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

Isak Silander, Thomas Hausmaninger+3 more|arXiv (Cornell University)|Apr 3, 2017

Spectroscopy and Laser Applications5 citations

TL;DR

This paper presents fast switching dual Fabry-Perot cavity optical refractometry (FS-DFPC-OR), a novel method that eliminates drift-induced errors in gas density and flow rate measurements by using two laser-locked cavities and rapid sequential measurements. The technique enables accurate, drift-free assessments in both open and closed systems, even under non-temperature-stabilized conditions, with minimal temperature dependence and high sensitivity to small leaks.

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.

Motivation & Objective

• To overcome the performance limitations of dual Fabry-Perot cavity refractometry caused by cavity drift.
• To enable accurate gas density and flow rate assessments in non-temperature-stabilized environments.
• To address the challenge of volumetric expansion in closed compartments by using paired measurements to determine relative volumes.
• To develop methodologies for detecting small gas leaks using triple-cavity evacuation techniques.
• To minimize temperature dependence in refractometric measurements through intrinsic design and calibration.

Proposed method

• Utilizes two narrow-linewidth fiber lasers locked to high-finesse Fabry-Perot cavities to enable stable, drift-free operation.
• Employs rapid switching between two measurement states: one for gas equilibration and one for system calibration.
• Applies Allan-Werle plots to verify drift-free performance over short measurement times.
• Performs a pair of sequential measurements to determine the relative volume ratio between the measurement cavity and the external compartment.
• Uses a triple-measurement protocol—two rapid successive measurements followed by a third after a delay—to assess leak rates.
• Leverages the intrinsic temperature insensitivity of the dual-cavity configuration to reduce thermal drift effects.

Experimental results

Research questions

• RQ1 How can cavity drift be effectively mitigated in dual Fabry-Perot cavity refractometry to enable accurate gas density measurements?
• RQ2 What measurement strategy allows for accurate assessment of gas number density in closed, finite-sized compartments despite volumetric expansion?
• RQ3 How can small gas leaks be detected with high sensitivity using optical refractometry?
• RQ4 What is the origin of the low temperature dependence in the proposed system, and how is it exploited for stable operation?
• RQ5 Can the method achieve high-precision measurements without requiring temperature stabilization?

Key findings

• The FS-DFPC-OR method achieves drift-free operation for short measurement times, as confirmed by Allan-Werle plots.
• The technique enables accurate determination of gas number density in both open and closed systems by using a pair of rapid sequential measurements to calibrate for volume ratios.
• Leak rates are assessed with high sensitivity using a triple-cavity evacuation protocol involving two fast measurements and a delayed third measurement.
• The system exhibits minimal temperature dependence due to the differential response of the two laser-locked cavities, which inherently compensates for thermal drift.
• The method is effective under non-temperature-stabilized conditions, making it suitable for field and industrial applications.
• The approach allows for precise and accurate assessment of gas flows, particularly from small leaks, without requiring prior knowledge of system geometry.