Phase and group refractive indices of air calculation by fitting\nof phase difference measured using a combination of laser and\nlow-coherence interferometry

The air refractive index is an important parameter in interferometric length measurements, since it substantially\naffects the measurement accuracy. We present a refractive index of air measurement method based on monitoring\nthe phase difference between the ambient air and vacuum inside a permanently evacuated double-spaced cell. The cell is placed in one arm of the Michelson interferometer equipped with two light sources—red LED and HeNe laser, while the low-coherence and laser interference signals are measured separately. Both phase and group refractive indices of air can be calculated from the measured signals. The method was experimentally verified by\ncomparing the obtained refractive index values with two different techniques