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Anti-reflection coating damage threshold dependence on substrate material

This paper presents a study of the laser damage threshold (LDT) of ion beam sputtered (IBS) Ta2O5/SiO2 AR coated and uncoated samples ranging from low grade plate glass through neutral density glass to laser grade fused silica substrates. We find that the damage threshold varies by just over a factor of 3 from 9 to 33 J/cm2 for 1064nm pulsed laser light with 20ns pulse length and 20 Hz repetition rate. The damage threshold of the uncoated samples ranges from 80 J/cm2 to greater than 200 J/cm2. We have estimated the single pulse temperature increase in the uncoated neutral density glasses to be as much as 400-500°C at the damage fluence. This is unexpected as multiple pulse exposure would rapidly bring the glass up to melting temperatures. A possible explanation is saturation of the absorption at high fluence levels resulting in lower temperature than estimated. We conclude that the absorption level in the IBS coating is too low to give a significant contribution to the LDT of the AR coated samples.