All-optical infrasound sensor based on a Fabry-Perot interferometer

Recent demands for infrasound measurement, whether for detecting violent atmospheric events or evaluating the environmental impact of wind farms, require new acoustic calibration standards in the frequency range below 2Hz. Expanding the frequency bandwidth of microphones remains a limited strategy since they are designed to filter out continuous components. An alternative method for measuring infrasound is proposed, which relies on a Fabry-Perot interferometer. This technique has been studied extensively for static pressure measurements with success. Therefore, adapting a Fabry-Perot refractometer to perform acoustic measurements is not only a technical challenge but also an opportunity to bridge the gap between static pressures and acoustic metrology. As acoustic movement involves both small pressure and temperature variations, it is important to account for their coupled effects in interferometer measurement results. Since the temperature variation field in the measurement system cannot be characterised experimentally, an analytical model has been developed. The chosen theoretical and technical solutions are validated by the good agreement between experimental results obtained with the refractometer carried out and other calibrated sensors in the frequency range from 40mHz to 1.5Hz. These results also highlight the improvements required to bring the device up to the level of an infrasound reference instrument.