Fused silica undergoes densification upon prolonged exposure to high e
nergy radiation and explanations in terms of changes in the silica tet
rahedral ring structure have been offered. Under laser irradiation, th
e densification is often less than 10 parts per million, necessitating
sensitive optical techniques for its measurement. This paper reviews
some of the key literature, with emphasis on recent studies involving
193 nm excimer laser irradiation, and presents recent evidence that th
e UV laser-induced densification involves optically-induced weakening
of bonds, via a two-photon absorption process, and subsequent structur
al relaxation of the glass. (C) 1997 Elsevier Science B.V.