Optical breakdown of transparent media with random inhomogeneities

A theoretical analysis is given of the influence of the overlap of temperature fields created by isolated inhomogeneities on the probability of optical breakdown of a transparent medium as a result of thermal instability. It is shown that the required probability has a number of universal properties governed by the average number of inhomogeneities present in the focal region and by the asymptotic behavior of the size distribution of the larger inhomogeneities. General expressions are obtained for determining in each concrete case the explicit dependence of the breakdown probability on the laser radiation energy density. It is shown that, in general, inhomogeneities cannot be replaced by an effective continuous background even when their concentration is high because the breakdown probability decreases slowly (as a power law) in the range of low laser radiation intensities.