Influence of radiation focusing and of surface finish of the optical system on the measurement of optical breakdown thresholds

It was found that when laser radiation is focused into a spot having a size of the order of several wavelengths a strong dependence is observed of the threshold power required for the breakdown of transparent dielectrics on the focusing depth in a sample. This dependence can have different forms and is determined by the aberrations of the entire optical path, including the resonator mirrors and the sample under test. The absolute values of the threshold power density can be found by measuring the size of the central maximum in the focal region inside the sample and the fraction of the energy concentrated in it. The threshold radiation power density is then independent of the focusing depth in the sample.