The Race to Make the Laser

By Jeff Hecht

An excerpt from Laser Pioneers by Jeff Hecht, ISBN 0 12-336030-7, published by Academic Press, 1991. Also available from Amazon.com

 

From Maser to Laser

Charles H. Townes invented the microwave-emitting maser in the early 1950s at Columbia University, with the help of Herbert Zeiger and James Gordon. Masers began proliferating in the last half of the 1950s, but even before then a few physicists were looking at prospects for amplifying stimulated emission at wavelengths much shorter than microwaves. In the Soviet Union, V. A. Fabrikant and his students filed a patent application dated June 18, 1951 on amplifying electromagnetic radiation from the ultraviolet through the radio spectrum, but their idea had little direct effect on laser research, even in the Soviet Union. In the United States, Robert H. Dicke developed in 1954 proposed the "optical bomb," in which a short excitation pulse would produce an inverted population, which would then generate an intense burst of spontaneous emission. In 1956, he filed a patent on using a pair of parallel mirrors, forming a Fabry-Perot interferometer, as a resonant optical cavity.


However, the first detailed proposal for building a laser -- which at the time they called an "optical maser" -- was published by Townes and Arthur L. Schawlow. Schawlow had been a postdoctoral fellow under Townes at Columbia until leaving to join Bell Labs in 1951, and they continued working together on a book on microwave spectroscopy, although not on masers. The two also maintained close personal ties -- Schawlow married Townes' younger sister -- and Townes consulted at Bell Labs. In 1957 both began thinking about the possibility of "infrared and optical masers," and after discussing the idea over lunch at Bell Labs, decided to collaborate. They spent several months working on the problem, as described in their interviews, which led to their famous paper "Infrared and Optical Masers," published in the December 1958 Physical Review.

That paper had a profound impact on American laser development. Preprints circulated at Bell Labs and Columbia before the journal came out, and formal publication was the starting gun for the great laser race that culminated in completion of the first laser. Not everyone realized its importance, however. Bell Labs attorneys did not think the idea was worth patenting, and filed a patent application only after Townes insisted. That led to U. S. Patent No. 2,929,922, issued in 1960.

However, Townes and Schawlow weren't the only ones working on the laser idea. Soon after he started thinking seriously about the laser, Townes talked with a 37-year old Columbia graduate student, Gordon Gould. Gould had been using a technique called optical pumping in his dissertation research under Polykarp Kusch, who shared the 1955 Nobel Prize in Physics with Willis Lamb, and Townes was interested in using the same technique to energize a laser. Gould dreamed of being an inventor, and he was hardly a typical establishment scientist of the placid 1950s. He and his first wife joined a Marxist study group in the mid 1940s. After the 1947 Soviet takeover of Czechoslovakia he left both the group and his wife, but that background haunted him during the anticommunist witch hunts of the 1950s. He had begun taking graduate courses at Columbia while teaching at the City College of New York, but lost that teaching job in 1954 after he refused to identify other members of the study group to a special committee of the New York board of higher education. That incident incensed Kusch, who secured a research assistantship so Gould could become a full-time graduate student.

Gould essentially went off and solved the physics problem that Townes had formulated. He wrote down his laser ideas -- including a definition of "laser" as Light Amplification by the Stimulated Emission of Radiation -- in late 1957, and had them notarized by a candy store owner named Jack Gould (no relation) in what he hoped was the first step to getting a patent.

The cavity problem

The path from maser to laser was far from obvious, because of the large physical differences between microwaves and visible light. Optical photon have thousands of times more energy, and microwaves are thousands of times longer. Among the trickier problems was developing a resonant cavity for laser oscillation.

Resonance occurs when a wave travels an integral number of wavelength as it makes a round-trip of the cavity. The simplest resonant cavity is a half-wavelength long, so a round trip equals one wavelength. Microwaves are measured in centimeters, and microwave cavities typically are on the order of a wavelength across, and enclosed on all sides. Visible light has wavelengths under a micrometer, so analogous cavities are obviously impractical.

The solution to that problem, recognized both by Townes and Schawlow and by Gould, was an optical device known as a Fabry-Perot interferometer. The Fabry-Perot is simply two flat mirrors mounted parallel to each other, separated by many thousands of wavelengths. Light bounces back and forth between them, through the laser medium, stimulating the emission of more light. In practice, one mirror reflects all incident light, while the other transmits some light to form the laser beam.

The allocation of credit for "inventing" the laser concept remains controversial. Townes and Schawlow have been widely honored by the scientific community, separately receiving Nobel Prizes in 1964 and 1981. Their Physical Review paper had a profound impact, and was the single biggest event triggering many research efforts that led to early lasers. Gould's notebooks and their offspring -- his patent applications and proposals for research funding -- had only minimal circulation, and essentially no impact on most of the scientific world.

Gould made less-important but nonetheless solid contributions in lasers and fiber optics, but by the mid-1970s he had become almost invisible in the laser world. However, he quietly continued pursuing his patent applications, and after a court in 1973 invalidated a patent issued to Townes and Schawlow, he was able to secure a series of patents on laser concepts and applications, starting in 1977. The delay made them far more valuable, with Gould and his legal team collecting many millions of dollars in royalties.

The Great Laser Race

Publication of the Schawlow-Townes paper was the starting gun for the race to build lasers. Some researchers, such as Townes, Theodore Maiman, and Nicolaas Bloembergen, had worked on microwave masers. Others, such as Peter P. Sorokin, Robert Hall, and C. Kumar N. Patel, came from other fields of physics after becoming intrigued with the laser concept.

Early efforts concentrated on materials whose energy-level structures already were well-known from spectroscopic studies. Ali Javan started working on the helium-neon gas laser at Bell Labs even before the Schawlow-Townes paper was published. At Columbia, Townes and two graduate students, Herman Z. Cummins and Isaac Abella, investigated a potassium-vapor scheme discussed in the Physical Review paper. In the Soviet Union, Nikolai Basov studied semiconductors. With a few exceptions, such as Bell Labs, most of the research was modestly funded.

The largest research program was sponsored by the Advanced Research Projects Agency of the Department of Defense, the agency chartered to support risky research with high potential rewards. It had a peculiar history. As he developed his laser ideas, Gordon Gould realized that he could not continue pursuing both them and his graduate work. He left Columbia to work for a small company on Long Island, TRG Inc., and soon interested his employer in lasers. The company used Gould's ideas as the basis for a $300,000 research proposal to ARPA. Pentagon officials, dazzled by visions of laser weapons, were so excited that they gave TRG a contract for $1 million. Such increases are extremely rare.
Gould, like Townes, initially concentrated on alkali metal vapors. The generous Pentagon funding let TRG investigate many laser candidates, but the program was ill-starred from the beginning, as Gould describes in his interview. Security restrictions came with the military money. Although the worst anticommunist hysteria had passed, the Marxist skeleton in Gould's closet was enough to prevent him from getting a security clearance. TRG scientists trying to build lasers could consult with Gould, but they could not tell him details about classified research. Moreover, alkali metal vapors would prove to be very difficult to make into lasers.

The First Laser

Among the maser materials considered for use in lasers was synthetic ruby, aluminum oxide doped with chromium atoms. The chromium lines were useful in masers, and their spectroscopy was well known. At Bell Labs, Schawlow considered ruby as a laser material, but in 1959 he publicly dismissed it as unsuitable. That opinion was based on inadequate data, and before long it was proved wrong.

Meanwhile, Theodore Maiman was trying to use his knowledge of ruby masers to make a laser at Hughes Research Laboratories in Malibu, California. He began working with ruby because it was well-known, at first thinking he could switch to a better material later, when he better understood laser requirements. However, he eventually convinced himself that Schawlow was wrong, and that ruby would make a good laser. It could not generate a continuous beam, but he decided pulses were good enough to demonstrate laser action. As he relates in his interview, he forged ahead, working alone, while Hughes management grew skeptical. By the time he succeeded in making the ruby laser work for the first time, on May 16, 1960, he was not supposed to be working on the program.

Maiman's laser, was small and elegant: a ruby rod, with its ends silvered to reflect light, which he placed inside a spring-shaped flashlamp. His success is undisputed, but he almost immediately ran into problems. The then-new Physical Review Letters summarily rejected his report of making an "optical maser" as "just another maser paper." The journal's founding editor Samuel Goudsmit, a theoretician best known as the co-discoverer of electron spin, had grown tired of the glut of maser papers arriving at his office, and decided that they no longer merited rapid publication in his journal. Moreover, the journal had just published another paper by Maiman on the spectroscopy of ruby -- work that led to his laser demonstration.

Hughes management reacted enthusiastically once the laser worked, and sponsored a full-fledged press announcement in early July. However, the public-relations photographer commissioned to immortalize the first laser on film wasn't satisfied with it. He thought the device was too small, and insisted that Maiman pose with a bigger flashlamp and ruby rod. Today Hughes is still distributing those pictures, showing Maiman with what isn't really the first ruby laser.

Maiman hurriedly prepared a concise 300-word report which was immediately accepted by the British weekly Nature. When efforts to convince Goudsmit of his error failed, the Nature paper, published August 6, 1960, became the first report of a working laser. Maiman later published a more detailed analysis in Physical Review.

Some laser pioneers recall when and how they heard the news of Maiman's laser. Other laboratories soon made their own ruby lasers -- although some used the flashlamp shown in the press release, rather than the one Maiman actually used. Schawlow's group at Bell Labs was among the first to get one working, but theirs was considerably larger than Maiman's. Soon afterwards, laser action on slightly different lines in "dark" or "red" ruby, which has a higher concentration of chromium ions than in the "pink" ruby used by Maiman, was reported by Bell Labs and another group at Westinghouse in Physical Review Letters.

Although Maiman had beat everyone else hands down in the great laser race, Townes, Basov and Aleksandr Prokhorov received the 1964 Nobel Prize for their work on laser theory. Understandably annoyed, Maiman points out that none of the theorists were able to make a working laser before he did. However, he has received the Japan Prize and been inducted into the National Inventors' Hall of Fame.


There's lots more in Laser Pioneers, including an extensive overview of laser history, a bibliography, and interviews or profiles of Charles Townes, Arthur Schawlow, Nicolaas Bloembergen, Gordon Gould, Theodore Maiman, Peter Sorokin, Ali Javan, Robert Hall, C. Kumar N. Patel, William Bridges, William Silfvast, James J. Ewing, John Madey, and Dennis Matthews. Order ISBN 0-12-336030-7 from your bookstore, buy direct from Academic Press on the Web, or order now from Amazon.com.

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