Methodology for internally referenced assessment of cavity deformation in Fabry-Perot refractometry

Fabry-Pérot (FP) cavity-based refractometry enables low-uncertainty realizations of pressure; however, its accuracy is fundamentally constrained by pressure-induced cavity deformation. We report a pressure-standard-independent method for in situ characterization of cavity deformation employing two connected refractometers using the Diffusion-Isolated Pressure Equilibration (DIPE) methodology, which allows two compartments containing dissimilar gases to reach thermodynamic pressure equilibrium while preventing gas mixing. By comparing the differential responses of two refractometers when used with identical and different gases (He and Ar), where one serves as the reference for the other, the cavity deformations of both cavities are determined with high accuracy without the use of a pressure standard.