Dynamic Demodulation Technique for Tandem Dual-Cavity EFPI Sensors Enabling Simultaneous Temperature Measurement in Harsh Environment

In this article, we proposed a dynamic demodulation technique for tandem dual-cavity extrinsic Fabry-Perot interferometer (EFPI) sensors, which enabled the simultaneous real-time temperature measurement in harsh environment. The concept of tandem dual-cavity EFPI was presented and used to design a pressure/temperature sensor as well as the dynamic demodulation method. A window function finite impulse response (FIR) digital filter was designed to separate the pressure sensitive EFPI (PS-EFPI) and thermal sensitive (TS-EFPI). The cross correlation method was used to demodulate the optical path difference (OPD) of cavities from the filtered PS-EFPI and TS-EFPI spectrum signals. An FPGA-based hardware system with maximum data processing speed up to 25 kHz was developed to provide high-speed computational resources for the filtering and demodulation processes. Characterization experiments indicated that the sensor and the proposed demodulation system were able to simultaneously detect static pressure up to 4 MPa and temperature up to 1200 °C. The pressure sensitivity varies from 0.92 nm/kPa (20 °C) to 1.18 nm/kPa (1200 °C), while the temperature sensitivity is 3.73 nm/°C. Dynamic pressure sensing experiments indicated excellent dynamic properties of demodulation system as well as sensor, the maximum sensitivity for sinusoidal dynamic pressure is 0.241 nm/kPa (at 10 Hz), while the minimum response time for step dynamic pressure is 37.4~\mu s (at 70 kPa). In this work, innovative progress has been made in realizing dynamic demodulation of dual-cavity EFPI-type sensor for harsh environment, which holds great potential for practical harsh environment applications, such as aero turbine engines and so on.