A significant problem in the use of excimer lasers is the limited lifetime of the cavity outcouplers. To maximize energy throughput some commercial lasers use the vacuum windows as the laser cavity, both the rear reflector and the outcoupler. In this case one is limited to materials for these optics that are resistant to chemical degradation as well as color center formation due to the high fluence of the laser beam (especially at deep UV wavelengths). The usual material employed is magnesium fluoride because it is transparent well below 193 nm, chemically resistant to fluorine etching (as well as HC1), and easy to figure. Unfortunately, it is difficult to make it in sufficient quantity and quality to supply the market. Often new optics out of the box have enough defects to form color centers almost immediately on irradiation by 193-nm radiation.

© 1990 Optical Society of America

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