Sapphire is an ideal visible-MWIR window due to its excellent optical and mechanical properties and its availability in large sizes up to 340-mm diameter boules. Anticipated applications for new, high performance optical systems call for even larger, 450-750 mm diameter, windows. The present effort has focused on producing 500-mm diameter sapphire boules using the Heat Exchanger Method. Three experimental growth runs demonstrated the feasibility of producing 500-mm diameter sapphire boules. Completely crack- free boules have not been grown, but large size sapphire pieces up to 400 mm by 280 mm have yielded from these experimental runs.
Recent interest in monitoring systems requires very large optical windows that are transmitting over a wide spectral range. Some of the other requirements involve durability, high strength and robustness to withstand severe environments. Therefore, sapphire has been required for these applications. The Heat Exchanger Method (HEM)<SUP>TM</SUP> has been used to produce very large sapphire crystals primarily for optical applications. Crystals of 20 cm and 25 cm diameter have been produced in production for over 20 years. Presently, 34 cm diameter boules have been adopted in production, and 50 cm diameter sapphire growth is currently in development. Results of progress and characterization data of the boules will be presented.
Sapphire has been used for many optical applications. However, smaller sizes have been used, even though the Heat Exchanger Method (HEM) has produced 20 cm diameter crystals. New generation systems require outstanding optical properties, high strength, and abrasion and thermal shock resistance. Therefore, the choice is limited to sapphire. Crystals up to 34 cm diameter, 65 kg have been grown by HEM, and it is planned to scale up the size to 50 cm diameter. In addition to larger size, the optical quality has been improved to cover the vacuum ultraviolet (VUV) and the near infrared wavelengths. Fabrication technology was advanced to fabricate larger size, higher precision optics cost effectively. Improved transmitted wavefronts and higher quality surfaces have been produced to address current applications.