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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

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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-

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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

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~ 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.