Fabry–Perot Refractometer for Infrasound Measurements

The objective of this work is to propose an all-optical measurement technique based on a Fabry-Perot (FP) refractometer for infrasound acoustic pressure. There is an increasing need for infrasound measurements to detect violent atmospheric events worldwide or to evaluate the environmental impact of wind farms. This implies new acoustic calibration requirements in a frequency range below 2 Hz, which is not currently covered by calibration and measurement capabilities (CMCs). There are some limitations to expanding the frequency bandwidth of usual acoustic sensors, such as microphones, in particular, because they are intrinsically designed for dynamic pressure measurements and, thus, for removing dc components. Alternative methods for measuring pressure variations are, therefore, of fundamental interest, particularly to establish a metrological link between static and dynamic pressures, which is currently missing. The proposed solution involves adapting an FP interferometer to measure pressure variations around atmospheric pressure in the infrasound range. Since acoustic movement involves both small pressure and temperature variations, taking into account the effects of both of them in the interferometer measurement results is one of the main challenges here. As the temperature variation field in the measurement system is difficult to characterize experimentally, modeling is necessary for data analysis. The experimental results are in agreement with both static pressure and acoustic calibrated reference sensors in a frequency range from 40 mHz to 1.5 Hz, with a maximum deviation of 3%. Therefore, solutions and improvements are proposed to bring this new sensor up to the level of an infrasound reference instrument.