Direct Measurement of Index of Refraction of Air by Means of High-Resolution Laser Interferometry

Abstract

Laser interferometers are precise distance measurement devices with resolution in nanometer or sub-nanometer region. If interferometric measurements are carried out under atmospheric conditions (usual situation in industry), they measure the optical path length of an unknown distance instead of its true geometrical value. It is caused by an index of refraction of air that introduces a multiplicative constant to measured results. If we would like to obtain correct values of the unknown distance the knowledge of the index of refraction is necessary. In this work, we present design and the first experimental results of two methods of the direct measurement of the index of refraction, where a Fabry-Perot (FP) interferometer is used as a detection system. The first method employs an evacuable cell inserted into the mirrors of the FP interferometer and the other method consists of a differential setup of two FP interferometers, one of which is permanently evacuated.