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Title: Lasers
Author: Hellman, Hal
Language: English
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*** Start of this LibraryBlog Digital Book "Lasers" ***
Lasers
by Hal Hellman
U.S. ATOMIC ENERGY COMMISSION
Division of Technical Information
_Understanding the Atom Series_
ATOMIC ENERGY COMMISSION
UNITED STATES OF AMERICA
The Understanding the Atom Series
Nuclear energy is playing a vital role in the life of every man, woman,
and child in the United States today. In the years ahead it will affect
increasingly all the peoples of the earth. It is essential that all
Americans gain an understanding of this vital force if they are to
discharge thoughtfully their responsibilities as citizens and if they
are to realize fully the myriad benefits that nuclear energy offers
them.
The United States Atomic Energy Commission provides this booklet to help
you achieve such understanding.
{Edward J. Brunenkant}
Edward J. Brunenkant, Director
Division of Technical Information
UNITED STATES ATOMIC ENERGY COMMISSION
Dr. Glenn T. Seaborg, Chairman
James T. Ramey
Wilfrid E. Johnson
Dr. Clarence E. Larson
[Illustration: LASERS]
by Hal Hellman
CONTENTS
INTRODUCTION 1
THE ELECTROMAGNETIC SPECTRUM 5
RADIO WAVES 9
LIGHT AND THE ATOM 14
WHAT’S SO SPECIAL ABOUT COHERENT LIGHT? 19
CONTROLLED EMISSION 25
A LASER IS BORN 29
LASING—A NEW WORD 32
SOME INTERESTING APPLICATIONS 34
A MULTITUDE OF LASERS 42
COMMUNICATIONS 48
A LASER IN YOUR FUTURE? 52
SUGGESTED REFERENCES 53
United States Atomic Energy Commission
Division of Technical Information
Library of Congress Catalog Card Number: 68-60742
1968; 1969(rev.)
[Illustration: _Nothing about lasers is more astonishing than their
ability to produce holograms, under arrangements such as shown
above. Two laser beams (of different colors) emerge from the curtain
(rear). They are optically combined (left center) and the combined
beam is then divided by prisms, mirrors and lenses so that part of
it shines on the figurines (foreground) and part on the square
holographic plate (right center). When the plate is developed (like
an ordinary photographic film), it will seem to have only a dull
gray surface until it is viewed with spatially coherent light (such
as from a laser or a beam through a pinhole) shining through it.
Then an amazing, multi-colored, three-dimensional image of the
figurines will be visible. (See page 19 and Figure 13.)_]
[Illustration: LASERS]
By HAL HELLMAN
INTRODUCTION
The transistor burst upon the electronic scene in the 1950s. Almost
overnight the size of new models of radios, television sets, and a host
of other electronic devices shrank like deflating balloons. Suddenly the
hard-of-hearing could carry their sound amplifiers in their ears.
Teenagers could listen to favorite music wherever they went. Everywhere
we turned the transistor was making its mark. There was even a proposal
before Congress to require that every home have a transistor radio in
case of emergency.
The next development to fire the imagination of scientists and engineers
was the laser—an instrument that produces an enormously intense
pencil-thin beam of light. Most of us have heard so much about this
invention it seems hard to believe that the first one was built only a
few years ago. We were told that the laser was going to have an even
greater effect on our lives than the transistor. It was going to replace
everything from dentists’ drills to electric wires. The whole world, it
seemed, eventually would be nothing but a gigantic collection of lasers
that would do everything anyone wanted. Roads would be blazed through
jungles at one sweep; our country would be safe once and for all from
intercontinental ballistic missiles; cancer would be licked; computers
would be small enough to carry in a purse; and so on and on.
Yet for the first couple of years the laser seemed able to do nothing
but blaze holes in razor blades for TV commercials. Somehow the device
never seemed to emerge from the laboratory, prompting one cynic to call
it “an invention in search of an application”.
Many of the wild claims came from misunderstandings on the part of the
press, others from exaggerations by a few manufacturers who wanted free
publicity. But with even less exotic devices than lasers, the road from
the laboratory to the marketplace may often be long and hard. Price,
efficiency, reliability, convenience—these are all factors that must be
considered. It soon became clear that with something as new as the
laser, much improvement was necessary before it could be used in science
and medicine, and even more before it could be used in industry.
It now seems, however, that the turning point has been reached. We have
seen laser equipment put on the market for performing delicate surgery
on the eye, spot-welding tiny electronic circuits (Figure 1), and
controlling machine tools with amazing accuracy (Figure 2).
[Illustration: Figure 1 _A commercial laser microwelder. A
microscope is needed for accurate placement of beam energy._]
The pace is quickening. At least a dozen manufacturers have announced
that they are designing laser technology into their products. These are
not laboratory experiments but practical products for measurement and
testing, and for industrial, military, medical, and space uses. The
Army, for example, has announced that it will purchase its first
equipment for use in the field: a portable, highly accurate range finder
for artillery observation.
Still, this hardly accounts for the $100,000,000 spent in one recent
year on laser research and development by some 500 laboratories in the
United States. The U. S. Government alone has spent about $25,000,000 on
laser research in a single year. Dozens, and perhaps hundreds, of other
applications are on the fire—simmering or boiling as the case may be.
Some require particular technical innovations such as greater power or
higher efficiency. Others are entirely new applications. One of the most
exciting of these is holography (pronounced ho LOG ra phy).
Holography involves a completely different approach to photography. In
addition to more immediate applications in microscopy, information
storage and retrieval, and interferometry, it promises such bonuses as
3-dimensional color movies and TV someday.
You have to see the holographic process in operation to believe it. One
moment you are looking at what appears to be an underexposed or lightly
smudged photographic plate. Then suddenly a true-to-life image of the
original object springs into being behind the negative—apparently
suspended in midair! Not only is the full effect of “roundness” and
depth there, but you can also see anything lying behind the object’s
image by moving your head, exactly as if the original scene containing
the object were really there.
Still another important field of application is that of communications.
Perhaps because it is less spectacular than burning holes in razor
blades, we haven’t heard as much about it. Yet there are probably more
physicists and engineers working on adapting the laser for use in
communications than on any other single laser project.
The reason for this is the fact that existing communications facilities
are becoming overloaded. Space on transoceanic telephone lines is
already at a premium, with waiting periods sometimes running into hours.
Radio “ham” operators have been threatened with loss of some of their
best operating frequencies to meet the demand of emerging nations of
Africa for new channels. Television programs must compete for space on
cross-country networks with telephone, telegraph, and transmission of
data. The increasing use of computers in science, business, and industry
will strain our facilities still further. Communication satellites will
help, but they will not give us the whole answer; and much development
work remains to be done on satellites.
[Illustration: Figure 2 _Precision control of a machine tool by
laser light._]
Why the interest in the laser for communications? In a recent experiment
all seven of the New York TV channels were transmitted over a single
laser beam. In terms of telephone conversations, one laser system could
theoretically carry 800,000,000 conversations—four for each person in
the United States.
In this booklet we shall learn what there is about the laser that gives
it so much promise. We shall investigate what it is, how it works, and
the different kinds of lasers there are. We begin by discussing some of
the more familiar kinds of radiation, such as radio and microwaves,
light and X rays.
THE ELECTROMAGNETIC SPECTRUM
Some 85% of what man learns comes to him through his vision in response
to the medium of light. Yet, ironically, it wasn’t until the end of the
17th century that he first began to get an inkling of what light really
is. It took the great scientific genius Isaac Newton to show that
so-called white light is really a combination of all the colors of the
rainbow. A few years later the Dutch astronomer Christiaan Huygens
introduced the idea that light is a wave motion, a concept finally
validated in 1803 when the British physician Thomas Young ingeniously
demonstrated interference effects in waves. Thus it was finally realized
that the only difference between the various colors of light was one of
wavelength.
For light was indeed found to be a wave phenomenon, no different in
principle from the water waves you have seen a thousand times. If you
stand at the seashore, you can easily count the number of waves that
approach the shore in a minute. Divide that number by 60 and you have
the frequency of the wave motion in the familiar unit, cycles-per-second
(cps).[1]
You would have to count pretty quickly to do this for light, however.
Light waves vibrate or oscillate at the rate of some 400 million million
times a second. That’s the vibration rate of waves of red light; violet
results from vibrations that are just about twice that fast.
With frequencies of this magnitude, discussion and handling of data and
dimensions are cumbersome and rather awkward. Fortunately there is
another approach. Let’s look again at our ocean waves. We see that there
is a regularity about them (before they begin to break on the shore).
The distance from one crest to the next is significant and is called the
_wavelength_. Water waves are measured in feet, and in comparable units
light waves are recorded in ten-millionths of an inch—again a very
cumbersome number. Scientists therefore use the metric system[2] and
have standardized a unit called the angstrom[3], which is equal to the
one-hundred-millionth part of a centimeter (10⁻⁸ cm). Thus we find, as
shown in Figure 3, that the visible light range runs from the violet at
about 4000 angstroms to red at about 7000 angstroms.
[Illustration: Figure 3 _The visible light spectrum ranges between
approximately 4000 and 7000 angstroms._]
Wavelength
(Angstroms)
Violet 4000-4300
Blue 4300-5000
Green 5000-5600
Yellow 5600-5800
Orange 5800-6100
Red 6100-7000
At roughly the same time that the wavelength of light was being
determined, the German-British astronomer William Herschel performed an
interesting experiment. He held a thermometer in turn in the various
colors of light that had been spread out by an optical prism. As he
moved the thermometer from the violet to the red, the temperature
reading rose—and it continued to rise as he moved the instrument
_beyond_ the red area, where no prismatic light could be seen.
Thus Herschel discovered infrared rays (the kind of heat we get from the
sun) adjoining the visible red light, and at the same time found that
they were merely a continuation of the visible spectrum. Shortly
thereafter, ultraviolet rays were found on the other end of the visible
light band.
One of the most fascinating movements in science has been the constant
expansion since then of both ends of the radiating-wave spectrum. The
result has come to be called the _electromagnetic spectrum_, which, as
we see in Figure 4, encompasses a wide variety of apparently different
kinds of radiation. Above the visible band (the higher frequencies), we
find ultraviolet light, X rays, gamma rays, and some cosmic rays; below
it are infrared radiation, microwaves, and radio waves. Only a small
proportion of the total spectrum is occupied by the visible band.
Another point of interest is the inverse relationship between wavelength
and frequency. As one goes up the other goes down.[4]
[Illustration: Figure 4 _Visible light region spans a tiny portion
of the total electromagnetic spectrum._]
Frequency (cps) Wavelength
Angstroms
Cosmic rays
10²² 0.0001
0.001
10²⁰ Gamma rays 0.01
0.1
10¹⁸ X rays 1
10
10¹⁶ Ultraviolet radiation 100
1,000
Visible light
10¹⁴ 10,000
Infrared radiation 100,000
Angstroms
0.01
10¹² Millimeter waves 0.1
10¹⁰ Microwaves, radar 1
10
10⁸ TV and FM radio 100
Short wave 1,000
10⁶ AM radio 10,000
Low frequency communications 100,000
10,000 = 10⁴ 1,000,000
These many kinds of rays and waves vary tremendously in the ways they
interact with matter. But they are all part of a single family. The only
difference among them, as with the colors of the rainbow, lies in their
wavelengths. In a few cases, as we shall see later, the mode of
generation is also different.
The band of radiation stretching from the infrared to cosmic rays has
been, up to now, largely the concern of physical scientists. Because of
their high frequencies, these radiations are generally handled, when
calculations or measurements must be made, in terms of wavelength. Radio
and microwaves[5], on the other hand, have been more in the domain of
communications engineers and are more likely to be discussed in terms of
frequency. Thus it is that your radio is marked off in kilocycles, or
thousands of cycles per second, while light is described as radiation in
the 4000 to 7000 angstrom band.
The relative newness of the various radiations has kept scientists busy
learning about them and, as information and experience have accumulated,
putting them to work.
RADIO WAVES
One of the first of the newly discovered electromagnetic radiations to
be put to work was the radio wave, which is characterized by long
wavelength and low frequency.[6] The low frequency makes it relatively
easy to produce a wave having virtually all its power concentrated at
one frequency.
The advantage of this capability becomes obvious after a moment’s
thought. Think for example of a group of people lost in a forest. If
they hear sounds of a search party off in the distance, all likely will
begin to shout in various ways for help. Not a very efficient process,
is it? But suppose all the energy going into the production of this
noise could be concentrated in a single shout or whistle. Clearly, their
chances of being found would be much improved.
[Illustration: Figure 5 _(a) Temporally coherent radiation. (b)
Temporally incoherent radiation._]
The single frequency capability of radio waves has been given the name
_temporal coherence_ (or coherence in time) and is illustrated in Figure
5. Part _a_ shows a single sine wave, the common way of representing
electromagnetic radiation, and particularly _temporally coherent
radiation_. In _b_ we see what _temporally incoherent radiation_ (such
as the mixed sounds of the stranded party) would look like.
It was on Christmas Eve 1906 that music and speech came out of a radio
receiver for the first time. Today the sight of someone walking, riding,
or studying with an earpiece plugged into a transistor radio is common.
But the early radio enthusiasts _had_ to wear earphones because it takes
considerable power to activate a loudspeaker and the received signal was
quite weak. Some means of increasing, or amplifying, the signal was
needed if the process was to advance beyond this primitive stage.[7]
The use of vacuum tube or electron tube amplifiers is so widespread that
it is unnecessary to explain their operations here in any detail. It is
important that the principle of amplification be understood, however.
The input or information wave causes the grid to act as a sort of faucet
as shown in Figure 6. That is, it controls the flow of electrons (the
current in the circuit) from cathode to anode. A weak signal can
therefore cause a similar, but much stronger, signal to appear in the
circuit. The larger signal is subsequently used to power a loudspeaker
in the radio set.
[Illustration: Figure 6 _Amplification by a three-element vacuum
tube._]
Power source
Cathode
Grid
Input wave
Anode
Output wave
The amplification principle can be applied in another equally important
way. Once a signal gets started in the circuit, part of it can be _fed
back into the input_ of the circuit. Thus the signal is made to go
“round and round”, continuously regenerating itself. The device has
become an _oscillator_, that is, a frequency generator that produces a
steady and temporally coherent wave. The frequency of the wave can be
rigidly controlled by suitable circuitry.
The oscillator plays a vital part in radio transmission, for a
transmitter beams energy continuously, not just when sound is being
carried. The oscillator generates what is called a “carrier wave”.
Information, such as speech or music, is carried in the form of audio
(detectable-by-ear) frequencies, which ride “piggyback” on the carrier
wave. In other words, the carrier wave is _modulated_, or varied, in
such a way that it can carry meaningful information. The familiar
expressions AM and FM, for example, stand for Amplitude Modulation and
Frequency Modulation—two different ways of impressing information on the
carrier wave. Figure 7 shows a basic and an amplitude- (or height-)
modulated wave.
[Illustration: Figure 7 _(a) Unmodulated radio wave._ _(b)
Amplitude-modulated wave carries information._]
The electron tube made its giant contribution to radio, television, and
other electronic devices by making it possible to generate, detect, and
amplify radio waves.
Because radio waves are easily controlled, something useful can be done
with them. Suppose we set up five radio transmitters, all beaming at the
same frequency. The waves might look like those shown in Figure 8.
Although the waves are temporally (or time) coherent, they are out of
step, and not _spatially coherent_. But since good control is possible
in radio circuits, we can force each antenna to radiate in _phase_ (that
is, in step) with the others, thus producing fully coherent radiation
(Figure 8).
[Illustration: Figure 8 _(a) Spatially incoherent radiation._ _(b)
Spatially coherent radiation._]
Such a process can increase the radiation _power_ to an almost unlimited
degree. But it does nothing to solve the problem of the limited total
carrying capacity of the radio spectrum.
The most obvious and best way out of this difficulty is to raise the
operating frequencies into the higher frequency bands. There are two
reasons for this. First, it is clear that the wider the frequency band
(the number of frequencies available) with which we work, the greater
the number of communication channels that can be created.
But second, and more important, the higher the frequency of the wave,
the greater is its information-carrying capacity. In almost the same way
that a large truck can carry a bigger load than a small one, the greater
number of cycles per second in a high frequency wave permits it to carry
more information than a low frequency wave.
However, high frequencies must be generated in different ways than low
frequency waves are; they require special equipment to handle them.
Radio waves are transmitted by causing masses of free electrons to
oscillate or swing back and forth in the transmitting antenna. (Any time
electrons are made to change their speed or direction they radiate
electromagnetic energy.)
Each kind of oscillator has some limit to the frequencies at which it
can operate. The three-element electron tube has been successfully
developed to oscillate at frequencies up to, but not including, the
vibration rate of the microwave region. Here ordinary tubes have trouble
for the unexpected reason that free electrons are just too slow in their
reactions to oscillate as rapidly as required in microwave transmission.
To overcome this obstacle, two new types of electron tubes were
developed: the klystron in 1938 and the traveling-wave tube some 10
years later. These lifted operation well up into the microwave region;
it was the klystron that made wartime radar possible. Today many
communication links depend heavily upon microwave frequencies.
At this point in our story we have a situation where low temporally
coherent radio waves and microwaves can be generated, but nothing of
higher frequency. Communications engineers have gazed wistfully, but
almost hopelessly, at light waves, whose frequencies are millions of
times higher than radio waves. Thus, just by way of example, some 15
million separate TV channels could operate in the frequency range
between red and orange in the visible band.
What, then, is the problem?
Why is light so much more difficult to handle?
LIGHT AND THE ATOM
Since light waves have such high frequencies, a different mode of
generation comes into play. We can no longer count on the controlled
movement of free electrons _outside_ atoms and molecules. Rather, light
and all the radiations in the higher frequencies are generated by the
movement of electrons _inside_ atoms and molecules.
Let us review momentarily the modern, albeit highly simplified,
conception of an atom. Remember that no one has yet seen one. We
describe the atom on the basis of how it acts, as well as how it reacts
to things scientists do to it.
For the present purpose, the best model we have of the atom is that of a
miniature solar system, with a nucleus or heavy part at the center and a
cloud of electrons dashing around the nucleus in fixed orbits.
The term “fixed orbits” is used advisedly.
Our planet moves in a certain orbit around the sun. If we attached a
large enough rocket to the earth we theoretically _could_ move it closer
to or farther away from the sun. In the atom, we have learned, this
cannot be done. An electron can only exist in one of a certain number of
fixed orbits; different kinds of atoms have different numbers of orbits.
We might think in terms of an elevator that can only stop at the various
floors of an apartment building. Each upper floor is like an orbit of
the electron. But you get nothing for nothing in the world of physics,
and just as it takes energy to raise an elevator to a higher floor, it
takes energy to move an electron to an outer orbit.
Hence the atom is said to be raised to higher _energy_ levels when an
electron is nudged to an outer orbit. The energy input can be of many
different kinds. Examples are heat, pressure, electrical current,
chemical energy, and various forms of electromagnetic radiation. If too
much energy is put into the elevator it goes flying out the roof. If too
much energy is put into the atom, one or more of its electrons will go
flying out of the atom. This is called _ionization_, and the atom, now
minus one of its negative electrons and therefore positively charged, is
called a positive _ion_.
But if the _right_ amount of energy is put into the atom, one of its
electrons will merely be raised to a higher energy level. Shown in
Figure 9, for instance, are the ground state (Circle No. 1) and two
possible higher energy levels. As you can see there are three possible
transitions.
[Illustration: Figure 9 _Schematic representation of the electron
orbits and energy levels of an atom. Each circle represents a
separate possible orbit and each arrow a possible energy level
difference._]
The higher energy levels are abnormal, or excited, states, however, and
the electron will shortly fall back to its normal (ground state) orbit
(assuming some other electron has not fallen into it first). In order
for the electron to do this (go back to its normal orbit), it must give
off the energy it has acquired. This it does in the form of
electromagnetic radiation.
The energy difference between the two levels will determine what kind of
radiation is emitted, for there is a direct correlation between energy
and frequency.[8] If the energy difference between the two levels is
such that the frequency of emitted radiation is roughly between 10¹⁴ and
10¹⁵ cycles per second, we see the radiation as light. When more energy
is added, the radiation emerges as ultraviolet or X rays. In other words
the higher the energy difference, the higher the frequency, and vice
versa. Thus it is that cosmic rays, with the highest frequencies known
to man, can pass right through us as if we weren’t there.
This simple picture of energy levels and associated frequencies doesn’t
quite hold for ordinary white light, however. Such light is generally
produced by a process called incandescence, which results from the
heating of a material until it glows. True, the atoms of the
incandescent material are being raised to higher energy levels by
chemical energy (as in fire), electricity (light bulb), or nuclear
energy (the sun). In a hot solid, however, the explanation becomes more
complicated. Many different electronic configurations are possible and
the differences in energy among the various levels (which can be many
more than the three shown in Figure 9) vary only slightly from one
another. The result is a wide band of radiation.
Thus, while the incandescent electric bulb is a great advance over fire,
it is still a very inefficient source of light. Because it depends upon
incandescence, a considerable portion of the electrical input goes into
the production of unwanted heat, for the bulb’s filament radiates in the
infrared as well as the visible region.
For providing illumination, the fluorescent tube is far more efficient
than the incandescent lamp: a 40-watt fluorescent tube gives as much
light as a 150-watt incandescent light. This is because its radiation is
more controlled, operating more in accord with our description of
electronic energy levels. Hence more of its output is in the desired
visual region of the spectrum.
In certain types of lighting, particular energy level changes may
predominate, leading to the characteristic colors of neon tubes and
vapor lamps. Although the resulting radiation bandwidth is narrow enough
in these devices to appear as a definite color instead of the broad
spectrum we know as white, it is still quite broad. In other words, the
radiation is still frequency incoherent—and it is still spatially
incoherent.
To understand this, let us return for a moment to the group of radio
antennas we showed in Figure 8. All of them, you will recall, could be
made to radiate in phase. In the production of light, however, each
antenna is replaced by a single atom!
This creates two problems. First, because the energy stored in the atom
is quite small, it comes out not as a continuous wave but as a tiny
packet of radiation—a _photon_.[9] It has an effect more like the hack
of an ax than the buzz of a power saw.
Second, atoms are notoriously “individualistic”. When a batch of atoms
in a material has been raised to higher energy levels there is no way to
know in what order, or in what direction, they will release their
energy.
This kind of process is called _spontaneous emission_, since each atom
“makes up its own mind”. All we know is that within a certain period of
time—a short period, to be sure—a certain percentage of these higher
energy atoms will release their photons.
[Illustration: Figure 10 _Ordinary light is a jumble of frequencies,
directions, and phases._]
What we have, then, is incoherent radiation—a jumble of frequencies (or
colors), directions, and phases. Such light, symbolized in Figure 10,
works well enough in lighting up this page, but is almost worthless as a
carrier of information (and in other ways, as we shall see shortly).
About the best that can be done with it is to turn it on and off in a
sort of visual Morse code, which is exactly what is done on the blinker
communication systems sometimes used for ship-to-ship communication.
In other words, ordinary light cannot be modulated as radio waves can.
It is of interest to note, however, that ordinary white light _can_ be
made coherent, to some extent, but at a very high cost in the intensity
of the light. For example, we might first pass the light through a
series of filters, each of which would subtract some portion of the
spectrum, until only the desired wavelength came through. As can be seen
in Figure 11, only a small fraction of the original light would be left.
[Illustration: Figure 11 _Obtaining coherent radiation the hard way.
Filters and pinhole block all but a small amount of the original
radiation._]
Incoherent
Filters
Coherent in time
Pinhole
Coherent in time and space
We would then have monochromatic (one color) light, which is temporally
coherent radiation, but it would still be spatially incoherent. In our
diagram, we show three monochromatic waves. If we then passed this light
through a tiny pinhole as shown, most of these few remaining waves would
be blocked; the ones that got through would be pretty much in step. (In
a similar manner, a true point source of light would produce spatially
coherent radiation; but, as in the process described here, there
wouldn’t be very much of it.)
We have, finally, obtained coherent light.
The important thing about the laser is that, by its very nature, it
produces coherent light automatically.
Now....
WHAT’S SO SPECIAL ABOUT COHERENT LIGHT?
So desirable are the qualities of coherent light that the complicated
filtering process described above has actually been used. For example,
one British experimenter, Dennis Gabor, used it in the 1940s in an
attempt to make a better microscope. But so great was the effort, and so
meager the resulting light, that this project was abandoned.
In the course of Dr. Gabor’s experiments, however, he did manage to make
a special kind of picture, using coherent light, which he called a
_hologram_. He derived the name from two Greek words meaning a _whole
picture_. We shall see why in a moment.
Ordinary black and white photographs merely record darks and lights, or
the intensity of the illumination, thereby providing a scale of grays,
nothing more. But because waves of coherent light consistently maintain
their relative spacing, they can be used to record additional
information, namely the distance from objects.
For example, if we shine a beam of coherent (laser) light between two
objects we can, knowing the light wavelength, determine the distance
between them to a high degree of accuracy. The basic idea is diagramed
in Figure 12. It can be seen that the number of waves times the
wavelength gives the precise distance (to within 1 wavelength of light)
from the laser source to each object. But this would be a difficult
process to implement.
A better way, and one that is already in operation, is to use
conventional methods to measure the approximate distance and use the
laser beam for precise or fine measurement. In the device shown in
Figure 2, the beam is split into two parts. One part is kept in the
instrument itself to act as a reference. The other is aimed at a
reflector, which sends it back to a detector in the main device, where
it is automatically compared with the reference beam. If the two beams
are in phase (that is, if their crests are superimposed), the waves
combine and produce a high intensity beam at the detector. As the
reflector moves closer to or farther away from the laser source the beam
intensity decreases and then increases again as the wave crests move in
and out of phase. The instrument counts the changes and displays the
distance the reflector moves, as a function of the wavelengths, on the
control cabinet meters.
[Illustration: Figure 12 _Principle of distance measurement using
coherent light. Wavelength times number of waves gives precise
distance between laser and object._]
Distance to be measured
Laser
Object No. 2
1 Wavelength
Object No. 1
Since the Word For the Interaction of the Waves in a System Like This
Is “Interference”, the Measurement Process Is Called _interferometry_
(Pronounced in Ter Fer Om E Try). Although Not New, It Can Now
Be Applied For the First Time in Machine Tool Applications,
Providing the Accuracy Needed in This Age of Space Technology and
Microminiaturization. Measurements With a Laser Interferometer Can
Be Made With an Accuracy of 0.5 Part Per Million at Distances Up to
200 Inches. Such Precision Was Previously Unheard of in a Machine
Shop Environment, Having Been Limited to Laboratory Measurements, and
Only at A Range of a Few Inches. Under Similar Laboratory Conditions,
Measurements by Laser Interferometry Now Detect Movements of 10⁻¹¹
Centimeter, a Distance Approaching the Dimensions of an Atomic Nucleus.
Now let us suppose we expand the laser beam as shown on page 22, and,
with the aid of a mirror, direct part of it (the reference beam) at a
photographic plate. The remaining portion of the diverging beam is used
to illuminate the object to be photographed. Some of this light (the
object beam) is reflected toward the plate and carries with it
information about the object, as indicated by the wavy line. In the
region where these two beams intersect, interference occurs, and a
sample of this interference is recorded within the photographic
emulsion. Where two crests meet a dark spot is recorded; where the waves
are out of phase the processed plate is clear. The result is a hologram,
a complex pattern of “fringes”, characteristic of the contour and light
and dark areas of the object, as well as its distance from the plate.
These fringes have the ability to diffract light rays. When light from a
laser, or a point source of white light, is directed at the hologram
from the same direction as the reference beam, part of the light is
“bent” so that it appears to come from the place once occupied by the
object. The result is a remarkably realistic 3-dimensional image.
There, in a nutshell, is the incredible new technique of holography. The
extreme order of laser light is illustrated by the regularity of the
dots on the cover of this booklet.
This strange kind of light provides us with yet other advantages.
Indeed, one of the most important is the fact that the energy of the
laser is not being sprayed out in all directions. All of it is
concentrated in the narrow beam that emerges from the device. And it
_stays_ narrow. Laser light has already been shone on the moon, the beam
spreading out to only a few miles in traveling there from earth. The
best optical searchlight beam would spread wider than the moon itself,
thus dissipating its energy.
It is for this reason, as well as its temporal coherence, that laser
light is being considered for communications. A narrow beam is
particularly important for space communications because of the long
distances involved.
But it is also possible to focus laser light as no light has ever been
focused before. At close range a laser beam can be focused down to a
circle just a few wavelengths across, concentrating its energy and
making it possible to drill holes only 0.0002 inch in diameter. The
photo on page 52 shows the exquisite control that can be exercised.
Let us see what this focusability means in terms of power. Consider, by
way of analogy, a dainty 100-pound lady in a pair of spike-heeled shoes.
As she takes a step, her weight will be concentrated on one of those
heels. If the area of the heel is, say, one quarter of a square inch (½
× ½ inch), the pressure exerted on the poor tile or carpet rises to 400
pounds per square inch (4 × 100) and if the heel is only ¼ inch on a
side, the pressure will be 1600 pounds per square inch!
[Illustration: Making and Viewing a Hologram]
MAKING A HOLOGRAM
Object
Object beam
Holographic plate
Mirror
Reference beam
Laser
VIEWING A HOLOGRAM
Hologram
Image
Eye
Coherent light source
What we are getting at, of course, is the fact that the coherence of the
laser beam permits it to be concentrated into a tiny area. Thus whatever
total energy is being sent out by the laser can be concentrated to the
point where its effective energy is tremendous. The sun emits some 6500
watts per square centimeter. Laser beams have already reached 500
_million_ watts per square centimeter.
But the power of the laser does not derive solely from its ability to be
focused. Even an unfocused beam is several times more powerful than the
sun’s output (per square centimeter).
[Illustration: Figure 13 _The typical hologram, looks like a
geometric design, but it contains more information than would an
ordinary photograph. The images below, made from a hologram, show
the detail, apparent solidity, and parallax effect of the
reconstructed light waves. The parallax effect is the ability to see
around the objects just as one could if they were really there. (See
frontispiece.)_]
[Illustration: Model tank]
[Illustration: Tank, from another angle]
The crucial difference between the sun’s light or any ordinary kind of
light and laser light lies in the extent to which the emission of energy
can be controlled. In the production of ordinary light the atoms, as we
know, emit spontaneously, or in an uncontrolled fashion. But if the
atoms could be forced to take in the proper amount of energy, store it,
and release it when we wanted them to, we would have _stimulated_,
rather than spontaneous, emission.
This, however, is practically the same as the amplification principle we
discussed earlier. In that case, a small radio signal is jacked up into
a large one by stimulating an available power source to release its
energy at the same wavelength and in step with the smaller signal.
The question is, how can we do this with light?
CONTROLLED EMISSION
The laser and its parent, the maser, can be traced back half a century
to its theoretical beginnings. The great physicist Albert Einstein is
most widely known for his work in relativity. But he did early and
important work on that other gigantic 20th century scientific
achievement, the quantum theory.[10] In one of his papers, published
first in Zurich, Switzerland, in 1916, Einstein showed that controlled
emission of light energy could be obtained from an atom under certain
conditions. When an atom or molecule has somehow had its energy level
raised, the release of this stored energy could be stimulated by
subjecting the atom or molecule to a small “shot” of electromagnetic
radiation of the proper frequency.
Einstein wrote that when such a photon of energy caused an electron to
drop from a higher to a lower orbit, the electron would emit another
photon of the same frequency and in the same direction as the one that
hit it.[11] In other words, the energy of the emitted photon would be
added to that of the photon that stimulated the emission in the first
place. Here, _potentially_, was light amplification. The three major
factors, absorption of energy, spontaneous emission, and stimulated
emission are diagrammed in Figure 14.
There the matter lay for more than 30 years.
In 1951 Charles H. Townes, then on the Columbia University faculty, was
interested in ways of extending to still higher frequencies the range of
microwaves available for use in communications and in other scientific
applications. Townes and other scientists who were interested in the
problem were to meet in Washington, D. C., on the 26th of April. The
night before the meeting he slept in a small Washington hotel; but he
awoke at 5:30—pondering, pondering the high frequency problem.
He dressed and took a walk, then sat on a park bench and savored the
beauty of azaleas in bloom. But all the while his mind was running over
the various aspects of the problem.
[Illustration: Figure 14 _An atom can release absorbed energy
spontaneously or it can be stimulated to do so._]
Before After
Excited state —–— —•—
Absorption ~~→
Relaxed state —•— —–—
Excited state —•— —–—
Spontaneous emission
Relaxed state —–— —•— ~~→
Excited state —•— —–—
Stimulated emission ~~→
Relaxed state —–— —•— ~~→
~~→
Suddenly the answer came to him.
Normally more of the molecules in any substance are in low-energy states
than in high ones. He would change the natural balance and create a
situation with an abnormally large number of high-energy molecules. Then
he would stimulate them to emit their energy by nudging them with
microwaves. Here was amplification.
He could even take some of the emitted radiation and feed it back into
the device to stimulate additional molecules, thereby creating an
oscillator. This _feedback_ arrangement, he knew, could be carried out
in a cavity, which would resonate (just like an organ pipe) at the
proper frequency. The resonator would be a box whose dimensions were
comparable with the wavelength of the radiation, that is, a few
centimeters on a side.
On the back of an envelope he figured out some of the basic
requirements. Three years, and many experiments, later the maser
(_m_icrowave _a_mplification by _s_timulated _e_mission of _r_adiation)
was a reality. The original maser was a small metal box into which
excited ammonia molecules were added. When microwaves were beamed into
the excited ammonia the box emitted a pure, strong beam of high
frequency microwaves, far more temporally coherent than any that had
ever been achieved before. The output of an ammonia maser is stable to
one part in 100 billion, making it an extremely accurate atomic
“clock”.[12] Moreover, the amplifying properties of masers have been
found to be very useful for magnifying faint radio signals from space,
and for satellite communications.
Ammonia gas was chosen for the first maser because molecules of ammonia
have two individual energy states that are separated by a gap
corresponding in frequency to 23,870 megacycles (23,870 million cycles)
per second. Ammonia molecules also react to a nonuniform electric field
in ways that depend on their energy level: low-level molecules can be
attracted and high-level ones repelled by the same field. Thus it is
possible to separate the low-energy molecules from the high, and to get
the excited molecules into the cavity without too much trouble.
This procedure for getting the majority of atoms or molecules in a
container into a higher energy state, is called _population inversion_
and is basic to the operation of both masers and lasers.
It should be noted that two Russians, N. G. Basov and A. M. Prokhorov,
were working along similar lines independently of Townes. In 1952 they
presented a paper at an All-Union (U.S.S.R.) Conference, in which they
discussed the possibility of constructing a “molecular generator”, that
is, a maser. Their proposal, first published in 1954, was in many
respects similar to Townes’s. In 1955, Basov and Prokhorov discussed, in
a short note, a new way to obtain the active atomic systems for a maser,
a method that turned out to be of great importance.
Thus on October 29, 1964, the Nobel Prize in Physics was awarded, not
only to Townes, but to Basov and Prokhorov as well. The award was for
fundamental work in the field of quantum electronics, which has led to
the construction of oscillators and amplifiers based on the “aser”
principle.
A LASER IS BORN
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._]
LASING—A NEW WORD
Now we can begin to put together the various processes and equipment we
have been discussing separately. Perhaps the best way to do this is to
look again at the word _laser_ and recall its meaning: _l_ight
_a_mplification by _s_timulated _e_mission of _r_adiation. Our objective
is to create a powerful, narrow, coherent beam of light. Let us see how
to do this.
In Figure 18 we imagine a laser crystal containing many atoms in the
ground state (white dots) and a few in the excited state (black dots).
Pumping light (wavy arrows in _a_) raises most of the atoms to the
excited state, creating the required population inversion.
[Illustration: Figure 18 _Sequence of operations in a solid crystal
laser. (a) Pumping light raises many atoms to excited state. (b)
Lasing begins when a photon is spontaneously emitted along the axis
of the crystal. This stimulates other atoms in its path to emit. (c)
The resulting wave is reflected back and forth many times between
the ends of the crystal and builds in intensity until finally it
flashes out of the partially silvered end._]
(a)
Ruby crystal
Pumping light
Atom in ground state
Excited atom
Partial reflecting mirror
Full reflecting mirror
(b)
Excited atom emits photon parallel to axis
(c)
_Lasing_ begins when an excited atom spontaneously emits a photon
parallel to the axis of the crystal (_b_). (Photons emitted in other
directions merely pass out of the crystal.) The photon stimulates
another atom in its path to contribute a second photon, in step, and in
the same direction.
This process continues as the photons are reflected back and forth
between the ends of the crystal. (We might think of lone soldiers
falling into step with a column of marching men.) The beam builds up
until, when amplification is great enough (_c_), it flashes out through
the partially silvered mirror at the right—a narrow, parallel,
concentrated, coherent beam of light, ready for....
SOME INTERESTING APPLICATIONS
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._]
A MULTITUDE OF LASERS
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._]
COMMUNICATIONS
Future deep space missions are expected to require extremely high data
transmission rates (on the order of a million bits[16] per second) to
relay the huge quantities of scientific and engineering information
gathered by the spacecraft. Higher data rates are necessary to increase
both the total capacity and the speed of transmission. By comparison,
the Mariner-4 spacecraft that sent back TV pictures of Mars had a data
rate of only eight bits per second—a hundred thousand times too small
for future missions. The use of lasers would mean that results could be
transmitted to earth in seconds instead of the 8 hours it took for the
photos to be sent from Mariner-4.
One of the problems to be solved in using lasers for deep space
communication, oddly enough, is that of pointing accuracy. Since the
beam of laser energy is narrow, it would be possible for the radiation
to miss the earth altogether and be lost entirely unless the laser were
pointed at the receiver with extreme precision. Aiming a gun at a target
50 yards away is one thing; aiming a laser from an unmanned spacecraft
100 million miles away is quite another. It is believed, however, that
present techniques can cope with the problem.
Another peculiarity of laser communication is that it will probably be
accomplished faster and more readily in space than here on earth.
Powerful though laser light may be, it is light and is therefore impeded
to some extent by our atmosphere even under good conditions. Data
transmissions of 20 and 30 miles have already been accomplished in good
weather with lasers.
But if you have ever tried to force a searchlight beam or shine
automobile headlights through heavy fog, rain, or snow, you will
appreciate the magnitude of the problem under these conditions. The use
of infrared frequencies helps to some extent, since infrared is somewhat
more penetrating, but the poor-weather problem is a serious one.
A possible solution is the use of “light pipes”, similar to the wave
guides already in use for certain microwave applications over short
distances. But as often happens, new developments create new needs; how,
for example, can we get the laser beam to stay centered in the pipe and
follow curves? A series of closely spaced lenses, about 1000 per mile,
probably would accomplish this, but too much light would be lost by
scattering from the many lens surfaces.
Scientists are experimenting with a new kind of “lens”, one that uses
variations in the density of gases to focus and guide the beam
automatically. Since there are no surfaces in the path of the light
beam, and since the gas is transparent and free of turbulence, the laser
beam is not appreciably weakened or scattered as it travels through the
pipe.
[Illustration: Figure 31 _Laser light beam being guided through a
“light pipe” by a gas “lens”. Heating coil (lower left) or mixture
of gases (lower right) are two possible ways of maintaining proper
density gradient in the gas._]
Figure 31 shows how the gas focusing principle might be used to guide a
beam through a curving pipe. The shading represents the density of the
gas. Several means have been developed to keep the gas denser in the
center than around the outside. When the pipe curves, the light beam
starts moving off the axis of the pipe. The gas then acts like a prism,
deflecting the light beam in the direction of the curvature of the
“prism”.
In communication between distant space and earth, a light pipe might be
a little cumbersome; hence it may prove necessary to set up an
intermediate orbiting relay station that will, particularly in cases of
poor weather, intercept the incoming laser beam and convert it to radio
frequencies that can penetrate our atmosphere with greater reliability.
Powering space-borne lasers will, of course, be a problem. Indeed one of
the major unsolved problems in production of spacecraft and long-term
satellites is the provision of an adequate supply of power. Fuel cells
and solar cells have helped but do not give the whole answer.[17]
One other approach has already been developed: a sun-pumped laser.
Sunlight focused onto the side of the laser (see Figure 32) provides the
pumping power, enabling the device to put out 1 watt of continuous
infrared radiation, enough for special space applications. Descendents
of this device could produce visible light if this is deemed desirable.
Another approach, using _chemical lasers_, is even more intriguing and
may have greater consequences. Chemical lasers will derive their energy
from their internal chemistry rather than from the outside. A mixture of
two chemicals may be all that is needed to initiate laser action aboard
a spacecraft or satellite. (Chemical lasers also offer the promise of
even greater concentrations of power than have been achieved heretofore,
which may make them useful in plasma research.)
With all these possibilities, it may still be that spacecraft will need
more power than is available on board. The narrow beam of the laser
offers one more fascinating possibility, especially in the case of
satellites relatively near earth. The light of a laser might actually be
used to beam energy to a receiver, either for immediate use or storage.
It would then become possible to “refuel” satellites at will, giving
them much greater capabilities.
If available laser power is great enough, laser beams might even be used
to push satellites back into their proper orbits when they begin to
wander off course, as they almost invariably do after a while.
[Illustration: Figure 32 _Artist’s rendering of sun-pumped laser as
it would operate in space. The sun’s rays are collected by a
parabolic reflector and are focused on the laser’s surface by two
cylindrical mirrors._]
Sun
Parabolic Collector
Hyperbolic-cylindric secondary mirror
Semi-circular-cylindric tertiary mirror
Laser beam
A LASER IN YOUR FUTURE?
Atomic energy, only a scientific dream a few short years ago, is now
providing needed power in many parts of the world. In the same way, the
laser, also an atomic phenomenon, has made its way out of the laboratory
and into the fields of medicine, commerce, and industry. If it hasn’t
touched your life as yet, you need only be patient. It will.
Indeed the most exciting probability of all is that lasers undoubtedly
will change our lives in ways we cannot even conceive of now.
[Illustration: Figure 33 _Tiny hole drilled in paper clip
demonstrates remarkable capability of laser beam. Paper clip is 1¼
inches long. Hole (top) was drilled by the laser microwelder shown
in Figure 1._]
SUGGESTED REFERENCES
Books
_ABC’s of Masers and Lasers_, Allan H. Lytel, Howard W. Sams and
Company, Inc., Publishers, Indianapolis, Indiana 46206, 1966,
96 pp., $2.25.
_The Laser: Light That Never Was Before_, Ben Patrusky, Dodd, Mead and
Company, New York 10016, 1966, 128 pp., $3.50.
_Masers and Lasers_, Manfred Brotherton, McGraw-Hill Book Company, New
York 10036, 1964, 224 pp., $8.50.
_Masers and Lasers_, H. Arthur Klein, J. B. Lippincott Company,
Philadelphia, Pennsylvania 19105, 1963, 184 pp., $3.95.
_The Story of the Laser_, John M. Carroll, E. P. Dutton and Company,
Inc., New York 10003, 1964, 181 pp., $3.95.
_Quantum Electronics: The Fundamentals of Transistors and Lasers_,
John R. Pierce, Doubleday and Company, Inc., New York 10017,
1966, 138 pp., $1.25.
_Lasers and Their Applications_, Kurt R. Stehling, The World
Publishing Company, Cleveland, Ohio 44102, 1966, 192 pp.,
$6.00.
_Understanding Lasers and Masers_, Stanley Leinwoll, Hayden Book
Companies, New York 10011, 1964, 96 pp., $1.95.
_Atomic Light: Lasers_, Richard B. Nehrich, Jr., Glenn I. Voran, and
Norman F. Dessel, Sterling Publishing Company, Inc., New York
10016, 1967, 136 pp., $3.95.
Articles—General and Historical
Advances in Optical Masers, A. L. Schawlow, _Scientific American_,
209: 34 (July 1963).
The Evolution of the Physicist’s Picture of Matter, P. A. M. Dirac,
_Scientific American_, 208: 45 (May 1963).
Filling in the Blanks in the Laser’s Spectrum, F. M. Johnson,
_Electronics_, 39: 82 (April 18, 1966).
The Amateur Scientist—How a persevering amateur can build a gas laser
in the home, C. L. Stong, _Scientific American_, 211: 227
(September 1964).
The Amateur Scientist—Homemade Laser, C. L. Stong, _Scientific
American_, 213: 108 (December 1965).
The Amateur Scientist—How to make holograms and experiment with them
or with ready-made holograms, C. L. Stong, _Scientific
American_, 216: 122 (February 1967).
The Maser, James P. Gordon, _Scientific American_, 199: 42 (December
1958).
The Quantum Theory: Early Years to 1923, Karl Darrow, _Scientific
American_, 186: 47 (March 1952).
Laser’s Bright Magic, T. Meloy, _National Geographic Magazine_, 130:
858 (December 1966).
Infrared and Optical Masers (original paper), A. L. Schawlow and C. H.
Townes, _Physical Review_, 112: 1940 (December 15, 1958).
Laser Market Enters Era of Practicality, W. Mathews, _Electronic
News_, 11: 1 (April 18, 1966).
Lasers, A. K. Levine, _American Scientist_, 51: 14 (March 1963).
Lasers, A. L. Schawlow, _Science_, 149: 13 (July 2, 1965).
Lasers and Coherent Light, A. L. Schawlow, _Physics Today_, 17: 28
(January 1964).
The Laser’s Dazzling Future, L. Lessing, _Fortune_, 67: 138 (June
1963).
Optical Masers, A. L. Schawlow, _Scientific American_, 204: 52 (June
1961).
Optical Pumping, A. L. Bloom, _Scientific American_, 202: 72 (October
1960).
Research on Maser-Laser Principle Wins Nobel Prize in Physics, J. P.
Gordon, _Science_, 146: 897 (November 13, 1964).
Resource Letter MOP-1 on Masers (Microwave through Optical) and on
Optical Pumping, H. W. Moos, _American Journal of Physics_,
32: 589 (August 1964), extensive bibliography. Available from
American Institute of Physics, 335 East 45th Street, New York
10017. Enclose stamped return envelope.
Advances in Holography, K. S. Pennington, _Scientific American_, 218:
40 (February 1968).
Applications of Laser Light, D. R. Herriott, _Scientific American_,
219: 140 (September 1968).
Holography for the Sophomore Laboratory, R. H. Webb, _American Journal
of Physics_, 36: 62 (January 1968).
Laser Light, A. L. Schawlow, _Scientific American_, 219: 120
(September 1968).
The Modulation of Laser Light, D. F. Nelson, _Scientific American_,
218: 17 (June 1968).
Articles—Special Subjects
Biological Effects of High Peak Power Radiation, S. Fine et al., _Life
Sciences_, 3: 209 (1964).
The Interaction of Light with Light, J. A. Giordmaine, _Scientific
American_, 210: 38 (April 1964).
Chemical Lasers, George C. Pimental, _Scientific American_, 214: 32
(April 1966).
Color Laser Stores Data, J. Eberhart, _Science News_, 90: 51 (July 23,
1966).
Communication by Laser, Stewart E. Miller, _Scientific American_, 214:
19 (January 1966).
Guidelines for Selecting Laser Materials, R. H. Hoskins, _Electronic
Design_, 13: _M_29 (July 19, 1965).
Holography: The Picture Looks Good, J. Blum, _Electronics_, 39: 139
(April 18, 1966).
How Dangerous Are Lasers?, L. H. Dulberger, _Electronics_, 35: 27
(January 26, 1962).
Injection Lasers, R. W. Keyes, _Industrial Research_, 6: 46 (October
1964).
Laser Potential in Deep-Space Link Grows, B. Miller, _Aviation Week
and Space Technology_, 84: 71 (January 31, 1966).
Laser Retinal Photocoagulator, N. S. Kapany et al., _Applied Optics_,
4: 517 (May 1965).
Laser Welding in Electronic Circuit Fabrication, J. P. Epperson,
_Electrical Design News_ (EDN), 10: 8 (October 1965).
The Light That Slices Inch into Millionths, (use of interferometry in
industry), _Steel_, 158: 38 (February 28, 1966).
The Optical Heterodyne—Key to Advanced Space Signaling, S. Jacobs,
_Electronics_, 36: 29 (July 12, 1963).
Photography by Laser, E. N. Leith and J. Upatnieks, _Scientific
American_, 212: 24 (June 1965).
Liquid Lasers, Alexander Lempicki and Harold Samelson, _Scientific
American_, 216: 81 (June 1967).
Plasma Experiments with a 570-kJ Theta-Pinch, F. C. Yahoda, et al.,
_Journal of Applied Physics_, 35: 2351 (August 1964).
A Sun-Pumped CW One-Watt Laser, C. G. Young, _Applied Optics_, 5: 993
(June 1966).
3-D Image Made at Home, _Science News_, 90: 185 (10 September 1966).
Scanning with Lasers, Robert A. Myers, _International Science and
Technology_, 65: 40 (May 1967).
Booklets
_Applications of Lasers to Information Handling_, The Perkin-Elmer
Corporation, Norwalk, Connecticut 06852, 1966, 32 pp., free.
Reprint of five talks given by company personnel.
_Laser Interferometer_, Airborne Instruments Laboratory, Division of
Cutler-Hammer, Inc., Deer Park, Long Island, New York 11729,
1965, 20 pp., free. Collection of article reprints.
_Laser: The New Light_, Bell Telephone Laboratories, Murray Hill, New
Jersey 07971, 19 pp., free. Full color, nontechnical brochure
presents some background, principles, and applications of the
laser.
[Illustration: _Argon laser, which emits high-power blue-green beam
continuously, has application in signal processing, communications,
and spectroscopy. This unit is being beamed through prisms that
separate its several discrete wavelengths of light, displayed on
card at left foreground._]
FOOTNOTES
[1]Sometimes referred to as _hertz_ (abbreviated Hz), for the 19th
Century German physicist Heinrich Hertz; 1000 Hz = 1000 cps.
[2]Devised in France and officially adopted there in 1799, the metric
system uses the meter as the basic unit of length and has been
proposed for all measurements in this country.
[3]Named for the Swedish physicist Anders J. Angstrom.
[4]The wavelength, indicated by the Greek letter λ (lambda) is related
to frequency (f) in the proportion λ (in meters) = 300,000,000/f.
(The number 300,000,000 is the velocity of light in meters per
second.)
[5]Microwaves are radio waves with frequencies above 1000 megacycles per
second.
[6]Ten to 30,000,000 kilocycles per second; this is low in the
electromagnetic spectrum, but not low in terms of the radio
spectrum, which has a low-frequency classification of its own.
[7]Primitive as early radios were by today’s standards, they brought a
new era to communication at the time. Unmodulated CW (continuous
wave) transmissions and crystal receivers were used to summon
rescuers in the _Titanic_ disaster of 1912, for example.
[8]Energy = h (Planck’s constant) × frequency. Planck’s constant is the
energy of 1 quantum of radiation, and equals 6.62556 × 10⁻²⁷
erg-sec.
[9]Each photon carries 1 _quantum_ of radiation energy, which is a unit
equal to the product of the radiation frequency and Planck’s
constant (see footnote page 15).
[10]Einstein was awarded the Nobel Prize in 1921 for his 1905
explanation of the photoelectric effect (in terms of quanta of
energy) and _not_ for his relativity theory.
[11]Einstein’s theoretical explanation applies in the case of
stimulation of a single atom. In practical stimulation,
directionality is enhanced by stimulating many atoms in phase.
[12]An atomic clock is a device that uses the extremely fast vibrations
of molecules or atomic nuclei to measure time. These vibrations
remain constant with time, consequently short intervals can be
measured with much higher precision than by mechanical or electrical
clocks.
[13]The 1966 Nobel Prize in Physics was awarded to Prof. Alfred Kastler
of the University of Paris for his research on optical pumping and
studies on the energy levels of atoms.
[14]See _Accelerators_, a companion booklet in this series, for a full
account of the Stanford “Atom Smasher”.
[15]For descriptions of fission and fusion processes, see _Controlled
Nuclear Fusion_, _Nuclear Reactors_, and _Nuclear Power Plants_,
other booklets in this series.
[16]A bit is a digit, or unit of information, in the binary
(base-of-two) system used in electronic data transmission systems.
[17]See _SNAP_, _Nuclear Space Reactors_ and _Power from Radioisotopes_,
other booklets in this series, for descriptions of nuclear sources
of power for space.
This booklet is one of the “Understanding the Atom” Series. Comments are
invited on this booklet and others in the series; please send them to
the Division of Technical Information, U. S. Atomic Energy Commission,
Washington, D. C. 20545.
Published as part of the AEC’s educational assistance program, the
series includes these titles:
_Accelerators_
_Animals in Atomic Research_
_Atomic Fuel_
_Atomic Power Safety_
_Atoms at the Science Fair_
_Atoms in Agriculture_
_Atoms, Nature, and Man_
_Books on Atomic Energy for Adults and Children_
_Careers in Atomic Energy_
_Computers_
_Controlled Nuclear Fusion_
_Cryogenics, The Uncommon Cold_
_Direct Conversion of Energy_
_Fallout From Nuclear Tests_
_Food Preservation by Irradiation_
_Genetic Effects of Radiation_
_Index to the UAS Series_
_Lasers_
_Microstructure of Matter_
_Neutron Activation Analysis_
_Nondestructive Testing_
_Nuclear Clocks_
_Nuclear Energy for Desalting_
_Nuclear Power and Merchant Shipping_
_Nuclear Power Plants_
_Nuclear Propulsion for Space_
_Nuclear Reactors_
_Nuclear Terms, A Brief Glossary_
_Our Atomic World_
_Plowshare_
_Plutonium_
_Power from Radioisotopes_
_Power Reactors in Small Packages_
_Radioactive Wastes_
_Radioisotopes and Life Processes_
_Radioisotopes in Industry_
_Radioisotopes in Medicine_
_Rare Earths_
_Research Reactors_
_SNAP, Nuclear Space Reactors_
_Sources of Nuclear Fuel_
_Space Radiation_
_Spectroscopy_
_Synthetic Transuranium Elements_
_The Atom and the Ocean_
_The Chemistry of the Noble Gases_
_The Elusive Neutrino_
_The First Reactor_
_The Natural Radiation Environment_
_Whole Body Counters_
_Your Body and Radiation_
A single copy of any one booklet, or of no more than three different
booklets, may be obtained free by writing to:
USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE 37830
Complete sets of the series are available to school and public
librarians, and to teachers who can make them available for reference or
for use by groups. Requests should be made on school or library
letterheads and indicate the proposed use.
Students and teachers who need other material on specific aspects of
nuclear science, or references to other reading material, may also write
to the Oak Ridge address. Requests should state the topic of interest
exactly, and the use intended.
In all requests, include “Zip Code” in return address.
Printed in the United States of America
USAEC Division of Technical Information Extension, Oak Ridge, Tennessee
Transcriber’s Notes
—Silently corrected a few typos.
—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
—In the text versions only, text in italics is delimited by
_underscores_.
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