Coating and substrate surface roughness

Mirror scattering, which is an important factor in a number of instruments including the ring laser gyro, is largely determined by the surface roughness. The question is to what extent is the latter influenced by the coating material and process. To answer this, a number of substrates were surface profiled using the (noncontact) Zygo optical heterodyne profilometer. Of these, two each were coated with one, two, and four half-waves of titania using the reactive ion plating (RIP) technique. A similar set of coatings using silica were produced as well as two multilayer stacks of titania/silica. Ail samples were surface profiled after the coating. Results indicate no statistically significant change in the rms surface roughness and, on the average, a slight increase in the peak-to-valley roughness. Experimental details and the results of both the optical heterodyne profilometry and Talystep measurements are discussed.