Application of lasers can be divided into two broad categories: (1)
commercial, industrial, military, and medical uses, and (2) scientific
research. In the first case, lasers are used to do something that has
been done in another way up to now (but not as well). Sometimes a laser
solves a particular problem. For example, one of the first applications
was in eye surgery, for “welding” a detached retina. The laser is
particularly useful here because laser light can penetrate transparent
objects such as the eye’s lens (Figure 19), eliminating the need to make
a cut into the eye.
[Illustration: Figure 19 _Diagram of human eye showing laser beam
focused on retina._]
Cornea
Lens
Optic Nerve
Beam angle
Fovea centralis
Iris
Image
Retina
Surgeons have long wanted a better technique for treating extremely
small areas of tissue. A laser beam, focused into a small spot, performs
perfectly as a lilliputian surgical knife. An additional advantage is
that the beam, being of such high intensity, can also sterilize or
cauterize tissue as it cuts.
The narrowness of the laser beam has made it ideal for applications
requiring accurate alignment. Perhaps the ultimate here is the
2-mile-long linear accelerator built by Stanford University for the
United States Atomic Energy Commission. “Arrow-straight” would not have
been nearly good enough to assure expected performance. A laser beam was
the only technique that could accomplish the incredible task of keeping
the ⅞ inch bore of the accelerator straight along its 2-mile length. A
remote monitoring system, based on the same laser beam, tells operators
when a section of the accelerator has shifted out of line (due for
example to small earth movements) by more than about ¹/₃₂ inch—and
identifies the section.[14]
Figure 20 shows the 2-mile-long “klystron gallery” that generates the
power for kicking the high-energy particles down the tube. The gallery
parallels the accelerator housing and lies 25 feet beneath it (Figure
21). The large tube houses the optical alignment system and supports the
smaller accelerator tube above. Target patterns dropped into the large
tube at selected points produce an interference pattern at the far end
of the tube similar to the one in Figure 13. Precise alignment of the
tube is achieved by aiming the laser at the center dot of the pattern.
Then the section that is out of line is physically moved until the dot
appears in the proper place at the other end of the tube. It is the
extreme coherence of the laser beam that makes this technique possible.
Having heard that laser light has bored through steel and is being used
in microwelding, some have asked whether the laser will ever be used to
weld bridge members and other structural girders. This is missing the
whole point of the laser: It would be like washing your floor with a
toothbrush (even one with extra stiff bristles)! There would be no
advantage to using lasers for large-scale welding; present equipment for
this operation is quite satisfactory and far less wasteful of input
power. The sensible approach is to use lasers where existing processes
leave something to be desired.
Until the advent of the laser, for example, there was no good way to
weld wires 0.001 inch in diameter. Nor was there a good way to bore the
tiny hole in a diamond that is used as a die for drawing such fine wire.
It used to take 2 days to drill a single diamond. With laser light the
operation takes 2 minutes—and there is no problem with rapid wear of a
cutting tool.
So much for the first category of application. In the second category,
namely use of the laser as a scientific tool, we enter a more
theoretical domain. Here we use coherent light as an extension of
ourselves, to probe into and to look at the world around us.
[Illustration: Figure 20 _A laser beam was used (and continues to be
used) for precise alignment of Stanford University’s 2-mile-long
linear accelerator. This view shows the aboveground portion during
construction._]
Much experimental science is a matter of cooling, heating, grinding,
squeezing, or otherwise abusing matter to see how it will react. With
each new tool—ultrafast centrifuges, high- and low-pressure and
extreme-temperature chambers, intense magnetic fields, atomic
accelerators and so on—more has been learned about this still-puzzling
world.
Since coherent light is something new, we can do things to matter that
have not been done before, and see how it reacts. The laser is being
used to investigate many problem areas in biology, chemistry, and
physics. For example, sound waves of extremely high frequency can be
generated in matter by subjecting it to laser light. These intense
vibrations may have profound effects on materials.
[Illustration: Figure 21 _Subterranean view of Stanford accelerator
housing. Alignment optics (laser systems) are housed in the large
tube, which also acts as support for the smaller accelerator tube
above it._]
[Illustration: Figure 22 _Laser beam spot as observed at the end of
the accelerator._]
In the chemical field the sharp beam and monochromatic energy of the
laser hold great promise in the exploration of molecular structure and
the nature of chemical reactions. Chemical reactions usually are set off
by heat, agitation, electricity, or other broadly applied means. None of
these energizers allow the fine control that the laser beam does. Its
extremely fine beam can be focused to a tiny spot, thus allowing
chemical activity to be pinpointed. But there is a second advantage: The
monochromaticity of coherent light also makes it possible to control the
energy (in addition to the intensity) of the beam accurately by simply
varying the wavelength. Thus it may be possible, for instance, to cause
a reaction in one group of molecules and not in another.
One application in chemistry that holds great promise is the use of
laser energy for causing specific chemical reactions such as those
involved in the making of plastics. Bell Telephone Laboratory scientists
have changed the styrene monomer (a “raw” plastic material) to its final
state, polystyrene, in this way. The success of these and similar
experiments elsewhere opens for exploration a vast area of molecular
phenomena.
In another scientific application, the laser is being used more and more
as a teaching tool. Coherence is a concept that formerly had to be
demonstrated by diagrams, formulas, and inference from experiments. The
laser makes it possible to see coherence “in action”, along with many of
the physical effects that result from it. Such phenomena as diffraction,
interference, the so-called Airy disc patterns, and spatial harmonics,
always difficult to demonstrate to students in the abstract, can now be
seen quite concretely.
Other interesting things can also be seen more plainly now. At the Los
Alamos Scientific Laboratory, laser light is being used to “look” at
plasmas; the result of one such look is shown in Figure 23. Plasmas are
ionized gaseous mixtures. Their study lies at the heart of a constant
search by atomic scientists for a self-sustained, controlled fusion
reaction that can be used to provide useful thermonuclear power. This
kind of reaction provides the almost unlimited energy in the sun and
other stars. It is more efficient and releases less radioactivity than
the other principal nuclear process, fission, which is used in
atomic-electric power plants.[15]
[Illustration: Figure 23 _Shadowgraph of deuterium discharge taken
in laser light. Turbulence of the plasma is clearly seen._]
Westinghouse Electric Corporation scientists, on the other hand, have
used the concentrated energy of the laser, not to look at, but to
_produce_ a plasma (Figure 24). They blasted an aluminum target the size
of a pinhead with a laser beam, thereby vaporizing it and creating a
plasma. The calculated temperature in the electrically charged gas was
3,000,000° centigrade. This is pretty hot, but still not hot enough for
a thermonuclear reaction.
[Illustration: Figure 24 _Plasma heating by laser light._]
Diamagnetic loop
Laser beam
Vacuum chamber
Magnetic field
Magnetic coils
Electrostatic probe
Plasma
Lens
Mirror
To vacuum pump
Camera
The temperature of a plasma necessary to sustain a thermonuclear
reaction is so high (above 10,000,000°C) that any material is vaporized
instantly on coming into contact with it. The only means developed so
far to contain the plasma is an intense magnetic field, or “magnetic
bottle”; containment has been accomplished for only a few thousandths of
a second at most. The objective of the Westinghouse research, which was
supported by the Atomic Energy Commission, was to study in detail the
interaction of the plasma with a magnetic field.
We do not have room to describe more applications in detail, but it may
be interesting to list a few other uses of lasers—some commercial and
some still experimental:
—Earthquake prediction.
—Measurement of “tides” in the earth’s crust under the sea.
—Laser gyroscopes.
—Highly accurate velocity measurement (useful in certain assembly line
and continuous manufacturing processes).
—Scanner for analyzing photographs of bubble chamber tracks and
astronomical phenomena.
—Computer output and storage systems; perhaps even complete optical data
processing systems.
—Lightning-fast printing devices.
—High-speed photography (Figure 25).
—Missile tracking and accurate alignment of antennas.
—Automatic flaw spotter for big radio antennas.
—Aircraft landing aid for poor weather conditions.
—Fast, painless dental drill.
—Cancer research.
[Illustration: Figure 25 _Twenty-two caliber bullet and its shock
wave are photographed from the image produced by a doubly exposed
laser hologram. The original hologram was exposed twice by a ruby
laser within half a thousandth of a second as the bullet sped past
at 2½ times the speed of sound._]