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.