Integrated fiber-optic Pitot tube sensor based on dual Fabry-Perot micro-cavities for airflow speed measurement

Abstract

In this paper, an integrated fiber-optic Pitot tube sensor based on differential pressure principle for airflow speed measurement is proposed and demonstrated. The sensor array, comprising dual miniature fiber-tip pressure sensing probes, is integrated in two thin pressure conduits located at the head of the Pitot tube. This allows direct measurement of total and static pressures at the very top of the probe, which contributes to mitigating pneumatic lag and enhancing the reliability of airflow speed measurements. Based on the differential pressure principle derived from Bernoulli's equation, the airflow speed can be determined by simultaneously monitoring the changes in FP cavity lengths of both sensing probes. The single miniature fiber-tip sensing probe of the sensor array is made by fusing a standard single-mode fiber (SMF) to a section of hollow-core fiber (HCF) with an outer diameter of 125 μm and fixing a 280 nm-thick gold-silver sensitive diaphragm of comparable diameter at the other endface of the HCF. The ultra-thin diaphragms produced by microelectromechanical systems (MEMS) have high uniformity and good airtightness, supporting consistent response and stable operation of both fiber-tip sensing probes at the same time during the controlled airflow tests. Experimental results demonstrate that the measurable range of the sensor array spans from 0 to ±300 Pa, exhibiting sensitivities of 251.41 nm/kPa and 248.73 nm/kPa, respectively, and resolutions of 0.48 %F.S. (1.43 Pa) and 0.46 %F.S. (1.37 Pa). To fulfill the demands of real-time speed measurement, we have developed a dual-channel white light interferometer (WLI) interrogator capable of acquiring data at a frequency of 100 Hz. When combined with the integrated fiber-optic Pitot tube sensor, this system enables the measurement of airflow speed ranging from 5.97 to 16.42 m/s, showing an error rate not exceeding 1.53 %F.S. compared to a standard anemometer. The primary characteristics of this work encompass compact and lightweight sensing structure, high-frequency scanning, and good measurement accuracy, rendering it ideally suited for applications in wind tunnel research.