February 1961 Popular Electronics
Table of Contents
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is a really good introduction to the way a laser works. The article
talks about a couple Bell Telephone scientists who pioneered long
distance laser communications back in 1960. Their experiments began
with rather large chemical lasers on the rooftops of buildings separated
by 25 miles. We've come a long way since then, with laser communications
now taking place within the confines of a semiconductor integrated
Introducing the Laser
Brightest Light in Electronics' Future
day last September, two Bell Telephone scientists, R. J. Collins
and W. S. Boyle, stood on a hill at the company's laboratory in
Holmdel, New Jersey. Beside them, mounted on a tripod, was a brass
cylinder a little bigger than a flashlight. At a precise moment,
one of them touched a button on some nearby electronic equipment.
Instantly, a brilliant red flash shot from one end of the cylinder.
Two other Bell scientists, D. F. Nelson and W. L. Bond, standing
on a rooftop 25 miles away, were able to see the flash with their
This accomplishment - transmission and detection
of a light flash over a 25-mile distance - seems unremarkable enough.
Yet Dr. George Dacey, Bell's Director of Solid State Electronics
Research, thought otherwise. Hearing of the experiment's success,
he made a simple but solemn pronouncement: "A new era of communications
A new kind of light never before seen on earth
is the product of the laser - a device which taps the power of the
electron's spin to generate a light beam of unparalleled intensity
and purity. What does the laser offer science? Just this:
- true amplification of light for the first time in history
- the first truly coherent (single-frequency) beam of light
ever produced by man
The weird light of the laser has a number of properties that may
well make it the most promising development in communications -
and in a few other fields as well- since De Forest put the grid
in the vacuum tube. Soon-to-be-available devices making use of the
laser's unique abilities include such wonders as:
What the Laser Is.
- super-precise radar with a beam hundreds of times narrower
than anything now available
- an atomic clock 1000 times more accurate than the best current
models which do not stray more than one second in one hundred
- a super heater that can pour out thousands of watts of energy
into an area the size of a pinhead
- a radio transmission system of such tremendous capabilities
that it could carry more than 10,000 simultaneous television
signals using only a single channel
The laser, for all its revolutionary
properties, actually stems from another development several years
old. As you may have noticed, there's a similarity between the words
"laser" and "maser," and the similarity is more than coincidence.
A laser is simply a maser capable of operating at frequencies within
the visible light range.
Dr. Charles Townes of Columbia
University - the inventor of the maser - suggested some time ago
that there seemed to be no reason why his device could not operate
in the visible light range. Now years of theoretical work by both
Hughes Research Laboratories and Bell Telephone Laboratories in
solid-state electronics have proven him right! (For details on the
maser, see April 1960 issue of POPULAR ELECTRONICS.)
spite of its tremendous promise, the laser is an extremely simple-looking
device. It is nothing more than a cylinder of synthetic ruby about
1/4" in diameter and 1-1/2" long, mounted in the center of a spiral
coil of glass. The coil is a xenon-filled flash tube, very much
like the ones used by photographers for taking flash pictures.
To operate the gadget, scientists send a jolt of current
through the gas-filled tube, setting off a brilliant flash of greenish
light. The electrons in the ruby absorb this light, and generate
energy at another frequency. To put it another way, the ruby absorbs
greenish light, only to give off a pure red ray. And the beam produced
by this atomic flashlight is capable of performing the feats mentioned
earlier - as well as a number of others - because it is unique in
several important ways. Let's see just what makes the laser's light
HOW THE LASER WORKS
laser is is small rod of synthetic ruby which absorbs light
energy at one frequency and emits light at another frequency
or color. Its operation depends on the fact that the ruby contains
chromium atoms which can be at any one of at least three different
energy levels, as illustrated at left.
The lowest level
- A - represents the area where the atoms will normally be.
If. however. a photon of light from outside the system hits
one of the chromium atoms, that atom absorbs light energy and
is lifted to a more excited stare, represented by level C. Almost
immediately, it falls back to level B, giving up a little of
the energy absorbed from the photon It remains at level B for
a relatively long period, as measured in atomic time - perhaps
as much as ten microseconds Eventually, it falls back to level
A, and in the process gives up the rest of the energy absorbed
from the photon. This emitted energy is in the form of red light.
The process described so far is normal fluorescence
- just like that which takes place in fluorescent lighting.
In the fluorescent bulb, ultraviolet light is used to excite
the atoms of fluorescent material, which then give off a white
light. But the separate quantities of light given off by the
electrons are not in phase. Instead. they are random. or - to
use the scientists' word - incoherent; in a way, they are similar
to radio noise. The laser, on the other hand, generates a coherent
signal - a signal of one frequency, with all electromagnetic
light radiation in phase.
An intense green light is
beamed at the ruby. Thi light "pumps" huge quantities of chromium
atoms into energy level C These atoms quickly fall to level
B, where they remain for a while. Occasionally, one atom spontaneously
fall back to energy level A, emitting red light. But there are
so many atoms now at energy level B that the spontaneously emitted
light from the atom that falls will almost certainly bump into
another chromium atom at level B This collision will cause the
second atom to give off its energy in phase with the first atom
The energy from the second atom bumps into another atom, and
The chain reaction builds rapidly. Because the
ends of the rod arc silvered, the emitted light bounces back
and forth, stimulating still more atoms to give up their energy.
Soon, tremendous quantities of red light are rushing back and
forth in the rod like water sloshing back and forth in a bathtub.
Finally, it reaches such a level of intensity that it bursts
t through one end of the rod (one end has less silver than the
other) and shines forth in a brilliant, coherent ray.
The light generated by the laser
is coherent. This means that all its rays are at one frequency.
Natural light, in contrast, whether produced by the sun, a light
bulb, or a match, is made up of rays of many different colors, or
frequencies. Even light sent through a colored filter contains many
frequencies, although far fewer than "white" light.
containing many frequencies is roughly comparable to a completely
un-tuned radio signal or a raucous noise. Such a hodgepodge signal
is impossible to control effectively. About the only thing you can
do to transmit information with such an undisciplined mixture -
whether light, radio frequencies or just plain noise - is to turn
it on and off to form a simple code. Ships, of course, have been
using blinker lights for years.
With the laser, we have
a coherent light source for the first time. We can control it in
the same sophisticated ways we take for granted in radio. In addition,
because of the extremely high frequencies at which light is transmitted,
we can perform a number of tricks impossible with radio.
For example, fantastic amounts of information can be packed
into one light beam. With such a system, we may some day transmit
thousands of television signals and hundreds of thousands of telephone,
teletype, and telegraph signals on a single laser beam!
In addition, the laser, by operating in the visible light spectrum,
vastly increases the number of useful frequencies we can put to
work. Heretofore, we have been able to use frequencies up to about
50,000 mc. (See chart above.) But even though this upper limit has
been gradually pushed back, the need for additional space to accommodate
the ever-growing load of world-wide communications has grown much
faster. Now the laser, in one jump, has extended the range of useful
frequencies tremendously. As Dr. Theodore H. Maiman of Hughes Aircraft
put it recently, "The laser jumps the gap from 50,000 million cycles
to 500,000 billion cycles, opening the way for a host of important
Heart of maser is silver-ended
ruby rod, placed in coiled, xenon-filled tube.
The coherence of laser light is responsible
for another useful property: it makes the laser beam far narrower
than any previously available. For example, a high-quality military
searchlight - the kind used to spot raiding aircraft during World
War II - produces a beam approximately one degree in width. One
mile from the light, the beam is about 85 feet wide. This may sound
impressive, but only until we compare it with the laser beam - which
will ultimately be able to illuminate a spot approximately 5 inches
in diameter a mile away!
Another comparison: the beam from
the military searchlight, if directed at the moon, would spread
to cover an area 3600 miles in diameter, bigger than the moon itself.
But the laser beam would illuminate a spot on the moon's surface
less than 10 miles in diameter, without any optical help at all.
And one scientist predicts that with a proper setup of lenses, the
diameter of the spot could be reduced to two miles!
spectrum, showing new frequency region opened by laser. Note laser's
wide operating range.
Of course, like every device, the laser has
its limitations. Even the higher microwave frequencies now in use
are partially blocked by clouds, dust, fog, and atmospheric moisture.
Laser beams, at still higher frequencies, are affected even more.
As a result, a point-to-point laser communications link would be
put out of commission by fog, or perhaps even by rain.
there are ways of getting around this problem. Bell scientists have
already demonstrated that laser beams, like microwaves, can be transmitted
through hollow pipes or "waveguides." Thus, communications engineers
may simply lay waveguides from city to city and literally pipe through
huge amounts of information, regardless of weather or other conditions.
What Lies Ahead?
As is the case with most
new developments, no one knows for sure in just how many ways the
laser will turn out to be useful. Dr. Townes predicts that it will
push back the frontiers of spectroscopy, revealing further secrets
about the basic nature of matter. Distances will be measured with
far greater precision than ever before, using Doppler-type radar.
And there will undoubtedly be many other as yet undreamed of applications
for this newest wonder child in the field of electronics.
When will the laser actually go to work? Although it is still
in the experimental stage, it should soon be earning its keep. Out
at Hughes Aircraft, Dr. Maiman is investigating laser radar. Because
of the extremely narrow width of the laser beam, such a radar would
be able to pinpoint the location of a distant target to within a
few feet, far more accurately than present-day equipment.
How great an impact is the laser likely to have on the field
of communications? Right now, it's anybody's guess. But those in
the field make no secret about the fact that they are tremendously
enthusiastic about this new gadget. With usable frequencies already
badly overcrowded in many regions of the present radio spectrum,
any system that promises to open up vast new chunks of space is
something to get excited about.
Perhaps the potential role
of the laser in communications is best illustrated with a remark
made by Dr. R. J. Collins of Bell Labs' laser development team.
Said Dr. Collins, "We're not ready to start replacing telephone
lines yet. But, "he added with a smile, "we're beginning to think