Relationships governing the cumulative effect in optical damage and its mechanism under conditions of multiphoton generation of colour centres

An investigation was made of the relationships governing the cumulative effect in optical damage to lead silicate glasses under irradiation conditions such that multiphoton excitation and coloration were detected reliably in experiments based on changes in the transmission and on the formation of colour centre tracks. It was established that the damage threshold reduction in multiple irradiation of glasses under these conditions was related to an increase in the role of thermal self-focusing because of accumulation of colour centres and an increase in the absorption coefficient of glass. A proposed theoretical model explains not only the shape of the accumulation curves, but also the influence of the irradiation spot diameter and of the efficiency of colour centre generation on the single and multiple breakdown thresholds. The model also accounts for the influence of variable-valence impurity ions on the characteristics of the accumulation process.