Investigation on Stability of Extrinsic Fabry–perot Interferometric Pressure Sensors for High-Temperature/High-Pressure Underground Applications

Long-term stability of extrinsic Fabry–Perot interferometric (EFPI) pressure sensors at high temperature/high pressure (HT/HP) liquid environments are discussed in this article. For HT/HP underground applications, a method of fabricating EFPI sensors with a high-energy CO2 pulse laser as heating source to thermally fuse tube and fiber is used. Through theoretical analysis and experiments, factors that influence stability of EFPI sensors at HT/HP liquid environments are reported. Based on theoretical analysis, numerical simulations, and experimental tests, methods and technologies that improve the stability of EFPI sensors at HT/HP liquid environments are proposed and demonstrated. First, a spindle-shaped diaphragm in the sensor head fabricated an optical route to improve energy distribution around tube. Second, placing EFPI sensors into a HT/HP environment to release residual stress left in the bonding points to eliminate drift when used in HT/HP environments. Third, carbon deposition of hermetic coatings was used to prevent liquid molecules from diffusing into the silica capillary tube. Finally, the long-term stability of EFPI pressure sensors at HT/HP liquid environments was tested through experiments. The results show that long-term drift of air gap of EFPI pressure sensors is less than 0.1% at 300 °C, and 30 MPa liquid environments is realized.