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In Situ Thermally Induced Attenuation in Sapphire Optical Fibers Heated to 1400°C



The purpose of this work was to determine the spectral loss of sapphire optical fibers as a function of temperature to determine the suitability of using sapphire optical fiber-based instrumentation in extremely high-temperature applications such as coal gasifers, or potentially in advanced high-temperature nuclear reactors. In this work, the broadband (450–2200 nm) optical attenuation of two commercially available sapphire optical fibers was monitored in situ as the fibers were heated in steps to temperatures up to 1400°C. The results presented in this work are the first known reporting of the broadband spectral loss in sapphire fibers (as opposed to bulk sapphire) as a function of temperature. The added attenuation in the sapphire fibers seemed to reach a steady-state at temperatures of 1300°C and below, and remained below 3.7 dB/m at all measurable wavelengths. At 1400°C, rapid increases in attenuation were observed that appeared to increase as the square root of heating time, indicating a diffusion-limited process. Cooling to room temperature in steps resulted in recovery of a large portion of the thermally induced attenuation, suggesting that the effects caused by heating at 1400°C introduced additional attenuation that was very temperature dependent. Based on comparison with previous work and on the spectral features of the added attenuation observed in this work, heating at 1400°C could have caused high-temperature oxidation of the sapphire fibers, which resulted in a temperature-dependent scattering loss mechanism.