November 1960 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
When really good researchers set out to write books on history,
they do not simply cull information from the publications of
fellow contemporary authors; instead, they look for sources
that were published during or around the time of the subject
being covered. Doing so helps minimize the possibility that
inaccuracies have crept into the knowledge pool and that information
other authors might have either deemed insignificant or have
missed can be recovered. With a bit of luck, sources are discovered
that have never been used before. That is part of my motivation
for going to the trouble of buying these vintage magazines and
posting articles like this one which reports on early maser
developments. It delves fairly deeply into the solid state physics
of rare earth minerals that some of the first masers and lasers
relied upon to function, including energy band diagrams and
If the "sugar scoop" antenna looks familiar, it might be
due to its rising to fame as the result of Dr.s Arno
Robert Wilson having serendipitously discovered the background
cosmic radiation of the universe's creation while using it after
Project Echo shut down. The discovery led to a
Nobel Prize in Physics in 1978.
Articles here on RF Cafe which mention the Dr. Robert W. Wilson and Dr. Arno
Penzias are The Maser & Sugar
Scoop Antenna: Receiver for Signals from Space,
Laboratories Project Echo,
The Amazing Maser: The Jewel That Conquers Space,
Signals from Sun and Stars, and
The Maser: Receiver for Signals from Space
By Martin I. Grace & Joseph G. Smith
Airborne Instruments Laboratory, Div. of Cutler-Hammer Inc.
Radar pulses bounced back from distant planets, communications
with space vehicles, receivers to listen for radio signals from
outer space, satellite-reflected telephone and TV microwave
signals - these are all possible because of the remarkable maser,
which uses atomic forces within a super-cooled ruby to amplify.
Some two-hundred times better than a good, conventional radar
receiver, the maser receiver affords scientists in the fields
of space communications, radar, and radio astronomy the possibility
of amazing improvements. This is because the maser receiver
is an almost theoretically perfect receiver insofar as sensitivity
and low noise are concerned. Radar range can be increased, or
for the same range, the transmitter power can be cut by a factor
of 10. Satellite and deep space communication will be extended.
Coast-to-coast and continent-to-continent microwave communication
links without repeater stations (i.e., via satellite reflector)
should be realized. Radio astronomers will be able to "see"
much deeper into space, helping to answer some of the basic
questions about the universe, and perhaps, discover another
civilization. This is not as preposterous as it sounds. Project
"Ozma" has been initiated to listen continuously for intelligent
transmissions of radio signals from outer space. The project
will use a maser for the ultimate in listening range.
Dr. R. H. Kingston prepares the maser for
the Venus observations at M.I.T. Lincoln Laboratory's Millstone
Hill Radar Observatory.
An excellent example demonstrating the ability of a maser
to amplify very weak signals was the Venus radar bounce disclosed
last year. In this experiment conducted at the Millstone Radar
Site of the MIT Lincoln Laboratory, a radar signal was beamed
at the planet Venus. A small portion was reflected back toward
earth where it was detected by a maser receiver. The scientists
involved in the bounce admit the project would have been unsuccessful
without the maser. Calculations showed that the transmitter
power necessary for the bounce, using a conventional receiver,
would have been so high that the air directly in front of the
antenna would have been ionized.
Prof. C. H. Townes of Columbia University coined the word "maser"
by taking the first letters of its more technical description,
"microwave amplification by stimulated emission of radiation."
Prof. Townes proposed a maser in 1953 and experimentally verified
it three years later with a maser which was essentially an ultra-stable-microwave
type oscillator. Based on these results, Prof. N. Bloembergen
of Harvard University proposed in 1956 a maser capable of continuous-wave
amplification and, within three months the theory was embodied
in an operating unit.
Lincoln Laboratory's Radar Observatory at
Westford, Massachusetts. Venus was "seen" with the maser-equipped
dish at the right. Frequency used was about 300-500 mc.; pulse
power was 265 kw.; dish size was 84 feet. Maser is at feed point
of dish antenna.
How It Works
The maser is a rather exotic piece of equipment, combining
a rare gem (usually ruby) and extremely low temperatures to
produce amplification. Unlike most other devices in electronics,
the maser makes use of bound electrons to accomplish amplification.
There is no flow of electrons as in a tube or transistor. It
is a property of these bound electrons that they spin on their
axes very much the same way as the earth spins on its axis.
We all know that a moving charge is a current and, associated
with every current is a magnetic field. The spinning electron
is charge in motion and this causes the electron to have a magnetic
field. Normally the spin is randomly oriented but when this
"electron spin" is placed in a d.c. magnetic field, the electron's
magnetic field normally aligns itself parallel to the d.c. field.
The action is similar to that of a compass needle.
Close-up of Harvard maser mounted at antenna
feed. Protective covers have been removed to show electromagnet
at bottom and dewar structure that is utilized. The receiving
waveguide horn is located at left.
The electron spin with its field aligned with the d.c. field
possesses a discrete amount of energy. Now, the electron spin
can take up another position, namely, against the field. Energy
must be expended in "flipping" the spin over. Thus, an electron
spin aligned against the d.c. field has more energy than one
aligned with it. In a substance containing many of these spins,
the condition pictured in Fig. 1 will exist. Some spins will
be aligned with and some against the d.c. field. It can be seen
that two definite energy levels exist and that the difference
in energy is directly proportional to the applied d.c. magnetic
Fig. 1. Electron-spin energy at various values
of magnetic field strength.
The above condition is for a substance possessing one net
electron spin per atom. If the substance possesses two net electron
spins per atom, it will have three energy levels - one for both
spins aligned with the d.c. field, one for both spins aligned
against, and one for one spin aligned with and one spin aligned
against the d.c. field. In the case of three net electron spins,
there are four energy levels and so on. Chromium-doped ruby,
which is the primary maser material today, has three net electron
spins per atom. Ruby is a single crystal of aluminum oxide with
a small percentage (.05%) of chromium. This small percentage
of chromium is what gives ruby its characteristic red color.
It is also the chromium which provides the three net electron
spins necessary for maser action.
Quantum physics states that the difference in energy (Δ
E) between two levels can be equated to frequency (f) by the
simple formula: Δ E = hf, where h is Planck's constant.
Now we can change the graph of energy levels and write frequency
in place of energy. Fig, 2 shows the energy levels for ruby
plotted in this manner.
Harvard University's 60-foot parabolic dish
with maser installed at feed point.
There are approximately 7 x 1010 electron spins
in a cubic centimeter of ruby and, of these, a certain percentage
of spins are in each of the four energy levels. The number of
spins in each level a function of the temperature of the crystal.
At room temperature the four levels are almost equally populated
but at liquid helium temperature, 4.2° above absolute zero.
the populations of the four levels are substantially unbalanced.
(Absolute zero is also known as zero degree Kelvin, or K, and
represents a temperature of 273 degrees below zero C or 460
degrees below zero F.) The solid bars in Fig. 3 represent a
typical energy population distribution for ruby. From the diagram
it can be seen that the lower energy levels are more heavily
populated than the higher ones.
Fig. 3. Energy-level spin population.
The operation of a 3-level maser can be explained from an
energy level diagram. The term "3-level maser" means that only
three energy levels are used in attaining maser operation even
though there may be a greater number of energy levels in the
material. If we inject into the crystal a strong r.f. signal
from a local oscillator, equal in frequency to the difference
between energy levels 1 and 3, an interesting phenomenon takes
place. Some of the electron spins in the lower level absorb
energy from the r.f. field and jump to level three. If the local-oscillator
r.f. signal, usually called the "pump signal," is strong enough,
the "pump" transition (level 1 to level 3) can be saturated,
i.e., the number of spins in level 3 can be made equal to that
in level 1. The total number of spins in the two levels has
not changed, but now they are equally populated. Levels 2 and
4 are unaffected since their natural frequencies have not been
involved. This "pumped" condition is shown in Fig. 3 by the
dashed bars. It can be seen that level 2 now possesses more
electron spins than level 1. This is contrary to the normal
H. E. D. Scovil of Bell Laboratories points
out to R. W. DeGrasse an input coax of the two-channel traveling-wave
maser designed for satellite communication. This maser is being
used with a horn-reflector antenna.
If a signal which is equal in frequency to the difference
between energy levels 1 and 2 is now fed into the crystal, spins
in level 2 will be stimulated to emit energy to the signal rather
than absorb energy from it. In the process they will flip over
and fall back down to level 1. This giving up of energy to the
signal frequency is amplification. By varying the d.c. magnetic
field, the operating point in Fig. 2 is varied, thereby changing
the spacing of the levels and "maser action" can be obtained
at other frequencies. This is the basic scheme of maser operation.
Sophisticated techniques have been developed using more than
three levels, double-pumping arrangements, and harmonic and
sub-harmonic schemes; but it all boils down to the same fact.
To accomplish amplification, a greater population must exist
in a higher energy level than in a lower one.
Fig. 2. Energy-level diagram for ruby showing
magnetic field and pump frequency needed to operate at 1800
The low-noise characteristic of a maser is more difficult
to explain but it is extremely important. In conventional receivers
the main source of background noise is from random emission
from hot cathodes, shot noise in tubes, and random thermal noise
in resistors. Since, with the maser, amplification occurs without
the use of hot cathodes and tubes, it is logical to expect that
noise from such sources does not exist. Also, the components
that could produce noise are at an extremely low temperature,
a few degrees above the point where all thermal motion ceases
to exist. This is only part of the low-noise story because,
for quantum-mechanical reasons, noise is even lower than the
helium-bath temperature would predict.
Types of Masers
There are two basic types of maser configuration, the cavity
maser and the traveling-wave maser, or TWM.
The first maser amplifiers constructed were cavity masers.
In this type, resonant circuits are used to inject the r.f.
signals into the ruby crystal. The early cavity masers consisted
of a single microwave waveguide cavity resonant at two frequencies;
the pump and the signal. The maser material was placed inside
the microwave cavity. This first type of cavity maser had the
disadvantage of operating at a single fixed frequency. Later,
cavity masers of a tunable nature were designed. A tunable maser
structure is shown in Fig. 4. It consists of a waveguide resonant
cavity which is resonant at the pump frequency, and a quarter-wave
coaxial resonator, resonant at the signal frequency. The coaxial
resonator is constructed inside the waveguide resonator. The
maser material is placed inside the resonant cavity. The r.f.
signals are coupled into the cavity by adjustable loops.
Fig. 4. Tunable maser cavity structure used
in Airborne Instruments Laboratories gear.
The cavity maser is a one-port amplifier; that is, the input
and output have the same common terminals. In order for this
type of amplifier to operate, a non-reciprocal device called
a "circulator" is necessary. The circulator acts as a traffic
cop (see Fig. 5). A signal that enters port 1 is directed to
port 2, a signal that enters port 2 is directed to port 3; and
so on around the loop.
Fig. 5. System diagram of maser operation.
The cavity maser has many inherent problems: (1) it must
be manually tuned; (2) the noise figure is degraded by "lossy"
input components; (3) the unit's stability is not too good;
and (4) saturation effects occur at relatively low power levels.
At the onset of saturation the maser loses gain and becomes
transparent, acting very much like a piece of transmission line.
The receiver's second stage is usually capable of picking up
the saturating signal and completing the reception. Once the
maser saturates, it takes a considerable amount of time after
the saturating signal is withdrawn to return to normal operation.
If, during this time, a low-level signal enters the system,
it is lost. This is the only drawback of the maser. Presently,
this "recovery" time is on the order of 50 milliseconds but
crystal-line materials are being developed which will reduce
this figure to insignificance.
The traveling-wave maser corrected most of the drawbacks
of the cavity type. It employs a transmission type of coupling
between the r.f. signal energy and the crystal rather than the
resonant technique used in the cavity maser. There are many
advantages in this type of unit. The overall system noise figure
is reduced by the elimination of the circulator, it is much
more stable, it can be electronically tuned, its bandwidth is
greater, and its saturation characteristic is improved.
In this method of operation (Fig. 6), the signal enters at
one end of the structure, travels along a transmission line
past a slab of ruby, picking up gain as it goes. The amplified
signal leaves at the other end. In a cavity maser the fields
are built up by a resonant technique to enhance the interaction
of the signal with the crystal. In the TWM, the signal reacts
very weakly with the crystal and the gain per inch of structure
is small. For a signal traveling at its normal speed, the speed
of light, the structure would have to be 50 to 70 feet long
to attain a gain of 20 db. This is not practical so the signal
is slowed down by using a series of metal rods forming a comb
structure in which the signal is bounced back and forth before
it emerges at the output. The signal is then slowed down, and
this increases the reaction time between the signal and the
ruby crystal. Gains of 25 db can be obtained in lengths of 5
to 10 inches in sections of this type. This technique is not
new, it has been used very effectively in many devices. The
traveling-wave tube is an excellent example of a device using
the same technique. The TWM is made oscillation-proof by the
inclusion of small ferrite isolators between each finger in
the slow-wave structure. These isolators look transparent to
a signal traveling in the forward direction but very "lossy"
to a signal traveling in the reverse direction. This eliminates
any regenerative effects which are the primary cause of instability
and oscillations. The pump power is introduced into the crystal
in the same resonant manner as in the cavity maser.
Fig. 6. Comb-type slow-wave structure designed
for traveling-wave maser of type employed in Bell Telephone
Laboratories and Airborne Instruments Labs. equipment.
In order to operate a maser, the ruby crystal must be kept
at a very low temperature, usually at or below 4.2° absolute.
The only substance that is liquid at such low temperatures is
helium. Liquid helium boils (that is, turns into a gas) at 4.2°
absolute. Because liquid helium boils at such a low temperature,
elaborate systems have had to be devised to contain it. In fact,
the whole science of cryogenics has developed around liquid
helium, its production, storage, and effect on other materials
at very low temperatures.
Complete ruby maser developed for Signal Corps by Hughes.
Assembly weighs 25 lbs. and is 30" high and 5" in diameter.
Double vacuum assembly contains liquid helium and nitrogen for
supercooling a half-inch square ruby crystal to only a few degrees
above absolute zero. The ruby and a 12-ounce permanent magnet
are inside the structure at the very bottom of assembly.
Most masers employ a double Dewar system to establish the
continuous cold necessary for operation. A functional sketch
of such a system is shown in Fig. 5 and a number of stainless-steel
double Dewars are shown in the photos. The double Dewar is nothing
more than one vacuum bottle inside another. The inner one is
filled with liquid helium and the outer is filled with liquid
nitrogen to reduce dissipation of the helium (liquid nitrogen
boils at 77° absolute). The maser structure is immersed in the
liquid helium, supported on long narrow stainless-steel rods
which reduce boil off. With proper design, a single charge of
liquid can last from 18 to 24 hours, after this the Dewar system
must be recharged. Cryogenic engineers are developing a closed-cycle
system which will allow continuous operation without recharging.
This type of system collects the helium gas as it boils off,
re-liquefies it, and returns it to the Dewar. When these closed
systems become readily available, we can expect to see greater
use of masers. Currently, the open helium systems are not very
desirable - recharging is tricky and costly, storage of the
liquids is difficult, and the supply of liquid helium is very
The operation of a TWM with a closed-loop refrigerating system
forms a very practical low-noise receiver. It can be remotely
operated for extended periods of time without maintenance.
The systems applications of masers have been slow in coming,
because many components in present systems were not designed
for use with an ultra-low-noise receiver. For a maser to be
useful, the noise contributions of the rest of the system should
be of the same order as that of the maser. It is for this reason
that the Bell Telephone Laboratories had to develop a special
low-noise antenna for use with the maser for project "Echo."
This is the first system engineered to take maximum advantage
of the maser's characteristics.
Among the many systems that can make good use of the low-noise
characteristics of a maser are: preamplifiers for radio astronomy
receivers, radar front ends, and numerous systems that do not
suffer from high background noise.
To take full advantage of a maser one would have to mount
it directly at the feed of an antenna. Giordmaine and Meyer
of Columbia University operated a 9000-mc. radio telescope with
a maser preamplifier mounted on the feed of a 50-foot parabolic
reflector at the Naval Research Laboratory. The effective noise
temperature of the complete system was 85°K with the maser contributing
about 4° maximum and the antenna 20°. The rest of the noise
contributions were due to input losses and the second stage.
If a typical X-band mixer had been used instead of a maser,
a noise temperature of 20000K would not be uncommon. This shows
an improvement on the order of 25 times in the reduction in
noise and hence increase in sensitivity with the use of a maser.
B. F. C. Cooper and J. V. Jelley operated a maser radio telescope
at 1420 mc. with the maser mounted at the feed of a 60-foot
reflector located at the Harvard College Agassiz Station. The
overall noise characteristics of this maser system were similar
to those of the X-band maser radio telescope mentioned previously.
A unique feature of this system was a closed-loop feedback system
to keep the maser gain constant.
The use of a maser as a preamplifier for a radar receiver
was first accomplished by Hughes Aircraft Company. A 10-db improvement
in the effective noise temperature was obtained as compared
to the normal receiver operation. The main problem with a maser
when used with a pulsed radar is that the maser saturates from
pulse leakage. To overcome this deficiency, a special low-loss
ferrite switch had to be constructed. This switch inserted about
30-db loss when the radar was in the transmitting state. In
the receiving state the switch loss was only about 0.25 db.
The overall 10-db improvement with the use of a maser almost
doubled the range of the radar.
The maser has proven that it can detect and amplify signals
better than any other type of receiver. We can expect the maser
to develop from a laboratory curiosity to the workhorse of ultra-low-noise
receiving systems. The major drawback to a maser is the necessity
of maintaining low temperature for operation. Hughes Aircraft
has operated a maser successfully at liquid nitrogen temperatures
using the present maser material (ruby).
In the future we can expect the development of new materials
that will permit operation at higher temperatures. For masers
operating at 4.2°K, there is a recovery time problem, which
will only be corrected with the development of new maser materials.
There are a number of new, materials that look promising for
4.2°K and higher temperatures, among them iron-doped sapphire
and iron- or chromium-doped titanium dioxide (known as rutile
or titania). For frequencies above 10,000 mc., there is no other
type of amplifier that can approach, even closely, the extremely
low noise and high sensitivity of a maser. Masers have already
been operated from 400 to 75,000 mc. Many laboratories are trying
to develop infrared and light masers.1 If successful,
these will be the first amplifiers for such types of electromagnetic
radiation. These are but a few of the things that can be expected.
The future for this amazing receiver is certainly beyond the
See "The Laser - a Light Amplifier" in our September issue.
Posted December 31, 2013