NIST Welcome

Aaron A. Sanders

National Institute of Standards and Technology

Boulder, Colorado 80303

It is my pleasure to welcome you to Boulder and the National Institute of Standards and Technology, formerly the National Bureau of Standards. Since I will be telling you later

about the changes associated with the new name, I will not take the time now to further discuss this excepting to assure you that the same work associated with NBS will continue uninterrupted in NIST.

We are especially pleased with the long relationship of NIST and this symposium, now in its 20th year. The information exchanged and documented through this conference has, we believe, become the major exchange and documentation of information in this important area of laser-induced damage in optical materials. In this regard we believe this symposium has played an important service to the development and use of lasers and laser systems throughout the world and we are pleased to have had a small part in this accomplishment. I have only been associated with the symposium since the early 1970's and in an official capacity only since the later 1970's. During this period I have been pleased to see the symposium maintain and build a truly international flavor and indeed its relevance by continually addressing new topics of interest to the research community. We have also, each year, experienced a growth in the number of quality papers. Much of the success of course is due to the hard work of our co-chairmen, both past and present, and indeed we all owe them our thanks.

As you know, the main part of NIST is located in Gaithersburg, Maryland, with the facilities here at Boulder being only a small part and about 500 people. The Group for which I am responsible is the Optical Electronic Metrology Group and is a group of about 45 people. We conduct research in laser measurements and standards, optical fiber, integrated optics, optical fiber sensors, and components for optical communications. Our work has been growing, due to new monies from the Congress and we expect additional growth in the next few years, especially in the area of optical communications. During this year we are putting in a chemical beam epitaxial system which will allow us to make multiple quantum wells, III-V integrated optic structures and long wavelength (1.3 and 1.55 micrometer) laser diodes and detectors. We will be using these facilities for our work in standards and measurement methods of these components. Our work in laser standards has been especially synergistic with the work of interest here at this symposium. Over the years we have developed a number of calorimeters used as national standards and have provided traceable measurement services to many of you during this period. We also, over a decade ago, developed a national standard

for measuring a few hundred kilowatts of cw laser power and are still providing measurement support with this system, although we have subsequently turned this system over to the Air Force. We are currently working on a national standard capable of measuring a few megawatts of cy power. Of additional interest may be our work on the measurement of fast laser pulses. We are pursuing both detector development, and both all optical and electro-optical sampling methods.

Standards are of course also very important in your work both physical standards such as measurement of power/energy, beam profile, pulse characteristics, etc. and procedural standards and measurement methods. We, of course, here at NIST work principally in development of physical standards, but we are very happy to and do participate in consensus standard - making bodies such as ASTM, ANSI, EIA, IEEE, etc. when we can make a contribution.

I hope your stay here in Boulder will be a pleasant one. If there is anything either I or my staff can do, please let me know.

Thank you and welcome.

Symposium Introduction and Projections for Future Research

Brian E. Newnam

Los Alamos National Laboratory

Los Alamos, New Mexico 87545

On behalf of the Symposium Steering Committee of Drs. Harold Bennett, Arthur Guenther, M.J. Soileau, and myself, I am pleased to welcome you to the 20th Annual Symposium on Optical Materials for High-Power Lasers. It is always a pleasure to return to Boulder, Colorado, and the excellent facilities of the National Institute of Standards and Technology.

Before projecting into the future, I have a status report about the Proceedings of recent Boulder Damage Symposia and announcements about the planned activities for this meeting. In the last few years, the timely publication of the conference Proceedings has been a matter of concern, but we believe that we finally have the matter under control. I am glad to say that the 1985 Proceedings were mailed this summer, and copies of the 1986 Proceedings are available at the registration desk for last year's attendees. The 1987 Proceedings are about to be published, although the manuscripts of a number of the papers are not included because they were not available to us on time. Nevertheless, we do encourage delinquent authors to send in late manuscripts, which will be included in the following year's publication. A deadline of January 15 has been set for delivery of your manuscripts to enable us to publish this year's proceedings within the year. Το contribute to early publication, we co-chairmen will remain in Boulder an extra morning after this symposium to summarize and document the technical highlights for inclusion with the proceedings.

Since this conference is sponsored by organizations of the U.S. Government, I am obligated to comment on the elegant clocks given to each registrant as a special conference memento. These are intended for your offices (not your homes) to advertise and promote these annual symposia.

Regarding the social events, do plan on attending the wine and cheese festival this evening. Also, you are invited to a special Banquet tomorrow evening to formally celebrate this Twentieth Laser Damage Symposium. As our after-dinner speaker, we are pleased to welcome back our colleague in damage, Dr. Jack Marburger. Although Jack is known to many of us for his definitive work on self focusing during the 1970's, he is now the very competent President of the State University of New York at Stony Brook.

Now, I wish to take this opportunity to discuss one topic that offers special opportunities for present and future research in the area of laser damage. This is the survival and performance of the optical components of free-electron lasers, known as FELS, which are now beginning to proliferate. First demonstrated by John Madey's group at Stanford University in 1977, this class of lasers was not of concern when research on the

causes of laser damage began at the end of the 1960's. For the last ten years, research centers in a number of countries have focused on the subtleties of FEL physics and

experimental demonstrations.

With continued developments, these lasers have the potential

to have very high wall-plug efficiency, even up to 40% when energy-recovery schemes can be used. Major goals of present FEL research and development are: (1) high-average power operation at visible and near-infrared wavelengths, and (2) extension of the shortwavelength operating limit into the vacuum ultraviolet.

Briefly, the optical gain of an FEL is due to the stimulated emission from pulses of relativistic (1 MeV to 1 GeV) electrons passing through a magnetic undulator. The length of the undulator can range from <1 to >10 m, and the mirrors of an FEL optical resonator are precisely spaced so that the optical pulses overlap the train of picosecond electron pulses. Present FEL dimensions can be quite large, but there is a great desire to compress all FEL dimensions to reduce the necessary electron energy. This should make FELS cheaper and more attractive for a host of applications.

In contrast with the relatively limited spectral range of operation of most other lasers, the inherently broad tunability of FELS requires use of many classes of optically transparent and reflective materials. Recent experiences with the near-infrared FELS operating at Los Alamos and Stanford University have shown that optical damage to the mirrors, intracavity optical elements, and output windows limits the output power. Since FELS operate in essentially the lowest-order spatial mode in the resonator with the beam waist centered in the undulator, the mirrors generally have to be spaced far enough apart to reduce the incident beam intensity below the threshold for damage or significant thermal distortion. This contradicts the desire for compact resonators, so the need for maximum mirror damage resistance is obvious. One resonator design to increase the beam divergence in the resonators uses metal-coated, intracavity hyperboloidal reflectors at grazing incidence angles from 80° to 88°. This allows the end mirrors, which may have paraboloidal or ellipsoidal surfaces, to be moved in closer to the beam waist which is located near the center of the undulator. Likewise, intracavity elements such as output couplers and diffraction gratings experience lower intensities with this configuration.

In addition to highlight intensities, the FEL optics must operate in the presence of several other radiation hazards. First, high-energy radiation, gamma rays, x-rays, and neutrons, are produced by collisions of the electron beam with the various materials in the beam line. These create color centers in dielectric materials associated with impurities or structural changes even in pure materials. Second, optical harmonics of the fundamental laser wavelength, generated by the oscillatory motion of the electrons in the undulator, extend into the vacuum ultraviolet. These photons can produce color centers, Third, at the very high-peak powers associated with the picosecond micropulses, stimulated Raman scattering and self focusing may produce phase distortion in window materials. Finally, thermal distortion must be considered as the lowest order threat, especially in high-average power devices.

too.