Damage Threshold Mechanisms
Absorption causes thermal laser damage. Bulk absorption can be avoided by the choice of non-absorbing materials and by making sure all materials are of the highest quality with no absorbing impurities. Localized absorption can come from inclusions in the coating (prevented by appropriate coating techniques) or from surface-absorbed contaminants.
Electron-avalanche induced damage comes from multiphoton absorption that ionizes the material, causing shock waves and vaporization. The damage can occur in surface imperfections, in one of the dielectric materials, or at the interface between materials. Since the processes are multi-photon, the probability of damage increases very non-linearly with laser power density (normally expressed in watts/cm2).
Careful surface preparation and extreme cleanliness are required prior to and during the deposition process. To minimize laser damage TecOptics developed proprietary polishing techniques that minimize surface polishing imperfections such as scratches and digs. Similarly, contaminants such as grease and residual polishing compound are meticulously removed by multiple cleaning steps. And finally, elegant coating design and production techniques shift the peak electric field intensities away from the vulnerable coating layer interfaces into the less troublesome low index of refraction layers.
TecOptics quoted damage levels correspond to a very low probability of damage and therefore can be regarded as safe operating levels.