April 1960 Popular Electronics
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Early masers (microwave amplification by simulated emission
of radiation), like the lasers (light amplification...), began
life with the requirement of a rare earth-based mineral at its
core - in this case a ruby. Early applications of the maser,
as reported in this 1960 Popular Electronics article, were centered
on radars. High amplification and high power was beyond the
capability of common semiconductors like Si, GaAs, or GaN. Required
substrate impurities, gate widths, and thermal control was well
beyond the state of the art of the day. As always, the early
pioneers like Dr. Charles H. Townes, inventor of the maser,
accomplished incredible feats with rudimentary tools, including
the venerable slide rule.
The Maser / Receiver Signals from Space in the
November 1960 Electronics World.
The Amazing Maser
This strange amplifier, the heart of which is synthetic ruby,
harnesses the energy of spinning electrons to increase the sensitivity
of receiving equipment up to 100 times
By Miles Dillard
Maser-operated radio telescope at the Naval
Research Laboratory has a range of more than seven billion light
On February 10th of last year, eight scientists gathered
at MIT's famous Lincoln Laboratory near Boston. They carefully
checked over the powerful research radar installed there, then
angled its huge dish antenna sharply into the afternoon sky.
At exactly 2:21 p.m., a pulse shot out from the antenna in the
direction of the planet Venus. Just under five minutes later,
an echo so faint as to be hardly recognizable came back to earth.
Man had made his first direct contact with the planets.
The scientific breakthrough that made it possible for an
electronic signal to complete the 55-million-mile round trip
from Earth to Venus was the invention of the "maser." This amplifying
device effectively makes radar sets up to 100 times more sensitive
than they were before, vastly extending their range. The world's
first true "atomic amplifier," the maser owes its amazing sensitivity
to the fact that it actually harnesses the tremendous internal
energy of the electron's spin.
maser," to give it its full name, is an odd but deceptively
simple-looking gadget. Its heart is a strip of synthetic jewel
which is suspended in a tank of liquid helium and maintained
at the almost unbelievable temperature of nearly 460°F below
zero! Add a few lengths of waveguide-electronic plumbing to
direct the radio waves in and out - and you've got the whole
While the maser's most publicized triumphs
to date have been associated with radar, its accomplishments
have by no means been limited to this field.
Searching Space. Columbia University's Dr. Charles H. Townes,
who invented the maser, and scientists at the Naval Research
Laboratory near Washington have built a maser-operated 50-foot
radio telescope which has a range three and one half times as
great as the best light telescopes - over seven billion light
years! This has opened up for exploration a total volume of
space about 40 times greater than that seen by the 200-inch
Mount Palomar telescope and earlier radio telescopes.
The inventor of the maser, Dr. Charles H.
Townes (above left), of Columbia University, and J. P. Cedarholm
of IBM inspect a gas maser clock which is used in scientific
The great sensitivity of the new 50-foot "space eye" is already
enabling astronomers to learn something about the surface of
Venus for the first time. This has been perhaps the most mysterious
of all the planets because it is eternally obscured by thick
clouds. But the maser telescope can pick up its feeble surface
radiation easily, and astronomers now know far more about it
than ever before. They have recently learned, for example, that
the surface of Venus is a sizzling 585°F - far too hot for the
existence of life as we know it.
But this is just the beginning. Dr. Frank D. Drake of the
National Astronomy Observatory at Green Bank, West Virginia,
predicts that within about a year a radio telescope with three
times the range of the 50-foot unit at the Naval Research Laboratory
and ten times the range of earlier radio telescopes will allow
scientists to "see" the actual surface of Venus and to determine
for the first time its speed of rotation - that is, the length
of its days.
This new instrument is already under construction at the
West Virginia observatory site. Its gigantic dish antenna will
be 600 feet in diameter; two football fields would fit end to
end across its span. Not only will it "see" nearby planets more
clearly, but its tremendous sensitivity will enable it to probe
30 to 40 times as much space as can now be explored with the
50-foot telescope. Its range may extend as far as 20 billion
light years! Astronomers think they will even be able to observe
the "edge of space," where radiation emitted at the time of
the formation of the universe may perhaps be detected or where
space may actually be seen to curve.
Disassembled maser is shown at left; scientist
R. W. DeGrasse of Bell Laboratories holds in his right hand
the strip of synthetic ruby which provides the amplification.
"Such observations," says Dr. Townes, "will quite possibly
indicate whether present ideas of an expanding universe are
correct, as well as providing a means of checking other cosmological
One of the maser's most obvious applications is in the field
of satellite communications. Since tons of fuel must be burned
for every pound of satellite put into orbit, scientists use
every conceivable trick to design the lightest possible equipment
for space probes. With maser amplifiers one hundred times as
sensitive as older types in ground listening posts, smaller
and lighter transmitters can be installed in satellites. Smaller
transmitters also use lighter batteries, saving more weight.
The cutaway view of a maser shows how the
input signal weaves in and out of the row of pins next to the
strip of ruby; each time the signal goes around a pin, additional
energy is radiated. resulting in increased amplification.
Thermal Noise. The maser - this weird piece of frigid hardware
- is able to perform its many tricks because its unique principle
of operation virtually eliminates that old bug-a-boo, thermal
Why is this so important? Let's take a look at the maser's
use in radar and find out. Figure 1 shows a radarscope with
an echo from a nearby airplane. The echo is strong and clear.
But notice the wiggly lines along the bottom of the scope trace.
Radar operators call this irregular pattern "grass." Engineers
call it thermal noise.
If there were no thermal noise, the scope trace with the
same echo would look like Fig. 2. Here, the transmitter pulse
and the echo is unchanged; the only difference is that there
is now no "grass," or thermal noise. It makes little difference
whether or not the thermal noise is there, as long as we get
a strong echo from a nearby target.
But what about that
echo from Venus? By the time a signal travels 55 million miles,
there isn't much of it left. On a maser radarscope, the trace
would look like Fig. 3. Without the maser, it would look like
Fig. 4. Where is the echo? Completely blanked out by thermal
Three Bell Labs scientists, Harold Seidel,
H. E. D. Scovil, and George Feher, are shown here with one of
their brain-children, an early solid-state maser.
How does the maser do away with thermal noise? To answer
this question, let's quickly review the cause of thermal noise.
If you could look inside the tubes and wires of your hi-fi
set, for example, you would see streams of electrons rushing
along in orderly groups. This flow of electrons is the "signal"
that eventually comes out of the speaker as music or speech.
But here and there a few electrons, stirred up by the heat present
in any circuit, scamper around aimlessly. This random movement
generates a small but measurable current of its own, which comes
out as noise. It is called thermal noise, or thermal agitation,
since it is caused by heat: the more heat, the more noise.
You can actually hear thermal noise on your hi-fi amplifier,
just as you can see it on a radarscope. With no signal applied
to your hi-fi set, turn up the volume control and put your ear
next to the speaker .. The hissing sound you hear is thermal
noise greatly amplified. Although such noise is rarely objectionable
in hi-fi amplifiers, it seriously limits the range of radar,
as we have seen.
Since the maser does not depend on electron
flow, there is little random noise created. And even the few
stray electrons that would normally wander about are much less
likely to do so when the maser is dipped in a chilling bath
of liquid helium. At temperatures. close to absolute zero (-473°F),
random electron movement becomes virtually non-existent. A radar
echo from Venus, minute signals from a star six billion light
years away, or a feeble message from a satellite with a small,
light-weight transmitter can come right in without competing
with amplifier noise.
Fig. 1. Radarscope picture of pulse and echo,
with thermal noise along bottom of the trace.
Fig. 2. Radarscope picture of same pulse
and echo, but without any thermal noise or "grass."
Fig. 3. Maser-operated radar has no thermal
noise, allowing very weak echo to be received.
Fig. 4. On conventional radar, thermal noise
completely swamps out echoes of low amplitudes.
How the Maser Works
The heart of the solid-state maser is a strip
of semiconductor material - usually synthetic ruby - placed
in a resonant chamber into which are piped the signal to be
amplified and the so-called "pump" signal.
Scientists tell us that materials such as synthetic ruby
contain electrons spinning at different rates, or to be more
accurate, at different "energy levels." Under normal conditions,
most electrons are at the lowest energy level, which is called.
"Energy Level 1." Fewer electrons are at Energy Lever 2, and
still fewer at Energy Level 3. When an electron "falls" from
a high level to a lower one, it gets rid of its excess energy
by radiating that energy in the form of microwave signals.
To illustrate how the maser works, a mechanical analogy
can be used. This analogy describes the operation of the "3-level
maser," the most common type.
Let us represent Energy
Level 1 by a tank of water, Energy Level 2 by a row of buckets,
suspended above the tank, and Energy Level 3 by a still higher
row of buckets. Valves in the bottom of the buckets on Energy
Level 3 are arranged so that they automatically keep the buckets
on Energy Level 2 full.
Each bucket on Energy Level
2 has a sensitive valve on its bottom which can be opened by
the slightest touch. The system also has a pump which pumps
water from the tank, keeping the buckets on Energy Level 3 full,
which in turn keeps the buckets on Energy Level 2 full.
The maser is now ready to operate. Tiny drops of water shooting
into the system from the outside will hit the valves on the
bottom of the buckets on Energy Level 2, releasing large amounts
of water. These small drops of water represent small incoming
signals, which in actual masers cause the electrons on Energy
Level 2 to drop to Energy Level 1, and thereby radiate their
excess energy. The amount of energy they radiate is far more
than the amount needed to trigger them. Therefore, a small signal
coming into the maser is amplified into a large one.
Because of complex technical considerations, it is easier
in practice to "pump" electrons up to Energy Level 3 and let
them drift down to Energy Level 2 than to pump them directly
to Energy Level 2. The "pump" used in an actual maser is an
oscillator operating at a frequency higher than the signal frequency
to be amplified. In practice, the cavity in which the maser
is placed must be resonant at the signal and pump frequencies.
The gas maser is a "2-level maser," which operates on
a slightly different but similar principle. The word "maser"
stands for "Microwave Amplification by Stimulated Emission of
Spinning Electrons. When Dr. Townes invented
the maser in 1954, he was not looking for a new type of amplifier.
In the process of using radio waves to study the structure of
gas molecules, he discovered that the energy of the spinning
electrons in the gas could be tapped under certain conditions,
and that it would give off microwave radiation similar to radar
waves. Eventually he found a way to make the electrons radiate
large amounts of energy when stimulated by small amounts, a
reaction similar in some ways to a vacuum tube's action in controlling
a large current flow with a small signal. Thus, a new amplifier
working on an entirely new principle was born.
first instrument was a "gas maser," as opposed to the solid-state
maser mentioned earlier. One of its first applications was in
the world's most accurate atomic clock. Because the energy radiated
by the maser's electron spin vibrates at an extremely constant
rate, he was able to build a clock regulated by these vibrations
that was accurate to within one second in a hundred years! With
similar clocks, scientists are now measuring the rotation of
the earth so accurately that soon we will know if it is actually
slowing down, as many think.
The gas maser has also
been used to confirm Einstein's theory of relativity concerning
the velocity of light. Earlier attempts had been hampered by
the lack of a timing device of sufficient accuracy. The maser
clock enabled scientists to prove conclusively that the theory
Although the gas maser operated perfectly
in atomic clocks and a few other devices, it was not a particularly
efficient amplifier, so investigators began looking around for
other materials to which the principle of the maser could be
applied. A team of Bell Telephone Laboratories scientists, headed
by Dr. H. E. D. Scovil, constructed a series of successful designs
which used semiconductor solids, some of them similar to those
used in transistors. So far, synthetic ruby has proved to be
one of the more effective materials, and many of today's atomic
amplifiers are made of this material.
The maser seems likely to be cast in a starring role when National
Aeronautics and Space Administration and Bell Telephone scientists
try to transmit high-frequency signals from coast to coast and
across the Atlantic by bouncing them off satellites. (See "Communications
Satellites-Key to World-Wide TV," POPULAR ELECTRONICS, March,
1960.) Specially designed maser amplifiers and powerful antennas
are now under construction at Bell Labs in New Jersey. And when
world-wide TV becomes a reality, the maser will play an important
If the space probe scheduled to be fired
into orbit around Venus this year is successful, scientists
on earth will listen to its cryptic messages with maser receivers.
And, of course, masers will be on hand when man himself takes
the big step into space and wants to communicate over vast distances
back to his home planet.
New uses, some based
on startlingly original concepts, are proposed regularly. For
example, work has begun on the development of masers operating
at frequencies so high that they are actually visible light.
Techniques for generating infrared and visible light rays and
for transmitting them like radio waves - although still far
in the future - may open up entirely new applications for the
Are masers likely to show up in our home
TV and radio receivers? Well, there are some tremendous technical
problems that have to be solved first. For example, the earth
gives off feeble radiations which can jam the super-sensitive
maser. Maser devices that have been successful so far overcome
this problem by using sharply directional antennas pointed up
and away from the earth's radiation. Consequently, maser-operated
home television receivers seem unlikely until the day comes
when our TV stations broadcast from satellites.
Although at this time we can only guess where future developments
may lead, we can be sure that the maser and its applications
will grow increasingly valuable - both here on earth and in
the empty vastness or space when man leaves his planet to explore
Posted July 24, 2012