Oral History Transcript — Dr. Alan White
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Alan White; April 30, 1986
ABSTRACT: Narrative account of discovery of the visible red HeNe laser in the Bell Laboratories Exploratory Development Group under Signal Corps contract, contributions of coworkers and supervisors, White's work on the arc lamp for cw pumping, collaboration with Dane Rigden (1962), knowledge of Javan-Bennett-Herriott infrared laser work and work of Spectra-Physics group, equipment and funding, atmosphere in the laboratory following the discovery.
You ask in your question set about the Fort Monmouth Signal Corps contract and how it led to our discovery of the visible red HeNe laser. The Signal Corps wanted a 1.15 micron HeNe laser for experiments. They first approached the Research Division for it, and either the Signal Corps or Research subsequently approached our group — the Exploratory Development Group — because we were specialists in gas discharges. We had developed gas discharge tubes such as diodes for telephone switches (“talking-path” diodes). This was several months after the Javan-Bennett-Herriott infrared laser was first operated.
The infrared laser produced a great deal of excitement throughout the laboratory. It was clearly one of the most interesting things going on and provoked a good deal of discussion. Here for the first time one had useful amounts of continous coherent optical power at frequencies three to four orders of magnitude higher than could be achieved with microwave tubes. At that time Dane Rigden and I were working on separate projects. Ray Sears and Gordon Cooper were the department head and supervisor, respectively. We were located at Murray Hill, but in Building 2, located at the other end of the MH complex from Building 1, where Javan, Bennett, and Herriott were working. Rigden was working on frequency shifters and modulators for lasers. This was a very difficult project and I don’t believe anything successful came of it at the time.
I was working on high pressure mercury arc lamps for pumping a cw ruby laser. Don Nelson and Bill Boyle in the Research Department had a program to get a cw ruby laser going, and their problem was to get a sufficiently bright cw pumping source. My job was to develop the arc lamp to pump it. Just how to go about getting a super-bright arc lamp was not obvious. Tungsten electrodes would vaporize, depositing tungsten on the glass so the envelope would blacken. I came up with specifications for a lamp that would be sufficiently bright, and it was made by the Hanovia Corporation, based in enough (a few hours) for them to do their experiment. A long lived (thousand hour) super-bright lamp has still not been made, and I believe the technological problems are almost insuperable.
After that, Rigden and I began to collaborate on the Signal Corps tube. We teamed up because I had gas discharge experience and he had experience in optics. In the course of studying the spectroscopy of the HeNe discharge, we made the discovery that the 6328 A neon line was one of the brightest in the helium neon discharge. It was much brighter than the same line in a pure neon discharge. We spent a day or so doing experiments and checking results and became convinced, after studying the energy levels, that the neon upper level was being pumped by the singlet metastable state of excited helium. This, of course, was precisely the same active in the 1.15 laser but with entirely different helium and neon levels involved. At this point, we started to work evenings and weekends to see if inversion existed and then try to get the red neon line to “lase.”
Most of the equipment was scrounged. We had some apparatus made in the shop, where facilities for making and pumping out this kind of tube were already in place because of our work on the 1.15 micron laser for the Signal Corps. We bought only the mirrors, which were a couple of hundred dollars at most, and could be squeezed into our budget. I would estimate that the entire project cost less than two thousand dollars. We encountered no objections to using shop facilities for getting the tube made but I believe budgeted support for a more expansive project probably wouldn’t have been forthcoming. In the same way managements attitude seemed to be “do what you like,” but I suspect if it had stretched on for several months with no results we would have been asked to drop the experiments.
The work actually took only a couple of months. I believe we started it about the time our internal memorandum of April 3, 1962, titled “DC Excitation of a HeNe Gas Maser” was written. The letter on the visible red line was received by the journal editor (Proceedings IRE) on June 1, 1962. Initially, I believe we worked with a 15 mm bore tube because that was the type we had made for the Signal Corps. In that tube, we were unable to measure gain in the red line, but we did see transparency, suggesting that the populations of upper and lower levels were nearly equal. Our next step was to make a 7 mm bore tube. My gas discharge experience led me to believe that a smaller bore tube would have higher excitation rates and might therefore have higher gain. I am not sure we would ever have gotten lasing in the larger diameter tube, but we never went back to find out.
Meanwhile, we had completed the Signal Corps, contract, took the laser tube down to Fort Monmouth, aligned it for them, and got it operating. We were aware of the tremendous amount of work Javan Bennett and Herriott had done on the IR laser— in particular Bennett’s very precise and detailed experimental work on state lifetimes, and Herriott’s work with the plane-parallel laser cavity. We couldn’t do that kind of work in evenings and weekends even if we had the equipment.
The simplest thing to do was to set up the tube, align the mirrors, and try it. The use of Brewster windows and spherical mirrors simplified our work immensely. The nearly confocal mirrors relaxed the alignment tolerances by orders of magnitude. Sometime after lasing on the red line was demonstrated, Bennett said something to the effect that there were two ways to confirm the possibility of inversion on a potential laser line. One was to make a methodical and detailed spectroscopic study. The other was to use an analog computer consisting of a gas discharge tube and two mirrors. And in fact, within six months a great many people were using the second method. I gave a talk on the red laser at the Electron Device Conference — that was probably around June 1962.
I believe the Spectra-Physics people first heard about our result there. I have heard stories that Spectra-Physics, Perkin-Elmer, and Raytheon were all working on the red line prior to the meeting, but I have no way to evaluate these claims. As far as I am aware, no other group came close to us on this. Arnold Bloom and W. Earl Bell from Spectra-Physics came to visit us at Bell Labs some time later, but we were then pushing ahead and weren’t too anxious to talk to them about our new results. They also were not very forthcoming about their work. (Bloom is a very competent physicist. Bell, I was told, was an engineer as well as a very good businessman).
A whole world opened up after we got our result. We wanted to learn how to increase the gain. We wanted to understand the mechanisms by which inversion is produced in a gas discharge. Gene Gordon took over from Cooper as supervisor a few weeks after we got our results and he was very enthusiastic about this work, and contributed greatly to the understanding of inversion and pumping mechanisms. We placed prisms in the cavity (this was Rigden's idea, I recall) to get additional transitions but were not successful at first because of poor quality optics. After the Spectra-Physics paper on the 3.39 micron line appeared (A.L. Bloom, W.E. Bell, and R. Rempel, “Laser Operations at 3.39 mocrons in a Helium-Noeon Mixture” Appl. Optics. 2 (1963), 317). We returned to this topic and got other visible transitions to work with. We studied interactions between transitions. We discovered the advantages of using the pure helium isotope instead of the naturally occurring helium mixture. We looked in detail at the 3.39 micron line because Gordon had developed a theory of high-gain laser lines and we did the experiments to check it out. We looked briefly at other gases, including mercury and iodine. Then we started working on schemes for stabilizing the laser frequency.
All of the work, dating from the time the red line lased was done with company money. It was as close to pure research as anything ever done in the Exploratory Development Department. It was a complete reversal of the usual policy of doing things only if they had some connection with application. We had a sort of carte blanche. A tremendous amount of support was now given to us. Darwin Perry had assisted us on his own time in the beginning and later set up a coating facility to make very high reflectivity mirrors for us. Warren Gronros handled the logistics of getting experimental apparatus made and also set up an optical shop to make high quality optics for us. Jack Morton came to see the results. He was very excited. He knew how to get publicity and he recognized that publicity was the way to insure funding for the project. Rudolph Kompfner came around several times, as did Bill Baker." Julius Molnar, the executive vice president, was fascinated by it. He had done his doctoral thesis, and his research at the labs on gas discharges many years before. He watched us working with the laser and said, “Do you mean to tell me they pay you for doing this?”
The sense of excitement was enormous. Anyone who worked on lasers in those days remembers it. It was partly the unexpectedness of it. There is always a certain amount of pleasure when speculations and hypotheses are confirmed by experiments. But there is a different kind of excitement. When something both new and significant is achieved. You have also to realize that this was the first time anyone saw really coherent visible light. The ruby laser had too many modes going to give this kind of coherence.
I might mention that the Research Division had a laser seminar that met about once a week. About twenty to thirty people attended, so everyone was familiar with the work others were doing. The motivation for our work was primarily scientific curiosity rather than applications. Optical communications of course was in the back of people’s minds, but to go from our laser to an optical communications system would have been extremely difficult and really outside our area of competence. We would have needed to deal with modulators, transmission lines, detectors, and receivers — all adding up to a lot of effort. And of course, as it turned out much later, optical communication was best done in the infrared at 1.3 or 1.5 microns.
(In response to a question about technical problems). At one point we went to a 3 mm bore tube to get higher gain and then alignment became a problem. The long thin spaghetti-like tubes were difficult to make and handle. Some of them were more than two meters long. They came from the glass blower anything but straight, and we frequently had to spend hours to get them straightened out and lined up with the mirrors. To sum up, Rigden and I initially had only evenings and weekends on which to work. These were extremely complex phenomena, and rather than attempt to do a thorough analysis beforehand, we said to each other, let’s set up the apparatus and look at the light emission with a spectrometer.
We did a lot of things that were not based on profound prior deliberations. In some instances it was a matter of doing some judicious “poking” and then looking to see whether the response confirmed your speculations. Not all of this is recorded in the laboratory notebooks.