July 1963 Popular Electronics
[Table of Contents]
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boy and his father had just witnessed a demonstration of one of
the most promising and fastest developing technological devices
ever conceived by man - the laser. In only three whirlwind years,
the laser - which gets its name from the initials of Light Amplification
by Stimulated Emission of Radiation - has moved out of the theory
stage, out of the laboratory curiosity category, and into a whole
new, exciting world of applications." That's the opening of an article
in the July 1963 edition of Popular Electronics
. I remember
when ruby lasers were the the rule rather than the exception for
lasers. Power levels were measured in units of 'Gillettes' in reference
in the number of razor blades they could cut through. Next came
chemical lasers with power levels in the megawatts and now even
gigawatts that can take out ICBM warheads as they reenter the atmosphere
and fry orbiting satellites. At the same time the realm of semiconductors
and microcircuits was turning out devices on the opposite scale
for use in communications, inertial navigation, and boardroom presentation
pointers. This article is an extensive recounting in layman's terms
the state of the art in 1963.
See all articles from Popular
The "Light" Fantastic
Laser Status Report
One development follows another
in rapid successionBy Ed Nanas
man behind the telescope-like device was ready. "I'll count down
so you won't miss the action," he said. "All set? Here we go - three,
two, one, FIRE!" A pencil-thin beam of red light shot out from a
six-inch ruby rod less than a half-inch in diameter. It smashed
through a stainless steel plate as if the tough metal wasn't even
there. Then it hit a balloon suspended some ten feet away, vaporizing
the rubber. And still the beam of light continued on its narrow
path, burning a small hole in a curtain at the far end of the room.
It was all over in a fraction of a second. "Wow! That light sure
packs a wallop!" exclaimed a teen-age radio amateur.
A c.w. gas laser recently demonstrated by Sylvania. An r.f. field
excites gas mixture to self-sustaining oscillation.
standing at the rear of the hall with his father, an electronics
engineer. "I've never seen anything like it!"
three years ago, nobody had seen anything like it!
The boy and his father had just witnessed a demonstration of one
of the most promising and fastest developing technological devices
ever conceived by man - the laser. In only three whirlwind years,
the laser - which gets its name from the initials of Light Amplification
by Stimulated Emission of Radiation - has moved out of the theory
stage, out of the laboratory curiosity category, and into a whole
new, exciting world of applications.
The laser has
the unique ability to generate and amplify light waves at specific
wavelengths just as radio waves are generated and amplified at specific
wavelengths. Light generated by a laser is known as coherent light
because it is "pure," or predominantly of one frequency. Making
the sun seem like a hand-held flashlight by comparison, the thin
beams of coherent light, which have already vaporized steel and,
in a recent experiment, illuminated the moon, can be used to:
• Transmit a billion simultaneous telephone conversations
on a single thread of light one millimeter in diameter -without
- Build laser radar systems, including portable range finders,
having resolutions more than 1000 times better than conventional
- Perform micro-surgery - delicate eye surgery has already
been demonstrated - precise enough to allow cutting of a single
- Reach billions of miles into space with a beam powerful
enough to guide a spaceship, communicate with life on planets
in other solar systems;
- Construct ultra-precise clocks, guidance systems, and laboratory
- Devise practical underwater communications and ranging systems
using recently developed techniques for generating green or
blue coherent light;
- Build new battlefield weapons, including an anti-missile
device and a form of "death ray";
- Greatly speed up the functioning of complex computers by
using lasers in conjunction with fiber optic paths to transmit
great masses of information within a computer or from one machine
- Investigate the possibility of transmitting electric power
- Speed up chemical processes thousands of times, such as
those that take place during photosynthesis.
Fantastic? Not at all; and the list is by no means complete. The
laser is indeed creating a revolution, and new discoveries relating
to the generation and application of coherent light are being announced
almost daily. Let's take a closer look at the phenomenon of laser
light and the mechanisms used to generate it.
Coherent vs. Incoherent.
TV via laser is an accomplished fact. Image of the young lady
is relayed (in this experiment) to an optical modulator which
impresses it on a c.w. laser beam. The beam strikes a photocell
in a telescope-like receiver (right, rear) and is converted
to the TV signal seen on monitor.
Highest power laser to date, the 350-joule Raytheon model below
may soon be dwarfed by powers as high as 3000 joules. Intense
beam of light is shown blasting its way through a steel girder
a quarter-inch thick.
Continuous wave (c.w.) lasers have reached powers of 9 watts
and are expected to go higher in the near future. Many of these
types use a dysprosium-doped calcium fluoride crystal as does
the RCA model below. The whole apparatus is enclosed by two
hemispherical mirrors which focus light on the crystal. The
crystal itself is obscured by the light source next to it.
Sunlight pumping of a solid-state laser is a new development
which may soon make it possible to put sun-powered laser aboard
satellites for communications, tracking, and geodetic measurements.
This device, designed by RCA, uses a 12" hemispherical mirror
to focus sun on calcium fluoride crystal rod.
Laser rifle is actually a compact rangefinder which fires a
pulse of coherent light, collects the light reflected back from
the target, and indicates exact distance to the target by computing
the elapsed time for pulse's round trip. The device has measured
up to seven miles.
A "Colidar" (for Coherent Light Detecting and Ranging), produced
by Hughes Aircraft Co., operates on the same principle as the
laser "rifle" opposite. Research is underway on other laser
ranging devices.Brightest stars of recent research
are semiconductor and liquid laser devices.
Liquid "frequency converter" that can change the frequency or
color of a laser beam.
One of the first gallium arsenide lasers produced by G.E. (others
were made by I.B.M. and M.I.T.). It is suspended in liquid nitrogen
to keep it cool when large excitation currents are passed through
I.B.M scientists examine a brand-new semiconductor laser announced
very recently: an indium phosphide type.
Light waves coming from
the sun or from incandescent or fluorescent lamps consist of a broad
band of frequencies all mixed together. In addition, light from
these sources can be considered as having been emitted from an infinite
number of sources, all of which have random phases and polarizations
with respect to one another. We call this kind of light incoherent.
A similar thing happens in the radio bands when lightning
is discharged. A whole host of frequencies are generated, and they
can be heard as noise or static on a radio receiver. As another
example, incoherent water waves on the surface of a pond can be
created by throwing in a handful of pebbles; coherent waves by dropping
in a single large size rock.
Both a radio transmitter
and a laser generate coherent radiation that is predominantly of
one specific frequency. The difference between the two is that the
radiation produced by the laser is very much higher in frequency
(and, therefore, much shorter in wavelength), so that it falls within
the optical portion of the electromagnetic spectrum.
The electromagnetic spectrum ranges from extremely low frequencies
where wavelengths (the distance between two specific crests or two
troughs in a given wave) can be measured in miles or meters, to
frequencies far above the visible band where wavelengths are measured
in microns (one-thousandth of a millimeter) and angstrom units (one
ten-thousandth of a micron). In terms of cycles per second, light
waves vibrate at an extremely rapid rate - 1015
be a rough figure.
At visible frequencies, radiation must
be generated on an atomic level - as in a laser. This is made possible
by the fact that the atoms of certain materials, when excited by
large doses of energy, emit light at one frequency or group of frequencies.
Thus, a substance (ruby, for example) made up of atoms which can
be excited, and which will, at a certain point, emit coherent light,
are used in lasers instead of the electron tubes used at lower frequencies.
The Amazing Laser.
The properties exhibited
by laser beams are much more startling than the foregoing explanation
indicates. Like ordinary light, they can be focused and modulated,
but there the similarity ends. A laser beam, because of its extremely
short wavelength and because it is generated at the atomic level
with all of the light energy in phase, is a very narrow beam of
extremely high energy. This energy, concentrated at a single point,
can burn through steel.
The fact that a laser beam can be
modulated is expected to be of great importance in future applications.
The reason is easy to understand. The transmission of a voice by
radio requires a band of frequencies several thousands of cycles
wide. The transmission of a television signal complete with sound
takes up six million cycles of the available spectrum. By and large,
the radio portion of the spectrum is now overcrowded, and the situation
is expected to get progressively worse.
The use of optical
frequencies for communications opens up great new vistas. In the
visible white-light portion of the spectrum alone, the number of
frequencies available is fantastic - 250 million megacycles! This
figure represents thousands of times more frequency space than in
all the radio frequency bands combined. One or two laser beams,
relayed as microwaves are now, could carry all of the communications
traffic in America from coast to coast - telephone calls, television
programs, computer data, and facsimile! Optical
The first successful pulsed optical maser
(laser) generated a peak power of about 10 kilowatts for very short
intervals. The newer lasers have now climbed much higher - recently
one was announced by the Korad Corporation with a peak of 500 megawatts
(500,000,000 watts) - all concentrated in one narrow beam of 7-nanosecond
duration. For the 500-megawatt pulse, it was calculated that the
electric field in the focused electromagnetic beam was on the order
volts per centimeter. The beam was observed
to cause ionization of the air in its focal path with a brilliant
blue flash, and spectacular damage was done to materials placed
at the focal point. Since these gigantic pulses of energy were for
extremely short intervals, the total pulse energy was only about
In only one year, the output energies of
pulsed lasers have spiraled up from 1 or 2 joules (one watt for
one second) to 350 joules. A thousand joules is just around the
corner and may be achieved by the time you read this. A glass laser
with a pulsed output of somewhere between 2000 and 3000 joules is
under development by American Optical Company, and may be announced
as early as August, 1963. Theoretically, there is no limit. Ten-thousand-joule-outputs
are predicted within the next year.
What do all these
figures mean? Just one joule - about the same amount of energy you
get from a flashlight bulb in several seconds of operation - is
enough to vaporize a hole through a 1/32-inch-thick sheet of steel
if it is transmitted in a concentrated burst of about a millisecond.
A 10-joule ruby laser, which is fairly common today,
operating at a wavelength of 0.7 micron, has a power density at
the center of the beam of about 1016
watts per square
meter. The power density at the surface of the sun is less than
watts per square meter. Thus, this rather modest
laser is capable of producing a power density 100-million times
that of the surface of the sun!
THE BASIC LASER
Basic configuration of a low-power c.w. gas
laser. The reflecting plates reflect back a large percentage of
energy; result is a small, continuous output.
The essential ingredients
of a laser are:
A resonant cavity:
Usually formed by two reflecting surfaces, such as precisely
parallel mirrors, one slightly less opaque than the other;
An active medium: Positioned inside
the cavity with its axis perpendicular to the reflecting surfaces.
It may be a (1) gas-a noble gas such as helium, mixed with neon,
and contained in a glass or a quartz tube; (2) crystal-a rod
of high purity ruby, glass or a rare earth material such as
calcium tungstate; (3) liquid-an organic liquid, such as benzene
or pyradine; (4) semiconductor-the newest of the lasers, a gallium-arsenide
Pumping power: Applied to the
active medium to excite its atoms. It may consist of: (1) high-power
lamps - used with crystal lasers; (2) concentrated sunlight
- also used with crystal lasers; (3) electrical or radio frequency
discharge - used with gas lasers; (4) direct electric current
- 10,000 to 20,000 amperes injected directly into the junction
of a diode laser.
The basic principles of laser light
generation are similar to those of the microwave maser. Atoms
in the active medium can possess different amounts of energy.
Ordinarily, an atom will occupy the lowest of several energy
levels, and is said to be in the ground state. But when "pumping"
power is applied, they get excited. That is, the atoms absorb
some of the photons (particles or "quanta" of light) from the
power source and jump to a higher energy level, like water being
pumped into a tank atop a standpipe.
At this higher
energy level, usually two steps above the ground state, the
atoms begin to relax. They fall to an intermediate energy level,
but still above the original lowest level. This intermediate
level is called the metastable condition, because the atoms
are more reluctant to leave it than they were to leave the higher
level to which they were originally excited. In order to make
their departure they must give up the light they absorbed. The
important thing is that the light they give up in dropping back
to the ground state is of a specific wavelength.
or later, (within a few microseconds), the first atoms begin
to drop from the metastable level. They are put to work in the
resonant cavity of the laser. Without the reflecting surfaces,
the light they emit would be mere fluorescence, like that of
a neon sign. But inside the resonant cavity of the laser, they
are bounced back and forth. With each pass parallel to the axis
of the active material, they stimulate other excited atoms in
the metastable level to give off their absorbed light much more
quickly than they would ordinarily. The stimulated light moves
in the same direction as the light stimulates it. With each
pass, the light gains more energy in an effect akin to a chain
In only 200 microseconds or so, the released
light waves, traveling in parallel and in phase back and forth
between the reflecting ends of the laser - you might call it
"feedback" - build up to an intensity great enough to escape
through the one end of the laser which is only partially opaque.
This output beam of light has, .most all of its intensity In
a very narrow cone. All its waves are in step; of the same phase
and frequency, It is coherent light.
A little over a year ago, the first crystal laser
was made to operate continuously (the high-power devices we have
been discussing are pulsed) at Bell Laboratories with power outputs
of a few milliwatts. Gas lasers and semiconductor lasers also have
been made to operate continuously with comparative power outputs.
As POPULAR ELECTRONICS goes to press, however, a new 9-watt continuous-wave
(c.w.) laser is about to be introduced for use in research related
to welding and other machine tool uses.
A 45-watt c.w. laser
may be another major development to be announced in 1963. Both this
unit, which is being researched at M.I.T.'s Lincoln Laboratory,
and the 9-watt c.w. laser use dysprosium-doped calcium fluoride
crystals rather than gases or semiconductors.
lasers are somewhat puny in their power outputs compared to the
pulsed type, they have immense advantages as carriers for communications
purposes. At optical frequencies and with narrow beam angles concentrating
all the radiated energy into a small cone, a television channel
could be established between the earth and Saturn with only about
600 watts, while a voice channel to the most distant planet, Pluto,
could be maintained with as little as five watts.
S. Bayley, of General Precision, Inc., one of some 400 organizations
conducting laser research, has said that an interstellar information
channel carrying one binary bit per second could be set up with
the star Altair (16.5 light-years away) with only 10 watts from
a laser. Already, General Electric has designed a burst communications
system using a rapid laser pulse of great power (rather than a continuous
wave) to carry vast amounts of data. LASER STATUS
Gallium arsenide laser, greatly enlarged
and shown in schematic form, looks like this. Coherent light is
emitted perpendicular to the front and back surfaces and along the
junction of device. Power Transmitters?
In the vacuum of space, attenuation is slight, governed primarily
by the degree of beam spread. Recently, Sperry Rand Corporation
has been able to achieve the minimum theoretical beam spread of
a point source of light -0.005°, or 10-4
radians - without
the use of external lenses. Previously, the already narrow laser
beams, on the order of 0.05°, were further focused down by what
amounts to an inverted telescope.
The ultra-small beam spread
of concentrated optical-frequency energy indicates that it will
be possible to transmit power over great distances with very little
loss; power for spaceships, for instance. It is now possible to
construct optical antennas, nothing more than a series of lenses,
to transmit laser beams which would lose only 1/30th of one percent
in a 20-mile hop - far less than present-day transmission lines.
Thus, if you had a laser putting out one million watts, a hundred
miles away you would receive 997,500 watts: still quite a bit of
power. Laser Radar.
One of the immediately
attainable applications of the laser is in a radar-like system for
measuring distance and velocity, and for tracking. Several such
systems already have been built or are in the works, including those
of Hughes, RCA, General Electric, and Sperry. For example, the RCA
tracking system, using a two-inch corner reflector, is expected
to achieve a range accuracy of six feet over 70 miles. The Sperry
system, using the Doppler effect, can measure the frequency shift
of vehicles traveling at 18,000 miles per hour, or as slowly as
0.2-inch per hour!
Such systems on the moon or in
a satellite hold tremendous promise for guiding spacecraft and rendezvousing
in space. By 1965, laser radars are expected to provide high-resolution
maps of the moon and Mars, yielding new information about their
surfaces that will make landing a man a fairly safe procedure. A
laser ranging and telescope system will go into operation this year
at Cloudcroft, N. M., to track satellites such as the new Discoverer
series; in that area of the country, the weather is generally clear
and lasers can be beamed into space from the ground.
Even the heavy-wheel gyroscope may be on its way out as a result
of laser technology. Sperry Gyroscope - the organization which invented
the gyro - has come up with a closed-circuit ring of lasers which
can be used as an automatic device for guiding ships, planes, missiles
and space vehicles. New laser Types.
of the most important developments in the rapidly changing field
of laser technology came late last year when three organizations,
I.B.M., General Electric, and M.IT., announced almost simultaneously
the development of a semiconductor laser.
of this type of laser-a gallium arsenide (GaAs) diode - are impressive
when compared to crystal and gas lasers. Semiconductor lasers approach
efficiencies of 100 percent as compared to a few percent for other
types; they are excited directly by electric current while other
lasers require bulky optical pumping apparatus; because they are
excited by an electric current, they can be easily modulated by
simply varying the excitation current.
diodes (research models are already available as relatively low-cost,
off-the-shelf items) consist of a layer of p-type gallium arsenide
and a layer of n-type gallium arsenide. When electrons in an intense
electric current, about 20,000 amperes per square centimeter, are
applied to the device, it emits coherent or incoherent light, depending
upon the diode type, from the junction between the two layers of
Current research is concerned with improving
the efficiency of semiconductor lasers, modulating them, and, in
a new twist, using them as "pumps" to improve greatly the efficiency
of other types of lasers.
Developed in a number of forms
are lasers which use rare earth chelates - molecules which completely
enclose each atom of a rare earth element such as europium. Chelates,
combined with plastic, a liquid or other medium, can be pumped to
produce laser action.
One of the most important new laser
techniques is a method for generating light of different frequencies
or colors from a single laser beam. By beaming coherent light through
a liquid such as nitrogen, light of other frequencies is obtained.
Laser frequencies can also be altered by heterodyning two beams
by mixing them within a crystal. These developments make it possible
to convert intense laser beams to any frequency; green, blue, or
from far infrared to near ultraviolet regions.
the year we have seen lasers operating at room temperatures rather
than having to be immersed in expensive cryogenic environments to
keep them cool. Lasers pumped by the sun, by cathode-ray fluorescence,
and by exploding wires, as well as by directly applied electric
current, have come into being in the past twelve months. Raytheon
and M.IT. have bounced a laser beam off the moon. Whereas early
last year you had to build your own laser if you wanted one - an
expensive and delicate process even for the most advanced electronics
engineer-you can now buy a wide variety of laser types.
Continuous wave crystal lasers use
configurations somewhat like this Bell telephone design. The neodymium
crystal is at one focus of a elliptical cavity, and the mercury
lamp at the other to concentrate pumping light.
The Modulation Problem.
on the future usefulness of laser beams in communications are highly
optimistic, much work remains to be done on developing practical
methods of modulation. Communications - including television signals
- have already been transmitted by laser in laboratory setups, but
thus far, only a small fraction of the fantastically large available
bandwidth in a laser beam has been utilized.
approaches to modulation can be divided into two groups: internal
modulation applied while the coherent light is being generated,
and external modulation applied to the light beam after it leaves
the laser. As we noted earlier, the gallium arsenide laser is relatively
easy to modulate using an internal technique. The excitation current
can be simply varied to produce modulation.
of internal modulation, used with other types of lasers, involves
changing the Q of the laser cavity with an electro-optical shutter
between the laser material and a reflecting end plate of the cavity.
This introduces a variable loss which causes large changes in the
level of operating power akin to amplitude modulation.
third approach is Stark-effect modulation, achieved by sending a
strong transverse electric field into the laser material. This field
causes line-splitting and frequency modulation of the output. A
similar technique, using a magnetic field, has also been used. It
is called Zeeman-effect modulation.
of the laser output can be accomplished using the Pockels effect,
in which the beam is passed through a piezoelectric crystal which
can be "strained" by an electrical field. Other external modulation
approaches include the Kerr effect (plane-polarization), varying
the pumping power, and mechanical means, such as the use of shutters,
graings, lenses, reflectors and ultrasonics.
demodulation, the radiation must be converted to electrical energy
in most cases. New phototubes, photomultiplier detectors, and photodiodes
have been developed for this purpose within the last six months.
With coherent radiation, the same techniques will work with light
beams that will work with microwaves, so it boils down to a difference
in detail, not principle. Superheterodyne techniques can be used
to convert the light into lower frequency signals, such as microwaves;
microwave detection equipment can then be employed. "Heterodyning"
is accomplished by "beating" one laser beam with another. The result
is a frequency equal to the difference between the two falling in
the microwave region. Lasers, Present and Future.
As indicated earlier, it isn't only the communications people who
are taking a close look at the laser. At Columbia-Presbyterian Medical
Center in New York, ophthalmologists already have used a ruby laser
beam to coagulate a human eye retina to prevent it from becoming
detached. Such an operation can be completed in less than 0.001
second, eliminating the possibility of damage due to eye motion
during the exposure.
The machining and welding potential
of the laser has already been demonstrated in certain applications.
At G.E., the surfaces of industrial diamonds have been vaporized
the instant the high-energy light beam strikes them. Production
lines are now being set up to use the laser beam in cutting "components"
to size for use in microcircuits, and to weld leads to semiconductors.
The laser also holds the potential of becoming the ultimate
anti-missile weapon. One proposal is to use high-power beams of
several lasers focused on the enemy missile with sufficient energy
to vaporize it. This would be a "clean" weapon compared to anti-missile
rockets with nuclear warheads and their attendant radioactive fall-out.
A new laser scheme which theoretically could generate
a billion joules or more is under development now. It involves the
separation and sorting of hydrogen spins, the physics of which are
too complex to go into in this article. Such a powerful laser could
transmit its beam through the atmosphere and earth cloud cover and
still deliver enough power at the impact point to vaporize a missile.
Power requirements could be sharply reduced, however, by
orbiting an anti-missile laser above the atmosphere. Laser light
could also be used as a spotlight from space for photography at
The laser is less than three years old,
yet we have already come a long way. The experts say that this is
one field in which we are well ahead of the Russians. To understand
the laser is to understand an important facet of the future of communications,
medicine, machining for industry, and the practical equivalent of
the legendary death ray.
Whatever use the laser is put to,
its impact on mankind will be great-comparable, perhaps, to the
discovery of atomic energy. When and how will the impact be felt?
Only time will tell. Posted