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OCR for page 34
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A carbon dioxide (CO2) laser
cuts heavy-duty circular saw
blades from '/4-inch steel
sheet. The powerful CO2 laser,
the workhorse of industry, has
been applied to a wide
assortment of tasks, ranging
from tough metal work to
delicate surgery.
34
I'....
lasers were invented
nt~sts streamed of harness
;mg the unique properties of
light to study the swift motion
of molecules, atoms, and
electrons. The military wanted
light's awesome power to
annihilate enemy tanks,
planes, and missiles. Once
the practical laser was built,
however, the "glamorous
blowtorch" began doing jobs
no one had dreamed of. Today
lasers play music, read price
tags, carry phone calls, cut
cloth, perform surgery, and
test the quality of air. And
although the military is still
waiting for a light-ray
weapon, lasers have become
standard research tools for
scientists and engineers in
laboratories around the world.
The word laser stands for light amplifica-
tion by stimulated emission of radiation, and
laser light is unlike any other. Light waves
from a laser all have the same frequency,
creating a beam with one characteristic color.
The light is also coherent, its waves traveling
in phase crest next to crest, trough next to
trough. Coherency intensifies the waves'
combined power, much as football fans
intensify their combined sound by chanting
in unison. In addition, the waves are almost
perfectly parallel and so travel in nearly the
same direction. This directionality keeps the
waves concentrated in a narrow beam that
widens only gradually over great distance.
Incoherent light waves from the sun, light
bulbs, and other nonlaser sources travel out
of phase at different frequencies in a beam
that quickly spreads and disappears from
sight.
Laser waves are no more powerful than
waves of other light. But, because of their
unique properties, they are easily focused to
a point that can vaporize diamond and steel.
Where continuous power of this magnitude
is needed, the carbon dioxide laser has been
the workhorse since it was introduced
commercially in 1967. It drills holes in hard
ceramics, cuts through composite materials,
and heat-treats metals to harden them. A
CO2 laser beam focuses to a fine point for the
delicate work of cutting cloth or drilling
holes in rubber baby-bottle nipples. Doctors
use the CO2 laser as a surgical knife; the laser
cauterizes blood vessels as it cuts, eliminat-
ing much bleeding. Because light from a CO2
laser is infrared and thus invisible, a red,
low-energy helium-neon laser is often used
to aim it.
Laser light is useful, too, in other areas of
medicine. Its single-frequency nature lets a
E N G I N E E R I N G A N D T H E A D VA N C E M E N T O F H U M A N W E L FA R E
OCR for page 35
laser zap one kind of tissue while causing
little harm to another. This is because some
materials absorb more light energy at one
particular frequency than at others. The
colorless central portion of the eye, for
example, absorbs little of the argon laser's
green light, which is readily absorbed by
blood-containing tissue at the back of the
eye. Doctors, therefore, use the low-powered
argon laser to spot weld detached retinas and
seal the leaking blood vessels that often blind
diabetics. Lasers also vaporize brain tumors,
perform delicate inner-ear surgery, and
remove warts and gynecological cysts. Laser
light goes inside the body through fiber-optic
endoscopes to burn fatty deposits out of
clogged arteries, pulverize kidney stones,
stanch bleeding stomach ulcers, and open
blocked fallopian tubes.
Coherent light is necessary for construct-
ing the three-dimensional pictures called
holograms. They are made by shining one
part of a laser beam directly onto photo-
graphic film while bouncing the other part
off an object and then onto the film. Waves
from the two beams interfere with each other
in complex patterns that are recorded on the
film. When the film is developed, these
patterns act like a complex system of micro-
scopic mirrors. They reflect back the object's
image if the original laser light is shined on
the film at the original angle. The patterns are
so intricate that they reflect a slightly differ-
ent image in slightly different directions. This
lets you see the object from different angles
and gives the image its three-dimensional
quality.
Holograms are difficult to counterfeit
and so are used as tamper-proof seals on
boxes of videocassettes and auto parts as
well as on credit cards and passports.
Double-exposure holograms are used widely
for quality control in, for example, the
aircraft tire industry. Disruptions in the
delicate wave pattern on a double exposure
of a tire reveal defective bulges only
6 millionths of an inch high. Since 1980
holograms have been used to direct the beam
in many laser scanners that read price bar
codes at checkout counters. Holograms on a
whirling disc bend a red helium-neon laser
beam in different directions, allowing it to
scan for the bar code up to 1,800 times per
second. The light pattern reflecting back to
L A S E R 5
A researcher aims the
beam from a YAG (yttrium-
aluminum garnet) laser at a
sample of gallium arsenide
in an experiment to
measure impurities in the
semiconducting material.
Rows of microscopic bumps
on this video disc carry digital
information that is read by
laser and translated by
. . ~ .
computer Into visual Images.
35
OCR for page 36
- ~
A fraction of a second after
the flash of an ultraviolet
laser beam, a "smoke" plume
erupts from the corneal
surface of the eye, shoots
upward, and turns into a
microscopic mushroom cloud
during a surgery experiment.
Laser beams of other
frequencies pass harmlessly
through the cornea to perform
surgery inside the eye.
36
sensors from any successful scan will
transmit the code.
The directionality of laser light makes it
very useful for aligning new buildings,
tunnels, and pipes as well as leveling and
grading land. And the ability to switch them
rapidly on and off lets lasers produce the tiny
pulses needed for timing measurements of
long distances. Lasers can generate pulses of
less than 0.1 billionth of a second far
shorter than those made by mechanical or
electrical switches. By timing pulses bounced
off reflectors placed on the moon by U.S.
astronauts and Soviet unmanned landers,
I laser instruments measure the earth-to-moon
distance with less than 1 centimeter of error.
Scientists use even shorter laser pulses to
observe the lightning movement of atoms,
molecules, and chemical reactions. The
pulses can, in effect, take "snapshots"
quickly enough to prevent blurring. One
laser system generates a pulse of just
6 quadrillionths of a second, fast enough to
take step-by-step shots of a chemical reaction
lasting only 100 quadrillionths of a second.
Lasers are also good for measuring very
slow movements. Geologists use them to
I measure the almost imperceptible creep
along the San Andreas Fault in California.
A two-laser device on one side of the fault
shoots red and blue beams at a reflector
farther up the fault on the other side. One
beam could be used to measure the distance.
But by comparing two beams of different
frequencies, geologists calculate how much
the atmosphere has slowed the beams on a
given day. With this information, they
compensate for measurements taken under
different atmospheric conditions. Shooting at
a reflector 5 kilometers away, the instrument
can detect a shift in the earth of only
2 millimeters.
The ability to develop lasers with special
talents opened the door to their use in
communications in the 1970s. The break-
through came with development of a
semiconductor laser that operates at room
temperature, is smaller than a grain of sand,
and produces a light frequency that travels
well through glass optical fiber. This laser
made it practical to use fiber-optic cables for
long-distance telephone lines that carry
thousands of calls at once. The installation of
fiber-optic telephone cables since then has
expanded long-distance telephone service
and reduced its cost.
E N G I N E E R I N G A N D T H E A D VA N C E M
The semiconductor laser quickly became
the key to compact disc (CD) recordings,
which store large amounts of information
and can be played at home. CDs were
invented in the Netherlands, and the first
audio CDs were introduced in Japan in 1982.
Information for a disc is translated into the
l's and 0's of digital code and then stamped
onto the upper side of the disc in a series of
long pits. To read the information, a pin-
point laser beam scans the bumps on the
underside. Flat surfaces between the bumps
reflect a strong return beam; bumps scatter
the light and weaken the beam. Sensors
detect differences between strong and weak
beams, interpreting them as digital code.
Music, video, and computer data can all be
stored in digital form on compact discs.
Other types of laser are especially useful
for probing the environment. Government
agencies and private organizations around
the world use ground-based and airborne
lasers to measure air pollution, monitor the
weather, and study climate. Laser instru-
ments have been used to study holes in the
ozone layers over the North and South poles,
particulates and gases over the Amazon rain
forests, and dust drifting across the Atlantic
from the Sahara.
Laser radar, or lidar (light detection and
ranging), detects airborne particles of dust,
moisture, and chemicals by measuring the
strength of laser light reflected back to the
; instrument. These fine particulates are
invisible to normal radar. Weather scientists
use lidar plus knowledge of the Doppler shift
to study wind speeds. Light reflected off
particles moving with the wind changes
frequency, which gets higher if the particles
are moving toward the observer and lower if
they are moving away. A similar frequency
shift causes a train whistle to sound higher
lo while a train approaches and lower as it
speeds away.
Another system, called differential
absorption lidar, uses laser beams at two
I frequencies to detect the presence of a gas in
the atmosphere and measure whatever is
there. The first beam uses lidar to measure
the light reflected back by particulates. The
second beam, at a wavelength absorbed by a
gas such as ozone, scans the same area. Some
of its light is absorbed by the gas but some is
reflected back by particulates. The amount of
gas in the air and its location are revealed by
comparing the return echoes of the two
beams.
1
.. E N T O F H U M 1\ N W E L FA R E
OCR for page 37
In the future, laser research will aim at
reducing the size and cost of lasers while
expanding their versatility. Technology is
being developed for arrays of tiny semicon-
ductor lasers which are limited in
size that develop enough power for devices
such as printers and facsimile machines. The
search for a powerful laser that can be tuned
to many frequencies has sparked interest in
the free-electron laser. This laser generates
light by sending electrons through a periodi-
cally alternating magnetic field. Its periodici-
ty or its strength can be altered to change the
frequency of the light beam.
At the same time, existing lasers are
being applied to many new tasks. Lasers will
find use in industries seeking greater speed
and efficiency. They fit well into automation
A laser beam shoots
moonward from an
observatory, left, in a test
to learn whether a laser
could hit a small target on
the moon. Laser beams from
this and another observatory
appear as two tiny dots of
light, above, sparkling on
the dark side of the earth
in a photo taken from the
moon by a television camera
on the Surveyor 7 lunar
probe. This test led to the use
of lasers for measuring the
-
earth to-moon distance.
schemes because they can be operated by
computer and used by robots. And, because
light beams are quickly redirected with
mirrors and fiber-optic cables, lasers can be
assigned new jobs without costly retooling.
The fastest growing field for lasers,
though, is undoubtedly medicine. Lasers are
being investigated for reshaping the cornea
to correct eye problems. They are being
tested for activating toxic anticancer drugs
that accumulate in tumors but pass through
the rest of the body. And lasers are being
used experimentally for drilling new blood
channels in weakened hearts and for other
techniques aimed at reducing heart disease,
the leading cause of death in the United
States.
LASERS I
37
Representative terms from entire chapter:
laser light
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