Development of sapphire windows for use in high-quality IR imaging systems

A polishing process for fabricating high-quality optical sapphire windows is presented. This process was successfully used to produce very high aspect ratio, as high as 1500:1, sapphire windows for use in optical imaging systems. During the development of the process as unusual print-through phenomenon was discussed. This replication of surface features on the blocking bodies, such as bumps or grooves, onto the polished window surface produced distortion in the final wavefront and degraded window performance. A method of bonding smaller, thick sapphire panes into a larger optical window is also presented. A glass fritting process utilizing two different temperature frits is discussed. The optical errors encountered during fritting which contribute to the optical error budget and methods for minimizing their effect on the wavefront of the final window are also reviewed. An 8' diameter X 0.435' optical sapphire window was produced using this fritting technology.