Ultra-Thin Sapphire Fiber Enabled High-Order Mode Suppression in All-Sapphire Extrinsic Fabry-Perot Interferometers for Harsh Environment Applications

An all-sapphire fiber extrinsic Fabry-Perot interferometer (EFPI) based on ultra-thin sapphire fiber is proposed and experimentally demonstrated in ultra-high temperature environments. The composite Fabry-Perot (FP) structure for simultaneous measurement of temperature and pressure is fabricated by directly bonding three polished sapphire wafers with low surface roughness. The sapphire fiber is packaged within a ceramic tube, and physically connected to the single mode fiber, achieving sensing signal transmission in ultra-high temperature zone. The high-order modes are effectively suppressed by adopting 30 μm-diameter sapphire fiber to pick up the sensing signals, improving the demodulation accuracy and stability. Results show that the sensing structure can operate stably in the temperature range of 20 °C to 1413 °C, enabling real-time demodulation for the cavity length of the FP cavity. The cavity length of air cavity varies linearly with pressure in the pressure range of 0 MPa to 1 MPa, exhibiting good repeatability. Based on its excellent performance, the proposed all-sapphire sensing structure is promising to achieve real-time and accurate pressure and temperature measurement in harsh environments.