Optical technique to sense thermal stress in sapphire

An optical technique for measuring surface stress in chromium-doped sapphire windows is reported. The approach utilizes the well-known effects of temperature and stress on the spectral profile of chromium ion fluorescence in crystalline sapphire. In this study, the sapphire samples were selectively doped with a surface concentration of chromium ions, which provided a direct measure of the stress and temperature in the surface region of the window. A series of fluorescence measurements were performed to calibrate the effects of temperature and mechanical stress on the spectral characteristics of the surface fluorescence. The results of this laboratory study are currently being developed into a dynamic, non-contact probe of stress in infrared seeker windows while under simulated conditions of flight.