High-temperature effects on the light transmission through sapphire optical fiber

Single crystal sapphire optical fiber was tested at high temperatures (1500°C) to determine its suitability for optical instrumentation in high-temperature environments. Broadband light transmission (450-2300 nm) through sapphire fiber was measured as a function of temperature as a test of the fiber's ability to survive and operate in high-temperature environments. Upon heating sapphire fiber to 1400°C, large amounts of light attenuation were measured across the entire range of light wavelengths that were tested. SEM and TEM images of the heated sapphire fiber indicated that a layer had formed at the surface of the fiber, most likely due to a chemical change at high temperatures. The microscopy results suggest that the surface layer may be in the form of aluminum hydroxide. Subsequent tests of sapphire fiber in an inert atmosphere showed minimal light attenuation at high temperatures along with the elimination of any surface layers on the fiber, indicating that the air atmosphere is indeed responsible for the increased attenuation and surface layer formation at high temperatures.