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OCR for page 123
interactions Betvveen
the Science and
Technology of Lasers
John R. W hinnery
In his history of lasers and masers, Mario Bertolotti (1983) asks
the question in one chapter title, "Could the laser have been
built more than 50 years ago?" The answer seems to be yes.
Certainly, the principle of stimulated emission was known, and
spectroscopists such as Ladenburg and Kramers understood
that this could lead to "negative absorption." The technology of
the 1930s was sufficient for some kinds of lasers the relatively
simple helium-neon laser, for example. But practical masers and
lasers did not come until physicists such as Charles H. Townes,
who understood the principle, were motivated to take advantage
of the unique properties of these devices by their work with
radar in World War II (Townes, 1984~. Since that time, there
has been continuing interaction between the science and its
applications, so the laser is an ideal example with which to study
the interrelations between engineering and science in a new
technology. ~
Nearly all new developments have a mix of science and
engineering, but in some the science seems to come first; in
others, the engineering. The science of thermodynamics is
generally considered to have developed through the efforts of
Carnot, joule, and Rankine to improve the steam engine (Mum-
ford, 1963, chapter 5; Bernal, 1970, chapter 2~. In contrast, the
major applications of electricity followed the scientific discover-
ies of Volta, Oersted, Faraday, and Maxwell (Bernal, 1970~. The
electrical example is an interesting one in its continuing inter-
iAlthough several historical examples are cited in this paper primarily
U.S. there is not room for completeness and fairness. Readers seeking further
detail are referred to Bertolotti (1983), Bromberg (1986), and Townes (1965).
323
OCR for page 124
124 JOHN R. WHINNERY
THE MASER AS THE
CRITICAL STEP
actions over the nearly two centuries since Volta. Consider, for
example, how the very practical effort to develop the incandes-
cent lamp led, through the Edison effect, to l. l. Thomson's
discovery of the electron. From this came not only the techno-
logical field of electronics but also the base for atomic physics
and much of the science of the twentieth century.
A more recent example, parallel in many ways to the laser, is
the transistor. Because of its base in solid-state physics, it is not
surprising that the transistor was developed by physicists, who
used the purified materials developed for microwave detectors
during World War II. Later developments were carried on by
scientists and engineers working together, with the result being
both a new technology and also dramatic contributions to
scientific instrumentation and computation.
Although we know now that many lasers are simpler than most
masers, the microwave amplifier using stimulated emission, or
maser, was, in fact, the first device to use stimulated emission in
a practical way. The science needed for this was, first of all, an
understanding of the energy levels of molecules and solids, but
the specific principle was that described in Einstein's 1916 work
on stimulated emission. The technology needed was that devel-
oped during World War II for microwave radar magnetron
and klystron sources, semiconductor detectors, and waveguid-
ing networks. But as Townes has described in his writings
(Townes, 1965, 1984), his understanding of the noise problems
of practical amplifiers provided the motivation for his seminal
contributions to the field. Bromberg has described how other
engineering knowledge motivated and became part of the field
(Bromberg, 19861.
In 1951 Purcell and Pound demonstrated population inver-
sion in a nuclear spin system by rapid reversal of the magnetic
field.2 Although a brilliant experiment, it was not intended as a
practical device; the first practical stimulated emission device
was the ammonia-beam maser of Gordon, Zeiger, and Townes
described in 1954. This was quite a complicated device, so the
three-level, solid-state maser proposed by Bloembergen and
demonstrated by Scovil, Feher, and Seidel became the important
quantum electronics device of the 1950s. The maser's promise
2Bibliographic information about work cited from this point on in this paper
will be found in Bertolotti (1983), unless otherwise stated.
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INTERACTIONS BETWEEN LASER SCIENCE AND TECHNOLOGY ~ 25
was as an oscillator of unusual stability and an amplifier with
extremely low noise characteristics. Since the simpler parametric
amplifier satisfies many of the low-noise requirements, the
maser remains a somewhat specialized device today, but it was
an important avenue to the laser.
Beginning with the announcement of the ammonia-beam laser,
there was speculation concerning the possibility of extending to
the infrared, the visible, and the ultraviolet. (The names
IRASER, LASER, and UVASER had brief currency for these
respective ranges.) Knowing now the simplicity of some lasers, it
is interesting to speculate about why it took so long to make the
extension or, indeed, why the maser had to come first. One
reason is psychological. In radio it had always been hard to make
the extension to higher frequencies from broadcast to ultra-
high frequency (UHF), from UHF to microwaves, and from
there into the millimeter-wave range. Consequently, most work-
ers assumed the same would be true as one moved into the
infrared and beyond. And of course, a good deal of understand-
ing and technique had to be developed before devices that now
seem "simple" really were so. Nevertheless, in 1958 Schawlow
and Townes wrote the classic paper setting down the recipe for
making a laser. The principles of that paper are still valid today.
It is also somewhat surprising that the first operating laser was
a ruby laser, since it is by no means the easiest material to make
lase. Ruby (chromium ions in an alumina lattice) is a system in
which the lower laser level is the ground state, so that more than
half the atoms must be pumped to higher levels before a
population inversion is possible (or nearly so; there is some fine
structure that helps a little). Nevertheless, Maiman's first ruby
laser was not an accident. Maiman had worked with ruby as a
material for masers, knew its energy levels well, and had
calculated the optical pumping power necessary for the laser
action. So for ruby, the laser was the natural progression from
the maser.
Once the possibility of lasing action was established, other
laser systems followed quickly. Many other ions in a variety of
host solids were shown to lase. Lasers with neodymium ions in
glass or yttrium-aluminum garnet (YAG) have proved among
the most important. The first gas laser was the infrared helium-
neon laser of pagan, Bennett, and Herriott in 1961. In 1962,
four groups announced lasing action in the semiconductor
LASERS
THE FIRST ROUND
_~
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26 JOHN R. WH~NNERY
gallium arsenide: Hall and colleagues at the General Electric
Company; Nathan and colleagues at IBM; Quist and colleagues
at the Lincoln Laboratories; and Holonyak and Bevacqua at the
Syracuse laboratories of the General Electric Company. Thus,
within 2 years after the first laser demonstration, three basic
categories of laser the ionic solid-state, gas, and semiconductor
lasers had been demonstrated.
LAS ERA
THE SECOND ROUND
It is not surprising that most of the persons mentioned so far are
physicists, since a sound knowledge of spectroscopy and either
discharge phenomena or solid-state physics was required to
understand the various classes of lasers. Engineering depart-
ments had become involved in the research on masers shortly
after the announcement by Gordon, Zeiger, and Townes in
1955, however, and there was little difference between the
courses taken by individual doctoral students in engineering
departments and those in physics departments. Moreover, the
major industrial research laboratories Hughes, the Bell Labo-
ratories, IBM, and others- made little distinction between the
assignments of doctorates with engineering degrees and those
with science degrees. So it is also not surprising that persons with
engineering degrees contributed significantly to the next round
of important laser discoveries.
Ion lasers remain the most important laser source for medium
powers at visible wavelengths. Lasing transitions in ionized
mercury had been reported in 1963, but in 1964 W. B. Bridges
and colleagues at Hughes Research Laboratories reported many
transitions in ionized noble gases, including the important
visible and ultraviolet lines in ionized argon. At about the same
time, C. Kumar N. Patel at the Bell Laboratories, in studying
molecular lasers, predicted and demonstrated the high effi-
ciency of the carbon dioxide laser with appropriate gases added.
Physicists and chemists, of course, continued to make major
contributions, as exemplified by the tunable dye laser demon-
strated by Sorokin and colleagues and by Schaefer and col-
leagues, and the chemical laser suggested by Polanyi and dem-
onstrated by Kasper and Pimentel.
Laser resonator theory is an example of a field developed by
physicists, engineers, and mathematicians, often in collabora-
tion. The open resonant systems proposed by Dicke and by
Schawlow and Townes had been suggested by the Fabry-Perot
interferometer familiar to spectroscopists. The first important
1
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INTERACTIONS BETWEEN LASER SCIENCE AND TECHNOLOGY ~ 27
analysis was a numerical analysis by Fox and Li, using Kirchhoff
diffraction theory. Analytic solutions using the paraxial approx-
imation were then supplied by Boyd and Gordon for rectangu-
lar coordinates, and by Goubau and Schwering for circular
cylindrical coordinates. The latter analysis had been motivated
by the problem of guiding microwaves, but it was also applicable
to optical resonators and lens waveguides. Herwig Kogelnik
then generalized this to show the propagation properties of
Gaussian beams through a variety of elements by means of the
ray matrices (Kogelnik, 1965~. Thus, during this round, major
contributions were made by individuals from different disci-
plines and often by teams of persons with various backgrounds
working together.
DEVELOPMENTS AND
NEW FRONTIERS
There was much development work to do before the newly
discovered lasers could have practical use. Engineers, some
co-opted from other fields and some graduated from the in-
creasing number of quantum electronics programs in engineer-
ing colleges, naturally took part in this developmental phase.
But physicists and others with science backgrounds did also.
Bromberg, from her interviews with E. I. Gordon, explains how
he first used his background in gaseous discharge phenomena to
study the physics of gas and ion lasers in his work at the Bell
Laboratories, but as time went on, he became increasingly
concerned with development problems related to the devices
needed for use with the lasers in practical systems modulators,
beam deflectors, acousto-optic switches, and the like (Bromberg,
1986~. Similar shifts in interest occurred in other development
groups.
The steady developments were punctuated by occasional
"breakthroughs," again coming from contributors from all
fields. The ultra-short optical pulse generation is one example.
Pulse trains from the helium-neon laser were generated by
Hargrove, Fork, and Pollak by the mode-locking technique,
which was suggested in part by Lamb's analyses of nonlinear
self-locking phenomena in gas lasers (Smith et al., 1974~. Anal-
yses of a variety of mode-locking techniques were made by Yariv
and by Harris and colleagues. Shortly after, DeMaria and
colleagues recognized that pulses as short as a picosecond were
possible by passively mode locking neodymium lasers through
the use of a saturable dye within the resonator. An important
extension that produced trains of picosecond pulses from tun-
OCR for page 128
128 JOHN R. WHINNERY
able dye lasers was demonstrated by Ippen, Shank, and Dienes,
and was later extended into the femtosecond regime by a
"colliding-pulse" ring configuration by Fork et al. (19831.3 Still
later, techniques of compressing these pulses by using the
nonlinear effects in fibers were demonstrated by Grischkowsky
and colleagues, leading to the very short pulses. Definitive
analyses of the passive mode-locking techniques were made by
Haus, Siegman and Kuizinga, and New.
Of the researchers abovementioned, Pollak, DeMaria, Ippen,
Shank, Dienes, Yariv, Harris, Haus, Siegman, and Kuizinga had
engineering degrees. A number of the key ideas in this field also
paralleled ideas developed earlier for microwave radar. A1-
though the parallels were sometimes not recognized until later,
they nevertheless helped in the understanding or extension of
the optical techniques. Thus, DeMaria noted the basic similarity
of his passively mode-locked laser system and the microwave
pulse generator demonstrated earlier by C. C. Cutler (1955~.
The pulse compression techniques, made possible by a fre-
quency chirp introduced by the nonlinear fiber, were related to
the pulse compression techniques of chirp radar.
The picosecond and femtosecond optical pulses have been
applied to a variety of purposes, but by far the most important
applications have been to the dynamic spectroscopy of fast
processes. Because of this work, chemistry, biochemistry, and
solid-state physics laboratories now have numerous setups for
the generation and measurement of these very short pulses in
order to study the extremely fast processes that may take place
in organic and inorganic molecules and solids. Other examples
exist, but this remains a textbook case of the way in which science
can create a field of engineering, which in turn makes possible
new science.
INTERACTION AMONG
TH E SOCI ETI ES
The first article on the ammonia maser of Gordon, Zeiger, and
Townes was published in Physical Review, but was later repro-
duced in the Proceedings of the Institute of Radio Engineers because
of the maser's recognized potential as a low-noise amplifier or
ultrastable oscillator. The Physical Review continued to accept
maser papers for some time, until the editors decided that the
field was well enough established that such papers should go to
3For later developments in the generation of short pulses, see the special issue
on picosecond phenomena in IEEE journal of Quantum Electronics, Vol. QE-l9,
1983.
11
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INTERACTIONS BE]WEEN LASER SCIENCE AND TECHNOLOGY ~ 29
more applied journals. (The decision, unfortunately, came just
in time for them to reject Maiman's paper describing the first
laser.) There was then a shift in the physics journals to the
f ournal of Applied Physics and, after 1962, to Applied Physics Letters.
The latter remains one of the most important journals for
publication of new results from laser research.
The Optical Society of America (OSA) quickly recognized the
impact that coherent light would have upon optical research and
published basic papers on this subject in the journal of the Optical
Society of America. Applied Optics was established in 1962, and
from its inception carried articles on lasers, optical resonators
and guides, and electro-optics. Laser papers became an estab-
lished part of the annual Optical Society meetings, and numer-
ous topical meetings, such as those on ultra-short pulse phenom-
ena and integrated optics, became the key meetings in those
fields. Optics Letters was established in 1977 and became a leading
medium for rapid publication in the field.
The Institute of Radio Engineers and its successor, the Insti-
tute of Electrical and Electronics Engineers (IEEE), encouraged
maser and laser papers following its reprinting of the ammonia
maser paper. These papers appeared either in the proceedings
or in the transactions of one of the professional groups. There
was, in fact, competition between different professional groups,
as Bromberg has described (Bromberg, 1986), until the Quan-
tum Electronics Council was established in 1965 with responsi-
bility for the journal of Quantum Electronics. The Council became
a professional group in 1978 and a society of the IEEE in the
same year; its name was changed to the Lasers and Electro-
Optics Society in 1985.
Since each of these major societies established a firm position
in research and development results on lasers and their appli-
cations, some vicious competition might have been expected.
However, the societies cooperated from the beginning. The first
Quantum Electronics Conference was sponsored by the Office
of Naval Research, but later conferences were cosponsored by
one or more societies, with the 1986 International Quantum
Electronics Conference cosponsored by the American Physical
Society, the IEEE, the Optical Society of America, and the
European Physical Society. The annual Conference on Lasers
and Electro-Optics is jointly sponsored by IEEE, the Optical
Society, the European Physical Society, and the Japanese Quan-
tum Electronics Joint Group. IEEE has also joined in the
sponsorship of several of the topical meetings initiated by the
Optical Society. Perhaps the most creative bit of cooperation was
the establishment of the Journal of Lightwave Technology by IEEE
OCR for page 130
~ 30 JOHN R. WH~NNERY
CONCLUSION
REFERENCES
and OSA jointly. This journal emphasizes optical communica-
tion systems, concepts, and devices.
There are other publications related to lasers in the United
States, other societies, and other examples of cooperation.
There are also parallels in other countries and international
meetings. However, these examples establish the existence and
the rewards of this excellent cooperation. Such cooperation
between scientific and engineering societies is, of course, not
unique to the laser field, but the degree to which it has
happened there is at least unusual.
Two coupled systems can, with constant coupling, exchange
activity from the first to the second and then back to the first in
a time determined by the coupling, and so on, as is apparent in
experiments with coupled pendulums. With diffused or random
couplings, the energy that begins in one system may become
more equally shared between the two after an initial transfer
period. The kinds of couplings we have seen between science
and engineering in the laser field have elements of the coupling
example first described, but more of the second. Clearly, the
richness and excitement of this field have come largely from the
interaction between engineering and science concepts, between
scientists and engineers, and between the professional societies.
Progress in laser science and engineering is an admirable model
for cooperation in other new technologies.
Bernal, J. D. 1970. Science and Industry in the Nineteenth Century. Blooming-
ton: Indiana University Press.
Bertolotti, M. 1983. Masers and Lasers An Historical Approach. Bristol,
England: Adam Hilger.
Bromberg, I. L. 1986. Engineering knowledge in the laser field. Technology and
Culture 27(4):798-818.
Cutler, C. C. 1955. Proc. IRE 43: 140- 148.
Fork, R. L., B. I. Greene, and C. V. Shank. 1983. IEEE l. Quantum Electron.
19(4):500-506.
Kogelnik, H. 1965. Appl. Opt. 4: 1562 - 1569.
Mumford, L. 1963. Technics and Civilization. New York: Harcourt Brace.
Smith, P. W., M. A. Duguay, and E. P. Ippen. 1974. Mode locking of lasers, in
Progress in Quantum Electronics, Vol.3, Part III. New York: Pergamon Press.
Townes, C. H. 1965. IEEE Spectrum 2:30 - 43.
Townes, C. H. 1984. IEEE T. Quantum Electron. QE-20:547-550.
Representative terms from entire chapter:
optical society
