Measurements of air group-refractive index using a tunable repetition rate of a femtosecond pulse train

Summary form only given. We propose a new method of direct group-refractive-index measurement using a repetition rate of a stabilized femtosecond pulse laser. For conventional phase-refractive-index measurement, the comparison between an optical path through air and an equivalent path in vacuum is necessary. In contrast, our new method utilizes an interferometer between adjacent femtosecond pulses in a mode-locked train without the necessity of a long reference arm in vacuum, which makes the setup simple. Tunable and stabilized repetition rate of a femtosecond pulse laser gives a high-accuracy in the measurement.