Degradation of vacuum-exposed SiO 2 laser windows

Abstract

Observations of a damage phenomenon at the surface of fused silica and crystalline quartz windows are presented. Uncoated windows were mounted at Brewster's angle to facilitate the introduction of a vacuum chamber directly into the cavity of an Ar-ion laser (488 - 514 nm). The transmission of these windows, prior to evacuating the chamber to less than 1 Torr, approaches the theoretical value of > 99.9%, remaining constant indefinitely. However in our normal usage, the chamber is evacuated (P < 10<SUP>-7</SUP> Torr), exposing the windows to high vacuum as well as UV borelight from the laser discharge. After several hours of operation, the intracavity power is observed to decrease monotonically (by approximately 15% per hour) accompanied by the development of a red fluorescence on the inside window surface where exposed to the visible laser radiation. Partial rejuvenation of the windows can be accomplished by reintroduction of gas into the vacuum chamber. Possible damage mechanisms are presented.