Here is a really good introduction
to the way a laser works. In this 1961 Popular Electronics article, author
Ken Gilmore discusses 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. Back in the day, most
people - including technical types - had never read or heard a description of how
a laser works, so this was a brand new concept. Lasers were a science fiction thing
used as weapons for battling aliens. We've come a long way since then, with laser
communications now taking place within the confines of a semiconductor integrated
February 1961 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Introducing the Laser
Brightest Light in Electronics' Future
By Ken Gilmore
One 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 naked
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 has begun!"
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:
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:
- true amplification of light for the first time in history
- the first truly coherent (single-frequency) beam of light ever produced by man
- super-precise radar with a beam hundreds of times narrower than anything now
- an atomic clock 1000 times more accurate than the best current models which
do not stray more than one second in one hundred years
- 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
What the Laser Is. 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.)
In 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
HOW THE LASER WORKS
A 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
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 so on.
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.
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 so different.
"Coherent" light. 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.
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
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 applications."
Heart of maser is silver-ended ruby rod,
placed in coiled, xenon-filled tube.
Narrow Beam. 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
Frequency spectrum, showing
new frequency region opened by laser. Note laser's wide operating range.
Piping it Through. 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.
But 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
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 about it."
Posted July 13, 2022
(updated from original post on