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Interview of Fred McClung by Joan Bromberg on 1985 March 6,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Composition of George F. Smith's laser group in the early 1960s; laser radar work at Malibu; work on Q-switching with Robert Hellwarth; discovery of the stimulated Raman effect, and some of the follow-up research; and the spirit of laser work at Hughes' laboratories in the early years.
I received my PhD at the University of Southern California with a thesis on the ferromagnetic properties of thin films. When I joined Hughes Aircraft Company, I was hired into a research group working in this area. Subsequently, however, several key people left and the company management decided not to pursue this work in the same direction it had taken before.
Hughes kept working on the thin film works for a while. However I looked for other projects to work on. Eventually I joined a lasers applications group that George Smith formed some months after Hughes announced the ruby laser in July 1962. It may have been in September. We tried to keep some distance from the research projects that Ted Maiman's group was working on. Ted did not have the resources to pursue all applications, and George Smith's group was tasked to look initially at applications for the laser.
Besides Smith and myself, there was a technician, Bob Halden, and an engineer Doug Buddenhagen, a person even more junior than I. About a year later, Buddenhagen left to go into another part of the company and take the ruby laser into a commercial product.
We chose to look first at optical ranging and radar. This was mostly an engineering effort and there were no very serious problems to overcome. We used one of the lasers that were available from Ted's group.
We did a lot of night work because we were using propagation distances of up to 7 miles and couldn't easily detect the beam in the daylight to do some of the visual alignments. We were aware that Eric Woodbury was also working on this problem in Hughes' Culver City operation, and that TRG was working in the general area. That competition was not the principal source of pressure on us, however; rather it was the deadline we set of having a paper ready for the IEEE meeting. We knew that eventually the actual commercial and military applications were going to be in Woodbury's province.
Maiman was an example of the freedom Hughes' scientists had then. His managers had no confidence that he could make a laser. The fact that he had no support from management and still could go on is indicative of our freedom then.
After we concluded the laser radar experiment, Bob Hellwarth and I then started talking about the giant pulse experiment. I went to see George Smith and I asked if I could work on the giant pulsed laser with Bob. He said, yes.
There was very little corporate direction going on; actually the scientists had the freedom to do much of what they thought was important within broad boundaries and the company supported the decisions of the scientists.
There was an air of excitement to learn as much as possible about the laser in those days. This was coupled with a spirit almost like a game, to do as much work as we could and get it into publication. It wasn't a "publish-or-perish" situation but just one of competition and excitement.
Hellwarth and I could see that the giant pulse would be important if we could do it. But there were a lot of subtle questions as to whether the physics was really what we envisioned it to be. Hellwarth did more analysis, and it looked more and more as if we were going in the right direction. I got equipment together and started collecting data.
Ted and others had written a proposal to the Air Force before he left, and after the laser was announced, the Air Force issued a contract to Hughes. It was pretty much a matter of, go out and do something with the laser to try to develop the technology. Irnee D’Haenens became defacto project manager, for the A.F. contract, after Ted left. I started charging our giant pulse work on that contract just a few weeks before we began to get results. Not it is often not possible to fix a date on which the result emerges, and it was not possible in the case of the Q-switched laser. Instead, the Hughes lawyer asked me to fix a date for patent purposes and the date I chose, though not picked with that in mind, turned out to be just a few days after I went on the contract.
Back to the giant pulse laser experiment. First we had to open up the laser cavity. The microwave maser people, like Ted, customarily coated the cavity with silver. Ted made his first laser by vacuum depositing a silver coating on the ends of the crystal and then scratching a hole of about a millimeter diameter in the soft silver coating. Alignment was just a matter of mechanically making the crystal ends parallel. Since I came more from the optics side (this was probably one reason why George involved me in the laser applications work), it was reasonable to attempt to align the laser with one external mirror. This was the first such work at Hughes.
Next we attempted to put a polarizer into the cavity but the cement which glued the two pieces of calcite polarizer together failed due to the high light fields inside the laser cavity. At that time, all this optical equipment was designed for low intensities. We then tried a Pockel's cell modulator which had electrodes on the ends of a KDP crystal with 1 cm dia holes to look through. These were attached to glass ends which were also cemented to the crystal, and these also failed. So we then went to a Kerr cell with nitrobenzene, which was self-healing if damage occurred. Immediately after the pulse we would see stuff dropping down through the liquid. We didn't know at the time that was seeing nonlinear optical effects, and we didn't worry about it, we just kept going.
At this time, we were part of a group of compatible people who were close socially at the laboratories. There was not a lot of top management involvement, just guidance. The team had a strong esprit des corps and in this sense, the work that all team members were doing was appreciated both within the group and by our management. Our Air Force funding was almost automatically renewed from year to year. Also the company was funding a new laser area, Don Forster's gas laser group, creating a little competition which spurred those of us working in the ruby lasers to do a bit more.
The story of the stimulated Raman effect: When Dan Weiner and I studied the spectrum of our pulsed ruby laser, we found that the line width was horribly broadened. We published a report saying, this is what we found but we don't understand why.
Hughes theorist Bill Wagner was aware of our results and didn't have an explanation. Hellwarth was also aware. No one, after the publication was out, responded with an explanation. (In hindsight, the explanation was stimulated Brillouin scattering.) This was late 1961 — early 1962. We finished the spectral broadening measurements and started on other things.
We were using multiple detectors to measure our output since we had beams out of both mirrors. We had about 90% reflectivity from one and 70% with the other, so we set up detectors to read both. This was partly because we had so much trouble with what we perceived as nonlinearities in the detectors. We put lots of filters in front of the detectors to diminish the intensity. All this time, our calibrations of the photo detectors gave enormous inconsistencies. There seemed to be some sort of power dependence showing up in the calibration.
I talked with Woodbury about these problems. Was it saturation in the filters, or nonlinear optical effects in the filters? Woodbury suggested we use 1/r2 attenuation. Finally, we scattered light from paper, using the 1/r2 attenuation and got consistent results from all detectors.
About 6 months later we had a visitor at the lab, perhaps NRL's Condon. He told us about work that Woodbury was doing at Culver City. Woodbury's group had looked at our attenuators and found that they were not wave-length neutral but had higher transparency at the near infrared than at the laser wavelength. I called Eric [Woodbury] and asked about it. Woodbury indicated that there was Ir energy coming out of the Q-switched ruby laser. I asked him if he'd mind if we worked on it. We had the personnel to apply, and we were very interested in the results. We then started to work together in collaboration with Woodbury.
Woodbury had used broadband detectors and verified there were lines different from the laser line. He found that there was a wavelength shift of about 1300 cm-1 and multiples of it. We talked about this. It was the magnitude of wavelength shift normally associated with Raman scattering but we couldn't understand why energy was collimated or occurred in multiples of 1300 cm-1.
Steve Schwartz, Dan Weiner, who had joined my group, and I had a 2-meter grating spectrograph that we'd been measuring the spectral broadening of the Q-switched laser with. We had the ruby line set at the long wavelength end of the spectrograph, and weren't looking at anything much longer than 100 Angstroms or so away from it. Stimulated Raman anti-stakes emission comes out in a cone in which there is very little energy on the axis. We'd actually seen some broad scattered light in the spectrograph but thought that it was ghosts, so we had not paid attention to them.
Now we redid these experiments, cranking the spectrograph so that the ruby line was at the short wavelength end. We also put in an infrared sensitive plate, although this was in fact unnecessary. We went into the darkroom and measured 3 or 4 discrete lines and confirmed that the energy was well collimated. Since the spectrograph had virtually no astigmatism, we could also measure beam width by tracing the far field pattern of the laser on the entrance slit as well as measuring wavelength shift.
Hellwarth was the theoretician we had been working with. He was at a meeting in Colorado. We tried to communicate with him but he was difficult to reach. I did talk once with him, but at that point, Bill Wagner stepped in and helped with much of the early theory discussions.
So Dan Weiner, Schwartz, and I collected data. We knew we had a wavelength shift. Woodbury thought it was a Raman shift, but we didn't understand why we were getting multiples of the Raman shift. We also were trying to find out which of the materials in the laser cavity was exhibiting the shift. The Kerr cell with its nitrobenzene was only one of the possibilities, even though it was a well-known Raman material. We needed more definitive proof than just that. We redesigned the Pockels cell to avoid the damage and changed the electronics to go back to the Pockel cell. When we did that, we no longer saw the stimulated Raman lines, so it definitely was the nitrobenzine.
In parallel, we were trying to determine Raman frequencies for some of the other materials in the cavity. Just about the time we got the Pockel's cell result, we also got these materials' frequencies measured. This clearly showed only nitrobenzene had the correct frequency. So the Pockels result was actually anti-climactic.
After that, we put other materials in the laser cavity, one after another, and measured characteristic Raman shifts for them. We were probing for why this was happening. These were IR active materials with anharmonic CH bonding and showed a weak absorption maximum near the 6943 ruby line. They all showed about 5% absorption. The questions were: what was the mechanism for the scattering, why were we getting about 30% conversion efficiency to the Raman wavelength, and why was it so intense when normal scattering cross sections were about 10-6 to 10-8.
Hellwarth was keeping up by telephone. He got back and said, "Well, this is just stimulated Raman scattering." None of us had identified it as stimulated before. I presented the paper announcing the effect at the OSA meeting in Rochester.
Right after the OSA paper, Dan Weiner and I crosschecked the Raman scattering cross section to see if the interpretation as stimulated Raman scattering could be explained by a straight forward theory that Nick Bloembergen had published shortly after we published our results. Our measurements didn't come close to the theory's predictions, by orders of magnitude. We started discussions with Bloembergen and some of his group but he didn't want to hear us. His position was that the theory was correct.
Our paper on the discrepancy was rejected because the referee said we speculated on possible explanations without proof. Our final Physical Review Letters paper therefore reported just the numerical discrepancy; Raman cross section too low by 2 orders of magnitude to give the power densities that actually were present. We gave a talk at the Puerto Rico meeting and there we did give our explanations. One was that there was a form of the light trapping the C.H. Townes had just postulated. Elsa Garmire was working on it, and we got the idea for this kind of explanation from talking with Townes.
Our paper resulted in an offsite meeting being called with Bloembergen, Townes, some ONR people, and others. Our power outputs, some tenths of joules, were exciting at the time, and these things tended to get classified. All of us there were working under contracts that, though they came from 2 or 3 sources, were basically out of the same area.
This was one of the few times I have ever felt my work directed. I said at the meeting that we looked for trapping, and thought we had seen it but weren't sure because our resolution was inadequate. I was told by the contract managers at the meeting to use our high speed streak camera to see whether the discrepancy might have been a time-dependent effect that had gotten averaged out. We were to take the camera and see if we could get the effect time resolved. Bloembergen meanwhile wanted to get better imaging, and was going to put in a spectral filter.
In any event, our high speed camera work didn't pan out, whereas Bloembergen found 10 micron spots, and thus showed self-focusing. He published this, but didn't refer to our Puerto Rico report.