Development and characterization of a Fabry-Perot refractometer for gas pressure measurement

This thesis work deals with the development and characterization of a single-cavity Fabry-Perot refractometer operating at 532 nm, designed to measure the pressure of pure gases between 100 Pa and 100 kPa. The aim of this new instrument is to improve on the best current measurement capabilities obtained in France by conventional methods based on the measurement of a force applied to a surface. First, the fundamental principles of refractometry are reviewed, detailing the relationships between the refractivity of a pure gas, its molar density and its thermodynamic pressure. The values of gas density and refractivity virial coefficients and their uncertainties have been extrapolated to the working wavelength and temperature of the refractometer developed at LNE-Cnam. After a detailed description of the Fabry-Perot cavity, measurements of its intrinsic parameters and their uncertainties (coefficient of thermal expansion, pressure-induced distortion coefficient, long-term drift and free spectral range) are presented. They make it possible to correct pressure measurement errors due to cavity deformations during the measurement procedure. Methods for controlling millikelvin temperature, stabilizing the gas pressure as well as limiting the thermal effects of gas expansion and compression are described. A metrological characterization of the instrument makes it possible to establish a complete uncertainty budget, and to identify the improvements needed to reach the level of uncertainty of the national references around atmospheric pressure and exceed them for lower pressures. Measurement reproducibility is assessed at 2.7 ppm. A comparison with a PG7607 reference pressure balance between 30 kPa and 100 kPa shows a relative deviation of less than 8 ppm, which is lower than the best current uncertainties. Finally, this thesis explores an innovative application of refractometry for measuring acoustic pressure in the infrasound range, enabling continuity between static and acoustic pressure measurements which is still missing nowadays.

refractivity, Fabry-Perot interferometer, Virial coefficient, Pressure metrology, thermodynamics, opto-acoustic sensor

PhD Thesis (FR) : https://theses.hal.science/tel-04352332v1/file/CNAM_REZKI_2023.pdf