Any organic or particulate residue from polishing or cleaning may absorb laser energy and is therefore a potential damage site. For this reason, the substrate and coating interface is a critical area in achieving high damage thresholds.
Making high-power optical coatings requires tight control of every aspect of production, from initial substrate manufacturing to final packing. Before the optical element even reaches the coating chamber, its surface quality, subsurface quality, and cleanliness must be assured.
Substrates for use with high-power optical coatings must be high-quality. This is particularly important for refractive, or transmissive, optics - these substrates must demonstrate extremely low intrinsic absorption at the relevant wavelengths. To maximize transmission, it is important for the substrate to have as few surface quality defects as possible, as they can act as potential damage sites during irradiation. Surface quality is specified in terms of a scratch and dig value (the scratch number in units of .001mm, the dig number in units of .01mm). High-power laser optics typically call for less than 20-10 or 10-5 scratch-dig surface values.
The substrate must also be free of subsurface defects. This can be avoided through proper machining, grinding, and polishing methods prior to cleaning and coating. The first step is to begin with a blank large enough to allow for all necessary substrate removal. When machining, coating technicians carefully choose the appropriate tool feed, tool speed, and coolant flow in order to reduce subsurface stress and damage. Grinding is then done in incrementally smaller steps so as to achieve a more controlled surface. Finally, roughly 0.01 - 0.03mm is removed by polishing, which is used to remove subsurface damage caused by the previous steps.