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Picometer metrology for precise measurement of refractive index, pressure, and temperature



Jack A. Stone Jr., Patrick F. Egan, Jay H. Hendricks, Gregory F. Strouse, Douglas A. Olson, Jacob E. Ricker, Gregory E. Scace


For several years we have been studying the use of Fabry-Perot interferometers for precise measurement of the refractive index of gasses, where the primary motivation has been to improve interferometer-based length measurement. Because the refractive index of a gas depends on its pressure and temperature, we can also use refractive index to monitor either of these quantities if the second is known. Recently we have embarked on a project that will utilize refractive index to infer pressure with a smaller measurement uncertainty than is currently possible, hoping to reach a relative standard uncertainty of 1.4x10-6. The projected uncertainty budget is currently dominated by uncertainty of the Boltzmann constant, but following the coming redefinition of SI units, the Boltzmann uncertainty will be replaced by uncertainty in the temperature scale, at which point refractive index measurements can be expected to play a central role in precise realization of thermodynamic temperature. Dimensional metrology with picometer uncertainties is the core of our technique and is the subject of this paper. Refractive index will be measured by comparing two precisely equal displacements (150 mm), where one displacement is in vacuum and the second is in helium and will appear to be slightly longer due to the refractive index. The two displacements must be compared with
Proceedings Title
NCSLI 2013 Workshop & Symposium, Proceedings
Conference Dates
July 14-18, 2013
Conference Location
Nashville, TN


dimensional metrology, pressure metrology, interferometry, Fabry-Perot
Created July 14, 2013, Updated February 19, 2017