Real-Time Measurements Improve High-Energy Laser Optical Coatings Published Sept. 25, 2007 By Materials & Manufacturing Directorate AFRL/ML WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- As part of an AFRL Small Business Innovation Research effort, MetaStable Instruments, Inc., reduced the time and expense required to optimize optical coating processes for aircraft and space components. This effort resulted in the successful development of a dynamic new technique that enables coating technicians to measure light absorption and scattering losses of less than one part per million--in real time, as the coating process is underway inside a vacuum chamber. The measurements collected for this effort, which represent the most sensitive ever achieved in vacuum conditions, could prove instrumental for ongoing development of high-energy lasers. By providing technicians a means to optimize materials processing and coating parameters on a routine basis, the new method enables very-low-absorption (VLA) optical thin film coatings to be produced in less time and at reduced cost to the Air Force. Highly reflective multilayer optical coatings are essential to high-power laser systems, such as the airborne laser. Because these lasers are so powerful, even small amounts of incident laser energy absorption can result in damage to the optical coatings. Since damaged steering optics produce distorted laser beams, VLA coatings--as well as accurate and timely nondestructive VLA measurements--are necessary to efficiently and cost-effectively optimize the optical thin film deposition process. MetaStable Instruments' new technique uses attenuated total internal reflection altered by evanescent coupling of light into a waveguide mode in the thin film. The fraction of light no longer reflected once the waveguide condition exists is a measure of the thin film's extinction coefficient. Researchers employed this technique outside the vacuum chamber, with an air gap between the measuring instrument and the film of interest, as well as inside the chamber, with a low-refractive-index solid layer (or layers) separating the instrument from the film. Consequently, coating technicians can now quickly and routinely optimize materials processing and coating parameters in order to minimize coating absorption. The new method causes light that is undergoing total internal reflection at the surface being coated to be evanescently coupled into a waveguide within the thin film. Absorption or scattering losses in the waveguide reduce the total internal reflection of the light. Therefore, measuring the reduction in internal reflection can determine the extinction coefficient of the thin film.