Simultaneous in-situ flow velocity and temperature sensing based on dual thermally heterogeneous Fabry–Perot interferometers

To enable in-situ temperature measurement under spatially nonuniform temperature fields while eliminating flow-velocity measurement errors caused by ambient-temperature interference, a simultaneous flow-velocity and in-situ temperature sensing technique based on dual thermally heterogeneous Fabry–Perot interferometers was proposed. In this method, the phase signals of two fiber Fabry–Perot interferometers (FPIs) with different thermal-coupling efficiencies were processed through weighted differential demodulation, enabling flow-velocity measurement with strong immunity to ambient-temperature interference. Meanwhile, the envelope phase was extracted from the Vernier spectrum formed by numerically superimposing the spectra of the two FPIs, with the Vernier effect enhancing the temperature sensitivity. The in-situ temperature was subsequently demodulated by combining the extracted envelope phase with the recovered flow velocity. Experimental results showed that the proposed method effectively suppressed ambient-temperature cross-sensitivity and enabled simultaneous measurement of flow velocity and in-situ temperature. Flow-velocity demodulation based on the weighted-differential phase achieved a standard deviation of 0.9919 m/s over the full range of 0–10 m/s. Owing to the Vernier effect, the average temperature sensitivity of the Vernier-envelope phase reached − 1.32 rad/ ∘ C, which was 1.83 and 1.28 times those of the short-cavity FPI ( − 0.72 rad/ ∘ C) and the long-cavity FPI ( − 1.03 rad/ ∘ C), respectively. The temperature measurement exhibited a standard deviation of 0.3851 ∘ C. In addition, the sensor demonstrated good repeatability and short-term stability. The proposed simultaneous flow-velocity and temperature sensing method also featured a compact structure, thereby providing technical support for minimally invasive fluid monitoring in industrial applications based on fiber-optic sensing.