Fabry-Pérot based refractometry : development of a transportable refractometer for assessment of gas pressure

Abstract: A unified description of physical phenomena through measurement science is one of the foundational pillars in a global society. The International System of Units (SI) is the most widely used system of units and since its redefinition in 2019, all units encompassed by it are based on fundamental physical constants. The units of the SI, such as the second, metre, and kilogram, are realized by the use of primary standards which are used, through calibration chains, to certify the accuracy of measuring devices in our society. Its redefinition enabled the realization of the SI-unit for pressure (pascal) in a novel way; instead of force per area (N/m2), it can alternatively be defined as an energy density (J/m3). Subsequently, this opened up for the use of optical realizations of the pascal (Pa). It has been prophesied that a possible means to do this is by assessing refractivity through the use of Fabry-Pérot (FP) refractometry. Although such instrumentation indeed can assess refractivity, it has unfortunately been found that they in practice are affected by various types of disturbances that aggravate assessments with the required uncertainty.This thesis describes the development of FP-based refractometers utilizing a novel measurement methodology, denoted gas modulation refractometry (GAMOR). By the use of rapid gas modulation and baseline interpolation, GAMOR has the ability to significantly reduce the influence of various types of disturbances, not least drifts and fluctuations. From this, two FP-based refractometers have been developed; one stationary, denoted the SOP, capable of assessing pressure with an uncertainty of [(10 mPa)2 + (10 × 10−6·P)2]1/2, and one transportable, denoted the TOP, with an uncertainty of [(16 mPa)2 + (28 × 10−6·P)2]1/2. Furthermore, it was shown that their mutual short-term precision is excellent, with a deviation of only 0.04 ppm when simultaneously assessing a pressure of 16 kPa.A major part of this thesis was devoted to the construction of the TOP and an investigation of its transportability and performance. It was used in a ring comparison with various pressure standards at four European national metrology institutes. It was concluded that, despite being transported, the performance remained virtually unchanged, and that, in the 10 – 90 kPa range, all the standards agreed within their uncertainties.These results indicate that FP-based refractometers utilizing the GAMOR methodology have the potential to act as transportable standards based on fundamental physical constants and paves the way for future research within the field.