Fabry-Perot interferometers can be used for very precise measurement of the refractive index of gasses. This can enable increased accuracy of interferometer-based length measurement. In addition, because the refractive index of a gas depends on its pressure and temperature, measurements of refractive index can be used to monitor either one of these quantities if the second is known. Recently we have embarked on a project with a goal of measuring pressure with a relative standard uncertainty below 1.4x10^-6. Dimensional metrology with picometer uncertainties is the core of this 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 < 3 pm uncertainty. Major challenges include many of the typical sources of error in dimensional measurement, such as Abbe errors, alignment errors, material dimensional stability, etc. Careful consideration must be given to second-order effects that are not normally large enough to merit mention. The proposed experimental design will minimize the major sources of error while providing additional metrology (including angle measurements with nanoradian precision) to correct residual errors.
Citation: NCSL International Measure
Pub Type: Journals
dimensional metrology, pressure metrology, interferometry, Fabry-Perot