August 1965 Electronics World
Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
|
Not so long ago the availability and usage of
lasers
was restricted to laboratory and military use, but today they
are ubiquitous in our world. CD, DVD, and Blu-ray players are
found in nearly every home and office. Laser pointers
(including the green ones being illegally
targeted on aircraft), laser distance measuring devices,
laser leveling and alignment tools, laser light shows, laser
cutting, even laser weapons can be found in many other venues,
and at relatively low costs (except the
weapons). Half a century ago, most lasers were fabricated
from rare earth elements whose output powers were measured in
a few milliwatts at most. The cost of even a low power laboratory
experimental device was thousands of dollars. I can remember
seeing something like a 0.5 mW helium-neon laser for hobbyists
offered by Edmund Scientific back in the 1970s for about $200,
and much more for a laboratory quality laser. Power supplies
were hundreds of volts. Typical laser pointers put out around
5 to 10 mW and are made from tiny chips of semiconductors
powered by pen cells or button cells. This article is a fairly
extensive report on the state of laser science in 1965.
Lasers
By Warren Groner Sr. Engineer, Electro-Optics Group Sperry
Gyroscope Co.
An authoritative explanation of the operation of solid and
gaseous lasers.

The significance of such effects as coherence, population
inversion, photon amplification, and stimulated emission is
made clear.
Editor's Note: This is the first in a group of three articles
on lasers. Our objective is to provide the reader with a clear
and accurate understanding of laser operation. Future issues
will feature articles covering the modulation and demodulation
of a laser beam, the injection laser, applications, and laser
measurements.
The first experimental production of electromagnetic waves
by Hertz in 1888 ushered in the age of radio communication.
The reader is surely familiar with the growth and importance
of the communications industry. The milestones which mark its
progress are numbered in consecutive orders of ascending frequency.
The reason for expanding the communications spectrum toward
the higher rather than the lower end is that the rate of information
(e.g., video signals) which can be transmitted is directly proportional
to the center frequency of the carrier wave. In addition, motivation
has been furnished for the expansion of the electromagnetic
spectrum by the greater atmospheric penetration of shorter wave-lengths,
by the increased directionality of transmitted power at higher
frequency, and the needs of various interested parties.
It is curious that ordinary light, recognized early in the
game as electromagnetic waves of extremely high frequency ( ≈1015
cps), was never exploited for communications purposes. To understand
the reason for this neglect and appreciate the importance of
the laser, it is necessary to consider what is required of a
transmitter in a communications link. An effective transmitter
is a generator of electromagnetic waves which radiates a significant
amount of coherent power in a narrow band of frequencies, including
the particular one of interest. But why is coherence desirable?

Fig. 1. The effects of temporal (in time)
and spatial (in space) coherence on three waves.
Need for Coherence
The need for coherence in an efficient generator is sometimes
overlooked by the radio or microwave engineer. For example,
the tacit assumption made when he designs or builds an antenna
for a receiving system is that increasing the area over which
the signal power is collected will increase the signal-to-noise
ratio at the detector. However, this is true only if the phase
of the incoming signal is constant or varies in a predictable
manner over all points of the antenna. If the phase of the incoming
signal changes in a random way from point to point over the
antenna, then the detector can sum only the absolute value of
the incoming power. The sum of phase angles will be zero, in
general, and all modulation will be lost. This correlation of
phase in the signal is just what is meant by coherence. An instantaneous
correlation of phase from point to point in space is called
"spatial coherence" and a consistent correlation in phase at
two neighboring points over a length of time is called "temporal
coherence" (see Fig. 1).

A technician, eyes protected by dark goggles
from the white-hot glare of light, monitors growth of a laser
crystal boule. Painstaking eight-hour process starts with a
crystal "seed" dipped into a crucible filled with molten raw
laser crystal materials. Fixed to a slowly rotating rod, the
seed is pulled up a half inch an hour. The molten metal adheres
to the seed and cools and hardens as it is drawn from the crucible.
The crystal looks like a small icicle before it is cut, machined,
and then polished into a usable, commercial laser rod.
The generators of radio, television, and radar signals exhibit
both temporal and spatial coherence in the emitted signal. Until
the advent of the laser, however, no sources of signal power
operating in what is known as the optical spectrum were coherent
in any but a statistical sense. This fact, more than any other,
has precluded the highly desirable use of these frequencies
for communications other than simple "on-off" switching.
The reason for the lack of coherence in optical sources other
than the laser is related to the lack of correlation among the
motions of the electrons, each electron behaving as a tiny oscillator
which emits light. For familiar light sources, such as tungsten
filaments and gas-discharge tubes, electrical energy is supplied
to create conditions favorable to the emission of light, e.g.,
by heating the filament or exciting the atoms in the gas. The
actual emission process, however, is uncontrolled. Each oscillator
radiates independently of its neighbors. Thus the emitted light,
which is just the sum of all the individual radiations, lacks
both spatial and temporal coherence. The idea of maintaining
a constant phase relation over the oscillators by stimulating
their emission with a wave of the frequency to be radiated was
first proposed by C.H. Townes in 1958. Two years later, utilizing
stimulated emission, Theodore Maiman achieved pulses of coherent
optical radiation from a single ruby crystal.
Since that time the development of the laser, an acronym
for Light Amplification by Stimulated Emission of Radiation,
has proceeded at a phenomenal pace. The intensity, monochromaticity,
and polarization of laser radiation has created potential applications
in medicine, industry, and scientific measurement, in addition
to the obvious application to communications. In conjunction
with this spectacular growth, recognition of the importance
of the laser, or optical maser as it is sometimes called, has
become widespread. A billion-dollar laser market by 1970 has
been predicted, and in 1964 Professor Townes was a co-recipient
of the Nobel Prize for his pioneering work in this field.
The importance of the laser to the engineer and technician
cannot be sufficiently stressed. Before learning the fundamentals
of laser operation, some preparation is required. The laser
is often referred to as a "quantum-electronic" device thus necessitating
the introduction of some important concepts of modem or quantum
physics.
Some Ideas of Modern Physics
In 1900 Max Planck formulated an accurate description of
the spectrum radiated from a black body. (A black body is a
perfect radiator or absorber of energy.) Planck offered the
revolutionary hypothesis that the energies of the electron oscillators
responsible for the radiation are quantized, that is, restricted
in certain integral multiples of a constant which now bears
his name.
Although not too well understood at the time, Planck's "quantum"
hypothesis was successfully employed by Albert Einstein in explaining,
among other phenomena, the photo-electric effect. In explaining
the photoelectric effect, Einstein extended the "quantum" concept
to the radiation itself, by assuming that light interacts with
electrons in a metal as if the light were itself composed of
discrete bundles of energy whose energy E is given by E = hf
where h = Planck's constant and f = frequency of radiation.
Consequently, a light beam can be thought of as a stream of
massless particles called photons which travel at the speed
of light. Each particle contains an energy of hf joules.

Fig. 2. The Bohr model of the hydrogen atom
showing electron jumping from one energy level to another and
emitting radiation.
In 1913, Niels Bohr gave the quantum theory a big boost by
proposing a quantized model for the hydrogen atom. In Bohr's
model the hydrogen atom is pictured as a small, positively charged
nucleus orbited by an electron. Bohr postulated that the rotational
energy of the orbiting electron can have only certain discrete
values. These values define a set of stable electron orbits,
that is, while an electron is rotating in a stable orbit it
does not emit radiation. A region which separates allowed energy
levels is called an energy gap. The electron could change energy
only in a jump in which it either absorbed or emitted a photon.
Thus, the conservation of energy for an electron jump may be
written as Em - En = hfm,n
where Em and En are two allowed energy
states for the orbiting electron, and fm,n is the
frequency of the emitted radiation. Fig. 2 shows this for energy
levels corresponding to m = 2 and n = 1.
Using Bohr's model, one could calculate the frequencies of the
emitted radiation and it was found that these frequencies agreed
almost perfectly with the observed characteristic spectrum of
hydrogen.
In the years following the introduction of Bohr's model,
quantum theory grew in significance and power. Many changes
were made and even a brief sketch of this development would
carry us too far afield for the task at hand. However, some
of the features of the Bohr atom provide a useful introduction
to the important concepts necessary to the description of laser
operation.

Fig. 3. Energy-level diagram for the Bohr
model of hydrogen atom. When electrons make transitions from
a higher energy level to a lower energy level, then radiation
having a certain specific wavelength (in angstroms) and frequency
is produced.
A graphical picture of the Bohr model in terms of energy
is given in Fig. 3, where energy is plotted on the vertical
scale. The horizontal lines are the allowed energy levels and
the vertical connecting lines represent examples of electron
jumps or "transitions" with the wavelength of the emitted radiation
given along the transition lines in angstroms (1 angstrom = 10-10
meter). Energy level E1 denotes the lowest energy
level or ground state for the atom. The other levels (E2,
E3 .... ) represent excited states. Such a representation
is called an energy-level diagram.
The usefulness of these diagrams in atomic physics may be
appreciated when one realizes that the energy-level diagram
for a given atom is peculiar to that type of atom. The energy-level
diagram for an atom is to the atomic physicist what the schematic
is to the electronics technician because such a diagram exhibits
important information about atomic behavior.
Lifetime & Population of Levels
One feature of an atom's behavior not included in an energy
diagram is the lifetime and population of levels. That is, if
the atom at some given time is in an excited state (its electrons
are at any level except the ground state), will it remain there
for all time if undisturbed, or will it spontaneously jump to
some other level and emit radiation? The answer to this question
is that there is always a tendency for an atom to return to
its lowest energy or ground state. Consequently it will spontaneously
make transitions downward until the ground state is reached.
For a gas of unexcited atoms almost all atoms will be in the
ground state.
The situation, however, is complicated by the fact that the
average time an atom remains in a state before decaying to some
lower level depends on what state it is in to begin with. Therefore,
in order to complete the picture, a set of numbers must be made
available which represents the mean lifetimes of the electron
in all its possible states before it decays spontaneously. In
general, these lifetimes are quite short, <10-8
second. However, there exist levels for which the lifetime is
considerably longer and these are known as metastable states.
It also must be remembered that each of the spontaneous transitions
must conserve energy and therefore is accompanied by the emission
of a photon. Photons emitted by spontaneous transitions are
called spontaneous emissions. Because one atom does not know
what another atom is emitting, there is lack of interaction
among atoms, and the resulting emission that is produced is
incoherent.
Stimulated Emission
Recalling what was said earlier about ordinary light sources,
it would appear that these sources rely on spontaneous emissions
for their output. Spontaneous transitions are not the only means
by which a particular atom may return to its ground state.
Consider the hypothetical case of a coherent light or photon
beam traversing atoms of a gas.
When the frequency of such a source coincides with one of
the frequencies of spontaneous emission, atoms are induced to
make transitions between two particular energy levels whose
difference is ΔE and satisfy the relation ΔE = hf.
An important result is that transitions from the upper to lower
energy states are induced in addition to those in the opposite
direction. In fact, the probability that the induced transition
will be in one direction rather than the other depends only
on which level the majority of atoms find themselves. When the
transition is from a lower to an upper energy level (accompanied
by the loss of one photon from the beam), it is called absorption,
and when the transition is from upper to lower energy levels
a photon is emitted and it is called induced or stimulated emission.
Note that in this case the emission was induced by the presence
of an energy source.

Experimental laser radar resembles a battery
of rocket launchers as it is set up for a test. In operation,
the laser beam flashes from the transmitting telescope (right)
and bounces back off a distant target into the receiving telescope
(left). Two smaller telescopes are used for alignment and photography.
If equal numbers of atoms are in each of two levels the beam
intensity will remain constant in traversing the gas. For more
atoms in the upper than in the lower state the beam will see
a net gain of photons or will be amplified by stimulated emissions
in traversing the gas. Because photon emissions will be induced
by interaction with the photons in the beam, which were assumed
coherent, they will all be in phase, and the amplified beam
will be coherent (see Fig. 4). In this way a single spontaneous
emission may be amplified into an intense coherent beam. The
remaining problem to solve is to obtain the higher concentration
of atoms in the upper energy level necessary for amplification.
This is known as population inversion, and this particular topic
will be treated for the particular case of ruby in the next
section.

Fig. 4. Atomic transitions producing emission
and absorption.
The Ruby Laser
Obtaining population inversion involves the addition of energy
to the gas. The process of populating an upper energy level
at the expense of a lower one is called pumping. One possible
method of pumping between two levels whose difference in energy
is ΔE is by supplying electromagnetic energy of the frequency
satisfying the relation hf = ΔE and thus raising the energy
by absorption. This method is efficient only at the start, when
the population of the lower level exceeds that of the upper.
As equal population is obtained the number of upward transitions
becomes equal to the number of downward transitions irrespective
of the pumping energy. Thus, to obtain inversion, a more sophisticated
technique is necessary, involving at least one intermediate
energy level upon which the pumped atoms may be stored.

Fig. 5. The three levels of the chromium
ion in a ruby rod which is involved in pumping and in laser
action.
A simple example of the three-level system is in the chromium
ions present in chromium-doped aluminum oxide, more familiarly
known as ruby. Before continuing, it must be observed that since
the essential property of a gas is that the constituents do
not interact with one another, a lightly doped (<<1%)
crystal is essentially a gas of dopant atoms in a rather special
container. However, their energy levels are somewhat modified
by the presence of the host material. A simplified energy level
diagram of the chromium ion in a ruby crystal is shown in Fig.
5. The intermediate level (3) is metastable with a lifetime
of approximately 10-3 second. The operation is as
follows:
A pumping light of frequency ƒ12 causes transition
between levels (1) and (2). The atoms in the excited (2) state
may return to the ground state spontaneously either directly
or by first stopping at the metastable state. Because the lifetime
of the metastable state is 100,000 times longer than that of
state (2), the atoms which fall there may be considered almost
stationary. The rate at which atoms find themselves in state
(3) is proportional to the rate at which they arrive in (2)
which, in turn, is proportional to the pumping power and independent
of the population. Thus, if sufficient pumping power is supplied,
the population of the (3) state will grow at the expense of
the (1) state without the limitation imposed on the two-level
system, and population inversion is obtained.
As long as the population is inverted, the ruby can be an
amplifier for radiation of frequency ƒ31 and,
as with any amplifier, adding a positive feedback loop can cause
sustained oscillation. In this case what is meant by positive
feedback is the return of some of the output light (ƒ31
radiation) into the ruby. This may easily be accomplished with
mirrors. In fact, by making use of the geometry of the mirrors
so that the feedback is directional, a resonant cavity is formed.
The amplified radiation, referred to as photon amplification,
will build up in a standing-wave pattern familiar in microwave
technology.

Fig. 6. (A) Output of ruby laser. Time scale
is 0.1 millisec./div. The envelope is the pump-lamp flash. (B)
Expanded portion of the trace with time scale of 1 μsec./div.
Incidentally, in the scope traces shown time proceeds from right
to left rather than left to right.
The resonant cavity is formed from the crystal itself by
carefully grinding and polishing the ruby and silvering its
ends. Because of the shortness of optical wavelengths, an essential
difference exists between our crystal cavity and the more familiar
microwave cavity. Calculating the wavelength from the energy-level
diagram, the wavelength corresponding to ƒ31
radiation is found to be 6943Å in vacuum. Thus a ruby
ground to form a cavity 7.3 cm. long (a typical size), has 100,000
nodes in the standing wave and will be resonant for every frequency
that satisfies the standing-wave condition: (n/2)λ = L
where λ is the wavelength and n an integer. For example,
taking n as 105, the difference between resonant
wavelengths Δλ is given by: (Δλ/λ) = (Δƒ/ƒ) =
1/n = 10-5. The cavity is resonant for
a large number of frequencies right around ƒ31
instead of being resonant for only one particular frequency
as in the microwave case.
Practical Considerations
Before turning to a description of operation of the helium-neon
gas laser, it is worth mentioning some practical considerations
which arise in carrying out the scheme just described. First,
the pumping power required to obtain population inversion for
a reasonable size crystal is considerable, and may be accomplished
only in brief bursts of light from a flash lamp. The operating
time of the ruby laser is therefore limited to a couple of milliseconds.
Second, while the ruby is lasing, the metastable state is being
depopulated by stimulated emission, and quickly (in 10-6
second) outruns the pump, causing lasing action to stop until
the pump can again create a population inversion. Hence, the
output of a ruby laser consists, typically, of a series of irregularly
spaced spikes about 10-6 second in duration in an
envelope defined by the pump lamp duration. Fig. 6 shows an
oscilloscope trace of the output of a photo detector receiving
light energy from a ruby laser.

Fig. 7. Operation of four-level system as
in gaseous laser.
Finally, we may consider the over-all efficiency of the ruby
laser by forming a percentage from the ratio of total output
of laser light energy to the electrical energy supplied to the
pump. This efficiency is typically less than 1% with most of
the lost energy heating the ruby. This makes cooling the crystal
an important practical consideration.
The Gaseous Laser
To understand the operation of the helium-neon gas laser,
a different means of obtaining population inversion as well
as of pumping must be considered. The modification of the energy-level
scheme is shown in Fig. 7. Note that a fourth or terminal level
has been added above the ground state. The population inversion
is now obtained between the (3) and (4) levels. The advantage
of the four-level scheme is that the initial population of the
terminal level is negligible compared to the ground state and
therefore inversion is more easily obtained (i.e., less atoms
in the (3) state are necessary for its population to exceed
that of the (4) state than that of the ground level) . This
reduces the pumping power required and opens up the possibility
of pumping by a different method, called electron-collision
pumping.
In quantum theory a striking analogy is found for an electron
beam traversing atoms of gas and that of a photon beam. Only
when the kinetic energy of the electrons coincides with the
differences in energy between any two levels are atoms induced
by collisions with the electrons to make transitions between
these levels. As in the electromagnetic case, the most probable
direction of the transition depends only on the relative populations
of the states. Hence, as an alternative, we may consider pumping
atoms of gas with accelerated electrons in a discharge tube
instead of by a light beam. These may be the result of a glow
discharge when r.f. energy is applied to the discharge tube.
The obvious advantage of this method is we can maintain this
energy constant over extended periods of time and obtain a continuous
laser output. This presupposes the availability of sufficient
pump energy for inversion.

Fig. 8. Example of resonant transfer by collision
of gas atoms.
The first successful operation of a laser by collision pumping
required the presence of two gases, such as helium and neon,
in the discharge tube to realize the proper energy scheme. A
slight digression is necessary to consider the transfer of energy
between atoms of different gases. A sort of resonance phenomenon
is encountered where energy transfer proceeds only when an energy
gap is shared. That is, an atom of one type of gas (A) in a
given energy level (2A) may transfer its energy to an atom
of another type (B) in a stage (1B) via a collision if, and
only if, there exist energy levels 1A and 2B such that E2A - E1A = E1B - E2B
(see Fig. 8). As before, the probability of the transition direction
is determined by the population of the levels.
In this gas laser, the higher and lower energy levels are
in different gases, and their population may be modified by
changing the relative concentration of the different gases in
the discharge tube. This additional control was fundamental
in achieving the first observed laser action in a gaseous mixture
of helium and neon.

Fig. 9. Energy levels of helium and neon
during laser action.
Fig. 9 shows the pertinent sections of the energy-level diagrams
for helium and neon with the transitions indicated. Note that
when the (3) level is well populated by pumping energy, there
is amplification for two different frequencies, ƒ34
and ƒ34' Note also that the alternate route
1, 2', 3', 4' should result in amplification at the frequency ƒ3'4'.
Lasing action has been observed at all of these frequencies
in helium-neon mixtures. Selection of oscillation between these
frequencies is accomplished by using feedback mirrors with reflectivities
at the different frequencies.
It must be pointed out that the above discussion by no means
covers all known lasers. In fact, the list of laser materials
grows on an almost weekly basis. Laser action has been observed
in other solids doped with small quantities of rare earth ions,
almost all the noble gases, and even in some special liquids.
In addition, the observation of coherent light generated by
injection currents in semiconductor diodes such as gallium arsenide
and gallium phosphide has added another important class of laser
(called injection lasers) to this fast-growing field.
It is interesting to compare in a general way the helium-neon
laser to the ruby crystal laser. In the first place, the atoms
in a gaseous discharge tube are more widely spaced than the
dopant atoms of even a slightly doped solid. A longer path length
in the discharge tube is usually necessary to obtain sufficient
optical gain for lasing. The helium-neon laser resonant cavity
is, therefore, generally larger than that of a ruby laser. However,
the over-all efficiency of the helium-neon laser (total laser
output power/pumping power) is considerably higher than that
of ruby. Consequently, power supplies for ruby lasers are most
often larger than those for the helium-neon type.
The most striking difference between these devices is in
their outputs. Whereas the ruby laser output is typified by
irregular spiking, the helium-neon laser is capable of a continuous
wave of extremely narrow bandwidth. This great disparity in
emissions has divided the two types of lasers into their respective
fields of application. Microsurgery, microwelding, micromachining,
and pulsed-type communications systems require high instantaneous
powers for short durations and consequently have adopted the
ruby laser. On the other hand, continuous communications requiring
sophisticated demodulation techniques such as heterodyning,
as well as the practical usage of light interferometry, find
the gaseous type of lasers well suited for this particular application.
Posted June 30, 2015