Radiation-induced self-trapped holes (STHs) in pure-silica optical fibers are argued to fall into two classes: inherent and strain-assisted ones. The inherent STHs are associated with small Si–O bond shortening and are of extremely low thermostability. The strain-assisted STHs occur in strongly strained silica network fragments and are metastable even at room temperature. The STH optical properties, such as suppression by pre-irradiation, interconversion upon temperature variation, etc., are reviewed based on the classification proposed.
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