Modeling Laser Damage Caused by Platinum Inclusions in Laser Glass

A fracture-mechanic theory suggests that crack-propagation rates are proportional to the square root of the inclusion radius and to the three-quarters power of the fluence. The theory does not yet account for pressure loss or convection heat transfer as platinum vapor flows into the crack, and needs to be developed further before accurate propagation rates can be predicted. However, a simple estimate obtained by multiplying the time that the platinum is in the vapor state by the sonic velocity in glass gives a conservative but reasonable result. At fluence levels of 2 J/cm², crack propagation of 4 μm per shot are predicted.

A crack-development hypothesis, based on shock-wave propagation, successfully predicts the shape of typical damage sites. The sites have an annular crack extending from the equator of the inclusion and planar crack lobes extending from the poles. The lobe at the pole exposed to laser light is larger than the lobe at the unexposed pole.

A numerical heat-transport model predicts temperature profiles in and around a metallic platinum inclusion imbedded in a phosphate laser glass matrix when illuminated with 1-μm laser light. The thermal model predicts that glass damage will occur if the platinum temperature exceeds the boiling point. Predicted fluence-damage limits of 1.4 J/cm² for a 1-ns pulse and 4.3 J/cm² for a 10-ns pulse agree with experimental data. The damage limit is independent of inclusion size above about 1 μm in diameter.