Controlling stress in sapphire optics

Precision optical fabrication is often influenced by surface stress introduced during processing. Various steps, such as lapping, grinding, polishing and coating, can influence optical figure and transmitted wave front in sapphire optics. The Twyman effect was used as a tool to measure the variation in stress from different processes and to investigate annealing treatments. Compressive stresses were generated by all fabrication techniques, however, the magnitude of stress varied considerably. The highest stress was generated during tile transition fi-sm the brittle to ductile node of removal; the lowest stress was observed during polishing with colloidal silica. Heat treatments were successful in removing machining stress from the parts. After heat treatment at 1450 degrees C, the remaining grinding-induced stress levels were too small to measure accurately.