Measurement of Sapphire Wafer Thermo-Optic Coefficient Using High-Temperature Optical Fiber Sensors

The thermo-optic coefficient (TOC) of sapphire wafers at temperatures ranging from 25 °C to 1500 °C is measured by an extrinsic Fabry-Perot interferometer capable of operating at high temperatures. Sapphire wafers with two thicknesses are compared in the experiment. The TOC of the sapphire wafer is retrieved based on the measured temperature response of the optical path length of the Fabry-Perot cavity and the known thermal expansion coefficient of sapphire wafer. Experimental results demonstrate that the TOC of sapphire wafer at 1550 nm wavelength has a quadratic dependence on temperature up to 1500 °C. The first and second-order TOCs of the sapphire wafer are calculated as 8.163× 10 -6 and 1.1075× 10 -8, respectively. The temperature response and sensitivity of the sapphire-wafer-based high-temperature sensors with the same structure can be inferred using the obtained sapphire TOC. The reported approach and results significantly simplify the design and calibration procedures of the high-temperature sensors made of sapphire wafers.