Method for the optical measurement of size and complex index of laser damage precursors in optical components

摘要

Laser damage in optical components is caused mainly by the presence of sub-micronic defects, inherent to the manufacturing process (metal or dielectric inclusions, fractures, bubbles). An improvement of the laser damage threshold requires an analysis of damage process and an identification of the laser damage precursors. However, the assumed nanometer size of such precursors makes their identification difficult by the usual optical methods. In this paper, we present a method to obtain the size and complex index of laser damage precursors in thin films or substrates. This method is based on the knowledge of three parameters accessible to measurements, which are the precursor density, the laser damage threshold and the precursor absorption. Density and threshold are extracted from the fit of laser damage probability curves with the use of a statistic model and absorption is obtained with photothermal measurements. From these measurements, an electromagnetic and thermal model permits to obtain an estimation of both complex index and size of the laser damage precursors. The different experimental and theoretical tools are described in this paper : laser damage testing apparatus, photothermal bench, stochastic model for the interpretation of laser damage probability curves, electromagnetic and thermal model. An application example is given : we present our first results in silica thins films where sub-micronic laser damage initiators at 1064nm have been highlighted and identified with our method.