Proceedings PaperFrontiers in infrared window and dome materials
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Optical-quality chemical vapor deposited diamond has optical, thermal, and mechanical properties similar to those of natural Type IIa diamond, with the exception of low strength in synthetic diamond. Clear windows up to 60 cm in diameter and 0.75 mm thick have been produced. Domes with a diameter of 60 - 75 mm and a thickness of 1 mm have been grown and machined on the outer surface by a laser process that reduces the surface roughness to 1 micrometer. Major challenges for diamond windows and domes are: (1) increasing the mechanical strength, (2) improving the growth rate, and (3) improving the efficiency of polishing. Four methods of attaching thin protective coatings of diamond to infrared windows have been demonstrated. Other protective coatings developed recently include boron phosphide, gallium phosphide, zinc sulfide, diamond-like carbon, amorphic diamond and polymers. Sapphire is the most durable, commercially available infrared window material, but its high temperature performance is limited by inability to withstand compression along the optical axis of the crystal. The compressive strength loss from the 20 degree Celsius value is 84% at 400 degrees Celsius and 97% at 600 degrees Celsius. Pros and cons of using ZnS, ZnSe, GaAs, GaP, Ge and Si as 2-color (midwave and long wave) windows are discussed. Engineering alternatives must be considered for extending the range of operating conditions for infrared window and dome materials.