Cumulative effects and time dependence of the optical damage threshold of solid transparent dielectrics exposed to coherent radiation

An experimental study was made of the dependences of the optical damage threshold of polymethylmethacrylate on the duration of exposure to laser radiation for two different sizes of focal spot. A theoretical interpretation of the experimental results is proposed, based on a model of the evolution of thermal instability at absorbing inclusions, which is generalized to the case when cumulative effects are substantial. This model can also explain experimental data obtained by other authors on cumulative effects.