Measurement of index of refraction of air by optical frequency method

Abstract

Laser interferometers are even more precise distance measurement devices with resolution up to sub-nanometer region. If the measurements are carry out under atmospheric conditions (usual situation in an industry), the interferometers measure 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 want correct values of the distance measurement the knowledge of the instantaneous value of the index is necessary. In the work, we present design and the first experimental results of method of the direct measurement of the index, where a Fabry-Perot (F.-P) interferometer is used as a detection system. The method employs a differential setup of two F.-P interferometers, where the cavity of the first is permanently evacuated and the other is on the air. The ultimate resolution of the measurement and the operating regime without need of a vacuum pump stay the method very advantage. The work includes comparison of the method with conventional refractometer where evacuatable cell is inserted into the measuring arm of Michelson interferometer. The comparison of the method with indirect measurement of the index with using Edlen formula is presented too.