Progress Toward Sapphire Optical Fiber Sensors for High-Temperature Applications

High-temperature measurements are of significant importance in various harsh-environment engineering fields, such as fossil fuel production, and the metallurgical and aviation industries. In recent years, there is a steady trend to shift from conventional electronic sensors to optical fiber sensors for high-temperature applications. In particular, optical fiber sensors are small in size, immune to electromagnetic interference, readily applicable for remote sensing, have high elasticity, and incorporate capabilities for multiplexing and distributed sensing. However, commonly used fused silica optical fiber sensors exhibit severe limitations at ultrahigh temperatures due to significantly degraded optical and mechanical properties at temperatures 1000 °C. The excellent optical transparency, thermal and chemical stability, mechanical robustness, and high melting temperature (~2040 °C) of single-crystal sapphire fibers (SFs) make them a strong candidate for sensing applications in high-temperature environments. Translation of the sensing schemes from mature silica fiber sensors to SF sensors has undergone tremendous growth and advancements in the past two decades. However, hurdles to the development of a near-term deployable SF sensing system have proven persistent due to the highly multimodal nature of SFs. This article reviews sensing techniques that have been implemented with SFs recently. The aim is to provide a comprehensive summary of past research on SF sensing systems. Perspectives on further research into the challenging yet promising arena are also discussed.