Prethreshold phenomena in laser damage of optical materials

A new method was used to investigate the laser damage on the basis of nonlinear scattering of light by the zone interaction between laser radiation and an optical material. This method revealed irreversible changes in the bulk of sapphire crystals and several glasses exposed to laser radiation below the threshold of visible damage. It was found that such irreversible changes were accumulated in a series of laser pulses which eventually caused visible damage. This effect was interpreted and its possible use as a test of the influence of defects on laser damage was suggested.