Fast Switching Dual Fabry-Perot Cavity Optical Refractometry - Methodologies for Accurate Assessment of Gas Density

TL;DR

This paper introduces a fast switching dual Fabry-Perot cavity optical refractometry method that achieves drift-free, accurate gas density and flow rate assessments by rapid measurements and calibration, even under non-temperature stabilized conditions.

Contribution

The paper presents a novel fast switching DFPC-OR technique that overcomes cavity drift limitations for precise gas density and flow measurements in various conditions.

Findings

Drift-free measurements achievable for short times using dual laser locking.

Methodology effectively assesses gas density and flow rates in open and closed compartments.

Technique exhibits minimal temperature dependence, enhancing accuracy.

Abstract

Dual Fabry-Perot cavity based optical refractometry (DFPC-OR) has a high potential for assessments of gas density. However, drifts of the FP cavity often limit its performance. We show that by the use of two narrow-linewidth fiber lasers locked to two high finesse cavities and Allan-Werle plots that drift-free DFPC-OR can be obtained for short measurement times (for which the drifts of the cavity can be disregarded). Based on this, a novel strategy, termed fast switching DFPC-OR (FS-DFPC-OR), is presented. A set of novel methodologies for assessment of both gas density and flow rates (in particular from small leaks) that are not restricted by the conventional limitations imposed by the drifts of the cavity are presented. The methodologies deal with assessments in both open and closed (finite-sized) compartments. They circumvent the problem with volumetric expansion, i.e. that the gas density in a measurement cavity is not the same as that in the closed external compartment that should be assessed, by performing a pair of measurements in rapid succession; the first one serves the purpose of assessing the density of the gas that has been transferred into the measurement cavity by the gas equilibration process, while the 2nd is used to automatically calibrate the system with respect to the relative volumes of the measurement cavity and the external compartment. The methodologies for assessments of leak rates comprise triple cavity evacuation assessments, comprising two measurements performed in rapid succession, supplemented by a 3rd measurement a certain time thereafter. A clear explanation of why the technique has such a small temperature dependence is given. It is concluded that FS-DFPC-OR constitutes a novel strategy that can be used for precise and accurate assessment of gas number density and gas flows under a variety of conditions, in particular non-temperature stabilized ones.