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High-LIDT Coatings: Historical Notes; Standards and Testing

Historical Notes

The production of optical coatings, which is both a science and an art, was already a mature technology prior to lasers. Well-developed, successful coating techniques continued to prevail in the first decades of high-energy lasers, but with mixed results for the laser-induced damage thresholds (LIDTs) of the coatings. The coatings of some manufacturers exhibited higher resistance to laser damage than those of other manufacturers, at least sometimes—and no one knew exactly why.

To a large extent, this stemmed from the proprietary, protected nature of manufacturers’ techniques, procedures and coating protocols and processes. But, concerning LIDTs, it wasn’t clear why one batch of coatings might be more resistant to laser-induced damage than another, much less why the coatings of one manufacturer tended to have higher LIDTs than those of another.

While the efforts of government research organizations and government-funded university and industrial labs toward development of high-energy lasers, starting in the 1960s, were multi-national in scope—with particularly strong activity in North American, Europe and East Asia—there was a lack of good communication between researchers from different laboratories, even within the same country. Better communication between researchers was necessary for high-energy laser research goals to go forward, and this need for better communication applied to the particularly vexing problems of laser-induced damage that were hindering the production of higher-energy laser pulses.

Out of this common concern regarding laser-induced damage, Art Guenther of the then Air Force Weapons Laboratory and Alex Glass of Wayne State University organized the first Boulder Damage Symposium. It was held in Boulder, Colorado, in 1969, and embraced their vision of an international forum for reporting and discussing research on laser damage to optical materials. The Boulder Damage Symposium, which is now the SPIE Laser-Induced Damage to Optical Materials conference, has continued every year since that first meeting in 1969.

Meanwhile, there was a growing need for a forum specific to laser-induced damage that would be more accessible to the East Asian research community.  Leadership in meeting this need was provided by the Shanghai Institute of Optics and Fine Mechanics (SIOM), which, since its beginnings going back to the mid-1960s, has become a world-class laboratory devoted to the investigation of high-energy laser optics and optical materials. It was SIOM’s Jianda Shao who spearheaded the organization of the SPIE/SIOM Pacific Rim Laser Damage conference which first took place in 2011 and has continued since then as an international forum offering enhanced opportunities for participation by Pacific Rim researchers.

Most of the advances worldwide in understanding of the mechanisms of laser-induced damage and how to design and produce high LIDT coatings have been reported at these conferences and in their proceedings.

Standards and Testing

LIDT test protocols differ mainly with respect to how many laser pulses at specific per-pulse fluences are used to irradiate each test site. Because of site-to-site variations in coating defects and defect densities, interpretations of damage test results are usually statistical in nature.

Laser damage standards have been established by the International Standards Organization (ISO). Keeping standards up to date is ongoing with the ISO and industry organizations like the American National Standards Institute’s Optics and Electro-Optics Standards Council through its TF7 task force on laser damage standards.

Producers and users of high LIDT coatings pay close attention to damage standards and test protocols – especially to how testing and test-laser parameters and conditions match those of a high-energy laser beam train in which an optic will be used. Spica Technologies Inc. and Lidaris Ltd. are companies devoted to providing LIDT testing, and their test services play an important role in the development of laser-induced-damage-resistant coatings.

Spica and Lidaris have supported the international laser damage competitions that Lawrence Livermore National Laboratory (LLNL) coordinates as part of the SPIE Laser-Induced Damage to Optical Materials conferences. These competitions use one damage test facility and protocol to reveal the range of LIDT results for the different coatings submitted by the competitors (with double-blind measures insuring that no one knows which coatings belong to each competitor).

Round-robin testing, on the other hand, compares LIDT results for the same coating as tested by different laboratories and companies, to help better define industry norms for LIDTs. A good example of this is a round-robin study organized by Laurent Lamaignere of CEA/Laser Megajoule (LMJ), which involved international cooperation among five laboratories: CEA/LMJ and Institut Fresnel in France, the Laboratory for Laser Energetics (LLE) and LLNL in the United States, and the Laser Research Center of Vilnius University in Lithuania.

[Note: The references and resources list of the article “Coatings for Petawatt-Class High-Energy Lasers” includes published papers specific to the topics of this online supplement.]

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