Fiber-optic Fabry-Pérot interferometric accelerometer with composite cavity and temperature calibration for high-temperature and high-pressure applications.

To address the demand for flow-induced vibration monitoring of steam generator heat transfer tubes in pressurized water reactors under high-temperature (350 °C) and high-pressure (17.5 MPa) conditions, a fiber-optic Fabry-Pérot interferometric accelerometer based on a composite Fabry-Pérot cavity structure is proposed. The sensor employs a symmetrically arranged multidirectional cantilever beam and a central proof mass to effectively reduce cross-axis sensitivity. Using a MEMS-based fabrication process, a three-layer sensing chip with a composite cavity is formed, mitigating the temperature drift problem of conventional single-cavity structures under elevated temperatures. A temperature calibration model is further incorporated to improve measurement accuracy. The optical path is folded by a 45° metallic mirror and hermetically sealed by laser welding, ensuring stable operation under high temperature, high pressure, and external mechanical shocks. Experimental results show that the sensor achieves a sensitivity of 4.53 nm/g, a resonant frequency of 7450 Hz, a cross-axis sensitivity as low as 0.281%, and a resolution of 4.4 mg, with an acceleration measurement range of ±238 g at room temperature. Under 350 °C and 17.5 MPa, the sensor exhibited cavity length drift below 0.1 nm during a 60-h stability test, demonstrating reliable dynamic performance and long-term stability in extreme conditions, which provides an effective tool for the continuous safety monitoring of critical heat transfer structures in pressurized water reactors.