Improve/develop metrology for VLA (very low absorption) Coatings

"Minimizing the optical absorption of thin films is crucial to producing low loss, high laser damage threshold coatings for mirrors, windows and other optical components. Determining absorption by measuring the loss of power in a reflected or transmittedlight beam is not practical because the single pass absorption is often only a few parts per million. Existing procedures illuminate the coating with a powerful light beam and use another light beam (mirage effect), photoacoutics or calorimetry to relatethe observed thermal phenomena to the absorption of the film. We propose a 100-fold enhancement in the sensitivity of all techniques by changing the illumination from near normal incidence to internal reflection beyond the critical angle. The resultingattenuated total internal reflection (ATR) increases the absorption of most coatings by more than three times. An additional 10 to 100-fold increase is available if the internally reflected beam is routed to experience multiple internal reflections, as incommercially available (ATR) plates. In combination, these changes increase the sensitivity of absorption measurements by 100 times or more. The technique can be used on the actual substrate, as the coating is being applied in a vacuum chamber and/or ona separate monitoring plate. Real time monitoring of absorption as coatings are being deposited will allow coaters to optimize coating processes and designs to produce improved coatin