _joules_ and _watts_. You are familiar
with a watt and have an idea of its magnitude: think, for example, of a
15-watt or a 150-watt bulb. A watt is a unit of _power_; it is the rate
at which (electrical) work is being done.
[Illustration: Figure 26 _High power is demonstrated as a laser beam
blasts through metal chain._]
The joule is a unit of _energy_ and can be thought of as the total
capacity to do work. One joule is equivalent to 1 watt-second, or 1 watt
applied for 1 second. But it can also mean a 10-watt burst of laser
light lasting 0.1 second, or a billion watts lasting a billionth of a
second.
In general, the crystal (ruby) lasers are the most powerful, although
other recently introduced materials, such as liquids (see Figure 27) and
specially prepared glass, are providing competition. With proper
auxiliary equipment, bursts of several _billion_ watts have been
achieved; but the burst lasts only about 100 millionths of a second. For
certain uses, that’s just what is wanted: a highly concentrated burst of
energy that does its work without giving the material being “shot” a
chance to heat up and spread the energy, perhaps damaging adjacent
areas.
[Illustration: Figure 27 _Active substance for a modern liquid laser
is made in an uncomplicated 10-minute procedure. Bluish powder of
the rare earth, neodymium, is dissolved in a solution of selenium
oxychloride and sealed in a glass tube._]
Since the joule gives a measure of the total energy in a laser burst it
is not applicable to CW output. Power in this area began low—in the
milliwatt (one thousandth of a watt) region—but has been creeping up
steadily. A recent gas laser utilizing carbon dioxide has already
reached 550 watts of continuous infrared radiation. This is the giant
44-footer shown in Figure 28. An advantage of gas (and liquid) lasers is
that they can be made just about as large as one wishes. By way of
comparison, the smallest gas laser in use is shown in Figure 29.
[Illustration: Figure 28 _A giant 44-foot gas laser produces 550
watts of continuous power and is expected to reach 1000 watts.
Glowing of the tube comes from gas discharge, not from laser light,
which is in the infrared region and cannot be seen._]
One of the least satisfactory aspects of the laser has been its
notoriously low efficiency. For a while the best that could be
accomplished was about 1%. That is, a hundred watts of light had to be
put in to get 1 watt of coherent light out. In gas lasers the efficiency
was even lower, ranging from 0.01% to 0.1%.
In gas lasers this was no great problem since high power was not the
objective. But with the high-power solid lasers, pumping power could be
a major undertaking. A high-power laser pump built by Westinghouse
Research Laboratories handles 70,000 joules. In more familiar terms, the
peak power input while the pump is on is about 100,000,000 watts. For a
brief instant this is roughly equal to all the electrical power needs of
a city of 100,000 people.
Two relatively new developments have changed the efficiency levels. One,
the carbon dioxide gas laser, is quite efficient, with the figure having
passed 15%. The second is the injection, or semiconductor laser, in
which efficiencies of more than 40% have been obtained. Unless
unforeseen difficulties arise this figure is expected to continue to
rise to a theoretical maximum of close to 100%.
[Illustration: Figure 29 _A miniature gas laser produces continuous
output in visible red region._]
The semiconductor laser is to solid and gas lasers what the transistor
was to the vacuum tube; all the functions of the laser have been packed
into a tiny semiconductor crystal. In this case, electrons and “holes”
(vacancies in the crystal structure that act like positive charges)
accomplish the job done by excited atoms in the other types. That is,
when they are stimulated they fall from upper energy states to lower
ones, and emit coherent radiation in the process. Aside from this the
principle of operation is the same.
The device itself, however, is vastly different. For one thing it is
about the size of this letter “o” (Figure 30). For another, it is
self-contained; since it can convert electric current directly into
laser light—the first time this has been possible—an external pumping
source is not required. This makes it possible to modulate the beam by
simply modulating the current. (A different approach has been to
modulate a magnetic field around the device. This, it turns out, can
also be done with some newer solid crystal lasers.)
An additional advantage offered by the semiconductor laser is
simplicity. There are no gases or liquids to deal with, no glassware to
break, and no mirrors to align. Although it will not deliver high power,
it can already deliver enough CW power for certain communications
purposes. Its simplicity, efficiency, and light weight make it ideal for
use in space.
[Illustration: Figure 30 _A tiny injection laser works in infrared
region. The beam is visible because photo was taken with infrared
film. The laser itself is a tiny crystal of gallium arsenide inside
the metal mount being held between the fingers._]