Of the above, vacuum-ultraviolet induced degradation of multilayer dielectric mirrors has been most problematic in FEL oscillators. For example, FEL researchers at the University of Paris found that titania-silica reflectors degraded after only a moderate exposure to ultraviolet synchrotron radiation in the ACO storage-ring FEL. Loss of oxygen from the coatings and carbon contamination were responsible. Similarly, the operating lifetime of the storage-ring FEL at Novosibirsk, USSR, has been limited by mirror degradation caused by VUV synchrotron radiation. VUV-induced degradation has also been observed in laser gyro reflectors. In this case, the He-Ne laser plasma radiates several lines below 100 nm, resulting in a slow deterioration in the titania/silica mirror reflectance. At this year's conference, Jim Early and Virgil Sanders of Los Alamos will present experimental data on the rate of degradation versus UV dose for a number of multilayer coating combinations. Apparently, certain materials, such as hafnium oxide, and optimized coating processes are more resistant to UV degradation. More work on this problem by members of the laser damage community could be very fruitful.

At the

Although multilayer dielectric mirrors can be produced with very high reflectance and are very convenient as transmissive output couplers, metal mirrors have survived best in FEL oscillators operating in the visible and infrared. Also, certain metals have a broad range of high reflectance which allows for continuous tuning of the FEL wavelength. However, for future high-average power FELS, the absorption of metal reflectors is still too large, and the resultant thermal loading could cause excessive surface distortion. One potential solution to this problem may be operation at cryogenic temperatures. 1981 Boulder Damage Symposium, Don Decker and V. Hodgkin of the Naval Weapons Center presented evidence that absorption of silver surfaces does indeed exhibit the predicted temperature dependence, decreasing from the values at 300 K to a factor of 5 lower at 77 K. However, with repeated thermal cycling, the absorption values were not reproducible, indicating that some surface restructuring was occurring. With such a high potential impact on laser power handling capability cryogenic metal mirrors should be pursued seriously.

Looking forward into the future, FEL technology will extend into the extreme ultraviolet below 100 nm. This will require resonator mirrors with adequately high reflectance both to minimize roundtrip losses and to control surface figure distortion due to absorption. Over the 30-100 nm and 10-14 nm spectral ranges, these requirements may be satisfactorily met by the multifaceted metal mirror design that I first described here during the 1985 Symposium. This year, Marion Scott will give a poster paper that describes Los Alamos progress to implement this design into a nine-faceted aluminum retroreflector that attained a measured reflectance of 89% at 58.4 nm. This is a factor of 3 higher reflectance than previously attained by any other mirror operatiang in this wavelength region. This work may lead to practical devices, but I do recommend that some of you try to devise other new mirror concepts for the XUV.

In summary, FELs pose a number of challenging problems for the optical and laser

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damage community. Since there is a wide array of potential applications awaiting these devices, it is important that damage-resistant optical elements be developed to further FEL utilization.

[1] Deacon, D.A.G.; De Angelis, A., eds. Applications of Free Electron Lasers, in Nucl. Inst. and Methods in Phys. Res. A239, No. 3(1984).

[2] Deacon, D.A.G.; Newnam, B. E., eds. Free-Electron Laser Applications in the Ultraviolet, Opt. Soc. Am., Washington, DC; OSA Tech. Digest Series, Vol. 4; 1984.

[3] Morin, P. "FEL Applications in the UV," Synchrotron Radiation News 1, 11; 1988.

[4] Deacon, D.A.G.; Haglund, Jr., R.F.; Newnam, B. E.; Schlossberg, H., eds. Special Issue on Physics of Free-Electron-Laser Applications, J. Opt. Soc. Am. B, 6, May, 1989.

Good morning and welcome to Boulder. This is truly a pleasure for all of us to be back here celebrating our 20th year at this venue. We thought it might be appropriate to share with you some of the background of this series of conferences. Some of this may be old news to a few of you, but I am sure much of this information will be new because of the longevity of this symposium. A review will point out why we are meeting here at the Boulder Laboratories of the National Institute of Standards and Technology, known as the National Bureau of Standards until recently. As you all know, laser action was first demonstrated in 1960 and their application exploded on the commercial market shortly thereafter. The language of commerce to many people is standards, standards of definitions, of tests, of performance specifications. When one purchases something, one needs to describe its characteristics and have both the supplier and the buyer agree on what the terminology means. That happens to be the business, in part, of standards. The National Institute of Standards and Technology has in its charter a responsibility to interact with standards committees in promulgating standards through certain procedures.

Back in those early days, the ASTM, the American Society for Testing and Materials, was involved in developing standards for the burgeoning laser industry. It formed a committee, a subcommittee of its F1 electronics committee, in the mid-sixties and it started to attack laser problems of slope efficiency, beam divergence and other quantities that were important to laser manufacturers and purchasers at that time. The committee consisted of buyers and suppliers, while government people were represented either by the Institute of Standards and Technology or people from the DoD or DoE. The objective of the group activities was to address the standards that were being proposed, tested, and evaluated. We found that when we got together to develop standards, we could not agree among ourselves what parameters should be measured as well as how they should be measured. We decided it would be appropriate to have what would be called a mini-symposium to discuss the physics of the issues under question.

test.

You must realize that standards are supposed to measure the quantity or object under Standards should not depend on the operator of the test or instruments used in the test or anything like that, they need to be test object sensitive. It is necessary to develop standards that have a good foundation based on an understanding of the physical and engineering principals that are behind the operative definition and the test procedures. Because of the newness of the field of lasers we had to start with definitions and data reduction methods. A series of round-robin tests was organized to insure that we were measuring the sample under test and not the performer of the test!

We were quite successful in holding several mini-symposiums on selected subjects like beam divergence, slope efficiency, crystal orientation, roughness, etc. Damage came along as an issue that was, of particular interest, in those early days of rather impure materials and great variability in what you could buy in the commercial sector. So it was decided in 1969 to hold a little mini-symposium that would discuss the physics of damage and then write our standard. We thought that would be the end of the subject. At that early meeting, I think there were thirty-five people. We found out that we knew even less than we thought we did when we went into the meeting. That is how this conference started. The proceedings of the first meeting were published by ASTM. However, few have seen it since the distribution was more limited than the subsequent proceedings, published as NBS Special Publications.

The history we have just reviewed explains why, in fact, we are here in Boulder at the Boulder Laboratories of the what was then the National Bureau of Standards. The meeting logo features the Flatiron mountains surrounding the Boulder Laboratories and is symbolic of the important role played by the National Institute of Standards and Technology in these meetings (Fig. 1).

The following figures contain some interesting statistics. Some of the early chairmen of the ASTM sub-committees include Johnnie Meyers, who now runs KIGER and makes optical and laser materials; Haynes Lee, who presently is executive director of the Laser Institute of America; John Detrio, who is at the University of Dayton Research Institute; and me (Fig. 2). One thing that is sure is that we have out-lived the sub-committee on lasers of the ASTM. The ASTM is still the sponsor of this conference because they, in fact, put one of their reference numbers on our proceedings and distribute the proceedings, not only here in the United States, but also in Europe. I think this participation has a lot to do with why we have had good foreign representation at this conference.

In 1969, Alex Glass and I took it upon ourselves to write up and put the proceedings together. A major player in attracting some of the speakers was Martin Stickley who was at that early meeting. As the meeting progressed and grew we needed additional help and were pleased to add Hal Bennett from the Naval Weapons Center and Brian Newnam from Los Alamos to the committee as co-chairmen. Alex was moving to the corporate aspect of lasers. We also added Dave Milam as a representative from Livermore and said goodbye to Alex as co-chairman. Alex, in fact, still acts as our treasurer, so he can maintain a connection with this particular meeting.

The co-chairmen primarily represented the DoD and the DoE, which had large programmatic interests in the damage question. We were starting to see a tremendous influx from the academic community and felt that it would be necessary to start bringing in some young blood to stimulate those guys who were becoming old war horses in this business. Thus, we added M.J. Soileau as a co-chairman last year to represent the academic community. All of these people have made significant contributions to this conference over the years. Aaron Sanders of the Boulder Laboratories is coordinator for

this meeting.

Over the years we have had sponsorship from those organizations that are listed in the proceedings. We have had additional support from organizations such as the Naval Weapons Center and the Air Force Weapons Laboratory. They provide in-kind services by which we were able to bring in staff personnel to help us operate the meeting. Last, Dave Milam is moving on to other areas at Lawrence Livermore National Laboratory and is being replaced by some virile new blood the very capable Lloyd Chase. Good-bye Dave, welcome Lloyd.

How has the meeting grown? As the chart shows, we have grown to about eighty papers this year and about 210 participants. I will be honest with you; one of the goals that we set for ourselves was to review the technical content of what has gone on at Boulder for twenty years. We found it necessary to only have two of those reviews this year. We will have the other two reviews next year because of lack of time due to the large number of papers submitted this year. There has been a healthy growth in both attendance and participation. Many of us feel the present level is about correct, since we don't want togo to multiple sessions and wish to have the meeting remain informal and conducive to discussion and openness (Fig. 3).

You might find the nations that have been involved in this conference over the last 20 years interesting. They are listed in the attendance lists at the back of our proceedings. I went through those attendance lists and checked off the participating countries. Of the more than 900 papers that have been presented in the first twenty years, about fifteen percent were from countries other than the United States. By the end of this meeting, we will probably have 3,000 participants over the twenty years and about seven percent from countries outside of the United States (Fig. 4).

What has been happening at the meeting? The graph represents a percentage of the total papers that were divided up among the four areas into one of which we try to place each contribution (Fig. 5). The areas include fundamental mechanisms, which concern primarily the interaction of light with optical media: mirrors and surfaces, which involve reflectivity, scattering, polishing techniques; thin films, which is self explanatory; and bulk materials and measurements, which covers new materials and the measurement of those properties important in the damage process. In the beginning there was no work on thin films, and everybody was interested in getting impurities or platinum inclusions out of laser glass. Issues like that generated a need for a data base, and that is why in the early years most of the papers dealt with materials and measurement issues. Mirrors and surfaces have been of fairly constant interest, as have fundamental mechanisms. We really do try to understand. A lot of the early papers in this particular area dealt with self focusing in materials and high-power effects such as multiphoton processes and their theoretical analysis. But as you have seen, we have realized that the area with the most leverage for improvement in optical performance of systems is in the thin films area. This area has grown to be the major portion of the meeting at the present time. The drivers in the early days were high-power issues because of ICF, the Inertial Confinement Fusion program of the DOE, and the high-energy area by the DoD. Commercial spin-off